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Graphene-Based Materials for Solar Cell Applications Zongyou Yin, Jixin Zhu, Qiyuan He, Xiehong Cao, Chaoliang Tan, Hongyu Chen, Qingyu Yan, and Hua Zhang* The increasing demand for energy consumption and the limited energy resources are the two main driving forces for the exploration of new energy-harvesting processes, among which new materials, especially nanomaterials, have been investigated in clean energy applications such as solar cells,[83–90] solar fuels,[91–96] and lithium-ion batteries (LIBs)[97–104] and [ 105–111 ] Here, we focus on the recent advances supercapacitors. of graphene-based materials for solar cell applications.

Graphene has attracted increasing attention due to its unique electrical, optical, optoelectronic, and mechanical properties, which have opened up huge numbers of opportunities for applications. An overview of the recent research on graphene and its derivatives is presented, with a particular focus on synthesis, properties, and applications in solar cells.

1. Introduction In the past decade, graphene, a 2D sheet composed of sp2bonded single-layer carbon atoms with the honeycomb lattice structure, has attracted great research interest in physics, chemistry, materials science, etc.[1–17] Until now, the electrical, optical, mechanical, thermoelectric, and magnetic properties of graphene have been intensively studied.[18–34] In order to mass produce high-quality 2D graphene nanosheets for potential applications, various strategies have been developed.[35–40] More importantly, graphene has been used in various applications. For example, its extremely high room-temperature carrier mobility (≈20 000 cm2 v−1 s−1)[41] makes graphene a promising candidate to replace the conventional semiconductor materials in the electric circuit.[18,42–46] Moreover, the high optical transparency of graphene (only 2.3% of incident light absorbed in the range from near-infrared to violet)[47] makes it promising for next-generation transparent conductive electrodes, which may replace traditional indium tin oxide (ITO) in optoelectronics, displays, and photovoltaics.[48–53] In addition, it is worth mentioning that the large 2D basal plane with huge surface area (≈2600 m2g−1)[54] and high chemical/thermal stability of graphene has been used as a template to synthesize versatile functional composites used for sensors,[24,55,56] catalysts,[57–60] optical modulators,[10,61,62] antibacterial activities,[63–65] surface enhanced Raman scattering (SERS) platforms,[66–68] electrochemical devices,[69–72] and energy storage.[73–76] Furthermore, the high flexibility makes graphene one of the most promising materials for flexible and rollable electronics applications.[49,51,77–82] Dr. Z. Yin, Dr. J. Zhu, Dr. Q. He, Dr. X. Cao, C. Tan, Prof. Q. Yan, Prof. H. Zhang School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore, 639798, Singapore E-mail: [email protected] H. Chen Division of Chemistry and Biological Chemistry Nanyang Technological University Singapore, 637371, Singapore

DOI: 10.1002/aenm.201300574

Adv. Energy Mater. 2014, 4, 1300574

2. Various Applications in Solar Cells With the unique properties, i.e., highly optical transparence, highly electrical conduction, and mechanical flexibility, graphene and its derivatives have been investigated extensively in the field of solar cells. Lots of impressive results have been reported, where graphene was used as the electrodes, i.e., transparent anodes,[49–51,82,112–117] non-transparent anodes,[118,119] transparent cathodes,[52,120–122] and catalytic counter electrodes,[117,123–127] as well as where graphene was used as the active layer, i.e., light harvesting material,[128–130] Schottky junction,[131–135] electron transport layer,[136–140] hole transport layer,[141–145] both hole and electron transport layer,[143] and interfacial layer in the tandem configuration.[146–148] Table 1 summarizes the previous work using graphene as the electrodes. 2.1. Conductive Electrodes 2.1.1. Transparent Conducive Anodes Recently, graphene has been successfully used as the transparent conductive anode for the flexible organic photovoltaic (OPV) cell with the configuration graphene/poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/copper phthalocyanine (CuPc)/C60/bathocuproine(BCP)/Al (Figure 1a).[82] Graphene was first deposited onto the Si/SiO2/ Ni film by chemical vapor deposition (CVD) and then transferred onto the transparent glass or polyethylene terephthalate (PET) substrates (Figure 1b). The obtained graphene/PET film showed its sheet resistance down to 230 Ω/sq with transparency of 72%. The solar cell incorporating CVD-grown graphene (referred to here as CVD-graphene) as the transparent conductive anode exhibited a power conversion efficiency (η) of 1.8%, which is comparable to the performance (1.27%) obtained in the device with the commonly used ITO electrode. Obviously, the flexibility of CVD-graphene device surpasses that of the ITO device because the former device can operate under bending

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up to 138° (Figure 1c), whereas the latter one degraded rapidly and irreversibly even under bending of 60° (Figure 1d). Consistently, the decay/decrease rate of the initial fill factor in the CVD-graphene device was much slower than that in the ITO one as the bending angle increased, and the fill factor of the ITO device rapidly decayed to 0 when it was bent to around 60° (Figure 1e). The lower performance under bending, i.e., lower flexibility, of the ITO device was caused by the generation of microcracks in the ITO film under mechanical stress, while no such microcracks were observed on the CVD graphene device (Figure 1f). In the area of flexible OPV devices, our group has used chemically reduced graphene oxide (rGO) as the transparent conductive anode in the device with configuration of rGO/ PEDOT:PSS/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/TiO2/Al. The performance of the OPV devices mainly depends on the charge transport efficiency and the transparency of the rGO electrodes, when the optical transmittance of rGO is above and below 65%, respectively. Impressively, the fabricated OPV device on rGO can sustain a thousand bending cycles at 2.9% tensile strain.[51] Iijima and co-workers reported a roll-to-roll method to transfer the 30-inch CVD-graphene monolayer film from the copper foil onto a flexible substrate, such as PET.[49] The graphene film exhibited 97.4% optical transmittance with sheet resistance as low as 125 Ω/sq. After a four-layer graphene film was achieved by layer-by-layer stacking, it was doped with HNO3, showing the further decreased sheet resistance (≈30 Ω/sq at ≈90% transparency), which is superior to the commercial transparent ITO electrode. Furthermore, such graphene electrode was capable of withstanding high strain.[49] All of the aforementioned reports have demonstrated the highly flexible nature of graphene, which shows its potential application as the transparent conductive anode electrode for flexible solar panels, since both CVD-graphene and solutionprocessed rGO have exhibited superior mechanical performance over ITO. For the flexible CVD-graphene based thin film, which is compatible with the roll-to-roll fabrication process developed by Iijima and co-workers,[80] it possesses not only better mechanical properties than ITO, but also superior sheet resistance and transparency compared to ITO when used as a transparent electrode. In addition to the OPV devices, graphene also shows potential as the transparent anode electrode for dye sensitized solar cells (DSSCs). In a recent report,[50] the graphene thin film, prepared by dip coating aqueous graphene oxide (GO) solution on quartz followed by temperature-controlled film drying and subsequent thermal reduction, exhibited a high conductivity of 550 S cm–1 and transparency of more than 70% in the wavelength range of 1000–3000 nm. A solid-state DSSC with configuration of graphene/TiO2/dye/spiro-OMeTAD/Au has been fabricated using the graphene film as the transparent anode (Figure 2a). The corresponding energy level diagram of the DSSC device is shown in Figure 2b. This is the first demonstration of a solid-state DSSC based on a graphene electrode with the power conversion efficiency (PCE) of 0.26%, which is still lower compared to the fluorine tin oxide (FTO)-based solid-state DSSC (0.84%; Figure 2c).

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Zongyou Yin studied at Jilin University in China for his B.E. and M.S., and completed his Ph.D. at Nanyang Technological University in Singapore (2008). He is currently working as a postdoctoral fellow in Prof. Hua Zhang’s group. His research interests include the synthesis and application of new functional nanomaterials based on graphene and its inorganic analogues. Hua Zhang studied at Nanjing University (B.S., M.S.), and completed his Ph.D. at Peking University (with Zhongfan Liu) in 1998. After postdoctoral work at Katholieke Universiteit Leuven (with Frans De Schryver) and Northwestern University (with Chad Mirkin), and working at NanoInk Inc. and the Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University (2006). His current research interests focus on synthesis of 2D and low-dimensional nanomaterials and carbon materials (graphene and CNTs), and their applications in nano- and biosensing, clean energy, etc.

2.1.2. Transparent Conducive Cathodes Graphene has also been developed as the transparent conductive cathode electrode for solar cells. Recently, our group has reported that rGO was used as the working electrode for electrochemical deposition of functional materials, which was further used as the cathode electrode for the hybrid solar cell application.[52] Briefly, the rGO thin film on quartz was obtained by two-step reduction of GO, i.e., using hydrazine vapor and then thermal annealing. It exhibited a sheet resistance of 420 Ω/sq at 61% transmittance. The n-type ZnO nanorods (NRs) were electrochemically deposited on the obtained rGO electrode. Then, the p-type poly(3-hexylthiophene) (P3HT) layer was spin-coated on the top of ZnO NRs to form an inorganic–organic material based hybrid solar cell. In addition, we found that the electrochemical deposition of ZnO strongly depends on the thickness, i.e., the sheet resistance, of the rGO film. The thicker rGO film with lower sheet resistance has advantages for the growth of high quality crystalline ZnO nanorods. The device with a layered structure of quartz/rGO/ZnO NRs/P3HT/PEDOT:PSS/Au was fabricated with a PCE of around 0.31% (Figure 3a).




Material

Electrode (Rs: sheet resistance; T: transmittance)

Type and configuration of solar cells

PCE [%]

Ref.

rGO

Transparent anode (Rs: 1.8 kΩ/sq at T: ≈70%)

Solid-state DSSC: glass/rGO/TiO2/dye/ spiro-OMeTAD/Au

0.26

[50]

rGO

Transparent anode (Rs: 3.2 kΩ/sq at T: 65%)

OPV: PET/rGO/PEDOT:PSS/ P3HT:PCBM/TiO2/Al

0.78

[51]

CVD-graphene


OPV: quartz/graphene/PEDOT:PSS/ CuPc:C60/BCP/Ag

0.85

[115]

rGO-CNT


OPV: glass/rGO-CNT/PEDOT:PSS/ P3HT:PCBM/Ca:Al

0.85

[112]

CVD-graphene


OPV: PET/graphene/PEDOT:PSS/ CuPc:C60/BCP/Al

1.18

[82]

CVD-graphene


OPV: quartz/graphene/MoO3+ PEDOT:PSS/P3HT:PCBM/LiF/Al

2.5

[113]

Tetracyanoquinodimethane (TCNQ)-graphene


OPV: glass/TCNQ-graphene/PEDOT: PSS/ P3HT:PCBM/Ca/Al

2.58

[116]

Au-doped graphene


OPV: Au-graphene/PEDOT:PSS/ P3HT:PCBM/ZnO/ITO

3.04

[117]

rGO

Transparent cathode (Rs: 0.42 kΩ/sq at T: 61%)

Hybrid solar cell: quartz/rGO/ZnO/ P3HT/ PEDOT:PSS/Au

0.31

[52]

Al-TiO2 modified graphene


OPV: Au/graphene/Al-TiO2/P3HT: PCBM/MoO3/Ag

2.58

[121]

CVD-graphene


Thin film solar cell: glass/graphene/ ZnO/ CdS/CdTe/graphite paste

4.17

[120]

CVD-graphene


Hybrid solar cell: glass/graphene/ PEDOT:PEG(PC)/ZnO/PbS QD(P3HT)/ MoO3/Au

4.2 (0.5)

[122]

Functiona-lized rGO

Catalytic counter electrode

Liquid DSSC: FTO/TiO2/dye/I3− /I1− mediated electrolyte/rGO

4.99

[124]

CVD-graphene


Liquid DSSC: FTO/TiO2/dye/I3− /I1− mediated electrolyte/graphene

5.73

[125]

TiN-rGO


Liquid DSSC: FTO/TiO2/dye/I3− /I1− mediated electrolyte/TiN-rGO

5.78

[127]

CNT-rGO paper


Liquid DSSC: FTO/TiO2/dye/I3− /I1− mediated electrolyte/CNT-rGO

6.05

[117]

Graphene platelets


Liquid DSSC: FTO/TiO2/dye/Co(III)/(II) mediated electrolyte/graphene

9.3

[126]

As another example, Jiang and co-workers have successfully demonstrated the CdTe thin film solar cell using CVD-graphene as the front transparent electrode.[120] The graphene film used in their work was grown on Cu foil by the low-cost CVD at ambient pressure and then it was transferred onto the glass substrate. Importantly, the layer number of graphene was well controlled by the inlet H2 flow rate during the CVD growth. The sheet resistance of the obtained graphene film is 220 Ω/sq at the transparency of 84%. As a proof of concept, the prototype device with configuration of glass/graphene/ZnO/CdS/CdTe/ (graphite paste) has been fabricated (Figure 3b), with PCE of as high as 4.17% (Figure 3c). CVD-graphene normally is hydrophobic, which hampers the direct growth of functional semiconductors on top through the hydrothermal synthesis method. Using the conductive polymer modification of CVD-graphene, Park and co-workers


successfully grew crystalline ZnO nanowires on a graphene substrate via the hydrothermal method.[122] The obtained graphene layer on glass after transferred from Cu foil exhibited the sheet resistance of 300 Ω/sq with transparency of 92%. Based on the obtained graphene/polymer/ZnO structures, a graphene-cathode based hybrid solar cell using PbS QDs and P3HT as the p-type active material and ZnO as n-type material has been developed (Figure 3d). The J–V characteristics of PbS QD and P3HT based hybrid solar cells are presented in Figure 3e,f, respectively. The obtained PCEs of PbS QD-based devices on ITO/ZnO, graphene/PEDOT:PEG(PC)/ZnO and graphene/RG-1200/ZnO were 5.1%, 4.2%, and 3.9%, respectively. The obtained PCEs of P3HT-based devices ITO/ZnO, graphene/PEDOT:PEG(PC)/ZnO, and graphene/RG-1200/ZnO were 0.4%, 0.3%, and 0.5%, respectively. It shows that the graphene electrode is comparable with ITO in these devices.



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Table 1. Previous work on graphene or reduced graphene oxide (rGO) used as electrodes in solar cells.

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Figure 1. a) Schematic representation of the energy level alignment (top) and the construction of heterojunction organic solar cell fabricated with graphene as anodic electrode: graphene/PEDOT/CuPc/C60/BCP/Al. b) Schematic illustration of the transfer process of CVD-graphene onto transparent substrate. c,d) The plots of current density vs voltage for c) graphene and d) ITO devices under 100 mW cm–2 AM1.5G spectral illumination at different bending angles. Insets show the experimental setup used in the experiments. e) The plot of fill factor vs bending angle for the graphene and ITO devices. f) Scanning electron microscopy (SEM) images of the surface structure of CVD-graphene (top) and ITO (bottom) devices after being subjected to the bending angles described in panels (c,d). Reproduced with permission.[82] Copyright 2010, American Chemical Society.

The sheet resistance and transparency of graphene used as the transparent electrode are two main factors that influence the performance of graphene-based solar cells. For example, the quartz/graphene/RG-1200/ZnO nanowires/P3HT/MoO3/ Au device exhibits a PCE of 0.5%,[122] enhanced by 61%

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compared to the device of quartz/rGO/ZnO nanorods/P3HT/ PEDOT:PSS/Au with PCE of 0.31%.[52] The main reason is that the CVD-graphene prepared in the former work has a much better sheet resistance of 300 Ω/sq at transparency of 92% compared to the sheet resistance of 420 Ω/sq at transparency of




REVIEW Figure 2. Illustration and performance of DSSC based on graphene electrodes. a) Schematic illustration of DSSC using graphene film as electrode. The four layers from bottom to top are Au, dye-sensitized heterojunction, compact TiO2, and graphene film. b) The energy level diagram of the DSSC with the configuration of graphene/TiO2/dye/spiro-OMeTAD/Au. c) Current density–voltage curves of the graphene-based cell (black) and FTO-based cell (red) illuminated under AM solar light (1 sun). Reproduced with permission.[50] Copyright 2008, American Chemical Society.

61% from the solution-processed rGO in the latter work. This indicates the crystal quality of CVD-graphene is higher than the solution-processed rGO since there are still oxygen functional groups in rGO.[149] Importantly, the CVD-graphene based ZnOP3HT hybrid solar cells exhibited higher PCE compared to the ITO electrode.[122,150] All the aforementioned results indicate the great potential of graphene as a new transparent conductive cathode material to replace the traditional ITO for the hybrid solar cell applications. 2.1.3. Catalytic Counter Electrodes As discussed above, graphene with its highly transparent conductive characteristics has been used as efficient transparent conductive electrodes in various photovoltaic devices. Much beyond that, graphene has also been used for the catalytic counter electrode in DSSCs with the prospect to replace the traditional expensive platinum (Pt) counter electrode. To date, much related research has been published, as listed in the Table 1. Some typical work will be reviewed in the discussion that follows. Aksay and co-workers use the functionalized graphene sheet (FGS) as a stand-alone catalyst in a DSSC to replace Pt.[124] The FGS-based cell gave a high efficiency of ≈5%, which is only 10% lower compared to the corresponding Pt-based cell (Figure 4c). The catalytic activity from FGS is attributed to its comparable charge-transfer resistance with platinum at an applied bias, and the introduced oxygen-containing functional groups on FGS (Figure 4a,b). Finally, they demonstrated that the FGS ink-casted plastic substrate can be used as an efficient counter electrode, eliminating the requirement for the common FTO substrate. Tailoring the functionalization or morphology of the FGS electrodes could decrease the charge-transfer resistance


and also facilitate the low-cost production of the catalytic, flexible, and conductive counter electrodes for DSSCs. Graphene nanoplatelets (GNPs) as a counter electrode material have been demonstrated with high electrocatalytic activity for the redox of Co(L)2 (L = 6-(1H-pyrazol-1-yl)-2,2′-bipyridine) by Grätzel and co-workers.[126] They observed that the exchange current density for the Co2+/3+(L)2 redox was 1 to 2 orders of magnitude larger than those for the conventional I3−/I− redox couple on the same electrode. In the Co2+/3+(L)2 redox system, the electrocatalytic activity of GNP-film cathode with optical transmission below 88% outperformed the activity of the Pt electrode. DSSCs with Y123 dye adsorbed on TiO2 photoanode achieved the energy conversion efficiency of 8–10% for both GNP and Pt-based cathodes. Importantly, the cell with GNP cathode is superior to that with Pt cathode particularly in the fill factor and the efficiency at higher illumination intensity (Figure 4d,e). In addition to pure graphene, graphene-based composites with controlled functionalization have also been developed for the catalytic counter electrodes. Chen et al. first realized the direct growth of vertically aligned carbon nanotubes (VACNTs) on ≈3 μm thick graphene paper (GP) by a thermal CVD method.[117] The as-prepared freestanding VACNT/GP film possesses superb flexibility and durability, which is attributed to the highly flexible GP (Figure 5a). When employed as the counter electrode in a DSSC, it displayed superior overall performance over the GP or the tangled CNT/GP film and only slightly lower efficiency compared to the Pt electrode (Figure 5b). A facile and versatile route has been developed to synthesize nanohybrids of titanium nitride (TiN) nanoparticles decorated on nitrogen-doped graphene (NG).[127] The resulting TiN/ NG hybrids exhibited comparable catalytic performance to the platinum (Pt), which is widely used as the counter electrode



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Figure 3. a) Current density–voltage (J–V) characteristics of ZnO/P3HT hybrid solar cells with rGO film as electrode. Inset: schematic illustration of the fabricated solar cell. Reproduced with permission.[52] b) Schematic diagram and c) J–V characteristics of the glass/graphene/ZnO/CdS/CdTe/(graphite paste) solar cell. Reproduced with permission.[120] d) Schematic illustration of glass/graphene/conductive polymer (PEDOT:PEG(PC) or Plexcore OC RG-1200)/ZnO/PbS QD(P3HT)/MoO3/Au hybrid solar cells with PbS QD or P3HT as the photoactive material. e) J–V characteristics of PbS QD based device with different polymer interlayers, demonstrating their performance comparable to that of a reference solar cell on ITO substrate. f) J–V characteristics of P3HT based device with different polymer interlayers, compared with a reference device on ITO. Insets in (e,f) show the cross-sectional SEM images of the device. Reproduced with permission.[122] Copyright 2012, American Chemical Society.

in DSSCs (Figure 5c,d). This work demonstrated that the TiN/ NG nanohybrids can be used as a low-cost counter electrode to replace the traditional Pt in DSSCs. Furthermore, the density functional theory (DFT) calculation indicated that the high catalytic performance of TiN/NG originates from the synergetic effect between TiN and NG. Based on the aforementioned promising results achieved from graphene and graphene-based composites, we can anticipate the great potential of graphene-based materials to replace the traditional expensive Pt electrode as a low-cost catalytic counter electrode in DSSCs.

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2.2. Active Layers 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,[143] and interfacial layer for tandem solar cells in the photovoltaic




REVIEW Figure 4. a) Schematic illustration of functional groups and lattice defects on an FGS. Epoxides and hydroxyls are on both sides of the graphene plane, while carbonyl and hydroxyl groups are at the edges. A 5–8–5 defect (green), and a 5–7–7–5 (Stone-Wales) defect (yellow) are also shown. Carbon, oxygen, and hydrogen atoms are gray, red, and white, respectively. b) Side view emphasizing the topography of the FGS. These schematics are representative of the functionalities on FGS but not an actual sheet, which would measure on the order of 1 μm across. c) J–V characteristics of DSSCs using thermally decomposed chloroplatinic acid and FGS counter electrodes. Active area is 0.39 cm2. Reproduced with permission.[124] Copyright 2010, American Chemical Society. d) J–V characteristics of DSSCs with Pt counter electrode. e) J–V characteristics of DSSCs with GNP counter electrode. Reproduced with permission.[126] Copyright 2011, American Chemical Society.

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.[128] 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. Importantly, the effect of graphene with different lateral size in OPV devices was studied.[130] In this work, the active layer of OPV device was composed of aniline-functionalized graphene



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Figure 5. a) Flexible GP. b) Current–voltage characteristics of four DSSCs using the Pt, GP, TCNT/GP film, and VACNT/GP film as counter electrodes. Reproduced with permission.[117] c) Schematic illustration of a DSSC using TiN/NG as the counter electrode. d) Characteristic J–V curves of DSSCs with TiN/NG or Pt counter electrodes measured under simulated sunlight 100 mW cm−2 (AM 1.5). Inset table: photovoltaic parameters of DSSCs with different counter electrodes. Reproduced with permission.[127]

(used as electron acceptor) and P3HT (used as electron donor). It was found that after optimization, the device with anilinefunctionalized 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 ANIGQD 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 GQDbased devices is expected to be further improved by choosing other proper functionalization systems. 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.[129] 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. 1300574 (8 of 19)


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.[152] 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).[133] 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




Materials Graphene QDs

Function

Configuration of solar cells

PCE [%]

Ref.

Sensitizer of dye

Liquid DSSC: FTO/TiO2/graphene QD dye/ I3−/I1− mediated electrolyte/Pt