Solution-Processed Graphene-Based Transparent Conductive Electrodes as Ideal ITO Alternatives for Organic Solar Cells

The isolation of free-standing graphene in 2004 was the spark for a new scientific revolution in the field of optoelectronics. Due to its extraordinary optoelectronic and mechanical properties, graphene is the next wonder material that could act as an ideal low-cost alternative material for the effective replacement of the expensive conventional materials used in organic optoelectronic applications. Indeed, the enhanced electrical conductivity of graphene combined with its high transparency in visible and near-infra - red spectra, enabled graphene to be an ideal low-cost indium tin oxide (ITO) alternative in organic solar cells (OSCs). The prospects and future research trend in graphene- based TCE are also discussed. On the other hand, solution-processed graphene com bines the unique optoelectrical properties of graphene with large area deposition and flexible substrates making it compatible with printing and coating technologies, such as roll-to-roll, inkjet, gravure, and flexographic printing manufacturing methods. This chapter provides an overview of the most recent research progress in the application of solution-processed graphene-based films as transparent conductive electrodes (TCEs) in OSCs. (a) Chemically converted graphene (CCG), (b) thermally and photochemically reduced graphene oxide, (c) composite reduced graphene oxide-carbon nanotubes, and (d) reduced graphene oxide mesh films have demonstrated their applicability in OSCs as transparent, conductive electrodes.

Transparent conductive electrodes (TCEs) with an outstanding combination of high electrical conductivity and good optical transparency have a crucial role in OSCs, as the first layer of the device that the incident solar electromagnetic radiation should transmit in order to be absorbed by the solar cell's active medium [13][14][15]. Conventionally, the most common materials are indium tin oxide (ITO) and fluorine tin oxide (FTO), which are widely used as the transparent electrode in many optoelectronic devices because of their good combination of high transparency and low resistance [16][17][18]. However, these metal oxides exhibit several issues due to the high cost, resulting mostly from the indium scarcity [19], their brittleness [20], the device degradation due to indium diffusion into the photoactive layer [21], and the requirement for high cost coating methods [20]. Significant effort toward the search of alternatives has therefore been motivated [15,[22][23][24] and several alternative materials have been proposed, including metallic nanowires (NWs), nanostructured carbon, and conductive polymers, among others [24,25]. Therefore, developing new materials combining most desirable properties for transparent electrodes will contribute to satisfy the increasing demand for lowcost solution-processed flexible devices. Due to its exceptional properties, graphene is highly attractive and is believed to be the next wonder material for optoelectronics; thus, triggering its application as a transparent electrode for flexible energy-harvesting devices [26].
Graphene production includes several methods. Depending on its quality, the properties of the synthesized graphene may differ. More specifically, the grain size, the shape, the thickness as well as the presence of defects influence graphene mechanical properties. From a highly ordered pyrolytic graphite (HOPG) through a mechanical exfoliation method ("scotch-tape"), originally graphene was produced [33]. Since then, the number of methods for graphene production has significantly increased. Some of them include liquid phase exfoliation (LPE) [34], chemical vapor deposition (CVD) [35], preparation from organic materials using solvothermal method [36], chemical cross-linking of polycyclic aromatic hydrocarbons [37], preparation from SiC through thermal decomposition [38], and carbon nanotube unzipping [39]. Although CVD method produces the less defective graphene films, it exhibits technical issues relating with the deposition of graphene films on flexible substrates making this method incompatible with roll-to-roll (R2R) mass production processes. On the contrary, the fact that graphene oxide (GO) can be produced in large quantities from low-cost graphite powder and its property to give stable dispersions in various solvents, make GO an ideal candidate for solutionprocessed graphene production [40]. On top of that, it can be the precursor for conductive inks production, adding to the printing electronics technology [27]. However, the introduction of oxygen functional groups on the GO lattice, disrupts the sp 2 conjugation system of the hexagonal graphene lattice, making GO an insulator. By removing the oxygen containing groups, GO can be partially reduced to conductive graphene-like sheets. The reduction process can be performed by chemical [41], thermal [42], or photochemical treatment [43], aiming to yield reduced graphene oxide (rGO) with similar properties to graphene [41].
This chapter covers the latest advances in solution-processed graphene-based thin films as the anode TCE in OSCs, substituting the conventional ITO. We highlight the latest advances on thermally, chemically and photochemically produced conductive graphene-based nanomaterials, as well as on graphene-based nanocomposites TCE films. In addition, we summarize some promising routes for the graphene-based TCEs treatment that advance their optoelectrical properties tailoring and achieve a balance between the sheet resistance (R s ) and transmittance of the solution-processed graphene-based TCEs. ITO replacement in graphene-based TCEs, establishes the era of lightweight, low cost, extended lifetime, and stability, as well as flexible and stretchable OSCs.

Applications of graphene-based TCE in OSCs
Numerous studies have aimed to investigate the key role of graphene-based materials as the anode (positive electrode) in OSCs. A TCE with efficient carrier transport and thus, high charge collection efficiency is required in OSC devices as outlined in Figure 1. Properties including high transparency (>80%), low sheet resistance (R s < 100/sq), suitable work function (4.5-5.2 eV) as well as low cost and compatibility with R2R fabrication processes characterize the ideal TCE material. Despite the unique optoelectrical properties (T > 90%, R s = 10-20 Ω/sq) of ITO and its ideal work function, the abovementioned ITO disadvantages have intensified the research on graphene-based TCEs in OSCs. In this context, solution-processed graphenebased films meeting the standards for low electrical resistance, high optical transparency and tunable WF must be developed.

Chemically treated rGO transparent conductive electrodes
In 2008, Wu et al. compared the electrical properties of rGO films prepared according to two different reduction methods: chemical and thermal treatment [44]. As pristine material, graphene oxide (GO) prepared by Hummers' method was used. The conductivity properties of GO reduced by vacuum annealing at 1100°C, by a combination of hydrazine (N 2 H 4 ) treatment, and Ar annealing at 400°C were explored. The results displayed that the vacuum annealing reduction resulted in rGO films with slightly better transparency and conductivity compared to the films reduced using the chemical and thermal treatment combination. In both cases, the film surfaces were free from spikes that can cause short circuit in optoelectronic devices. This is an advantage of graphene-based electrodes over carbon nanotube or metal nanowire mesh electrodes, which require a thick, spin-coated polymer buffer layer in order to prevent shorts, generated by rough surfaces (spikes). The transmittance and R s with respect to the rGO film thickness with the two different fabrication methods are depicted in Figure 2. In a general term, for <20 nm film thickness, the optical transmittance was 480%, while the R s varied from 5 to 1 MΩ sq −1 . Bilayer small molecule OSCs were fabricated on rGO/quartz and on ITO/glass substrates. The fabricated cells of the structure rGO/copper phthalocyanine (CuPc)/fullerene (C 60 )/bathocuproine (BCP)/Ag achieved a power conversion efficiency (PCE) of ~0.4%, whereas the ITO-based devices exhibited a PCE of ~0.84%. The observed PCE difference is attributed to the higher R s of graphene-based electrodes compared to the ITO electrodes.
Yun et al. investigated the potential use of a solution-processed rGO thin film as transparent electrodes and characterized the effect of rGO films by tuning the film thickness and annealing treatment on the cell performances in OSCs [45]. Using a p-toluenesulfonyl hydrazide (p-TosNHNH 2 ) reducing agent, GO reduction took place. rGO dispersions with high concentrations and thin film processability were prepared as a result of the hydrazone groups attached to the edge and basal plane of graphene. In this way, issues created using other film preparation techniques (i.e., transfer problems during vacuum filtration) or high temperature reduction methods could be overwhelmed [46,47]. They also evaluated the morphological and optical properties of the solution-processed rGO thin films by tuning the fabrication conditions, such as spin-coating cycles and annealing temperatures. All rGO thin films fabricated through a spinning method showed a highly uniform morphology with thickness controllability. In addition, sheet resistance was efficiently decreased from ~103 to 10 kΩ sq −1 by the trade-off between the coating cycles and the annealing conditions. Finally, the effects of rGO-based electrodes on the OSC performances by controlling the number of coating cycles were studied. OSCs performance was improved upon decreasing the sheet resistance, for 200°C as the annealing temperature and seven times coated rGO film, achieving a PCE of 0.33%.
In 2015, Moaven et al. reported the synthesis of an rGO/Ag nanocomposite electrode appropriate for OSCs [48]. The reported technique is applicable to flexible substrates by spin coating of an aqueous solution of rGO/Ag nanocomposite on polyethylene terephthalate (PET) substrate in ambient conditions. The optical and electrical properties were determined by tuning the Ag concentration of the rGO/Ag nanocomposite and the electrode thickness. Flexible electrodes were prepared with R s as low as 83 kΩ sq −1 and with a transmittance of 47%. Finally, OSCs onto PET, based on rGO/Ag anode electrode in different ratios were fabricated. The highest PCE observed was 0.24% onto PET flexible substrates which was improved by 25% compared to the conventional ITO/glass (0.18%).
Huang et al. in another article, demonstrated the preparation of highly conductive and transparent graphene-based electrodes with tunable WFs by mixing single walled carbon nanotubes (SWCNTs) with chemically reduced GO [49]. More specifically, dry powders of GO and SWCNTs were directly dispersed in anhydrous hydrazine. The yielded composite was coated in different thicknesses (1-5 layers) and the optical and electrical properties of the fabricated films were characterized. The transmittance was directly affected by the number of spin-cast layers, as presented in Figure 3. By increasing the number of spin-cast layers from one to five, the optical transmittance at 550 nm was decreased from 88.8 to 58.7%. On the other hand, sheet resistance values of the rGO-SWCNT films were conversely affected upon the increase of the number of spin-cast layers reaching R s values as low as 254 Ω sq −1 . Furthermore, OSCs were fabricated using rGO-SWCNT films as the anode TCE. Through doping with alkali carbonates (Li, Na, Cs, etc.), it is possible to tune the WF of the solution-processed rGO-SWCNTs to match the lowest unoccupied molecular orbital (LUMO) of PC 61 BM. In this way, an improved ohmic contact with the active layer can be achieved, leading to increased charge injection and better device performance. On top of that, SWCNTs can act as conductive percolation paths that short circuit the rGO sheets. Inverted OSCs with structure of rGO-SWCNT/PEDOT:PSS/ P3HT:PC 61 BM/V 2 O 5 /Al were fabricated to demonstrate the applicability of WF tuning of rGO-SWCNT. The devices based on P3HT:PC 61 BM, incorporating the four-layer rGO-SWCNT film exhibited a maximum PCE of 1.27%. Additionally, an excellent flexibility even under bending angles of more than 60° was observed.

Thermally annealing rGO transparent conductive electrodes
The first application of solution-processed thermally reduced GO TCEs in OSC devices was performed by Yin et al. in 2010 [50]. In more detail, GO films were firstly spin-coated on SiO 2 /Si substrates, followed by reduction through thermal annealing at 1000°C in the presence of Ar/H 2 . Polymethylmethacrylate (PMMA) (~300 nm thick) was subsequently coated to be used as an intermediate transfer substrate before the final transfer of rGO films onto polyethylene terephthalate (PET) substrates. Flexible poly(3-hexylthiophene) (P3HT):phenyl-C 61 -butyric acid methyl ester (PC 61 BM)-based OSCs were fabricated on the rGO-coated PET substrates, and the electrical characteristics of the OSCs were investigated with respect to the thickness of the rGO electrode (Figure 4).
Higher device performance dependence on the rGO film R s was observed when the rGO films optical transmittance exceeded 65%, while the light transmission efficiency dominated the OSCs performance in lower optical transmittance (<65%). The optimum power conversion efficiency (PCE) was achieved for device 3 (PCE = 0.78%), slightly lower than the one reported for CVD-graphene/PET-based photovoltaic devices (1.18%) [15]. This results from the lower R s of the CVD grown graphene compared to the thermally derived rGO film.
rGO films were also investigated regarding their flexibility as displayed in Figure 5. BHJ OSC electrical properties incorporating rGO films as electrodes demonstrated excellent tolerance under high bending conditions and multiple number of bending cycles. The critical point beyond which the fabricated OSCs with rGO/PET films exhibited degradation due to bending depends on the rGO film thickness. In more detail, thicker rGO electrodes led to higher stability, when the OSCs were subjected to tensile stress. After various bending cycles, an electrical degradation of rGO-based OSCs was observed, which was analogous to the increased R s of the rGO/PET films. In particular, after 1600 bending cycles, the R s of the rGO/PET electrode increased from 16 to 18 kΩ sq −1 and 3.2 to 3.5 kΩ sq −1 , respectively, for the 4 and 16 nm thick rGO films.
In another study, Geng et al. reported a simple method for preparing graphene TCEs using seriatim a chemical and a thermal reduction method [51]. More specifically, a converted graphene (CCG) suspension obtained via controlled chemical reduction of exfoliated GO in the absence of dispersants was used as the pristine material and then, upon thermal annealing for 15 min at three different temperatures (200, 400, and 800°C) under vacuum in a furnace tube, the sp 2 carbon networks of the graphene sheets were recovered, with the resulting CCG films exhibiting an R s of the order of 103 Ω sq −1 at 50% transparency (at 550 nm). Owing to the apparently greater extent of sp 2 carbon networks restoration during the two-step reduction of the CCG films, each thermally annealed CCG film exhibited lower R s than the thermally annealed GO as depicted in Figure 6. The transparency of the CCG films decreases linearly with respect to the volume of the CCG suspension used to prepare the vacuum-filtered film, as also displayed in Figure 6. By increasing the annealing temperature, the transparency of the CCG films decreased due to thermal improvement of the sp 2 carbon networks in the CCG sheets.
The recession of the R s observed in Figure 6, as the annealing temperature increases, is attributed to the smaller induced distance owing to the elimination of the functional groups between the CCG sheets layers, which leads to facilitated charge carrier transfer across the CCG sheets. It is observed that interlayer distance in CCG-A800 films is reduced to 0.354 nm, approaching the value of bulk graphite. The transparency reduced from this point, while the R s was reduced linearly to the order of 10 3 Ω sq −1 , demonstrating that reduction impacted successfully and uniformly from the outermost to the inner layers [51]. The short circuit current density, overall power conversion efficiency (n), and sheet resistance R SR for devices 1 and 3, respectively, as a function of the number of bending cycles. Reprinted with permission from Yin et al. [50]. Copyright 2010 ACS American Chemical Society.
An important conductivity improvement was induced due to the structural changes in the CCG resulted from the thermal reduction. In particular, sp 2 carbon network recover was crucial for increasing the charge carrier transport in individual CCG layers, while the interlayer distance was decreased to a level close to the value of bulk graphite; thereby, improving the charge carrier transport across the CCG layers. As a proof of concept, CCG films were used as TCEs in P3HT:PC 61 BM-based OSCs achieving a maximum PCE of 1.01%, which is ~50% decreased compared to the PCE value in ITO-based devices, as presented in Figure 7.

Photochemically treated rGO transparent conductive electrodes
For the first time, Kymakis et al. presented a facile, laser-assisted technique to prepare transparent and highly conductive graphene-based films on top of flexible substrates, produced by spin-casting [52]. The experimental setup was realized with (1) a Ti:Sapphire pulsed laser source, (2) a 10 mm diameter lens, and (3) a high precision X-Y computer controlled translation stage where the film was placed and translated across the focused laser beam (Figure 8).  During the reduction process, the fs laser beam irradiated the as-spun GO layers, while it was translated across the layers. The reduction of GO can be obvious. Scan lines obtained upon irradiation with 100 fs pulses at different fluencies indicating the gradual color change due to the reduction are also presented in Figure 8. The brownish color of the as cast film was gradually turned into black, which is a strong indication that GO is rapidly reduced via the laser treatment in air, without using any reducing chemical agent. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis demonstrated that no or minor ablation effects occur during the reduction process.
The scope behind the production of solution cast laser reduced GO (LrGO) electrodes is to prepare highly flexible OSCs that can be utilized for compact R2R solar modules. In this context, the LrGO films were used to fabricate flexible OSC devices (Figure 9) in order to determine their photovoltaic characteristics and identify the combination of transparency and R s that provides the best performance. Compared to the thermally reduced GO-based  OSC devices (PCE ~ 0.78%) [50], LrGO-based OSCs exhibited 41% efficiency improvement for a 16.4 nm thick film having an R s of 1.6 kΩ sq −1 and 70% transparency. It is clear that the higher efficiency is attributed to the LrGO film, revealing the superiority of the laser ablation method, in contrast to the chemical method.
Since rGO TCEs presents low performance in terms of conductivity and transmittance (R s ~1 kΩ sq −1 , 70% transmittance) compared to the ITO TCEs (R s ~15 Ω sq −1 , 90% transmittance), new efforts are performed to improve graphene-based TCE performances. One very promising technique is the employment of a mesh structure with periodic lines on the rGO film [53], with OSC devices based on graphene mesh electrodes (GMEs) already exhibiting performances comparable to those using conventional ITO TCEs [54,55]. Therefore, it is possible to control the films R s and transparency by tailoring the grid width, spacing, and thickness [56].
In the same year, Zhang et al. demonstrated the preparation of graphene mesh electrodes (GMEs) by using the standard industrial photolithography and O 2 plasma etching process as illustrated in Figure 10 [57]. The R s and the transparency of the graphene TCEs before the mask-based etching were 150 Ω sq −1 and ~8%, while after the etching technique, the electrodes R s and transparency were measured to be 750 Ω sq −1 and 65%, respectively. The GME transparency was directly designated by (a) pit depth and (b) period of the mesh, while its conductance was mainly affected by the mesh (a) pit depth and (b) linewidth. By adjusting the duration of the O 2 process, the pit depth was controlled, with 4-10 min of O 2 plasma etching time required for high transparent mesh pits. To further investigate the appropriateness of the GMEs, OSC devices of the structure GME/PEDOT:PSS/P3HT:PC 61 BM/LiF/Al were fabricated. The devices employing optimum pit depth GMEs exhibited a PCE equal to 2.04% decreased by ~33% compared to the conventional ITO-based OSCs (PCE ~3%).
In 2015, Konios et al. presented a different laser-assisted method to directly pattern a mesh on the surface of rGO films deposited onto flexible substrates, for the production of large area reduced graphene oxide micromesh (rGOMM) electrodes [58]. By using this technique, the use of complicated photolithographic [59], ion beam [60], chemical etching [61], template [62] and O 2 plasma methods [52] was overtaken. The proposed technique is a one-step method, Figure 10. Illustration of the fabrication steps for the preparation of GMEs. Reprinted with permission from Zhang et al. [57]. Copyright 2013 Springer Berlin Heidelberg. since no TEM grid patterning mask [63], and no transfer step are needed. In addition, the method does not use any photo-resistive material [64] or pre-patterned elastomeric stamps [65], while the application of a fs laser pulses allows the patterning of micro size holes on top of any flexible low cost material. In addition, this technique can be easily controlled over the entire illuminated area induces minimum thermal damage in the surrounding layers and more importantly is compatible to R2R production processes.
The electrode transparency can be accurately controlled with a small R s increase, successfully handling with the existing trade-off between transparency and electrical conductivity. The rGO thin films optoelectrical properties of depended on the interplay between the periodicity and the geometrical characteristics of the mesh pattern structure. The rGO mesh electrode experimental setup and SEM images of the laser-induced patterns are presented in Figure 11. The proposed technique main advantage is that it permits fine-tuning of the optoelectrical properties via variation of the irradiation dose (energy, number of pulses) and/ or the periodicity and thus the neck width of the mesh. A significant improvement of the transmittance by ~65% was performed after laser treatment. As a proof of concept, reduced GO micromesh (rGOMM) were employed as the anode TCE in flexible devices based on poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl20,10,30-benzothiadiazo-le)] (PCDTBT): [6,6]-phenyl C71 butyric acid methyl ester (PC 71 BM) and compared with those deposited on ITO (Figure 12).  The optimum photovoltaic parameters for the rGOMM-based devices were extracted for transparency ~59.1% and R s ~565 Ω sq −1 , with the resulting PCE of 3.05%, the highest reported so far for flexible OSC devices incorporating solution-processed graphene-based electrodes. Another important issue investigated in this study was the determination of the effectiveness of the proposed method when tested in large area photovoltaic cells, as presented in Figure 12. This is very important for upscaling from lab solar cells to solar modules. In this context, stress tests demonstrated that the photovoltaic performance deterioration for both rGOMM and ITO electrodes tested was almost the same; for 135 mm 2 active area PCE reduction measured was 63.2% for ITO and 64.9% for rGOMM, respectively. Therefore, the proposed method can be effectively applied when upscaling to large area photovoltaic cells or solar modules without compromising the photovoltaic efficiency compared with the widely commercialized ITO transparent electrode.
During the last 5 years, an intensive research effort was conducted in the field of solutionprocessed graphene-based TCEs (Figure 13). The optoelectrical properties of graphene-based TCEs in OSCs are presented in Table 1. The increase of the PCE of OSC devices incorporating graphene-based TCEs has been improved from 0.13 to 3.7% for rigid devices, while the progress on the flexible graphene-based devices was also incredible with an efficiency improvement from 0.78 to 3.05%.

Conclusion
In this chapter, the concept of solution-processed graphene-based films as anode electrodes in OSCs has been presented, with respect to the fabrication techniques used: (a) thermally reduced graphene, (b) chemically and photochemically rGO, (c) composite rGO-CNTs, and (d) rGO mesh  rGO-SWCNTs 0.331 65.8 [49] rGO mesh electrodes 0.700 65 [57] LrGO mesh electrode 0.565 59.1 [58] rGO/Ag NPs 83 47 [48] Reprinted with permission from Petridis et al. [26]. Reproduced by permission of The Royal Society of Chemistry. films. The application of rGO-based TCEs in OSCs was also demonstrated. The successful transferring of such schemes in large scale poses a significant engineering challenge and ongoing effort is needed for preparing high efficient printed optoelectronic devices such as flexible and stretchable OSCs. Despite graphene-based materials advantages regarding the manufacturing and purchase cost, compatibility with flexible materials, WF tuning (by controlling graphene size, layer and through functionalization), transparency, and solubility, significant improvement is required in order to overcome the corresponding properties of ITO films. Even though the current performance of graphene-based TCE OSCs approaches that of the conventional ITO technology, there is more space for the improvement of OSCs adapting graphene as an anode electrode.
Future research efforts should be focused on (a) inventing cost-effective techniques for large-scale graphene or rGO production, (b) effectiveness improvement of the GO reduction methods, (c) enhancing the electrical conductivity of the graphene-based TCEs without disrupting their transmittance, and (d) improving the graphene-based films morphology. For improving the TCE conductivity, metal nanoparticles (e.g., Au, Ag, and Al) with different sizes and shapes can be incorporated in the graphene-based electrodes, leading to the production of graphene-NPs electrodes with even lower resistance and high mechanical stability; and thus, may improve the overall stability of the devices, contributing to the lifetime enhancement. On top of that, hybrid films consisting of rGO and 1D conductive bridging materials, such as metal nanowires, which exhibit better mechanical properties than ITO films, can significantly decrease the film resistance. The metal nanowires can provide electric pathways among the graphene layers and thus may enhance the conductivity, without sacrificing their optical properties, leading to improved performance of graphene-based TCEs. The incorporation of metal-graphene-based TCEs into optoelectronic devices demonstrates their potential for ITO replacement in a broad range of optoelectronic applications.