Bridging efficiency and stability in organic solar cells: architectural design, degradation insights, and emerging mitigation strategies

Abstract

Organic solar cells (OSCs) have received much interest because of their high power conversion efficiencies (PCEs >20%), solution processability, light-weight nature, flexibility, and possibility for low-cost fabrication. Nevertheless, their commercialization is still a significant challenge due to the inability to stabilize their long-term operation. Degradation of the device can be caused either by intrinsic factors (such as the morphological instability of the bulk heterojunction (BHJ) active layer and electrodes and interfacial diffusion of the transport layers) or extrinsic factors (such as exposure to oxygen, moisture, and light, mechanical stress, etc.). These problems severely reduce the device life. This review gives an overview of the current developments in enhancing the stability of OSCs via encapsulation, introduction of additives, interfacial engineering, optimization of the active layer, and the rational molecular design of acceptor and donor materials. It focuses on the principles of degradation and possible ways to improve the efficiency and durability of organic photovoltaics. To conclude, this review identifies the prospects of developing stable, eco-friendly and efficient OSCs in the future and provides comprehensive insights into the degradation processes and mitigation plans. Because of their ability to combine efficiency, durability and environmental sustainability, OSCs can be highly significant in the transition towards next-generation renewable energy systems.

Organic solar cells (OSCs) have received much interest because of their high power conversion efficiencies (PCEs >20%), solution processability, light-weight nature, flexibility, and possibility for low-cost fabrication.

1. Introduction

The world's energy use is continuously growing, leading to a great demand for sustainable and renewable energy sources. Due to depleting fossil fuel supplies and growing concerns over climate change, solar energy has become one of the most promising clean and renewable sources of energy globally. Solar cell technologies have gained considerable research interest as methods for directly converting sunlight into electricity, and therefore, they have the potential for sustainable electricity generation. Organic solar cells (OSCs) have been receiving a lot of interest in the field of emerging PV technologies due to their low energy consumption and manufacturing cost, mechanical flexibility, lightweight design and short energy payback times. These special properties make OSCs an appealing alternative to traditional PV technologies, such as silicon-based photovoltaic panels, especially in flexible electronics, stand-alone power systems, indoor energy harvesting and PV building-integrated applications, where the use of conventional rigid silicon-based solar panels is not practicable.^1,2

There are generally three generations of solar cell technologies. Current first-generation PV modules are mainly crystal silicon-based, with laboratory power conversion efficiencies of up to 26%; these are the dominant type of PV module in the market based on installed capacity, but the production of these modules involves energy-intensive wafer processing and requires strict module architectures. Second-generation technologies are based on amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium gallium selenide (CIGS); they use less materials to achieve efficiencies of 10–23% and can be designed flexibly.³ Third-generation technologies include quantum dot solar cells, dye-sensitised solar cells (DSSCs), perovskite solar cells (PSCs) and organic solar cells (OSCs); they are characterised by their low-temperature and solution-processable production methods, potential for short energy payback times, and ability to be designed for specific spectral response, geometry and form. Of these technologies, OSCs occupy a special place because of their mechanical flexibility, tunable optical transparency, lightweight structure, and ability to be manufactured using roll-to-roll technology on flexible plastic substrates, but they also have a much shorter life span than commercially available silicon photovoltaic (PV) modules.^4,5Fig. 1 shows the developmental route to advanced PV systems based on semiconductor materials, in which the emergence of the new solar cell technologies can be observed.

Fig. 1. Generational evolution of solar cell technology. Redrawn and modified from ref. 6.

One of the major developments in solar technology is the use of carbon-based materials in organic photovoltaics (OPVs). Unlike conventional inorganic solar cells, OPVs are largely organic semiconductors, including polymers and small molecules. The creation of OPVs began when the photovoltaic effect of organic substances was found.^7,8 In 1986, the first advancement of OPVs was the creation of the so-called Tang cell, a two-layer, perylene diimide and copper phthalocyanine device. This discovery introduced the concept of bulk heterojunctions and made the creation of OPVs possible. The BHJ active layer facilitated the creation of bi-continuous networks between acceptor and donor materials, and these networks substantially increased charge generation and exciton dissociation. With these important findings, OSC research has experienced major leaps forward in the last few decades because of improvements in material design and device optimisation with a view to achieving higher PCEs.^9,10 The technology utilizes organic semiconductor materials as the photoactive layer, which has several advantages over perovskite- and silicon-based solar cells. OSCs are readily movable and have a high flexibility; hence, they can be used in handheld electronic gadgets and wearable appliances. They also allow cheap production methods and are thus less expensive to manufacture.^11,12 Moreover, OSCs can be programmed to absorb defined colours of light, thereby enabling them to be programmed to look and behave in any way desired. Also, OSCs are more productive in low-light environments.¹³ Laboratory-scale work has recently demonstrated OSC power conversion efficiencies of more than 20%, especially in optimised single-junction and tandem designs; these values are comparable to those offered by current inorganic photovoltaic systems.^14,15Table 1 presents a comparison between OSCs and other technologies.

Table 1. Comparison of OSCs with other solar cell technologies.^3–15.

Features	OSCs	Silicon solar cell	PSCs	DSSCs
Efficiency (lab scale)	Moderate (>20%)	Very high (∼26%)	Very high (>26%)	Moderate (∼12–14%)
Cost of fabrication	Low	High (wafer based)	Low (solution based)	Low
Operating stability	Low-medium	Very high (20–25 years)	Medium (sensitive to light, heat, etc.)	Medium
Flexibility	Very high	Low (rigid wafers)	High (thin film, roll to roll possible)	High
Toxicity issue	Some solvents toxic	Minimal	Lead content, solvent use	Electrolyte leakage possible
Scalability potential	Developing stage	Already fully commercialized	Promising, but needs stability improvement	Limited
Energy payback time	Very short	Longer (1–2 years)	Very short (months)	Short
Commercial maturity	Emerging	Fully commercial	Pre-commercial	Limited niche
Key limitations	Stability and degradation	High energy processing cost	Instability, ion migration	Electrolyte evaporation, sealing

The PCE of OSCs depends on three main parameters: open-circuit voltage (V_oc), fill factor (FF), and short-circuit current density (J_SC).¹⁶ To optimize OSCs, researchers have dedicated efforts to determining the best donor and acceptor materials with similar energy levels. Some fullerene derivatives have found wide use as electron acceptors, such as PC₆₁BM, PC₇₁BM and ICBA, based on their high mobility in terms of electrons.¹⁷ These fullerene acceptors (FAs) do not absorb as much light in the near-infrared and visible regions, which limits the sunlight that the acceptors can absorb. To overcome this, new donor materials were designed, and high-performance donors including PTB7 and PffBT4T-2OD have evolved. These materials contain conjugated backbones (alternating electron-accepting and electron-donating units) that enhance light absorption in the visible range and accelerate charge transfer. Thus, OSCs with PCEs as high as 12.1% have been achieved.¹⁸

Meanwhile, non-fullerene acceptors (NFAs), which can be selectively fine-tuned to particular chemical structures, have several benefits over the existing fullerene-based materials, including increased light absorption, adjustable energy levels, and reduced recombination losses.¹⁹ In recent years, many NFAs, such as Y6 and ITIC, have been developed, and they represent a new type of research on OSCs.²⁰ Moreover, the introduction of high-performance donor materials, like PM6 and D18, and streamlining of the device structure and working processes led to performance improvements.²¹ Because of innovations such as single-junction OSCs, PCEs of greater than 19% have been achieved.²² In tandem OSCs, efficiency has been greatly increased. A long NIR absorbent sub-cell was evolved into a prototype tandem OSC that exhibited a respectable PCE of 15.9% through the incorporation of narrow-bandgap NFAs and expansion of the charge recombination region in tandem. In addition, a high PCE of 20.27% was achieved when TiO_x and PEDOT : PSS interconnecting layers were constructed. This was the first tandem OSC that attained the landmark of 20% efficiency, demonstrating the outstanding progress of tandem OSC technology in terms of performance and practical application.²³

The continuous increase of OSC efficiency over the last decade reflects a shift from fullerene-based acceptors to more advanced NFA systems, as well as the gradual enhancements in molecular architecture, morphology, and interfaces. Initially (2010–2016), the use of fullerene derivatives, such as PCBM, dominated OSCs, achieving efficiencies in the range of 7–11% because of the limited absorption band, inefficient energy-level selection, and limitations in the tuning of electronic properties.^24–26 In 2017, the coaxial implementation of early NFAs and, most notably, the ITIC family, made huge strides in spectral coverage, as well as in reducing voltage losses, which resulted in efficiencies of about 10–12%.^27,28 In 2018, ternary OSC strategies and reaction optimization of donor–acceptor pairings were adopted again and further increased charge transport and exciton dissociation to efficiencies approaching 12–15%.^29–32 The period from 2019 to 2021 saw a significant advance in OSC performance due to the universal use of high-performance NFAs, particularly the Y6 series of molecules. These materials allowed lower non-radiative recombination, better morphology of the BHJ, and enhanced energy-level alignment, reaching 15–16% efficiency in 2019 and 17–18% in 2021.^33–38 Simultaneous progress in donor–acceptor molecular packing, refined interfacial layers and optimized charge extraction also stabilized device operation and increased the fill factors to efficiencies of the order of 17% in 2020.^39–42 Further tuning of the fine morphology and crystallinity maximization, together with increased control over the miscibility between acceptor and donor, resulted in efficiencies of up to 18.7% in 2022^43–45 and the minimization of energy loss and optimization of donor–acceptor miscibility achieved efficiencies of up to 19.3% in 2023.^46–48 These advances suggest the significance of nanoscale phase separation and controlled crystallization in realizing effective exciton dissociation and charge collection. The NFA-based OSC technology is mature, as shown by the recent breakthroughs in 2024–2025. Crystallization control and interfacial dipole engineering were some of the strategies that made efficiencies of ∼19.5% possible in 2024.^49–51 This was subsequently optimized with the inclusion of thermally activated delayed fluorescence materials to reduce energy loss, and novel additive engineering using non-halogenated processing and two-step crystallization methods to promote film ordering, with efficiencies of up to ∼20–21% reported in 2025.^52–54 Furthermore, the realization of perovskite-based tandem designs has driven the performance threshold to 26.7%. Hybrid tandem designs have been able to push the limit of performance to 26.7 percent.⁵⁵ All in all, the combination of these developments highlights the fact that synergistic optimization of molecular structure, film morphology, and interfacial energetics is a major factor behind the efficiency growth rate of current non-fullerene OSCs, as depicted in the efficiency trend graph shown in Fig. 2.

Fig. 2. Evolution in power conversion efficiencies of OSCs. Data taken from ref. 16–55.

Nevertheless, these amazing efficiency results are surrounded by ongoing issues of stability of operation and the durability of devices. In contrast to inorganic photovoltaics, OSCs are associated with rapid performance degradation when subjected to environmental stress factors such as light, oxygen, moisture and high temperatures. The low stability of OSCs prevents their commercialization because commercial OSCs must last longer than 5 years.⁵⁶ Nevertheless, a number of studies have clearly shown that stability, and not efficiency, has become the most significant constraint to viable OSC implementation. When OSC devices are stressed by real-world environmental factors like oxygen, moisture, thermal cycling and extended illumination, performance degradation is reflected in very low rates. Weak van der Waals forces in the organic materials and metastable nanoscale morphologies are some of the contributors to performance degradation mechanisms that are still to be curtailed.⁵⁷

The degradation pathways in OSCs have been studied extensively, and numerous new approaches have been generated to improve their stability. A few studies have indicated very stable OSCs. Du et al. demonstrated an NFA-based OSC with a lifetime that was estimated to be about 10 years.⁵⁸ By putting forward a new non-fullerene trimer acceptor, TYT, Lee and co-workers described an organic solar cell that was much more efficient and stable. The power conversion efficiency (PCE) reached 18.15% when this acceptor was blended with PM6 in forming the active layer, and the device was very stable, with a T₈₀ life of 8454 hours.⁵⁹ Therefore, a review on recent gains in the stability of OSCs is required, from which researchers can gain a better understanding and address the stability bottleneck preventing commercialization. This review aims to give a critical and thorough examination of the stability and degradation pathways of OSCs. The review systematically deals with both intrinsic degradation mechanisms, including active-layer morphology evolution and material photochemical instability, and extrinsic mechanisms such as environmental exposure and interfacial failure. Moreover, the latest developments in stabilization approaches, including encapsulation technologies, device engineering, and molecular design, are presented in detail. By correlating degradation mechanisms with material properties and device architectures, this review will provide a clear understanding of the existing challenges and future directions for achieving long-lived and high-performance OSCs. This combined outlook is needed to direct future research to the rational design of stable organic photovoltaic systems that can be implemented on a large scale.

2. Fundamentals of organic solar cells

OSCs are a type of PV device that harnesses sunlight to produce electricity through organic semiconducting materials, typically conjugated polymers or small molecules. In contrast to inorganic solar cells (e.g., silicon), OSCs have excitonic photophysics and use highly engineered layered structures to enable light absorption, exciton dissociation and charge transport.⁶⁰

2.1. Basic device architecture

An average organic solar cell (OSC) is made up of a multilayer design such as that shown in Fig. 3, which is designed in such a way to maximize light capture, charge separation, and carrier harvesting. In the majority of designs, incident sunlight is absorbed via a transparent anode like indium tin oxide (ITO), which not only passes light but also gathers holes created in the photoactive material.⁶¹ Below the anode is the hole-transport layer (HTL), which is a thin layer of interfacial material that selectively extracts holes and rejects electrons and eliminates recombination losses; commonly used materials here are conductive polymers (such as PEDOT : PSS) or inorganic oxides engineered to have good energy alignment and stability.⁶² The active layer is the heart of the device and is usually a bulk heterojunction film of an electron-donating polymer and an electron-accepting substance that provides the highest possible exciton dissociation, forming an interpenetrating network of acceptors and donors with nanoscale phase separation. This BHJ structure is very effective for enhancing charge generation and transport over planar heterojunctions and has been the central focus of creating PCEs of more than 19% in recent non-fullerene acceptor systems.⁶³ An electron-transport layer (ETL) is a selective electron extractor placed just below the active layer and is designed to harvest electrons and suppress holes, with inorganic ETLs being particularly effective in inverted architectures because they show preferential conduction band levels and better stability.⁵⁷ Ultimately, the stack is topped by a low work function metal cathode (e.g. silver or aluminium), which gives the stack effective electron collection capability and acts as the back contact of the device.⁶¹

Fig. 3. Organic solar cell device structure. Redrawn and modified from ref. 61.

2.2. Roles of the multilayers

Light absorption and charge generation are the key functions that are carried out by the active layer in OSCs. The active layer is usually based on a bulk heterojunction mixture of acceptor and donor semiconductors that receive incident photons and create bound electron–hole pairs called excitons. A nanoscale morphology of the acceptors and donors selectively encourages exciton dissociation at the interface areas, and free charges are transported to their corresponding electrodes. This layer mainly depends on its morphology, phase separation, and energy-level matching to control open-circuit voltage, short-circuit current and PCE, with current and future designs of the system relying on advanced and improved versions of the BHJ allowing OSC efficiencies of nearly 20% to 25% in tandem and ternary configurations.⁶⁴ Nevertheless, this layer is also exposed to photochemical and thermal damage, where domain coarsening and traps can drastically decrease carrier mobility and cause loss of performance over time. Extensive evaluations of active layer molecular arrangement are clear on the significance of a stable arrangement in terms of both high performance and longevity of the device.⁵⁷

OSC electrodes, which are charge collectors, need to be electrical conductors and have an ideal balance of energy levels to reduce energy losses and achieve maximum carrier efficiency. A clear bottom electrode, usually indium tin oxide (ITO), gathers holes (in conventional designs) and allows easy light penetration into the gadget. The upper electrode (which is commonly a metal, e.g. aluminium, silver or gold) gathers electrons or holes, based on the polarity of the device (conventional vs. inverted).⁶⁵ The properties of the electronic contact are directly affected by the choice of electrodes and electrode interface engineering, and it affects the efficiency of charge extraction and long-term stability. As an example, traditional low work function cathodes (e.g., Ca/Al) may oxidize at ambient temperatures, and their devices are also quickly degraded. One of the strategies that has been proven to increase the operation lifetimes is the introduction of inverted architectures using air-stable electrodes and optimized interfacial layers.⁶⁶ ETLs (e.g., ZnO and SnO₂) allow the electrons to be extracted toward the cathode, whereas the HTLs (e.g., PEDOT : PSS and doped metal oxides) permit the holes to be extracted toward the anode while blocking the recombination of electrons. Interfacial layers play a vital role in determining the level of energy alignments across the device stack and hence minimizing recombination losses and maximizing carrier collection. Notably, another role of transport layers is to ensure the stability of the device by limiting the diffusion of the active layer and exposure to environmental stressors. Indicatively, solution-processed, solution-to-solution, solution-to-gas, solution-to-liquid, solution-to-dry, solution-to-solid, solution-to-biological, and solution-to-metal transfers have enabled the fabrication of solution-based transport layers with high efficiency and higher stability by enhancing charge collection and mitigating interfacial degradation.⁶⁷ Also, other recent studies on hybrid hole-transport materials have indicated that charge transport and environmental robustness can be optimized by modulating HTL composition at the same time.⁶⁸ Functional layers and their role in OSCs are illustrated in Table 2.

Table 2. Functional layers and their roles in OSCs.

Layers	Typical materials	Primary function	Major stability issues	Ref.
Transparent electrode	ITO, FTO	Light transmission, charge collection	Indium diffusion, brittleness	65 and 69
HTL	PEDOT : PSS, MoO3, NiOx	Hole extraction, electron blocking	Hygroscopicity, acidity, interfacial reactions	70
Active layer (BHJ)	PM6, PTB7, Y6, ITIC	Light absorption, exciton dissociation	Photo-oxidation, phase separation, morphology instability	71
ETL	ZnO, SnO2, TiOx	Electron extraction, hole blocking	UV-induced defects, interface trap states	67 and 70
Metal electrode	Ag, Au, Al	Charge collection	Oxidation, metal diffusion	12

Although layered OSC structures provide improved performance, they are sensitive to light, heat, oxygen and moisture, with each component getting affected differently, resulting in degradation. As a case in point, the dynamics of the acceptor–donor reorganization in the BHJ active layer lead to morphological changes in the active layers, which reduce its performance over time; this means that the stability of the active layers is essential for sustained operation. Some PEDOT : PSS systems are rather sensitive to environmental factors, and interfacial degradation occurs when electrodes are exposed to humid and light environments; moreover, chemical reactions on the ETL/active layer interface can introduce trap states that accelerate device aging.⁷⁰ The mechanisms of degradation require the design of materials, interface engineering, and encapsulation approaches to enhance the working life of OSCs, which has been a major obstacle to commercialization despite breakthroughs in efficiency and stability studies.⁷²

2.3. Organic semiconductor materials for active layers

The photoactive layer is the working part of an OSC and is the region where light absorption, exciton formation, charge separation and charge transport take place. The characteristics of organic semiconductors used play a major role in the performance of this layer. In modern OSCs, the active layer is usually composed of two different classes of organic semiconductors: conjugated polymer donors and small-molecule acceptors, most of which are NFAs. The advantages of these materials for PV usage stem from a combination of electronic, optical, morphological, and stability-related properties, which together control the PCE and operational lifetime.⁷³ The energy levels of an efficient organic semiconductor should allow the efficient transfer of charges across the donor–acceptor interface and have an open-circuit voltage high enough for practical applications. Exciton dissociation and charge separation with minimum energy loss are achieved by appropriate alignment of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels. Moreover, a high and broad optical absorption is necessary to achieve the highest solar photon harvesting. Organic semiconductors have an outstanding molecular tunability that can be rationally designed for the desired electronic structure and absorption profile, which is not possible in most inorganic semiconductors. Another key criterion is high charge-carrier mobility, which is important because the efficient transport of electrons and holes minimizes recombination losses and enhances charge collection. In addition, the active-layer materials should have good film-forming properties and self-organize into desirable nanoscale morphologies. The creation of an interpenetrating donor–acceptor network with domain sizes that are on the order of the exciton diffusion length is especially relevant for efficient charge generation.⁷⁴

Conjugated polymer donors are one of the two main types of semiconductors used in OSC active layers. They consist of materials with extended π-conjugated backbones, which enable efficient hole transport and absorption of visible light, such as P3HT, PTB7-Th, PM6, and D18. The vast majority of high-performance polymer donors use donor–acceptor copolymer architectures where alternating electron-rich and electron-deficient units are used. This molecular design approach allows precise control of frontier molecular orbital energies, optical bandgaps, and intermolecular interactions. The chemical structure of the backbone and side chains can be modified to improve the absorption, charge transport, and blend morphology. The planarization of polymer backbones allows for strong π–π stacking interactions, which are beneficial for charge-carrier transport and for improving hole mobility. At the same time, side-chain engineering affects molecular packing and solubility, as well as compatibility with acceptor materials, which, in turn, affects the nanoscale organization of the bulk heterojunction. Importantly, the conformational rigidity of highly planar conjugated backbones can also enhance photostability by minimizing conformational disorder and bond rotation or distortion-induced degradation pathways.^75,76

The second big class of active-layer semiconductors is small-molecule acceptors, such as non-fullerene acceptors, which have largely superseded fullerene derivatives in state-of-the-art OSCs. Some examples are the ITIC family and Y-series materials like Y6, BTP-eC9, and L8-BO. Typical molecules are built with fused-ring electron-deficient cores with strong electron-accepting groups at the end of the molecules, creating A–D–A (acceptor–donor–acceptor) or similar structures. The molecular structure provides for strong intramolecular charge–transfer interactions, which give rise to a broad, strong absorption stretching from the visible to the near-infrared. The large absorption coefficient of NFAs greatly improves light harvesting and complements the absorption spectra of polymer donors. Also, they have well-defined molecular structures that enable them to pack tightly together with each other and transport electrons effectively. The fused-ring structure is very rigid and planar, which favors good electronic coupling between adjacent molecules, and it also inhibits conformational changes that might lead to photodegradation. These have been important factors driving the dramatic increase in OSC efficiency over the last 10 years.^77,78

In addition to optical absorption and charge transport, the electronic quality of the semiconductor materials is also significantly affected by the density of defect states and non-radiative recombination pathways. Minimizing non-radiative voltage loss is a key design goal in high-performance OSCs, as these losses directly reduce device efficiency. Materials exhibiting low non-radiative recombination typically possess fewer energetic trap states and lower structural disorder, resulting in enhanced charge-carrier lifetimes and improved photovoltaic performance. The minimization of non-radiative losses also helps improve operational stability, as the formation of traps can be enhanced by photo-oxidation, chemical degradation, and morphological evolution. Therefore, in recent years, molecular design strategies have increasingly been developed that aim to optimize both electronic quality and durability.⁷⁹

The other crucial factor is that the donor and acceptor materials are able to keep the bulk heterojunction morphology stable during device operation. Thermal phase separation, domain coarsening, and molecular diffusion can cause the desired nanoscale morphology for efficient charge generation and transport to be lost. In recent years, there have been efforts to overcome these challenges by developing multicomponent and high-entropy active-layer systems, where multiple donors and acceptor materials are mixed to improve configurational entropy and thus suppress morphological evolution. Such approaches illustrate how active-layer design has evolved beyond simple efficiency optimization toward a broader framework in which electronic structure, molecular packing, morphology, and stability are considered as interdependent parameters.⁸⁰ In general, the advantages of active layers in OSCs are tunable energy levels, high optical absorption, efficient charge transport, solution processability, the ability to form optimized bulk heterojunction morphologies, and the potential for enhanced photochemical and morphological stability. These properties ultimately dictate the efficiency and operational stability of organic solar cells, and structure property stability relationships are key to the design of next-generation photovoltaic materials. These properties render conjugated polymers and SMAs very promising materials for next-generation OPVs. Improvements in efficiency, operational stability, and manufacturability on a large scale remain crucial tasks that require continuing molecular engineering of donor and acceptor semiconductors.⁸¹ The representative organic semiconductors utilized in OSC active layers and their salient features are displayed in Table 3.

Table 3. Representative organic semiconductors used in OSC active layers and their key characteristics.

Materials	Category	Role	Key advantages	Ref.
P3HT	Polymer donor	Donor	Good processability, stable morphology	82
PTB7-Th	Polymer donor	Donor	Low bandgap, broad absorption	83
PM6	Polymer donor	Donor	High mobility, excellent compatibility with NFAs	84
D18	Polymer donor	Donor	High fill factor and photocurrent	85
PC71BM	Fullerene acceptor	Acceptor	High electron mobility	86
ITIC	Non-fullerene acceptor	Acceptor	Tunable energy levels, strong absorption	87
Y6	Non-fullerene acceptor	Acceptor	NIR absorption, low energy loss, high efficiency	88

The choice of donor and acceptor semiconductors and their associated optoelectronic properties has a strong impact on the performance of OSCs. The efficiency and stability of the devices have been greatly enhanced by recent developments in molecular engineering, including the creation of new high-performance donor polymers and non-fullerene acceptors. Thus, the optimization of active-layer materials is a key strategy for the further development of next-generation organic photovoltaic technologies.^84–88

2.4. Synthesis methods for organic active layers

High-quality fabrication of organic active layers is a key process to achieve efficient and stable organic solar cells. The nanoscale morphology of the donor–acceptor blend directly dictates exciton dissociation, charge transport and recombination losses. As a result, a lot of research has been devoted to the development of deposition and post-treatment methods that can control the molecular organization, crystallinity and phase separation of the photoactive layer. Solution-processing techniques are especially appealing because of their low cost of fabrication, ability to be applied to flexible substrates, and potential for large-area manufacturing. The choice of processing conditions has a significant impact on the formation of films and thus has a decisive influence on the performance and operational stability of the devices.⁸⁹

Given its simplicity, reproducibility, and capability for producing highly uniform thin films, spin coating is still the most widely used laboratory-scale deposition method for OSC fabrication. The self-assembly of the donor and acceptor materials during spin coating results in the formation of a bulk heterojunction network due to the rapid evaporation of the solvents. The optimization of solvents, coating parameters and drying kinetics allows precise control of the domain size and molecular packing. Although spin coating has been essential for record power conversion efficiencies, the significant material wastage and limited scalability present challenges for its industrial application.⁹⁰

For large-scale manufacturing, blade coating and slot-die coating are good alternatives to spin coating. Blade coating enables the thickness of the coating to be controlled precisely by regulating the coating speed and the temperature of the substrates, and slot-die coating provides continuous coating with low material loss and is compatible with roll-to-roll production. The techniques also allow controlled drying dynamics and molecular aggregation to achieve optimized donor–acceptor morphologies. In recent years, improvements in processing methods have made blade-coated and slot-die-coated devices as efficient as their spin-coated counterparts, and they are important for commercial OSC production.⁹¹

One of the most promising methods to control the morphology of organic active layers is by engineering the solvent additives. High-boiling-point additives like 1,8-diiodooctane (DIO), chloronaphthalene (CN) and diphenyl ether (DPE) selectively affect the aggregation behavior of donor and acceptor materials in film formation. These additives influence the evaporation rate of the solvent and the molecular self-organization, which leads to optimal phase separation, increased crystallinity and well-balanced charge transport pathways. This means that solvent additives are often used to increase the power conversion efficiency, while reducing charge recombination and increasing morphological stability. The strategy has emerged as an important tool for the realization of high-performance non-fullerene organic solar cells.⁹²

Besides traditional solution processing methods, a number of advanced fabrication and post-processing techniques have been developed to further optimize the morphology of the active layer and the stability of the devices in OSC. Precise control of the film thickness, molecular orientation and interfacial quality is possible with vacuum-based techniques, such as thermal evaporation and organic vapour phase deposition, which allow the formation of highly uniform and reproducible photoactive layers. Because vacuum-based deposition methods, especially thermal evaporation, can be used to carefully control film thickness, molecular organization, and interfacial quality, they have also been used to fabricate highly regulated small-molecule organic solar cells. A more recent method for morphology engineering, sequential deposition or layer-by-layer (LbL) deposition, has been developed to provide better control of the distribution of donors and acceptors in the film and of the vertical scale, as well as reducing interdiffusion, which can adversely affect charge transport. In addition, post-deposition treatments like thermal annealing and solvent-vapour annealing are important for controlling molecular packing, crystallinity and nanoscale phase organization. They facilitate desirable charge–transport pathways, increase morphological robustness and prevent structural degradation during prolonged service. Therefore, the precise control of film formation and morphology evolution is still required for achieving efficient device performance and operational stability, which can help develop next-generation OSC technologies.^57,93

2.5. Conventional and inverted structures of OSCs

There are two broad categories of OSCs, traditional and inverted OSCs, with inverted OSCs being far more stable. The structure of a conventional organic photovoltaic (OPV), as shown in Fig. 4(a), has at least five layers. The anode is a transparent indium tin oxide (ITO) electrode, which gathers holes. Over this is a hole-transport layer (HTL) that helps in the injection of holes but prevents movement of electrons. The active layer is the main component of the device; it consists of a conjugated electron-donor polymer diluted with an electron-accepting substance, such as fullerene or some other n-type semiconductor, normally in an equal mass ratio.⁹⁴ The combination of ionomers comprising of poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate), i.e., PEDOT : PSS, is the most utilized HTL in the conventional architecture as it has the capacity to attain compatibility with ITO as well as superb adjustment in the energy level.⁹⁴ The most conventional active layer consists of P3HT and an organophilic fullerene: PC₆₁BM or a fullerene of a larger size such as PC₇₁BM.⁹⁵Fig. 4 shows the device structures.

Fig. 4. OSCs architectures: (A) conventional structure and (B) inverted structure. Redrawn and modified from ref. 95.

For OPV to function, photoactive organic semiconductor materials and interface layers (HTL and ETL) are crucial. This is because the energy level between the metal contacts and organic semiconductors is influenced by the charge injection induced by the photon in these solar cells, which is sensitive to the physical and chemical properties of the interfaces. Another OPV structure that BHJ cells possess is the inverted architecture Fig. 4(b). Special attention was paid to inverted OPV cells (i-OPV) because they are more stable, and in most cases, they are not degraded even in the absence of encapsulation, and within several days. Furthermore, in i-OPV, metal oxides are the materials of choice as an interface layer because, unlike the organic interface layer, they have better physical and chemical properties. These devices typically consist of the substrate, ITO (transparent cathode), ZnO (ETL), active layer, MoO₃ (HTL), and Ag (anode), which have demonstrated increased aging resistance and PCE.⁹⁶ The increased stability is associated with the use of a high work function metal as an upper electrode, which simplifies the manufacturing process and achieves lower production costs. Moreover, the inverted architecture is more flexible and generates a higher photocurrent. However, the device's operation is highly sensitive to the type of ETL material and how this material interacts with the photoactive BHJ layer.⁹⁷ A PEDOT : PSS coating can be introduced for achieving greater stability in inverted frameworks; similarly, the utilization of metal nanoparticles⁹⁸ and metal oxides, such as WO₃, NiO_x, MoO₃ and V₂O₅, can enable desirable electronic attributes, low optical absorption in the visible wavelength range and a high level of technology compatibility.⁹⁹

The PEDOT : PSS, in a mass ratio ranging from 1.00 : 6.11 or 1.00 : 6.92 with some suppliers, is commonly utilized in the standard organic solar cell schemes. It is preferred because it has a number of desirable qualities. An example here is that it has a high work-function that is compatible with the HOMO level of most donor polymers, enables more than 80% of visible light to pass through, provides good electrical conductivity, and can be used to smooth the surface of the ITO layer.¹⁰⁰ Nevertheless, PEDOT : PSS also has some limitations. It can exhibit poor electrical performance due to its hydrophilic characteristics and the relatively low morphology of its film. In addition, the interface between ITO and PEDOT : PSS is not completely stable, and thus can lead to chemical reactions that would eventually degrade the device. Because of the high acidity of PEDOT : PSS, the ITO layer can lose indium and tin atoms at its surface. These atoms can find their way into the PEDOT : PSS layer during thermal annealing, which leads to instability of the device and its degradation over time.¹⁰¹

Inverted device configurations have been created to improve device stability and prevent the problems mentioned previously. This kind of construction avoids oxidation problems by using high work function metals (which are air-stable) like silver (Ag) or gold (Au) as the top anode to collect holes and ITO as the cathode to collect electrons. The transparent electron-conducting layer can be made of metal oxides, such as zinc oxide (ZnO) or titanium oxide (TiO_x), which solves the problem of acidic PEDOT : PSS on ITO or FTO.⁹⁵ A comparison of the inverted and conventional OSC device structures is given in Table 4.

Table 4. Comparison of conventional vs. inverted OSC architectures.^94–101.

Aspect	Conventional OSC	Inverted OSC
Device configuration	ITO/HTL/active layer/ETL/low-work-function metal (Al, Ca) cathode	ITO (modified cathode)/ETL/active layer/HTL/high-work-function metal (Ag, Au)
Charge extraction	Holes collected at bottom; electrons at the top electrode	Electrons collected at bottom; holes at the top electrode
Electrode stability	Reactive cathode prone to oxidation and corrosion	Air-stable top electrode enhances durability
Sensitivity to oxygen/moisture	Highly sensitive; faster degradation in ambient air	Improved resistance to oxygen and moisture ingress
Interfacial stability	PEDOT : PSS can etch ITO and induce instability	Metal-oxide ETLs (ZnO, SnO2, and TiO2) provide robust interfaces
Thermal and operational stability	Moderate stability: interfacial reactions accelerate aging	Superior thermal and operational stability
Fabrication compatibility	Often requires vacuum deposition of reactive metals	Compatible with solution processing and scalable fabrication
Suitability for flexible devices	Limited due to electrode reactivity	Highly suitable for flexible and large-area devices
Overall stability outlook	Lower long-term stability	Preferred architecture for high-stability OSCs

2.6. Device structures (single/bilayer/BHJ/tandem)

2.6.1. Single-layer OPV cells

The simplest type of structure is the single-layer OPV cell, which is composed of one active organic layer with two electrodes. The components of the donor and the acceptor must be included in the same film in this arrangement, and charge separation is carried out predominantly at the interfaces or by the internal fields at the interfaces, as shown in Fig. 5(a). Although PCEs of early single-layer OPVs were very low, more recent developments have demonstrated that PCEs of over 10% can be achieved, with better photostability and simplified active-layer processing, in the context of the continued design of materials.¹⁰² Although these advances have been achieved, the inherent weakness of single-layer cells is the absence of a defined donor–acceptor interface, which limits the efficiency of exciton dissociation and separation of charges. Single-layer and single-component OPVs simplify the architecture and are more stable intrinsically but have efficiencies that are typically lower than those of bulk heterojunction and tandem devices.¹⁰³

Fig. 5. Device structures: (a) single-layer OPVs, (b) bilayer OPVs, (c) BHJ OPVs and (d) tandem OPVs. Redrawn and modified from ref. 103.

The single-layer architecture also offers useful results in terms of inherent material behaviour and stability. Since only one component is used or the components are homogeneously mixed in the active layer, information on a single layer has helped understand the underlying processes of exciton diffusion, charge transport restraints, and molecular photostability, which is used to design other more intricate systems. OSCs with optimized molecular structures have shown lower non-radiative losses and better device life than blended systems, while single-layer designs have been found to be capable of contributing to the realization of stable, low-complexity OPV systems in the future.¹⁰² A transparent electrode is usually a transparent conducting oxide, which serves as the anode of the device. This layer is permeable to light and capable of collecting charge carriers. A light-absorbing polymer or small molecule that acts as the donor typically produces electrons, and a light-absorbing material, which is typically a derivative of fullerene, serves as an acceptor to capture the electrons and allows the separation of charge. The metal electrode works as the cathode and is usually manufactured using aluminium or silver, which assist in gathering the electrons and closing the electrical circuit.¹⁰³

2.6.2. Bilayer OPV cells

Bilayer organic photovoltaic (OPV) devices are constructed as thin-film solar cells that are composed of two layers of organic semiconductors between two electrodes. The two layers normally involve an electron-donating (p-type) layer and an electron-accepting (n-type) layer, as illustrated in Fig. 5(b). The electron-donating layer in these types of structures can usually be made of a conjugated polymer or a small molecule with low ionization potential, which can easily lose electrons upon exposure to light. The electron-accepting layer, however, tends to be a fullerene derivative or a non-fullerene acceptor with a higher ionization potential that facilitates the process of electron capture into the electron-accepting layer and allows charge separation in the gadget.¹⁰⁴ This planar heterojunction provides a direct interface for exciton dissociation: excitons produced in the electron-donating layer diffuse to the acceptor and donor interfaces where they transfer electrons to the acceptor and leave the holes in the donor. Studies that have looked at sequentially deposited bilayer systems have indicated that with careful control of the deposition and interfacial morphology, power conversion efficiencies (PCEs) can reach levels (e.g., PCE of 7.1% in PCDTBT/PC₇₀BM systems) approached by more complex bulk heterojunction devices without difficulty in fabrication, and a clear separation of material domains is achievable.¹⁰⁵

The bilayer architectures is useful in the study of interfacial energetics and charge–transfer processes as their donor/acceptor interface is defined. This enables researchers to independently tune every layer of the stack, the levels of energy, and the morphology, and bilayer systems can be used to study fundamental processes involving the diffusion of excitons and interfacial recombination. In addition, bilayer transport layer engineering, including the engineering of bilayer hole transport layers (HTLs), has demonstrated enhanced device stability and operational robustness, with record efficiencies (>17%) achieved in inverted OPV designs, demonstrating the significance of layered interfacial design not only in performance but also in durability.¹⁰⁶

2.6.3. BHJ OPV cells

A type of thin-film solar cell made up of an interpenetrating array of electron-donating and electron-accepting materials is called a bulk heterojunction OPV cell. Electron-donating and electron-accepting materials are typically mixed in a solution and spin-coated onto a substrate in a bulk heterojunction OPV cell.¹⁰⁷ Effective charge separation and transfer are made possible by the mixture, which is an overlapping mesh of acceptor and donor materials. The mixture of the materials that give and receive electrons is enclosed by the anode and cathode electrodes, as shown in Fig. 5(c). The most common and high-performance organic solar cell architecture is bulk heterojunction OPVs.⁸ Electron-acceptor and electron-donor materials are closely mixed in BHJ devices, forming an interpenetrating network that offers an extremely high donor–acceptor interfacial area of excitons that dissociate across the active layer. BHJ systems with optimized non-fullerene acceptors and donor polymers have recently demonstrated PCEs exceeding 19% in single-junction devices, and they are on the path to commercialization.¹⁰⁸ However, the BHJ architecture also brings about complicated stability and degradation issues because the structure has a metastable morphology and a large interfacial surface. The use of thermal annealing, solvent additives, and processing methods plays a great role in phase separation, domain purity, and the recombination pathways, thus affecting long-term stability. Recent works highlight the importance of morphological control and energetic optimization of BHJ interfaces to prevent nonradiative recombination and disorder, and further characterization of the nanoscale energetic landscape of the BHJ interface and its relationship to device performance has indicated future stability-oriented BHJ designs.¹⁰⁹

2.6.4. Tandem OPV cells

Tandem OPV cells are a relatively recent technology that uses two or more sub-cells connected in series to increase the technology's efficiency. By stacking two or more sub-cells with complementary absorption spectra on top of one another, tandem organic photovoltaics aim to overcome the performance limitations of single-junction devices, as illustrated in Fig. 5(d). In order to maximise the use of incident light and reduce thermalisation losses, the individual sub-cells are made to absorb different portions of the solar spectrum, frequently by combining photoactive systems with a wider bandgap and a narrower bandgap. Studies of tandem OPVs have shown that deep optical and electrical matching between sub-cells can result in a dramatic overall performance increase; for example, tandem structures using organic or hybrid systems may see a 30% overall performance increase over single-junction OPVs.¹¹⁰

In tandem structures, effective charge recombination and spectral matching of sub-cells are important for achieving high performance and minimizing electrical losses. State-of-the-art designs, such as perovskite organic tandem cells, integrate the potential of organic materials (mechanical flexibility and solution processability) with the desirable optoelectronic characteristics of the perovskites to achieve PCEs that are superior to all-organic systems. They offer the possibility of achieving material and interlayer engineering control of stability. Further PCE improvements of the perovskite/organic tandem cells are possible, with a maximum calculated PCE exceeding 31% via the optimization of the bandgaps of the sub cells. These hybrid tandem strategies are also considered viable avenues to next-generation OPVs that are more efficient and operationally robust.¹¹¹

2.6.5. Short comparison of device structures

We present a comparison based on the main performance-related parameters, such as structural advantages, intrinsic limitations, possible power conversion efficiencies, and stability issues. This systematic comparison will give a clear understanding of the effects of device architecture on efficiency and long-term operational stability, thereby informing choices regarding appropriate OPV structures for the future research and development of PV technologies. To highlight the differences between the different organic photovoltaic (OPV) cell designs, a comparative overview of single-layer, bilayer, BHJ and tandem OPV cells is outlined in Table 5.^102–111

Table 5. Comparison of organic photovoltaic (OPV) device architectures.

Structure	Advantages	Disadvantages	Core characteristics	Stability	Ref.
Single-layer	Simple design, easy fabrication	Low mobility, limited absorption, low efficiency	Single blended donor–acceptor film between two electrodes	Low	102 and 103
Bilayer	Better charge separation, improved absorption, higher efficiency than single-layer	Limited mobility, exciton diffusion limits thickness, more complex than single-layer	Sequential donor and acceptor layers forming a heterojunction	Moderate	104–106
Bulk heterojunction	Efficient exciton dissociation, improved charge transport, high efficiency	Morphology control is critical, recombination losses possible	Interpenetrating donor–acceptor network, typically solution-processed	Variable	107–109
Tandem	Broader light absorption, higher efficiency	Complex fabrication, current-matching issues	Stacked sub-cells with complementary spectral absorption	Moderate high	110 and 111

2.7. Layer-by-layer OSCs

Layer-by-layer (LbL) organic solar cells are composed of active layers, interface layers, and electrodes. Light is absorbed by the active layer, and the photons are converted into free charges. An interface layer exists between the anode/cathode and the donor/acceptor layer and is used to construct hole/electron transport paths, respectively. LbL organic solar cells have been developed with a hierarchical design of donor and acceptor layers to facilitate light absorption at different wavelength ranges. In general, the donor material absorbs shorter wavelengths, and the acceptor material contributes to the longer-wavelength absorption, resulting in the formation of excitons (bound electron–hole pairs). These excitons migrate towards the donor–acceptor interface, where charge separation takes place. The layer thickness of organic semiconductors is important, as the diffusion length of excitons in these materials is normally less than 5–20 nm, meaning that it is essential to keep the thickness of the layers within this range to minimize recombination losses. The energy-level offset at the interface causes dissociation of the exciton, with the transfer of an electron to the acceptor LUMO and a hole to the donor HOMO. The separated charge carriers then migrate through dedicated transport pathways, with electrons moving toward the cathode through the acceptor layer and holes traveling to the anode through the donor layer. Furthermore, the optimized vertical phase separation in the LbL structure provides a favorable energy-level gradient, which enhances the built-in electric field and facilitates efficient directional charge transport.¹¹² The basic structure and working principle are shown in Fig. 6.

Fig. 6. Working principle of LbL OSCs. Redrawn and modified from ref. 112.

The basic working mechanism of both bulk heterojunction (BHJ) and layer-by-layer (LbL) architectures of organic solar cells (OSCs) is identical, but there are several significant differences between the two. First, the sequentially deposited donor and acceptor layers enable independent optimization of the light absorption, which can be precisely controlled by varying the material and the layer thickness. Second, the donor–acceptor interface frequently results in a transitional pseudo-BHJ region that fosters efficient exciton dissociation and charge generation. Third, the clear vertical phase separation property of LbL devices forms donor-rich regions close to the anode and acceptor-rich regions close to the cathode, which allows for directional charge transport and enhanced charge collection. Lastly, the relatively ordered morphology of the LbL active layers is believed to be closer to a thermodynamically stable state that can help mitigate morphological degradation and potentially lead to greater long-term stability of the devices than their conventional BHJ counterparts.¹¹³ To achieve better molecular packing and donor–acceptor interaction, an alloyed polymer donor strategy has been used to improve all-polymer LbL solar cells. The resulting morphology facilitates the dissociation of excitons, transport of charges and charge collection, while reducing the recombination losses, resulting in a power conversion efficiency (PCE) of 18.34%.¹¹⁴

Interfacial engineering has also been one of the most important developments for the advancement of LbL photovoltaics. Optimized donor–acceptor interface morphology enhances exciton dissociation, charge transport and carrier extraction, and reduces recombination losses. Precise control of the interface proved to be crucial for the development of high-performance LbL OSCs with PCE values >19.2%, emphasizing the significance of vertical phase distribution and balanced charge mobility in materials.¹¹⁵ Thermally activated delayed fluorescence (TADF) materials have proven to be an effective tool to manage excitons in LbL OSCs. The TADF molecule BN-STO was used to achieve a PCE of 19.65% by its incorporation into devices, which reduced non-radiative energy losses via reverse intersystem crossing (RISC), increased exciton utilization, and improved charge generation.^52,116

Sequentially processed OSCs have also been further developed by morphology regulation through crystallinity engineering. A highly crystalline third component enhanced molecular ordering, donor/acceptor interpenetration, and charge transport and prevented recombination to achieve efficiencies of over 20%. These results emphasize the need for morphology optimization and sequential deposition for the fabrication of high-performance LbL and pseudo-planar heterojunction solar cells. Several representative examples of normal and inverted layer-by-layer organic solar cells are discussed in Table 6, demonstrating the benefits of sequential deposition for controlling the morphology of the active layer and vertical phase separation. The LbL architectures allow the creation of donor-rich and acceptor-rich transport pathways, which aid in the dissociation of excitons, extraction of charges, and reduction of bimolecular recombination, in comparison with conventional bulk heterojunction devices.¹¹⁷

Table 6. Representative recent advances in layer-by-layer (LbL) organic solar cells.

Active layer	Device strategy	Structure	PCE (%)	Key features	Ref.
PM6 : L8-BO + BN-STO	TADF incorporation	Conventional	19.60	Reduced energy loss	52
PBQx-TCl : PM1/PY-DT	Spin-coating method	Conventional/Inverted	18.34	Improved morphology	114
PM6 : BTP-eC9	Spin-coating technology	Conventional	19.20	Enhanced charge transport	115
PM6/L8-BO + DMAC-DPS	TADF-mediated dissociation	Conventional	>19.0	Enhanced exciton utilization	116
D18/L8-BO/L8-BO-C4	Pseudo-planar heterojunction (PPHJ)	Inverted	20.24	High crystallinity	117
D18-Cl : BTP-4F-P2EH	Additive-assisted LbL	Inverted	20.80	Bulk p–i–n morphology	118
D18/L8-BO	Dual-additive engineering	Inverted	19.59	Reduced recombination	119
(D18-Cl : PY-Cl)/(PM6 : PY-SSe)	Quaternary-LbL processing	Inverted	19.46	improved efficiency and stability	120

2.8. Semitransparent organic solar cells

The remarkable progress in OSCs, with certified power conversion efficiencies (PCEs) now over 20%, has ignited interest in semitransparent OSCs (ST-OSCs) as a next-generation PV technology. The device structure is a conventional inverted organic solar cell structure ITO/PEDOT/PM6/PNDIT-F3N/Ag. The transparent anode is made of indium tin oxide (ITO), and the hole-transport layer (HTL) is made of PEDOT, which can extract holes efficiently and decrease interfacial recombination. The PM6 blend is used to create the photoactive layer, where the light is absorbed, the exciton is created, and the charge is separated and transported. PNDIT-F3N serves as the electron-transport layer, which can facilitate selective electron extraction and enhance the interfacial energy-level alignment.

Last but not least, the cathode is a silver (Ag) electrode, as shown in Fig. 7(a), which has the ability to collect charges efficiently.¹²¹ The multi-layer structure facilitates efficient transport of the photogenerated charges and helps to improve the PV efficiency. ST-OSCs are engineered to produce electrical power and be partially transparent to visible light, which makes them uniquely suitable for applications like building-integrated photovoltaics (BIPVs), greenhouse energy systems, smart windows and wearable electronics, as shown in Fig. 7(b). The design of ST-OSCs, however, has to balance the photovoltaic performance with the average visible transmittance (AVT) of the device, which necessitates optimization of the device architecture, material selection, and optical management strategies.¹²²

Fig. 7. (a) Schematic of the inverted organic solar cell device structure, and (b) representative applications of semitransparent organic solar cells.¹²³.

In order to overcome this problem, much research has been conducted on active-layer engineering. Wide-bandgap donor polymers and near-IR-absorbing non-fullerene acceptors allow efficient harvesting of UV and near-IR photons, while reducing absorption in the visible. Furthermore, layer-by-layer deposition methods and donor-content optimization have shown enhanced charge transport, morphology control, and light utilization efficiency (LUE). Recent research has demonstrated that rational active-layer engineering can not only improve transparency but also improve PV performance, leading to efficient semitransparent devices with desirable optical properties.¹²⁴

In addition to active-layer optimization, transparent-electrode engineering and optical-management engineering are also becoming important methods to enhance the performance of ST-OSC. Advanced architectures of the dielectric/metal/dielectric electrodes can selectively reflect photons from the near-infrared into the active layer, while maintaining the transmission of visible photons. These help to improve not only photocurrent generation but also the colour rendering and infrared rejection properties, which are essential for smart window and building-integrated applications. The introduction of optical management layers with ultrathin metallic electrodes has paved the way for the development of multifunctional PV window technologies with simultaneously enhanced efficiency, transparency, and thermal–management properties.¹²⁵ In order to give a general idea of the recent progress in the development of semitransparent organic photovoltaics, a summary of representative ST-OSC devices reported in the recent literature is presented in Table 7. Key performance indicators, such as power conversion efficiency (PCE), average visible transmittance (AVT), light utilization efficiency (LUE), device characteristics and architectural design strategies, are compared. These are examples of the ongoing development of the trade-off between transparency and photovoltaic performance achieved by active-layer engineering, optical management, and scalable approaches to device fabrication.

Table 7. Representative semitransparent OSC performance metrics.

Device structure	Strategy	AVT (%)	PCE (%)	LUE (%)	Ref.
PM6 : BO-4Cl/L8-BO	Layer-by-layer donor-layer dilution	27.6	14.04	3.88	124
PM6 : L8-BO + Na3AlF6/ZnSe	IR-reflector bilayer (top electrode capping)	30.57	15.19	4.64	125
ITO/PEDOT : PSS/PCE10-2F : Y6/PDINO/Ag	Employing sequential deposition (SD) to fabricate the active layer	50.05	10.01	5.01	126
ITO/PEDOT : PSS/PTB7-Th : BZO-4Cl/PDIN/MoO3/Ag/Cu	Employing a wavelength-selective active layer	43.08	9.33	4.02	127
ITO/PEDOT : PSS/PCE10-BDT2F : Y6/PDINO/Ag	Employing a wavelength-selective active layer	41.08	10.85	4.45	128
ITO/ZnO/PL2 : Y6 : PC61BM/MoO3/Ag	Employing a wavelength-selective active layer	40.40	9.91	4.00	129
ITO/PEDOT : PSS/PBDTTTPT:N3/PDIN/Au/Ag	Employing a wavelength-selective active layer	35.70	12.26	4.37	130
ITO/PEDOT : PSS/PCE10-2F : PM6 : Y6/3-PDINO/Ag	Employing ternary active-layer materials	36.57	12.25	4.48	131

In order to improve the efficiency and transparency balance of semitransparent OSCs, significant efforts have been made in the development of advanced materials and device-engineering techniques, as summarized in Table 7. However, performance alone is not enough to assess the usefulness of these devices. ST-OSCs suffer from additional stability issues due to the high UV transmission, which can promote photo-oxidative degradation inside the active layer, and the thin transparent electrodes, which are more prone to oxidation, electromigration and morphological degradation. Hence, the strategies discussed in this review for enhancing the stability (interfacial engineering, morphology stabilization, encapsulation technologies, and rational molecular design) are particularly relevant to semitransparent architectures. Optimization of efficiency, transparency and operational durability should therefore be pursued in future research to accelerate the commercialization of energy-generating windows and façade-integrated photovoltaic systems.¹³²

2.9. Working principle

OSCs function based on a series of photophysical and electronic reactions that are fundamentally distinct from the reaction mechanisms in traditional inorganic photovoltaic devices. Photoexcitation of organic semiconductors involving tightly bound excitons instead of free charge carriers is caused by the molecular nature and low dielectric constant of the organic semiconductors.¹³³ The general working principle of OPVs can be summarized in four key steps:

2.9.1. Light absorption

Organic photovoltaic (OPV) cells are made so that light impacting the cell's active layer is absorbed by the organic semiconducting materials, initiating the photovoltaic conversion process. Because organic materials have a low dielectric constant, photon absorption excites electrons in the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), creating electron–hole pairs as tightly bound excitons.¹³³ Photons whose energies are equal to or higher than the optical bandgap of the absorber will be useful, and bandgap engineering is a very important design factor to optimize solar energy harvesting.¹³⁴ Molecular parameters that determine the efficiency of light absorption in organic semiconductors are molecular weight, chemical structure, backbone planarity, and molecular orientation in the solid state. High-absorption conjugated polymers include conjugated polymers that have a very high absorption coefficient and that are tuneable to maximize light absorption with high absorption coefficients in the visible and near-infrared. These materials are also widely researched because they can enhance photocurrents in OPV systems.¹³⁵

Besides molecular design, the morphology of the active layer is also vital for light-absorption efficiency. The active layer of OPVs is usually a bulk heterojunction blend that is made up of an electron-donor material and an electron-acceptor material. Such a nanoscale interpenetrating network enhances the interfacial area and ensures that excitons produced when the light is absorbed can travel to a donor and acceptor interface within their short diffusion length. To maximize optical absorption and promote efficient exciton dissociation, proper phase separation and domain purity are both needed at the same time.¹³⁶ In an attempt to improve light uptake in OPV cells, several optical management techniques have been invented. Increasing the optical path length of incident light in the active layer by light-trapping mechanisms, like textured substrates, reflective electrodes and photonic crystal structures, increases absorption without raising material thickness. It has also been demonstrated that the addition of plasmonic nanoparticles, e.g. gold or silver, can make the absorption of light more efficient by means of localized surface plasmon resonance, a phenomenon that localizes electromagnetic fields around the active layer. The plasmonic effects can be very useful for enhancing photocurrent generation on optimization.¹³⁷

2.9.2. Charge separation

An internal electric field is present in the active layer that assists in parting electron–hole pairs. Generation of this electric field is due to the different energy levels of the acceptor and donor in the active layer.¹³⁸ This separation of these charges is known as electron–hole pair separation, and it occurs following absorption of light by the organic semiconductor in the active layer. The significance of this separation is that the generated charges may flow into their respective electrodes and an electric current is generated. The active layer in an organic photovoltaic cell typically comprises a mixture of an electron-donor substance and an electron-acceptor substance, creating a heterojunction at which charge separation occurs. The donor substrate is typically a conjugated polymer with a high ionization potential, i.e. it can readily lose an electron. In contrast, the acceptor material, i.e. fullerene-based or non-fullerene compounds, possesses a large electron affinity and is therefore able to accept electrons effectively.¹³⁹ As a result of the absorption of a photon in the active layer, an exciton is produced. This exciton transfers to the acceptor and donor interfaces, where it is partitioned into free charge carriers. Once this separation is achieved, the electron and hole move to their respective electrodes.¹⁴⁰ The active layer structure is also critical in this process; acceptor and donor materials should be mixed in a close manner to allow free charges to break out easily. Solvent processing and thermal annealing are some processes that are regularly employed to regulate and enhance morphology, assisting donor and acceptor phases to establish a properly connected network of heterojunctions.¹⁴¹

The electronic properties of the acceptor and donor materials also determine the efficiency of charge separation. The materials should have the right energy levels to enable them to separate charges effectively. The difference in energy levels of these materials must be sufficient to stimulate movement of electrons and holes to their respective electrodes.¹⁴² The general operation of OPV devices can be described with the assistance of Fig. 8. To enhance charge separation of OPV cells, researchers have developed the concept of using non-fullerene acceptors and the design of tandem cell configurations that facilitate multi-exciton dissociation processes that ultimately increase the overall device efficiency.¹⁴⁰

Fig. 8. Schematic showing the mechanism of working OSCs. Redrawn and modified from ref. 8.

2.9.3. Charge collection

Electrons and holes that have been torn off are gathered by means of electrodes, which are usually made of a transparent conductive material that is sprayed on the surface of the active material. The principle of OPV cells is that a current will be produced because of charge movement of the two separated charges to the corresponding electrodes. The process of charge collection is a crucial process, and the effectiveness of the process directly affects the general functionality of the device.⁸ Separated charges in OPV cells, which are produced because of charge separation, get to the electrodes of the cell via the organic semiconducting material in the active layer. Electrodes are normally composed of conductive substances such as indium tin oxide (ITO), aluminium, or silver, and they receive the produced charges.¹⁴³

2.9.4. Electrical output

The electrical output of an OPV cell is the outcome of its charge separation and collection processes. The electrical output efficiency of a device is one factor to consider when evaluating its overall efficacy. An OPV cell's fill factor, short-circuit current density, open-circuit voltage, and PCE determine its electrical performance.¹⁴⁴

3. Stability and degradation pathways

The achievements over the past five years in the field of OSCs have been dominated by efficient non-fullerene acceptors (NFAs), ternary blend systems, tandem device architectures, and morphology control strategies. All these advances have enabled OSCs to achieve efficiencies of over 20%, bringing them closer to commercial competitiveness. While there are improvements in device performance, the biggest hurdle for widespread adoption is device stability. In the last few years, much research has been undertaken, and the results have made clear that the durability of the product and achieving a long service life under realistic operating conditions are more important issues than further improvements in peak efficiencies. This has resulted in the recognition of degradation processes and the need to understand and mitigate these processes, especially from lab-to-practice. This is now a significant research area in OSC research.¹⁴⁵

There are usually two types of OSC degradation: intrinsic and extrinsic. Intrinsic degradation occurs due to intrinsic chemical and physical instability of the donor and acceptor materials, and extrinsic degradation occurs due to environmental stress factors such as oxygen, moisture, temperature, light exposure, mechanical stress, degradation of electrodes, and degradation of interfacial layers. However, these degradation processes are often very interdependent and non-exclusive. For example, oxygen can enter a device from the air and directly interact with the device to promote intrinsic chemical degradation. Similarly, molecular diffusion, phase segregation and morphological coarsening can occur, either as a result of thermal stress or as a result of the intrinsic molecular diffusion, phase segregation and morphological coarsening by gradual thermodynamic relaxation. Therefore, it is important to understand how the intrinsic material properties interact with external environmental factors that can impact the operational reliability and lifetime of an OSC.^{69,71,146,147}

The following subsections discuss OSC degradation from a mechanistic point of view, covering the chemical and physical processes that underlie OSC degradation, the experimental evidence used to identify the processes, and the quantitative parameters used to describe the degradation behavior. This is not a degradation classification based on external stressors, but rather the molecular causes of degradation and how they change over the lifetime of the device. Reductions in PV performance parameters like J_SC, V_OC and FF can occur through different degradation pathways, so an understanding of these degradation pathways is necessary for establishing clear structure performance relationships and designing molecular and device engineering strategies for effective devices. These mechanistic insights are essential for proper diagnosis of degradation, prediction of long-term operational stability, and proper selection of mitigation strategies.¹⁴⁸ The main degradation mechanisms mentioned in this section are summarized in Fig. 9. The following are major factors that make OSCs unstable.

Fig. 9. Degradation factors that affect device stability. Redrawn and modified from ref. 61.

3.1. Photochemical degradation

Photochemical degradation is the process whereby structural distortion or even cleavage of light-harvesting materials occurs under light, resulting in a steep drop in the performance of the OSC, as illustrated in Fig. 10(a). One of the most likely intrinsic degradation mechanisms that have been noted to be highly problematic in organic solar cells is photochemical degradation, since the photoactive material is under constant exposure to high photon flux when operating. Photon absorption is crucial to exciton formation and the generation of charge carriers, but longer durations of illumination may lead to irreversible changes in both donor and acceptor materials, both chemically and structurally. Such alterations have a direct negative impact on molecular integrity and optoelectronic efficiency of the active layer, which reduces the long-term stability of the device and its commercial feasibility.^63,78 The conjugated electron systems of polymer donors and non-fullerene acceptors undergo degradation processes mainly due to their high degree of delocalization, which is the reason behind the mechanism of efficient charge transport. When photoexcited, the singlet and triplet excited states undergo unwanted photoredox reactions, and in the presence of residual oxygen, such reactions can produce reactive oxygen species, such as singlet oxygen, superoxide anions and hydroxyl radicals, which assault the conjugated backbone and side chains of the organic semiconductors. Consequently, polymer backbone oxidation, chain scission, and carbonyl or hydroxyl defects are formed, and conjugation is disturbed, raising the level of energetic disorder. These chemical modifications slow carrier mobility, increase trap-assisted recombination, and ultimately cause progressive loss of efficiency.¹⁴⁹

Fig. 10. (a) Diagram of OSC degradation pathways brought about by heat and light. (b) Schematic of the degradation of OSC efficiency under continuous light irradiation. Redrawn and modified from ref. 57.

Besides intrinsic excited-state chemistry, photochemical degradation is highly affected by trace environmental species that are contained in the device stack. In nominally encapsulated OSCs, diffusive access of small quantities of oxygen and moisture to weakly crystalline or amorphous sections of the bulk heterojunction can occur, and they react with photoexcited states and promote oxidative damage.¹⁵⁰ Intermolecular interactions and the electronic structure can be altered in the presence of water and oxygen molecules without the creation of strong covalent bonds, and this can cause the bleaching of absorption features and a decrease in conductivity. In addition, the diffusion of extrinsic species into metastable BHJ morphologies with the assistance of moisture has been demonstrated to increase photochemical.¹⁵¹ Alkyl side chains in polymer donors, which are necessary to ensure solubility and morphological control, are especially prone to oxidation and form peroxides and radical intermediates, which, in turn, target the conjugated backbone.¹⁵² On the same note, high-performance NFAs can also be subjected to light-induced structural rearrangements, bond cleavage, or isomerization under extended light illumination, which changes molecular packing and aligns energy levels.⁶³

Any change in energy of donor–acceptor recombination and higher non-radiative recombination can result in quantifiable losses. Some of the factors that can explain such losses include light-induced trap states and alterations of the HOMO energy levels.¹⁵³ Notably, photochemical degradation is mostly identified as interactions with thermally propelled morphology development and interfacial instability, which can further promote overall degradation. It is crucial that this complex interplay can be addressed using chemically robust molecular design, radical formation suppression, side-chain stabilization, and enhanced interfacial engineering in order to create intrinsically photostable OSCs that can provide sustained high-performance.⁵⁷ Research has demonstrated that OSC degradation processes usually take place in two phases, the first being a rapid decrease in performance, followed by a gradual linear decrease. The first sharp drop in performance, commonly known as burn-in, can be attributed to an amalgamation of different aspects inside the system, which include defect-mediated recombination of charges and morphological alterations in the active layer.¹⁴⁸ The aging process is influenced by extrinsic factors such as light and heat, which exacerbate performance reduction in the early phase at a rapid rate. When the morphology is slowly stabilized and finally stabilizes near the equilibrium, the degradation rate is reduced, leading to a smaller and more linear decay in the PCE.¹⁵⁴ Because the operational lifetime of OSCs is a parameter that is best assessed by comparison with a standard, T₈₀ has been defined as the time at which the PCE of an OSC drops to 80% of its original value. To date, the lifespan of OSCs remains less than is required for commercialization.¹⁵⁵

The term burn-in loss in organic solar cells refers to the fast and severe initial decrease in the performance of a particular device during the initial stages of its operation. This stage, which is commonly marked by a steep, almost exponential reduction in power conversion efficacy, commonly happens within the initial tens to hundreds of hours. To a significant extent, as much as 40–50% of the original device efficiency may be lost prior to the onset of the more gradual degradation regime, as illustrated in Fig. 10(b). Photostability is an inseparable phenomenon, which is essentially photo-induced and material-dependent, and its extent and duration differ among various OSC systems.¹⁵⁶ Although external conditions, such as oxygen and moisture, can accelerate the general degradation, the burn-in is particularly evident during illumination.¹⁵⁷ The mechanisms that are involved in the genesis of burn-in loss include complicated photochemical depreciation activities in the active layer. Irreversible chemical and structural modifications, including the oxidation of the polymer backbone, chain scission, and the creation of defects, are caused by prolonged exposure to light, and this undermines the molecular integrity and optoelectronic activity. Such alterations frequently result in the formation of reactive oxygen species that assault the conjugated systems of organic semiconductors, interfere with charge transport, and enhance recombination.¹⁴⁹ On the device level, the burn-in is realized in the form of a reduction of the main photovoltaic characteristics. In particular, the fill factor can commonly be reduced by higher series resistance, lower shunt resistance and degradation at interfacial contacts, which can be worsened by oxygen-induced defect states and energy level disruption. Light-induced trap states and HOMO energy level alterations are associated with V_OC losses, whereas a decrease in J_SC can be caused by low absorption efficiency, short exciton diffusion length, and low carrier mobility.¹⁵⁸ Photo-dimerization of fullerene acceptors, such as PC₆₁BM, has also been reported to play a major role in performance loss during burn-in ref. 159.

Photochemical degradation can occur in organic solar cells but can be suppressed with good encapsulation or by operating in an inert atmosphere. Under an N₂ atmosphere, Clarke et al. studied the photostability of some NFAs (O-IDTBR, EH-IDTBR, ITIC and ITIC-M) and found that long-term degradation and burn-in loss were observed in the devices based on ITIC and ITIC-M. In situ Raman spectroscopy showed that the characteristic vibration bands changed significantly during photoaging, suggesting conformational twisting between the terminal acceptor units and the conjugated backbone, as shown in Fig. 11(b). The DFT-simulated Raman spectra shown in Fig. 11(c) also showed that the higher the dihedral angle of the vinylene linkage, the more the structure is distorted and the possibility of bond rupture is increased. These molecular changes negatively affect the device's performance and photostability, leading to a gradual decrease in device performance over time.¹⁶⁰

Fig. 11. Photodegradation of ITIC under an inert atmosphere: (a) molecular structure showing key Raman-active vibrational modes, (b) in situ Raman spectral evolution during the photoaging process, and (c) DFT-simulated Raman spectra showing the effect of increasing dihedral angle along the vinylene linkage. Redrawn and modified from ref. 160.

Although both systems showed a significant improvement in the photovoltaic performance of ITIC and ITIC-M based devices, they still showed significant burn-in losses and long-term degradation under an inert atmosphere. In situ Raman spectroscopy indicated that the main photochemical degradation route is conformational twisting of the terminal acceptor groups and, possibly, bond scission in the conjugated backbone, as shown in Fig. 11(a). The resulting structural changes affect the molecular conjugation, charge transport, and ultimately the device stability and degradation.

3.2. Thermal degradation

Since the solar cells work under continuous light, a constant heating effect at the working temperature is inevitable. However, the OSCs are not always lucky because they face the problem of thermal instability. This is because, although most of the materials used in OSCs have decomposition temperatures that are much higher than the working temperatures, the device's thermal instability is closely linked to physical deterioration. The architecture and characteristics of the active layer may be altered at high temperatures. The long-term durability of organic solar cells and their economic viability are seriously threatened by thermal deterioration, which is commonly illustrated by the devices' declining performance at high temperatures.¹⁴⁵ Specifically, irreversible functional changes in device components are caused by exposure to heat, in contrast to photodegradation caused by exposure to light. Stress due to temperature may be caused by the surrounding environment, the operating temperature of the device, and especially due to self-heating when subjected to intense sunlight, where the acquired energy is transformed into heat. Such elevated temperature boosts a range of degradation mechanisms, which affect the inherent stability of the materials and interfaces in the OSC architecture. Thermal degradation is a fundamental concept to understand and reduce the impact and extend the lifespan of OSCs and guarantee their stable use in the real world.¹⁴⁷

Thermal degradation in OSCs occurs through a complex series of mechanisms, with the main ones being the modification of the morphology of the active layer and the appearance of defect states. When temperatures are high, it can cause major morphological restructuring of the active layer, such as phase separation, aggregation, and crystallization of the active materials.¹⁶¹ These transformations, especially at temperatures above the glass transition temperature of the constituent elements of the blends, may disorient the desirable nanostructure that is essential for the generation and transport of charge. Moreover, thermal stress causes aging-induced defect states (charge traps) to be formed in the active layer, which raises the trap density and increases non-radiative recombination.¹⁶² Heating may disrupt the carrier transport layers and contact interface in addition to the active layer. The adhesion between the active and carrier transport layers may be affected by a change in the active layer's shape, which could provide a barrier to electron extraction.¹⁶³ According to a recent study by Uddin et al., thermal degradation disrupts the charge extraction process during the device operation of both PTB7-Th : PC₇₁BM-based fullerene and PTB7-Th : 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithiophene-based non-fullerene devices.¹⁶⁴ PEDOT : PSS, which is used to form carrier transport layers, can exhibit reduced electrical conductivity when heated. However, the heating may also be used to enhance the rate of diffusion of the metal atoms brought about by metal electrodes to reduce the functioning of the device since the majority of the diffusion processes are temperature sensitive. This defect formation can severely reduce the open-circuit voltage and reduce charge carrier mobility. High temperatures may also increase the diffusion of materials at the interfaces, which can further damage devices. Since assessing the thermal stability of OSCs under controlled high-temperature conditions is essential, standardized testing protocols, such as ISOS-D tests, have been developed.^63,165

Schaffer et al. found that the thermally induced morphological instability in MDMO-PPV : PCBM blend films was very pronounced when heated continuously at 110 °C. As the annealing time increased, the degree of phase segregation increased, which meant that the morphology continued to evolve as a result of the thermal stress. The extended thermal treatment causes the formation of larger domains, which can disrupt the optimized BHJ morphology and have a negative impact on charge generation and transport processes and thus on device performance.¹⁶⁶

For high-efficiency PM6 : Y6 organic solar cells, thermal stability remains a major challenge because prolonged heat exposure can induce significant morphological evolution within the bulk heterojunction (BHJ) active layer. Detailed flash DSC investigations have shown that the glass transition temperature of the PM6 : Y6 blend is substantially higher than typical device operating temperatures, indicating that large-scale molecular diffusion is not the primary origin of thermal degradation. Instead, the instability is mainly associated with the crystallization tendency of the Y6 acceptor, which promotes progressive donor–acceptor demixing and disrupts the optimized nanoscale morphology required for efficient charge generation and transport. As illustrated in Fig. 12, the initially intermixed PM6 : Y6 domains at room temperature gradually evolve with increasing annealing temperature. At 50 °C, partial diffusion of Y6 into PM6-rich regions becomes evident, while annealing at 100 °C promotes crystallization-induced purification of Y6 domains, leading to enhanced phase segregation. Upon further thermal treatment at 200 °C, large and distinct Y6-rich aggregates are formed, accompanied by purification of the surrounding PM6 domains, resulting in pronounced coarse phase separation. Such thermally induced morphological restructuring substantially reduces the donor–acceptor interfacial area and creates unfavorable pathways for charge separation and transport, ultimately leading to severe device performance losses.¹⁶⁷

Fig. 12. AFM-IR overlay images illustrating the nanoscale morphology of the PM6 : Y6 (1 : 1) blend after thermal annealing at (a) room temperature, (b) 50 °C, (c) 100 °C, and (d) 200 °C. In the images, the PM6-rich regions are marked in red, while the Y6-rich regions are shown in green, highlighting how the phase distribution evolves with increasing temperature. Redrawn and modified from ref. 167.

To isolate the thermal effects, the organic solar cells were heated from 10 °C to 130 °C in 2 hours in an N₂ atmosphere with in situ J–V measurements. The V_OC decreased almost linearly with both devices, and J_SC, FF, and overall PCE of PBDBTCl-DTBT degraded significantly (10.21% to 5.36%), showing poor thermal stability. For comparison, the thermal robustness of PBDB-TF-T1 was relatively high, with its J_SC and FF values maintained at 11.37% and 9.45%, respectively, with a smaller efficiency drop. V_OC showed reversible temperature-dependent recombination effects when it was cooled, while J_SC and FF exhibited minor irreversible decreases. The overall performance loss is attributed primarily to recombination and changes in resistance, with the PBDB-TF-T1 solar cell showing greater stability over the entire thermal test range, as presented in Fig. 1, where both solar cells were thermally aged under nitrogen, but with in situ J–V measurement. The J–V characteristics display prominent temperature-dependent variations, degradation in V_OC, J_SC and FF during heating and recovery in V_OC (partial) during cooling. In general, PBDB-TF-T1 had better thermal stability throughout the entire aging process than PBDBTCl-DTBT. The thermal aging of J–V curves of the PBDBTCl-DTBT : BTP-4F-12 (a, c) and PBDB-TF-T1 : BTP-4F-12 (b, d) solar cells is displayed in Fig. 13 as a function of temperature, ranging from 10 °C to 130 °C and back to the initial conditions.¹⁶⁸

Fig. 13. J–V curve evolution of the (a) PBDBTCl-DTBT : BTP-4F-12 solar cells and (b) PBDB-TF-T1 : BTP-4F-12 solar cells, illustrating the performance change as the temperature rises from 10 °C to 130 °C. (c) J–V curves of the PBDBTCl-DTBT : BTP-4F-12 solar cells and (d) PBDB-TF-T1 : BTP-4F-12 solar cells at their fresh, 130 °C, and final aged state following cooling. Redrawn and modified from ref. 168.

The development of thermally resistant cathode interlayers has been shown to be a material-level approach to mitigating interfacial degradation. Comparatively, the efficiency of PDINN-based devices remained high after being heated at 110 °C, with a PCE more than 15% higher than that of PFN-Br based devices, despite having similar initial efficiencies. This enhanced stability was attributed to strong hydrogen-bonding interactions between the amine groups of PDINN and the Y6 acceptor, which enhance the integrity at the interface and prevent thermally induced degradation of the material. By contrast, the weaker ionic interactions in PFN-Br led to rapid degradation at high temperatures, demonstrating the importance of the molecular interactions at the interfaces for enhancing the thermal stability of non-fullerene organic solar cells.¹⁶⁹

3.3. Morphological instability

The effect of time on the active layer blend morphology can adversely affect the performance of the device. Morphological instability of organic solar cells is a major challenge to long-term operation and is defined by the irreversible transformation of the nanostructure of the active layer over time. This instability may be in the form of a perturbation of the finely tuned donor–acceptor interpenetrating network, without which efficient exciton dissociation and charge transportation are restricted.¹⁶¹ This morphological degradation is caused by processes such as phase separation, aggregation and crystallization of the active materials or even the development of existing domains, as shown in Fig. 14. The causes of these changes can usually be enhanced temperatures, light exposure, and effects of annealing solvents, which result in loss of the most suitable morphology initially developed for high power conversion efficiency. Ultimately, these changes in the active layer structure minimize the interface area of charge generation and escalate recombination losses, thus affecting the efficiency of the device.¹⁷⁰ Such morphological rearrangements are especially susceptible to thermal stress. The thermodynamic drive towards larger, less connected domains of donor and acceptor materials can be accelerated by higher temperatures during device operation or storage. That may cause rough phase segregation, in which the optimal bicontinuous network folds up into big, isolated areas, restricting the movement of excitons to the donor–acceptor interface and charge transport routes.¹⁵⁸ The glass transition temperature of the active layer components is very important; when the operating temperature is close to or even above this temperature, the molecular mobility becomes higher, and the morphology becomes more easily altered. The chemical structure of the polymer and fullerene components, as well as their miscibility and annealing conditions during the production of the devices, also affect the rate and degree of morphological evolution. Fine-scale morphology changes may also take place during operation even below the glass transition temperature, which leads to performance losses.¹⁷¹

Fig. 14. Schematic showing morphological instability of OSCs, structural consequences, and device performance loss. Data taken from ref. 170.

The effect of morphological instability on device performance is severe, and it has a direct effect on the essential photovoltaic parameters. Abrupt, high changes in series resistance may be a result of disconnected charge transport channels and high bulk resistance in the active layer. Moreover, open-circuit voltage is also capable of being minimized through modification of the energy levels at the interfaces or an augmented recombination of charges in a less optimized morphology.¹⁵³ Elevated current density tends to decrease because of poorer exciton generation performance, shorter exciton diffusion length, and poorer charge collection due to the changed nanostructure. Morphological degradation also forms trap states that result in enhanced non-radiative recombination and constrained charge-carrier mobility, which in turn affects the fill factor. Thus, stabilization and preservation of the active layer morphology is important to achieve OSCs with long lifetimes.¹⁷²

Analytical transmission electron microscopy (TEM) was used to perform high-resolution spectroscopic imaging and identify key morphological features that control the generation and transport of charges in the BHJs of P3HT : PCBM. The as-cast film, as shown in Fig. 15(a), shows a highly intermixed morphology, with a large number of regions featuring mixed donor–acceptor domains and a large amount of interfacial area between the P3HT-rich and PCBM-rich domains. Such morphology is expected to favour exciton dissociation and charge separation, but the presence of mixed amorphous phases is predominant, which prevents the establishment of efficient charge–transport pathways, thus reducing the extraction of carriers. Thermal annealing, on the other hand, leads to phase segregation, forming larger and purer P3HT-rich and PCBM-rich domains, and minimizing the interface area. This reduction in donor–acceptor interface could reduce the likelihood of charge generation; however, the increased structural order and domain purity would promote charge transport via percolation pathways. The thermally annealed device shown in Fig. 15(b) thus shows an improvement in photocurrent and fill factor (FF) over the as-cast device, indicating the need for a balance between charge-generation and charge–transport processes through morphology optimization. The J–V curves for the devices are shown in Fig. 15(c).¹⁵⁸

Fig. 15. Morphology visualization using high-resolution spectroscopic imaging with an analytical transmission electron microscope for a sample P3HT : PCBM: (a) as-cast and (b) thermally annealed, and (c) J–V curves for devices fabricated under the same conditions. Redrawn and modified from ref. 158.

In a complementary study, it was also shown that the incorporation of a symmetric phenolic compound into the donor : acceptor (p–i–n) interlayer of pseudo-planar PM6 : BTP-eC9 solar cells is important for morphology stabilization. The additive proved to be effective in minimizing the disruption of the underlying donor layer caused by the solvents in a sequential deposition process, while maintaining a desirable vertical phase distribution and retarding the evolution of the morphology. This led to the devices achieving not only higher power conversion efficiencies but also greater operational stability, underscoring the efficacy of morphology-locking strategies to prevent degradation in non-fullerene organic solar cells.¹⁷³

3.4. Interfacial degradation

The interfaces between dissimilar materials in organic solar cells are also major determinants of device stability and performance. These heterojunctions control charge extraction, recombination kinetics, and energy-level alignment and are therefore highly susceptible to degradation during operational stresses. In contrast to the bulk active layer, which enjoys the stabilizing influence of donor–acceptor intermixing, interfaces are subjected to sharp chemical and electronic discontinuities that can increase the rates of failure processes.¹⁷⁴ The common inverted OSC design comprises a bulk heterojunction active material between two transport layers on the surface and two electrodes at the interfaces. Photon absorption and exciton generation are regulated by the BHJ layer, whereas efficient extraction of electrons and holes is ensured by interfacial layers that create quasi-ohmic contacts with the electrodes. In turn, device performance and operation life can be incredibly impaired by any chemical, morphological, or electronic instability at these interfaces.¹⁷⁵

Within the context of fullerene-based OSCs, in inverted structures, much effort has focused on optimizing the electron-transporting interfacial layers (ETLs), most prominently ZnO as the most common ETL in inverted structures. ZnO has high electron mobility, good band alignment with other common acceptors, and is easily solution-processable by sol–gel methods and low-temperature annealing (approximately 130 °C).¹⁷⁶ Nonetheless, ZnO has inherent constraints such as low mechanical flexibility and high photocatalytic performance under UV light.¹⁷⁷ Such photocatalytic effects can cause oxidative decomposition of the adjacent active layer of the organic material, resulting in interfacial chemical degradation, trap state formation, and slow decay of photocurrent and fill factor as the device is operated. Other interfacial modifiers, such as amino acids, have been used extensively to tune electrode work functions by forming interfacial dipoles, which include polyethylenimine (PEI), ethoxylated PEI (PEIE), PFN and PDIN. These materials have the potential to greatly reduce the work function of ITO and other conductive substrates and can therefore increase the selectivity of electrons and allow efficient extraction of charge in fullerene-based OSCs.¹⁷⁸ In this work, the importance of the interfacing layers for selective charge extraction and the formation of efficient ohmic contact between the active material and the electrodes in organic solar cells is highlighted. ZnO, PEI and PEIE are the most common electron-transport layers used in fullerene-based devices, but they have different performance in non-fullerene devices. In contrast, while the J–V characteristics of devices based on PEI/PEIE are found to be S-shaped, suggesting the presence of charge-extraction barriers and carrier accumulation at the interfaces, devices based on ZnO show little such effect and are therefore highly effective, as shown in Fig. 16. Of particular interest are these interfacial losses, which increase with thermal annealing, indicating lower interfacial stability under such conditions. Devices based on fullerene acceptors with a combination of ZnO and PEI/PEIE, on the other hand, exhibit similar photovoltaic performance, revealing the different interfacial needs of the non-fullerene acceptor systems and thus the importance of interface engineering for high efficiency and thermal stability.¹⁷⁹

Fig. 16. Instability of the interfaces in organic solar cells. (a) Schematic of an inverted OSC and chemical structures of PCBM and ITIC. (b) J–V characteristics of the non-fullerene OSCs using ZnO, PEI, and PEIE before and after thermal annealing (160 °C for 10 min). (c) J–V curves for fullerene-based OSCs with interlayers of ZnO and PEI/PEIE. Redrawn and modified from ref. 179.

Other interfacial degradation mechanisms are due to the reactions and diffusion of chemicals. To provide one example, acidic and hygroscopic PEDOT : PSS, frequently utilized as a hole-transport layer in standard architectures, has the capability to etch ITO and allow indium to diffuse into the active layer, raising series resistance and interfacial recombination. On the same note, migration of metal electrodes (e.g. Al or Ag migration) due to thermal or electrical stress can degrade the diffusion interface between the electrode and active layer, resulting in burn-in losses and long-term instability. The entrance of moisture and oxygen at the defective interfaces also enhances photo-oxidation, creating interfacial trap states that affect charge extraction.¹⁸⁰ Another disastrous problem is morphological instability at interfaces. Interlayer intermixing, roughness development, and phase segregation at the electrode can change local electric fields and break percolation routes of charge carriers. The nanoscale morphology of NFA-based OSCs is critically dependent on processing conditions; consequently, even a slight interfacial diffusion or minor rearrangement of the dipoles can cause performance deterioration. All these morphological and chemical modifications result in increased non-radiative recombination, reduced built-in potential, and, ultimately, failure of the device.¹⁸¹ In general, interfacial instability in OSCs is a result of a set of mismatches in energy levels and dipole instability, photocatalytic and chemical reactions at the metal oxide interfaces, diffusion of the interlayer or electrode species, and interfacial morphological evolution under thermal and working conditions. Knowledge of such degradation pathways is critical for the design of robust interfacial materials that exhibit enhanced chemical inertness, stable energy alignment, and suppressed interdiffusion, and thus for the design of long-lived next-generation organic photovoltaics.¹⁷⁹

3.5. Oxygen and water

Penetration of water and oxygen into unencapsulated organic solar cells is one of the greatest challenges, particularly for cells that are produced and operated under ambient conditions. These species can diffuse deep inside the OSC through microscopic defects and interfaces, initiating various degradation pathways that greatly decrease device performance and lifetime, as illustrated in Fig. 10. Molecules of water, especially, may enter the active layer and form electronic trap states because of their low ionization energy.¹⁸² These traps are counterproductive for efficient charge generation as they trap photogenic carriers, which eventually diminish the photocurrent of a device. During diffusion of water into the active layer of a polymer, water may react with the polymer backbone to produce hydroxide ions (OH⁻), and cause swelling of the polymer and distortion of its structure. Most materials used in active layers are chemically sensitive to water; for example, conventional fullerene acceptors exhibit irreversible and severe changes in their highest occupied molecular orbital (HOMO) upon exposure to water.¹⁸³ Although the energy levels tend to vary more reversibly and less severely in the presence of moisture in non-fullerene acceptors (NFAs), they are, however, not resistant to degradation.¹⁸⁴

Hydrolysis is also increased in the presence of water when hygroscopic hole-transport layers like PEDOT : PSS are used. The absorption of moisture changes the work function of PEDOT : PSS to the vacuum level, which negatively impacts charge extraction and favours the oxidation of the other electrodes, which, in typical device designs, is aluminium. Water often seeps in at the edges of a device, reducing effective area and lowering the PCE.¹⁸⁵ This may result in an S-shaped compression in the current–voltage (I–V) characteristics, which is a sign of impaired charge extraction.¹⁸⁶ Moreover, interfacial delamination is enhanced upon water infiltration, especially at the interface of the PEDOT : PSS and the active layer, which further degrades the performance of the device. Strategies to avert water-related degradation encompass a reduction in the hygroscopicity of PEDOT : PSS, with the aid of perfluorinated ionomer (PFI) additives, or substituting PEDOT : PSS with hydrophobic HTLs. Other recent works have also emphasized that some electron transport layers (ETLs), including ZnO, could become volatile in the presence of water and form aggregates in the active layer, further accelerating degradation because of their inappropriate chemical stability.¹⁸⁷

Another typical extrinsic degradation factor that has been noted to significantly decrease OSC lifespan is oxygen penetration. Photo-oxidation of active-layer materials and oxidation of the low-WF electrodes are the predominant processes involved in oxygen-mediated degradation, and high temperatures tend to accelerate the reactions. The metal-oxide layers formed on the low-WF electrodes through oxidation reduce their conductivity. These insulating layers create charge-extraction barriers between the metallic electrodes and the active layer. Consequently, oxygen and water are then allowed to diffuse into the active layer and throughout the whole device through electrode pinholes or grain boundaries (Fig. 17).¹⁸⁰ The process of oxidation of OSCs in the dark occurs because p-type doping of O₂ on the donor polymer causes the number of mobile holes to exceed that of electrons, leading to the establishment of an electron extraction barrier layer.¹⁸⁸ This can be reversed in a vacuum or inert atmosphere and does not require chemical oxidation of the materials. However, in the presence of light and O₂, permanent absorption bleaching takes place because of the degradation of conjugated bonds and loss of chromophores.¹⁵¹ This oxidation reaction begins with diffusion of free radicals and continues to side-chain degradation, leading to a significant reduction in photon absorption (photobleaching) and a reduction in J_SC. Higher recombination and density of traps, as well as the development of bandgap defects, can also be witnessed (Fig. 17).¹⁸⁹

Fig. 17. Diagram showing moisture and oxygen effects on an OSC device. Redrawn and modified from ref. 180.

The degradation rate of organic solar cells can be greatly accelerated by exposure to air and water, which can affect the active layer both in terms of its electronic properties and its morphology. Apart from these electronic modifications, the effects of oxygen and water vapor exposure on the electronic structures of the non-fullerene acceptors ITIC and o-IDTBR, and the occurrence of water vapor-induced aggregation of PC₆₁BM and progressive phase separation of the PBDB-T : ITIC blends with air aging, were also reported by Liu et al.¹⁸⁴ This morphology evolution decreases the donor–acceptor interfacial area and disturbs the charge–transport pathways, which hinders exciton dissociation, charge collection, and overall device performance.^190,191

To study the stability of the device in the presence of water and oxygen, the OSCs were fabricated in a conventional device structure of ITO/PEDOT : PSS/PAL/PDINN/Ag. Unencapsulated devices were then left in the dark in an inert glovebox atmosphere or in air to assess the degradation due to moisture and oxygen exposure. Ambient air led to significant degradation of the devices, with unencapsulated OSCs experiencing more than 50% loss in initial PCE, while devices stored under an inert atmosphere maintained more than 95% of their initial performance. Degradation was mostly caused by the penetration of oxygen and water into the photoactive layer. Interestingly, the devices featuring blended photoactive layers showed greater stability of V_OC than their bilayer counterparts, which is related to reduced water- and oxygen-induced energetic disorder. In contrast, bilayer devices with an atomic surface layer of Y6 suffered higher loss of V_OC because of broadening of the electronic density of states. Degradation trends in J_SC and FF for both device architectures were found to be similar upon air exposure.¹⁹²Fig. 18 shows how well bilayer passivation active layers (PALs) work to prevent environmentally induced deterioration. Compared with blended PALs, the bilayer PAL construction offers a more effective barrier against moisture and oxygen ingress, as seen in Fig. 18(a). This leads to significantly lower performance losses and greater retention of photovoltaic characteristics under air exposure. Additionally, the long-term stability data (Fig. 18(b)) show that devices with bilayer PALs maintain a much higher percentage of their initial efficiency over 672 hours under both inert (N₂) and ambient storage conditions. This underscores the crucial role that interfacial passivation engineering plays in reducing degradation pathways caused by oxygen and water and enhancing OSC operational durability.

Fig. 18. (a) Scheme of water and O₂ diffusion into the OSC and the corresponding J–V curves with blended and bilayer PALs stored in air. (b) Performance evolution of OSCs with blended (blue and teal curves) and bilayer (orange and purple curves) PALs stored in a glovebox (N₂; solid–symbol curves) and fume hood (air; open-symbol curves) for 672 hours. Redrawn and modified from ref. 192.

3.6. Mechanical degradation

OSC devices should be stable during transport and when used outdoors in order to produce commercially viable OSCs on a large scale. This is only possible when such devices can be protected against mechanical degradation, although this aspect has not been given serious consideration in any research studies that have been carried out to date. Mechanical degradation can also take place at the photoactive layer of polymer–fullerene blends, device electrodes, interfaces and interfacial layers, as shown in Fig. 19. These flexible organic solar cell modules must usually sustain a significant number of mechanical forces such as scratching, bending, deforming, and shearing during manufacturing and on-site installation.¹⁹³ Exposed mechanical pressure can result in the destruction of the device when the devices are weak. Moreover, during outdoor use, their structure can be degraded due to cracking, delamination, and other processes as a result of mechanical forces. Mechanical degradation can also lead to splitting and delamination at the edges of the OSC modules, which can result in the ingress of water and oxygen and also degrade the performance and stability of the entire module.⁶¹

Fig. 19. Diagram showing mechanical degradation effects. Redrawn and modified from ref. 194.

The performance of a mechanical device is related to its stability. A potential benefit of organic solar cells is the use of ductile and stiff active layers; however, managing these two mechanical properties is difficult because the photoactive layers of the BHJ are very sensitive to changes.¹⁹⁵ Balancing these mechanical degradation parameters requires expert knowledge in degradation mechanisms; this would enable stable equipment functioning throughout its service life. Polymer–fullerene blends have been researched to determine their mechanical characteristics such as stiffness and strength, and they were proven to be not highly effective in raising the elastic modulus or lowering the onset fracture strain.¹⁹⁶

One of the major challenges in flexible OSCs is mechanical degradation, which can lead to cracking, delamination at interfaces, and permanent degradation. To make it possible to compare the mechanical stability of different OSC systems, the flexible photovoltaic fatigue factor (F) was developed, providing a common benchmark that considers bending strain, cycle number, and device performance retention. This is done under standard conditions of 1% strain over 1000 bending cycles to enable quantitative comparison of fatigue resistance of materials and device architectures. The two-layer neutral mechanical plane model also demonstrates that the strain in the active layer may vary considerably from the nominal strain of the substrate. The neutral plane moves towards the device stack as the thickness of the substrate decreases, resulting in increased local strain within the photoactive and transport layers. As a result of these localized stresses, microcracks are often created and, particularly in mechanically weak layers or interfaces with poor bonding, charge transport is disrupted, series resistance increases, and degradation of the efficiency accelerates. In summary, the results correlate the device architecture and strain distribution with fracture behavior, offering design guidelines for designing more mechanically durable flexible OSCs.^197,198

4. Strategies to improve stability

The instability of OSC materials is a known issue. Degradation of both the outer structure and the inner structure of the device can take place at the same time. Both extrinsic and intrinsic stability are thus relevant for the successful functioning of organic solar cells. It should be possible to prepare OSCs with large-size modules that withstand degradation caused by photochemical oxidation, irradiation, thermal, and mechanical stress. These degradation factors should not affect the electrodes, interfaces, interfacial layers or the most important photoactive layer.^146,147 To address the instability parameters of the OSCs in relation to lifetime working, different approaches have been undertaken in the literature. These are addressed in the subsequent sections.

4.1. Device encapsulation

Encapsulation is a key measure that is employed to make devices stable with regard to extrinsic degradation factors. The encapsulation layer usually adheres to the uppermost part of the device, as illustrated in Fig. 20. Encapsulation provides security for the device against extrinsic degradation forces, such as oxygen and water, and mechanical uncertainties, such as scratches and bending. Also, the photovoltaic characteristics of the device following its encapsulation are not influenced too tremendously. These hermetically sealed devices exhibit amazing levels of stability in the presence of water or under constant light. The encapsulation material must exhibit certain virtues, like easy processability, high transmittance, electro-conductivity and hydrophobicity, and it should be ultra-stable to UV, oxygen, and water, and have excellent mechanical flexibility.¹⁹⁹ Inorganic materials, e.g., Si, SiN_x and Al₂O₃, are often employed as encapsulants as they offer excellent durability because of their high mechanical strength and transmittance, together with high water, thermal and corrosion resistance. Nevertheless, even these inorganic encapsulants exhibit certain unavoidable shortcomings. To start with, the costly processing technologies, e.g., atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD), increase the barriers to the usage of these inorganic encapsulants considerably. Secondly, the universal nature of most inorganic encapsulants in flexible electronic devices is severely limited by the lack of flexibility. More to the point, the quality of film encapsulation is mostly predetermined by the regulation of substrate and deposition conditions, without which inefficient encapsulation is bound to emerge.¹⁵¹

Fig. 20. Structure of OSCs with device encapsulation. Redrawn and modified from ref. 200.

Organic encapsulant materials, in contrast, possess more advantages, such as variable molecular structures, easy processability, and super-flexibility. Indicatively, since it is one of the most commonly used organic encapsulants, ethylene vinyl acetate (EVA) is inexpensive, resistant to wear, has a long lifetime, and can withstand different climatic conditions.²⁰¹ Based on this, the copolymer of ethylene and methyl acrylate, EMA, was developed as a much more thermally stable material that demonstrated higher chemical resistance; this material is now one of the most promising alternatives to EVA.²⁰⁰ Besides the above heat- or UV-irradiation-activated encapsulants, the thermoplastic polymer polyvinyl butyral has also been developed as an encapsulant due to its superior integration with modules and high-water resistance (hydrophobicity).²⁰² In the recent past, poly(hexafluorobutyl) acrylate (PHFBA) nanocoating formed in situ has emerged as an excellent encapsulant, which was manufactured through initiated chemical vapor deposition (i-CVD). Multilayer encapsulation technology, which is typically composed of alternating organic and inorganic encapsulants, is regarded as a viable approach to improve durability and offer effective encapsulation. Qin et al. managed to apply a multilayer encapsulation material to the Y-series-based, highly stable device, SiO_xC_y/a-SiN_x(O) : H films.²⁰³ The device comprised SiO_xC_y/a-SiN_x(O) : H films and provided an excellent lifetime of over 1000 h at a temperature of 25 °C and humidity of 50%, demonstrating that a range of affordable materials could be utilized for the commercial encapsulation of organic photovoltaics in the future.²⁰⁴ Although encapsulation of devices can create a physical barrier to the diffusion of molecular oxygen into a device, the encapsulation technology significantly raises the cost of fabrication, and OSCs are hardly a cheap renewable energy source.¹⁵¹

Encapsulation can shield devices from external environmental factors that can cause degradation, such as oxygen and moisture, and also provide protection from mechanical damage, such as scratches and bending. Besides protecting, the encapsulation process should maintain the PV performance of the PV device as close to the original as possible. Based on these requirements, the necessary properties of the encapsulation materials are summarized in Table 8. The most important parameter among these is the water vapor transmission rate (WVTR), which refers to the ability of water vapor and oxygen to penetrate the encapsulation layer and affect the underlying active layers, which affects device stability.¹⁹⁹

Table 8. Specifications and requirements for encapsulating materials for the protection of organic devices.¹⁹⁹.

Characteristics	Specification of requirements
Water vapor transmission rate	10−4–10−6 g m−2 day−1
Oxygen transmission rate (OTR)	10−3–10−5 cm3 m−2 day−1 atm−1
Glass transition temperature (Tg)	<−40 °C (during winter in deserts)
Total light transmission	>90% of incident light
Water absorption	<0.5 wt% (20 °C/100% RH)
Tensile modulus	<20.7 mPa (>3000 psi) at 25 °C
UV absorption degradation	None (>350 nm)
Hydrolysis	None (80 °C, 100% RH)

Recently, atomic layer deposition (ALD) has been shown to be a promising approach for improving the operational stability of non-fullerene acceptor (NFA) organic solar cells (OSCs) using inorganic barriers. Zimmerman et al. showed that OSCs could maintain stable performance at the maximum power point in ambient air for more than 1000 h, with negligible performance degradation even when encapsulated with Al₂O₃ layers grown by ALD at a relatively low temperature (80 °C), demonstrating the excellent barrier properties of ALD-grown films against oxygen and moisture ingress. However, the study also found that increasing the thickness of the barrier does not necessarily lead to a more durable device as the intrinsic tensile stresses of the ALD films are in the range of 400–500 MPa, and they crack and delaminate when the barrier thickness is more than about 300 nm. Moreover, although low-temperature ALD barriers proved to be sufficiently stable under ambient conditions, reliable damp-heat resistance was obtained by increasing the deposition temperature beyond the temperature limit of conventional p–i–n NFA-based OSC architectures. These results suggest that the trade-off between gas-barrier performance, mechanical strength, and thermal compatibility should be optimized in order to achieve long-term stable OSC device performance under realistic operating conditions.²⁰⁵

In order to improve the operational endurance of organic solar cells while retaining excellent photovoltaic efficiency, an ultrathin Al₂O₃ encapsulation layer formed by atomic layer deposition (ALD) is essential. The findings show that while ALD processing at moderate temperatures (especially 80 °C) maintains device efficiency, higher deposition temperatures cause notable performance losses, most likely as a result of thermal degradation and morphological disruption inside the photoactive layer. More significantly, compared to their unencapsulated counterparts, Al₂O₃-protected devices show exceptional stability during long-term maximum power point tracking, maintaining almost their initial efficiency for more than 1000 hours under ambient circumstances, as shown in Fig. 21(a–c). Additionally, the accelerated aging results demonstrate how well the ALD barrier works to prevent moisture and oxygen from entering organic photovoltaic materials, which are two of the main causes of chemical deterioration. These results highlight the significance of low-temperature ALD encapsulation as a successful tactic for reducing environmental deterioration and prolonging the high-performance OSCs' operational lifetime. While ALD-derived Al₂O₃ barriers are among the best encapsulation techniques for improving OSC durability, several other ways have also been investigated to reduce degradation caused by moisture and oxygen. In order to give a more comprehensive view, Table 9 lists the main encapsulation techniques for organic solar cells that have been described, emphasizing their main components, benefits, drawbacks, and general efficacy in enhancing device stability.^205–207

Fig. 21. (a) Dependence of OSC power conversion efficiency (PCE) on the ALD deposition temperature of the Al₂O₃ gas-diffusion barrier (GDB), (b) normalized PCE of OSCs with and without Al₂O₃ GDB during continuous operation in ambient air and (c) the normalized PCE of encapsulated OSCs stored under ambient and damp-heat (70 °C/70% RH) conditions without illumination.²⁰⁵.

Table 9. Overview of recent encapsulation strategies employed in organic solar cells (OSCs), highlighting their key materials/approaches, advantages, and associated limitations.

Encapsulation strategy	Key materials/approach	Advantages	Limitations	Ref.
Glass encapsulation	Glass cover with UV-curable epoxy	Excellent O2/H2O barrier; high durability	Rigid; unsuitable for flexible OSCs	47
Thin-film encapsulation (TFE)	Organic/inorganic multilayers	Flexible with excellent barrier properties	Complex fabrication	199
ALD-based encapsulation	Al2O3 gas diffusion barriers	Superior moisture/oxygen protection	Thermal and mechanical stress issues	205
Flexible polymer barriers	PET/PEN with barrier coatings	Lightweight; roll-to-roll compatible	Higher WVTR than glass	206
Sandwich/top encapsulation	Flexible barrier films with multilayer architecture	Enhanced environmental stability and flexibility	Additional processing steps	207

4.2. Ternary blends

The use of OSCs with a three-component blend has been of great interest in the last couple of years, as exemplified in Fig. 22. This kind of blend normally comprises a donor material, an acceptor material and a third component. The addition of a third element allows the range of light absorption to be extended and materials with varying bandgaps to be coupled while maintaining the comparatively straightforward processing environment of binary devices. OSCs with a third constituent tend to exhibit higher PCEs compared to conventional binary blends.²⁰⁸ A rapid development of ternary OSCs has thus been experienced and has drawn a lot of research attention. More recently, other groups have documented ternary OSCs capable of reaching PCE values of over 18%, which is a milestone in the commercial viability of such OSCs.^209,210

Fig. 22. Diagram of tandem cells and ternary solar cells. Redrawn from ref. 208.

The synergistic processes of exciton generation, charge transport, and morphological changes within the active layer also play a crucial role in the outstanding performance of ternary organic solar cells, in addition to the presence of an extra photoactive material. Through intermolecular interactions, energy-level alignment, and nanoscale phase separation, ternary blends have more degrees of freedom. Thus, the improved efficiency and stability of ternary OSCs are primarily believed to be due to three complementary mechanisms: alloy formation, Förster resonance energy transfer (FRET), and morphology-mediated charge transport. Together, these mechanisms help to ensure efficient photon harvesting, inhibit charge recombination, and increase the morphological stability of the bulk heterojunction in thermal and operational stress situations.²¹¹

In general, ternary OSCs can be classified as two-donor/one-acceptor (D1 : D2) and one-donor/two-acceptor (D : A1). A judiciously designed third component can control the nanoscale morphology, prevent charge recombination, and enable exciton dissociation while avoiding increased complexity of the device. Thus, ternary blending is becoming one of the most significant ways to overcome the efficiency-stability problem that has prevented OSC technologies from being used in practice. The secondary donor is usually introduced to improve the absorption profile of the host donor and for donor-phase compatibility in the bulk heterojunction in D1 : D2 systems. It has recently been demonstrated that the charge transport properties can be enhanced and the trap-assisted recombination and thermally induced phase-segregation in the morphology of the active layer can be reduced by alloy-like donor phases obtained by mixing structurally compatible donor materials. This donor alloy engineering has resulted in spectacular improvements in device performance and operational lifetime, with several donor ternary systems being found to be more efficient and thermally stable than binary systems. In addition, the use of compatible guest donors has been demonstrated to achieve better molecular packing and charge percolation pathways, and thus better carrier mobilities and higher fill factors.^212,213

In a similar manner, D : A1 ternary systems have been the topic of much interest because of their capacity to widen light absorption and enhance the organization of the acceptor domains. The guest acceptor in these systems can also serve as an energy-transfer mediator and morphology regulator, thus promoting efficient harvesting of excitons and energy disorder reduction and avoiding non-radiative voltage losses. The inclusion of alloy-like acceptor phases, especially for structurally related non-fullerene acceptors, has been found to be an effective strategy to improve charge transport and morphological stability. Recent reports have demonstrated that minimization of molecular diffusion and phase separation during rational acceptor-alloy engineering can yield efficiencies of up to 20% or higher, and can also improve thermal and operational stability. The results show that ternary blending is a promising mitigation approach to realize high-performance and durable OSCs, which is a versatile platform.^214,215

As an example, a group of researchers headed by Sun proposed NFA-based ternary OSCs named BTP-F. The material has the same fundamental unit as the commonly used acceptor BTP-eC9, with the addition of a fluorinated end group that facilitates the creation of a more homogeneous acceptor phase in blends with PM6. Consequently, the charge carrier mobility is enhanced, the morphology of the active layer is optimized, and the non-radiative recombination is decreased, which eventually results in a spectacular PCE of 18.45%, which is one of the highest values recorded in single-junction OSCs.²¹⁶ In the same way, the other NFA, G19, created by Ge et al., when combined with the high PCE binary system D18-Cl, enabled a high charge-transfer rate and good phase morphology in the ternary mixture, leading to a significant PCE of 18.53%.²¹⁷ A research group, headed by Ding, found a PCE of 18.69% in a ternary OSC structure of ITO/PEDOT : PSS/D18-Cl : N3 : PC.61BM/PDIN/Ag.²¹⁸ Elsewhere, Lu and colleagues reported very efficient ternary OSCs with a PCE of 19.39% by using a 2 TT non-fused ring electron-acceptor.²¹⁹ In addition to the efficiency boost, optical, thermal, and mechanical stability of the photoactive layer in OSCs can also be enhanced by adding a third component (cross-linkers, compatibilizers, derivatives of fullerene, nanoparticles, insulating polymers, or buffer-layer materials).²²⁰ Another example is the addition of low-bandgap polymer and fullerene derivatives and a small portion of alkane dithiols in the casting of the film. This technique increased the PCE by enabling the development of an optimized BHJ morphology through effective morphological control even when thermal annealing was out of the question. The ternary blend strategy has thus turned out to be a potent approach to improving the performance of OSC devices using the enormous number of materials that are available. The addition of a third component into the host mix generally forms a more complicated morphology than the traditional binary systems. Among the benefits of this method are the stabilization of the morphology of binary systems, reduction of light-induced morphological degradation, and enhancement of hydrophobicity, among others. Understanding the morphology of ternary blend OSCs (TB-OSCs) is also necessary to further optimize the performance of such devices.^221,222

Ternary OSCs also show less energy loss, recombination inhibition, and better charge-carrier transport, which results in increased fill factor, open-circuit voltage, and short-circuit current density. It is notable that ternary devices that use ITOA have established a PCE of 18.62% in the PM6 : Y6 system and 19.33% in the PM6 : BTP-eC9 system, the highest efficiencies to date. In the meantime, ITOA-based ternary devices without post-processing have reached a PCE of 18.04%, providing a new series of OSCs with a bright future of scalable commercial realization. Stability experiments also demonstrated that ITOA has a prominent positive impact on the stability of ternary OSC devices, which explains the significance of prudent material design for the development of OSC technology.²²³

In addition to the increase in efficiency, the ternary strategy has been shown to be very promising in terms of operational stability of OSCs. Table 10 summarizes some of the results of the recent studies. The alloy-like morphology of the ternary blends has a higher glass transition temperature (T_g) and lower molecular diffusivity than binary blends, which hinders thermally driven changes in morphology that result in loss of efficiency during extended service time. All these results have shown that ternary blending is an efficient technique and is a viable morphological stabilization approach to address one of the most challenging hurdles for successful commercialization of OSC technology.

Table 10. Stability performance of representative ternary OSC systems.

Blend system	Config.	Initial PCE	Stress type	Duration	PCE retained	Stability mechanism	Ref.
PBDT-TAZ : PTB7-Th : NOE10	D1 : D2 : A	8.5%	Thermal, 65 °C	300 h	18% enhancement	Burn-in-free, long term stability	224
PM6 : BTP-eC9: isoIDTIC	D : A1 : A2	19%	Thermal-aging	254 h	T 80 extended: 101 h → 254 h	High-Tg guest suppresses de-mixing	225
PM6 : BTP-eC9: 2Qx-C3	D : A1 : A2	19.03%	Thermal, 85 °C, N2	1205 h	T 74.1 = 1205 h	Dimer acceptor anchors donor chains	226
PM6 : BTP-eC9: DY-P2EH	D : A1 : A2	19.09%	Thermal, 85 °C, dark, N2	1100 h	>85% retained	Parallel-connected BHJ, stable morphology	227
PM6 : BTP-eC9: o-BTP-eC9	D : A1 : A2	19.5%	Operational stability	NR in main article	Enhanced operational stability vs. binary	Optimized miscibility and morphology control	228

4.3. Optimizing the active layer

A prominent component of organic solar cells is the bulk heterojunction photoactive layer, which comprises acceptor and donor materials. The chemical structure of the acceptor and the general geometry of the device have a significant influence on the photostability of the devices. Thermal stability, however, is more influenced by physical factors, including the molecular weight (MW) of the organic materials, especially the MW of the donor in the blend. As such, it is important to select and mix matching materials with care to ensure a balance in morphology, which eventually enhances the stability of the active layer.⁵⁴ The majority of the active layers are based on the donor–acceptor design, which is usually based on the famous P3HT/PCBM system. Even though the overall plan of photoactive layers is comparable, the morphologies of the photoactive layers may vary vastly with respect to domain size and interpenetrating networks.²²⁹ Because excitons have a limited lifetime and a short diffusion length of about 10 nm, nanoscale morphology is crucial for enabling their movement to the interface, where separation of charge and transport to the electrodes can be assisted. However, domains that are too small may promote charge recombination. Acceptor and donor domains should ideally be between 10 and 20 nm for BHJ OSCs to perform well.²³⁰

4.3.1. Acceptor materials

Fullerene derivatives (PC₆₁BM, PC₇₁BM, and ICBA) have been the most frequently used traditional acceptors in organic solar cells. Their widespread use is due to their high electron mobility and good solubility as well as their great compatibility with usual donor materials, which makes it possible to create effective interpenetrating networks with these materials. However, acceptors based on fullerene are susceptible to photo-oxidation when they are exposed to oxygen, which is extremely hard to eliminate completely, and this results in deterioration of the performance. In addition, the high crystallinity and molecular mobility of fullerene derivatives often result in morphological instability, particularly under thermal stress, causing phase segregation and deterioration of the optimized bulk heterojunction structure. To overcome these limitations, extensive efforts have focused on the development of cross-linking strategies in which additional functional components are incorporated into the active layer to immobilize fullerene molecules, suppress morphological evolution, and enhance the long-term thermal and operational stability of organic solar cells.²³¹ Also, at high temperatures above the crystal temperature (T_g), the active layer may be unstable due to morphological changes because of the high crystallinity and diffusivity of fullerene derivatives.²³²

However, in recent years, attention has been shifted to non-fullerene acceptors because of their controllable crystallinity, controllable energy state, and extended absorption spectra. PCEs of over 19%²³³ have been achieved with these materials. As an illustration, the introduction of the PCBM with the polymer acceptor N2200 in blends with PBDB-T and PtzBI showed greater long-term stability, regardless of the conditions, under inert, ambient and thermal stresses. In this regard, the introduction of the non-fullerene acceptor SF-HR into P3HT-based OSCs led to better thermal and burn-in stability.²³⁴ Di-PBI is another example that was found to be more thermally stable than PC71BM, maintaining its original PCE of over 70% at 100 °C after extensive heating, unlike PC71BM-based devices, which quickly degraded. These results are backed by several papers showing that non-fullerene acceptors have a high potential for enhancing the performance and long-term stability of OSCs.²³⁵ The creation of extremely stable acceptor materials has emerged as a crucial tactic for concurrently enhancing organic solar cells' operational longevity and efficiency. Although many acceptors have high initial power conversion efficiencies, photochemical deterioration, morphological instability, and environmental stress frequently restrict their long-term performance. Table 11 highlights representative high-stability acceptor systems, including their active layer compositions, photovoltaic performance, degradation characteristics, and reported device lifetimes, to give a detailed comparison of recent advancements in this field. This study provides important insights for the creation of next-generation OSCs by highlighting the connection between material design, degradation resistance, and long-term device stability.

Table 11. Relevant performance parameters of acceptor materials with high stability.

Active layer materials D : A	PCE (%)	Method	Degradation	Lifetime	Ref.
P3HT/PC61BM	3.93	Open-cage fullerenes as additives	19%	150 °C, 144 h	236
PDTSTzTz/PC61BM	3.83	Annealing at high temperature	26.3%	180 °C, 10 min	237
PBDT-TAZ : NOE10	8.1	Chain engineering strategy	3%	65 °C, 300 h	238
PTB7-Th/IEICO-4F	11.6	Black phosphorus nanoflakes as morphology modifier	27%	150 °C, 3 h	239
PCE11/PC71BM	9.2	Piperazine as phase stabilizers	Almost unchanged	1000 h illumination, 65 °C annealing	240
PM6 : TB-4Cl	14.67	Thermal-annealing treatment	Retained around 75% (TB-4Cl) and 60% (Y6)	Heated at 100 °C in nitrogen for 110 h	241
ITO/s-MoOx	17.3	Ultrathin solution-processed MoOx hole extraction layer	20% loss (T80)	600 h at 85 °C	242
PM6 : D18 : PY-IT	16.07	Ternary blend morphology stabilization	Retained 80% of initial PCE	1500 h at 120 °C	243

A combination of efficiencies of over 15% and extrapolated T80 lifetimes of over 25 000 h, which equates to an average lifetime exceeding 16 years, has been achieved for binary OSCs based on an oligomer acceptor.²⁴⁴ Much attention has been given to structural engineering of fullerene derivatives as well as to the introduction of materials that enable cross-linkage and prevent molecular diffusion to stabilize the morphology of the active layer. The use of PEG-capped fullerene derivatives (PCBM-PEG) and PC61BPF greatly increased the stability of P3HT : PCBM devices, whereas modified fullerene acceptors (pyrrolidinofullerene (PyF5) and methanofullerene (FAP1)) allowed the devices to retain more than 93% of their initial PCE after thermal aging, in comparison with only 19% retention in conventional PC60BM-based devices.²⁴⁵ Polymerizable fullerene acrylates also showed very high thermal stability, with the most stable devices losing only 0.3% efficiency after 35 h at 150 °C, compared with the reference devices, which lost 88.2% efficiency after 35 h at 150 °C.²⁴⁶ Polymerizable fullerene acrylates further demonstrated exceptional thermal stability, with the best-performing devices retaining 99.7% of their initial efficiency after 35 h at 150 °C, whereas the reference devices retained only 11.8%, highlighting the importance of morphology locking in maintaining both efficiency and operational stability.²⁴⁷

The development of non-fullerene acceptor (NFAs) materials has transformed OSC research, boosting the power conversion efficiencies from 10 to 19% and also providing new avenues for stability optimization. NFAs have strong absorption in the visible and near-infrared regions, in contrast to fullerene acceptors, and highly tunable molecular architectures that enable precise control over molecular packing, crystallinity, and intermolecular interactions. The latter include, among others, acceptors based on ITIC, which have been examined in great detail as models for the study of structure–stability relationships. Molecularly engineered NFAs are expected to be very stable, as evidenced by the ITIC family of OSCs, with projected lifetimes of nearly 10 years. In addition, slight changes in the ITIC end groups, like fluorination and methylation, were discovered to have significant effects on the photostability, with fluorination enhancing the resistance to light-induced degradation. Controlled crystallinity of the acceptor has also been shown to be important for thermal stability; after 96 h of annealing at 150 °C, the PCE of OSCs using mF-ITIC as an acceptor was only reduced by 8%, whereas those using ITIC and oF-ITIC were reduced by 18% and 33%, respectively.⁵⁸ Furthermore, non-fused acceptors like PTIC exhibited better photostability, with retention of ∼72% of the initial PCE after continuous irradiation for 500 h, compared to the PCE retained by the fused and semi-fused analogues of 20–30%. Substituting selenium into the ITIC core further improved the intermolecular interactions and increased the device lifetime from ∼400 h for the ITIC-based devices to ∼5600 h. These results clearly show that rational molecular engineering is not only able to attain higher photovoltaic efficiencies, but also effectively prevents chemical and morphological degradation, which is a fundamental strategy to achieve a better balance between efficiency and durability in next-generation organic solar cells.²⁴⁸

4.3.2. Donor materials

The contribution of the conjugated polymer or small molecule donor (DP) to stability and performance is substantial in both fullerene- and NFA-based OSCs. So, much effort has been dedicated to the development of novel polymer structures to enhance the performance of OSCs in terms of PCE and stability. If both donor and acceptor are stable, then the donor–acceptor BHJ active layer can be stable, provided it has an interpenetrating network morphology. Donor polymers can be stabilized by engineering their side chains, but not to a great extent. The solubility of polymers is highly sensitive to variations in the side chains' length, position, bulk, and type, which will ultimately influence both PCE and OSC stability. But when deterioration occurs, it can often cause breakage of the side chain, which can lead to charge trapping and further deterioration. A UV-light degradation study was performed on four polymers based on PBDT, which all included benzodithiophene-co-thienothiophene (BDT-TT) as the backbone and various types of side chains: OSCs containing alkoxy side-chain-substituted polymers were more stable than those with alkylthienyl-substituted polymers, possibly as a result of a photochemical reaction triggered by the UV light.²⁴⁹

Donor materials used in organic solar cells are crucial for the efficiency and durability of the device. Photo-induced and thermally induced degradation should receive proper consideration during the design of donor materials. Sensitivity to temperature can be minimized by thermal treatment and photochemical cross-linking, physical degradation can be limited by photochemical cross-linking, and photo-oxidation can be reduced by meticulously designing the molecule and engineering side-chains. Measures that enhance the stability of the donor include enhancing the molecular crystallinity and backbone rigidity, adding antioxidant functional groups, and encouraging copolymerization. Even very minor changes in structure, a few twists in the torsion of the side chains, have been demonstrated to have a significant effect on thermal stability.²⁵⁰ A summary of different donor strategies and their role in stability improvement is given in Table 12.

Table 12. Summary of donor strategies.

Stability strategy	Material/system	Performance metric	Stability improvement	Ref.
Side-chain engineering	BDTT-TR vs. BDTT-S-TR vs. BDTT-O-TR	Thermal stability	Different meta-position bridging atoms significantly impact stability	251
Antioxidant side chains	Polymer with BHT units	Oxidation inhibition	Prevents backbone structure oxidation	233
Block copolymerization	P3HT-b-PTB7-Th	90% PCE retention after 150 h	>80% degradation for single-polymer devices	252
Fluorine backbone	PTB7 and PTB7-Th	85% PCE after 1 h burn-in	>30% degradation without fluorine	251
Side-chain regularity	Less regular placement	Structural ordering	Maintains ordering during thermal degradation	233

New material design strategies have also been developed to make donors more stable. Indicatively, all-conjugated block copolymers, such as P3HT-b-PTB7-Th, were developed that showed a remarkable durability of approximately 90% of the initial PCE after long-term operation, in comparison to P3HT-based and PTB7-Th-based devices, which showed severe degradation. It has also been shown that device stability is strongly influenced by molecular weight, while a controlled reduction in crystallinity can be used to achieve an increase in mechanical compliance without significant restriction of the optoelectronic properties. Also, making polymer chains more flexible has been proposed as an effective method for enhancing the mechanical strength and general stability of a device.²⁵³ Improved device stability was achieved due to the increased photostability of the crosslinked donor polymer, as OSCs with crosslinked PM6-Br showed much better operational stability under continuous 1 Sun irradiation for 1000 h. The results underscore the importance of crosslinking as a potential solution to increase the photostability of OSCs, thereby enhancing their commercial viability.²⁵⁴

Covalently tethering the BTR-Cl monomer led to the innovative design of ‘face-to-face’ type giant dimeric donors DZ-1 and DZ-2. The different rhodanine-based terminals dramatically changed the interaction and thermally driven assembly properties of the molecules. The molecular stacking of DZ-2 in the blend film was moderate, and the miscibility was also compatible, resulting in a higher PCE of 13.27%, which is similar to the PCEs of OSCs based on BTR-Cl : Y6 (13.83%), demonstrating that the excellent performance of face-to-face type dimeric donors could be maintained. Importantly, the increased molecular size of the giant dimeric donors enhanced the T_g and reduced the molecular diffusion in the blend films. Moreover, the ternary PM6 : DZ-2 : L8-BO OSCs led to an enhanced PCE of 19.07% and greater device stability, as the 3D charge-transport channel was created and molecular diffusion was suppressed. This study shows that the design strategy for giant molecule donors can be developed to create high-performance and stable OSCs.²⁵⁵

4.4. Enhancement of stability through interfacial engineering

Interfacial engineering is an important technique for improving the performance and PCE of OSCs by ensuring the optimization of charge transport, reducing recombination, and eliminating cell degradation processes at key interfaces in the structure. Also, beyond this simple design, interfacial layers are being viewed as imperative at both the anode and cathode interfaces as components of the bulk heterojunction design of OSCs. Structure is commonly constructed in either the traditional or inverted design. In the traditional design, holes are gathered using the bottom electrode, which is typically ITO. The reversed geometry, conversely, permits collection of the electrons by the bottom electrode.^95–97 Strongly charge-selective interfacial layers are required to ensure better device efficiency and reduce charge recombination at interfaces. For the hole transport layers, to achieve effective hole transfer to the anode, alignment of the energy levels of the donor material (polymer) with those of the donor must be achieved. The conduction band of the HTL also needs to be high to ensure that the electrons cannot reach the anode. The same thing can be said for electron transport layers. So, to prevent recombination of excitons at the electrode, interfacial materials are required to possess a broader bandgap compared to acceptor and donor materials. Most of the popular interfacial materials, however, tend to deteriorate with time, resulting in decreased OSC performance. This is why the long-term stability of OSC devices has been investigated using different alternative materials.²⁵⁶

4.4.1. Stability and optimization of the hole transport layer (HTL)

The primary objective of increasing hole-transport layer stability is to significantly increase the overall stability of devices. An optimised, highly stable interface of the HTL with the active layer leads to less interfacial degradation through a reduction in defects and pinholes. It is also used as a shield against moisture and oxygen that may destroy organic materials. Furthermore, an advanced HTL facilitates effective hole transportation and extraction, which reduces charge recombination losses. Optimization can include the use of passivation layers or additives to reduce trap states of the active layer surface, resulting in an increase in efficiency and long-term stability.²⁰⁴ PEDOT is the most common HTL material due to its high optical density, mechanical flexibility, and its ability to be used with bulk heterojunction layers. PEDOT : PSS can be used in systems that have enhanced performance and high open-circuit voltage (V_OC), primarily because of its high collection barrier. Nevertheless, its acidic and hygroscopic properties can be destructive as it corrodes the underlying ITO layer, which diffuses into the photoactive layer. Consequently, numerous other types of HTL materials have been considered for OSC use.²⁵⁷ One of these alternatives is metal oxide-based HTLs, which have undergone extensive research as substitutes for PEDOT : PSS. Metal oxides that are often employed include MoO₃, NiO and V₂O₅, due to their electronic structures and energy-level alignment. These are solution-processed materials and can be made using large-scale roll-to-roll processes. The best metal oxide is MoO₃ because of its good conductivity, excellent electronic characteristics, and high transparency and stability, which allow holes to be extracted faster and enhance the efficiency of the OSC with respect to PEDOT : PSS.²⁵⁸ MoO_x HTLs have been used to produce devices that exhibit increased stability at high temperatures (300–420 K). Although NiO-based HTLs show better stability and performance, they need high annealing temperatures to construct the NiO thin films, which complicates and increases the cost of processing. V₂O₅ is also a good HTL that provides protection against moisture-driven surface reactions and therefore enhances efficiency and stability. Devices based on V₂O₅ HTLs have demonstrated better PCEs than those based on PEDOT : PSS and can act as optical spacers, improving light absorption and photocurrent generation.²⁵⁹

Graphene oxide (GO) has been studied as an HTL material due to its high transparency and good mechanical strength. Its electrical conductivity is, however, hindered by the oxygen-bearing functional moieties of the epoxy and hydroxyl groups on its surface and the carboxylic acid groups at its edges. O₂ gas increases the amount of oxygen in the circuit and decreases conductivity, which makes GO more insulating. This problem can be solved by chemically or thermally reducing GO to eliminate the oxygen groups and partially reorganize its conductive skeleton. Reduced graphene oxide (rGO) has been demonstrated to be more conductive and better in OSC devices.²⁶⁰

A conjugated polyelectrolyte (CPE-Na) was used as a hole-transport layer (HTL) modifier to improve the efficiency and stability of organic photovoltaic (OPV) devices. Two methods were explored: blending CPE-Na with PEDOT : PSS and adding CPE-Na as a bilayer under PEDOT : PSS on the ITO substrate. The power conversion efficiency and fill factor were increased in CPE-Na based devices compared to PEDOT : PSS based devices, and the devices were also found to be highly stable. The reference device achieved T₈₀ after 2500 h, while the CPE-Na-based devices had T₉₀ throughout the same period. The impedance spectroscopy and photoinduced transient analyses also revealed an enhancement of charge transfer and a decrease in carrier recombination, suggesting that CPE-Na is a promising HTL modifier for non-fullerene OPVs.²⁶¹ The wettability problems of the solution-processed NiOx HTLs were overcome by incorporating PMMA into NiO_x nanoparticle dispersions, which resulted in better interfacial and electrical properties. Consequently, the PM6 : Y6-based inverted OSC showed a PCE of 15.11%. Improved ambient stability was thus demonstrated to be a key strategy for high-performance and stable devices, which was achieved by HTL surface engineering.²⁶²

Recently, modification of the HTL at the interface with the SAM has proven to be an efficient approach to enhance OSC performance and stability. Specifically, the addition of myristic acid (MA) to 2PACz-based HTLs was shown to improve the surface coverage, interfacial compatibility, and hydrophobicity of the 2PACz, which resulted in reduced non-radiative recombination and increased charge extraction. Thus, the efficiencies of D18 devices were found to be 19.93%, and enhanced stability and wide applicability were further demonstrated in a D18 : L8-BO ternary system, with efficiencies higher than 20% being achieved, indicating the potential of composite SAM interfaces for high efficiencies and enhanced stability of OSCs.²⁶³

4.4.2. Stability and optimization of the electron transport layer (ETL)

In organic solar cells, the ETL is positioned at the interface between the photoactive layer and the metal, with the ETL creating an interfacial dipole that regulates the vacuum level and enhances the separation of electrons. This configuration minimizes power wastage and inhibits charge recombination by creating an effective, barrierless electron-flow channel. LiF and calcium are common ETL materials in traditional OSCs, whereas ZnO is very common in inverted structures. These materials, however, have the problem of low stability under ambient conditions; LiF and calcium react readily with oxygen and moisture, and ZnO can easily degrade in air and light.²⁶⁴ In order to overcome these disadvantages, different approaches have been devised to transform ZnO, such as chemical composition tuning, alternative fabrication schemes, insertion of a buffer layer, and interface engineering. The introduction of fullerene-based self-assembled monolayers, amino acid-based modifications, and metal-doping (e.g. with aluminium or magnesium) approaches, etc., have greatly enhanced the conductivity, PCE, and lifetime. Other ETL materials, such as CrO_x, Cs₂CO₃ and TiO₂, have also shown better device performance. Graphene oxide (GO) and interlayers including polyethylenimine-modified ZnO have also been demonstrated to improve the photostability and efficiency.²⁶⁵

Although much has changed, the long-term stability of high-performance OSCs is a major challenge, especially for commercialization. Interfacial layers are very significant in determining the overall efficiency and durability of a device, as one of the main modes of failure is degradation by moisture. Inorganic HTLs are capable of high PCE, but their fabrication typically involves high-temperature vacuum operations, which cannot be used with roll-to-roll production. Accordingly, more studies have been devoted to creating organic HTLs with high transparency and good thermal stability, low toxicity and cost, tunable energy levels, and large-scale printing protocols.¹⁸² Meanwhile, ETLs should be high-mobility electron carriers, and their energy levels should be properly aligned, with the LUMO slightly lower than the conduction band of the active material to maximize the efficiency of electron transport to the cathode.⁶⁷

The inverted OSCs use a commonly employed ETL, ZnO, which has good transmittance, suitable energy levels for electron transfer with common acceptors, and high conductivity. This ETL, however, has a lot of defects that reduce the performance of the devices and eventually lead to degradation of the device. Surface defects, such as oxygen vacancies and Zn²⁺ interstitials, can act as recombination centres for the charge carriers in the ZnO. Furthermore, surface defects can be sites for oxygen and water adsorption, resulting in the generation of ROS upon electron transfer from the photo-excited active-layer material to the adsorbed molecular oxygen, which would eventually destroy the active-layer material. Modification of the ZnO layer to decrease the Zn interstitial may be another path to decrease the chemisorption of oxygen on the surface.²⁶⁶

Other ETLs such as SnO₂ and TiO₂ are also used in OSCs. The chemical stability of SnO₂ against UV, besides its excellent charge conductivity and transparency, makes it more suitable for NFA-based OSCs. However, recent reports of OSC performance and stability with TiO₂ have been largely attributed to its porous surface, which makes achieving a good thin ETL difficult, and multiple-phase formation, which easily changes depending on the coating conditions, as well as low electron mobility.²⁶⁷

Modification of the surface properties of ZnO has proven to be a promising approach to enhance the efficiency and operational stability of inverted organic solar cells (OSCs). Recently, a rationally designed molecule (NMA) was developed to fabricate hybrid ETLs with ZnO. The carboxylic acids on NMA can bind strongly to the surface of ZnO, which allows effective passivation of the defect sites and suppression of the photocatalytic activity of ZnO, while the fused naphthalene diimide core allows for efficient electron transport. Moreover, the amine functional side chain ensures good interfacial compatibility with the active layer, leading to good morphology and charge extraction. Consequently, NMA-modified devices showed lower charge recombination, higher photovoltaic performance, and much higher operational stability under continuous illumination. It is noteworthy that devices with the D18 : N3 system realized a power conversion efficiency of 18.20% along with an extrapolated T80 lifetime of 7572 h, which equates to an estimated outdoor operational lifetime of about 5 years. This work suggests that molecular interface engineering can be a key strategy for addressing the long-standing efficiency-stability compromise in OSCs, while simultaneously reducing the degradation pathways induced by the presence of ZnO.²⁶⁸

The impact of interfacial engineering in improving OSC stability is illustrated in Fig. 23. The addition of a zwitterionic NMA interlayer, which enhances interfacial characteristics and inhibits defects that cause deterioration, between the ZnO electron-transport layer and the active layer is shown in panel (a). The MPP tracking data in panel (b) illustrate the improved operating stability of the redesigned device, while panels (c) and (e) indicate the concurrent improvement in photovoltaic performance. Additionally, the comparison in panel (d) shows that interfacial modification is a promising approach to reducing OSC deterioration, since it can successfully strike a desired balance between high efficiency and longer device lifetime.

Fig. 23. Strategy for interface engineering in inverted organic solar cells via the molecule NMA, which can enhance both the efficiency and stability of these devices. (a) Architecture of the NMA device and molecular structure of NMA. (b) Operational stability comparison of devices with and without NMA modification under continuous illumination. (c and e) Current density–voltage characteristics, which confirm the improvement of the photovoltaic performance after the modification of the surface of ZnO. (d) Comparison of the device stability with representative OSCs reported in the literature.²⁶⁸.

The power conversion efficiency of the i-OSCs used in an indacenodithieno[3,2-b]thiophene-alt-5,5-di(thiophen-2-yl)-2,2-bithiazole (PIDTT-DTBTz) donor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) acceptor was enhanced from 4.20% in pristine ZnO to 5.01% and 5.37% in ODT- and OTMS-treated ZnO-based devices, respectively. In addition, the photostability of the device improved substantially. Therefore, the devices with ODT and OTMS retained 76% and 89% of their initial PCE, respectively, after 48 h of irradiation, while the pristine ZnO-based device retained only 66% of the initial PCE after 48 h of irradiation. This is due to enhanced charge transfer, reduced bimolecular recombination, reduced trap-assisted recombination, and an improved interface between the ETL and the active layer, which leads to improved PCE and extended lifetime of the i-OSCs. Furthermore, the OTMS-treated ZnO ETL-based i-OSC can be seen as more stable and efficient devices than the ODT-treated ZnO ETL-based devices.²⁶⁹

4.5. Alternative electrodes for inverted structures

In organic solar cells, photo-active material is usually put between a low work function metal cathode and a high WF transparent conducting oxide anode. There are two main architectural structures: the traditional one and the inverted one. The basic distinction is the direction of charge collection, which has a strong influence on the environmental and operating characteristics of the device.²⁷⁰ In the traditional design, a low WF cathode is deposited as the top-most layer and is therefore exposed directly to exterior forces like oxygen and moisture, resulting in quick oxidation and degeneration. On the other hand, the inverted structure inverts the positions of the electrodes, with the cathode at the bottom of the active layer, and the top anode is a high WF, air-stable noble metal, such as silver or gold. This structure automatically protects the more active components and inhibits the subsequent horizontal movement of water and oxygen into the device interior.²⁵¹

To overcome this problem, inverted device structures have been designed; i.e. the positions of the anode and cathode have been switched. In this design, the cathode is piled under the exposed surface and therefore will not be in direct contact with oxygen and moisture, resulting in enhanced air stability in the OSCs. Indicatively, initial investigations revealed that OSCs with an inverted device architecture of ITO/ZnO/P3HT/PC₆₁BM/PEDOT : PSS/Ag were able to retain 95% of the original PCE of the device after 1 hour of air exposure when compared to conventional devices (ITO/PEDOT : PSS/P3HT/PC₆₁BM/ZnO/Al) which retained 20% of their initial PCE after 1 hour of air exposure.²⁷¹ Equally, devices using the PTB7/PC71BM system were reported to maintain 95% of initial PCE in the case of inverted devices after 48 hours of exposure to air, whereas the conventional device did not manage to retain 30% without encapsulation. Although the advantages of air stability are obvious, the inverted device structure cannot be considered a universal solution to all problems with degradation. Although it is a good method of avoiding air permeation, the inverted structures are susceptible to other degradation conditions like light and heat as compared to the conventional structures.²⁷² Comparative analysis of systems with PCDTBT : PC₇₁BM showed that traditional devices are inherently more stable than the inverted devices under a controlled atmosphere with less than 0.1 ppm oxygen and water during tests under continuous 1 Sun illumination.²⁷³ The traditional devices presented a predicted lifetime of 20 years, which was much longer than the 7 years for the inverted devices. This emphasizes that although inverted structures can result in a reduction in extrinsic degradation by air, intrinsic degradation processes, including morphological instability and chemical degradation, especially during light and heat stress, can be even more intense in some inverted architectures.²⁵¹

Recent studies still focus on the interaction of device structure and material intrinsic stability and extrinsic protection. Examples of unstable Y6-based organic solar cells include thermal degradation processes, such as polymer orientation changes and demixing, which contribute to V_OC and FF losses. Also, photochemical degradation schemes of materials for the active photochemical layers, e.g. the Y6 vinyl oxidation that generates trap states, are paramount, regardless of the device structure.²⁷⁴ Thus, although structural changes, such as inverted geometry, have great benefits with respect to air stability, a comprehensive strategy that combines the design of high-quality materials with effective encapsulation strategies and optimization of device design is needed to resolve the intricate mechanisms of degradation in OSCs. This involves using high-tech barrier materials, including multi-layered polymer films and atomic layer deposition (ALD), and using getters to remove remaining oxygen and moisture, which are key to increasing the service life of both standard and inverted micro-fabricated systems in real-world environments.²⁷⁵

Two-dimensional (2D) MoO₃ nanoflakes have been investigated for use as anode buffer layers (ABLs) to eliminate the poor coverage and interface defect problems that are usually found in conventional MoO₃ nanoparticle films. Owing to their enhanced surface uniformity and compact morphology, the optimized 2 mg mL⁻¹ MoO₃ nanoflake layer promoted efficient hole extraction, suppressed charge recombination, and improved light harvesting, resulting in superior device performance. The findings highlight the promise of 2D MoO₃ nanostructures as a high-performance ABL to improve the efficiency and stability of inverted OSCs.²⁷⁶ The wettability and charge–transport properties of solution-processed hole-transport layers in inverted OSCs have been shown to be enhanced by interfacial engineering with an Al₂O₃ nanocrystal conjunction layer. The Al₂O₃/PIDT-F : POM structure improved the interfacial contact, promoted hole extraction, and reduced charge recombination and consequently achieved the highest PCE of 18.4%. Furthermore, the devices with this architecture demonstrated improved efficiency and stability over traditional MoO₃-based devices, indicating the potential of solution-processed oxide interlayers for high-performance inverted OSCs.²⁷⁷

Simultaneous active-layer and electrode engineering has been proven to be a promising approach to improve the efficiency and durability of flexible OSCs. The energy-level alignment, molecular packing, and morphology of the films of the D18 : L8-BO system were optimized by the incorporation of the wide-bandgap acceptor F-GS-ISO, leading to a certified PCE of 19.46% and enhanced mechanical robustness of the active layer. The flexibility of OSCs was combined with a highly conductive PEDOT : PSS transparent electrode prepared by in situ doping to yield flexible OSCs with a certified PCE of 18.28%, demonstrating the synergistic effect of optimizing the active layer and electrode to obtain high-performance flexible photovoltaics.²⁷⁸

4.6. Use of additives and stabilizers

The introduction of additives and stabilizers into the active layers of OSCs is one of the key strategies to reduce degradation and improve the performance and lifespan of devices. These materials are carefully selected to overcome weaknesses in the OSC architecture and are mainly selected to enhance morphological stability, inhibit chemical degradation and enhance charge transportation. As an example, the photoactive blend morphology can be kinetically entrapped by non-volatile additives in the film-making process, avoiding unwanted phase separation or aggregation, which commonly results in reduced efficiency and stability, especially in thermal stress situations.¹⁹⁹ Moreover, some additives may serve as radical scavengers or UV-blockers and directly intercept photochemical degradation processes or inhibit bond cleavage by light in the active materials, thus increasing the photostability of the OSCs.⁷⁸

In addition to preserving the morphology and avoiding photodegradation, stabilizers and additives are also important to make the active layer components and their interfaces intrinsically stable. A case in point is to passivate trap states that appear due to material defects or degradation products, for example those caused by the oxidation of vinyl in Y6-based devices, which otherwise interfere with charge transport and extraction.²⁷⁹ The crosslinking agents may be incorporated strategically as thioctic acid (TA) to form in situ crosslinked matrices in the active layer that can effectively freeze the nano-morphology of flexible OSCs and confer excellent mechanical strength and deformability.²⁸⁰ This leads to mechanical and thermal stability, since a stable morphology is less likely to reorganize under stress. Careful choice and integration of these chemical and morphological stabilizers is thus essential to extending OSC lifetimes to meet the demands of commercial applications and supplementing the development of interfacial engineering and encapsulation schemes. PFI also augments the stability and longevity of OPV devices in addition to augmenting performance. We conclude from our degradation analysis that PFI suppresses the diffusion of water into and out of the hygroscopic PEDOT : PSS layer, which retards the degradation of both the PEDOT : PSS layer and aluminium electrode. These tests show the processes and prospects that should be considered in developing components that prevent the breakdown of OPV devices.¹⁵⁷ The incorporation of stabilizers is an acceptable way for inhibiting degradation as it is inexpensive and simple to scale up. β-Carotene (BC) is a cheap and green naturally occurring antioxidant that can increase the stability of OPV devices to a significant degree. The accumulated power generation with the addition of BC is improved by a factor of 6 (because of the stability of burn-in and lifetime) for PTB7 : PC₇₀BM devices and by a factor of 21 for P3HT : PC₆₀BM devices owing to a longer lifetime. The singlet oxygen generated by PC₆₀BM sensitizes PTB7 : PC₇₀BM devices, thereby demonstrating that BC is an effective mitigation measure against burn-in ref. 149.

Additive engineering has grown from a mere morphology control strategy to one of the most successful methods to simultaneously enhance the efficiency and operational stability of organic solar cells (OSCs). Conventional high-boiling-point solvent additives like 1,8-diiodooctane (DIO) were found to facilitate donor–acceptor phase separation and the transport of charges, but were frequently found to cause morphological degradation and reduce the stability of the devices due to their residual content in the photoactive layer.²⁸¹ Therefore, considerable research has focused on the development of other additives that are able to optimize molecular packing, while reducing stability issues. These include 1,4-difluorobenzene (DFB), tetrahydrofuran (THF), 1,3,5-trichlorobenzene (TCB) and trimethylbenzene (TMB), which are representative examples of molecules that effectively control the nanoscale morphology and crystallization behavior, resulting in enhanced charge-carrier mobility and reduced recombination losses. The improvement in device performance is reflected in the progressive increase in the PCE from below 10%, for early additive-assisted devices, to more than 19%, for state-of-the-art non-fullerene OSCs. Furthermore, additives like TCB and related aromatic compounds have shown excellent operational stability under maximum power point tracking (MPPT) conditions, emphasizing the importance of controlling intermolecular interactions and stabilizing the morphology to improve device lifetimes.^48,282–284

The development of solid additives has further broadened the scope of additive engineering, offering more durable and robust molecular interactions within the bulk heterojunction. For example, P-Cl (2-(4-phenoxybenzylidene)-1H-indene-1,3(2H)-dione) and PQ (9,10-phenanthrenequinone) are effective suppressors of phase segregation, decrease energetic disorder, and enhance the morphological robustness under thermal and light-induced stress.²⁸⁵ PID((2-(4-phenoxybenzylidene)-1H-indene-1,3(2H)-dione)-processed PM6 : Y6 devices showed only 2% loss of PCE after 1200 h at 85 °C,²⁸⁶ while PQ-containing devices exhibited a loss of only 7% of their initial PCE after thermal aging.²⁸⁷ All these results, illustrated in Table 13, show that the present design approach of additive manufacturing has moved beyond efficiency improvement to ensuring long-term morphological stability, environmental compatibility and operational durability. Therefore, the replacement of volatile halogenated solvent additives with non-halogenated, less toxic and solid-state additives is a crucial step towards the development of commercially viable OSC technologies that can simultaneously achieve high efficiencies and long service lives.

Table 13. Summary of additive-engineering strategies for enhanced efficiency and stability of organic solar cells.

Additive	Active layer	PCE (%)	Condition	Lifetime	Ref.
1,8-Diiodooctane	PTB7-Th : PC71BM	7.8	MPPT	<60%, 120 min	281
1,4-Difluorobenzene	J71 : m-ITTC	12.38	Dark	87%, 200 h	282
Tetrahydrofuran	D18-Cl : Y6	17.70	80 °C	60%, 150 min	283
1,3,5-Trichlorobenzene	PM6 : BTP-eC9	19.31	MPPT	T80 = 1000 h	48
TMB	PM6 : BTP-eC9	18.61	MPPT	88%, 200 h	284
P-Cl	PM6/BTP-eC9	19.10	MPPT	91%, 500 h	285
PID	PM6 : Y6	16.3	85 °C	98%, 1200 h	286
PQ (9,10-phenanthrenequinone)	PM6 : Y6	15.2	85 °C	>93%, 100 h	287
DTH	PM6 : L8-BO	18.51	Illumination	79.0%, 800 h	288

In recent years, the focus has turned to additives that are more environmentally friendly and non-volatile to solve environmental and stability issues. Highly efficient PM6 : BTP-eC9 devices, with PCE up to 20.0%, have been obtained without the use of conventional halogenated solvents thanks to the use of green solvent additives like isopropanol (IPA). Likewise, newly developed thienyl alkyl thienyl additives such as DTH can improve the crystallinity of both the donor and acceptor phases, leading to excellent photostability (79% PCE retention after 800 h of continuous illumination).²⁸⁸

4.7. Film post-treatment for morphology control

Different post-treatment techniques are available to obtain optimum phase separation and high crystallinity in the processed active layer. The strategies are easy to run, although parameters that are relevant like annealing temperature and operation time must be optimized precisely. Post-treatment of the as-cast active layer film is crucial for both optimisation of the initial PV performance and kinetically locking in of a morphology that is more resistant to degradation during PV operation. The three main post-treatment methods, thermal annealing (TA), solvent vapour annealing (SVA) and vacuum drying (VD), each cause different morphological changes due to different physical mechanisms, and the resulting stability is likewise mechanism- and system-dependent.^78,289

TA is used to ensure the controlled crystallisation of both the donor polymer and the NFA acceptor, which requires sufficient thermal energy for molecular motion and reordering above the kinetically frozen as-cast morphology. The most important stability advantage of TA is that it pushes the active layer towards a more thermodynamically equilibrated morphology before it is even used in the device, minimizing the driving force for further evolution during device operation at typical operating temperatures (50–85 °C). Devices treated with SVA using an appropriate solvent vapour (e.g., chlorobenzene or toluene) exhibited significantly higher initial PCE and long-term morphological stability than devices that were not treated with SVA or treated with TA or VD; devices treated with SVA retained approximately 95% of the initial PCE after 70 days of ambient storage and 75.6% after 35 h of continuous illumination, compared to 65.4% for the reference devices that were not treated with SVA.²⁹⁰ The SVA mechanism is based on selective swelling and molecular reordering of the NFA phase, which causes the orientation of the NFA crystals to be face-on and helps to increase the exciton dissociation at the donor–acceptor interface without inducing gross phase separation.²⁹¹

VD post-treatment enhances the removal of solvent from the as-cast film, which decreases the concentration of residual processing solvent (usually chlorobenzene or chloroform) that can act as a plasticiser, lowering the effective T_g of the blend, and thus increasing the rate of morphological evolution during early device operation. VD can lead to more complete removal of the solvent, minimizing the plasticisation effect and thus increasing the onset temperature for morphological instability, which can result in improved burn-in stability in some systems. Upama et al. showed that light-soaking-induced degradation in PBDB-T : ITIC devices could be reduced by TA post-treatment, which was suggested to be due to a reduction in the shallow trap density caused by TA, which would otherwise lead to a fast recombination of carriers during the initial light-soaking time.¹⁵⁴

Solvent-assisted post-treatment is an effective strategy to reduce morphology-related losses in OSC. For example, BTR-Cl : Y6 based all-small-molecule devices treated with carbon disulfide (CS₂) exhibited more favourable donor–acceptor phase separation and molecular organisation compared to the conventional thermal annealing method, leading to reduced charge recombination and increased charge transport. Hence, the optimized morphology increased both the fill factor and current density and resulted in a PCE of 15.5%. The findings show the possibility of using solvent treatments to adjust the morphology of the active layer and thus optimize device performance.²⁹² SVA is an effective method to reduce the morphology-related losses in all-polymer solar cells. In PM6 : PY-IT blends, in situ structural characterization showed that SVA induces molecular reorganization by solvent swelling, recrystallization, and molecular rearrangement, which leads to a decrease in the crystal plane spacing, an increase in crystallinity, and optimized phase separation. These results demonstrate that thermally pre-annealed devices treated with chloroform SVA showed a 15.1% enhancement in PCE, emphasizing the importance of controlled post-treatment for optimizing the morphology of the active layer and enhancing the charge–transport properties.²⁹³

Another popular post-deposition process to optimize OSC active layers is thermal annealing. Controlled heating increases the order of molecules, increases π–π stacking interactions, and promotes the formation of favorable donor–acceptor networks. Such structural modifications generally increase charge-carrier mobility and decrease the number of traps, which contributes to recombination, resulting in more efficient devices. Besides, the thermal annealing process can also improve the stability of the film by driving the film to a more thermodynamically favorable morphology that is less prone to phase separation under long-term operating conditions. The annealing process is highly dependent on material composition, annealing temperature and duration, and optimization of the process is crucial to achieve high efficiency and long device lifetime.⁵⁷ In addition to traditional coating and post-treatment techniques, novel printing techniques like inkjet printing, spray coating and gravure printing are being investigated for scaling OSC production. These methods provide digital patterning, low material usage, and roll-to-roll compatibility. Printing-based manufacturing, along with solvent engineering, thermal annealing, and new morphology control methods, is likely to be a key enabler in the future for moving organic PV technologies from lab-to-fab efficiencies to commercialization.⁹³

Post-treatment is a useful approach to stabilize bulk heterojunction morphology and inhibit phase separation in OSCs. Thermal annealing maximizes molecular packing and crystallinity, resulting in more stable charge–transport pathways and reduced burn-in degradation. The vapor annealing of solvents enhances the phase distribution at the nanoscale so that there is no excessive growth of domains as they age. Crosslinking fixes the morphology, restricting molecular diffusion under thermal and illumination stress. In general, the post-treatment procedures increase the photostability by maintaining an ideal donor–acceptor network and reducing long-term morphological development.⁷⁸

5. SWOT analysis

OPV cells could be a sustainable and environmentally friendly alternative to conventional solar cells, having low production costs and design flexibility. But they also have problems with efficiency, durability, and competition from other proven renewable energy technologies. A detailed SWOT analysis of OPV cells, identifying the major strengths, weaknesses, opportunities, and threats in this technology, is shown in Fig. 24. OPV cells have several advantages, including low production costs and design versatility, which make them a promising option for various applications, including consumer products and emerging industries. The opportunities presented by growing demand for renewable energy sources, favorable government policies and incentives, and increasing consumer awareness of sustainability and environmental impact are all factors that could drive the growth and adoption of OPV cells in the future. But the drawbacks and vulnerabilities of OPV cells cannot be overlooked.⁸

Fig. 24. SWOT analysis of organic solar cells. Modified and redrawn from ref. 294.

The lower efficiency and durability of OPVs compared to conventional solar cells is a problem for commercialization, and it is possible that they will not be able to compete very well with the established renewable energy technologies. Moreover, technological constraints and research challenges of OPV cells demand substantial investment and research, which could delay adoption and commercialization. Last but not least, the regulatory framework for renewable energy technologies can evolve and introduce uncertainty and risk for the rollout of OPV cells. The results also highlight the importance of SWOT analysis and stakeholder-led workshops for future developments in the field. These assessment tools allow the systematic identification of technological gaps, emerging opportunities, and key challenges, and facilitate collaboration between researchers, industry partners, and policymakers. Moreover, they can help influence research priorities and secure focused funding to drive technology innovation and commercialization.²⁹⁴

5.1. Strengths

The fabrication of OSCs is carried out by solution-based processes, such as blade coating, slot-die coating, and roll-to-roll printing, at temperatures under 150 °C, which means that the energy payback time is less than one year compared to the energy-intensive vacuum processing of conventional silicon photovoltaics. This manufacturing advantage is being increasingly shown at technologically relevant scales. With advanced multifunctional interfacial engineering, inverted OSCs have been realized that achieve a champion PCE of 19.47%, with excellent operational stability of 94.2% after 8904 h under ISOS-D-1 tests and 81.5% after 7724 h of maximum power point tracking.⁵⁰ Moreover, steady progress in the control of the morphology of the active layer has led to unprecedented performance in devices. One such approach was the binary OSC, which was able to reach a PCE of 21% and an exceptional fill factor of 83.2%, one of the highest PCE and FF values reported so far, with the help of an acenaphthene-assisted two-step crystallization process.⁵⁴

Non-fullerene acceptors (NFAs), such as the Y-series and BTP-eC9 derivatives, possess strong visible-to-near-infrared (NIR) absorption (molar absorption coefficient >10⁵ M⁻¹ cm⁻¹) that allows efficient photon harvesting from thin active layers while retaining good charge–transport properties.⁶³ The optical bandgap of organic semiconductors can be precisely constructed by molecular design, allowing both narrow bandgap absorbers, for tandem photovoltaic devices, and wide bandgap materials, for indoor energy harvesting applications, to be realized. This unique tunability can enable OSCs to respond to a wide variety of application scenarios and is a key benefit over traditional inorganic PV technologies. Recently, the development of semitransparent, LBL OSCs has proven that donor-layer dilution engineering can be used to achieve a proper balance between transparency and photovoltaic performance. For instance, Xu et al. fabricated a semitransparent LbL OSC with a PCE of 14.04% and a LUE of 3.88%, which demonstrates the promise of this approach for building-integrated PV (BIPV) applications.¹⁶⁴

In situ crosslinked matrices of the active layer can be used to effectively prevent the evolution of the optimized bulk-heterojunction nanomorphology, while at the same time enhancing mechanical robustness. These crosslinked networks provide enhanced deformability and mechanical failure resistance, allowing seamless integration of efficient OSCs onto curved and textile surfaces without compromising device performance.²⁸⁰ Another major benefit of OSCs is their compatibility with high-throughput roll-to-roll and printing-based manufacturing techniques, allowing continuous large-area production without material waste and with lower fabrication costs. The lab-to-fab gap has been narrowed recently, and it has been proven that high-efficiency OSCs can be transferred to industrially relevant printing processes. Moreover, attention has shifted to recyclable device architectures and circular manufacturing strategies, showing the potential of OSCs as a sustainable photovoltaic technology, which has a lower environmental impact and better end-of-life management.²⁹⁵

5.2. Weaknesses

Although OPVs are making significant strides towards efficiencies greater than 20% in power conversion, there remains a significant efficiency gap between OPVs and established photovoltaic technologies. The efficiencies of perovskite-silicon tandem devices have crossed 33%, and certified silicon heterojunction cells have reached 26.8%. This difference is even more pronounced at the module level, as the performance of OPVs is affected by interconnection losses, non-uniform coating, and series resistance. Furthermore, high non-radiative voltage losses remain a key limitation of the maximum open-circuit voltage and overall efficiency of OPVs. Thus, further improvements in molecular design, morphology control, and scalable manufacturing are essential to bridge the performance gap and enhance the competitiveness of OPV technology.²⁹⁶ One of the major drawbacks of this OPV technology is the lack of adequate long-term stability in operating under real-world conditions. While major advances have been made, with some state-of-the-art NFA-based devices having lifetimes of a decade or more, and advanced acceptor systems having T₈₀ values greater than 8000 h, these are still short of the operational lifetime of commercial PV modules, which is 25–30 years. The major sources of this stability gap are photo-oxidation, thermally induced morphology evolution, interfacial degradation, and entry of oxygen and moisture during long-term operation. Therefore, enhancing intrinsic material stability and device durability remains a crucial step for the commercialization of large-scale OPVs.^161,162

One of the major drawbacks of OPVs is the loss of efficiency during scale-up from laboratory cells to large-area modules. The series resistance, non-uniform coating, interconnection losses, and challenges in maintaining optimal bulk-heterojunction morphology over large areas can all lead to lower module performance. The scale-up challenges of achieving these improvements in coating processes, ink formulations and manufacturing control are still critical to commercial deployment.¹⁷² The use of halogenated solvents is another major drawback of OPVs; there are still many high-efficiency devices that require environmentally harmful processing chemicals to get the best morphology and device performance. While significant progress has been made with green-solvent alternatives, it is still a challenge to achieve the efficiency and processing reliability of halogenated systems for large-scale commercialization.^53,289

5.3. Opportunities

Perovskite organic tandem solar cells hold great promise for OPV technology that may overcome the efficiency ceiling of single-junction solar cells. Tandem architectures take advantage of the near-infrared absorption of narrow-bandgap organic absorbers and the strong visible-light absorption of wide-bandgap perovskites to achieve more efficient use of the solar spectrum. This approach has recently been shown to be highly promising, with certified efficiencies of over 26% for perovskite organic tandem devices. Further advances in material design, interfacial engineering, and tandem integration are anticipated to lead to even better performance, making OPVs a promising candidate for next-generation high-efficiency PV technologies.⁵⁵ The rising BIPV market is a significant commercial prospect for OPVs. The lightweight, flexible, semitransparent, and aesthetically tunable properties allow them to be easily integrated into windows, facades, skylights, and more, where silicon photovoltaics are less viable. Together with the growing demand for buildings with net-zero energy consumption, these special characteristics make OPVs promising future candidates for BIPV and agrivoltaics applications.¹¹

As the number of IoT devices, wearable electronics, and autonomous sensors continues to grow, there are huge potential opportunities for OPVs. Thanks to their light weight, flexibility, and solution processability, they are especially well suited for self-powered electronic systems, especially when used under low-intensity indoor lighting. In addition, the spectral tunability and ability to be used on curved and wearable surfaces make OPVs an attractive energy source for next-generation wearable and indoor electronics.¹³ Moreover, recent advances in scalable manufacturing and device integration are driving OPVs from the laboratory research stage to real-world applications, opening new opportunities for commercialization and mass deployment.²⁹⁷

5.4. Threats

The main competitive advantage of crystalline silicon PVs is their high efficiency, outstanding long-term stability, and continually decreasing manufacturing costs. Commercial silicon modules have efficiencies that are routinely more than 28% and have operational lifetimes of more than 25 years, which is much longer than has been achieved with OPVs. Until significant gains in module efficiency, stability, and scale-up are realized, OPVs will remain uncompetitive compared to more mature silicon PV technologies.²⁹⁸ Perovskite solar cells (PSCs) are making significant strides, posing a significant competitive threat to OPVs. PSCs are already able to reach certified single-junction efficiencies above 27%, and perovskite silicon tandem devices have already surpassed 34%, surpassing current OPV technologies. Furthermore, recent advances in module-level manufacturing and reliability testing are driving the commercialization of PSCs. While OPVs still have the benefits of mechanical flexibility, light weight, and potentially easier recycling routes, further advancements in the efficiency and stability of perovskite cells could further drive competition for the emerging thin-film PV markets.^299,300

The absence of long-term field validation, bankability certification, and manufacturing readiness are still big risks for OSC commercialization. While the current PV technologies include 25 year performance warranties, OSCs have not yet proven to operate for comparable periods in the field. Moreover, standardization issues, mass production, and investor scepticism remain stumbling blocks to market uptake. Additional progress in durability, certification processes and scalable production technologies will be needed to overcome these challenges.^145,233

6. Challenges and outlook

Although tremendous progress has been made in the field of organic solar cells (OSCs), with power conversion efficiency (PCE) increasing from less than 0.001% to more than 20%, there are still many challenges that need to be overcome before the technology can be commercialized.³⁰¹ The main challenge is the high price and synthetic complexity of the current PV materials. Future studies should be directed towards the design of low-cost materials with simplified structures and scalable synthetic methods, as many of the high-performing donor/acceptor materials are costly, complex, and low-yielding.³⁰² Thus, further work is needed to synthesize low-cost materials that have simpler molecular structures and are easier to fabricate on a large scale. It is particularly desirable to have non-fused and dimeric acceptors and structurally simple polymer donors, because they avoid complex ring-fusion reactions and improve molecular stacking for fibrous morphology with favourable exciton dynamics.³⁰³ For example, several non-fused acceptors synthesized through only a few reaction steps have demonstrated promising efficiencies in both single-junction and tandem devices, while simple donor polymers, such as PTQ10, have achieved efficiencies exceeding 16%, highlighting the feasibility of combining low synthetic complexity with high device performance. Future material design should prioritize synthetic accessibility, cost-effectiveness, and scalability, alongside efficiency optimization, to accelerate industrial adoption.³⁰⁴

The major challenge to the widespread deployment of OSC technologies is operational stability over the long term. The degradation of devices is caused by several interconnected mechanisms, such as ingress of oxygen and moisture, photooxidation, thermal stress, ultraviolet irradiation, electrode diffusion, interfacial degradation, metastable morphology, and mechanical deformation during operation. Light-induced degradation may lead to photochemical and photophysical changes, and high temperatures may promote evolution of the nano-morphology in the active layer. Burn-in degradation is another long-term problem that leads to significant performance degradation at the start of life, and the phenomenon of burn-in is not yet fully understood. Fullerene dimerization, trap-assisted recombination, broad donor polydispersity, impurity-induced degradation pathways, and morphological instability have been suggested as possibilities.¹⁷³ Recent non-fullerene acceptor-based OSCs have achieved projected operational lifetimes of up to 10 years, while some reports have suggested extremely long extrapolated outdoor lifetimes, but it is difficult to translate these results into reliable real-world performance. Future OSC research needs to focus on the creation of standardized stability protocols and the development of predictive models of degradation that can reliably predict the degradation of OSC under various outdoor conditions based on accelerated aging data.

One of the simplest degradation pathways is related to the instability of the active layer, which is a thermodynamic instability of the conventional bulk heterojunction (BHJ) blends used. Donor and acceptor materials are kinetically trapped instead of thermodynamically equilibrated, which means that external stresses like illumination and heat can give rise to phase segregation, domain coarsening, and undesirable morphological change, thus leading to low charge-generation efficiency and device performance. In order to overcome this problem, single-component materials (SCMs) have been developed, which are composed of a donor and an acceptor in a single molecule or polymer. These architectures not only prevent large-scale phase separation but can also promote exciton generation, exciton dissociation, and charge transport within a single molecular structure. Furthermore, SCMs can address the problem of exciton diffusion length and morphological instability, and allow simplified fabrication of devices and better long-term stability. However, several challenges regarding molecular design complexity, synthetic availability, and optimization of nanoscale morphology still need to be addressed before SCMs can be considered as a viable alternative to the traditional BHJ system. Polymerized acceptors, covalently linked donor–acceptor systems, and self-assembled semiconductor networks could be promising routes to intrinsically stable active layers in future OSC technologies.²⁴⁷

Besides morphological stabilization, suppression of photo-oxidation is crucial to provide long-term device durability. Because oxygen-induced degradation is one of the primary failure modes in OSCs, encapsulation methods, radical scavengers, singlet oxygen quenchers, ultraviolet stabilizers, and antioxidant additives have been the focus of much attention for extending the lifetime of devices. Bitingly, natural photosynthetic systems have developed very efficient ways to counteract oxidative damage over billions of years, offering much inspiration for the development of future OSCs. Moreover, electrode stabilization, interfacial engineering, and morphology-freezing methods have proven to be very promising for decreasing degradation rates and enhancing operational reliability. Stability engineering for future use of these types of molecules will need to include molecular design, optimization of the interface, encapsulation technologies, and biomimetic protection processes to control several degradation pathways.

Lastly, sustainability aspects are likely to gain more significance with the commercialization of OSC technology. Much has been done to ensure the stability of the devices, but relatively little work has been done on end-of-life management and environmental impacts. There is great potential for biodegradable and recyclable OSCs to overcome the increasing PV waste problem and to integrate OSC technology with the principles of the circular economy. Interestingly, the same environmental factors that cause degradation of the device can also promote biodegradation of the material after disposal (oxygen and moisture). Thus, the development of future strategies could be directed towards selectively slowing degradation over the operational lifetime, but allowing controlled degradation at the end. The next generation of OSCs should thus be assessed beyond efficiency and stability, also in terms of sustainability, recyclability, environmental compatibility and impact in the life cycle, fostering the emergence of truly green photovoltaic technologies.³⁰⁵ Most record-efficiency OSCs are produced by spin coating under controlled laboratory conditions, while scalable methods like blade coating, slot-die coating, gravure printing and roll-to-roll processing are used for industrial production. The solution-to-solid transformation that takes place during large-area processing differs considerably from that of small-area devices and has a significant impact on molecular packing, phase separation, crystallization behaviour, and vertical composition distribution inside the active layer. Therefore, to achieve high efficiencies over large areas, it is important to understand the mechanisms controlling morphological evolution during film formation.³⁰⁴

It is hoped that the combination of in situ characterization tools, advanced process modelling, and machine-learning-enabled optimization will become more important in the future to close the gap between laboratory-scale performance and the scale-up to industrial manufacturing. In the future, further innovation in OPVs will depend more and more on the use of AI, innovative materials, and automated experiments. The use of data-driven methods, such as machine learning, generative models, and large language models, is expected to speed up the identification of high-performance donor and acceptor materials and provide a more detailed understanding of structure–property stability relationships. In addition, AI-driven molecular design, coupled with high-throughput synthesis, in situ characterization, and scalable manufacturing processes, could drastically reduce the design-fabrication-validation cycle. The interdisciplinary advances are expected to enable simultaneous optimization of efficiency, operational stability, and manufacturability, which will speed up the commercialization of next-generation OPV technologies.³⁰⁶

Progress in advanced functional materials, photovoltaic technologies, defect passivation, and electrochemical energy systems has reinforced the importance of rational device architecture, interface engineering, and materials optimization for improving efficiency and long-term operational stability.^307–311 These developments offer valuable design principles for organic solar cells, where suppressing degradation and enhancing device durability are key requirements for practical commercialization.^312–316

7. Conclusion

Scientific research into materials design and device engineering has made significant breakthroughs that have enhanced the efficiency and the stability of organic solar cells. Maximization of interfacial layers has been shown to be an effective method for reducing charge recombination and maximising energy-level alignment, and close regulation of the morphology of BHJ has facilitated balanced exciton dissociation and charge transport. Non-fullerene acceptors are an important advancement, providing increased control of energy levels, expanded absorption and much improved thermal and photochemical stability. Increased backbone rigidity, improved design of side-chain structures, the efficient use of copolymer solutions, and tuning of molecular weight achieved through advanced strategies in the design of donor materials have further enhanced the device stability under operating conditions.

Irrespective of these developments, there are still several issues to overcome before OSCs can be used for broad commercial applications. Operational stability under real-world conditions, such as prolonged light exposure, ingress of oxygen and moisture, and thermal cycling, should also be enhanced. Future studies must aim at creating materials with intrinsic stability against photo-oxidation as well as scalable processing methods that do not affect the nanoscale morphology. Advanced characterization and modelling will be needed to gain a clearer insight into degradation pathways at both molecular and device levels of understanding. Also, it will be essential to combine mechanical strength and high efficiency, which is crucial in developing flexible and wearable devices. In general, a consistent convergence between molecular engineering, morphological control, and optimization of the structure of the device is an evident route toward the development of highly efficient organic solar cells that are commercially viable. Through long-term cross-disciplinary collaboration, OSC technology has great potential to achieve competitiveness and sustainability in photovoltaic technology.

Conflicts of interest

There are no conflicts to declare.

Biographies

Biography

Muhammad Irfan.

Mr. Muhammad Irfan was born in 2002 in Layyah, Punjab, Pakistan. He obtained his Bachelor of Science degree in Chemistry from Government College University Faisalabad in 2024. He is currently pursuing his MPhil in Analytical Chemistry under the supervision of Prof. Dr Muhammad Ramzan Saeed Ashraf Janjua. His research interests include organic photovoltaic technologies, machine-learning-assisted design and the prediction of functional dyes, and advanced energy materials. His work involves the application of computational chemistry, particularly density functional theory (DFT), and data-driven approaches to investigate structure–property relationships and optimize the performance of emerging photovoltaic and dye-based systems.

Biography

Akash.

Mr Akash received his BS (Hons) degree in Chemistry in 2022 from Govt. Municipal Graduate College, Jaranwala Road, Abdullahpur, Faisalabad, an affiliated college of the Government College University Faisalabad. He is currently an MPhil scholar in Analytical Chemistry at Government College University Faisalabad under the supervision of Prof. Dr Muhammad Ramzan Saeed Ashraf Janjua. His research focuses on the computational chemistry aspects of organic electronic materials for photovoltaic applications, with particular emphasis on quantum chemical design using density functional theory (DFT/TD-DFT), transition density matrix analysis, structure–property relationships, and nonlinear optical (NLO) properties. His broader research interests include organic solar cells, advanced characterization techniques, and the application of machine-learning approaches for the discovery of next-generation photovoltaic materials.

Biography

Nimra Sultan.

Ms Nimra Sultan completed her MPhil in Physical Chemistry in 2024 from Government College University Faisalabad. She is currently pursuing her PhD in Chemistry at the same institution under the supervision of Prof. Dr Muhammad Ramzan Saeed Ashraf Janjua. Her research is focused on the computational physical chemistry aspects of organic electronic materials for photovoltaic applications, with particular emphasis on quantum chemical design using density functional theory (DFT/TD-DFT), transition density matrix analysis, and structure–property relationship investigations. She has contributed to several peer-reviewed publications, and her broader research interests include organic solar cells, advanced characterization techniques, and the application of machine-learning approaches for next-generation materials discovery.

Biography

Muhammad Ramzan Saeed Ashraf Janjua.

Prof. Dr Muhammad Ramzan Saeed Ashraf Janjua was born and raised in Sargodha, Pakistan. He began his professional career in the pharmaceutical industry, serving as a Medical Representative at Johnson & Johnson and later as a Pharma Associate at Aventis Pharmaceuticals between 2001 and 2005. He subsequently pursued higher education, earning an M.Sc. in Physical Chemistry from the University of Sargodha and a PhD in Physical Chemistry from Northeast Normal University, China, in 2010 and completed a postdoctoral fellowship at the University of Coimbra, Portugal. He is currently a Professor in the Department of Chemistry at Government College University Faisalabad (GCUF), Pakistan. Previously, he served as Assistant Professor, Associate Professor, and Professor at the King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia. His research focuses on computational chemistry, quantum chemical modelling, functional materials, nonlinear optics, and organic photovoltaics. He has published more than 180 research articles in international journals and has made significant contributions to the theoretical design of advanced materials for energy and optoelectronic applications.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.