Progress of Photocapacitors

Abstract

In response to the current trend of miniaturization of electronic devices and sensors, the complementary coupling of high-efficiency energy conversion and low-loss energy storage technologies has given rise to the development of photocapacitors (PCs), which combine energy conversion and storage in a single device. Photovoltaic systems integrated with supercapacitors offer unique light conversion and storage capabilities, resulting in improved overall efficiency over the past decade. Consequently, researchers have explored a wide range of device combinations, materials, and characterization techniques. This review provides a comprehensive overview of photocapacitors, including their configurations, operating mechanisms, manufacturing techniques, and materials, with a focus on emerging applications in small wireless devices, Internet of Things (IoT), and Internet of Everything (IoE). Furthermore, we highlight the importance of cutting-edge materials such as metal–organic frameworks (MOFs) and organic materials for supercapacitors, as well as novel materials in photovoltaics, in advancing PCs for a carbon-free, sustainable society. We also evaluate the potential development, prospects, and application scenarios of this emerging area of research.

1. Introduction

The urgent need to transition from fossil fuels to renewable energy sources has spurred the development of cutting-edge energy conversion and storage technologies.¹⁻¹² Photovoltaic (PV) systems have emerged as a leading solution to address the growing demand for carbon-free energy, with applications in various technical domains, such as photoelectrochemical water splitting (PEC),¹³⁻¹⁶ photocatalysis,¹⁷⁻²² and photoelectrochemical redox flow batteries.²³⁻²⁶ However, the inherent variability and unpredictability of solar radiation pose significant challenges to the widespread deployment of solar power, necessitating high-efficiency energy conversion and low-loss energy storage technologies.

Photocapacitors (PCs) offer an innovative energy conversion and storage technology by combining a photovoltaic or energy harvesting unit with a supercapacitor (SC) or an energy storage unit. This dual-use system allows for efficient generation and storage of power in a single device, making it suitable for a wide range of applications.^27,28 PCs are based on the central concept of a self-charging capacitor that can directly store the electrical energy produced by photovoltaic cells, as proposed by Miyasaka and Murakami.²⁹⁻³² This integrated system allows for efficient generation and storage of power, making PCs suitable for a wide range of applications in next-generation electronic devices and systems, particularly as energy demands increase and the need for sustainable, self-sufficient power sources becomes more critical.³³⁻³⁷

A photocapacitor comprises a photovoltaic or energy harvesting unit coupled to a supercapacitor (SC) or energy storage unit.^32,38−41 The energy from a light source (solar or artificial) is transformed into electrical energy by the PV unit.⁴²⁻⁴⁷ The photogenerated charges are channeled into the SC unit, where they are stored at the electrodes of the supercapacitor.⁴⁸⁻⁵² The components of the PV unit vary depending on the technology. First- and second-generation solar cells can be adapted to photocapacitors but on limited architectures.^9,53−57 Third-generation photovoltaic technologies, including organic photovoltaics (OPVs),⁵⁸⁻⁶¹ perovskite solar cells (PSCs),⁶²⁻⁶⁵ dye-sensitized solar cells (DSCs),^10,66−71 and quantum-dot solar cells (QDSCs),⁷²⁻⁷⁵ are preferred for developing PCs due to their ease of fabrication, compatibility with various architectures, and cost-effectiveness. These technologies consist of a photoactive material or working electrode (WE), a redox electrolyte or hole transport material (HTM), and a counter electrode (CE). The supercapacitor unit is composed of two electrodes containing electroactive materials capable of storing charge as an electric double layer (EDL), a membrane separator, and an ion-conducting electrolyte.⁷⁶⁻⁷⁹

While the highest reported charge storage efficiency of an integrated photocapacitor is approximately 20%,²⁸ further improvements in the intrinsic properties of the active materials, interface quality, and device integration are needed to enhance overall efficiency and commercial viability. Factors that influence a PC’s overall photoconversion and storage efficiency include the bandgaps of various semiconductors, hole–electron recombination, and the quality of multiple interfaces. Optimizing these factors is crucial for boosting the efficiency of PCs beyond 25% and enabling their commercial availability.³²

Photocapacitors present an elegant solution for the development of self-powered electronic devices in various applications, including the Internet of Things (IoT) and the Internet of everything (IoE). The IoT encompasses sensors, actuators, wireless communication networks, and data processing, leading to energy regulation and the creation of intelligent buildings.⁸⁰⁻⁸⁵ The widespread deployment of IoT devices in agriculture, health, and business will significantly benefit society by enhancing energy efficiency.⁸⁶⁻⁸⁹ Typically, these electronic devices require an energy source, such as batteries, supercapacitors, or separate photovoltaic and energy storage units.⁹⁰ PCs represent a compact and efficient alternative to these independent energy sources, enabling the development of self-powered devices and systems.⁹¹⁻⁹⁴

The adoption of PCs also circumvents the need for two physically separate devices for energy conversion and storage, reducing space, cost, and weight while boosting the overall package efficiency. As illustrated in Figure 1, PCs can be designed as compact devices that facilitate the paradigm shift in the electronic era. The potential applications of PCs expand further when the concept of IoT is extended to the Internet of Everything (IoE), which encompasses intelligent connections among people, electronic gadgets, and data. Widespread adoption of photocapacitors will result in significant societal benefits, including energy savings, increased use of renewable energy, self-powered artificial intelligence, enhanced connectivity, and data transfer.

Figure 1.

Schematic illustrating the integration of photocapacitors from individual components into a singular device capable of light harvesting and charge storage. Multiple applications are enabled by the use of photocapacitors, and their development will lead to more efficient and sustainable energy consumption.

Moreover, the production of photocapacitors can be entirely adapted to a circular economy, making them an excellent sustainable alternative to current storage technologies. Their eco-friendly nature aligns with global efforts to mitigate climate change and promote sustainable development.

This review provides a comprehensive overview of photocapacitors by examining three aspects: photoelectrode and capacitive materials, PC characteristics, and photoelectronic device systems. First, we summarize the research status in materials, components, and device engineering of photocapacitors, including photovoltaic characteristics and supercapacitor materials. We also discuss the bandgaps of various semiconductors, hole–electron recombination, and factors that influence the overall efficiency of PCs. Second, we review known methodologies to determine and enhance the performance of photocapacitors. These approaches include optimizing the intrinsic properties of the active materials, improving interface quality, and refining device integration. We also highlight potential strategies for developing highly efficient light conversion integrated systems, paving the way for further advancements in photocapacitor technology. Lastly, we explore the future applications of photocapacitors in sustainability and their potential impact on various industries. We assess the prospects of PCs in IoT and IoE systems, intelligent buildings, agriculture, health, and business. We also discuss the role of PCs in promoting energy efficiency, self-powered artificial intelligence, enhanced connectivity, and data transfer, ultimately contributing to a more sustainable future.

In summary, photocapacitors represent a promising avenue for the development of efficient energy conversion and storage technologies. By providing a comprehensive overview of photocapacitor materials, characteristics, and systems, this review aims to inspire further research and development in this emerging field. The ongoing advancements in photocapacitor technology will not only contribute to the global transition toward renewable energy sources but also enable the realization of innovative applications in various sectors, paving the way for a sustainable future.

2. Applications of Photocapacitors

2.1. Indoor Applications

Lithium-ion batteries (LIBs) are the most widely used energy storage technology due to their high energy density. Despite this, they present several disadvantages, including relatively high prices, inadequate safety precautions, and a limited supply of lithium and cobalt, whose mining is frequently associated with exploitative working conditions and a significant environmental impact.⁹⁵ Moreover, they cannot endure numerous charge/discharge cycles, limiting their lifespan and leading to replacement regularly, resulting in billions of hard-to-recycle batteries per year.

Employing an electrical double layer (EDL) or a supercapacitor (SC) in locations that demand fast, powerful, and secure energy storage devices can overcome these problems.⁹⁶ Due to their lower energy density than batteries, supercapacitors can be charged entirely or discharged in seconds or minutes. However, larger power output can be achieved for brief periods.

Supercapacitors can be charged sustainably by coupling them with low-cost, solution-processed photovoltaic cells. The resulting device, a photocapacitor (PC), will pave the way for the development of self-sufficient gadgets that function as an independent power source, requiring nothing more than solar or indoor light and thus playing an essential role in the transition to renewable energy sources for a wide range of applications.⁹⁷⁻¹⁰²

The photovoltaic unit of the incorporated device rapidly charges the supercapacitor unit upon illumination by any available indoor light or solar irradiance (when outdoors). Small electronic devices, such as the digital display of a pregnancy test kit, biosensors, wearable electronics, and Bluetooth-enabled devices, can be powered by the energy produced and stored in the photocapacitor. Some other applications include power grids, emergency door sensors in transport media, pulse power in communication devices, large-scale energy supply systems, and photochromic applications.^103−113 As a result, PCs can potentially replace batteries with the added benefit of substantially higher stability than standard coin cells or LIBs.

The Internet of Things devices and ecosystems need to be able to sense, process data, and communicate with each other.^114,115 Wireless machine learning and artificial intelligence are achieved by smart sensing and local data processing. These can be performed with low-power microprocessors and microcontrollers, determining the power requirements of the IoT devices.^80,81 For instance, the power consumption of passive and active radio frequency identification (RFID) tags, as well as a wireless communication protocol that ensures data connectivity ranges from μW to W (Figure 2).¹⁰² IoT devices with limited sizes ranging from 1 mm to 10 cm can be embedded on various substrates (glass, flexible substrates, clothing, fibers, etc.).^116,117 As a result, their energy supply systems must be lightweight, adaptable, and miniaturized.

Figure 2.

Schematic representation of the illuminance levels in different settings. As depicted in the diagram, most sensors and communications protocols require power from 10 μW to 100 mW. Efficient PCs with enough active areas can meet the power requirements of each application. The design of flexible photocapacitors broadens their ambient light applications and wearable electronics.

The light intensity attained by ambient light sources ranges from 50 to 300 lx in residential settings, 300–500 lx in offices, and 1000 lx in industrial environments, much lower compared to the illumination levels at 1 sun illumination of 100–110 000x (at AM1.5G, 1000 W m^–2).¹¹⁸ The emission spectra vary between each lamp source. CFL Fluorescent and LED-based lights emit mainly in the visible region, which provides an excellent opportunity to develop photocapacitors tuned to harvest the light from their surroundings efficiently.

Modern LED sources offer efficacies up to 100 lm/W, with an irradiance around 300 μW cm^–2 at 1000 lx conditions.¹¹⁹ Employing a highly efficient PV unit designed for indoor-light harvesting with a PCE of around 35% will readily deliver 105 μW cm^–2.¹²⁰ Conventional sensors require from 100 μW to 1 mW, depending on the operation mode.¹²¹ Ambient light sources from 100 to 1000 lx can provide from 25–300 μW cm^–2, sufficient to power IoT devices.⁹⁸ The Shockley–Queisser efficiency limit for indoor photovoltaics under 500 lx from a white-LED source is approximately 52%.⁹³ Further advances in the following years will bring ambient photovoltaics closer to 50% PCE, allowing them to produce approximately 150 μW cm^–2.

As indicated in Figure 2, the power consumption of electronic devices and communication systems can range from low power in the range of μW to high power depending on the application (hundreds of W). The purpose of photocapacitors in this context is to replace traditional energy supply systems with specialized power supplies for each application.

Many architectures have been used on rigid and flexible substrates to fabricate photocapacitors. The hybrid four-terminal (4-T) architecture comprises the supercapacitor and solar cell individually constructed and then connected via wiring (Figure 4). The key challenges with a hybrid design are the need for external wiring and solid-state electronic components that are undesirable for integrated applications. In contrast, the integrated 3-T and 2-T architectures comprise a single device in which the energy harvester and storage unit share two or more terminals (see Figure 4). Due to the compactness of the completed device, these architectures will be the focus of future research as they overcome the need for external wiring. To increase the overall efficacy of the photocapacitor device, however, the direct coupling or stacking of materials requires additional engineering to reduce charge losses between the interface of the PV and SC units. In addition, further research must address the need for a diode material between the photovoltaic and supercapacitor components to prevent charge recombination from the supercapacitor to the PV. Generally, using a diode will result in a significant voltage drop, and this issue must be addressed by developing novel materials with diode-like behavior.

Figure 4.

Schematic representation of the photocharging and dark discharge processes in a photocapacitor, and integration of a PC into different terminal configurations: 2-terminal, 3-terminal, 4-terminal.

Operating photocapacitors in high-power applications offers a more compact and dependable alternative to large-area and bulky battery systems. The PCs can be coupled to solar modules and used as a fast-response energy storage unit alongside battery units to increase the total power supply.¹²²

2.2. Flexible Photocapacitors

As previously stated, the critical focus for numerous applications will be the construction of high-performance flexible photocapacitors. The PV community has prioritized the development of flexible photovoltaic cells in recent years, opening the path for flexible PCs.^123−127 Flexible substrates have been widely investigated for photocapacitors as they enable applications where flexibility and lightweight are essential requirements. Despite current flexible integrated devices exhibiting lower total energy conversion and storage efficiency than their rigid counterparts, innovative architectures have been proposed and efforts made toward more efficient and stable self-powered devices.

The manufacturing of flexible devices typically involves roll-to-roll or slot-die coating techniques, which can be complemented by spray processing, inkjet printing, and 3D-printing techniques.¹²⁸ Additionally, 3D printing can provide materials with enhanced surface area/charge storage properties and can be performed conveniently on top of the PV unit.¹²⁹ For instance, porous carbon materials for energy storage applications have been successfully 3D-printed,¹³⁰ highlighting the potential of this technique to integrate capacitive materials on top of the photovoltaic unit in a monolithic PC arrangement (2T or 3T) that benefits from compact packaging and requires fewer materials. In addition to producing membranes with custom-sized pores, 3D printing processes can be employed to deposit the solid electrolyte. The critical challenge is developing adequate inks/slurries of active materials, particularly those with rheological specifications that allow for high-quality printing.¹³¹

Moreover, employing conductive polymers for the SC unit and as components of the PV unit can facilitate the production of flexible photocapacitors.^115,132 Typically, flexible devices employ polymer substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN),¹³³ and polyimide (PI),¹³⁴ with a thin layer of a transparent conductive oxide, usually ITO; however, fabric materials and different paper materials have also been investigated.^135,136 Low-temperature techniques for the annealing of the components of the PV unit represent the bottleneck for high-performing flexible photovoltaics compared to their rigid substrate counterparts.^{123,124,137−139} For instance, the commonly employed semiconductor TiO₂ requires high annealing temperatures, which are incompatible with flexible substrates.¹²⁵ Alternative processes have been developed to meet the need for low-temperature annealing. One possible alternative is using Flash Infrared Annealing (FIRA), a cost-effective technique easily adapted to scale production lines.¹⁴⁰

Flexible photocapacitors for IoT applications have been developed by coupling commercial a-Si and GaAs and next-generation solar cells (DSC, PSC, and OSC) to flexible supercapacitors. Fu et al. introduced a flexible fiber-PC to power an LED in 2013.³⁵ They employed anodic deposition to coat a stainless steel wire with polyaniline. This fiber electrode served as an electrode for the DSC photovoltaic unit and the SC part. The integrated power supply presented an overall energy conversion efficiency of up to 2.1%. A series of four flexible fiber DSC and four flexible supercapacitors were connected through a hybrid approach. The integrated system was charged under 1 sun illumination and reached 2.34 V in 50 s.

A free-standing, highly conductive PEDOT:PSS film was developed as an effective alternative to metal-coated substrates as the common electrode for a flexible large area P3HT-ICBA OSC and a symmetric supercapacitor with a PVA-H₃PO₄ electrolyte. The 1 cm² solar cell with the laminated PEDOT:PSS electrode delivered 3.84% PCE, the supercapacitor showed 58% energy storage efficiency, and the overall efficiency of the integrated photocapacitor was 2%. By adding an insulating layer on top of the nonshared PEDOT:PSS electrode of the supercapacitor, the energy storage unit could be folded on top of the solar cell, further reducing the metal dimensions of flexible photocapacitors.¹⁴¹

Moreover, flexible photocapacitors can facilitate the creation of a vast array of self-powered portable devices, including wristbands, wearable accessories, and skin sensors (Figure 2).¹⁴² Z. Wen et al. demonstrated a fiber-shaped PC with a DSC as the PV unit.¹⁰⁵ The resulting textile can easily be woven into smart clothes. The system was connected through a 4-T approach.

Furthermore, a blocking diode was introduced to avoid undesired current discharge to the DSC unit. Three switches were used to control the charging process of the system; when the solar cell charges the series of supercapacitors, a voltage of 1.8 V is reached in 69 s.¹⁰⁵

Another example of fiber-shaped integration of a flexible DSC and a microsupercapacitor has been reported by W. Song et al.¹⁰⁷ Their system comprised three fiber DSCs connected in series between them and then connected to a series of microsupercapacitors through external wiring. The photocharging of the microsupercapacitor to 1.8 V was achieved in 30 s. Switches allow electricity to flow from the solar cell to the supercapacitor and then to the load, ensuring proper charging and discharging. The flexible energy supply system was used to power an electronic watch.¹⁰⁷

Chen et al. coaxially integrated a DSC and a supercapacitor into a fiber-shaped photocapacitor: a titanium wire surface, modified with perpendicularly aligned TiO₂ NTs and horizontally aligned CNT sheets acted as the electrodes for the energy conversion and storage units, completed by gel electrolytes. The DSC and SC showed an efficiency of 2.73% and 75.7%, respectively, with a maximum of 1.2% for the overall efficiency.¹⁴³

Recently, Liu et al. presented a novel design consisting of a self-supported graphene/CNT hollow fiber: PANI was deposited on both the inner and outer surfaces of the fiber with high mass loading. Such structure enabled a supercapacitor with a large specific capacitance of 472 mF cm^–2, which replaced the platinum wire in the fiber-shaped DSC that achieved 4.2% PCE; the integrated photocapacitor exhibited an overall efficiency of 2.1%.¹⁴⁴

A similar approach to designing wearable electronics has been reported by C. Li et al.¹¹³ They demonstrated the versatility of flexible photocapacitors with a self-powered wearable gadget that monitors physiological signs when in contact with the skin. In their study, a series of flexible perovskite solar cells were integrated through wiring with a lithium-ion capacitor and connected to a strain sensor that monitors the pulse signal and finger motion. The integrated system delivers an overall efficiency of 8.41% with a high output voltage of 3 V at a discharge current density of 0.1 A g⁻¹.

W. Jin et al. constructed flexible photocapacitors employing OSCs as the PV units connected to flexible supercapacitors. The materials were printed on different substrates with metal-embedded transparent conductive electrodes and then stacked vertically to form an integrated device.¹⁰³ Under 1 sun illumination, the device charges with an effective photocharge rate of 1.86 mV s^–1 (photocurrent output 3.8 mA). The SC unit discharged with an effective galvanostatic rate of 56 A s^–1, leading to an overall efficiency of 5.02%. Since these performances were insufficient to power electronic devices effectively, a module consisting of (2 series x 4 parallel connections) was fabricated through nickel wiring and silver paste. The module demonstrated a charging rate under 1 sun illumination of 1.03 mV s^–1 (photocurrent output 7.8 μA), with an overall efficiency of 4.94%.¹⁰³

Gao et al. combined a high-performance, cotton-textile asymmetric supercapacitor with a flexible solar module to build a self-charging power pack using scalable roll-to-roll manufacturing.¹⁰⁴ The integrated device was capable of delivering an open circuit voltage of 3 V under illumination and was able to power the LED for 10 min after the light was switched off.¹⁰⁴

Recently, Liu et al. reported an efficient ultrathin flexible photocapacitor with an overall efficiency approaching 6% and a total thickness below 50 μm: a 3 μm thick OSC was integrated on top of a 40 μm CNT/polymer-based supercapacitor on a 1 μm PET substrate. Besides the high performance, the integrated device also showed high operational stability, retaining over 96% of its initial efficiency after 100 charge/discharge cycles and mechanical robustness, losing only 5% of its initial efficiency after 5000 times bending at a radius of around 2 mm.¹⁴⁵

3. Integration of Photocapacitors

Numerous advantages are associated with using of photocapacitors as sustainable and compact solutions for self-powered devices, as evidenced by their multiple applications. Several research fields are required to design and manufacture the various components (PVs and SCs) of the dual-system, resulting in a more efficient and stable device. This section will discuss the operating principles of photovoltaics, supercapacitors, and other components.

3.1. Emerging Photovoltaics

Solar or photovoltaic cell technologies can convert light directly into electricity due to the photovoltaic effect. When a semiconductor material is exposed to light, the absorption of photons with energy equal to or greater than the bandgap (E_g) of the material leads to the excitation of an electron or other charge carriers to a higher-energy state. The photogenerated electron–hole pairs are separated, the electrons are extracted as current by an external circuit, and the holes are quenched at the positive terminal. Silicon-based technologies comprise the first generation of photovoltaic technologies (PVs), with crystalline silicon (c-Si) with a bandgap of E_g 1.1 eV showing the highest power conversion efficiency (PCE) of 26.7% under 1 sun illumination.¹⁴⁶ Amorphous silicon (a-Si:H) with an E_g of 1.6 eV¹⁴⁷ is frequently used despite the lower PCE of 11.9% due to it is lower cost compared to c-Si.¹⁴⁶ Under low light conditions of 1000 lx, a-Si:H can deliver up to 21% PCE.¹⁴⁷ Second generation photovoltaics include GaAs, CdTe, and CIGS, among others. Unfortunately, these materials are costly, require limited resources, and are difficult to integrate into photocapacitors.¹⁴⁸ Third generation or emerging technologies (Figure 3), such as organic photovoltaics (OPVs), perovskite cells (PSCs), and dye-sensitized cells (DSCs), can be manufactured at lower cost, with Earth-abundant materials and solution-processed techniques, easily adapted to industrial production. As a result, they are the favored photovoltaic technology for producing photocapacitors. Table 1 summarizes the photovoltaic parameters of the most common PV technologies.¹⁴⁹

Figure 3.

Top left: Different photovoltaic technologies. First-generation solar cells based on c-Si and a-Si, second-generation solar cells: GaAs and CIGS, among others, and third-generation or emerging solar cells: OPVs, PSC, and DSC or QDSC with similar structures. Top right: Shockley–Queisser limit for photovoltaic technologies. Bottom: comparison of the performance of batteries vs supercapacitors.

Table 1. Comparison of Photovoltaic Parameters of Different PV Technologies under the Global AM1.5 Spectrum (1000 W m^–2) at (ASTM G-173-03 Global or IEC 60904-3:2008).

Technology	Area (cm2)	VOC (V)	JSC(mA cm–2)	Fill factor (%)	PCE (%)	Ref
c-Si	79	0.738	42.65	84.9	26.7	(146)
a-Si	1.001	0.896	16.36	69.8	10.2	(177)
GaAs (thin film)	0.998	1.1272	29.78	86.7	29.1	(178)
CIGS	1.043	0.734	39.58	80.4	23.35	(179)
OPV	0.0473	0.871	26.75	79	18.43	(180)
PSC	0.09597	1.179	25.8	84.6	25.7	(181)
DSC	0.1155	1.0648	18.049	78.97	15.178	(167)

Organic solar cells (OSCs) rely on carbon-based semiconductors, such as conjugated polymers, for their active layers. Organic semiconductor films generate strongly bound excitons with limited diffusion length upon light irradiation. OSCs comprise an electron donor, typically benzodithiophene and difluorobenzothiadiazole-based organic semiconductors, with fullerene-based electron acceptor semiconductors. Planar architectures can be achieved by successfully depositing the donor and acceptor, forming a heterojunction.¹⁵⁰ In contrast, bulk heterojunctions are formed when the donor–acceptor molecules are mixed, reducing the path the exciton has to travel within its lifetime to be effectively separated.¹⁵¹ Due to their availability, nontoxicity, and low-temperature, low-cost manufacturing, OSCs have attracted significant interest. They can be printed on large areas and flexible substrates.^152−155 Continuous efforts in the research and development of materials have pushed the single junction efficiencies to values as high as 18.2% under 1 sun illumination,⁵³ and 28.8% under 1000 lx illumination from a 2600 K LED lamp.¹⁵⁶

Perovskite solar cells (PSCs) have attracted significant interest from the academic and industrial sectors due to their ease of fabrication based on solution processing. They have reached remarkable power conversion efficiencies of over 25% in single junction and above 31% in tandem with silicon solar cells.⁵³ Organic halide perovskites such as CH₃NH₃PbI₃, which act as the light absorber, present unique advantages such as high extinction coefficient, bandgap tunability with perovskite composition, low carrier recombination rate, and carrier diffusion length on a micron scale.^157−159 PSCs are composed of an electron transport layer (ETL), a hole transport material (HTM), and metal contacts. The ETL is the light-harvesting layer that generates charge carriers upon light absorption. The electron–hole pairs split, and the HTM scavenges the hole carriers. Different architectures such as mesoscopic, planar, and inverted, have been investigated in the past decade. Perovskite photovoltaics can reach PCEs above 40% under 1000 lx under indoor illumination.¹⁶⁰ Furthermore, PSC technology has been demonstrated to be scalable to large-area modules, adopting deposition techniques compatible with industrial manufacturing,^161−163 and compatible with flexible substrates.⁶⁵

Dye-sensitized solar cells (DSCs) were the first solar cells among third-generation PV to be considered for integration with energy storage devices in photocapacitors.^29,38,164 Due to their simple design, abundant, nontoxic materials, and relatively easy manufacturing process,¹⁶⁵ they result in low production costs, variety in the choice of substrates (either rigid or flexible, e.g., lightweight plastic or metal foils, wires or fibers) and active materials with tunable optical properties, such as color and transparency.^10,166 DSCs comprise a wide bandgap semiconductor such as TiO₂ with dye molecules (sensitizers) adsorbed onto its surface, a counter electrode (CE) and a redox mediator between the electrodes. The dye molecule generates photoinduced electrons upon irradiation, which are injected into the TiO₂ conduction band. The redox shuttle facilitates dye regeneration and the transfer of positive charges (holes) from the WE to the CE. Advanced molecular systems, including panchromatic rigid-structure dyes, alternative hole transport materials (HTMs), and design flexibility, have increased power conversion efficiencies (PCEs) up to 15% under AM1.5G conditions,¹⁶⁷ and remarkably to a record 34.5% under 1000 lx ambient light illumination.⁶ The superior performance of DSCs under low light^6,168−171 make them a suitable technology for indoor light harvesting and, ultimately, a technology with great potential for the development of highly efficient photocapacitors that harvest indoor light to power electronic devices located in ambient settings.

Quantum dot solar cells (QDSCs) employ nanocrystalline semiconductor quantum dots (QD) as sensitizers or photoactive material.^172−175 QDs feature size-tunable bandgap and size-dependent behavior, absorbing light over a broad range of wavelengths and generating multiple excitons for each photon absorbed. They can be synthesized at relatively low temperatures and then processed by solution techniques,¹⁷⁶ and have achieved record efficiencies as high as 18.1% employing Pb- and Cd-free-based QD sensitizers.⁵³

3.2. Supercapacitors

The leading energy storage technologies used in a wide range of applications include batteries, conventional capacitors, and supercapacitors (SC), also known as ultracapacitors.¹⁸² Some of the applications employing these technologies as power supply include (but are not limited to) consumer electronic devices, household gadgets, power generation, computation, wireless devices and chargers, electric vehicles, stationary grid storage, industrial systems, medical sensors, among others. The primary difference between a battery and a supercapacitor is their capabilities: batteries offer a higher energy density (implying that they can store more energy per unit weight, typically a few hundred Wh kg^–1). Supercapacitors offer a higher power density (they can release more power over a short duration of time, usually tens of thousands of W kg^–1),¹⁸² as shown in Figure 3.

Supercapacitors store and release charge via electrical double layer (EDL) formation.¹⁸³ During the charging process, the Helmholtz compact layer is formed at the electrode’s interface accumulating opposing charges from the ionic electrolyte (Figure 5), followed by a diffuse layer of charges in the immediate environment of the Helmholtz layer (hence double layer). A pseudocapacitive behavior is observed when there is faradaic charge accumulation from redox reactions at the electrolyte-electrode interface.¹⁸⁴ Due to their fast charge/discharge rates, SCs are well-suited for applications such as regenerative braking-capturing and storing braking energy produced in trains, trucks, buses, and automobiles and then releasing it on a need basis. They are ideal for fast-charging backup power. One of the most significant advantages of supercapacitors over batteries is their capacity to provide short-term, high-power bursts.

Figure 5.

Schematic guideline showing the mechanism, materials, and characterization techniques of photovoltaic and supercapacitor devices.

In contrast, batteries store charge by diffusion-controlled faradaic oxidation and reduction.¹⁸⁵ Ion diffusion over the active material layer slows charging and discharging to tens of minutes or hours, while SC may charge/discharge in seconds. This difference gives batteries a high energy density, making them appropriate for long-term power applications (e.g., mobile phones and laptops, among others).

The use of toxic or limited components such as cobalt and lithium can be avoided in SCs, which is one of the main concerns regarding the sustainability of lithium-ion batteries. In addition, SCs require minimal maintenance and have ultralong operational life as they can endure a million charge–discharge cycles without degradation. Furthermore, SCs can be safely operated over a vast temperature range of −40 to +60 °C without significant performance variation.^186,187 When used in conjunction with batteries, supercapacitors complement and extend battery life at an affordable price, and under certain conditions, they can also act as an adequate battery replacement. One of these scenarios is when SCs are coupled to a PV unit to create photocapacitors that can store the charges produced by light harvesting and serve as a battery replacement.

In terms of material characterization, both batteries and supercapacitor materials can be tested in the classical three-electrode setup in an electrochemical cell with the active material as the working electrode, compared to a counter and reference electrode. They can also be tested in two-electrode systems, depositing the active materials in two electrodes (symmetrical supercapacitor) or studying different materials in each electrode (asymmetric supercapacitor).

Activated carbon (AC) is the predominant active material in commercial supercapacitors, with capacitance values ranging from 100 to 400 F g^–1.²¹² Many alternative materials to activated carbon (AC), such as carbon nanotubes (CNTs), graphene (G), (reduced) graphene oxides (RGOs), transition metal oxides and chalcogenides such as RuO₂, MnO₂, and MoS₂ offer higher capacitance. However, the synthetic ease, scale-up, and expense of these materials offer substantial barriers to their use.²¹³ Carbon-based materials are usually characterized by a high effective surface area (a few thousand m² g^–1), resulting in high capacitance (few hundreds of F g^–1) and high power (10 kW kg^–1).^214−217

Transition metal oxides such as RuO₂, MnO₂, NiO, and metal sulfides such as MoS₂, Cu₂S, among others, with nanostructured morphologies show high capacitance and energy density due to their pseudocapacitive behavior or their ability to reversible undergo oxidation and reduction processes (Table 2).^{188,189,218−220} These metal oxides or chalcogenides have redox active metal centers (e.g., Mo, Ni, Fe, V, Cu) with partially filled d orbitals, capable of undergoing reversible reduction or oxidation as shown in eq 1:

1

Table 2. Comparison of the Capacitive Performance of Different Materials.

Positive electrode	Voltage (V)	Electrolyte	Capacitance (F g–1) (Current, A g–1)	Energy (Wh kg–1)	Power (kW kg–1)	Retention (%) (cycles)	Ref
NiO	0.4	KOH 1 M	1776 (1)	16.5	89	97.9 (1000)	(188)
CuS	0.3	KOH 6 M	833 (1)	–	–	75.4 (500)	(189)
PEDOT	0.8	H2SO4 1 M	198 (0.5)	4.4	40.25	86 (12000)	(190)
MnO2	1.8	Na2SO4 1 M	365 (0.25)	22.5	146.2	90.4 (3000)	(191)
MnO2/CNT	2	H3PO4/PVA	18 (0.267)	42	19.3	98 (500)	(192)
Mn3O4/G	1.8	KCl/PAAK	72.6 (0.5)	32.7	9.0	86 (10000)	(193)
RuO2/G	1.8	H2SO4/PVA	175 (0.5)	19.7	6.8	95 (2000)	(194)
PANI/CNT/G	1.6	H2SO4 1 M	107 (1)	20.5	25	91 (5000)	(195)
MnO2/Ni foam	2.0	Na2SO4 0.5M	41.7 (1)	23.2	1.0	83.4 (5000)	(196)
ZCOSH	0.43	KOH 2 M	116.3 (1)	41.4	0.80	92.4 (3000)	(197)
ZnO/AC	0.8	KOH 6 M	50.9 (2 mA cm–2)	4.52	1.62	–	(198)
PPy/GO/ZnO	0.9	KOH/PVA	123.8 (1)	–	–	92.7 (1000)	(199)
Ni(OH)2/AC/CNT	1.6	KOH 6 M	82.1 (0.5)	32.3	0.50	83.5 (1000)	(200)
NiO-CFP	1.8	KOH 2 M	240 (4)	105	12.7	68 (5000)	(201)
Fe2O3-CF	2	LiClO4/PVA	74	33.1	1.32	92 (10000)	(202)
Mn2O3/C	1.8	Na2SO4 1 M	122 (2.5)	54.9	22.6	97 (5000)	(203)
NiCo2O4@GQD	1.6	KOH 2 M	107 (1)	38	0.8	71.8 (3000)	(204)
NiCo2O4@PPy	1.6	KOH/PVA	102.5 (8 mA cm–2)	58.8	10.2	82.9 (10000)	(205)
NiCoMn-S-1.5	1.6	KOH 6 M	111.6 (1)	36.3	0.75	93.9 (3000)	(206)
CuCo2O4/CuO	1.7	KOH 2 M	–	41.76	7.86	104.7 (5000)	(207)
NiSe-Se/Ni foam	1.6	KOH 1 M	84.1 (4 mA cm–2)	29.9	0.59	95.1 (10000)	(208)
Au@GGO-ZnCo2O4	1.4	KOH/PVA	113.8 (10 mA cm–2)	31	2.12	97 (5000)	(209)
NiBxOy	1.6	KOH (6 M)	–	42.4	0.8	97 (10000)	(210)
Co-Ni-S/CNT	1.6	KOH 6 M	178.6 (1)	63.5	0.8	83 (10000)	(211)

Conducting polymers (such as poly(3,4-ethylenedioxythiophene) (PEDOT), poly(aniline) (PANI), poly(pyrrole) (PPy)) are also capable of showing a pseudocapacitive behavior.^190,221,222 By applying a voltage or current, they endure reversible doping by the electrolyte anions.²²³ Conducting polymers offer several benefits, including high electrical conductivity in the doped state, high chemical stability, and rapid charge–discharge kinetics. The swelling and shrinking of polymers over long charge/discharge cycles limit the system’s supercapacitance, making polymer-based supercapacitors challenging to commercialize.

Asymmetric supercapacitors use the same charge-storage mechanism with different active materials at the cathode and anode.²²⁴ In contrast, hybrid supercapacitors have distinct active materials with different charge-storage processes. They can have a different ratio of redox-active sites on the electrode material, separate redox-active electrolytes, or the same material with different surface functional groups.²²⁵ Combining a pseudocapacitive electrode with an EDL electrode provides a method for improving SC performance, as well as operating voltage and dark-discharge duration. The EDL electrode allows a high power capability, while the pseudocapacitor electrode provides a high energy density, thereby extending the device’s discharge time^226,227

The electrolyte is essential for charge separation since it contains compensating anions and cations that diffuse to the electrode surface upon charging. The choice of the electrolyte and the ionic salts significantly influence the SC’s performance and considerably affects the operating window. Due to their strong ionic conductivity, low cost, and low toxicity, aqueous electrolytes are the most common choice for SCs. The thermodynamic breakdown of water limits the voltage window of aqueous-based electrolytes to 1.2 V. (water splitting).²²⁸ However, designing asymmetric supercapacitors with different materials in each electrode can boost the operational voltage further than 1.2 V even with aqueous electrolytes.²²⁹

Organic electrolytes benefit from a working voltage of 2.5–3 V and longer life cycles than aqueous electrolytes.²³⁰ However, organic electrolytes present lower specific capacitance due to lower ionic conductivity than their aqueous counterparts.^231−234 The potential window can be extended up to 4.5 V by employing ionic liquid (IL) electrolytes. Additionally, their low volatility and flammability can boost the lifetime of the SCs.²³¹ Most ILs present high viscosity and expensive costs, limiting their practical applications.^231,232

Generally, the design of SCs with gel or solid electrolytes is preferred to increase the cycling stability, mechanical strength, and flexibility of the devices.²³⁵Table 3 presents typical examples of the capacitance parameters reached by supercapacitors employing different solvents for the electrolyte. Several binders and polymers can be employed to improve the contact of the hydrogels with the surface of the electrode, such as alginate, carboxy methyl cellulose (CMC), xanthan gum, chitosan, polyethylene glycol (PEO), poly(methyl methacrylate) (PMMA), poly(acrylic acid) (PAA), poly(amine-ester) (PAE), poly(vinyl alcohol) (PVA), among others.^236,237

Table 3. Capacitance Parameters of Supercapacitors Employing Aqueous (aq), Organic (org), Ionic Liquid (IL), and Solid-State Electrolytes (SSEs).

Electrolyte	Electrode material	Voltage (V)	Capacitance (F g–1) (Current, A g–1)	Power (kW kg–1)	Retention (%) (cycles)	Ref
H2SO4 0.5 M (aq)	RuO2/G	1.2	479(0.25)	0.6	94 (1000)	(238)
H2SO4 1 M (aq)	PANI/RGO/CeO2	1.7	684(1)	0.85	92(6000)	(239)
H2SO4 1 M (aq)	RuO2	1.5	150(1.25)	0.937	95 (10000)	(240)
KCl 3 M (aq)	PPy/ASA	3	804 (2)	6	93.6(5000)	(241)
Na2SO4 1 M+ K3Fe(CN)6 0.3 M (aq)	CoMoP4@MnO2/Ni	2.4	116.7 (1)	0.984	96.8(10000)	(242)
Na2SO4 1 M (aq)	C@Mn3O4	2.7	109 (1)	1.35	94.2(6000)	(243)
LiTFSI 3 m + PEO 30g/L (aq)	AC	2.4	125(0.5)	11.45	92(10000)	(244)
KOH 1 M + Choline 0.1 M (aq)	SC-Se-750-M	1.3	48.9(0.5)	20.4	94.1(10000)	(245)
LiClO4 1 M (aq)	MnO2–NPG	1.8	193(2)	25	85(2000)	(246)
NaClO4 17 m (aq)	AC	2.3	33.0 (1)	16.7	85(20000)	(247)
LiTFSI 0.5 M (GBL) (org)	MnO2/RGO/CNT	2	41 (0.1)	6.3	35(5000)	(248)
TEABF4 0.7 M (AND) (org)	AC	3.5	20	3.1	74 (35000)	(249)
EMIM-TFSI (1 M) (ACN) (org)	Mo2Ti2C3	3	152	22	86(5000)	(250)
SBPBF4 1 M (EC:MPN) (org)	AC	2.3	104	–	98(1500)	(251)
SBPBF4 1.5 M (PrC) (org)	AC	3.5	122(0.1)	6.938	59.5(2500)	(252)
TEMABF4 1 M (PrC) (org)	AC	3.2	104(0.1)	–	44.2(2500)	(253)
LiPF6 1 M (EC:DEC) (org)	AC	3	126(1)	2.243	–	(254)
NaPF6 1 M (EC:DMC:PrC:EA) (org)	C(Mo2C)	3.4	120	90	–	(255)
DmFc 0.2 M + TBAClO4 1 M (THF) (org)	CNTs	2.1	61.3(1)	1.04	88.4(3000)	(256)
EMIMBF4 (IL)	G	3.5	192(5)	–	90(1E6)	(257)
EMIMFSI (IL)	AC	3	120(0.5)	ND	90(5000)	(258)
PMPFSI (IL)	AC	3.5	98(0.5)	ND	80 (5000)	(258)
P4444FuA (IL)	AC/CNTs	3	10(1)	13.3	82(1000)	(259)
BMIMPF6 (IL)	Csponge	4	290(0.1)	ND	90(5000)	(260)
[EMIM+TMA]+[BF4]− (IL)	AC	3.5	182(1)	7.5	84(5000)	(261)
BQ/PYR14TFSI (IL)	AC	3	156	ND	50(1000)	(262)
DEMEBF4 (IL)	AC	2.5	25.4		85(5000)	(263)
PVA/LiCl (SSE)	NiCo2S4/CF	0.7	360	0.3	90(5000)	(264)
PVA/KOH (SSE)	CuMnO2	0.7	272(0.5)	7.56	80(18000)	(265)
PHPA/LiClO4 (SSE)	AC	2.5	111(0.25)	6.51	80(11000)	(266)
PVA/H2PO4 (SSE)	N-carbon	1	260(0.5)	5.8	86(10000)	(267)
PANI/Zr-MOF (SSE)	AC	0.8	647(1)	1	91(5000)	(268)
PEGBEM-g-PAEMA/EMIMBF4 (SSE)	AC	2	55.5(1)	0.9	75(5000)	(269)
PEO-NBR/EMIMTFSI (SSE)	G	2.5	208(1)	5.87	93.7(10000)	(270)
PVDF-co-HFP/EMIMNTf2 (SSE)	AC	2.5	153(0.05)	6.25	97(10000)	(271)
PVDF/TEABF4 (SSE)	G	3	28.46(1 mA)	7.5	91(10000)	(272)
TBAPF6/PMMA/PrC/ACN (SSE)	CNTs	2	34.2(0.63)	21.1	94(500)	(273)
PEO/MgTFSI/PrC (SSE)	AC	2	22.3(0.1)	6	100(2000)	(274)

The membrane, whose primary function is to separate the electrodes to prevent the stored charges on each electrode from recombining and avoid short circuits, is another crucial component of a well-performing SC. Membranes must be nonconductive and offer minimal resistance to the diffusion of ions in the electrolyte. They must be chemically and thermally stable, inert to electrode materials and species or redox mediators in the electrolyte, and thermally stable. A high level of mechanical stability is essential to prevent shrinking. In addition, wettability is crucial because it affects the stability under lengthy cycles of the SC unit, affecting the internal resistance and, consequently, the lifetime of the SC.^{228,275−277}

The membrane’s pore size impacts its conductivity and affinity for the electrolyte solvent. Good swelling can be obtained with pore sizes around 1 μm with an overall porosity between 40 and 60%.²⁷⁵ Several materials have been used as membranes for SC, such as glass, different kinds of paper, ceramic materials, ion-exchange membranes such as Nafion, PVDF, PVDF-HP, polyethylene (PE), polypropylene (PP), cross-linked polymers, chitosan, cellulose, and chitin among others.^275,278 The membrane thickness should be lower than 25 μm, but thinner films may not have mechanical strength and present low cyclability. Employing some of the solid electrolytes mentioned above can avoid the need for membranes acting as a separator between the electrodes.²³⁰

3.3. Integration of Photocapacitors

As previously discussed, photocapacitors (PCs) can harvest light by employing a photovoltaic unit. So far, third/next-generation photovoltaics (DSC, QDSC, OSC, PSC) have been demonstrated to be promising candidates to design photocapacitors in a vast range of architectures, including rigid and flexible substrates, thereby expanding the range of potential applications to include wearable and portable electronics.^279−281

The main components of a PC are depicted in Figure 4. The PV unit requires a transparent conducting substrate (TCO, e.g., ITO or FTO) that can be rigid or flexible. The photoactive layers are deposited on the substrate, followed by the deposition of the redox couple, gel electrolyte, or a hole transport layer (HTL) such as spiro-OMeTAD. Depending on the PC configuration, the counter-electrode (CE) of the PV unit can also be shared with the SC unit. The CE should preferably be a conducting material coated onto a current collector, such as transparent conductive oxides (TCOs), carbon paper or foam, or any inert metal foam. The SC electrode is usually a “Janus”-type electrode with the capacitive material (carbon-based, metal oxides, chalcogenides, conducting polymers, among others) coated over the rear side. The bottom electrode of the supercapacitor unit serves as the energy storage layer. The electrodes are separated by a membrane, which is essential to prevent charge recombination in the SC. The SC device is filled with an ion-conducting electrolyte and sealed.

The photocharging of the device is induced by light absorption by the PV unit. Charge separation occurs in the photoactive material (dye, organic semiconductor, QD, or metal-halide perovskite). The photogenerated electrons are injected from the LUMO/conduction band (CB) of the dye/QD/perovskite into the CB of the metal oxide (e.g., TiO₂) and subsequently transported to the current collector. The holes in the HOMO/valence band (VB) of dye/QD/perovskite are scavenged by the hole transport layer (HTL) or reduced by the species in the redox electrolyte. Photocharging requires connecting the photovoltaic electrode to the supercapacitor unit and storing the photogenerated electrons as charge, balanced by cations from the SC unit’s electrolyte. The dark-discharge is achieved by disconnecting the photovoltaic unit from the supercapacitor part of the integrated device. This procedure transfers the stored energy to an external load, supplying power to an electronic device. A rectifying diode should be installed between the PV and SC units to prevent charges from recombining with the PV unit rather than the external load. However, most investigations on photocapacitors avoid utilizing the rectifying diode since it causes a voltage drop of several hundred millivolts. To advance the rapidly expanding field of photocapacitors, developing novel materials or PC designs with a diode-like terminal and minimal voltage loss is essential. To maximize the benefits of a photocapacitor, it is essential to extract all charges during the dark discharge through an external circuit that delivers the power needs of an electronic device, thereby avoiding the supercapacitor’s self-discharge to the electrolyte. Table 4 summarizes the photovoltaic and capacitance parameters of photocapacitors incorporating different photovoltaic technologies for the light-harvesting unit and various capacitive materials for the energy-storage unit.

Table 4. Comparison of Photocapacitors with Hybrid Photovoltaics and Various Supercapacitor Materials.

PV type	PCE (%) (sun)	Supercapacitor electrode	Electrolyte	Voltage (V) (sun)	Dark discharge (s)	Capacitance (mF cm–2)	Ref
Si	17.8 (1)	a-MoOx	NaSO4 0.1M	0.6 (0.6)	330	34	(284)
Si	–	Si	EMIMBF4/PC/PEO	0.58 (0.65)	∼8	0.014	(306)
Si	15.69 (1)	RGO	SiO2-BMIMTFSI	0.38 (1)	>10 day	0.0002	(307)
SiNW/PEDOT:PSS	13 (1)	PPy	H3PO4/PVA	0.55 (1)	>40000	234	(308)
SiNW/PEDOT:PSS	12.37 (1)	G	H2SO4 1M	0.5 (1)	12	16.37	(309)
DSC	0.048	CNTs/MnO2	TEABF4 1M/ACN	0.932 (1)	∼380	13.1	(310)
DSC	9.5 (1)	MoS2	H3PO4/PVA	∼0.65 (1)	∼25	18.51	(311)
DSC	–	AC	TEABF4 15 wt %/PC	0.45 (1)	∼1500	690	(38)
DSC	6.10 (1)	CNTs/PANI	H3PO4/PVA	0.72 (1)	144	–	(296)
DSC	4.9 (1)	Co-NiOx	KOH 1M	0.8 (1)	∼270	32 F g–1	(31)
DSC	4.37 (1)	PEDOT	LiClO4 0.5M/MPN	0.69 (1)	150	520	(312)
DSC	3.17 (1)	TiO2	Li2SO4 2M	0.61 (1)	∼18	1.289	(313)
DSC	2.8 (1)	RGO	BMIMTFSI/THF	0.6 (1)	20	0.14	(314)
DSC	–	PEDOT/CNTs	LiOTf/PC	0.88	67	610	(315)
DSC	2.4 (1)	PPy/RGO	KOH/PVA	0.5 (1)	∼20	124.7 F g–1	(316)
DSC	2.25 (1)	AC	Pyr14TFSI/PEO/benzophenone	2.45 (1)	1000	–	(317)
DSC	2.25 (1)	AC	Pyr14TFSI	2.45 (1)	∼1.75 h	–	(318)
QD/DSC	6.11 (1)	PEDOP/MnO2	PMMA/BMIMOTf 1M/PC	0.72 (1)	132	183 F g–1	(294)
QDSC	2.75 (1)	NiCo-MOF	S 1M/Na2S 1M	0.83 (1)	∼175	588	(319)
QDSC	3.45 (1)	CNTs	PMMA/LiOTf/PC	0.5 (0.1)	75	150 F g–1	(293)
QDSC	1.83 (1)	Ni/C	Na2S 0.8M/S 0.8M/KCl 2M	0.16 (1)	140	–	(288)
QDSC	3.94 (1)	AC	KOH 2M	0.62 (1)	>120	132.83	(295)
QDSC	–	PEDOT	HEMIMBF4/PVP	0.33 (1)	55	0.667	(320)
OSC	1.01 (1)	CNTs	H3PO4	0.4 (1)	∼310	77 μF cm–1	(321)
OSC	3.44 (1)	PEDOT:PSS	H3PO4/PVA	0.80 (1)	100	–	(141)
OSC	1.8 (1)	CNTs	NaCl 1M	0.92 (388 lx)	3.57 h	–	(322)
OSC	3.39 (1)	CNTs	H3PO4/PVA	0.6 (1)	∼35	28 F g–1	(299)
OSC	7.85 (1)	RGO	H3PO4/PVA	0.727 (1)	∼110	144 F g–1	(297)
OSC	9.75 (1)	PEDOT:PSS/CNTs	H2SO4/PVA	0.734 (1)	183	250	(323)
OSC	2.5 (1)	Ti3C2Tx	TT/PEGDA/EMIMTFSI	0.8 (1)	∼320	410 F cm–3	(324)
OSC	1.57 (1)	G	TEABF4/PC	2.3 (1)	240	–	(325)
OSC	6.7 (1)	PEDOT:PSS/Ti3C2Tx	H2SO4 1M	3 (45k lx)	250	93	(326)
PSC	11 (1)	AC/RGO-PEDOT	H2SO4/PVA	0.9 (1)	>400	71.54	(327)
PSC	7.79 (1)	AC/MnO2	LiCl/PVA	0.84 (1)	>500	61.01	(289)
PSC	12.5 (1)	N-MC	H2SO4/PVA	1.0 (1)	>75	31	(282)
PSC	22.44 (1)	AC	KOH 6M	1.1 (1)	211.5	–	(28)
PSC	2.5 (1)	PANI/CNTs	H2SO4/PVA	0.7 (1)	275	–	(328)
PSC	13.66 (1)	RGO	H3PO4/PVA	0.91 (1)	∼130	142 F g–1	(297)
PSC	8.9 (1)	AC	H3PO4/PVA	0.91 (1)	44	∼17.5	(329)
PSC	14.14 (1)	AC	KOH/PVA	0.68 (1)	20	13.6 F g–1	(330)
PSC	5.6 (1)	Co9S8-MnO2	H3PO3/PVA	0.63 (1)	70	–	(331)
PSC	6.37 (1)	AC/PEDOT	LiClO4/MAI/iPrOH	0.7 (1)	38	12	(332)
PSC	12.54 (1)	MoO3	H2SO4/PVA	0.68 (1)	∼200	43	(333)
PSC	7.79 (1)	AC/MnO2	LiCl/PVA	0.84 (1)	∼20	61.01	(334)
PSC	14.13 (1)	RGO	H2SO4/PVA	0.75 (1)	45	–	(335)
PSC	6.1 (1)	NC	TEOS/TEABF4/H3PO4	1.2 (1)	∼50	–	(336)

PCs can be categorized based on the wiring between their components (PV and SC). The most immediate strategy to charge an energy storage system (such as batteries and supercapacitors) by an energy generation system (solar cell, thermo/piezo/tribo-electric device, among others) is to connect the two individual devices through an external wire in a 4-terminal (4-T) configuration as depicted in Figure 4.

Even though in a 4-T arrangement, each unit can be improved separately, this strategy presents some drawbacks, such as intricate packaging, energy losses due to the resistance of the external wires, and lower overall efficiency.²⁸² In addition, such externally connected systems tend to be bulky and not flexible, reducing the range of possible application fields.

Similar to tandem photovoltaics, photocapacitors can be integrated into a 2-terminal (2-T) configuration, often referred as monolithic arrangement, where the materials from the PV unit and SC are stacked together or just or separated by a membrane as shown in Figure 4.²⁸³ Miyasaka et al. introduced the first 2-T photocapacitor in 2004.³⁸ They incorporated a DSC and a carbon-based supercapacitor counter-electrode. The system was photocharged at 1 sun illumination to a voltage of 0.45 V and showed a capacitance of 0.69 F cm^–2.

The 2-T layout has the benefit of requiring fewer materials and substrates. Due to the limitations of the 2-terminal structure, which include low charging voltage, high resistance, low charge–discharge efficiency, and self-discharge to the photoactive side of the device, 2-T architectures are typically not employed.²⁸⁴ These limitations prompted the development of the 3-terminal (3-T) architecture, in which the PV and SC devices share a common electrode but operate independently.²⁸⁴

The early 2-T structure, adopted in 2004 in the first demonstration of a photocapacitor with a DSC as the photogeneration device,³⁸ was rapidly surpassed by the more effective 3-T design.²⁹ As a result of improved electron and hole transfer in the charge–discharge process, the 3-T design showed a photocharged voltage under 1 sun of 0.8 V and energy density of 47 μWh cm^–2. Since then, a plethora of materials and configurations have been investigated.^14,285−288

Bagheri et al. developed a 3-T PC employing a DSC and an asymmetric SC with cobalt-doped nickel oxide (Ni(Co)O_x) and AC as positive and negative electrodes.³¹ The PC generated a photovoltage of 0.8 V and overall energy conversion of 0.6% when exposed to 1 sun illumination for 500 s. A monolithic PC with a perovskite light harvester and a carbon-based supercapacitor was reported by Liu et al.²⁸⁹ Under 1 sun illumination, the integrated stacked device achieved a voltage of 0.84 V with an overall conversion efficiency of 5.26% and an energy storage efficiency of 76%.

Skunik-Nuckowska et al. developed a 3-T PC employing a solid-state DSC and ruthenium oxide as the intermediate electrode and ruthenium oxide/FTO as the second supercapacitor electrode.²⁹⁰ The system delivered 3.26 F cm^–2 with Coulombic efficiency of 88% and a charged state at 0.88 V when illuminated at 1 sun. A 3-T Pc employing a symmetric PProDOT-Et₂ supercapacitor and an N3-based DSC was reported by Hsu et al.²⁹¹ The PC exhibited a photocharged voltage of 0.75 V under 1 sun illumination, with a capacitance of 0.48 F cm^–2 and an energy density of 22 uWh cm^–2. A double-sided electrodeposited PPy/RGO as an intermediate electrode in a 3-T architecture was employed by Lau et al.²⁹² The PC delivered 124.7 F g^–1 under 1 sun illumination, with a retention of 70% after 50 consecutive cycles.

Narayanan et al. reported an integrated 3-T PC with a plasmonic QDSC delivering a 3.45% PCE. They employed a functionalized MWCNT symmetric EDL supercapacitor delivering a specific capacitance of 150 F g^–1 and reached a photocharged voltage of 0.5 V under 0.1 mW cm^–2 illumination.²⁹³

Das et al. presented another 3-T PC design, combining a QDSC (with CdS QDs and hibiscus dye as sensitizers) with a bifunctional poly(3,4-ethylenedioxypyrrole) (PEDOP)@manganese dioxide (MnO₂) electrode as CE and symmetric supercapacitor.²⁹⁴ The solar cell unit yielded a PCE of 6.11%. The PC reached a photocharged voltage of 0.72 V under 1 sun illumination and delivered 183 F g^–1 and power density of 360 W kg^–1 at a discharge current density of 1 A g^–1. Zheng et al. recently employed a similar device architecture in a PC that combines a CdS/CdSe QDSC and an AC-based supercapacitor with a common electrode.²⁹⁵ Under 1 sun, the QDSC delivered a 3.94% PCE and could charge the supercapacitor up to a voltage of 0.62 V. The integrated device achieved 2.66% overall efficiency and demonstrated good stability, retaining 76.7% of its initial overall efficiency after 100 charge and discharge cycles.

The development of solid-state photocapacitors will accelerate the widespread adoption of IoT applications that are economically viable on a commercial scale. An all-solid-state 3-T integrated device using a DSC and an MWCNT-based supercapacitor, with a PVA/H₃PO₄ gel electrolyte, was reported by Yang et al.²⁹⁶ The PC was photocharged to 0.72 V under 1 sun illumination with an energy storage efficiency of 84%. Adding polyaniline (PANI) to the MWCNT film boosted the specific capacitance from 48 to 208 F g^–1. An alternative solid-state photocapacitor was developed by Kim et al. They monolithically integrated a high-performance OSC PV unit based on PTB7-Th/PC71BM, with a carbon-based SC and a PVA/H₃PO₄-based solid-state electrolyte. The PC achieved a storage efficiency and overall efficiency of 64.59% and 5.07%, respectively, with a photocharged voltage of around 0.9 V under 1 sun illumination.²⁹⁷

Another solid-state photocapacitor with a PSC as a PV unit and a PEDOT-carbon composite material for the common electrode was reported by Xu et al.²⁹⁸ The PC delivered a maximum overall energy conversion and storage efficiency of 4.7%. Liu et al. integrated a PSC and a solid-state supercapacitor on the same FTO glass substrate through a common carbon-based electrode,²⁸⁹ reaching an energy storage efficiency of 76% and an overall efficiency of 5.26%. Combining a PVA/H₃PO₄-based solid-state supercapacitor and a PSC with 1 cm² active area, Kim et al. achieved a high storage efficiency of 80.3% and overall efficiency of 10.97%.²⁹⁷

The highest overall efficiency for a quasi-solid state photocapacitor integrating a PSC reached 11.5% in a monolithic stacked architecture with an N-doped carbon SC. This remarkable overall efficiency derives from the high PCE of the solar cells, i.e. 12.5%, the high storage efficiency of the supercapacitor, i.e. 92%, and the minimized internal energy losses due to the monolithic integration.²⁸² In general, the performance of solid-state photocapacitors is poor. This is because gel and polymer electrolytes have limited conductivity, which reduces capacitance and charge storage efficiency. Future research should concentrate on creating solid electrolytes with enhanced conductivity and electrode-wetting properties.²⁹⁹

Low ambient light levels are anticipated for many applications of photocapacitors, such as sensors for the Internet of Things and wearable electronics. The spectrum of ambient light varies depending on the source, so the design of photocapacitors must account for the IoT application’s power requirements and match the spectrum of the nearby light source. Lechêne et al. reported an example of PCs optimized for indoor operation. They designed a 4-T PC based on an OSC with PCDTBT:PC71BM and a carbon-based SC. The overall efficiency increased from 1.57% under 1 sun to 2.92% under simulated indoor lighting of 310 μW cm^–2 from a CFL lamp.³⁰⁰ A similar improvement with indoor illumination was observed by Jin et al.³⁰¹ They employed an OSC module (ITO/ZnO/PBDB-T:ITIC) with PEDOT:PSS and an asymmetric supercapacitor sharing the PEDOT:PSS electrode. The integrated photocapacitor achieved a photocharged voltage of 1.5 V under 1000 lx from a LED light source with an integrated power of 304.6 μW cm^–.²

Typically, the photocharged voltage measured in the storage unit is below 1 V. This voltage might be insufficient to power most applications, including sensors or low-power electronics. An effective strategy to get higher voltages is to connect several PV units in series, design an integrated PC with PV and SC modules, and employ microsupercapacitors with higher surface areas. Sun et al. designed a 2-T configurated photocapacitor delivering a high energy density (up to 32.3 μWh cm^–2) and high output voltages of around 2 V.¹⁰⁶ The system employed a PSC integrated with a micro-SC. The power pack powered an array of red and white LEDs, a micromotor, and a timer. Gao et al. reported a 3-T integrated system delivering an open-circuit voltage up to 1.8 and 4.7 V for an array of three PC in series, with an energy density of 0.18 Wh m^–2, and an overall efficiency of 7.0%.³⁹ In a 3-T planar configuration on rigid substrates, Xu et al. combined a CH₃NH₃PbI₃-based PV with a polypyrrole-based supercapacitor; the energy pack had an open circuit voltage of 1.45 V and an overall efficiency of 10%.³⁰²

Scalia et al. demonstrated high-voltage (2.45 V) photocapacitors enabled by four series-connected DSC modules with an IL electrolyte and activated carbon SC unit.³⁰³ Chien et al. used a string of 8 OSCs based on the P3HT:PC60BM bulk-heterojunction structure, connected in series on the same ITO glass substrate to increase the V_OC up to 5 V. The photovoltaic unit was integrated with graphene-based supercapacitors, providing about 2.5 mF cm^–2 of capacitance on the same substrate, with graphene as the common electrode.³⁰⁴ A different configuration was proposed by Dong et al.,³⁰⁵ in which a flexible printable DSC and a supercapacitor with reduced graphene oxide electrodes and a polymer electrolyte were fabricated side by side on the same PEN-ITO substrate. The device showed charging potentials up to 1.8 V and exceptionally stable performance under various bending and tilting tests in outdoor conditions.

Song et al. obtained the highest overall efficiency to date among all reported investigations on various photocapacitor configurations. They presented a novel approach to manufacturing photocapacitors with low-loss energy storage. The PC was charged to 1.1 V under 1 sun illumination and demonstrated a storage efficiency of 20.53% and an overall efficiency of 18.34%.²⁸ Future research on engineering should combine cutting-edge materials with lower energy loss integration methods between the PV and SC units, enhancing performance beyond 25% under direct sunlight and 40% under ambient light conditions.

4. Methods and Techniques for Characterizing Photocapacitors

When describing and characterizing the performance of a photocapacitor, several aspects come into play, ranging from the overall efficiency of the photocapacitor to the system operating voltage. Each parameter ultimately defines the potential application scenario or optimization pathway of a photocapacitor. In this section, we discuss the characterization of the PV and SC units, the behavior of the integrated PC device, and the need for a protocol to measure and report results in the growing photocapacitor area.

4.1. Performance Assessment

The overall efficiency of the integrated photocapacitor is derived by multiplying the independent efficiencies of the solar cell and supercapacitor components.²⁸⁰Equation 2 is used to determine the power conversion standard current–voltage measurements (I–V curves).

2

where V_OC, J_SC, FF, and P_in represent the open-circuit voltage, the short-circuit current density, the fill factor, and the incident light power density, respectively. As shown in Figure 5, V_OC is the maximum voltage at open circuit conditions (at J = 0). At the same time, the J_SC denotes the highest current attainable by the solar cells when the applied bias is 0. Several techniques, such as external quantum efficiency (EQE), transient measurements, electron impedance spectroscopy (EIS), intensity modulated photovoltage spectroscopy (IMVS), intensity modulated photocurrent spectroscopy (IMPS), can be employed to characterize the behavior of photovoltaic cells.³³⁷

In determining the total efficiency of the integrated PC module, the capacitance of the SC also plays a significant role. The capacitance of a charged electrode can be estimated by the contributions of the Helmholtz double layer. Following the Stern model, the ions are assumed to be point charges attracted by electrostatic forces in the inner layer. In the near vicinity, the diffusive layer is composed of solvated anions and cations that counterbalance the charged electrode. In eq 3 C_H and C_diff represent the inner layer and diffusive layer capacitance, respectively.

3

The specific capacitance can be calculated from different electrochemical methods. The mathematical equations below (eqs 4–6) deliver the specific capacitance obtained from galvanostatic charge–discharge measurements, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS), respectively.^226,338

4

5

6

In eq 4, i represents the constant current applied during the GCD measurement and dV/dt represents the slope of the GCD discharge. In eq 5, the capacitance is obtained by integrating the typical I–V voltammogram, divided by the potential window. Alternatively, the capacitance can be determined from EIS measurements, where in eq 6f represents the frequency and Z the real part of the impedance. The capacitance of a symmetric supercapacitor with equal mass loading of the same material can be obtained from measurements on a single electrode as shown in eq 7.

7

The following eqs 8–10 can be used to calculate the output energy (E_output), the energy density (E_A, in Wh g^–1) normalized by the mass loading, and the power density (P_A, in W g^–1) of the supercapacitor during charging, where V represents the potential window of the SC and R the internal resistance of the supercapacitor.

8

9

10

The total efficiency of the combined photocapacitor module can be calculated using eq 11, where A_PV is the solar cell’s surface area and E_in is the amount of energy received during charging (eq 12). The overall efficiency is calculated by dividing the energy produced during illumination by the energy stored during photocharging.^97,282,339

11

12

The overall efficiency of a PC is determined by the photoconversion efficiency, the supercapacitor performance, and the interface between them, which is crucial for advanced stacked systems. Assuming that the supercapacitor unit presents a constant capacitance, the maximum efficiency of a photocapacitor system is proportional to the solar cell’s PCE. Therefore, an efficient solar cell is a prerequisite for an efficient photocapacitor system. The Shockley-Queisser limit determines the performance of a PV unit, which depends on the material’s bandgap as shown in Figure 5.³⁴⁰ The photovoltaic community efforts in recent years have resulted in very efficient technologies with efficiencies over 25% under sunlight and indoor PCEs reaching around 40% as shown in Figure 5. The integration of the SC affects the time to reach the maximum value of the PC efficiency, excluding parasitic currents in the supercapacitor unit.³⁴¹ The low overall efficiencies of integrated PCs are mainly caused by energy losses between the PV and energy storage units. Future research should focus on performance coupling and new circuit design.³⁴²

It is feasible to enhance the performance of the energy storage device by increasing its energy and power density. Hybrid SCs that employ pseudocapacitive materials to build asymmetric SCs can provide superior energy storage performance. Additionally, increasing the effective surface area of electrode materials can boost the energy density of SCs.³⁴³ An increase in surface area can be achieved by improving the electrode’s 3D porous structure to maximize the electrode/electrolyte interface (wettability), ion diffusion, and charge transfer.³⁴⁴ Nanoporous materials with pore sizes between 2 and 50 nm can reach high capacitance and effective ion pathways for fast kinetics, making them ideal for surface utilization and performance enhancement.²⁸² EIS measurements can be implemented to gain a better understanding of the interfacial process and estimate energy losses in the PV and SC units, and for proposing further approaches to decrease the energy losses in the integrated device.

Optimizing the PV unit materials, structure, and interfaces reduces bulk and nonradiative recombination and increases charge transfer. Additionally, optimizing the architecture and arrangement of the integrated devices can reduce the series resistance of stacked or parallel-connected devices, reducing energy losses. Monolithic devices with a common electrode reduce energy losses and provide enough energy storage and charge extraction for high energy conversion efficiency and reliability.³⁴⁵ The absence of wiring between the two devices simplifies the architecture and reduces the PC’s internal resistance.²⁹⁹

4.2. Operating Voltage of the Integrated Devices

The voltage of a supercapacitor device is determined by the charge separation at each electrode, resulting in a potential difference across the whole device.³⁴⁶ Techniques such as CV and GCD can measure the maximum operating voltage of supercapacitor materials in a three-electrode setup or on entire devices (2-electrode setup).^327,346 However, maximal voltage testing can result in cell damage. An alternative option is to apply a lower voltage to the device and gradually increase it until a change appears at the potential window’s edge. As discussed earlier in section 3.2, the electrolyte solvent and the supercapacitor’s design significantly influence the operating voltage of the SC. For instance, the thermal breakdown potential of water at room temperature restricts the operating voltage of SCs employing aqueous electrolytes to values lower than 1.2 V.²²⁵ In traditional symmetric supercapacitors, the electrode materials and mass loading are identical; as a result, its stable potential window includes only a narrow potential range. Using the differing potential windows of asymmetric electrodes can maximize their working voltage during the charge and discharge cycles to voltages greater than 2 V.^225,347 Employing organic solvent electrolytes can expand the potential window of supercapacitor devices between 2.3 and 4 V. Using room temperature ionic liquids, the potential window can further shift from 3 to 6 V.

The open-circuit voltage of a PV unit depends on several components but mainly on its technology. For instance, in a DSC the V_OC depends on the energy difference between the Nernst redox potential (E⁰) of the redox mediator and the quasi-Fermi level (E_F,q) of the electrons in the TiO₂. At the same time, this is dependent on the photocurrent and the electron recombination rates.^348−352 For OPVs, the V_OC depends on the energy difference between the HOMO (highest occupied molecular orbital) of the donor molecule and the LUMO (lowest unoccupied molecular orbital) of the acceptor molecule and also relies on the external quantum efficiency (EQE).^353−358 The V_OC of PSCs depends on the bandgap of the photoactive material (perovskite)^359,360 and is greatly affected by defects and traps.^361−363

The supercapacitor unit essentially determines the operating voltage of an integrated PC in an ideal photocapacitor with minimal energy losses. This can be inferred since the current densities transferred from the PV to the SC during the photocharging process are sufficient to reach the supercapacitor’s maximum voltage (Figure 6). However, most PC studies show operating voltages lower than the SC unit’s capability. This voltage loss can be attributed to energy losses at the interface or wiring between the PV and SC unit or recombination of the stored charges in the SC unit with the PV unit. As mentioned earlier, charge recombination from the SC to the PV unit can be avoided by connecting the PV and SC through a rectifying diode. Unfortunately, the diode will inherently cause a voltage drop across the system, compromising the integrated PC device’s ultimate voltage output. New approaches must be studied in the future to develop PCs able to exploit the maximum voltage range of the SC unit.

Figure 6.

Schematic overview of the characterization of an integrated PC. The photovoltaic and charge storage efficiencies must be measured for the entire system to calculate a PC’s overall efficiency. The dark discharge must be recorded under strict dark conditions, and the stability must be reported following multiple photocharge–discharge cycles.

Using photocapacitors to power electronic devices will require a constant operating voltage of 3–6 V, requiring the connection of various SCs in series to increase the operating voltage of the individual units. Overcharging may harm the SC’s lifespan, and a malfunctioning unit may influence the output voltage. This emphasizes the urgency to design photocapacitors with high voltage ranges.^280,302,343 Moreover, one of the most significant difficulties that must be addressed is the design of PC devices that can maintain a constant voltage for longer discharging intervals than a few minutes, as shown in the majority of photocapacitors described in the literature.

One approach to increase the discharging time of a photocapacitor is to employ hybrid supercapacitors with a capacitive electrode and a battery-type electrode with ion intercalation. This strategy can boost energy storage and discharge times to achieve the needed 6–8 h dark lapse duration when ambient light is not available. One approach to greatly enhance the energy density of the supercapacitor unit is the design of a sodium/potassium/zinc or magnesium-ion supercapacitors, which can reach a high energy density ranging from 5 to 200 Wh kg^–1 and high power from 0.1 to 30 kW kg^–1.^{113,343,364−370,226,227} A wide variety of materials such as MoO₂, Fe₄O₄, Li₄Ti₅O₁₂ (LTO), and carbon-based materials doped with Li-ions, among others, can be employed for ion intercalation and thus can be employed to develop hybrid SCs.^371,372

4.3. Protocol and Standardization

While standard methods for PV characterization and electrochemical procedures for SCs are well established, the new field of integrated photocapacitors requires a standardized methodology for evaluating and reporting the integrated devices’ energy harvesting and storage properties.²⁸¹ Regardless of the photocapacitor’s architecture, the independent photovoltaic behavior of the PC unit when it is merely harvesting light without being coupled to the SC unit must be reported (J–V, IPCE, etc.) as depicted in Figure 6. Additionally, the SC unit’s independent capacitive or pseudocapacitive behavior when it is charged by a potentiostat (CV, GCD, EIS) must also be reported with the electrode’s mass loading, thickness, and active area. These parameters should be compared to the characterization of the complete photocapacitor to address how the photovoltaic and capacitive behaviors change when the PV and SC are coupled. Special attention must be given to the dark self-discharge time after charging the SC unit via light harvesting (photocharging). The photogenerated charges under various ambient light conditions are sufficient to charge the SC (at a lower voltage than under 1 sun conditions). As a result, if strict dark conditions are not followed to study the self-discharge of photocapacitor devices, a “constant” voltage can be measured over an extended period of time.

The capacitance retention and cyclability of the integrated PC device should be investigated, as the vast majority of research only describes the stability of the SC unit. In addition, it is crucial to determine the overall performance throughout a broad temperature and voltage operating range. This information is required to address safety concerns, specifically because energy storage devices can induce heat losses due to the increased resistance of electrodes and connections between PV and SC units and electrolyte degradation during extended charge/discharge cycles.³⁷³

5. Future Outlook

As we look toward the future of photocapacitor technology, a multitude of opportunities and challenges lie ahead.³⁷⁴ To unlock the full potential of photocapacitors and ensure their widespread adoption,³⁷⁵ future research could encompass key technical issues and continue to push the boundaries of our understanding of the underlying chemistry and materials science.^376−380

One critical area of focus could be the development of advanced materials for both the photovoltaic and supercapacitor components of photocapacitors. Novel materials, such as metal halide perovskites, organic semiconductors, and two-dimensional materials, offer unique properties that could significantly improve the performance and sustainability of photocapacitor devices.^381−391 Furthermore, exploring the potential of sustainable, abundant, and low-cost materials, such as Earth-abundant metal oxides and sulfides, carbon-based materials, and bioderived polymers, for use in photocapacitor components might be a fruitful direction (Figure 7).^392−399

Figure 7.

Sustainability of photocapacitors. Through Earth-abundant materials, third-generation photovoltaics can be manufactured with green chemistry techniques. Additionally, the charge storage unit can integrate multiple waste and bioderived materials. These features contribute to the viability of PC production and integration into a circular economy.

Fundamental research will play a vital role in driving the progress of photocapacitors.^115,400 By gaining a deeper understanding of the underlying chemical reactions, charge transfer mechanisms, and material properties, researchers can optimize device performance and identify novel materials and configurations to further improve the efficiency and stability of photocapacitors. This knowledge will also facilitate the development of predictive models, which can help guide the rational design of photocapacitor devices with tailored properties for specific applications.

Another important avenue of research is the investigation of novel device architectures and integration strategies that enable the efficient coupling of photovoltaic and supercapacitor components. Optimizing the interfacial charge transfer and minimizing energy losses will be crucial for enhancing the overall performance of integrated devices.²⁸⁰ The development of innovative fabrication techniques and scalable manufacturing processes will also be essential in making photocapacitors a viable option for a wide range of applications.

The potential of photocapacitors in powering the Internet of Things (IoT) and the Internet of Everything (IoE) is immense. As the electronic industry expands and the demand for portable, sustainable power sources grows, the commercial prospects for photocapacitors become increasingly favorable. The deployment of photocapacitors in self-powered electronic devices could lead to significant societal benefits, such as energy savings, increased use of renewable energy, enhanced connectivity, and improved data transfer.

The integration of ambient photocapacitors with machine learning (ML) and artificial intelligence (AI) technologies offers immense potential for developing self-powered, intelligent systems that can harness ambient light to power the next generation of smart IoT devices. This combination of cutting-edge technologies has the potential to revolutionize various sectors, such as agriculture, health, business, and environmental monitoring, ultimately leading to a more sustainable and connected future.

Ambient photocapacitors, which capture and store energy from ambient light sources, provide a sustainable and distributed energy solution for IoT devices. As electronic miniaturization and energy storage advancements continue, these devices can enable IoT networks to rely on renewable energy sources. Concurrently, ML and AI advancements are transforming data processing, analysis, and decision-making in IoT networks. By embedding these capabilities into photocapacitor-powered IoT devices, we can create intelligent systems that autonomously learn, adapt, and respond to their environment.

The convergence of ambient photocapacitors, ML, and AI allows for the development of self-powered, context-aware devices that actively respond to their surroundings. For example, in agriculture, these smart IoT devices could optimize irrigation and fertilization strategies based on soil conditions, crop health, and weather patterns. In healthcare, wearable devices powered by ambient photocapacitors could continuously track vital signs for early detection of potential health issues.

To fully harness the potential of integrating ambient photocapacitors with ML and AI, future research should focus on refining materials, fabrication methods, and device architectures for photocapacitors, as well as optimizing algorithms and computational resources for on-device ML and AI processing. Overcoming these challenges and fostering interdisciplinary collaboration will drive the development of self-powered, intelligent systems that will shape the future of IoT applications and contribute to a more sustainable, connected world.

In conclusion, the future of photocapacitor technology is bright, with abundant opportunities for innovation, growth, and positive impact. By addressing the key technical challenges, conducting fundamental research, and exploring novel materials and device configurations, we can unlock the true potential of photocapacitors and make them an integral part of our sustainable energy landscape. By exploring new materials, refining device architectures, and integrating with advanced technologies such as machine learning and artificial intelligence, photocapacitors have the potential to reshape numerous sectors and contribute to a more sustainable and connected future. Further research and collaboration across various disciplines will be crucial in unlocking the full potential of this versatile technology.

Biographies

Dr. Natalie Flores-Diaz received her Master’s degree in Chemistry from Universidad de Costa Rica (UCR), Costa Rica, in 2017 under the supervision of Prof. Leslie W. Pineda. She obtained her Ph.D. in Chemistry and Chemical Engineering from the École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, in 2021 under the supervision of Prof. Anders Hagfeldt. In 2021, she joined Prof. Marina Freitag as a postdoctoral researcher, funded by the EU Horizon 2020, MSCA. Her research focuses on the development of photovoltaics for various applications.

Dr. Francesca De Rossi holds a Ph.D. in Microelectronics Engineering from the University of Rome Tor Vergata, Italy. From 2015, she worked as a Technology Transfer Fellow at SPECIFIC Innovation and Knowledge Centre, Swansea University, UK, in the PV team led by Prof. T.M Watson, focusing on printable perovskite solar cells (PSCs), and leading the activity on PSC long-term stability. In 2019, she joined Prof. Brunetti at the Centre for Hybrid and Organic Solar Energy (CHOSE) as a senior researcher, funded by EU H2020 APOLO project on smart designed, fully printed flexible PSCs and Italian Space Agency Perovsky project on smart materials and PCSs for space applications. She is currently working on flexible PSCs with a focus on sustainability and space applications and on supercapacitors on flexible substrates.

Dr. Aparajita Das is currently a National Postdoctoral Fellow at IIT Madras, India. In 2011, Das received her Master’s degree in Chemistry from Sambalpur University, India. Das worked on quantum dot solar cells (QDSCs) with a special emphasis on photosupercapacitors with Prof. M. Deepa during 2015–2019 and received her doctoral degree from IIT-Hyderabad, India. She has worked extensively on understanding the charge recombination dynamics in photoelectrochemical cells, which included QDSCs and photosupercapacitors.

Prof. Melepurath Deepa received her Master’s degree in Chemistry in 1997 from the University of Delhi and received her Ph.D. from CSIR-National Physical Laboratory and Delhi University, India, in 2004. Currently, she is a professor in the Department of Chemistry, IIT Hyderabad. Before joining IIT Hyderabad, Deepa was a scientist (Electronic Materials Division) in CSIR-National Physical Laboratory, New Delhi, during 2004–2009. Her expertise lies in the field of applied electrochemistry. Her research group focuses on developing efficient solution-processed solar cells, electrochromic devices, lithium-based batteries, and high-performance supercapacitors. She also is working toward designing dual-function devices by integrating solar cells with electrochromic devices as well as supercapacitors.

Prof. Francesca Brunetti, FRSC, received her Ph.D. in Telecommunications and Microelectronics from the University of Rome Tor Vergata in 2005. In 2005, she was awarded of a Marie Curie Individual Fellowship spent in the Institute for Nanoelectronics of the Technical University of Munich, Germany. In 2006 she became a researcher in the Department of Electronic Engineering of the University of Rome Tor Vergata, and since 2018, she is an associate professor at the same Department. Cofounder of the Centre for Hybrid and Organic Solar Energy at the University of Rome Tor Vergata (CHOSE, www.chose.it), her current research is focused on the analysis, design, and manufacture of electronic and optoelectronic devices through the use of nanomaterials (carbon nanotubes and graphene), organic semiconductors, and perovskites realized on rigid and flexible substrates. In particular, she is working on third-generation organic and perovskite solar cells and modules realized mainly on flexible substrates. Recently, she started an activity on the realization of printed flexible supercapacitors on paper. In addition to being coordinator of several national and international projects, she has authored more than 110 papers and 6 patents. She is an Associate Editor of Sustainable Energy and Fuels, a Royal Society of Chemistry Journal.

Dr. Marina Freitag is a reader in Energy Materials at Newcastle University and a Royal Society University research fellow. She is working on novel light-driven technologies that integrate coordination polymers to increase hybrid PV stability and performance in ambient conditions. Her research on hybrid molecular devices started during her Ph.D. studies at Rutgers University in New Jersey, USA, from 2007 to 2011. Dr. Freitag relocated to Uppsala University for a postdoctoral research position (2013–2015), which resulted in a breakthrough discovery known today as “zombie solar cells.” Dr. Freitag continued on this research with Prof. Anders Hagfeldt at the École Polytechnique Fédérale de Lausanne (EPFL) (2015–2016). She was appointed as an assistant professor at Uppsala University in Sweden from 2016 to 2020. Her work has been recognized by the Göran Gustaffsson Young Researcher Award 2019 and the Royal Society of Chemistry Harrison-Meldola Memorial Prize 2022.

Author Contributions

CRediT: natalie flores-diaz conceptualization, visualization, writing-original draft, writing-review & editing; Francesca De Rossi data curation, formal analysis, project administration, visualization, writing-original draft, writing-review & editing; Aparajita Das writing-original draft, writing-review & editing; Melepurath Deepa visualization, writing-original draft, writing-review & editing; Francesca Brunetti conceptualization, formal analysis, funding acquisition, resources, supervision, visualization, writing-original draft, writing-review & editing; Marina Freitag conceptualization, funding acquisition, methodology, project administration, resources, supervision, validation, visualization, writing-original draft, writing-review & editing.

The authors declare no competing financial interest.

Special Issue

Published as part of the Chemical Reviewsvirtual special issue “Emerging Materials for Optoelectronics”.