Review Article A Review on the Efficiency of Graphene...

Review ArticleA Review on the Efficiency of Graphene-BasedBHJ Organic Solar Cells

Alejandro Manzano-Ramiacuterez1 Edgar J Loacutepez-Naranjo2

Wole Soboyejo3 Yunny Meas-Vong4 and Bertrand Vilquin5

1CINVESTAV-IPN Unidad Queretaro 76230 Santiago de Queretaro QRO Mexico2Universidad Autonoma de Nayarit Ciudad de la Cultura Amado Nervo sn 63190 Tepic NAY Mexico3Princeton Institute of Science and Technology of Materials (PRISM) Bowen Hall 70 Prospect Street Princeton NJ 08544 USA4CIDETEQ Parque Tecnologico Queretaro Sanfandila 76703 Pedro Escobedo QRO Mexico5Univertiste de Lyon Ecole Centrale de Lyon Institut des Nanotechnologies de Lyon 36 Avenue Guy de Collongue69134 Ecully Cedex France

Correspondence should be addressed to Alejandro Manzano-Ramırez amanzanocinvestavmx

Received 19 May 2015 Revised 30 August 2015 Accepted 31 August 2015

Academic Editor Michal Otyepka

Copyright copy 2015 Alejandro Manzano-Ramırez et al This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Graphene a material composed of one-atom-thick planar sheets of sp2-bonded carbon atoms with a two-dimensional honeycombstructure has been proposed for many applications due to its remarkable electronic optical thermal and mechanical propertiesIts high transparency conductivity flexibility and abundance make it an excellent material to be applied in the field oforganic photovoltaic cells especially as a replacement for transparent conducting oxide electrodes However graphene has beendemonstrated to be useful not only as substitute for indium tin oxide electrodes but also as cathode electron acceptor holetransport and electron extraction material Thus in this work we summarize and discuss the efficiency of bulk heterojunctiondevices using graphene as a main constituent

1 Introduction

Organic solar cells (OSCs) belong to the class of photovoltaicdevices known as excitonic solar cells which are char-acterized by strongly bound electron-hole pairs (excitons)that are formed after excitation with light Strongly boundexcitons exist in these materials as a consequence of thelow dielectric constants in the organic components whichare insufficient to affect direct electron-hole dissociation asit is found in their high dielectric inorganic counterpartsIn excitonic solar cells exciton dissociation occurs almostexclusively at the interface between two materials of differentelectron affinities (andor ionization potentials) the electrondonor and the electron acceptor To generate an effectivephotocurrent in these organic solar cells an appropriatedonor-acceptor pair and device architecturemust be selectedTwo main approaches have been explored in the effort to

develop viable devices the donor-acceptor bilayer and the so-called bulk heterojunction (BHJ) which is represented in theideal case as a bicontinuous composite of donor and acceptorphases thereby not only maximizing the donoracceptorinterface for efficient exciton dissociation but also forminga nanoscale interpenetrating network for charge transport tothe electrodes [1 2] (Figure 1) Organic solar cell research anddevice development still have a long way to go to competewith inorganic solar cells The efficiency of inorganic devicescan top 20 and the development of inorganic thin-layer andmultijunction instruments will likely lead to even better per-formance In contrast the best OSCs based on the BHJ con-cept operate at amaximumofsim80 efficiencyHowever dueto its low cost lightweight high availability of materials andeasy manufacturing the OSCs technology turns very appeal-ing as it could theoretically produce highly efficient devices[3ndash5]

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 406597 15 pageshttpdxdoiorg1011552015406597

2 Journal of Nanomaterials

Ano

de

Cath

ode

Solar light

h+ eminush+ eminush+ eminush+ eminus

(a) (b) (c) (d)

Figure 1 BHJ cell operation mechanism (a) Exciton generation (b) exciton dissociation (c) charge formation and (d) charge transport andcollection

Based on the simple consideration that low band gapmaterials have the possibility to improve the efficiency ofphotovoltaic devices due to a better overlap with the solarspectrum it is expected that for amaximumphoton harvest-ing in the OSCs low band gap materials will be employedThe use of this type ofmaterials results in higher currents thatcan lead to higher power conversion efficiency (PCE) Poly-3-hexylthiophene (P3HT) one of the most used materials asdonor in a BHJ device exhibits a band gap of 650 nm (19 eV)and thus only has the possibility to harvest up to 224 ofthe available photons giving a maximum theoretical currentdensity of 143mAcmminus2 If P3HT is applied in a BHJ devicecoupled with the most commonly used acceptor (ie phenyl-C61-butyric acid methyl ester PCBM) the band gap extends

to 1000 nm allowing for absorption of 53 of the availablephotons giving a maximum current density of 339mAcmminus2However in practical terms these values cannot be achievedand the real performance of a P3HT PCBM system is aroundits optimal of about 5 which is lower than a quarter of asilicon solar cellrsquos optimal [6 7]

Graphene a zero-band-gap semiconductor is a rising starin materials science (Figure 2) It shows a two-dimensional(2D) structure consisting of sp2-hybridized carbon and itis also the thinnest known material in the universe andthe strongest ever measured [8ndash11] It exhibits remarkableelectronic and mechanical properties and a considerablenumber of new optical effects that have not been observed inother materials [12 13] Its remarkably high carrier mobilitywide absorption spectral range andhigh optical transparencyindicate graphene as a cost-effective and abundant materialthat qualifies for application in optoelectronic devices suchas displays light-emitting diodes and solar cells [14ndash17]

Different routes are available for the preparation ofgraphene [12 18 19] The first involves chemical vapor depo-sition (CVD) of monolayer of graphite on transition metalsurfaces [20 21] The second route is the micromechanicalexfoliation of graphite and involves peeling of the graphenefrom graphite using ldquoScotchrdquo tape [22] The ldquoScotchrdquo tapeis then dipped in acetone to release the graphene whichis subsequently captured on a silicon wafer with a SiO

2

Figure 2 Representation of graphene structure

(silicon dioxide) layer on top The third route involvesthe epitaxial growth of graphene on electrically insulatingsubstrates like silicon carbide [23] However the second andthird methods are unsuitable for large-scale preparation ofgraphene for the fabrication of polymer nanocomposites [19]Thus it appears that the only routes to prepare bulk quantitiesof graphene and chemically modified graphene (CMG) arefrom graphene oxide (GO) and graphite intercalation com-pounds (GICs)The presence of hydroxyl and epoxide groupson the graphitic basal planes and carboxyl and carbonylgroups on the edges of layers makes GO water dispersible[24] Thus on sonication in aqueous media GO exfoliatesreadily into colloidal suspensions of single graphene oxidelayers Chemical reduction of GO present in the colloidalsuspension with reducing agents is necessary to convert theelectrically insulatingGO layers back to conducting graphene

Journal of Nanomaterials 3

suitable for composite applications The final route by whichbulk quantities of graphene can be prepared is by thermallyreducing GO This technique involves rapidly heating GO ininert (argon or nitrogen) environment to produce thermallyreduced expanded graphene oxide (TRGO) [25 26] Dueto its wrinkled nature TRGO do not collapse back to GObut remain highly agglomerated However McAllister et al[27] observed that after TRGO was dispersed by ultrasoni-cation in appropriate solvents 80 of the flakes were singlegraphene sheets

Derivatives of graphene such as GO and reducedgraphene oxide (rGO) are also being widely investigatedfrom the point of view of primary and practical applications[16] rGO can be easily prepared by wet processes such asspin- dip- and spray-coating to give large-area GO filmsand by hydrazine vapor treatment to reduce the GO filmsSecondary treatments such as thermal annealing have beenindicated to increase the photocurrent and device efficiencyby removing defects and optimizing the morphology ofgraphene-based cell constituents [2 16]

Regarding the PCE in BHJ devices the highest value wasobserved to be sim74 for the device with ITOPEDOT PSSPBTTT-3 PC

70BMCaAl (indium tin oxidepoly(34-ethyl-

enedioxythiophene) poly(styrenesulfonate)poly(25-bis(3 tetradecylthiophen-2-yl)thieno(32-b)thiophene)3 phenyl-C70-butyric acid methyl estercalciumaluminum) structure

[16] Photovoltaic devices based on graphene are still notas efficient as for example devices with ITO electrodesor organic acceptors mainly because the conductivities ofgraphene are relatively low [9 28] Theoretical PCEs as highas 12 have been calculated for graphene-based organicdevices [29] and it has also been proposed that graphenemay favor charge carrier multiplication [30] Graphene hasproved to be useful not only as substitute for ITO electrodesbut also as cathode electron acceptor hole transport andelectroextraction material (Figure 3) [15 31 32] Thereforethis review is intended to examine the efficiency of differentgraphene-based devices and the newest trends in order tomaximize their efficiencies

2 Electrode Material

In BHJ devices the conventional transparent conductiveelectrode used to couple photons into the active layer is exten-sively the ITO-based electrode that shows a sheet resistanceof less than 100Ωsq and an optical transparency of sim90[2 33] ITO electrodes consume as much as 30 of the fabri-cation cost in solar cells and their use seems to be increasinglyproblematic due to the limited indium source susceptibilityto ion diffusion into polymer layers instability in acid or baseand the brittle nature of metal oxide [34ndash36] AdditionallyKrebs et al reported that the energy involved in processingthe ITO electrode accounts for 87 in roll-to-roll basedOSCs fabrication [37]Thus a substitute for ITO with similarperformance but lower cost is clearly needed In this regardgraphene has risen as a promising electrode material due toits high theoretical surface area of 2630m2g and ability tofacilitate electron or hole transfer along its two-dimensional

Substrate

AnodeHole transport layerActive layerElectron extraction layerCathode

Acceptor material

Donor material

Figure 3 Configuration of a typical BHJ solar cell

surface [13 22 35] The high transparency conductivityflexibility and elemental abundance of graphene indicate thatit is an excellent replacement for transparent conductingoxide electrodes [38ndash40] Graphene transparent electrodescan be expected to be applied not only for the substitutionof ITO that is usually used as anode but also for cathode forwhich low work function metals are used [41 42]

However for chemically processed graphene mainly thecontact resistance of the small graphene flakes and theinsulating property of the graphene chemically reduced fromgraphene oxide limit the device performance The structuraldefects and lateral disorder of such chemically exfoliatedgraphite affect the carrier mobility of the film negatively[43] Therefore although graphene could provide a low-cost alternative for ITO there is still tremendous roomfor improvement in terms of enlarging the lateral scale ofgraphene in these devices [44]

21 Anode Xu et al [45] reported on polymer organic photo-voltaic cells using pure graphene films as the transparentanode directly fabricated from graphene solutions by spin-coating with P3HT as the electron donor and PCBM asthe acceptor The graphene films reduced with hydrazineand annealed at temperatures of 300 500 and 700∘C hadtransmittance of 75 73 and 69 at a wavelength of550 nm respectively Under these experimental conditionsthe reduced graphene films could have a sheet resistanceof 179 kΩsq and a conductivity of 223 Scm which iscomparable with that obtained using more complicatedgraphene composites The current density-voltage (119869-119881)characteristic of the device (hydrazine + 700∘C) underillumination of simulated solar light shows a short-circuitphotocurrent density (119869sc) of 118mAcm2 with an opencircuit voltage (119881oc) of 046V a calculated filling factor (FF)of 2458 and an overall PCE of 013 The ITO deviceused as a reference was fabricated under the same conditionsand had a PCE of 359 The low PCE of the graphene-based device is likely due to the hydrophobic graphenefilm surface which makes it rather hard to get a uniformpoly(34-ethylenedioxythiophene) poly(styrenesulfonate)(PEDOT PSS) layer So it is highly possible that withoptimized graphene-electrode conductivity and graphenefilm surface wetting property the performance of thesegraphene-electrodes cells could be improved significantly[45] Although this approach provides a cost-effective simple


solution-based process the overall PCE of the graphene-based device is lower than that reported for the best OSCsbased on the BHJ concept (sim80)

Yin et al [34] indicated that the performance of theorganic devices mainly depends on the charge transportefficiency through rGO electrodes when the optical transmit-tance of rGO is above 65 However if the transmittanceof rGO is less than 65 the performance of the device isdominated by the light transmission efficiency that is thetransparency of rGO films In this work highly conductiverGO film was transferred onto a polyethylene terephthalate(PET) substrate which was used as the transparent electrodeThe authors reported that lowering the rGO sheet resistancevia increasing the rGO thickness notably enhanced thecurrent density of devices and thus the overall PCE In orderto study the effect of rGO thickness on the performance ofthe devices rGO films with different thickness were prepared(4 nm 10 nm 16 nm and 21 nm) The highest optical trans-mittance 88 was obtained in the 4 nm thick rGO films butthe sheet resistance dramatically increased to 160 kΩsqThehighest PCE obtained in the flexible rGOPET devices was078 and corresponded to the 16 nm thick rGO cells whichshowed the highest 119869sc and 119881oc These devices showed anexcellent stability after applying the bending-induced tensionstress and their performance was well maintained even afterbending a thousand times [34]

Wang et al [46] described the fabrication and characteri-zation of transparent graphene-constructed films (TGFs) tobe used as hole collecting electrodes in organic solar cellsThe transparency of TGFs on quartz was tuned by controllingthe film thickness by varying the solution concentrationDifferent thicknesses (30 22 12 and 4 nm) had transmittanceof 55 66 80 and 90 respectively at a wavelength of 500 nmThe electrical characteristics of the as-prepared 30 nm thickTGFs were evaluated by a four-point probe measurementwhich showed resistance of 16 kΩsq and a conductivityof 206 Scmminus1 To demonstrate how the TGFs performed asanodes organic solar cells were fabricated using a blendof P3HT and PCBM The photoactive composite layer wassandwiched between TGFsquartz and Ag electrode TheTGFs on quartz had a sheet resistance of 18 kΩsq and atransmittance of 85 at 500 nmThedeviceswere illuminatedwith monochromatic light and the highest external quantumefficiency (EQE) of around 43was achieved at a wavelengthof 520 nm This efficiency is comparable to the highestEQE value of 47 for a reference device fabricated undersimilar conditions but with ITOglass as the anode Thecurrent-voltage (119868-119881) characteristics of the device showed adistinct diode behavior under monochromatic light with awavelength of 510 nm In this case PCE was of 153 andunder simulated solar light PCE was 029 [46]

Wang et al [44] produced large area continuous trans-parent and highly conducting few-layered graphene films(FLGFs) by chemical vapor deposition method to be used asanode for application in photovoltaic devices In the solution-processed organic solar cells the anode layer should possessgood hydrophilicity to allow the spreading of the hole trans-port layer In order to tune the surface wettability without

degrading the conductivity the graphene film was noncova-lent modified with pyrene buanoic acid succidymidyl ester(PBASE) as a result the PCE was of 171 This performancecorresponds to sim552 of the PCE of a control device basedon an ITO anode The electronic energy levels of graphenebefore and after modification with PBASE were investigatedthe results indicated that graphene maintained a nearly zero-gap condition after the treatmentThus since the main draw-back of graphene film is its poor hydrophilicity which led toa negative effect on the construction of solution-processeddevices PBASE treatment resulted to be an effective methodto overcome this issue successfully [44]

Gomez De Arco et al [47] reported on the implementa-tion of continuous highly flexible and transparent graphenefilms obtained by CVD and used as transparent conductiveelectrodes The thickness of CVD graphene was on theorder of 1ndash3 nm According to the authors the use of CVDgraphene is more attractive than the other graphene filmsfor example those that are formed by stacked micrometersized flakes that suffer from flake to flake contact resistanceand high roughness In the case of CVD graphene thegrain boundaries have the advantage of being formed insitu during synthesis such process is expected to minimizecontact resistance between neighboring graphene domainsand may result in smoother films with better conductingproperties Solar cells made with CVD graphene exhibitedperformance that compares to ITO device and surpasses thatof ITO devices under bending conditions exhibiting PCE of118 which is nearly 93 of that shown by the ITO deviceContrary to the conventional materials used in BHJ OSCs inthis case the multilayered configuration employed a copperphthalocyanine (CuPc) fullerene (C

60) and bathocuproine

(BCP) mixture as active layer [47]Jung et al [48] reported the characteristics of two dif-

ferent types of OSCs fabricated on HNO3- (nitric acid-)

treated and untreated multilayer graphene (MLG) transpar-ent electrodes prepared using chemical vapor depositionand a multitransfer process MLG films were formed bystacking graphene films three times on the same substrateand were used as the device anodes OSCs fabricated onuntreated transparent electrodes had a 119881oc of 0575V a 119869sc of808mACM2 FF of 435 and a PCE of 202 In contrastapplication of HNO

3-treated films as transparent electrodes

led to an improved performance with a 119881oc of 0602V a119869sc of 826mAcm2 FF of 575 and a PCE of 268 Theresults suggested thatHNO

3treatment is a promisingmethod

improving the performance of large-area OSCs [48]Choe et al [49] presented the results of applying mul-

tilayer graphene (MLG) films as transparent conductiveelectrodes The MLG films were synthesized into a CVDchamber under a flow of H

2in Ar gas mixture at different

growth temperatures MLG films prepared at 800 900 and1000∘C showed the sheet resistances of 1730plusmn600 990plusmn400and 610plusmn140Ωsq and the mobility of 660plusmn270 1030plusmn440and 1180plusmn 260 cm2V s respectivelyThe performance of thedevices with 1000∘C grown MLG films was found to be thebest with a PCE of sim13 The PCE was further enhancedwhen a hole-blocking titanium suboxide (TiO

119909) layer was


Table 1 PCE of BHJ OSCs using graphene as anode

Material Device conformation Highest PCE ReferenceTGFs QuartzTGFsP3HT PCBMAg 153 [46]FLGFs FLGFsPEDOT PSSP3HT PCBMLiFAl 171 [44]Graphene QuartzgraphenePEDOT PSSP3HT PCBMLiFAl 013 [45]rGO PET rGO PEDOT PSSP3HT PCBMTiO

2Al 078 [34]

Graphene PET CVD graphenePEDOT PSSCuPcC60BCPAl 118 [47]

MLG GlassMLGPEDOT PSSP3HT PCBMTiO119909Al 26 [49]

CCG CCGPEDOT PSSP3HT PCBMAl 101 [51]MLG MLGMoO

3+ PEDOT PSSP3HT PCBMLiFAl 25 [36]

MLG MLGPEDOT PSSP3HT PCBMCaAl 268 [48]MLG GlassMLGPEDOT PSSP3HT PCBMCaAl 117 [50]

inserted in the device structure resulting in a PCE of sim26which is a significantly higher efficiency compared to theother graphene-based photovoltaic cells [49]

Choi et al [50] fabricatedOSCs based onMLGelectrodeswhich were prepared using the CVD method and a mul-titransfer process on a glass substrate The MLG electrodetransferred onto a glass substrate showed a sheet resistanceof 374 plusmn 3Ωsq and an optical transparency of 842 Thesheet resistance of the MLG (4 layers) film was higher thanthose reported elsewhere because in this case no chemicaltreatment was performed on the filmThe transparency of theMLGfilmwas fairly constant regardless of wavelength unlikethe conventional ITO film showing transparencymodulationin the blue wavelength region To prepare the MLG electrodeon glass a thin layer of graphene was grown on Cu foilsusing CVD the transfer process was repeated four timeson the transparent glass substrate until the MLG film wasobtained The PCE of the BHJ device was 117 indicatingthat MLG films are a promising indium-free transparentelectrode substitute for the conventional ITO electrode foruse in cost-efficient OSCs [50]

Wang et al [36] developed a direct layer-by-layer (LBL)transfer method of graphene sheets to fabricate MLG filmsStacking multilayers together and doping them renderedhigher extrinsic conductivity HCl was used to dope theindividual layers during the transfer process followed byHNO

3doping at the surface Different types of devices were

fabricated to evaluate the effect of the various treatmentsproposed in this work To improve the hydrophilicity ofgraphene a thin layer of (sim20 A) molybdenum oxide (MoO

3)

was evaporated on the graphene The as-grown graphene-based devices showed a poor performance yielding a lowPCE of 035 After modifying the graphene with MoO3 +PEDOT PSS double interfacial layer the PCE was dra-matically increased and 123 was achieved Finally the4-layer acid-doped graphene solar cell exhibited the bestperformance in this work with PCE of 25 which is about833 that of the control device fabricated on ITO The LBLacid-doped four-layer graphene film has a sheet resistance ofsim80Ωsq and a transmittance of sim90 at 550 nm [36]

Geng et al [51] reported a simple method for preparingtransparent conductive films using a chemically convertedgraphene (CCG) suspension via controlled chemical

reduction of exfoliated GO in the absence of dispersant to beused as transparent electrodes in BHJ devices After vacuumfiltration of the reduction-controlled CCG suspension andsubsequent thermal annealing (optimal conditions were15min at 800∘C) of the CCG films high conductive filmswith a low sheet resistance on the order of 103Ωsq and hightransparency (80 at 550 nm) were obtained Thermallyannealed CCG film showed a sheet resistance that was lowerthan that of the thermally annealed GO films apparently dueto the greater extent restoration of the sp2 carbon networksduring the two-step reduction of the CCG films Devicesprepared with the CCG films as transparent electrodesexhibited PCE of 101 plusmn 005 which correspond to halfthe efficiency of solar cell devices in which the electrode wasmade using ITO [51]

Table 1 summarizes the best PCE achieved by the differentresearch groups presented in the previous section as well asthe device conformation that rendered such PCE

Although the overall PCE obtained by devices usinggraphene-based materials as anode has not surpassed that ofcells using traditional materials research groups around theworld have already identified the main aspects to improvegraphene-based materials performance It has been pointedout that lowering graphene sheet resistance will increase theoverall PCE of the BHJ devices In order to lower the sheetresistance different strategies can be followed for exampleincreasing the thickness of the graphene film This goal canbe achieved following a simple procedure that consists in themodification of the solution concentration from which thefilm is obtainedThis route constitutes a very good option dueto its simplicity and to the fact that no additional chemicalsare needed contrary to other techniques that require highlydangerous chemical agents to dope graphene Thicknesscontrol also has an impact on the transparency of the filmA higher transparency will originate a better light trans-mission and a higher PCE Thus a proper balance betweentransparency and sheet resistance needs to be achieved toimprove the performance of the device Scientists have alsoidentified that the optimization of the graphene conductivityand graphene films surface wetting has a positive effect on theperformance of graphene-based electrodes Thermal treat-ments (ie annealing) also play an important role regardingfinal efficiency by this simple procedure notable increments


Table 2 PCE of BHJ OSCs using graphene as cathode

Material Device conformation Highest PCE ReferenceMLG MLGWPF-6-oxy-FP3HT PCBMPEDOT PSSAl 123 [14]MLG GlassITOZnOP3HT PCBMMLG 250 [52]SLG ITOPEDOTCuPC C

60 TPBiSLG 022 [53]

rGO PET alkali + rGO-SWCNTP3HT PCBMV2O5Al 127 [55]

SLG GlassITOZnOP3HT PCBMPEDOT PSSAu + SLG 27 [54]

have been achieved In the specific case of anode-materialsgraphene constitutes a viable option to substitute traditionalmaterials first due to its higher abundance compared toindium and second to the considerable PCE reached so far

22 Cathode Although several studies have reported onthe graphene-based electrode for replacing the ITO bottomelectrode in organic photovoltaic devices the use of thegraphene top electrode is still very limited [52]The followingsection presents the researchworks related to this topic foundin the scientific literature Table 2 summarizes the best PCEachieved by devices using graphene as cathode

Lee et al [52] reported a simple lamination process fordepositing a graphene electrode on top of a polymer pho-tovoltaic device replacing the conventional metal electroderesulting in a semitransparent inverted polymer solar cellwith a PCE of over 75 of that of the standard opaquedevice using an Ag metal electrode which was fabricatedfor comparison For the preparation of the graphene topelectrode a graphene film was grown on copper foil usingCVD processes Since a single layer of graphene does nothave sufficiently high sheet conductivity multilayer stackingof the graphene film was required to obtain higher sheetconductance The Ag-based device yielded a PCE of 330and in the best case (10-layers device) the semitransparentdevice achieved a PCE of 250 when illuminated (AM15100mWcm2) from the ITO side By contrast while illumi-nated from the graphene side the semitransparent deviceconsisting of 8 layers of graphene exhibited an optimal PCEof 204 [52]

Jo et al [14] fabricated inverted-structure OSCs withgraphene cathodes The graphene film used in this work waswork function-engineered with an interfacial dipole layer toreduce the work function of graphene which resulted inan increase in the built-in potential and enhancement ofthe charge extraction thereby enhancing the overall deviceperformance MLG film was grown through CVD on nickelfilms as reported elsewhere The work function of theuntreated MLG film was 458 plusmn 008 eV which is close to thework function of highly ordered pyrolytic graphite (45 eV)The work function of the MLG film was reduced by theuse of different interfacial layers made of poly-ethyleneoxide (PEO) poly[99-bis((6rsquo(NNN-trimethylammonium)hexyl)-27-fluorene)-alt-(99-bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-9-fluorene)] dibromide (denoted by WPF-6-oxy-F) and Cs

2CO3 These interfacial layers formed

interfacial dipoles that pointed away from the MLG surfacethereby decreasing the MLG work function WPF-6-oxy-F

was found as the best material because it reduces the effectivework function of MLG to a value that is close to the LUMOof the acceptor material Therefore the best PCE (123)was obtained with WPF-6-oxy-F as the interfacial layer Theperformance of the OSCs with MLG electrodes is expectedto depend strongly on the quality and properties of theMLG film The CVD-synthesized MLG films exhibited highsheet resistance in the range of 850 to 520Ωsq while thetransmittance changed from 90 to 85 at a wavelength of450 nm [14]

Cox et al [53] demonstrated that a laminated single-layer graphene (SLG) grown through chemical vapor depo-sition can be used as a cathode for organic photovoltaicdevices The PCE of graphene devices was limited by theirseries resistance indicating that the preparation of graphenesamples with lower sheet resistance could improve deviceperformance In this work aluminum control devices wereused to evaluate the performance of the graphene OSCs Thegraphene devices exhibited a PCEof 002and the aluminumcontrols showed a PCE of 041 The 119881oc for both types ofinstruments was approximately 045V This large efficiencydisparity in conjunction with similar 119881ocrsquos indicated thatparasitic series resistance dominated the characteristics ofthe graphene devices To minimize the influence of the par-asitic series resistance the devices were studied under low-intensity monochromatic illumination At incident power of46mWcm2 the graphene device exhibits a PCE of 022[53]

Liu et al [54] reported the fabrication of a semitrans-parent device with a SLG synthesized on copper foils withCVD method used as top electrode The device was opti-mized by tuning the active layer thickness and changing theconductance and the work function of single-layer grapheneby doping Au nanoparticles and PEDOT PSS For each syn-thesis condition three identical cells were fabricated and theaverage PCE was calculatedThe devices were illuminated oneach side under an AM15 solar simulator with the intensityof 100mWcm2 The maximum PCE of the semitransparentcell was 27 for an active area of 20mm2 when illuminatedfrom the graphene side which is much higher than the valuefrom the ITO side (22) It was also found that the PCE ofthe device decreased from 3 to 23 with the increase ofthe size from 6 to 50mm2 which was attributed to both theincreased series resistance and the decreased edge effect of thedevice [54]

Huang et al [55] found that the work functions (120601119908) of

solution-processable functional graphenecarbon nanotube-based transparent conductors were readily manipulated


Table 3 PCE of BHJ OSCs using graphene as acceptor material

Material Device conformation Highest PCE ReferenceSPFGraphene ITOPEDOT PSSP3OT SPFGrapheneLiFAl 140 [56]SPFGraphene GlassITOPEDOT PSSP3HT SPFGrapheneLiFAl 069 [58]SPFGraphene ITOPEDOT PSSP3HT SPFGrapheneLIFAl 110 [64]SPFGraphene GlassITOPEDOT PSSP3HT SPFGrapheneLiFAl 088 [61]Graphene GlassITOPEDOT PSSP3HT PDI-G hybridLiFAl 104 [63]SPFGraphene GlassITOPEDOT PSSP3HT SPFGrapheneLiFAl 1046 [66]SPFGraphene ITOPEDOT PSSP3HT PCBM SPFGrapheneAl 140 [67]Graphene GlassITOPEDOT PSSC

60-G P3HTAl 122 [62]

BCFG-F GlassITOPEDOT PSSP3HT BCFG-FLiFAl 110 [60]SPFGraphene GlassITOPEDOT PSSP3HT MDMO-PPV SPFGrapheneLiFAl 1506 [65]Graphene GlassITOPEDOT PSSP3HT PCBM grapheneLiFAl 070 [59]

varying between 51 and 34 eV depending on the nature ofthe doping alkali carbonate salt used Highly conductive andtransparent graphene-based electrodes with tunable workfunctions were prepared by combining single walled carbonnanotubes (SWCNTs) with chemically reduced graphene(rGO) Composite films were doped with alkali carbonateswhich were decomposed into alkali oxides by means ofthermal annealing that covered the carbon-based materi-als Graphene-based electrodes possessing lower values of120601

119908were used as cathodes in inverted-architecture poly-

mer photovoltaic devices to effectively collect electrons Asexpected increasing the number of spin-layers from oneto five provided thicker carbon composite films therebydecreasing the optical transmittance at 550 nm from 888 (1layer) to 587 (5 layers)Therefore an optimal rGO-SWCNTthickness exists for optimal PV performance The bestPCE of the P3HT PCBM photovoltaic device was achievedwhen incorporating the four-layer rGO-SWCNT (127)[55]

According to the results presented in the previous sectionthe control of the work function of graphene layers consti-tutes the main strategy to improve the conductivity and theoverall power conversion efficiency of BHJ solar cells usinggraphene as cathode material This goal may be achievedfollowing different procedures for example using interfaciallayers or doping graphene films with different kind of agentsOther strategies to enhance the PCE include the tuning ofthe active layer thickness to maximize light absorption andcharge transport Regarding the preparation of the grapheneelectrode CVD is the most used route while lamination isthe most commonly method used to deposit the electrodeHowever the high processing temperature required for CVDlimits the growth of graphene to a restricted number ofsubstrates which could have a negative impact on theperformance of the BHJ device Thus it is clear that furtherenhancement in the conductivity of the graphene electrode isneeded and constitutes the next step in order to increase theoverall performance of the cathode-graphene-based devicesIn the same way the development of alternative methods forpreparing large-area graphene films is required sincemassivepreparation of graphene films would be needed to satisfyglobal demand

3 Active Layer Material

The active layer of an ideal BHJ solar cell is defined as abicontinuous interpenetrating network of donor and accep-tor at the nanometer scale with maximum interfacial areaIn this case excitons can always reach the DA interfacewithin their limited diffusion length and charge separationcan occur efficiently So far the most successful active layerwith BHJ architecture is based on either soluble P3HT orpoly-3-octylthiophene (P3OT) as the donor and PCBM orPC70BM (66-phenyl-C

70-butyric acid methyl ester) as the

acceptor Solar cells based on P3HT have exhibited PCEvalues approaching 5 depending on the architecture of thedevice On the other hand it is known that PCBM is notthe optimum acceptor material and limits the efficiency ofthe organic devices Thus new materials for both donor andacceptor with better HOMOLUMOmatching stronger lightabsorption and higher charge mobility with good stabilityare much needed [6 56 57] In this section the use ofgraphene as acceptor material is described and the PCEand the conformation of BHJ devices using graphene aresummarized in Table 3

Liu et al [58] used graphene as the acceptor material inBHJ organic photovoltaic cells To achieve good dispersionof graphene a solution-processable functionalized graphene(SPFGraphene) was prepared by a two-step method con-sisting of an oxidation step followed by an organic func-tionalization step The oxidation treatment using a modifiedHummersrsquo method of the flake gave out graphene oxidesheets having a thickness of sim07 nm and a dimension ofseveral hundreds of nanometers Then a phenyl isocyanatetreatment gave out SPFGraphene that could be dissolved inorganic solventThe active layer was prepared by spin-coatingusing a P3HTSPFGraphene solution in 12-dichlorobenzene(SPFGraphene content 0 25 5 10 and 15) Thecurrent-voltage (119869-119881) curves of the photovoltaic devices weredetermined using an AM15G standard operating with anillumination intensity of 100mWcm2 Under illuminationthe device based on pristine P3HT gave PCE of 0005 whilethe P3HT10-SPFGraphene based device gave a maximumPCE value of 015 which is much higher than that ofthe control device based on pristine P3HT The authors


also reported that after annealing at 160∘C for 10min a topPCE of 069 for the P3HT5-SPFGraphene was registered[58]

Yu and Kuppa [59] investigated BHJ devices employinggraphene nanosheets in the active layer Solar cells based onP3HT PCBM graphene showed device physics significantlydifferent from traditional BHJs and displayed a monotonicincrease in performance with graphene concentration ThePCE of a device based on a P3HT PCBM blend at a 20 2concentration ratiowas about 02while the addition of onlysmall amounts of graphene (concentration ratio 20 2 001for P3HT PCBM graphene) led to a better performance ofabout 07 Hence although the introduction of graphenesheets may have had a slightly negative effect on the 119881ocand FF the subsequent improvement in charge mobility andincrease in 119869sc compensatedmore for these drawbacks and ledto an increase in efficiency [59]

Ye et al [60] grafted a benzoic acid group onto pristinegraphene (BCFG graphene functionalized with benzoicacid) to improve its solubility and open a band gap of theresultant graphene BCFG was esterified with 1H1H2H2H-perfluoro-1-octanol to obtain BCFG-F which was blendedwith P3HT to be used as the active layer in BHJ devicesSolar cells of P3HTBCFG-F with functionalized graphenecontent of 5 wt 75 wt 10wt and 125 wt (ratio toP3HT) were fabricated PCEs of 024 046 11 and 048 wereobtained for these devices respectively In order to realize thedependence of the photovoltaic performance on the BCFG-Fcontent the surface morphology of the composites used asactive layer was investigated It was found that in compositefilms containing 5wt of BCFG-F only a few functionalizedgraphene sheets could be seen embedded in the P3HTmatrixTherefore for low BCFG-F content it cannot form enoughP3HTBCFG-F interfaces at which the separation of excitonsoccurs and continuous pathways in the P3HT matrix for theeffective electron transport In the composite filmwith 10wtBCFG-F there were more graphene sheets than in the filmwith 5wt and they were well dispersed in the P3HTmatrixto form abundant P3HTBCFG-F interfaces for chargegeneration and interpenetrating networks Finally for thefilm of the composite with a 15 wt content the morphologyof the film demonstrated great agglomerated forms byaggregated graphene sheets which had a deleterious effecton exciton separation and charge transport in the active layer[60]

Liu et al [61] designed an organic photovoltaic devicebased on an acceptor of solution-processable functionalizedgraphene (SPFGraphene) The tested devices were basedon heterostructure polymer- (P3HT-) graphene compositelayers as active layer Different active layers were synthetizedby means of spin-coating from a solution of 15mgmLP3HT in chlorobenzene with SPFGraphene content of 0wt1 wt 25 wt 5wt 10wt 125 wt and 15wt Dopingof graphene into P3HT resulted in appropriate energeticdistance between HOMO and LUMO of the donoracceptorfor a high open circuit voltage and provided higher excitondissociation volume mobility of carrier transport for a largeshort-circuit current density The device containing only10wt of graphene showed the best PCE (088) an open

circuit voltage of 077V and a short-circuit current densityof 372mAcm2 SPFGraphene content was the main factorimproving PCE For smaller concentrations such as 1 wtand 25 wt the SPFGraphene film was too small to form acontinuous donoracceptor interface and the transport path-way for the active layer P3HT matrix Therefore the electroncannot effectively meet the donoracceptor interface andtransport smoothly through the active layer However if SPF-Graphene is further increased to a concentration of 10 wtthe SPFGraphene film can form a continuous donoracceptorinterface and produce a better way to be transportedsmoothly through P3HT matrix improving the electronictransport However if there is a further increase in theconcentration of SPFGraphene such as 125 wt and 15wtthen the aggregation of SPFGraphene occurred reducingholemobility and decreasing the number of extracted carriers[61]

Yu et al [62] developed a lithiation reaction to covalentlyattach monosubstituted fullerene (C

60) onto graphene (G)

nanosheets to be used as electron acceptors in P3HT-basedBHJ solar cells Graphene nanosheets were produced throughreduction of GO in pure hydrazine and purified with tolueneand methanol Using C

60-G P3HT (1 1 wtwt) C

60 P3HT

(1 1 wtwt) and C60G mixture (ie 12 wt graphene

physical blend without chemical functionalization) P3HT(1 1 wtwt) as a control the device performance of a series ofBHJ solar cells using such materials as active layer was testedunder AM15 illumination A PCE of 047 was obtainedfor the C

60 P3HT system while the C

60-G P3HT system

showed a PCE of 122 The observed enhancement couldbe attributed to the improved electron transport due tothe presence of the C

60-grafted graphene In the case of

the C60 graphene device the simple blending of C

60with

the graphene cannot ensure a good interfacial contact tofacilitate strong electronic interactions for efficient electrontransport between the two components To make it evenworse graphene sheets easily aggregated in the compositefilm due to poor miscibility to show a detrimental effect onthe charge separationtransportTherefore the overall deviceefficiency was not improved (044) [62]

Liu et al [56] reported the fabrication and performance ofdevices with BHJ structure using an organic SPFGraphene aselectron-acceptormaterial and P3OT (poly(3-octylthiophene25 diyl)) and P3HT as donor material Devices with SPF-Graphene content of 1 wt 5wt and 15wt (ratio toP3OT) were fabricated The authors indicated that thefunctionalized graphene material could be dispersed intoorganic solvent to form a homogeneous solution Controlledannealing improved the device performance and the bestPCE of 14 (5wt SPFGraphene) was obtained usingsimulated 100mWcm2 AM15G illumination after annealingduring 20min at 160∘C Although the reported PCE ismoderate it is comparable with most of the best devicesusing materials other than fullerenes as electron acceptorsFurthermore there is much room for improvement such asoptimizing the graphene content annealing temperature andtime further controlling the size and functionalized degreeof graphene sheets and the device fabrication and structure[56]


Wang et al [63] studied the role of graphene (rGO) asatomic and structural scaffold in the nucleation and assemblyof organic nanostructures The photovoltaic properties ofthe PDI (NN1015840-dioctyl-34910-perylenedicarboximide) andgraphene hybrids were investigated to determine whetherthey offer enhanced performances over those of the indi-vidual constituents like PDI and G PDI-G hybrids wereprepared by the hydrothermal treatment of GO and PDIin DMF The authors also investigated the performance ofphysically mixed PDI + G composites made of PDI and Gthis system was used as a control to compare with the PDI-Ghybridwires in order to find out if the coating of G around thePDI wires offers any enhanced performance Using BHJ cellstested under 100mWcm2 AM15 illumination it was foundthat PDI-G hybrid wires based cells showed a PCE of 104while PDI cells presented a PCE of only 0064 [63]

Liu et al [64] studied a BHJ device with an activelayer consisting of P3HT and SPFGraphene According totheir results adding graphene to the P3HT induced a greatquenching of the photoluminescence of the P3HT indicatinga strong electronenergy transfer from the P3HT to thegraphene The device containing only 10 wt of grapheneshowed the best performance with a PCE of 11 under sim-ulated AM15G conditions at 100mWcm2 after an annealingtreatment at 160∘C for 10min The annealing treatment at160∘C greatly improved the device performance Howeverovergenerous conditions such as those at 210∘C resulted in adecrease in the device efficiency (057) [64]

Wang et al [65] reported on the fabrication of BHJdevices based on P3HT SPFGraphene doped with poly-2-methoxy-5-(3070-dimethyloctyloxy)-14-phenyleneviny-lene (MDMO-PPV) and P3OT In this work the amountof MDMO-PPV incorporated to the P3HT SPFGrapheneactive layer was varied from 0 to 2mg while the amount ofP3OT varied in the range from 0 to 3mg For further charac-terization the solar cells were illuminated through the sideof the ITO-coated glass plate AM15G standard operatingwith an illuminated intensity of 100 mW xenon lamp wasused as a broadband light source The authors pointed outthat nondoped P3HT SPFGraphene showed an efficiencyof 1046 By incorporating different amounts of P3OT thebest efficiency obtained was of 112 when 1mg of P3OTwas added On the other hand a top efficiency of 1506 wasregistered when 15mg of MDMO-PPV was used [65]

Wang et al [66] fabricated solar cell devices based onSPFGraphene oxide and P3HT with different contents ofSPFGraphene oxide (0 25 5 75 10 125 and15)The 120587-conjugation of graphene enables the polymers tobe used as an active material for various functional devicesincluding photovoltaic devices The pure P3HT thin filmshowed strong photoluminescence between 600 and 800 nmwith excitation at 450 nm however the photoluminescenceis remarkably reduced after doping of SPFGraphene oxideshowing that efficient energy transfer occurred along theP3HTSPFGraphene oxide interface The overall perfor-mance of the P3HTSPFGraphene oxide based devices wasmuch higher than that of the pristine P3HT-based oneThe improved photovoltaic performance was attributed to

the addition of the SPFGraphene oxide stating that there isan obvious charge transfer from the P3HT donor to the SPF-Graphene oxide acceptor To recover the conjugated structureand conductivity of the graphene sheet through removingthe organic functional groups from the SPFGraphene oxidesheet a thermal annealing process was carried out at 160∘Cat different times The best PCE was obtained for a devicecontaining 10wt of SPFGraphene annealed during 10min(1046) [66]

Liu et al [67] tested a SPFGraphene in P3HTPCBMphotovoltaic devices Tested devices took advantage of theelectron-accepting feature of fullerenes and the high electrontransport capability of graphene The photovoltaic perfor-mance was investigated for devices containing 0 1 255 10 125 and 15 of graphene The best results wereobtained with P3HT PCBM (1 1) mixture with 10wt ofgraphene with a PCE of 14 at illumination at 100mWcm2AM15 Results indicated that the addition of grapheneenhanced the performance of polymer photovoltaic devices[67]

Different methods to obtain graphene-based materialshave been developed (eg adhesive tape epitaxial growthand micromechanical cleavage) However most of thesemethods are not suitable for low-cost large-area optoelec-tronics In this tenor organic solution-processable materialsare preferred for electronic applications due to their solubilityusing common solvents which allow them to be used inpolymer-composite based materials preparation Solution-processable functionalized graphene (SPFGraphene) is byfar the most used functionalized graphene-based (Figure 4)material employed in the development of BHJ devicesAlthough the performance of the cells using this materialis lower than the most efficient BHJ cells tested it is clearthat the use of graphene significantly increases the efficiencycompared to the reference devices (ie without graphene)reported in each experiment The key to improve the PCEof SPFGraphene based materials seems to be related to theimprovement of its miscibility which will assure a gooddispersion of the graphene in the polymer matrix A bettermiscibility will also avoid agglomeration and its detrimentaleffects on charge separationtransport Thus in this way thehole mobility and the number of extracted carriers will beincreased An improved solubility will also ensure a goodinterfacial contact to facilitate strong electronic interactionsfor a better charge generation Another important factorto be considered is the graphene content that needs to beenough to form a continuous donoracceptor interface andan efficient transport pathway for the active layer Finallyannealing has proved to be an efficient treatment to increasepower conversion efficiency since it considerably increasesthe 119881oc of the device

4 TransportExtraction Layer Material

The interfaces between active layer and anode and cathodeplay an essential role in determining the overall deviceperformance of organic electronics [68] In a simple BHJdevice both the donor and acceptor phases are in direct


Table 4 PCE of BHJ OSCs using graphene as HTLs and EELs

Material Function Device conformation Highest PCE ReferenceGO HTLs ITOGOP3HT PCBMAl 350 [78]GO HTLs ITOPEDOT PSS GOP3HT PCBMLiFAl 380 [86]GO HTLs ITOGO NiO

119909P3HT PCBMLiFAl 348 [79]

FLGs HTLs GlassITOFLGs-PEDOT PSSP3HT PCBMCaAl 370 [85]mrGO HTLs GlassITOmrGOP3HT PCBMLiFAl 398 [81]GO-OSO

3H HTLs ITOGO-OSO

3HP3HT PCBMCaAl 437 [82]

GO HTLs ITOGOP3HT PCBMLiFAl 325[83]GO-Cs EELs ITOPEDOT PSSP3HT PCBMGO-CsAl 308

GOGO-Cs HTLsEELs ITOGOP3HT PCBMGO-CsAl 367UVozone-G HTLs ITOUVozone-GP3HT PCBMLiFAl 300 [80]rGO EELs GlassITOPEDOT PSSP3HT PCBMrGO-pyrene PCBMAl 389 [84]

2120583m

Figure 4 SEM micrograph of SPFGraphene

electrical contact with the cathode and anode electrodesleading to recombination of carriers and current leakageTo minimize such detrimental effects hole transport layers(HTLs) and electron extraction layers (EELs) are usedHTLs must be wide band gap p-type materials in thisrespect several inorganic transition metal oxides such asmolybdenum nickel rhenium vanadium or tungsten oxides(V2O5 ReO

3 WO

3 and MoO

3with NiO being the most

effective) have been reported yielding efficiencies greater than5 In a detrimental manner inorganic HTLs are depositedusing vacuumdepositionmethods that are incompatible withsolution-processable printable electronic techniques used tofabricate photovoltaic devices [69ndash71]

The most commonly employed material used as HTLsin polymer solar cells is semiconducting PEDOT PSSwhich is used between the ITO anode and the active layerPEDOT PSS has the advantages of being deposited fromsolution and serve tominimize the detrimental effects of ITOroughness and to align the work function of P3HT and ITOfor more efficient collection of holes However it is usuallydeposited from highly acidic aqueous suspensions that areknown to corrode ITO at elevated temperatures and can alsointroduce water into the active layer degrading the deviceperformance [68 72 73]

On the other hand certain metal fluorides n-type semi-conductors (eg TiO

2 TiO

119909 and ZnO) n-type organic

semiconductors (eg 29-dimethyl-47-diphenyl-110-phe-nanthroline BCP) and conjugated polymer electrolytes havebeen used as the EELs [74ndash77] Materials used as EELs needto have a low work function for electrons to be efficientlytransported to the cathode

Thus several research groups motivated by the need toovercome the limitations of PEDOT PSS as the conventionalHTLs by finding a suitable solution-processable alternativethat is compatible with OSCs materials and fabricationtechniques and by the need to find new materials to serveas both HTLs and EELs that could not only simplify thematerials design and device fabrication but also allow for aprecise control of the energy barrier for electron and holeextraction by tuning its work function via for examplecontrolled functionalization have reported their findingswhich are summarized in the following section and inTable 4

Li et al [78] used graphene oxide thin films as thehole transport and electron extraction layer in OSCs Toevaluate the performance of GO devices a set of two otherdevices was fabricated for comparison One set was ITO-only control devices and the other was the conventionalOSCs incorporating PEDOT PSS as the hole transport layerThe photovoltaic characteristics of the fabricated deviceswere characterized under simulated AM15 illumination at100mWcm2 ITO-only device exhibited an average PCE of18 The insertion of a 2 nm thick GO thin film betweenITO and P3HT PCBM led to an enhancement in the PCE to35while the typical PEDOT PSS typical device had showna 36 PCE Thus the incorporation of GO deposited fromneutral solutions between the photoactive P3HT PCBMlayer and the transparent and conducting ITO led to adecrease in recombination of electrons and holes and leakagecurrents resulting in a dramatic increase in the device PCEto values that are comparable to devices fabricated withPEDOT PSS as the hole transport layer [78]

Ryu and Jang [79] reported a solution processed GONiO119909 andGONiO

119909bilayer used as an anode interfacial layer

in a BHJ device In order to obtain the optimized thicknessof GO layer cells with different numbers of GO layers werefabricated by spin-coating The best cell performance was


achieved for the cell with 3 GO layers exhibiting a PCEof 275 which was superior to that of the cell withoutHTL (233) On the other hand the device with a solutionprocessed NiO

119909layer showed an improved PCE of 310 To

further enhance cell performance of the device with GO andNiO119909 devices with GONiO

119909andNiO

119909GOwere fabricated

Cell performance of the OSC with NiO119909GO as HTLs was

similar to the device with a NiO119909 However the device with

a GONiO119909layer showed an enhanced cell performance

indicating a 15 increase in PCE compared to the cellwithout HTLThese results indicated that the NiO

119909functions

both as a hole transport and an electron extraction layer Thedevice with a GONiO

119909showed a PCE of 348 The cell

efficiency is correlated with the matching of energy levelsbetween ITO hole transport layer and P3HT and thus a well-matched GONiO

119909showed the best PCE [79]

Kwon et al [80] fabricated organic devices using UVozone-treated graphene sheets as HTLs A PEDOT PSS-coated ITO device was used as reference and in the case ofthe UVozone-treated graphene devices the graphene wastransferred onto the ITO glass following exposure to UV-ozone with a power of 30mWcm2 for 1 3 5 7 and 9minCells were tested under AM15G 100mWcm2 illuminationIn the best case (UV-ozone treated device for 9min) thesheet resistance increased from 11 kΩsq to infinity and thetransmittance at 550 nm decreased from 952 to 938due to the formation of carbon-oxygen functionalizationlayers Although the device with 9min treated graphene wasexpected to show the best efficiency (PCE of 146) dueto its highest work function (485 eV) the best PCE wasobtained from the cell with 5min treated graphene (300)probably because the graphene sheet was damaged by UV-ozone exposure after 7min which roughened the surfacecreating defects [80]

Jeon et al [81] used moderately reduced graphene oxide(mrGO) as an alternative to the PEDOT PSS HTL in BHJOSCsmrGOfilmswere fabricated by simple and fast thermaltreatment of solution processed mrGO Spin-coated mrGOlayers were thermally treated at 150 250 and 350∘C for 10minin air Devices without and with thermal treatment at 150 and250∘C showed a well-formed 2-dimensional nanosheet Onthe other hand in the case of mrGO films thermally treatedat 350∘C some aggregation of nanosheets and relativelynonuniform morphologies were observed The thicknessesof nanosheets of mrGO without and with thermal treatmentat 150 250 and 350∘C were estimated to 118 107 087 and085 nm respectively The decrease in the thickness of singlesheet resulted from the removal of oxygen groups indicatingthat solution processed mfGO films were reduced by simplethermal treatment Devices with thermally treated mrGO at250∘C exhibited the best PCE (398) compared to devicescontaining conventional PEDOT PSS HTLs which showed385 Furthermore mrGO-based cells showed superiorstability compared to conventional devices under atmospherecondition without any encapsulation process [81]

Liu et al [82] developed a sulfated graphene oxide (GO-OSO3H) with ndashOSO

3H groups attached to the carbon basal

plane of reduced GO surrounded with edge-functionalized

ndashCOOH groups to be used as HTLs Different devicesbased on GO and GO-OSO

3H were fabricated varying the

thickness of the HTLs from 2 to 6 nm The best results wereobtained when using 2 nm thick HTLs Because of the poorconductivity of GO an increase in the GO thickness from2 to 6 nm increased the series from 31 to 64Ω cm2 andhence decreased the PCE By contrast the performance ofthe GO-OSO

3H-based devices is nearly independent of the

HTLs thickness over the range from 2 to 6 nm Comparedto that of GO the better conductivity of GO-OSO

3H led to

an improved PCE of 437 versus 334 of GO The controldevices based on PEDOT PSS exhibited a PCE of 439Therelatively poor photovoltaic performance observed for theGO device can be attributed to high series resistance associ-ated with the insulating GO Compared with the GO-baseddevices their GO-OSO

3H counterparts exhibited a much

lower series resistance and hence a significantly improvedPCE [82]

Liu et al [83] reported on the functionalization of GO toproduce both hole and electron extraction materials for BHJsolar cells GO was tested as HTLs and GO-Cs in which theperiphery ndashCOOH groups in GOwere replaced by ndashCOOCsas EELs GO itself has a work function of 47 eV whichmatches the HOMO level of P3HT for hole extraction Byreplacing the periphery ndashCOOH with the ndashCOOCs groupsthrough charge neutralization the work function of the GO-Cs can be reduced to 40 eV which matches the LUMO levelof PCBM for efficient electron extraction To investigate thehole extraction ability of GO three different devices with thesame active layer cathode and EELs were compared Thedevice with GO as HTLs showed a PCE of 325 which wassuperior to that of the device using PEDOT PSS (315)implying that GO is an excellent hole extraction material forBHJ solar cells Similarly to evaluate the ability of GO-Csas the electron extraction layer four devices were fabricatedwith the same anode hole extraction layer active layer andcathode In this case the device using LiF showed the bestPCE (315) However the cell based on GO-Cs exhibited afairly comparable PCE (308) showing the great capabilityof GO-Cs as electron extraction material in BHJ solar cellsFinally based on the excellent capabilities demonstrated forGO andGO-Cs a pair of BHJ devices with bothmaterials wasfabricated For this purpose both the normal and invertedconfigurations were investigated In both cases the devicesexhibited a better performance (367 and 297 resp) thanthat of the control device (120) based on Cs

2CO3and

PEDOT PSS [83]Qu et al [84] reported a graphene fullerene compos-

ite (rGO-pyrene-PCBM) in which PCBM was attachedonto rGO via the noncovalent functionalization approachrGO-pyrene-PCBM was successfully applied as EELs forP3HT PCBM BHJ devices affording a PCE of 389 whichis enhanced by sim15 compared to that of the referencedevice without EELs (339) Additionally the comparativedevices incorporating the rGO or pyrene-PCBM componentas EELs showed dramatically decreased PCE 253 and218 respectively which are much lower than that of thereference device indicating the importance of composite


formation between rGO and pyrene-PCBM components forits electron extraction property [84]

Nguyen et al [85] demonstrated the use of a few-layeredgraphene nanosheets- (FLGs-) PEDOT PSS composite asan effective HTL that improves the hole transporting abilityin an organic device with which the PCE was enhancedfrom 310 to 370 FLGs are considered to be a classof graphene materials namely 2D graphitic crystals withextraordinary electronic transport properties varied sys-tematically by the number of graphene layers FLGs weresynthetized via exfoliation from expanded graphite since thismethod produces a low defects material with an excellentconductivity To demonstrate the application of the FLGs-PEDOT PSS composite as HTLs FLGs and PEDOT PSSwere mixed at different volume ratios (11 21) as a partof a BHJ device based on P3HT PCBM All samples weremeasured under light illumination of AM15 (100mWcm2)By increasing the vv ratio of FLGs and PEDOT PSS PCEincreased to 347 (11) and 370 (21) Due to partiallycontaining oxygen groups on the graphene edges of FLGsit was expected that the work function of the FLGs wasin the range of 44 eV (graphite) to 51 eV (acid-oxidizedMWCNTs) therefore possessing an energy level close tothat of P3HT Furthermore no significant differences werefound in surface roughness between the pure PEDOT PSSand the FLGs-PEDOT PSS composite films indicating thatthe contact resistance between the active layer and the holetransport layer was unchanged in all devices As a result thesignificant improvement of current density is attributed tohigher hole mobility of FLGs as compared with PEDOT PSS[85]

Yin et al [86] reported a high-efficiency devicemadewithhydrophilic GO doped in PEDOT PSS composites used asHTLsThe PCE of the nondoped composites was of 21 andincreased to 38 for the doped composites under AM15G100mWcm2 illumination in air Preannealing of GO madePCE 18 times that of the device based on PEDOT PSS Thelow price and ease of preparation make soluble graphenea promising layer to be used in photovoltaic applications[86]

GO-based materials are the most used replacementfor PEDOT PSS which is the most commonly used holetransport layer material despite its highly acidic nature andhygroscopic properties which leads to a poor long termstability of the device A major advantage of GO-basedmaterials relies on the fact that they can be deposited fromneutral aqueous suspensions and yield efficiency values thatare comparable to devices fabricated with PEDOT PSSnot to mention that the term stability of the device is notcompromised using this type of suspensions On the otherhand the removal of oxygen from the structure of GOinduces the transition of GO from an electrical insulator toa semiconductor Thus strategies to obtain functionalizedGO using reagents with low toxicity constitute one of themain trends in this field research Regarding EELs materialsthe main goal seems to be to obtain low work functionhighly conductive materials to improve electrons mobilitywhich can achieve doping graphene with different chemicalagents

5 Conclusions

In this report the various applications of graphene as aconstituent of BHJ OSCs were discussed focusing on the PCEachieved by the graphene-based devices Results indicate thatthere is a great room and need for the improvement of BHJdevices to competewith inorganic cellsHowever there is alsoevidence that graphene and its derivatives can be successfullyused as a main component of the different layers that consti-tute OSCs giving the possibility to reduce the dependence onthe materials traditionally employed As for transparent elec-trode material graphene has demonstrated great potentialwithout the detrimental aspects of its traditional analoguesthe main key to achieve a better performance relies onincreasing the transparency and decreasing the sheet resis-tance of the graphene films As for cathodematerial themainaim is to control the work function mainly using interfaciallayers or doping graphene films Furthermore its outstandingproperties such as its band structure and dispersion abilityhavemade it also adequate to be used as an acceptor materialThe improvement of graphene miscibility to assure a gooddispersion represents an important issue in order to increasethe performance of the devices using graphene as a mainconstituent of the active layer Additionally its excellentcharge extraction performance has made it viable to be usedas transportextraction material in BHJ cells The main goalin this area is to obtain HTL-materials that can be depositedfrom neutral aqueous suspensions and highly conductiveEEL-materials using different functionalization agents

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] B C Thompson and J M J Frechet ldquoPolymer-fullerene com-posite solar cellsrdquo Angewandte ChemiemdashInternational Editionvol 47 no 1 pp 58ndash77 2008

[2] X Wan G Long L Huang and Y Chen ldquoGraphenemdashapromising material for organic photovoltaic cellsrdquo AdvancedMaterials vol 23 no 45 pp 5342ndash5358 2011

[3] T L Benanti and D Venkataraman ldquoOrganic solar cells anoverview focusing on active layer morphologyrdquo PhotosynthesisResearch vol 87 no 1 pp 73ndash81 2006

[4] M Joslashrgensen J E Carle R R Soslashndergaard et al ldquoThe state oforganic solar cellsmdashameta analysisrdquo Solar EnergyMaterials andSolar Cells vol 119 pp 84ndash93 2013

[5] Z Abdin M A Alim R Saidur et al ldquoSolar energy harvestingwith the application of nanotechnologyrdquo Renewable and Sus-tainable Energy Reviews vol 26 pp 837ndash852 2013

[6] E Bundgaard and F C Krebs ldquoLow band gap polymers fororganic photovoltaicsrdquo Solar Energy Materials and Solar Cellsvol 91 no 11 pp 954ndash985 2007

[7] N Yeh and P Yeh ldquoOrganic solar cells their developments andpotentialsrdquo Renewable and Sustainable Energy Reviews vol 21pp 421ndash431 2013

[8] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009


[9] MYang YHou andNAKotov ldquoGraphene-basedmultilayerscritical evaluation of materials assembly techniquesrdquo NanoToday vol 7 no 5 pp 430ndash447 2012

[10] Z Liu D He Y Wang H Wu J Wang and H WangldquoImproving photovoltaic properties by incorporating both SPF-Graphene and functionalized multiwalled carbon nanotubesrdquoSolar Energy Materials and Solar Cells vol 94 no 12 pp 2148ndash2153 2010

[11] S Stankovich D A Dikin G H B Dommett et al ldquoGraphene-based composite materialsrdquo Nature vol 442 no 7100 pp 282ndash286 2006

[12] G Jo M Choe C-Y Cho et al ldquoLarge-scale patterned multi-layer graphene films as transparent conducting electrodes forGaN light-emitting diodesrdquo Nanotechnology vol 21 no 17Article ID 175201 2010

[13] Y Zhu S Murali W Cai et al ldquoGraphene and graphene oxidesynthesis properties and applicationsrdquo Advanced Materialsvol 22 no 35 pp 3906ndash3924 2010

[14] G Jo S Na S Oh et al ldquoTuning of a graphene-electrodework function to enhance the efficiency of organic bulk hetero-junction photovoltaic cells with an inverted structurerdquo AppliedPhysics Letters vol 97 no 21 pp 213301ndash213303 2010

[15] G V Dubacheva C-K Liang and D M Bassani ldquoFunctionalmonolayers from carbon nanostructuresmdashfullerenes carbonnanotubes and graphenemdashas novel materials for solar energyconversionrdquo Coordination Chemistry Reviews vol 256 no 21-22 pp 2628ndash2639 2012

[16] A Iwan and A Chuchmała ldquoPerspectives of applied graphenepolymer solar cellsrdquo Progress in Polymer Science vol 37 no 12pp 1805ndash1828 2012

[17] X Du I Skachko A Barker and E Y Andrei ldquoApproachingballistic transport in suspended graphenerdquoNatureNanotechnol-ogy vol 3 no 8 pp 491ndash495 2008

[18] X Wan Y Huang and Y Chen ldquoFocusing on energy and opto-electronic applications a journey for graphene and grapheneoxide at large scalerdquo Accounts of Chemical Research vol 45 no4 pp 598ndash607 2012

[19] R Sengupta M Bhattacharya S Bandyopadhyay and A KBhowmick ldquoA review on the mechanical and electrical prop-erties of graphite and modified graphite reinforced polymercompositesrdquo Progress in Polymer Science vol 36 no 5 pp 638ndash670 2011

[20] M Eizenberg and J M Blakely ldquoCarbon monolayer phasecondensation onNi(111)rdquo Surface Science vol 82 no 1 pp 228ndash236 1979

[21] T Aizawa R Souda S Otani Y Ishizawa and C OshimaldquoAnomalous bond of monolayer graphite on transition-metalcarbide surfacesrdquo Physical Review Letters vol 64 no 7 pp 768ndash771 1990

[22] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[23] C Berger Z Song X Li et al ldquoElectronic confinement andcoherence in patterned epitaxial graphenerdquo Science vol 312 no5777 pp 1191ndash1196 2006

[24] S J Park and R S Ruoff ldquoChemicalmethods for the productionof graphenesrdquoNature Nanotechnology vol 4 no 4 pp 217ndash2242009

[25] P Steurer R Wissert R Thomann and R Mulhaupt ldquoFunc-tionalized graphenes and thermoplastic nanocomposites basedupon expanded graphite oxiderdquo Macromolecular Rapid Com-munications vol 30 no 4-5 pp 316ndash327 2009

[26] H C Schniepp J-L Li M J McAllister et al ldquoFunctionalizedsingle graphene sheets derived from splitting graphite oxiderdquoThe Journal of Physical Chemistry B vol 110 no 17 pp 8535ndash8539 2006

[27] M J McAllister J-L Li D H Adamson et al ldquoSingle sheetfunctionalized graphene by oxidation and thermal expansion ofgraphiterdquo Chemistry of Materials vol 19 no 18 pp 4396ndash44042007

[28] J-M Yun J-S Yeo J Kim et al ldquoSolution-processable reducedgraphene oxide as a novel alternative to PEDOTPSS holetransport layers for highly efficient and stable polymer solarcellsrdquo Advanced Materials vol 23 no 42 pp 4923ndash4928 2011

[29] V Yong and J M Tour ldquoTheoretical efficiency of nanostruc-tured graphene-based photovoltaicsrdquo Small vol 6 no 2 pp313ndash318 2010

[30] T Winzer A Knorr and E Malic ldquoCarrier multiplication ingraphenerdquo Nano Letters vol 10 no 12 pp 4839ndash4843 2010

[31] P V Kamat ldquoGraphene-based nanoarchitectures Anchoringsemiconductor and metal nanoparticles on a two-dimensionalcarbon supportrdquo Journal of Physical Chemistry Letters vol 1 no2 pp 520ndash527 2010

[32] Y H Hu H Wang and B Hu ldquoThinnest two-dimensionalnanomaterialmdashgraphene for solar energyrdquoChem Sus Chem vol3 no 7 pp 782ndash796 2010

[33] S Bae H Kim Y Lee et al ldquoRoll-to-roll production of 30-inchgraphene films for transparent electrodesrdquoNature Nanotechnol-ogy vol 5 no 8 pp 574ndash578 2010

[34] Z Yin S Sun T Salim et al ldquoOrganic photovoltaic devicesusing highly flexible reduced graphene oxide films as transpar-ent electrodesrdquo ACS Nano vol 4 no 9 pp 5263ndash5268 2010

[35] J Wu H A Becerril Z Bao Z Liu Y Chen and P PeumansldquoOrganic solar cells with solution-processed graphene trans-parent electrodesrdquo Applied Physics Letters vol 92 Article ID263302 2008

[36] Y Wang S W Tong X F Xu B Ozyilmaz and K PLoh ldquoInterface engineering of layer-by-layer stacked grapheneanodes for high-performance organic solar cellsrdquo AdvancedMaterials vol 23 no 13 pp 1514ndash1518 2011

[37] F C Krebs M Joslashrgensen K Norrman et al ldquoA completeprocess for production of flexible large area polymer solar cellsentirely using screen printing-First public demonstrationrdquo SolarEnergy Materials and Solar Cells vol 93 no 4 pp 422ndash4412009

[38] R R Nair P Blake A N Grigorenko et al ldquoFine structureconstant defines visual transparency of graphenerdquo Science vol320 no 5881 p 1308 2008

[39] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquo Nature vol 438 no 7065 pp 201ndash204 2005

[40] A K Geim and K S Novoselov ldquoThe rise of graphenerdquo NatureMaterials vol 6 no 3 pp 183ndash191 2007

[41] K Naito N Yoshinaga E Tsutsumi and Y Akasaka ldquoTranspar-ent conducting film composed of graphene and silver nanowirestacked layersrdquo Synthetic Metals vol 175 pp 42ndash46 2013

[42] X Wang L Zhi and K Mullen ldquoTransparent conductivegraphene electrodes for dye-sensitized solar cellsrdquoNano Lettersvol 8 no 1 pp 323ndash327 2008

[43] C Gomez-Navarro R T Weitz A M Bittner et al ldquoElectronictransport properties of individual chemically reduced grapheneoxide sheetsrdquo Nano Letters vol 7 no 11 pp 3499ndash3503 2007


[44] Y Wang X Chen Y Zhong F Zhu and K P Loh ldquoLargearea continuous few-layered graphene as anodes in organicphotovoltaic devicesrdquo Applied Physics Letters vol 95 no 6Article ID 063302 2009

[45] Y Xu G Long L Huang et al ldquoPolymer photovoltaicdevices with transparent graphene electrodes produced by spin-castingrdquo Carbon vol 48 no 11 pp 3308ndash3311 2010

[46] X Wang L Zhi N Tsao Z Tomovic J Li and K MullenldquoTransparent carbon films as electrodes in organic solar cellsrdquoAngewandte Chemie vol 47 no 16 pp 2990ndash2992 2008

[47] L Gomez De Arco Y Zhang C W Schlenker K Ryu ME Thompson and C Zhou ldquoContinuous highly flexible andtransparent graphene films by chemical vapor deposition fororganic photovoltaicsrdquo ACS Nano vol 4 no 5 pp 2865ndash28732010

[48] Y U Jung S Na H Kim and S Jun Kang ldquoOrganic photo-voltaic devices with low resistance multilayer graphene trans-parent electrodesrdquo Journal of Vacuum Science amp Technology AVacuum Surfaces and Films vol 30 no 5 Article ID 0506042012

[49] M Choe B H Lee G Jo et al ldquoEfficient bulk-heterojunctionphotovoltaic cells with transparent multi-layer graphene elec-trodesrdquo Organic Electronics vol 11 no 11 pp 1864ndash1869 2010

[50] Y-Y Choi S J Kang H-K Kim W M Choi and S-I NaldquoMultilayer graphene films as transparent electrodes for organicphotovoltaic devicesrdquo Solar Energy Materials and Solar Cellsvol 96 no 1 pp 281ndash285 2012

[51] J Geng L Liu S B Yang et al ldquoA simple approach for preparingtransparent conductive graphene films using the controlledchemical reduction of exfoliated graphene oxide in an aqueoussuspensionrdquo Journal of Physical Chemistry C vol 114 no 34 pp14433ndash14440 2010

[52] Y-Y Lee K-H Tu C-C Yu et al ldquoTop laminated grapheneelectrode in a semitransparent polymer solar cell by simultane-ous thermal annealingreleasing methodrdquoACS Nano vol 5 no8 pp 6564ndash6570 2011

[53] M Cox A Gorodetsky B Kim et al ldquoSingle-layer graphenecathodes for organic photovoltaicsrdquo Applied Physics Letters vol98 no 12 Article ID 123303 2011

[54] Z Liu J Li Z-H Sun G Tai S-P Lau and F Yan ldquoTheapplication of highly doped single-layer graphene as the topelectrodes of semitransparent organic solar cellsrdquo ACS Nanovol 6 no 1 pp 810ndash818 2012

[55] J-H Huang J-H Fang C-C Liu and C-W Chu ldquoEffectivework function modulation of graphenecarbon nanotube com-posite films as transparent cathodes for organic optoelectron-icsrdquo ACS Nano vol 5 no 8 pp 6262ndash6271 2011

[56] Z Liu Q Liu Y Huang et al ldquoOrganic photovoltaic devicesbased on a novel acceptor material graphenerdquo Advanced Mate-rials vol 20 no 20 pp 3924ndash3930 2008

[57] B Thompson and J Frechet ldquoPolymerndashfullerene compositesolar cellsrdquo Angewandte Chemie International Edition vol 47no 1 pp 58ndash77 2008

[58] Q Liu Z Liu X Zhang et al ldquoOrganic photovoltaic cells basedon an acceptor of soluble graphenerdquoApplied Physics Letters vol92 no 22 Article ID 223303 2008

[59] F Yu and V K Kuppa ldquoEnhancement in the performance oforganic photovoltaic devices with pristine graphenerdquoMaterialsLetters vol 99 pp 72ndash75 2013

[60] L Ye T Xiao N Zhao et al ldquoDerivitization of pristine graphenefor bulk heterojunction polymeric photovoltaic devicesrdquo Jour-nal ofMaterials Chemistry vol 22 no 33 pp 16723ndash16727 2012

[61] Z Liu D He Y Wang H Wu and J Wang ldquoSolution-processable functionalized graphene in donoracceptor-typeorganic photovoltaic cellsrdquo Solar Energy Materials and SolarCells vol 94 no 7 pp 1196ndash1200 2010

[62] D Yu K Park M Durstock and L Dai ldquoFullerene-graftedgraphene for efficient bulk heterojunction polymer photovoltaicdevicesrdquoThe Journal of Physical Chemistry Letters vol 2 no 10pp 1113ndash1118 2011

[63] S Wang B M Goh K K Manga Q Bao P Yang and K PLoh ldquoGraphene as atomic template and structural scaffold inthe synthesis of graphene-organic hybridwirewith photovoltaicpropertiesrdquo ACS Nano vol 4 no 10 pp 6180ndash6186 2010

[64] Q Liu Z Liu X Zhang et al ldquoPolymer photovoltaic cellsbased on solution-processable graphene and P3HTrdquo AdvancedFunctional Materials vol 19 no 6 pp 894ndash904 2009

[65] J Wang Y Wang D He et al ldquoPolymer bulk heterojunctionphotovoltaic devices based on complex donors and solution-processable functionalized graphene oxiderdquo Solar Energy Mate-rials and Solar Cells vol 96 no 1 pp 58ndash65 2012

[66] J Wang Y Wang D He et al ldquoComposition and anneal-ing effects in solution-processable functionalized grapheneoxideP3HT based solar cellsrdquo Synthetic Metals vol 160 no 23-24 pp 2494ndash2500 2010

[67] Z Liu D He Y Wang H Wu and J Wang ldquoGraphene dopingof P3HTPCBMphotovoltaic devicesrdquo SyntheticMetals vol 160no 9-10 pp 1036ndash1039 2010

[68] H Ma H Yip F Huang and A K Jen ldquoInterface engineeringfor organic electronicsrdquoAdvanced Functional Materials vol 20no 9 pp 1371ndash1388 2010

[69] V Shrotriya G Li Y Yao C-W Chu and Y Yang ldquoTransitionmetal oxides as the buffer layer for polymer photovoltaic cellsrdquoApplied Physics Letters vol 88 no 7 Article ID 073508 2006

[70] M D Irwin D B Buchholz A W Hains R P H Changand T J Marks ldquop-Type semiconducting nickel oxide as anefficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cellsrdquo Proceedings of the National Academyof Sciences of the United States of America vol 105 no 8 pp2783ndash2787 2008

[71] J Meyer R Khalandovsky P Gorrn and A Kahn ldquoMoO3films

spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronicsrdquoAdvancedMaterials vol 23 no1 pp 70ndash73 2011

[72] Y-H Kim S-H Lee J Noh and S-H Han ldquoPerformance andstability of electroluminescent device with self-assembled layersof poly(34-ethylenedioxythiophene)ndashpoly(styrenesulfonate)and polyelectrolytesrdquo Thin Solid Films vol 510 no 1-2 pp305ndash310 2006

[73] J Van De Lagemaat T M Barnes G Rumbles et al ldquoOrganicsolar cells with carbon nanotubes replacing In

2O3Sn as the

transparent electroderdquo Applied Physics Letters vol 88 no 23Article ID 233503 2006

[74] R Steim F R Kogler and C J Brabec ldquoInterface materials fororganic solar cellsrdquo Journal of Materials Chemistry vol 20 no13 pp 2499ndash2512 2010

[75] J Y Kim S H Kim H-H Lee et al ldquoNew architecture forhigh-efficiency polymer photovoltaic cells using solution-basedtitanium oxide as an optical spacerrdquoAdvancedMaterials vol 18no 5 pp 572ndash576 2006

[76] T-Y Chu J Lu S Beaupre et al ldquoBulk heterojunction solar cellsusing thieno[34- c]pyrrole-46-dione and dithieno[32- b2101584031015840-d]silole copolymer with a power conversion efficiency of 73rdquo


Journal of the American Chemical Society vol 133 no 12 pp4250ndash4253 2011

[77] ZHe C Zhong XHuang et al ldquoSimultaneous enhancement ofopen-circuit voltage short-circuit current density and fill factorin polymer solar cellsrdquo Advanced Materials vol 23 no 40 pp4636ndash4643 2011

[78] S-S Li K-H Tu C-C Lin C-W Chen and M ChhowallaldquoSolution-processable graphene oxide as an efficient hole trans-port layer in polymer solar cellsrdquo ACS Nano vol 4 no 6 pp3169ndash3174 2010

[79] M S Ryu and J Jang ldquoEffect of solution processed grapheneoxidenickel oxide bi-layer on cell performance of bulk-heterojunction organic photovoltaicrdquo Solar Energy Materialsand Solar Cells vol 95 no 10 pp 2893ndash2896 2011

[80] K C Kwon W J Dong G H Jung J Ham J-L Lee and S YKim ldquoExtension of stability in organic photovoltaic cells usingUVozone-treated graphene sheetsrdquo Solar Energy Materials ampSolar Cells vol 109 pp 148ndash154 2013

[81] Y-J Jeon J-M Yun D-Y Kim S-I Na and S-S Kim ldquoHigh-performance polymer solar cells with moderately reducedgraphene oxide as an efficient hole transporting layerrdquo SolarEnergy Materials and Solar Cells vol 105 pp 96ndash102 2012

[82] J Liu Y Xue and L Dai ldquoSulfated graphene oxide as ahole-extraction layer in high-performance polymer solar cellsrdquoJournal of Physical Chemistry Letters vol 3 no 14 pp 1928ndash1933 2012

[83] J Liu Y Xue Y Gao D Yu M Durstock and L DaildquoHole and electron extraction layers based on graphene oxidederivatives for high-performance bulk heterojunction solarcellsrdquo Advanced Materials vol 24 no 17 pp 2228ndash2233 2012

[84] S Qu M Li L Xie et al ldquoNoncovalent functionalization ofgraphene attaching [66]-phenyl-C61-butyric acid methyl ester(PCBM) and application as electron extraction layer of polymersolar cellsrdquo ACS Nano vol 7 no 5 pp 4070ndash4081 2013

[85] D D Nguyen N H Tai Y L Chueh et al ldquoSynthesis ofethanol-soluble few-layer graphene nanosheets for flexible andtransparent conducting composite filmsrdquo Nanotechnology vol22 no 29 Article ID 295606 2011

[86] B Yin Q Liu L Yang et al ldquoBuffer layer of PEDOTPSSgraphene composite for polymer solar cellsrdquo Journal of Nano-science and Nanotechnology vol 10 no 3 pp 1934ndash1938 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



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CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


Ano

de

Cath

ode

Solar light

h+ eminush+ eminush+ eminush+ eminus

(a) (b) (c) (d)

Figure 1 BHJ cell operation mechanism (a) Exciton generation (b) exciton dissociation (c) charge formation and (d) charge transport andcollection

Based on the simple consideration that low band gapmaterials have the possibility to improve the efficiency ofphotovoltaic devices due to a better overlap with the solarspectrum it is expected that for amaximumphoton harvest-ing in the OSCs low band gap materials will be employedThe use of this type ofmaterials results in higher currents thatcan lead to higher power conversion efficiency (PCE) Poly-3-hexylthiophene (P3HT) one of the most used materials asdonor in a BHJ device exhibits a band gap of 650 nm (19 eV)and thus only has the possibility to harvest up to 224 ofthe available photons giving a maximum theoretical currentdensity of 143mAcmminus2 If P3HT is applied in a BHJ devicecoupled with the most commonly used acceptor (ie phenyl-C61-butyric acid methyl ester PCBM) the band gap extends

to 1000 nm allowing for absorption of 53 of the availablephotons giving a maximum current density of 339mAcmminus2However in practical terms these values cannot be achievedand the real performance of a P3HT PCBM system is aroundits optimal of about 5 which is lower than a quarter of asilicon solar cellrsquos optimal [6 7]

Graphene a zero-band-gap semiconductor is a rising starin materials science (Figure 2) It shows a two-dimensional(2D) structure consisting of sp2-hybridized carbon and itis also the thinnest known material in the universe andthe strongest ever measured [8ndash11] It exhibits remarkableelectronic and mechanical properties and a considerablenumber of new optical effects that have not been observed inother materials [12 13] Its remarkably high carrier mobilitywide absorption spectral range andhigh optical transparencyindicate graphene as a cost-effective and abundant materialthat qualifies for application in optoelectronic devices suchas displays light-emitting diodes and solar cells [14ndash17]

Different routes are available for the preparation ofgraphene [12 18 19] The first involves chemical vapor depo-sition (CVD) of monolayer of graphite on transition metalsurfaces [20 21] The second route is the micromechanicalexfoliation of graphite and involves peeling of the graphenefrom graphite using ldquoScotchrdquo tape [22] The ldquoScotchrdquo tapeis then dipped in acetone to release the graphene whichis subsequently captured on a silicon wafer with a SiO

2

Figure 2 Representation of graphene structure

(silicon dioxide) layer on top The third route involvesthe epitaxial growth of graphene on electrically insulatingsubstrates like silicon carbide [23] However the second andthird methods are unsuitable for large-scale preparation ofgraphene for the fabrication of polymer nanocomposites [19]Thus it appears that the only routes to prepare bulk quantitiesof graphene and chemically modified graphene (CMG) arefrom graphene oxide (GO) and graphite intercalation com-pounds (GICs)The presence of hydroxyl and epoxide groupson the graphitic basal planes and carboxyl and carbonylgroups on the edges of layers makes GO water dispersible[24] Thus on sonication in aqueous media GO exfoliatesreadily into colloidal suspensions of single graphene oxidelayers Chemical reduction of GO present in the colloidalsuspension with reducing agents is necessary to convert theelectrically insulatingGO layers back to conducting graphene










Substrate


Acceptor material

Donor material























119909) layer was




2Al 078 [34]











3)









60 TPBiSLG 022 [53]

















60-G P3HTAl 122 [62]





















60 P3HT




60-G P3HT system




60with


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials












Substrate


Acceptor material

Donor material























119909) layer was




2Al 078 [34]











3)









60 TPBiSLG 022 [53]

















60-G P3HTAl 122 [62]





















60 P3HT




60-G P3HT system




60with


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





















119909) layer was




2Al 078 [34]











3)









60 TPBiSLG 022 [53]

















60-G P3HTAl 122 [62]





















60 P3HT




60-G P3HT system




60with


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






2Al 078 [34]











3)









60 TPBiSLG 022 [53]

















60-G P3HTAl 122 [62]





















60 P3HT




60-G P3HT system




60with


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






60 TPBiSLG 022 [53]

















60-G P3HTAl 122 [62]





















60 P3HT




60-G P3HT system




60with


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






60-G P3HTAl 122 [62]





















60 P3HT




60-G P3HT system




60with


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials













60 P3HT




60-G P3HT system




60with


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials


















3H HTLs ITOGO-OSO




2120583m



3 WO

3 and MoO





2 TiO












119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







119909andNiO






119909functions















3H led to





2CO3and








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials








5 Conclusions




References













































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







































































2O3Sn as the
























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Review Article A Review on the Efficiency of Graphene...

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Transcript of Review Article A Review on the Efficiency of Graphene...