Flexible Graphene Electrode-Based Organic Photovoltaics withRecord-High EfficiencyHyesung Park,†,‡,§,# Sehoon Chang,‡,§ Xiang Zhou,‡ Jing Kong,*,† Tomas Palacios,*,†

and Silvija Gradecak*,‡

†Department of Electrical Engineering and Computer Science and ‡Department of Materials Science and Engineering, MassachusettsInstitute of Technology, Cambridge, Massachusetts 02139, United States

*S Supporting Information

ABSTRACT: Advancements in the field of flexible high-efficiency solar cells and other optoelectronic devices willstrongly depend on the development of electrode materialswith good conductivity and flexibility. To address chemical andmechanical instability of currently used indium tin oxide(ITO), graphene has been suggested as a promising flexibletransparent electrode but challenges remain in achieving highefficiency of graphene-based polymer solar cells (PSCs)compared to their ITO-based counterparts. Here wedemonstrate graphene anode- and cathode-based flexiblePSCs with record-high power conversion efficiencies of 6.1 and 7.1%, respectively. The high efficiencies were achieved viathermal treatment of MoO3 electron blocking layer and direct deposition of ZnO electron transporting layer on graphene. Wealso demonstrate graphene-based flexible PSCs on polyethylene naphthalate substrates and show the device stability underdifferent bending conditions. Our work paves a way to fully graphene electrode-based flexible solar cells using a simple andreproducible process.

KEYWORDS: Flexible solar cell, polymer solar cell, graphene anode, graphene cathode

Flexible organic and hybrid organic−inorganic solar cellsrequire both the flexible photoactive media1,2 and flexible

electrodes with good conductivity and transparency. So far,indium tin oxide (ITO) has been an electrode material ofchoice for studies focusing on optimizing the morphology andchemistry of the photoactive media3 in polymer solar cells(PSCs). For flexible applications, graphene has been proposedas a promising replacement for ITO due to its mechanical andchemical robustness, excellent electrical and optical properties,and potentially low-cost processing.4,5 Recent studies havedemonstrated dramatic improvements in the efficiency ofPSCs6,7 that ascertain the bright future toward powerconversion efficiency (PCE) ≥ 10%, a threshold consideredfor industrial applications.2 In PSCs, and more generallyorganic solar cells (OSCs), one of the electrodes typicallyconsists of a transparent conductor, among which ITO is mostwidely used due to its good optical transparency and electricalconductivity. However, due to its nonuniform absorption,chemical and mechanical instability, as well as high cost ofindium,8 several alternative materials have been proposed,including carbon nanotube9,10 or metallic nanowires net-works.11 Furthermore, even at small mechanical stresses,microcracks are produced in ITO resulting in increased filmresistance and decreased device performance,12 thus limiting itsapplications for flexible OSCs.Owing to the unique optoelectronic properties of graphene5

several works on graphene-based OSCs have been reported,

demonstrating the feasibility of graphene in transparentelectrode applications.3,12−26 While the initial demonstrationshave been promising, performance of graphene-based devicesstill falls short (<3%) of recent advances accomplished in ITO-based devices (8−9%).6,7 Therefore, to demonstrate grapheneas an emerging alternative to ITO it is inevitable to improve thecurrently low efficiency of graphene-based solar cells. More-over, to test feasibility of graphene for flexible applications highperformance and stability of such devices must be investigated.In this work, we demonstrate high-efficiency graphene anode-and cathode-based PSCs with PCEs comparable to their ITO-counterparts. We also demonstrate graphene-based flexiblePSCs on polyethylene naphthalate (PEN) substrates and showthe device stability under different bending conditions.The overall PSC device structure used in this work and the

corresponding band diagram are shown in Figure 1a,b. Tofabricate efficient graphene-based OSCs we used photoactivemedia composed of a blend of low bandgap semiconductingpolymer donor thieno[3,4-b]thiophene/benzodithiophene(PTB7) and acceptor [6,6]-phenyl C71-butyric acid methylester (PC71BM) prepared using mixed solvents of chloroben-zene:1,8-diiodoctane (CB:DIO, 97:3 vol %).6 The holeinjection layer poly(3,4-ethylenedioxythiophene):poly-

Received: May 27, 2014Revised: August 8, 2014Published: August 20, 2014

Letter

pubs.acs.org/NanoLett

© 2014 American Chemical Society 5148 dx.doi.org/10.1021/nl501981f | Nano Lett. 2014, 14, 5148−5154

(styrenesulfonate) (PEDOT:PSS) was deposited on thetransparent graphene electrode. To ensure uniform coverageover the graphene surface, we used modified PEDOT:PSS withisopropyl alcohol (IPA) at 3:1 (v/v) ratio. Prior to the activedevice layer deposition, graphene/PEDOT:PSS must becovered by an additional electron blocking layer (MoO3).This process is critical in graphene-based bulk heterojunctionPSCs since the additional MoO3 layer prevents chargerecombination occurring at the graphene/PEDOT:PSS andthe polymer blend interface.27 We also note that although ITOdoes not require additional MoO3 layer, ITO control deviceswere fabricated with PEDOT:PSS/MoO3 for direct comparisonwith graphene-based devices.We discovered that one critical aspect of the device

fabrication is the thermal annealing of MoO3 layer beforespin-coating PTB7:PC71BM. When directly applyingPTB7:PC71BM to PEDOT:PSS/MoO3 on ITO or graphenewithout thermal annealing, considerable degradation in deviceperformance was observed, which did not occur onPEDOT:PSS only device. Figure 1c presents the currentdensity−voltage (J−V) characteristics of PSCs fabricated onITO substrates, in which addition of MoO3 layer (non-annealed) results in detrimental effects on the short-circuitcurrent density (JSC, 2.6 mA/cm2), open-circuit voltage (VOC,0.57 V), and fill factor (FF, 31.3%). However, significantimprovements in JSC (16.1 mA/cm2), VOC (0.68 V), FF(60.7%), and the resulting increase in PCE were observed afterthermal annealing of MoO3 electron blocking layer (Figure 1dand Supporting Information Table S1).Graphene-based PSCs were fabricated under the same

experimental conditions and the resulting J−V characteristics

are compared with that of ITO reference device in Figure 1d.The graphene electrode was made by stacking threemonolayers of graphene film prepared by low pressure chemicalvapor deposition with a typical sheet resistance of ∼300 Ω/sqand transmittance of ∼92% at λ = 550 nm.13 Withincorporation of appropriate PEDOT:PSS and thermallytreated MoO3, we observed record-high efficiency fromgraphene (PCE = 6.1%) approaching to ITO reference device(PCE = 6.7%) (Supporting Information Table S1).Because the efficient graphene PSC with PTB7:PC71BM was

accomplished only in the presence of MoO3 electron blockinglayer with subsequent thermal treatment, we investigated theeffect of CB:DIO solvent on MoO3 via several routes. First, thesurface morphology of MoO3 film (20 nm, on glass) wascharacterized using atomic force microscopy (AFM) after thethermal and solvent treatments (Figure 2a−d). Figure 2a showsthe surface topography of as-evaporated MoO3 film is smoothwith root-mean-square (rms) roughness of ∼1 nm. The thermaltreatment alone does not affect significantly topography ofMoO3 (Figure 2b). However, noticeable change in the surfacemorphology was observed after spin-coating CB:DIO onnonannealed MoO3 (Figure 2c), indicative of adverse effectsof the solvent. In contrast, after thermal treatment andsubsequent CB:DIO spin-coating on MoO3, the surface profile(Figure 2d) is similar to that of annealed MoO3 (Figure 2b)indicating that annealed MoO3 film becomes robust upon thesolvent treatment.We further characterized the effect of solvent treatment on

MoO3 via scanning transmission electron microscopy (STEM),as shown in Figure 2e−h, which revealed similar structuralchanges. STEM image of as-deposited MoO3 is largely

Figure 1. Device structure, energy levels, and performance of graphene- and ITO-based PSCs on quartz. (a) Schematic of a graphene anode-basedPSC in which additional MoO3 electron blocking layer is inserted between the conventional PEDOT:PSS hole injection layer and thePTB7:PC71BM polymer blend. MoO3 layer prevents the charge recombination occurring at the PEDOT modified graphene and polymer interface.(b) Corresponding flat-band energy level diagram. (c) AM1.5G J−V characteristics of ITO-based PSCs: ITO/PEDOT:PSS(40 nm)/(MoO3(20nm)/PTB7:PC71BM(200 nm)/Ca(25 nm)/Al(80 nm). The reference PEDOT:PSS-only device without the MoO3 layer is shown in black. WhenMoO3 film is inserted without any specific treatment to the conventional ITO/PEDOT:PSS-based polymer solar cell, significant degradation in thedevice performance is observed (blue). (d) AM1.5G J−V characteristics of graphene and ITO anode-based PSCs: anode (graphene or ITO)/PEDOT:PSS/MoO3(annealed at 150 °C for 10 min)/PTB7:PC71BM/Ca/Al). After the thermal treatment of MoO3 electron blocking layer (black),the ITO-based solar cell performance is similar to the one without the MoO3 (black in (c)). Graphene-based PSC exhibits device performancecomparable to that of an ITO reference cell fabricated under the same experimental conditions (red).

Nano Letters Letter

dx.doi.org/10.1021/nl501981f | Nano Lett. 2014, 14, 5148−51545149

featureless (Figure 2e) with minimum contrast differencesindicating a uniform structure. Significant difference is observedfrom the as-deposited MoO3 film after CB:DIO treatment(Figure 2g). Numerous contrast features with sizes on the orderof 50−100 nm were observed, which indicates that solventtreatment of the pristine MoO3 electron blocking layersignificantly degrades its structural uniformity. In contrast,solvent treatment did not significantly alter the morphology ofannealed MoO3, as shown in Figure 2h, indicating that theannealing prior to CB/CIO treatment makes the MoO3 filmmore robust and resistant to the solvent erosion. Both the AFMand STEM studies reveal that annealing step stabilizes theMoO3 structure and is thus crucial for the improved deviceperformance. The MoO3 morphology and device performanceannealed at different temperatures are shown in SupportingInformation Figure S1. On the basis of these results, annealingtemperature of 150 °C was chosen by considering the solventresistance of MoO3 film to CB:DIO and the glass transitiontemperature of PEN (155 °C) substrate for flexible devices.Next, ultraviolet photoelectron spectroscopy (UPS) was

performed to investigate the electronic structure of MoO3 filmafter the thermal and solvent treatment. Figure 2i,j shows theHe I UPS spectra in its full scan, including the photoemissiononset. For the pristine MoO3 film (Figure 2i), the photo-

emission onset occurs at 15.97 eV corresponding to a workfunction of 5.25 eV, and the solvent treatment shifts thephotoemission onset to a higher binding energy (16.43 eV)leading to downward-shift of the vacuum level and reduction inthe work function (4.79 eV). Annealing the films does notcause significant changes in the UPS spectra with thephotoemission onset of 15.94 eV (work function = 5.28 eV)and 16.46 eV (work function = 4.76 eV) before and after thesolvent treatment, respectively (Figure 2j). For both as-evaporated and annealed cases, MoO3 is slightly n-dopedafter the CB:DIO spin-coating as a result of oxygen vacancies inthe bulk of the material due to the adsorption of various speciesfrom the solvent onto MoO3.

28 Despite the slight decrease inwork function of both types of the film, the work functionvalues still remain relatively high suitable for the holetransporting material. Therefore, we suggest that the degradeddevice performance (Figure 1c, blue) primarily originates fromthe drastic morphology changes of MoO3 resulting upon theinteraction with CB:DIO (Figure 2a−h), which can beremedied via simple anneal treatment introduced in this work.In addition to the graphene anode-based devices discussed

above, we also investigated inverted cathode-based PSCconfiguration that can provide improved device stability byavoiding easily oxidized low work function metal electrodes

Figure 2. Effect of solvent and anneal treatment to MoO3 film: AFM, STEM, and UPS spectra of MoO3 (20 nm). AFM images of (a) as-depositedMoO3 film; (b) annealed MoO3 film at 150 °C for 10 min; (c) CB:DIO treated as-deposited MoO3 film; (d) CB:DIO treated MoO3 film afterannealing at 150 °C for 10 min. (e−h) corresponding STEM images from (a−d). As shown in (c,g), significant morphological changes are observedon the pristine MoO3 film (prior to annealing) after exposing to the solvent, which are mostly absent once the MoO3 film is thermally treated. Suchchanges are absent for annealed MoO3 film regardless of solvent treatment, as shown in (f,h). The circular features in (g) with diameters of a fewhundred nanometers are evaporation marks left behind by the solvent drying. (i) UPS spectra of as-deposited MoO3; the photoemission onset isshifted toward the lower binding energy after the solvent treatment. The corresponding work function change is from 5.25 to 4.79 eV. (j) UPSspectra of annealed MoO3 shows similar trend as the pristine MoO3 in which the photoemission onset shows shifts toward the lower binding energyafter the solvent treatment. The corresponding work function change is from 5.28 to 4.76 eV.

Nano Letters Letter

dx.doi.org/10.1021/nl501981f | Nano Lett. 2014, 14, 5148−51545150

such as Al or Ca.8,29 In this structure, n-type semiconductingmetal oxides such as ZnO or TiOx can be utilized as an effectiveelectron transporting path from the photoactive polymer layerto the cathode. After achieving uniform coverage of the electrontransporting ZnO layer directly on graphene surface by a simplespin-coating method (see Supporting Information Figure S2),graphene cathode-based inverted PSCs and their ITO-basedcounterparts were demonstrated with the following devicestructure: graphene (or ITO)/ZnO/PTB7:PC71BM/MoO3/Ag. The device structure, corresponding band diagram, andmeasured J−V characteristics are shown in Figure 3. Asillustrated in Figure 3c, both graphene- and ITO-based invertedsolar cells show great similarities in device performance withPCEs of 6.9 and 7.6%, respectively. We note that the invertedcathode-based PSCs fabricated on graphene electrodes withoutthe ZnO layer exhibit negligible photoresponse (SupportingInformation Figure S3), indicating the importance of electrontransporting layer on the graphene cathode.With growing interests in flexible electronic devices,30 as the

final step we have explored the potential of our devicestructures to realize flexible graphene-based PSCs. We haverealized both anode- and cathode-based device architectures onPEN substrates and tested their performance under mechanicaltensile bending conditions. For that purpose, the devices weresubjected to consecutive flexing cycles at 5 mm radius,corresponding to the strain of ∼4.3%. The resulting graphenePSCs on PEN substrates show excellent device performance forboth anode (PCE = 6.1%, Figure 4a and SupportingInformation Table S1) and cathode (PCE = 7.1%, Figure 4band Supporting Information Table S2) configurations. Ourgraphene-based flexible PSCs are robust under mechanicaldeformations, which is highly desirable for low-cost productionssuch as roll-to-roll processing and applications that requireflexibility. As shown in Figure 4c, the device (graphene anode)did not display any significant performance changes up to 100tensile flexing cycles. Key photovoltaic parameters at

intermediate flexing cycles are shown in Figure 4e,f andSupporting Information Table S3.We demonstrated highly efficient flexible graphene-based

PSCs with record PCE of 7.1%. The performances of ourgraphene-based devices are the highest reported in theliterature: Figure 5 summarizes the device performance ofgraphene-based OSCs with various polymer acceptors reportedin the literature along with our results for both normal(graphene anode) and inverted (graphene cathode) cells withlow bandgap PTB7. The successful demonstration of bothtypes of graphene electrode-based flexible devices in this workis an important milestone toward fully graphene electrodeintegrated flexible solar cells. This work is accomplished viathermal treatment of MoO3 hole transporting layer ongraphene anode, which resolves the compatibility issue ofpristine MoO3 with CB:DIO solvent for polymer deposition.Furthermore, we developed a simple spin-coating method fordirect deposition of ZnO electron transporting layer ongraphene for the cathode application. The demonstration ofhigh-efficiency graphene-based flexible PSCs with remarkablemechanical robustness opens up a bright future for variety ofgraphene-based flexible optoelectronic devices.

Methods. Graphene Synthesis. Graphene films weresynthesized via low pressure chemical vapor deposition onCu foil (25 μm in thickness). The growth chamber wasevacuated to a base pressure of ∼50 mTorr and the Cu wasannealed at 1000 °C for 30 min under hydrogen gas (9 sccm,∼300 mTorr). Subsequently, methane gas (17 sccm, ∼800mTorr) was added and the graphene growth was performed for30 min. Graphene sheets were transferred to quartz substratesusing PMMA13 and the PMMA layer was consequentlyremoved by acetone. Repeated transfers were performed toobtain three-layer graphene stacks.

PEDOT:PSS Modification for Graphene film. PEDOT:PSS(Clevios P VP AI 4083) solution was modified by isopropylalcohol via thorough mixing. After filtering the PEDOT:PSS(0.45 μm), it was mixed with IPA (3:1, v/v ratio) followed by

Figure 3. Graphene and ITO cathode-based PSCs: cathode (graphene or ITO)/ZnO(20 nm)/PTB7:PC71BM(200 nm)/MoO3(20 nm)/Ag(100nm). (a) Schematic of inverted PSCs and (b) corresponding flat-band energy level diagram. (c) AM1.5G J−V characteristics of a graphene-baseddevice, demonstrating comparable performance to that of an ITO reference cell.

Nano Letters Letter

dx.doi.org/10.1021/nl501981f | Nano Lett. 2014, 14, 5148−51545151

24 h of rigorous stirring at room temperature. This mixedsolution was spin-coated in air at 4000 rpm for 60 s andannealed at 170 °C for 5 min in air.Solar Cell Fabrication. Prepatterned ITO substrates (Thin

Film Devices, 150 nm thick, 20 Ω/sq, 85%T) were cleaned bysonication in soap water, DI water, acetone, and isopropanol,followed by oxygen plasma cleaning. Patterned graphenesubstrates were rinsed by acetone and IPA. ZnO layer wasdeposited by spin-coating 300 mM of zinc acetate dehydratemethanol solution followed by annealing at 175 °C for 10 min.Polymer blend solution was prepared by dissolving PTB7 (12mg/mL, 1d-material) and PC71BM (40 mg/mL, Sigma-Aldrich) in CB:DIO (97%/3% by volume) and mixing themat 2:1 volume ratio. The mixed solution was spin-coated at1000 rpm for 2 min in nitrogen-filled glovebox. MoO3, Ag, Ca,and Al were thermally evaporated through the shadow mask at

1.0 Å/s. MoO3 was annealed at 150 °C for 10 min. The devicearea (1.21 mm2) was defined by the overlap between the topand bottom electrodes. The substrate size was 12.5 × 12.5 mmfor both ITO and graphene.

Measurement and Characterization. Current−voltagecharacteristics of the solar cells were recorded using a Keithley6487 picoammeter source-meter in a nitrogen-filled glovebox. A100 mW/cm2 illumination was provided by a 150 W xenon arc-lamp (Newport 96000) equipped with an AM 1.5G filter. Forthe bending test of flexible graphene anode devices, J−Vmeasurements were recorded after 20, 50, and 100 compressiveflexing cycles at ∼5 mm radius.The surface morphologies of MoO3 were characterized from

Digital Instruments Veeco Dimension 3100 AFM operated intapping mode. STEM images were obtained using a JEOL2010F with an accelerating voltage of 200 kV. MoO3 (20 nm)

Figure 4. Graphene-based flexible PSCs on PEN. (a) AM1.5G J−V characteristics of a representative graphene anode-based device fabricated onPEN: graphene anode/PEDOT:PSS/MoO3/PTB7:PC71BM/Ca/Al. (b) AM1.5G J−V characteristics of a representative graphene cathode-baseddevice fabricated on PEN: graphene cathode/ZnO/PTB7:PC71BM/MoO3/Ag. (c) AM1.5G J−V characteristics of the champion graphene devicesbefore and after different flexing cycles, displaying no significant changes in the device performance after the mechanical deformation. (d) A digitalphotograph of a flexible graphene PSC. (e,f) Photovoltaic performance characteristics (JSC, VOC, FF, and PCE) of the devices displayed in (c). Linesserve as a guide to the eye.

Nano Letters Letter

dx.doi.org/10.1021/nl501981f | Nano Lett. 2014, 14, 5148−51545152

films were deposited on rigid TEM grids with 30 nm SiNxwindows and were subjected to identical solvent and thermaltreatment as the AFM analysis.UPS Measurement. After thermally evaporating the MoO3

(20 nm) on Au-coated glass substrate, the samples weretransferred to UPS analysis chamber (<10−10 Torr) withoutexposing it to air. UPS was done using a He I (21.22 eV)radiation line from a discharge lamp with an experimentalresolution of 0.02 eV.

■ ASSOCIATED CONTENT*S Supporting InformationMoO3 annealing with varying temperature, ZnO ETLdeposition with various solvents, inverted cathode-baseddevices fabricated on graphene electrodes without the ZnOlayer, and additional figures, tables, and references. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: (J.K.) jingkong@mit.edu.*E-mail: (T.P.) tpalacios@mit.edu.*E-mail: (S.G.) gradecak@mit.edu.Present Address#School of Energy and Chemical Engineering, Ulsan NationalInstitute of Science and Technology (UNIST), Ulsan 689−798,Republic of KoreaAuthor Contributions§H.P. and S.C. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by Eni S.p.A. under the Eni-MITAlliance Solar Frontiers Program. The authors acknowledgeaccess to Shared Experimental Facilities provided by the MIT

Center for Materials Science Engineering supported in part byMRSEC Program of National Science Foundation under awardnumber DMR - 0213282. The authors also gratefullyacknowledge Professor Vladimir Bulovic for the use ofexperimental equipment for organic solar cell fabrication andtesting, and Patrick Brown for UPS measurement.

■ REFERENCES(1) Li, G.; Zhu, R.; Yang, Y. Nat. Photonics 2012, 6, 153−161.(2) Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia, S.;Williams, S. P. Adv. Mater. 2010, 22, 3839−3856.(3) Park, H.; Chang, S.; Smith, M.; Gradecak, S.; Kong, J. Sci. Rep.2013, 3, 1581.(4) Bae, S.; Kim, H.; Lee, Y.; Xu, X. F.; Park, J.-S.; Zheng, Y.;Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.;Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Nat. Nanotechnol.2010, 5, 574−578.(5) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Nat. Photonics2010, 4, 611−622.(6) He, Z. C.; Zhong, C. M.; Su, S. J.; Xu, M.; Wu, H. B.; Cao, Y. Nat.Photonics 2012, 6, 591−595.(7) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano,A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll,M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.;Barlow, S.; Graham, S.; Bredas, J.-L.; Marder, S. R.; Kahn, A.;Kippelen, B. Science 2012, 336, 327−332.(8) Jørgensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol.Cells 2008, 92, 686−714.(9) Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H. J.;Dennler, G.; Sariciftci, N. S.; Hu, L. B.; Gruner, G. Appl. Phys. Lett.2006, 88, 233506.(10) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.;Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511.(11) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj,P. N.; Blau, W. J.; Boland, J. J.; Coleman, J. N. ACS Nano 2009, 3,1767−1774.(12) De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.;Thompson, M. E.; Zhou, C. W. ACS Nano 2010, 4, 2865−2873.(13) Park, H.; Brown, P. R.; Buloyic, V.; Kong, J. Nano Lett. 2012, 12,133−140.(14) Wang, Y.; Tong, S. W.; Xu, X. F.; Ozyilmaz, B.; Loh, K. P. Adv.Mater. 2011, 23, 1514−1518.(15) Wang, Y.; Chen, X. H.; Zhong, Y. L.; Zhu, F. R.; Loh, K. P. Appl.Phys. Lett. 2009, 95, 063302.(16) Choe, M.; Lee, B. H.; Jo, G.; Park, J.; Park, W.; Lee, S.; Hong,W.-K.; Seong, M.-J.; Kahng, Y. H.; Lee, K.; Lee, T. Org. Electron. 2010,11, 1864−1869.(17) Park, H.; Rowehl, J. A.; Kim, K. K.; Bulovic, V.; Kong, J.Nanotechnology 2010, 21, 505204.(18) Lee, Y.-Y.; Tu, K.-H.; Yu, C.-C.; Li, S.-S.; Hwang, J.-Y.; Lin, C.-C.; Chen, K.-H.; Chen, L.-C.; Chen, H.-L.; Chen, C.-W. ACS Nano2011, 5, 6564−6570.(19) Zhang, D.; Choy, W. C. H.; Wang, C. C. D.; Li, X.; Fan, L. L.;Wang, K. L.; Zhu, H. W. Appl. Phys. Lett. 2011, 99, 223302.(20) Zhao, Z. Y.; Fite, J. D.; Haldar, P.; Lee, J. U. Appl. Phys. Lett.2012, 101, 063305.(21) Hsu, C.-L.; Lin, C.-T.; Huang, J.-H.; Chu, C.-W.; Wei, K.-H.; Li,L.-J. ACS Nano 2012, 6, 5031−5039.(22) Park, H.; Howden, R. M.; Barr, M. C.; Bulovic, V.; Gleason, K.;Kong, J. ACS Nano 2012, 6, 6370−6377.(23) Jo, G.; Na, S. I.; Oh, S. H.; Lee, S.; Kim, T. S.; Wang, G.; Choe,M.; Park, W.; Yoon, J.; Kim, D. Y.; Kahng, Y. H.; Lee, T. Appl. Phys.Lett. 2010, 97, 213301.(24) Cox, M.; Gorodetsky, A.; Kim, B.; Kim, K. S.; Jia, Z.; Kim, P.;Nuckolls, C.; Kymissis, I. Appl. Phys. Lett. 2011, 98, 123303.(25) Shin, K.-S.; Jo, H.; Shin, H.-J.; Choi, W. M.; Choi, J.-Y.; Kim, S.-W. J. Mater. Chem. 2012, 22, 13032−13038.

Figure 5. Device performance of graphene-based OPVs with differentactive layers. Summary of PCEs of graphene electrode-based organicsolar cells from literature.3,12−26 Triangles stand for graphene anodedevices and circles represent graphene cathode devices withcorresponding references indicated for each result. Stars indicate theresult from this work, illustrating dramatic improvement in the deviceperformance achieved in our work.

Nano Letters Letter

dx.doi.org/10.1021/nl501981f | Nano Lett. 2014, 14, 5148−51545153

(26) Zhang, D.; Xie, F.; Lin, P.; Choy, W. C. H. ACS Nano 2013, 7,1740−1747.(27) Park, H.; Shi, Y. M.; Kong, J. Nanoscale 2013, 5, 8934−8939.(28) Meyer, J.; Shu, A.; Kroger, M.; Kahn, A. Appl. Phys. Lett. 2010,96, 133308.(29) Hau, S. K.; Yip, H. L.; Baek, N. S.; Zou, J. Y.; O’Malley, K.; Jen,A. K. Y. Appl. Phys. Lett. 2008, 92, 253301.(30) Angmo, D.; Krebs, F. C. J. Appl. Polym. Sci. 2013, 129, 1−14.

Nano Letters Letter

dx.doi.org/10.1021/nl501981f | Nano Lett. 2014, 14, 5148−51545154