The facile chemical functionalization enables graphene as a new candidate for active material in photovoltaic device applications.[128-130] Moreover, the work function of graphene can be adjustable by doping with another semiconductor to form a Schottky junction if their energy band structures match.[131-135] With high charge mobility, graphene and rGO are good candidates for the electron transport layer,[136-140] hole transport layer,[141, 142, 144, 145] both hole and electron transport layer, and interfacial layer for tandem solar cells in the photovoltaic field.[146-148] The previously reported photovoltaic devices that use graphene or graphene-based composites in active layers have been summarized in Table 2. Some typical researches will be reviewed as follows.
2.2.1 Light-Harvesting Materials
GO is easy to be functionalized based on various requirements since it has various functional groups. For example, Chen and co-workers functionalized GO sheets with phenyl isocyanate, which changed hydrophilic GO surface to hydrophobic one. The resultant solution-processed functionalized graphene (SPFGraphene) was mixed with poly(3-octylthiophene) (P3OT) to form the P3OT/SPFGraphene composites, which were then used as the active layer material in the bulk heterojunction (BHJ) OPV device (Figure 6a–d). The annealing conditions are critical for better performance of the device, since annealing can remove the functional groups from graphene sheets and enhance the crystallinity of P3OT. Based on their result, the optimized annealing condition, 160 °C for 20 min, was applied for fabrication of the OPV device, which achieved the best power conversion efficiency of 1.4%. This work indicated that the functional graphene can serve as a competitive alternative to [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor for high-performance OPV devices.
Figure 6. a) The idealized chemical structures of graphene and P3OT. b) Schematic illustration of the device with P3OT/graphene thin film as the active layer and the structure ITO (ca. 17 Ω/sq)/PEDOT:PSS (40 nm)/P3OT:graphene (100 nm)/LiF (1 nm)/Al (70 nm). c) Energy level diagram of P3OT and SPFGraphene. d) Schematic representation of the reaction of phenyl isocyanate with GO to form SPFGraphene. Reproduced with permission.
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Importantly, the effect of graphene with different lateral size in OPV devices was studied. In this work, the active layer of OPV device was composed of aniline-functionalized graphene (used as electron acceptor) and P3HT (used as electron donor). It was found that after optimization, the device with aniline-functionalized graphene quantum dots (ANI-GQDs) and P3HT showed enhanced efficiency as compared to the one with aniline-functionalized graphene sheets (ANI-GS) and P3HT. The corresponding current density versus voltage curves of ANI-GQDs-P3HT and optimized ANI-GS-P3HT based devices are plotted in Figure 7a. The maximum power conversion efficiency is 1.14% obtained from ANI-GQDs with 1 wt% of ANI-GQD and P3HT, which is much higher than 0.65% obtained from the optimized ANI-GS devices with 10 wt% ANI-GSs and P3HT. This is attributed to the improved morphological and the optical characteristics in ANI-GQDs. The performance of GQD-based devices is expected to be further improved by choosing other proper functionalization systems.
Figure 7. a) J–V characteristics of the photovoltaic devices based on ANI-GQDs with different amount of GQDs in ANI and ANI-GS with 10 wt% GS in ANI (under optimized condition) annealed at 160 °C for 10 min. Reproduced with permission. Copyright 2011, American Chemical Society. b) J–V characteristics of a typical nanocrystalline TiO2 solar cell sensitized by GQDs, in the dark and under illumination, respectively. Reproduced with permission. Copyright 2010, American Chemical Society.
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In addition, Li and co-workers presented a novel solubilization strategy for synthesis of graphene nanostructures through a bottom-up method instead of the common top-down method based on the exfoliation of graphite. Solution-processable black GQDs with uniform size were synthesized through solution chemistry, which were then used as a sensitizer for solar cells (Figure 7b). However, a much low current density was observed, which was attributed to the low affinity of GQDs on TiO2 surface due to the physical adsorption, and the consequent poor charge injection. In the future, synthesis of hydrophilic graphene nanostructures or realization of the chemical bonding between graphene nanostructures and TiO2 surface is expected to improve the device performance.
2.2.2 Schottky Junctions
The metallic graphene can form the Schottky junction with semiconductor and employed as the active layer for solar cells. Qin and co-workers have developed a simple and scalable patterning method for graphene using electron-beam or ultraviolet lithography followed by a lift-off process. The patterned graphene was used for fabrication of CdSe nanobelt (NB)/graphene Schottky junction solar cells. An ideal Schottky junction was formed between metallic graphene and semiconducting CdSe NB, which facilitates the electron-hole separation and diffusion driven by the built-in potential between graphene and CdSe. Accordingly, an excellent photovoltaic with an open-circuit voltage of ≈0.51 V, a short-circuit current density of ≈5.75 mA cm–2 and an overall solar energy conversion efficiency of ≈1.25% has been obtained.
Similarly, solar cells based on Schottky junctions between graphene sheets (GSs) and n-type doped Si (n-Si) have been developed (Figure 8a). In these examples, the GS film not only serves as a transparent electrode for light transmittance, but also is used as the Schottky junction layer for the electron–hole separation and hole transport. This means that the photogenerated carriers are separated by the built-in field, while the electrons and holes are diffused to GS and n-Si, respectively (Figure 8b). The solar energy conversion efficiency is 1.65% and 1.34% for the devices with junction areas of 0.1 cm2 and 0.5 cm2, respectively (Figure 8c). Although the efficiency of such GS/n-Si Schottky junction devices is still far lower than that of pure silicon thin-film solar cells (≈12%),[153-155] it provides an improved understanding of the effects on solar cell performance from the electronic coupling, surface passivation, doping, and junction formation. Therefore, more advanced and efficient graphene/silicon-based architectures with improved solar cell performance are possible.
Figure 8. a) Schematic illustration, b) energy diagram, and c) room-temperature J–V characteristics of a GS/n-Si Schottky junction solar cell. In (b,e), ΦG and Φn-Si are the work functions of graphene and n-Si, respectively; V0 is the built-in potential. Vbias is the applied voltage for solar cells. Φb is the barrier height. χ is the electron affinity. EC, EV, and EF correspond to the conduction band edge, valence band edge, and Fermi level, respectively, and Eg is the bandgap. Reproduced with permission.
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Furthermore, Hebard and co-workers reported that the performance of Schottky solar cells based on the graphene-Si system can be greatly enhanced after proper doping of graphene. Figure 9a shows the schematic illustration of the Schottky solar cell composed of doped graphene/n-Si. The graphene sheet was first doped with bis(trifluoromethanesulfonyl)-amide[((CF3SO2)2NH)] (TFSA). This TFSA doped (p-type doping) graphene exhibited lower sheet resistance. Importantly, the TFSA doping further increased the work function of graphene, thus increasing the built-in potential between the doped-graphene and n-Si in solar cells (Figure 9b). As shown in the J–V characteristics before and after doping of the graphene sheets with TFSA (Figure 9c), Jsc, Voc, and fill factor (FF) increase from 14.2 to 25.3 mA cm–2, 0.43 to 0.54 V, and 0.32 to 0.63, respectively. The increases in Jsc and Voc boost the power conversion efficiency from 1.9% to 8.6% (Figure 9c), which is the highest power conversion efficiency reported for graphene-based solar cells to date. Additional characterization of this device is presented in Figure 9d, showing the external quantum efficiency (EQE) before and after doping of graphene. The pristine device shows an EQE near 50% in the wavelength range of 400–850 nm, indicating the significant electron−hole pair generation and the subsequent facile collection of electrons and holes by the corresponding electrodes. After the TFSA doping, the EQE was significantly increased to a value of ≈65% in the abovementioned wavelength range, which is due to the more efficient charge separation and charge collection as a result of the increased built-in potential and reduced sheet resistance.
Figure 9. a) Schematic illustration of graphene/n-Si (left) and TFSA doped graphene/n-Si (right) Schottky solar cells. b) The band diagram at the graphene/n-Si interface before (left) and after (right) the doping. c) J–V curves of graphene/n-Si (blue curve) and doped-graphene/n-Si (red curve) Schottky solar cells under the AM 1.5 G illumination. The green curve is the J–V curve for doped-graphene/n-Si device in dark. d) The plot of external quantum efficiency (EQE) vs wavelength (λ, nm) for the pristine and TFSA-doped graphene/n-Si solar cells. Reproduced with permission. Copyright 2012, American Chemical Society.
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Based on the aforementioned discussion, graphene, which works as the electron acceptor after being functionalized with organic materials or as the Schottky junction metal layer combined with the inorganic semiconductors, has presented promising performance in solar harvesting. Since the performance is still low compared to the current equivalent solar cells without graphene, lots of work should be done for the improvement of performance, such as the design of graphene-based architectures with specific physical properties at heterojunctions, the chemistry at interface of the graphene-based hybrids, and the exploration of advanced materials and processing technologies. Combining low cost, mass production, high electron mobility with efficient charge transport, and 2D features with facile functionability, graphene is expected to play an increasingly important role in both research and industry for sustainable clean energy by partially or even fully replacing organic and/or Si in photovoltaic cells in the future.
2.2.3 Charge Transport Layers
184.108.40.206. Electron Transport Layer. Taking the advantage of high electron mobility of graphene, Jiang and co-workers reported that rGO can be used as the 2D electron transport channel in rGO-TiO2 nanocomposite based DSSCs, showing better performance than 1D CNT-TiO2 composite based DSSCs (Figure 10 a–d). In graphene composite electrodes, the particles can anchor onto graphene better, and the photogenerated electrons can be easily captured and transferred by the graphene. However, when the composite of TiO2 with 1D CNT is formed, there is less contact/connection between them. Therefore, the transfer barrier is large, resulting in the severe recombination of electrons and holes. Based on the operational principle of the device (Figure 10e), the introduced 2D graphene performs as an electron transport layer, i.e., accepts electrons from TiO2 and then transfers them quickly to the FTO electrode. Therefore, the recombination of electrons and holes is suppressed. In Figure 10f, the photocurrent–voltage characteristics of DSSCs with different electrodes are presented. The best performance is demonstrated by the device with the photoanode layer of TiO2 with loading of 0.4% rGO (i.e., ≈0.6% GO loading since GO will lose weight after reduction). It means there is an optimal rGO loading window in terms of the PCE of the devices because too little graphene loading weakens its electron transporting effect, while too much graphene loading reduces the dye adsorption onto TiO2. Moreover, the excessive graphene can act as a kind of recombination center instead of providing an electron pathway, thus easily triggering the short circuit.
Figure 10. Schematic illustrations of a,c) 1D and b,d) 2D nanomaterial composite electrodes. e) Operational principle of the device in term of charge transfer behavior in the DSSC. f) Photocurrent–voltage characteristics of different electrodes. The sensitizer is N3 (ruthenium dye). The cell active area is 0.20 cm2. The light intensity is 100 mW cm–2. Electrodes 1–4 are pure TiO2, TiO2 with loading of 0.6% GO, TiO2 loading of with 2.5% GO, and TiO2 loading of with 8.5% GO, respectively. Reproduced with permission. Copyright 2012, American Chemical Society.
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As another example, graphene as the electron transport layer (ETL) in OPV has been reported by Heeger and co-workers. Instead of the usual solution-processed coating to prepare graphene film, they developed a novel facile stamping process to directly transfer graphene onto the bulk heterojunction (BHJ) layer prior to the top Al cathode deposition. Such stamping nanotechnology is able to transfer large-area, single-layer CVD-graphene onto specific regions of a substrate. In this work, the graphene was first doped/oxidized by HNO3, which is also called graphene oxide (GO). In order to compare with the traditional TiOx ETL used in OPV, the pure GO, pure TiOx, and GO/TiO2 double layer used as the ETL in OPV and the device without the ETL were all studied. Consistently, the OPV with ETL has much higher power conversion efficiency (PCE) than that without the ETL. Importantly, the PCE of a device based on a GO/TiO2 ETL was increased by 6.8% compared to the pure TiO2 ETL based device. The improved BHJ device performance is attributed to the fact that introduction of the GO/TiO2 ETL layer, as compared to the pure TiO2 or GO ETL layer, increases the hole blocking barrier (Φh) and simultaneously shifts Evac downwards, thereby decreasing the electron injection barrier in the BHJ device. The larger hole blocking barrier facilitates the photogenerated hole to transport towards the ITO anode. The smaller electron injection barrier enables the photogenerated electron to transport efficiently towards the Al cathode. Hence, such a synergistic effect enhances the short-circuit current density (Jsc) and thus the PCE of GO/TiO2 ETL-based BHJ devices.
220.127.116.11 Hole Transport Layer. Different from graphene, which is a highly efficient electron transporter, the functionalized GO and GO-based composites showed excellent hole transport properties in photovoltaic devices. Huang and co-workers have combined GO and SWCNTs as the hole transport layer for P3HT:PCBM-based polymer solar cells (Figure 11a). They found that the addition of the proper amount of SWCNTs into GO can significantly improve the conductivity of the GO film as the hole transport layer. Such GO:SWCNT composite films reduce the hole transport resistance and facilitate the hole transport from the active layer to the anode. The results shown in Figure 11b demonstrate that the GO:SWCNTs composite film based device exhibits higher performance than the pure GO film based one. Note that the GO:SWCNT composite can offer comparable performance to the conventional PEDOT:PSS-based devices. At the same time, Chhowalla and co-workers also reported that the thin GO film with thickness of ≈2 nm on ITO can work as an effective hole transport layer in polymer solar cells (Figure 11c), showing the comparable values of efficiency to devices fabricated with PEDOT:PSS. They also found a clear trend of decreasing PCE with increasing GO film thickness (Figure 11d). As a result, the thinnest GO film yielded the best performance. This is mainly attributed to the increased serial resistance in the thicker GO film, which decreases Jsc and FF and also slightly lowers its transmittance.
Figure 11. a) Schematic illustration and b) current density–voltage characteristics of ITO/GO/P3HT:PCBM/Ca/Al device based on unmodified ITO (inverted triangle), GO layer (open circle, spin coated from 0.15 wt% dispersion), PEDOT:PSS layer (diamond), and GO:SWCNT layer (solid circle, spin coated from 0.15 wt% GO dispersion, GO:SWCNTs = 1:0.2, w/w). Reproduced with permission. c) Schematic illustration and d) current density–voltage characteristics of ITO/GO/P3HT:PCBM/Al devices with different GO thickness. Reproduced with permission. Copyright 2010, American Chemical Society.
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In order to lower the resistance of the GO film as the hole transport layer in the polymer solar cell, rGO was used as the hole transport layer. In this work, hydrazine was used to reduce GO (referred to as r-GO). However, since hydrazine is highly toxic, a new reductant, p-toluenesulfonyl hydrazide (p-TosNHNH2), was developed to reduce GO. The obtained pr-GO not only exhibited comparable reduction degree as that of r-GO, but also gave much smoother thin-film morphology. The obtained highest PCE (3.7%) is from the pr-GO based device, which is even higher than that obtained from the traditional PEDOT:PSS based device (3.6%). In terms of the device stability after exposure to the air, the device with pr-GO shows a much longer life time than those with PEDOT:PSS. The aforementioned results clearly demonstrate that the chemically reduced GO using p-TosNHNH2 as a novel reductant is a promising interfacial material and a practical replacement for the conventional PEDOT:PSS. It promises the realization of highly efficient, highly stable, and low-cost polymer solar cells by using an environmentally friendly, high-throughput, roll-to-roll manufacturing.
18.104.22.168. Hole and Electron Transport Layer. The work function of GO can be easily tuned by simple chemical modification. Dai and co-workers recently reported GO and modified GO as hole and electron transport layers, respectively, in the single solar cell configuration. As shown in Figure 12a,b, by replacing the periphery –COOH groups with –COOCs groups, the work function of the cesium-neutralized GO (GO-Cs) can be reduced to 3.9–4.1 eV from 4.6–4.8 eV for pure GO. As a result, the work function of GO matches both the ITO anode and the HOMO level of P3HT for efficient hole extraction, while the work function of GO-Cs matches both Al and the LUMO level of PCBM for efficient electron extraction (Figure 12b).
Figure 12. a) Chemical structures and synthetic route of GO and GO-Cs. b) Energy level diagrams and c) device structures of the normal device and the inverted device with GO as hole-extraction layer and GO–Cs as the electron-extraction layer. d) Current density-voltage curves of the normal device (left) and inverted device (right) with GO as the hole-extraction layer and GO–Cs as the electron-extraction layer. Reproduced with permission.
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Excellent hole/electron extraction capabilities have been demonstrated for GO/GO-Cs based polymer solar cells. Both the normal device with the configuration of ITO(anode)/GO/P3HT:PCBM/GO-Cs/Al(cathode) and inverted device of ITO(cathode)/GO–Cs/P3HT:PCBM/GO/Al(anode) were investigated (Figure 12c). The observed good performance for both the normal and inverted devices containing GO and GO–Cs demonstrates the capability of hole and electron extraction from GO and GO–Cs, respectively. The slightly lower performance from the inverted device, compared with the normal device, is mainly due to its smaller open-circuit voltage (Voc) (Figure 12d). This study implies that the chemically engineered GO is promising for both hole and electron transport materials in solar cells.
22.214.171.124. Interfacial Layer in Tandem Solar Cells. Huang and co-workers found that the mixed solution of GO and PEDOT:PSS exhibits a one to two order of magnitude increase in viscosity compared to either pure GO or PEDOT:PSS. Although GO is electrically insulating, the thin film made from the mixture of GO and PEDOT:PSS possesses one order of magnitude improvement in conductivity as compared to the PEDOT:PSS film. Therefore, the GO/PEDOT:PSS composite based thin film is adhesive, transparent, and conducting, which is well suited for the mechanically and electrically connecting part, i.e., the interfacial layer, in tandem solar cells. As a proof of concept, such a composite thin film has been used as the sticky interconnect to increase the charge transport and the adhesion between the two subcells in the tandem solar cell configuration (Figure 13a). As shown in Figure 13b, the Voc values of separately prepared front and back subcells are 0.59 and 0.53 V, respectively. The Voc of the tandem cell is 0.94 V, i.e., 84% of the total Voc of the subcells. This proves that two subcells have been successfully connected in series by using GO/PEDOT as the interfacial layer. The PCE of the final tandem cell with GO/PEDOT is calculated to be 4.14%, which is higher than that of the single rear and front cells (2.92% and 3.75%, respectively). Importantly, it is much higher than that of the tandem cell with only PEDOT:PSS as the interfacial layer (see the blue solid-triangle curve in Figure 13b). This study proves the importance of the composite film, with both enhanced viscosity and conductivity characteristics, used for the interfacial layer in tandem cells.
Figure 13. a) Cross-sectionalal SEM image of the fabricated tandem device. The two subcells and the interconnect layer can be clearly distinguished. Scale bar = 150 nm. b) J–V curves of separately prepared front cell (black), rear cell (red), tandem cells with PEDOT:PSS (blue) and GO/PEDOT:PSS gel (green) as the sticky interfacial layer, respectively. Reproduced with permission. Copyright 2011, American Chemical Society.
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In addition, the same research group also demonstrated that the water-processable GO:SWCNT thin film can be used as an effective interfacial layer to construct tandem polymer solar cells in both regular and inverted geometry by all-solution processing routes. Although the same polymer is used in both subcells, the significantly improved PCEs from the subcell (3.41%) to the serially connected regular tandem cell (4.1%) and from the subcell (2.9%) to the inverted tandem cell (3.5%) show that the GO:SWCNT thin film serves as the good mechanical separator and electrical interconnect of the two subcells. Because of its high transparency in the near infrared (NIR) region, the GO:SWCNT-based interconnect should work well with the complementary tandem cell if a low bandgap polymer is used. By using p- or n-doped SWCNTs with GO, the charge recombination in the interconnect layer is expected to be further improved.
In addition to GO, the CVD-graphene can also be employed as an interfacial layer in tandem organic solar cells. Loh and co-workers reported that the work function of graphene (4.2 eV) can be increased by coating MoO3 with a bulk work function of 6.76 eV. By using such MoO3/graphene as the interfacial layer in series/parallel tandem solar cells (Figure 14a), the performance can be effectively improved. For two-terminal series-connected tandem cells, MoO3/graphene acts as a recombination center for the extraction of electrons from the bottom cell and holes from top cell. As for three-terminal parallel connected tandem cells, the sandwiched MoO3/graphene/MoO3 interfacial layer only extracts the holes from the two end ITO and LiF/Al cathodes (Figure 14b). Finally, the MoO3/graphene thin film was demonstrated as interfacial layer in tandem solar cells, which harvest wide spectral solar energy by stacking different bandgap photoactive materials. A high Voc of 1 V and a high Jsc of 11.6 mA cm-2 were obtained in series and parallel connection, respectively (Figure 14c,d). The values of Voc and Jsc in the tandem cell are very close to the sum values of Voc and Jsc from two single subcells in series and parallel connections, respectively, further confirming the good ohmic contact at the active layer/MoO3-modified graphene interface. In this case, the graphene interfacial layer is capable of multiplying the open circuit voltage and short circuit current density by linking the subcells properly. The performance of tandem cells could be further enhanced if the efficiency of two subcells could be improved.
Figure 14. a) Schematic illustration of the structure of photovoltaic device. b) Energy level diagram of the tandem photovoltaic cell connected in series and parallel configuration. c) J–V characteristics of the reference single cell (ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al (bottom cell), ITO/MoO3/ZnPc:C60/LiF/Al (top cell)) and series connected tandem cell under light illumination. d) J–V characteristics of the reference single cells (characterized individually from the parallel connected tandem cell) and parallel connected tandem cell under light illumination. The theoretical J–V curve of the tandem cell is also constructed by summing the J–V curves of the single cells (the line with hollow squares). Inset graphs show the optimized thickness of MoO3 in the tandem device. Reproduced with permission.
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With their solution processability, modifiability in electrical properties, transferability between substrates, and chemical/thermal stability, graphene and graphene-based nanomaterials have been successfully demonstrated in the feasibility of ultrathin films ranged from single-layer to few-layer thickness, which were then utilized as charge (electron/hole) transport layers in single and/or tandem OPV solar cells as discussed above. Either using low work-function graphene based nanomaterials as the ETL or high work-function graphene-based materials as the HTL, or even both functions in one tandem solar cells, graphene-based nanomaterials have exhibited obvious advantages in device performance enhancement over the traditional ETL materials of ZnO or TiO2 and HTL materials of MoO3 or PEDOT:PSS. In this aspect, the role of graphene-based materials is expected to play an important role in the configuration of OPV solar cells, aiming for higher performance devices in future.