Recent Advances in Graphene-Enabled Materials for Photovoltaic Applications: A Comprehensive Review

Abstract

Graphene’s two-dimensional structural arrangement has sparked a revolutionary transformation in the domain of conductive transparent devices, presenting a unique opportunity in the renewable energy sector. This comprehensive Review critically evaluates the most recent advances in graphene production and its employment in solar cells, focusing on dye-sensitized, organic, and perovskite devices for bulk heterojunction (BHJ) designs. This comprehensive investigation discovered the following captivating results: graphene integration resulted in a notable 20.3% improvement in energy conversion rates in graphene-perovskite photovoltaic cells. In comparison, BHJ cells saw a laudable 10% boost. Notably, graphene’s 2D internal architecture emerges as a protector for photovoltaic devices, guaranteeing long-term stability against various environmental challenges. It acts as a transportation facilitator and charge extractor to the electrodes in photovoltaic cells. Additionally, this Review investigates current research highlighting the role of graphene derivatives and their products in solar PV systems, illuminating the way forward. The study elaborates on the complexities, challenges, and promising prospects underlying the use of graphene, revealing its reflective implications for the future of solar photovoltaic applications.

1. Introduction

Nonrenewable energy resources have a limited lifespan and cause worldwide environmental pollution. The reserve of these resources is limited, but their end products accelerate climate change and environmental degradation. Renewable energy sources are the best alternative to conventional ones as they are eco-friendly and long-term sustainable. The vast untapped energy could be harvested from solar radiation, wind, hydro, and geothermal sources. Solar energy is ample on the earth’s surface and can be effectively transformed into electrical power through a suitable photovoltaic cell setup. Solar radiation is directly converted into electric energy through photovoltaic (PV) solar cells.¹ A solar PV system generally consists of several components, including a broad panel, converter, and storage devices. The conversion of solar radiation into electric energy is also influenced by the characteristics of the material employed in the device. A variety of solar cells were developed to improve efficiency. However, solar cells constantly undergo modifications and updates to enhance durability, cost-effectiveness, and solar-to-electrical power conversion efficacy.^2,3

The energy transformation efficacy of a solar cell depends upon the materials used to develop the device.⁴ It was reported that TiO₂,^5,6 ZnO, SnO₂, and organic polymers have demonstrated good optical and electronic characteristics and are found suitable as photoanode materials for dye-sensitized solar cells (DSSCs).^5,7 Silicon (Si)-based solar cells are first-generation PV cells. It is reported that the micro-crystalline silicon cell efficiency is 11.9%, while the thickness of the solar radiation interacting film is approximately 2 μm. A significant enhancement in the fill factor plays a crucial role in achieving the highest efficiency on record.⁸ Second-generation solar cells are fabricated by a thin film made of thin layers of materials such as amorphous silicon, copper gallium selenide (CIGS),⁹ and gallium arsenide (GaAs).¹⁰ Advanced photovoltaic technology needs a more flexible, environmentally stable, highly photocatalytic nature and minimum electrical resistive materials. This Review comprehensively analyzed the prospect of third-generation solar cells synthesized by an ultrathin, high-conducting transparent material. Quantum-dot-sensitized solar cells (QDSSCs),¹¹ dye-sensitized solar cells (DSSC),⁴ and perovskite solar cells¹² are viable alternatives to conventional silicon solar cells. This analysis underscores the benefits and constraints of solar cells, with a particular emphasis on the imperative to enhance power conversion efficiency (PCE). This can be achieved by integrating light conversion (LCs) phosphor materials that enable the utilization of the complete solar spectrum.¹¹ Graphene (GA) has recently become a showcase performer in the limelight for solar-based devices.¹³

Figure 1 depicts the carbon allotropes and successors that can be classified as 0D (fullerene), 2D (GA), and 3D (carbon nanotube).¹⁴ GA is a successor of carbon, including carbon nanotubes, fullerenes, and carbon black.^14,15 Since armchair graphene nanoribbons are semiconducting, they are used in PV applications.¹⁶ Notably, it has high electrical and optical properties required in various commercial applications.^17,18 GA is widely applied for the PV domain due to its high mechanical strength,¹⁹ zero band gaps, swift carrier mobility, and desirable thermal properties.²⁰ Variations in molecular architecture provide different stability and characteristics to GA. Due to the meeting of the conduction band and valence band at the Dirac point (Fermi level), GA has been termed as a semimetallic semiconductor where charge carriers exhibit linear electronic dispersion.²¹ The charge carriers in GA show high velocities and high mobility at 298 K. GA and its derivatives also have impressive optoelectronics, optical characteristics, and luminance properties with high thermal and mechanical strength, which varies.²² The double bond between carbon–carbon atoms has high resistance to gas penetration and high surface area.²³ GA’s internal structure, electromechanical property, and thermal response provide the main advantage for introducing its derivatives. The PCE of solar cells depends upon the transparency and thickness of the electrodes of the material. The thickness of the GA sheets varies from nano to centimeters based on the adopted synthesis process. This Review discusses the various synthesized techniques, their assistance, and restraints. This Review focuses on the research and development of graphene’s potential in PV devices. Section 2 covers the synthesis methods of GA and its nanocomposites. Section 3 details the evaluation of GA for PV applications. Section 4 of this Review emphasizes the significance of genetic algorithms in perovskite solar cells. Section 5 delves into the intricacies of solar cell recycling, while section 6 concludes the study and outlines its future ramifications.

Figure 1.

Allotropes of carbon and their successor.²⁴ Reprinted with permission from ref (24). Copyright 2010 Nature.

The structures of graphite and their derivatives, graphene oxide (GO), reduced graphene oxide (rGO), and GA, are shown in Figure 2. Graphite derivatives have been obtained, depending upon the synthesized techniques.^24,25 GO is the first derivative of graphene, also known as graphitic acid or graphitic oxide. Significant changes occur in graphite because of the functional groups in its basal plane and edges. It has various functionalities like epoxy, carboxyl, and hydroxyl.²⁶ These groups provide larger spacing between two consecutive layers, making interlayer spacing almost twice that of graphite.²³ Several techniques eliminate these oxygen functionalities, so the surface area of rGO increases.²⁵ As a result, it becomes more thermally conductive, chemically reactive, and stable.²⁷

Figure 2.

Preparation technique of derivatives of GA from graphite.²⁴ Reprinted with permission from ref (24). Copyright 2010 Nature.

Similarly, graphene quantum dots (GQDs) are related to zero-dimensional GA nanostructures and have surfaced as an emerging and promising category of materials.²⁸ They have lateral dimensions ranging from 2 to 20 nm, which provides better photoluminescence properties, high transparency, sensitivity, and a large surface area.²⁹ These properties give new hope for a remarkable and fast display system.³⁰ In general, GA is found in single or multilayer formations with a separation equal to the diameter of a carbon atom.

2. Selection of Synthesis Method for GA and Its Nanocomposites

The synthesis technique depends on the fundamental properties of GA, such as morphology, interlayer separation, crystal size, electron mobility, thermal behavior, solubility, and optical response. Two top-down and bottom-up approaches are primarily followed for GA synthesis.³¹ The graphite turns into GO through a top-down strategy³² by chemical oxidization or electrochemical and mechanical exfoliation. After that outcome, the product can also be reduced thermally and chemically to get the other derived rGO of graphite. In the mechanical exfoliation method, GA is produced by peeling the outer layer of graphite with a scorching tap.³³ Graphite intercalation compounds (GIC) form through mechanical energy by overcoming the van der Waals attraction force among layers. Other GA synthesis methods, that is, bottom-up, started from gas molecules as a precursor.³⁴ Chemical vapor deposition (CVD)³⁵ and atomic layer deposition (ALD)³⁶ are widely utilized techniques for GA synthesis. In the epitaxial growth technique, annealing a high-temperature silicon carbide (Si–C) crystal leaves a carbon-rich GA layer growing on its surface.³⁷Figure 3a shows different synthesis mechanisms of GA.

Figure 3.

(a) Mechanisms of the synthesis of graphene (top-down approach).³⁸ (b) Mechanisms of the synthesis of graphene (bottom-up approach).³⁸

GO is prepared by a chemical oxidization technique like the Hummers method.³⁹ Numerous functional groups containing oxygen atoms become affixed to the GO plane during oxidation. These functional groups could be replaced by thermal reduction.³² The modified Hummers method can apply the steps mentioned above.⁴⁰ In this technique, GO is generated through a mixture of graphite, concentrated acids, and KMnO₄ in slow stirring under controlled temperature until the color changes from purple to brown.⁴¹ Further, H₂O₂ was mixed in the solution. Due to the high-level oxidation of graphite, the color changes from brown to yellow.⁴² GO is rinsed repeatedly with distilled water until the pH level reaches 4. During oxidization, the active oxygen group molecules relate to the GO surface by adsorption (noncovalent bond) or covalent bonds.⁴³ GO sheets comprise different oxygen groups, for example, epoxy, carboxyl, carboxylic acid, and hydroxyl.^44,45 These functional groups enhance the processing ability of GO for various applications.²¹ Functional groups prevent atom aggregation and increase dispersion stability in multiple solvents.⁴⁶ This functionality enhances the strong reactivity and the solubility of nanomaterials in water and organic solvents by specific techniques such as chemical and thermal processing.⁴⁷ Functionality plays a critical role in attaining magnificent properties and emerging applications. Much research has been done on its mechanical behavior, such as fracture and tensile strengths, Young’s modulus, etc.⁴⁸ It was reported that the fracture strength of the monolayer GA sheet is 130 GPa, with a Young’s modulus of 1.0 TPa, which shows the material film is defect-free.⁴⁹ It was described that the elastic modulus of the GA strip without restraint suspension is 0.25 TPa.⁵⁰ Moreover, GA dispersion in inorganic nanomaterials enhanced the mechanical stability of synthesized GA-based nanocomposites.^51,52 Although possessing biocompatibility, high optical transparency, and nontoxicity, the reduced thermal stability and compromised carrier mobility remain significant issues for GO.⁵³

Following the chemical synthesis process, the oxygen functionalities present in the inner part of an aromatic area of GO can be eliminated to produce rGO.³⁵ rGO is derived from GO by thermal annealing or chemical reduction. 200 mL of deionized water is mixed with 100 mL of GO suspension and stirred well to achieve a nonhomogeneous dispersion. This solution is stirred at 400 rpm for 72 h to produce rGO.⁵⁴

Following the reduction of GO, the elimination of functional groups leads to an increase in the surface area of rGO. However, this increase is accompanied by a decrease in the solubility of rGO. Enhancing the solubility of rGO necessitates chemical modification.⁵⁵ During the chemical modification process, there is an augmentation in functionality, thereby enhancing dispersion within solvents and improving solubility properties.⁵⁶ Due to chemical modification, the noncovalent functional group attached to rGO reinforces electrical properties without a change in structure.⁵⁷ Oxygenated functional groups are removed to restore π-bond networks.⁵⁸ The diverse oxygen functional sets could be detached via other techniques, thermodynamically and kinetically. The carboxyl and hydroxyl groups can be eradicated through heat treatment, whereas epoxy groups are removed utilizing the hydrazine compound.⁵⁹ The outcome of this reduction technique is called rGO; the most apparent is the derivative of GO, which shows characteristics similar to those of GO.⁶⁰ However, the π-network is redeveloped during the reduction of the material’s conductivity.

Graphene is an arrangement of nanosize atoms in monolayers or multilayers. The sp² hybrid carbon is arranged in a honeycomb structure.⁶¹ Most researchers established that GA is a perfect material for electrical conduction.⁶² It behaves like a zero bandgap semiconductor, thus facilitating the transport of excessive electrons.⁶³ At room temperature, the reported value of charge mobility was 15 000 cm²/(V·s).⁶² The charge carrier mobility increases tremendously by minimizing the doping at the ambient condition observed at about 200 000 cm²/(V·s).⁶⁴

Furthermore, it was also described that in GA sheets, the mobility of the charge carrier is independent of the temperature.⁶⁵ The thermal conductivity of freely suspended monolayer GA was 5000 W/(m·K) using confocal micro-Raman spectroscopy.⁶⁶ This thermal characteristic is notable for electronic applications, fuel cells, and batteries because this feature of GA provides thermal stability and durability. Furthermore, GA shows a high optical conductive response. It was observed that many GA nanocomposites exhibit optical transparency.^67,68

The chemical vapor deposition (CVD) approach is well-recognized as a prominent technology for preparing graphene-based materials. This method enables the fabrication of single- or multiple-layer structures that exhibit notable characteristics such as enhanced electrical conductivity, increased electron mobility, substantial surface and volume ratio, and notable optical response.⁶⁹ The optical characteristics of a material indicate its ability to effectively absorb solar radiation and transform the energy carried by photons into either heat or electrical energy.⁷⁰ As per the surface expanse, GA has a larger surface area than other allotropes of carbon. The large surface area of GA also provides strong interfacial interaction with other materials, atoms, or matrices during the fabrication of nanocomposites. Moreover, with the presence of minimum functionality, the large surface of GA inhibits or minimizes the cluster of particles in nanocomposites.^71,72

GA quantum dots (GQD) are 0D particles with quantum significance and energy traps, providing photoluminescence attributes.⁷³ The contribution of these optical characteristics in photovoltaics means developing advanced electronics for solar cells. The prism array encapsulation structure of polymer thin film coating on solar cells improves its optical energy conversion efficiency due to size spacing variation and structure quality.⁷⁴ Graphene coatings can also be utilized for nonreflective coatings for solar cells. GQD can be synthesized by either the top-down (Table 2) or the bottom-up (Table 1) method.⁷⁵ In the top-down method, it is prepared by chemical or thermal exfoliation. This approach is favorable for fine-size production and excellent morphology. In the bottom-up technique, CVD is preferred for a homogeneous control size layer.²⁸

Table 2. A Brief Account of the Evolutionary Path of Top-Down Graphene.

	standard size
technique	vertical depth	horizontal scale	advantage and limitation	ref
nanolayer delamination	multilayer graphene	dimensions ranging from micrometers (μm) to centimeters (cm)	advantage: the production process yields large-sized and unmodified graphene sheets, ensuring high quality and versatility for various applications	(33)
			limitation: however, the production scale remains very small, which limits the overall quantity of graphene sheets that can be produced at a time
graphite ultrasonication	formation of both single-layer and multiple-layer graphene	in range of micrometers (μm) or even smaller, submicrometer (subμm) sizes	advantage: the graphene produced is unmodified and cost-effective, making it accessible for various applications without additional processing costs	(89)
			limitation: however, the production process suffers from a low yield of graphene, and the separation of the material may be challenging, impacting overall efficiency and scalability
electrochemical delamination/modification of graphene	monolayer and low or multilayer graphene	typically in the range of 500–700 nm	advantage: the single-step functionalization and exfoliation process results in highly conductive graphene with improved properties for various applications	(90)
			limitation: however, the cost of ionic liquids used in the functionalization process can be a significant factor, potentially increasing the overall production cost of the functionalized graphene
graphite superacid dissolution	primarily yields single-layer graphene	ranging from 300 to 900 nm	advantage: the production of unmodified graphene is scalable, allowing for large-scale manufacturing to meet market demands	(91)
			limitation: however, the process entails the use of hazardous chloro-sulfonic acid, which poses safety and environmental concerns; moreover, the cost and complexity of acid removal during the production process can impact the overall economic feasibility and sustainability of the process

Table 1. A Succinct Account of the Origins and Development of Bottom-Up Graphene.

	standard size
technique	vertical depth	horizontal scale	advantage and limitation	ref
restricted self-organization	single-layer graphene	hundreds of nanometers	advantage: thickness control enables precise adjustments, ensuring uniformity and desired material properties	(40,81)
			limitation: even with precise thickness control, defects could impact the overall quality and performance of the material
CVD	graphene sheets with a minimal number of layers	very large scale, measured in centimeters	advantage: the product exhibits large size and high quality due to careful production	(82−84)
			limitation: the small production scale may limit overall output and availability
arc discharge	graphene including single-layer, bilayer (consisting of two layers), and few-layer forms sheets	few hundred nanometers to a few micrometers	advantage: the process can produce approximately 10 g/h of graphene, allowing for a reasonable production rate	(85)
			limitation: however, the low yield of graphene and the presence of carbonaceous impurities may impact the overall quality and efficiency of the production process
epitaxial development on silicon carbide (SiC)	few-layer graphene sheets	up to several centimeters	advantage: the production process yields an extensive area of pure graphene, providing ample material for various applications	(34,60,86)
			limitation: the production scale remains relatively small, limiting the quantity of graphene produced at a time
deconstruction of carbon nanotubes	multiple-layered nanoribbons	few micrometers long	advantage: the size control achieved by selecting starting nanotubes allows for tailored and specific graphene dimensions, catering to diverse application needs	(87)
			limitation: however, the process involves using expensive starting materials, which can increase production costs; the resulting graphene may also contain oxidized impurities that could affect its properties and performance in certain applications
reducing the oxidation of carbon monoxide (CO)	formation of multiple-layered graphene sheets	submicrometer range	advantage: the graphene sheet remains unoxidized, preserving its unique properties and ensuring high conductivity and strength	(88)
			limitation: however, a notable limitation is the contamination of the graphene sheet with a-Al2O3 and a-Al2S, which can adversely impact its performance in certain applications and require additional purification steps

In the bottom-up technique, the CVD process is generally used to yield pure, large-area, and high-performance GA. In this technique, the GA layer is grown on the substratum of metallic or semiconductor wafers by deposition of hydrocarbon vapor. There is a possibility that more than one gaseous vapor (precursor) decomposes on the metallic surface as per the demand of the device. The substrate surface acts as a catalyst; when the precursor flows on the stimulus, it gets deposited, and a chemical reaction occurs. The selected catalyst is either monomaterial or an alloy of more than one metal, per the requirement. The deficient d orbital metals (e.g., Co, Ni, and Cu, etc., are transition metals in metallic catalysts) promptly absorb and react with deposit carbon atoms, giving good performance. Polycrystalline GA material is generated through the absorption and concentration of carbon facilitated by metallic catalysts. The significance of polycrystallinity is the resultant thermal stability.³⁵ To provide hole and electron transportation, doping of GA is done with metallic atoms such as gold, platinum, and boron to create p-type GA, while copper, nitrogen, and aluminum atoms are for n-type GA.⁷³

It is reported that high sustainability and nontoxicity properties are most beneficial in the case of dye-sensitized solar cells (DSSCs).⁷⁶ Recently, many researchers have testified that using GA as a quantum dot increases the power transformation competence of solar PV cells.⁷⁷ The synthesized GA properties depend upon the flow of hydrocarbon gas vapor (precursor), the pressure of a gas, the temperature, and the property of the metallic material surface.⁷⁸ Therefore, the GA structure aggregation of carbon molecules must be avoided. It was reported that the 30-in. GA layer was deposited onto the Cu wafer for roll-to-roll production by the CVD technique.⁷⁹

Temperature is an essential factor in the CVD technique. Stefanos et al.³⁶ reported that between the temperature range 970–1070 °C, the activation energy enhances about 3.01 eV, and the growth rate increases. No extra nucleation above or below the initial layer has been detected in the uniformly grown single layer on the substrate. A highly pure and flawless GA layer typically grows under low-pressure conditions. The nucleation density is obtained from 5 to 80 nuclei/10 000 μm². The growth rate of the GA lies between 150 and 1400 μm² per 20 min.³⁶

Arif et al.⁸⁰ reported negligible water intercalation within graphene oxide layers; the relative humidity is very low, about ∼30%. The water adsorption on the surface and wettability are as low as the thickness of graphene layers. In the single layer (which had a thickness of 1.3 nm), the relative humidity was observed at 10%, while in the double-layer graphene (which had a thickness of 2.6 nm), the relative humidity was 80%. This improvement occurs because of the embolism of H₂O between GA layers. It was also observed that in the range of low humidity regime (5–50%), the friction force occurs from 5.9 to 8.2 nN, while the regime range increases up to 50–80%, and the friction force simultaneously rises from 8.9 to 16.5 nN.³⁶Figure 4 shows that the GA thin sheet grows on a metallic surface by the CVD technique: part (i) shows the precursor, part (ii) represents the absorption of carbon atoms, part (iii) denotes segregation, and part (iv) denotes the catalytic process.⁸⁰

Figure 4.

GA thin sheet grown on a metallic surface by the CVD technique.⁷³ Part (i) shows the precursor, part (ii) represents the absorption of carbon atoms, part (iii) denotes segregation, and part (iv) denotes the catalytic process.

3. Evaluation of GA in Photovoltaic Application

3.1. GA in Organic Solar Cells

Organic solar cells (OSCs) are more susceptible to damage under atmospheric conditions than are their inorganic counterparts. Evaluating durability involves assessing chemical stability concerning air-induced material effects, degradation from reactions with water and oxygen, and potential deterioration of electrode materials. Previous research has highlighted degradation sources in OSCs as shown in Figure 5, including donor–acceptor phase separation, photo-oxidation of active layers and donor materials facilitated by water and oxygen diffusion, and potential interface failures, particularly between the hole transport layer (HTL) and the ITO anode.⁹²

Figure 5.

Schematic of organic solar cells with graphene.⁹⁷

Addressing these challenges, the integration of carbon nanomaterials emerges as a promising solution due to their inherent barrier properties, impeding the diffusion of atmospheric elements into the device layers. Studies showcasing graphene-based materials (GRMs) in OSCs have reported enhanced long-term stability, notably in terms of preserving power conversion efficiency (PCE) over time. Recent studies have reported the long-term stability of GRM-based OSCs in terms of PCE changes.

The electrodes of an organic solar cell are a crucial element, as they are composed of materials that possess high conductivity and optical transparency. The preferred electrodes in organic solar cells are indium-doped tin oxide (ITO) and fluorine-doped tin oxide. Park et al.⁹³ reported that market-available ITO offers 80% transmittance in industrial applications. At the same time, on glass coating, the resistivity is about 10–15 Ω/□. Fluorine-doped tin oxide is also a candidate similar to ITO for electrodes. However, the major challenge with ITO is its cost as it is a rare earth metal, it is sensitive to higher and lower pH values, and it has brittleness, manufacturing issues, etc.⁹⁴ The solar cell industry requires flexible (avoiding brittle ITO layers), durable, conductive, thermally stable materials with high optical transparency to substitute ITO. Thus, metal grids, organic compounds, carbon nanotubes, and metal oxides are used as electrode ITO alternatives. However, due to poor optical transparency, such materials are not viable candidates for the electrode of organic PV cells.^93,94 However, due to the great properties of GA, it can be preferred for the electrodes of organic solar cells (OSCs).^95,96

The comparative performance study of different materials used for OSC electrodes with and without GA shows that GA-based electrode performance easily exceeds that of ITO.⁹⁸ GA is a far better material for both anode and cathode than ITO. Graphene-based electrodes are discovered to enhance the solar conversion efficiency of OSCs.⁹⁹ GA can be utilized for a cost-effective fabrication process for OSCs at a large scale, making GA a suitable candidate for substituting ITO.^100,101 It is observed that the GA properties depend on the synthesis mechanism. CVD is a widely applied synthesis mechanism of GA to achieve a dynamic optical nature. The transmittance (T) is a significant parameter of photovoltaic systems. The value of T decreases with the increases in the width of the GA sheet, its conductivity, and the purity of materials. However, the transmittance of GA samples synthesized by CVD is poor as compared to the Hummers method, but the purity of GA is higher in CVD.^82,83 GA synthesized by CVD has better transparency and conductance properties than those synthesized by the Hummers method. GA manufactured through the Hummers technique comprises sp³-carbons and structural defects.¹⁰² This confines the π-bond linkages and reduces its charge carrier mobility. Also, CVD generates bigger GA sheets (<30 in.) with large conductivity and governable sheet numbers, which results in solar device performance.⁸⁴ These properties of synthesized GA play an excellent role in organic solar devices.¹⁰³ It is reported that OSCs are generally fabricated through CVD.¹⁰⁴ At the top layer, CVD-synthesized GA is used due to its higher transparency. Solar cells having a four-layered GA structure, good air sustainability, and high flexibility provide the efficiency of photo energy conversion up to 3.2% with 90% transmittance of GA-based electrodes. This multilayered transparent CVD synthesized by a GA-based electrode provides a long-life work capability.⁹⁶

Li et al.⁵² reported that multilayer GA is superior due to its conductivity, low transmittance, and good stability in air. The liquid exfoliation is preferred to synthesize the monolayer, bilayer, and trilayer GA, respectively. The single-, bi-, and trilayer GA synthesized by liquid exfoliation offers resistances of ∼150, 20, and 8 kΩ and transparencies of 93%, 88%, and 83%, respectively.⁵² Thus, the best equilibrium among transmittance (T), resistivity (R_s), and conductivity (σ) are essential for a transparent conductive electrode. This obtained value showed that transparency decreases with an increment of R_s (Figure 6a).⁵² The R_s and T values are associated with 2D conductivity (σ_2D) and optical conductivity (σ_o). However, conductivity depends on the synthesis technique of GA. The transmittance can be formulated as given below:⁸⁴

1

where I_o represents the free space impedance, σ_o is optical 2D conductivity, σ_2D is direct current (DC) conductivity, and R_s is the electrode sheet resistance proportional to its transparency.¹⁰⁵ It is reported that the resistance of the electrodes depends upon the number of layers.¹⁰⁶ However, substrate-induced chemical doping will convert semimetallic GA to metallic GA, improving the DC conduction sufficiently to produce suitable GA realistically.¹⁰⁷

Figure 6.

(a) Sheet resistance (R_s) (*PVA-poly(vinyl alcohol)). (b) Optical transmittance (T) of GA gained from diverse methods. (The purity of Ni and Cu in CVD is approximately 99.99%, and so they are utilized as the thin foil on solar cells.)

The optical transmittance (T) of GA film is the crucial parameter for its performance, which is a reciprocal to conductivity and thickness, as shown in Figure 6b. The GO-based electrodes show higher T than CVD GA due to their morphological defects that help in the T of light. GA shows more than 90% T with a sheet resistance of 100–400 (Ω/□). Still, its major drawback is a low sensitivity to defects and impurities.¹⁰³ Metal nanoparticles have T = 88% with a sheet resistance R_s = 1–150 Ω/□, along with the constraints of high temperatures for sintering.¹⁰⁸ In comparison, carbon nanotubes show a transmittance of 82–88% and sheet resistance (R_s) = 300 Ω/□, along with an increase in resistance values at the junction of the tubes.^101,109,110

3.2. GA as a Carrier Receiver for PV Cell

Inorganic materials utilized in solar cells possess the characteristic of efficiently absorbing solar radiation, augmenting their capacity to convert solar energy into electrical potential. The energy conversion process relies heavily on the intrinsic structure of band connections within materials. The donor and acceptor materials should possess a band alignment configuration that must be aligned in the molecular orbital that is least occupied (LUMO).¹¹¹ In acceptor and donor materials, the potential must be about 0.3 eV. To achieve good solar energy absorption, the donor element’s band gap should not be higher than 1.5 eV.¹¹² The 2D structure is consistently supported as a receiver. Its fast charge transfer property is more compatible and suitable for donor candidates. This property shows coordination of the energy band gap.^113,114

GA is an essential building block element in which carbon may be present in the form of several allotropes. Fullerene is one of the examples of carbon allotrope molecules. The fast charge receiving property distinguishes it from the rest of the materials. With the help of this property, organic solar cells (OSCs) could be modified. Chu et al.¹¹⁵ observed that the phenyl-C₆₁-butyric-acid methyl-ester (PCBM) was implemented for changing OSCs. In such ways, the GA application reveals a superior combination with donner candidates due to their inherent physical and chemical attributes.¹¹⁶ The combination of the P3DT [poly(3-dodecylthiophene)]:GA depicted astonishingly less PL emissions in the about 800 nm range. Also, between 400 and 700 nm, material absorbance has been high as compared to P3OPT [poly(3-octylthiophene)].^113,114 P3HT (poly(3-hexylthiophene)) combined with GA performed a remarkable photovoltaic effect.¹¹⁷ It was reported that the electrons and holes were transmitted through the material structure; selecting those materials with the highest occupied molecular orbital (HOMO) or LUMO levels¹¹⁸ is necessary for a flawless transition. The thin film of GO exhibits electron blockers and good hole transfer in organic PV cells. Feng et al.¹¹⁹ synthesized a PV device employing a photoactive P3HT:PCBM layer between the GO layer by synthesis of neural solution. The first layer is synthesized by indium tin oxide (ITO), which is highly conductive and see-through to provide fast radiation transmission and reduce the leakage current and recombination of electrons with holes,¹¹² as depicted in Figure 7. The results of the GO layer arrangement as a hole transport layer are an improvement of the PV performance proficiency as compared to the conventional hole transport layer photovoltaic device, using synthesis with PEDOT (C₂H₄O₂C₄H₂S) and PEDOT:PSS as the hole transportation sheet or film.¹¹⁸Figure 7b depicts that the maximum power rectangle of ITO is less than that of GO incorporation. GO incorporation eliminates the rapid erosion of ITO electrodes in PEDOT:PSS; the cause of deterioration is the acid nature of PEDOT:PSS. Figure 7c represents the open and short circuit characteristics curve; the thin film of GO improves the power conservation efficiency. This means that if the thickness is reduced, the peak power rectangle graph of the PV cell is enhanced along with the thinnest film of GO with enhanced FF and J_sc. Figure 7 depicts the energy band diagram of ITO PVCs.

Figure 7.

(a) ITO/GO/P3HT depiction: PCBM/Al photovoltaic device. (b) Behavior of PV devices without a hole transport layer as I–V curves. (c) I–V characteristics of the ITO/GO/P3HT:PCBM/Al devices with GO thickness variation.¹²⁰

Figure 8a shows that the molecules depicted from L to R are PCBM, P3HT, P3OPT, Circ-HBC, and Tria-HBC. Figure 8b illustrates the energy disparity between the donor and acceptor LUMO.^113,114 The LUMO donor and acceptor should have an energy difference of 0.3 eV for efficient electron transfer. The bandgap energy of the donor LUMO should be near 1.5 and 1.9 eV for P3PH.¹¹³

Figure 8.

(a) The molecules depicted from left to right are PCBM, P3HT, P3OPT, Circ-HBC, and Tria-HBC. (b) The energy difference between acceptor and donor LUMO.

The fundamental configuration of the heterojunction solar cell consisting of graphene and silicon adheres to a TCO (ITO)/n-Gr/p-cSi/Ag arrangement, as depicted in Figure 9a. Significantly, the device comprises a silicon dioxide (SiO₂) layer that functions as an insulator for carrier transport from the transparent conductive oxide (TCO) contact to the p-type crystalline silicon (p-cSi) wafer. This arrangement guarantees that transportation exclusively takes place through the graphene layer.

Figure 9.

Graphene-based ITO photovoltaic cells and their energy band diagram.¹²¹

The solar cell with a graphene/silicon heterojunction is studied in this work. The simulation provides an illustrative depiction of the device setup (Figure 9a). Additionally, the forward-biased junction’s band diagram is depicted, where Φ_G, Φ_Si, and χ_Si represent the work functions of graphene and silicon and the silicon electron affinity, respectively (Figure 9b).

3.3. GA for DSSC

Dye-sensitized solar cells, a part of the third generation of solar cell technology, are characterized by their use of thin films. The primary function of the electrode in these photovoltaic cells is to extract and collect the charge carriers. Unlike conventional solar cells, DSSCs operate differently. In these cells, photo carriers are created by dye. Initially, DSSCs were constructed using large-surface FTO plates. The thin TiO₂ film is coated on the FTO plate. This plate is a molecular sensitizer, and it is dipped into photosensitizer with another electrode like platinum. The FTO plate acts as an anode.⁵ The electrodes are interconnected within the DSSCs setup, where FTO is the counter electrode. It is essential for the work function of the synthesized electron extraction material film to be low. This characteristic facilitates the easy transfer of electrons, ensuring efficient functioning within the solar cell system. It was reported that GA was used in all parts of DSSCs (Figure 10a); with the use of GA, the photo carriers drifted fast toward lithium fluoride,¹¹⁴ and conjugated polymer electrolytes,¹¹⁸ TiO₂, TiO_x film as n-type semiconductor, GA as an electrode for DSSCs,^122,123 and n-type organic semiconductors¹¹⁵ are used as the electron transference film included in the PV cell.

Figure 10.

(a) Dye-sensitized solar cells (DSSCs)⁴ and (b) incorporation of GA in all parts of DSSCs.¹²⁴

It is also reported that GA as an electrode for DSSCs using ZnO improves the performance of PV devices by 36%.¹²⁵ The synthesis of GA with a width of 1 nm may be straightforwardly introduced as a channel of electron transportation in the center of the BHJ device phase.¹²⁶ PCE of these PV cells is enhanced up to 6.72% with FF = 64% and a revised value in J_sc = 13 mA/cm². In comparison, in the photovoltaic device fabricated without a GA electron transport layer (Figure 10b), the PCE is 5.35%, FF = 58%, and J_sc = 10.66 mA/cm². Figure 11a depicts the maximum power gain of the various solar cells of different material combinations. It is also reported that using GA-based ETL, the stability improved; in terms of PCE decrement with GA-based ETL, it is about 3%, while without GA-ETL, the decrement percentage of PCE is about 56% obtained in the same manner as TiO_x or GA-coated substrate, and along with GO/TiO_x, the electron transport deposit exhibits the maximum 7.5% PCE value as shown in Figure 11b.

Figure 11.

(a) Current density–voltage characteristics of BHJ without an ETL sheet and with ETL sheets of TiO_x, GO, and GO/TiO_x; and (b) PCE versus voltage with a GA probe in DSSCs.¹²⁶

The probes of thin-film PV cells can be developed employing material consisting of fluorine-doped tin oxide (FTO) for organic photovoltaics, DSSCs, and hybrid perovskites. Usually, the implementation of silicon PV cells becomes costly on an industrial scale.¹²⁷ Dye-sensitized cells are a rational replacement for silicon solar cells. Also, DSSC exhibited better results than conventional semiconductors. The research findings indicate that the efficiency of conversion of DSSCs exhibits improvement when the percentage of graphene oxide (GO) reduction increases.¹²⁸ This improvement can be attributed to GO’s improved conductivity and charge mobility, following the elimination of oxygen during the reduction process.¹²⁹ The energy generation principle of DSSC is based on the photoelectric production of an electron using dye. For synthesized DSSC, a skinny transparent layer of fluorine-doped tin oxide (FTO) coated on titanium dioxide (TiO₂) comprises a translucent anode.^130,131 This combination provides a large porous surface that performs well after the emergence of molecular sensitizer solution or photosensitive dye solution with the platinum material plate on which iodide electrolyte spreads and both are combined.⁷⁶ Usually, the solution vessel is closed off to ensure that electrolytes do not leak.

The operating concept of DSSC differs from that of normal solar cells. It is based on the photoelectric production of an electron by dye-like photosynthesis.⁷⁸ Photoexcitation of the sensitizer dye causes electron transfer inside the TiO₂ nanoparticles.¹³² Following charge transfer, the dye’s ground state is excited by gaining electrons from the electrolyte reductant, which is then replenished by electrolyte oxidant reduction at the counter-electrode.¹³² To obtain extraordinary performance, GA could be used in each dye part. The oxygen functional group’s accomplishment is vigorous in catalytic reactions. Electrolyte GA’s redox chemical effect is more favorable than platinum electrodes. As compared to pure GA, doped GA performed better as counter electrodes. The scaffold shape of 3D and A is also suitable for counter electrodes.⁷⁸ In dye solar cells, PCE is enhanced by using rGO because of its high charge mobility.¹³²

4. GA for Perovskite Solar Cells

Recently, halide perovskites proposed a possible material as cost-effective and more efficient for use in PV cells.¹³² The first-time perovskite organo-lade halide (CH₃NH₃PbX₃) (X = bromine (Br) and iodine (I)) materials were used in DSSCs. With the application of inorganic materials such as Br and I, the power conversion efficiency was enhanced by 3.1% with Br and 3.8% with I, respectively.^132,133 Due to specific changes in material selection and the synthesis technique of the perovskite film, the recombination of electrons is reduced, which enhances its PCE value by an average of 16.6%.¹³⁴ Wang et al.¹³⁵ reported that in the analysis of various parameters such as lifetime, carrier diffusion length, etc., of triiodide CH₃NH₃PbI₃ and mixed halide CH₃NH₃PbI_3–xCl_x perovskite layer, the result shows that the diffusion length of CH₃NH₃PbI_3–xCl_x is larger than 1 μm (100 nm for MAPBI₃ and >1 μm for MAPbICl₂), there is a high carrier mobility, and there is a significant absorption coefficient that improves PCE performance of the solar cell.¹³⁶ Liang et al.⁴¹ reported that the substrate’s additive materials and film preparation technique greatly impact the perovskite surface, facilitating a high PCE of up to 12%.

The metal oxides require a high temperature for the charge transfer process. The physical structure of the PSCs is based on DSSC designs that need higher temperature fabrication owing to the usage of metal oxides that act as charge-carrying ingredients depicted in Figure 12. Different designs, such as organic photovoltaics, have addressed this issue because of their easy and low-temperature manufacturing with flexible structures.¹³⁸ Liu et al.¹³⁹ described that by adopting triple material S-phenyl-C₆₁-butyric-acid methyl-ester (PCBM),¹⁴⁰ C₆₀, and LiF layers cathode buffer as a component in perovskites solar cell,¹⁴¹ the PCE could be improved.¹⁴² In such devices, PEDOT:PSS is arranged as an electron–hole transporter.¹⁴³ The thin LiF film promotes the carrier collection process that improves the PCE of perovskite cells by 15%.¹⁴⁴ Constructing a perovskite solar device using a simple composite approach with a reduced-GA scaffold (rGS) provided a large surface area.¹⁴⁵ This scaffold is an interfacial region among the electron transport and absorber layers in perovskite solar cells. The use of rGS was enhanced by a 17.2% PCE of PSC with a 27% overall improvement in solar cells’ performance.¹⁴⁵ The light-trapping competence of the perovskite device and the increased superficial area in interaction with the perovskite coating contribute to a rise in the device’s external quantum efficiency (EQE).¹⁴⁶Figure 13 shows reduced hysteresis effects and a more extended durability than those of TiO₂-based solar PV devices.

Figure 12.

Schematic diagram of graphene incorporated in PSCs. Abbreviations listed: poly-ethylene terephthalate (PET), titanium dioxide (TiO₂), and gold (Au).¹³⁷

Figure 13.

(a) EQE graph, (b) J–V measurement, and (c) an ambient environment stability of PV cells after encapsulation through UV-cured epoxy.¹⁴⁵

Figure 13a represents that the electrical quantum efficiency of reduced graphene oxide is comparably high for GO film and TiO₂. It may occur due to the absence of oxygen atoms. Figure 13b depicts the comparative study between TiO₂, GA, and rGS power outcomes. The area under the curve of both biasing of rGS is large as compared to TiO₂ and GA. Further, Figure 13c denotes the PCE of all three diverse types of solar cell materials; undoubtedly, the rGS shows a response as compared to others because it does not have any oxide form. It would give good performance after a long period of use.¹⁴⁷Table 3 compares the performance of various PV cell configurations manufactured with graphene, its derivatives, and other materials.

Table 3. Comparison of PV Device Performance Manufactured with Graphene, Its Derivatives, and Other Materials.

		device performance parameters
cell configuration	role of GA	Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)	synthesis methods	ref
FTO/graphene/titanium dioxide/perovskite/spiro-OMeOTAD/gold	ETM	21.9	1.04	73	15.6	not available	(138)
FTO/conductive titanium dioxide/mesoporous titanium dioxide/graphene oxide/perovskite/spiro-OMeOTAD/gold	ETM	17.06	0.937	63.5	10.15	facile electrochemical method	(163)
FTO/conductive titanium dioxide/mesoporous titanium dioxide/methylammonium lead iodide/reduced graphene oxide/gold	HTM	11.5	0.95	60.54	6.62	reduction method	(164)
FTO/conductive titanium dioxide/reduced graphene oxide: mesoporous titanium dioxide/perovskite/spiro-OMeOTAD/gold	ETM	21.98	1.11	80	19.53	adapted Hummers method	(165)
ITO/tin dioxide: graphene quantum dots/methylammonium lead iodide/spiro-OMeTAD/gold	ETL	23.05	1.134	77.8	20.31	the chemical-bath deposition, spin-coating method, and atomic layer deposition (ALD) processes	(166)
graphene/molybdenum trioxide/poly(3,4-ethylenedioxythiophene):methylammonium lead iodide/zinc-stabilized sulfonate/fullerene (C60)/bathocuproine (BCP)/aluminum (Al)/lithium fluoride (LiF)	ETL	21.2	0.96	70	14.2	chemical vapor deposition (CVD) technique and the Lewis base adduct method	(167)
ITO/poly (3,4-ethylenedioxythiophene) having poly (styrene sulfonate) and graphene oxide (PEDOT:PSS-GO)/methylammonium lead iodide-chloride (CH3NH3PbI3–xClx)/PCBM/spiro-OMeTAD/bis(4-tert-butylphenyl)-benzidine (sBphen)/silver (Ag)	HTL	22.06	1.03	71	16.11	adapted Hummers method	(168)
graphene/molybdenum trioxide mixed with PEDOT:PSS/methylammonium lead iodide (CH3NH3PbI3)/C60/bathocuproine (BCP)/lithium fluoride (LiF)/Al	ETL	21.5	1.06	75	17.1	Lewis bases adduct method	(169)
FTO/conductive titanium dioxide/mesoporous titanium dioxide/graphene oxide-lithium/perovskite/spiro-OMeOTAD/gold	ETM	19.61	0.859	70.3	11.8	solution-process methods	(170)
FTO/ITO/graphene oxide (GO)/methylammonium lead iodide-chloride (CH3NH3PbI3–xClx)/phenyl-C61-butyric acid methyl ester (PCBM)/zinc oxide (ZnO)/gold (Au)	HTM	17.46	1.00	71	12.4	modified Hummers method (MHM) solution-precipitation process	(168)
FTO/titanium dioxide/titanium dioxide-perovskite/perovskite/graphene	ETL	16.7	0.943	73	11.5	MHM	(171)
FTO/titanium dioxide/perovskite/spiro-OMeOTAD/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/graphene	ETL	17.75	0.945	71.72	12.03	chemical vapor deposition (CVD) method layer-by-layer stacking method	(172)
ITO/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/self-stacked solvated graphene (SSG)/methylammonium lead iodide-chloride (CH3NH3PbI3–xClx)/phenyl-C61-butyric acid methyl ester (PCBM)/aluminum (Al)	HTM	22.6	0.92	70.1	14.51	chemical vapor deposition (CVD), simple vacuum filtration, and the hydrothermal approach	(173)
graphene/molybdenum trioxide mixed with poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)/methylammonium lead iodide(CH3NH3PbI3) /fullerene (C60)/bathocuproine (BCP)/lithium fluoride (LiF)/aluminum (Al)	ETL	22.5	1.03	74.3	17.3	one-step fast deposition crystallization method	(174)
reduced graphene oxide (rGO) coated with mesoporous titanium dioxide (mp-TiO2), deposited on top of a blocking layer of titanium dioxide (TiO2)/methylammonium lead iodide (CH3NH3PbI3)/spiro-OMeTAD/lithium bis (trifluoro-methane sulfonyl)imide (Li-TFSI)/tert-butyl pyridine (TBP)/silver (Ag)	ITEM	20.02	0.89	71.2	12.7	sequential deposition method and spin-coating method	(164)
doped amorphous silicon (a-Si) solar cell with an interdigitated back contact (IBC) structure, utilizing a transparent conductive oxide (TCO) layer	ETL/NIL	42.3	0.744	83.8	26.3 ± 0.5	PECVD method or hot-wire CVD method	(175)
PERL and PERT(either localized p+ doping (PERL) or diffused p+ doping (PERT)/inverted pyramid, passivation	NIL	42.20	706.0	82.80	24.70	sequential deposition method	(176)

PSCs have impressive performance that outperforms organic PV cells, quantum dots PV cells, and DSSCs.¹⁴⁸ However, certain drawbacks have also been observed in PSC devices’ material stability and mechanism. The performance of these devices decreases over time because of various influencing parameters such as exposure to thermal and moisture conditions, chemical reactions at the interfaces, etc.¹⁴⁸ The GA-based integration with PSCs helps to improve its device stability and performance. Table 4 reviews the GA-based PSCs device accomplishment and work strength.

Table 4. A Brief Review of the GA-Based PSCs Device Performance and Its Stability¹⁴³.

device architecture	types	device stability performance	ref
FTO/titanium dioxide (TiO2)//methylammonium lead iodide (MAPbI3) with graphene nanofibers/spiro-OMeTAD/gold (Au)	perovskite	the device’s initial PCE was found to be 94% after 150 h of working and reduced to 89.5% after 300 h having 85% RH	(12)
ITO/graphene oxide (GO)/methylammonium lead iodide (MAPbI3) with graphene oxide (GO)/phenyl-C61-butyric acid methyl ester (PCBM)/silver (Ag)	perovskite	more than 2000 working hours, the power conversion efficiency (PCE) of the device was 80% of its initial PCE under 50% RH	(177)
FTO/block titanium dioxide/mesoporous titanium dioxide/formamidinium-methylammonium lead iodide-bromide (FA0.85MA0.15Pb(I0.85Br0.15)3) with nitrogen-doped reduced graphene oxide (N-rGO)/spiro-OMeTAD/gold (Au)	perovskite	not available	(178)
FTO/conductive titanium dioxide/mesoporous titanium dioxide/methylammonium lead iodide (MAPbI3) with graphene quantum dots (GQD)/spiro-OMeTAD/gold (Au)	perovskite	not available	(179)
FTO/alpha-iron(III) oxide (α-Fe2O3)/methylammonium lead iodide (MAPbI3) with nitrogen-doped surface-functionalized graphene quantum dots (NSGQDs)/hole transport layer (HTL)/gold (Au)	perovskite	humidity stability improved by 78% after 400 h, and its thermal stability improved by 84% after 300 h compared to the initial PCE condition	(180)
FTO/conductive titanium dioxide/mesoporous titanium dioxide with graphene/perovskite/graphene oxide/spiro-OMeTAD/gold (Au)	ETM TiO2	under continual light exposure at the peak power point, the device retained over 88% of its initial PCE after 16 h of testing; when it was exposed to thermal stability testing at 60 °C in an oven scenario, it lost 15% of its PCE efficiency	(181)
FTO/conductive titanium dioxide/mesoporous titanium dioxide with graphene/graphene oxide-lithium/methylammonium lead iodide (MAPbI3)/spiro-OMeTAD/gold (Au)	ETM TiO2	not available	(182)
FTO/conductive titanium dioxide/mesoporous titanium dioxide with graphene/methylammonium lead iodide (MAPbI3) /spiro-OMeTAD/gold	ETM TiO2	the device performance was 93% after 1 week of working in dry conditions at 30 °C in a dark place	(183)
FTO/conductive titanium dioxide/mesoporous titanium dioxide with graphene/zirconium dioxide (ZrO2)/methylammonium lead iodide (MAPbI3)/carbon (C)	ETM TiO2	not available	(184)
FTO/conductive titanium dioxide/reduced graphene oxide with lithium-doped mesoporous titanium dioxide/(FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/gold (Au)	ETM TiO2	not available	(185)
FTO/conductive titanium dioxide/reduced graphene oxide with titanium dioxide/(FAPbI3)0.85(MAPbBr3)0.15/spiro-OMeTAD/gold (Au)	ETM TiO2	not available	(186)
FTO/conductive titanium dioxide/mesoporous titanium dioxide/graphene oxide-lithium/methylammonium lead iodide (MAPbI3)/spiro-OMeTAD/gold (Au)	ETM TiO2	the stability of the device is improved when exposed to white light from a light-emitting diode (100 mW cm–2) at room temperature for a continuous period of 60 h	(187)
FTO/nickel oxide (NiO)/graphene oxide (GO)/perovskite/graphene oxide-lithium (GO-Li)/titanium oxide (TiOx)/aluminum (Al)	ETM TiO2	after 15 days of working, the device lost 30% of its efficiency with the ambient conditions of humidity and temperature	(188)
FTO/zinc oxide (ZnO) with graphene (G)/zinc oxide (ZnO)/methylammonium lead iodide (MAPbI3)/spiro-OMeTAD/gold (Au)	ETM ZnO	not available	(189)
FTO/conductive nanographene (NG) with zinc oxide nanorod nanocrystals (ZnO NR NCs)/methylammonium lead iodide (MAPbI3)/spiro-OMeTAD/silver (Ag)	ETM ZnO	not available	(190)
FTO/zinc oxide (ZnO)/multilayered graphene (MLG)/methylammonium formamidinium perovskite (MAFA)/spiro-OMeTAD/gold (Au)	ETM ZnO	the performance of the device was reduced by 7% of the initial PCE under continuous light within 300 working hours	(190)
FTO/graphene-tin dioxide (G-SnO2)/methylammonium lead iodide (MAPbI3)/spiro-OMeTAD/gold (Au)	ETM SnO2	90% of the initial PCE value was maintained after 300 h of exposure with humidity levels of 40 ± 5% and room temperature	(191)
EMMBF4/Ag/graphene nanosheets (GNs)/tin dioxide (SnO2)/C60 self-assembled monolayer (C60-SAM)/methylammonium lead iodide (MAPbI3)/spiro-OMeTAD/gold (Au)	ETM SnO2	not available	(192)
ITO/tin dioxide nanoparticles with niobium doping (SnO2:NGO) /Rb0.05(FA0.83MA0.17) 0.95Pb(I0.83Br0.17)3-cesium iodide (CsI)/spiro-OMeTAD/gold (Au)	ETM SnO2	the addition of NGOs did not impact the device stability under ambient conditions	(193)
FTO/graphene-embedded tin dioxide (G@SnO2)/cesium formamidinium methylammonium perovskite (CsFAMA-perovskite)/spiro-OMeTAD/gold (Au)	ETM SnO2	not available	(194)
PEN (polyethylene naphthalate)/ITO (indium tin oxide)/graphene-embedded tin dioxide (G5@SnO2)/cesium formamidinium methylammonium perovskite (CsFAMA-perovskite)/spiro-OMeTAD/gold (Au)	ETM SnO2	not available	(146)
ITO/poly(3,4-ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS)/methylammonium lead iodide-Chloride (MAPbI3–xClx)/reduced graphene oxide:phenyl-C61-butyric acid methyl ester (rGO:PCBM)/polyethylene naphthalate (PEN)/silver (Ag)	PCBM	50% of initial PCE was maintained after 50 working hours with prolonged solar illumination and relative humidity (>50%)	(195)
ITO/graphene quantum dots:phenyl-C61-butyric acid methyl ester (GQD:PCBM)/methylammonium lead iodide (MAPbI3)/spiro-OMeTAD/gold (Au)	PCBM	the device maintained more than 80% of the original PCE value for 300 working hours with simulated sunlight and full ultraviolet component presence	(196)
APTES-functionalized graphene (APTES-GR)/phenyl-C61-butyric acid methyl ester:graphene quantum dots (PCBM:GQDs)/gold (Au)	PCBM	not available	(197)
ITO/titanium dioxide (TiO2)/methylammonium lead iodide chloride (MAPbI3Cl3–x)/reduced graphene oxide (rGO)/spiro-OMeTAD/gold (Au)	HTM spiro-OMeTAD	the device containing rGO shows higher stability while kept under constant sunlight in a nitrogen-filled glovebox	(198)
ITO/tin dioxide (SnO2)/methylammonium formamidinium perovskite (MAFA-perovskite)/reduced graphene oxide (rGO) mixed with spiro-OMeTAD/gold (Au)	HTM spiro-OMeTAD	the efficiency of 75% of its initial PCE values was maintained after 500 working hours in ambient conditions (40–60% humidity)	(199)
ITO/poly(3,4-ethylene dioxythiophene):polystyrenesulfonate:graphene (PEDOT:PSS:G)/methylammonium lead iodide (MAPbI3)/phenyl-C61-butyric acid methyl ester (PCBM)/ytterbium (Yb)/aluminum (Al)	PEDOT:PSS	enhanced stability of device at room operation conditions	(200)
ITO/graphene oxide (GO) mixed with poly(3,4-ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS)/(FAPbI3)0.85(MAPbBr3)0.15/phenyl-C61-butyric acid methyl ester (PC61BM)/bathocuproine (BCP)/silver (Ag)	PEDOT:PSS	not available	(201)
ITO/poly(3,4-ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS)/graphene oxide (GO)/methylammonium lead iodide (MAPbI3)/phenyl-C61-butyric acid methyl ester (PCBM)/silver (Ag)	PEDOT:PSS	the device performed at 83.5% of the original PCE after working for 39 days in the air	(202)
ITO/poly(3,4-ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS)/strontium-functionalized reduced graphene oxide (SrGO)/methylammonium lead iodide (MAPbI3)/phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine (BCP)/silver (Ag)	PEDOT:PSS	the device stability is enhanced in the ambient environment as per the ISOS-D-1 protocol	(203)
ITO/reduced graphene oxide (rGO)/copper thiocyanate (CuSCN)/methylammonium lead iodide (MAPbI3)/phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine (BCP)/silver (Ag)	CuSCN	90% of the device’s initial PCE efficiency was maintained in AM 1.5 sunlight exposure for about 100 working hours	(204)
FTO/titanium dioxide (TiO2)/cesium formamidinium methylammonium lead iodide bromide (CsFAMAPbI3–xBrx)/copper thiocyanate (CuSCN)/reduced graphene oxide (rGO)/gold (Au)	CuSCN	the device maintained 95% of PCE after aging for 1000 h under full-sun light exposure at 60 °C temperature	(205)
FTO/titanium dioxide (TiO2)/cesium formamidinium methylammonium lead iodide bromide (CsFAMAPbI3–xBrx)/copper thiocyanate (CuSCN)/graphene (G)/gold (Au)	CuSCN	the device maintained more than 94% of its actual PCE in relative humidity of 50% in dark conditions	(206)
ITO/graphene oxide (GO)/methylammonium lead iodide (MAPbI3)/fullerene (C60)/bathocuproine (BPhen)/silver (Ag)	GA-based material as HTM	not available	(207)
ITO/amorphous graphene oxide (a-GO)/methylammonium lead iodide-chloride (MAPbI3–xClx)/phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine (BCP)/silver (Ag)	GA-based material as HTM	the device retained 90% of its initial power conversion efficiency (PCE) after operating for 30 days in a dry nitrogen gas atmosphere	(208)
ITO/reduced graphene oxide (rGO)/methylammonium lead iodide (MAPbI3)/phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine (BCP)/silver (Ag)	GA-based material as HTM	the device preserved 62% of its power conversion efficiency (PCE) after 140 h of exposure to light without encapsulation under ambient conditions with around 50% humidity	(209)
ITO/reduced graphene oxide (rGO)/methylammonium lead iodide (MAPbI3)/phenyl-C61-butyric acid methyl ester (PCBM)/silver (Ag)	GA-based material as HTM	the device sustained a 50% power conversion efficiency (PCE) after 1000 h of continuous operation without encapsulation at 25 °C and 30% relative humidity	(210)
ITO/fluorine-functionalized reduced graphene oxide (F-rGO)/methylammonium lead iodide (MAPbI3)/phenyl-C61-butyric acid methyl ester (PC61BM)/bathocuproine (BCP)/silver (Ag)	GA-based material as HTM	the power conversion efficiency (PCE) remained at 72% of its initial value for a duration of 30 working days under ambient conditions, with a humidity level of 50% and a temperature of 25 °C	(211)
FTO/titanium dioxide (TiO2)/methylammonium lead iodide (MAPbI3)/reduced graphene oxide (rGO)/gold (Au)	GA-based material as HTM	the device maintained 50% of its initial power conversion efficiency (PCE) value for 30 days in an open ambient atmosphere without any encapsulation	(212)

Ramamoorthy et al.¹⁴⁹ reported that GA mixed TiO₂ thin layer blocking enhanced the efficiency of the DSSC cells. The porous nature of TiO₂ helps attach more dye molecules to its surface, improving the PCE. The Ti-GA composite functioned as a blocking layer, the metallic contact between TiO₂ and FTO glass, improving the device’s performance by easing the interfacial resistance. The efficiency is also dependent on the doping percentage of GA in TiO₂. It was reported that the device’s efficiency was enhanced from 2.49% to 5.09% due to fabrication in the blocking layer form.¹⁴⁹

Several techniques, namely, coating, atomic layer deposition, electrochemical coating, and sputtering, are followed to develop blocking layers.¹⁵⁰ The efficiency of DSSC depends upon back charge transfer losses at direct (for TiO₂/dye/electrolyte) and indirect (TCO/electrolyte) pathway interfaces, respectively. The thin blocking layer suppressed this back-charge transfer and enhanced the performance of DSSC.^127,151 It was also observed from the I–V characteristic that the PCE of the blocking layer solar cell obtained 5.09%. That shows the 49% increment without blocking layer cells.¹⁵¹

The electron transport in TiO₂ thin films is inhibited due to charge confining, recombination, and scattering effects of grain boundary.^152,153 Adding rGO into TiO₂ films can efficiently address these issues and enhance radiation absorbance and carrier mobility with less recombination of charge carriers as compared to native TiO₂ films.¹⁵⁴ Photoanodes comprising Au nanoparticles (GNPs) in the TiO₂ layer unavoidably experience oxidation at high temperatures; hence, GNPs turn into reduced GO.¹⁵⁵ Morphological analysis revealed that defectless thin layers were produced due to the integration of rGO. The band gap value found in the TiO₂–rGO layer was 3.34 eV, while it was 3.37 eV for TiO₂.¹⁵⁴

5. GA and Other Members of 2D Materials for Advanced Solar Cells

GA belongs to the 2D materials family; other 2D materials have gained attainment because of their optical and electronic characteristics for the evolution of PV cells and other optoelectronic devices. Some 2D materials with zero band gap material act as semiconductors. Such semiconductors are also utilized as thin, flexible PV cells.

Figure 14 depicted 2D materials for photovoltaic applications, categorized into GA and GA derivatives, and other 2D materials, including GA, transition-metal dichalcogenides (TMDCs), black phosphorus (BP), and boron nitrides. These materials play essential roles in enhancing the performance and stability of thin-film solar cells, presenting exciting opportunities for advancements in solar energy technology.

Figure 14.

ITO and FTO-based electrodes in the photovoltaic device.¹⁵⁶

Several reported works show that 2D materials, like TMDCs^157,158 and black phosphorus,¹⁵⁹ have evolved rapidly because of their exceptional mechanical characteristics, light weight, high tensile strength, and high young modulus. This helps in designing flexible and stretchy next-level electronic devices.¹⁶⁰ Si-based PV cells have gained popularity in renewable technology due to their high PCE and cheap electric power generation. The GA is also utilized as transparent and conductive electrodes in solar cell technology. The conductivity of GA improves with a rise in the number of layers due to poor transparency. Thus, a proper trade-off should be done to achieve the optimum performance of GA electrodes.¹⁶⁰

The sheet resistance of GA is lowered through its doping process to optimize the inner potential of Si-PV cells. The ambipolar characteristic of gallium arsenide (GA) enables the incorporation of both an anode and a cathode, as well as electron transport layers (ETL) or hole transport layers (HTL), in organic and perovskite photovoltaic (PV) cells. Furthermore, there is a strong likelihood that gallium arsenide (GA) will play a crucial role as a ternary substance in organic photovoltaic (PV) cells and as a material employed for stabilization.¹⁶¹ In recent years, significant research has been conducted on using 2D materials in solar cell applications, particularly on molybdenum disulfide, molybdenum diselenide, and tungsten diselenide.¹⁶² These materials exhibit advantageous properties due to their elevated absorption coefficient within the visible and infrared regions of the solar spectrum. The optimization of the work function, as well as the bandgap of transition metal dichalcogenides (TMDCs), has been found to have potential applications in organic and perovskite photovoltaic (PV) cells.¹⁶² The photovoltaic cells based on 2D heterostructures have superior absorption coefficients, elevated radiative efficiency, and well-defined interfaces, resulting in the highest power-to-weight ratio of the active material. The integration of roll-to-roll treated graphene oxide into current photovoltaic technologies is currently a prominent area of research.¹⁶²

6. Conclusions and Future Remarks

This Review comprehensively analyzes the utilization of GA and its derivatives GO, rGO, and GDs over conventional solar photovoltaic cells. GA has exceptional characteristics, including the absence of bandgaps, elevated carrier mobility, expansive surface area, optical responsiveness, and advantageous thermomechanical properties. The various synthesis techniques of GA are described with their limitations. This Review presents several significant findings and offers insights for future research. The transparency and transmittance of GA as the monolayer or multilayer depend on their preparation technique. The many techniques of GA preparation, like the Hummers method, epitaxial growth, and CVD technique, are proposed in this Review. By the adoption of an appropriate synthesis technique, the transparency of the GA layer can be increased, which would improve the PCE of the solar cell. GA offers a 2D arrangement of carbon atoms, a large surface area with transparency capable of encapsulating solar cells.

Regardless of remarkable progress in GA-based solar cells, the mass production of graphene is still more challenging. The introduction of thin homogeneous GA layers on the substrate is another issue. Currently, many researchers are working on GA and other 2D materials to develop new synthesis methodologies of GA derivates and novel fabrication techniques of PV cells to achieve cost-effective, flexible, foil-type thin advanced PV cells that would be easy to install and are also ecofriendly.

Glossary

Nomenclature

T

transmittance

R_s

sheet resistance

2D

two-dimensional

0D

zero-dimensional

PV

photovoltaic

DC

direct current

BHJ

bulk heterojunction

GA

graphene

GO

graphene oxide

CVD

chemical vapor deposition

DSSCs

dye-sensitized solar cells

LUMO

lowest unoccupied molecular orbitals

HOMO

highest occupied molecular orbitals

PCBM

phenyl-C₆₁-butyric acid methyl-ester

PEDOT:PSS

poly(3,4-ethylene-dioxy-thiophene):poly-styrenesulfonate

P3HT

regioregular poly(3-hexylthiophene-2,5-diyl)

ITO

indium tin oxide

FTO

fluorine-doped tin oxide

TCO

transparent conductive oxide

OSCs

organic solar cells

PCE

power conversion efficiency

TiO₂

titanium dioxide

ZnO

zinc oxide

SnO₂

tin(IV) oxide/stannic oxide

rGS

reduced-GA scaffold

EQE

external quantum efficiency

rGO

reduced graphene oxide

ALD

atomic layer deposition

QDSCs

quantum dot solar cells

GPa

gega pascal

TPa

tera pascal

OPVs

organic photovoltaics

ETM

electronic transfer material

HTM

hole transfer material

Elect

electrode

ETL

electron transport layer

HTL

hole transport layer

PVA

poly(vinyl alcohol)

Si

silicon

TMDCs

transition-metal dichalcogenides

GNPs

photoanodes comprising Au nanoparticles

Au

gold

Cu

copper

Ni

nickel

GIC

graphite intercalation compounds

LiF

lithium fluoride

TiO_x

titanium oxide

The authors declare no competing financial interest.