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Keywords:

  • graphene;
  • graphene derivatives;
  • solar cells;
  • applications

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Various Applications in Solar Cells
  5. 3 Conclusions and Outlook
  6. Acknowledgements
  7. Biographies

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

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Various Applications in Solar Cells
  5. 3 Conclusions and Outlook
  6. Acknowledgements
  7. Biographies

In the past decade, graphene, a 2D sheet composed of sp2-bonded 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]

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 supercapacitors.[105-111] Here, we focus on the recent advances of graphene-based materials for solar cell applications.

2 Various Applications in Solar Cells

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Various Applications in Solar Cells
  5. 3 Conclusions and Outlook
  6. Acknowledgements
  7. Biographies

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.

Table 1. Previous work on graphene or reduced graphene oxide (rGO) used as electrodes in solar cells
MaterialElectrode (Rs: sheet resistance; T: transmittance)Type and configuration of solar cellsPCE [%]Ref.
rGOTransparent anode (Rs: 1.8 kΩ/sq at T: ≈70%)Solid-state DSSC: glass/rGO/TiO2/dye/spiro-OMeTAD/Au0.26[50]
rGOTransparent anode (Rs: 3.2 kΩ/sq at T: 65%)OPV: PET/rGO/PEDOT:PSS/P3HT:PCBM/TiO2/Al0.78[51]
CVD-grapheneTransparent anode (Rs: 0.25 kΩ/sq at T: 92%)OPV: quartz/graphene/PEDOT:PSS/CuPc:C60/BCP/Ag0.85[115]
rGO-CNTTransparent anode (Rs: 0.6 kΩ/sq at T: 87%)OPV: glass/rGO-CNT/PEDOT:PSS/P3HT:PCBM/Ca:Al0.85[112]
CVD-grapheneTransparent anode (Rs: 3.5 kΩ/sq at T: 89%)OPV: PET/graphene/PEDOT:PSS/CuPc:C60/BCP/Al1.18[82]
CVD-grapheneTransparent anode (Rs: 0.08 kΩ/sq at T: 90%)OPV: quartz/graphene/MoO3+ PEDOT:PSS/P3HT:PCBM/LiF/Al2.5[113]
Tetracyanoquinodimethane (TCNQ)-grapheneTransparent anode (Rs: 0.278 kΩ/sq at T: 92%)OPV: glass/TCNQ-graphene/PEDOT: PSS/P3HT:PCBM/Ca/Al2.58[116]
Au-doped grapheneTransparent anode (Rs: 0.293 kΩ/sq at T: 90%)OPV: Au-graphene/PEDOT:PSS/P3HT:PCBM/ZnO/ITO3.04[117]
rGOTransparent cathode (Rs: 0.42 kΩ/sq at T: 61%)Hybrid solar cell: quartz/rGO/ZnO/P3HT/PEDOT:PSS/Au0.31[52]
Al-TiO2 modified grapheneTransparent cathode (Rs: 1.2 kΩ/sq at T: 96%)OPV: Au/graphene/Al-TiO2/P3HT: PCBM/MoO3/Ag2.58[121]
CVD-grapheneTransparent cathode (Rs: 0.22 kΩ/sq at T: 84%)Thin film solar cell: glass/graphene/ZnO/CdS/CdTe/graphite paste4.17[120]
CVD-grapheneTransparent cathode (Rs: 0.3 kΩ/sq at T: 92%)Hybrid solar cell: glass/graphene/PEDOT:PEG(PC)/ZnO/PbS QD(P3HT)/MoO3/Au4.2 (0.5)[122]
Functiona-lized rGOCatalytic counter electrodeLiquid DSSC: FTO/TiO2/dye/I3/I1 mediated electrolyte/rGO4.99[124]
CVD-grapheneCatalytic counter electrodeLiquid DSSC: FTO/TiO2/dye/I3/I1 mediated electrolyte/graphene5.73[125]
TiN-rGOCatalytic counter electrodeLiquid DSSC: FTO/TiO2/dye/I3/I1 mediated electrolyte/TiN-rGO5.78[127]
CNT-rGO paperCatalytic counter electrodeLiquid DSSC: FTO/TiO2/dye/I3/I1 mediated electrolyte/CNT-rGO6.05[117]
Graphene plateletsCatalytic counter electrodeLiquid DSSC: FTO/TiO2/dye/Co(III)/(II) mediated electrolyte/graphene9.3[126]

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-ethylenedioxythiophene):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 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).

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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.

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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 solution-processed 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).

image

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.

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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).

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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.

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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.

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% 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 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 ZnO-P3HT 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.

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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) JV 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) JV characteristics of DSSCs with Pt counter electrode. e) JV characteristics of DSSCs with GNP counter electrode. Reproduced with permission.[126] Copyright 2011, American Chemical Society.

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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).

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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 JV 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]

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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 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.

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 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.

Table 2. Summary of graphene and graphene-based composites used in active layers for solar cells
MaterialsFunctionConfiguration of solar cellsPCE [%]Ref.
Graphene QDsSensitizer of dyeLiquid DSSC: FTO/TiO2/graphene QD dye/I3/I1 mediated electrolyte/Pt<0.1[129]
Aniline-modified GO QDsElectron acceptorOPV: ITO/P3HT/ANI-GQDs/Al1.14[130]
rGO (called graphene in the cited paper)Electron acceptorOPV: ITO/PEDOT:PSS/P3OT:graphene/LiF/Al1.4[128]
Graphene–CdSe nanobeltsSchottky junction layerSchottky junction solar cell: Si/SiO2/graphene-CdSe nanobelt/In-Au1.25[134]
Graphene/n-SiSchottky junction layerSchottky junction solar cell: Ti/Pb/Ag/n-Si/graphene/Au1.65[133]
HNO3-modified-graphene/Si pillar arraySchottky junction layerSchottky junction solar cell: Ti/Pb/Ag/Si pillar array/graphene/Ti/Au7.7[135]
Graphene-TiO2Schottky junction layerLiquid DSSC: FTO/TiO2-graphene/dye/I3/I1 mediated electrolyte/Pt6.06[132]
TFSA-doped graphene/n-SiSchottky junction layerSchottky junction solar cell: In/Ga eutectic/n-Si/TFSA doped graphene/Cr/Au8.6[131]
rGO (called graphene in the cited paper)Electron transport layerOPV: ITO/PEDOT:PSS/P3HT:PDIa)-graphene/LiF/Al1.04[136]
rGOElectron transport layerLiquid DSSC: FTO/rGO/TiO2/dye/I3/I1 mediated electrolyte/Pt1.68[140]
rGOElectron transport layerLiquid DSSC: FTO/rGO/TiO2/dye/electrolyte/Pt6.97[139]
HNO3 oxidized CVD-grapheneElectron transport layerOPV: ITO/PEDOT:PSS/PCDTBTb)/PC71BMc)/GO/TiOx/Al7.5[151]
rGO-P3HTHole transport layerOPV: ITO/PEDOT:PSS/rGO-P3HT/C60/Al0.61[144]
GOHole transport layerOPV: ITO/GO/P3HT:PCBM/Al3.5[141]
p-TosNHNH2 reduced GO (pr-GO)Hole transport layerOPV: ITO/pr-GO/P3HT:PCBM/Ca/Al3.7[142]
GO-SWCNTHole transport layerOPV: ITO/GO-SWCNT/P3HT:PCBM/Ca/Al4.1[145]
GO and cesium-neutralized GO (GOCs)Hole and electron transport layerOPV: ITO/GO/P3HT:PCBM/GOCs/Al3.67[143]
MoO3-grapheneInterfacial layerSeries tandem solar cell: ITO/PEDOT:PSS/P3HT:PCBM/graphene-MoO3/ZnPc:C60/LiF/Al2.3[146]
  Parallel tandem solar cell: ITO/ZnO/P3HT: PCBM/MoO3-graphene-MoO3/ZnPc:C60/LiF/Al2.9 
ZnO-GO-PEDOT:PSSInterfacial layerSeries tandem solar cell: ITO/PEDOT:PSS/P3HT:PCBM/ZnO-GO-PEDOT:PSS/P3HT:PCBM/Ca/Al4.14[148]
GO-SWCNTsInterfacial layerRegular series tandem solar cell: ITO/GO: SWCNTs/P3HT:PCBM/ZnO-GO:SWCNTs/P3HT:PCBM/Ca/Al4.1[147]
  Inverted series tandem solar cell: ITO/ZnO/P3HT:PCBM/GO:SWCNTs/ZnO/P3HT:PCBM/GO:SWCNTs/Al3.5 
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.

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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.[128]

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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 (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.

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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.[130] Copyright 2011, American Chemical Society. b) JV characteristics of a typical nanocrystalline TiO2 solar cell sensitized by GQDs, in the dark and under illumination, respectively. Reproduced with permission.[129] 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.[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.

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 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.

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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.[133]

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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.[131] 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 JV 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.

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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) JV 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 JV 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.[131] 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

2.2.3.1. 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).[139] 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.

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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.[139] 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.[151] 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.

2.2.3.2 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.[145] 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.

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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.[145] c) Schematic illustration and d) current density–voltage characteristics of ITO/GO/P3HT:PCBM/Al devices with different GO thickness. Reproduced with permission.[141] 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.[142] 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%).[142] 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.

2.2.3.3. 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.[143] 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).

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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.[143]

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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.

2.2.3.4. 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.[148] 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).[148] This study proves the importance of the composite film, with both enhanced viscosity and conductivity characteristics, used for the interfacial layer in tandem cells.

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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) JV 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.[148] 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.[147] 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.[146] 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.

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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) JV 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) JV characteristics of the reference single cells (characterized individually from the parallel connected tandem cell) and parallel connected tandem cell under light illumination. The theoretical JV curve of the tandem cell is also constructed by summing the JV 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.[146]

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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.

3 Conclusions and Outlook

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Various Applications in Solar Cells
  5. 3 Conclusions and Outlook
  6. Acknowledgements
  7. Biographies

It is promising that graphene, as the transparent electrode material, has exhibited superiority in that it is highly flexible, an abundant carbon source, and has high thermal/chemical stability, compared to the traditional ITO. In particular, the flexible transparent electrodes show applications not only in solar cells, but also in flexible touch screens, displays, printable electronics, flexible transistors, memories, etc.[49-51, 79, 156-158]

In addition to working as transparent electrodes, graphene, GO, and their derivatives show many other important applications that include being electron/hole transporters and serving as interfacial layers and Schottky junction layers in photovoltaics devices. 2D GO is capable of π–π stacking and hydrogen bonding. This makes it possible to use such a 2D scaffold as the template to self-assemble GO-based novel inorganic, organic, and inorganic-organic hybrids with multifunctionalities[54, 159-164] for applications in photovoltaics. On the other hand, to enrich the application of graphene, processes on bandgap opening have always attracted the attention of scientists. To date, many methods have been investigated to engineer the band structure of graphene, including inducing a quantum confinement effect by reduction of graphene lateral size to form nanoribbons[165-169] or nanomesh,[170-175] introducing foreign elements,[176-178] and employing a strain effect from the substrate.[179-182] We believe that graphene will play more and more important roles in solar cells and other fields, such as energy storage, optoelectronics, electrics and sensing, in the near future.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Various Applications in Solar Cells
  5. 3 Conclusions and Outlook
  6. Acknowledgements
  7. Biographies

This work was supported by MOE under AcRF Tier 2 (ARC 10/10, No. MOE2010-T2–1–060), Singapore National Research Foundation under CREATE programme: Nanomaterials for Energy and Water Management, and NTU under the Start-Up Grant (M4080865.070.706022) in Singapore.

Biographies

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Various Applications in Solar Cells
  5. 3 Conclusions and Outlook
  6. Acknowledgements
  7. Biographies
  • Image of creator

    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.

  • Image of creator

    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.