ff

lla

Accepted 19 November 2011Available online 9 December 2011

Keywords:

ule

ted

t oc

fci

as molybdenum oxide (MoO3) and tungsten oxide (WO3) were used as buffer layer between the ITO

anode and active layer. The effect of MoO3 and WO3 thickness on the performance of the photovoltaic

attracttion cconve

interface [15]. Poly styrene sulfonic acid (PEDOT:PSS) has been

usinges ofcy of. The

function of each layer was discussed and the thickness of them

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/solmat

Solar Energy Mater

n Corresponding author. Tel.: 98 311 7932428; fax:98 311 7932435.

Solar Energy Materials & Solar Cells 98 (2012) 379384structure investigated in this research. ITO coated glass substratesali.hassanzadeh@gmail.com (A. Hassanzadeh).2. Experimental details

Fig. 1 illustrates the schematic of organic photovoltaic cells

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.solmat.2011.11.036

E-mail addresses: m_gh_v@yahoo.com,

m.ghasemi@sci.ui.ac.ir (M. Ghasemi Varnamkhasti),

hfallah@sci.ui.ac.ir (H.R. Fallah),

mmostajab@yahoo.com (M. Mostajaboddavati),

Rasoolgha@yahoo.com (R. Ghasemi),was optimized. Finally, the more suitable material to use as anodebuffer layer in our cells was determined.to change the interface between active layers and electrodes.Modication of the anode is one of the methods to improve theperformance of organic solar cells. Often, a thin buffer layer isused to obtain good band structure matching at the anode/donor

In this paper, organic photovoltaic cells were fabricatedCuPc as donor and C60 as an acceptor layer. The inuencvarious metal oxide layers on power conversion efciensmall molecule organic photovoltaic cells were investigatedorganic photovoltaic cells is still low. Several methods such asannealing treatment [6], enhancement of photon absorption [7]and introduction buffer layers [810] have been used to improvethe performance of organic photovoltaic cells. In the case of CuPc/C60 bilayer organic photovoltaic cells, exciton blacking andelectron transporting layers between the acceptor and the cath-ode have been shown to improve the device performance[1114]. To improve charge collection, buffer layers are inserted

organic photovoltaic cells [16,17]. It was shown that some metaloxides such as NiO [18], MoO3 [19] and V2O5 [20] are suitablesubstitutions for PEDOT:PSS in polymer organic solar cells. To thebest of our knowledge according to literature, less attention hasbeen paid to compare the inuence of various metal oxides undersame conditions as anode buffer layer on small molecule organicsolar cell performance. Moreover, the function and operationmechanism of these metal oxide buffer layers are still in debate.Anode buffer layer

Small molecules

Molybdenum oxide (MoO3)

Tungsten oxide (WO3)

1. Introduction

Organic photovoltaic cells haveto their potential for low fabricamechanical exibility [15]. Powerbetter hole transport. Also the devices performance was analyzed based on the surface roughness of

bare ITO, and ITO, which covered with WO3 and MoO3. It was found that the anode buffer layer

thickness is a very important factor in controlling the electrical characteristics of the organic

photovoltaic devices. It is shown that the best results are obtained with a 4 nm MoO3.

& 2011 Elsevier B.V. All rights reserved.

ed much attention dueost, light weight andrsion efciency of the

widely used as anodic buffer layer because of achieving goodadjustment of the work function and a good hole transport. But ithas some disadvantages like high series resistance, chemicalreaction with ITO anode, acidicity and degradation under UVillumination, which are limiting factors for performance ofOrganic photovoltaic cellsdevices was investigated and compared. The thickness of each buffer layer was optimized to haveComparison of metal oxides as anode buphotovoltaic cells

Mohsen Ghasemi Varnamkhasti a,n, Hamid Reza FaRasool Ghasemi a, Ali Hassanzadeh c

a Department of Physics, University of Isfahan, Isfahan, Iranb Quantum Optics Research Group, University of Isfahan, Isfahan, Iranc Department of Chemistry, University of Urmia, Urmia, Iran

a r t i c l e i n f o

Article history:

Received 3 October 2011

Received in revised form

13 November 2011

a b s t r a c t

In this work, small molec

hetrojunction were fabrica

the anode and the highes

buffer layer is necessary. Eer layer for small molecule organic

h a,b, Mojtaba Mostajaboddavati a,

organic photovoltaic cells based on copper phthalocyanine (CuPc)/C60. To have a good band structure matching between the work function of

cupied molecular orbital of the organic material the introduction of a

ency of devices shows a strong improvement when the metal oxides suchials & Solar Cells

(FF) and power conversion efciency (Z), which are obtained fromJV characteristics are presented in Table 1. Further improvementsin photovoltaic device performance can be obtained by placing abuffer layer between the ITO and CuPc layer. The comparison of thephotovoltaic properties of the devices prepared on MoO3 and WO3coated anodes lets us to nd that the photovoltaic performance ofthe devices is substantially improved in comparison to the photo-voltaic cells with ITO alone. As seen, the JV characteristics ofphotovoltaic devices signicantly depend on the thickness of theMoO3 and WO3 buffer layers. As shown in Table 1, power conver-sion efciency of photovoltaic cells without anode buffer layer(0.045%) is much lower than that of the other devices with an anodebuffer layer. The insertion of MoO3 and WO3 leads to an initial

Fig. 2. JV characteristics under illumination for photovoltaic devices withdifferent thicknesses of (a) MoO3 and (b) WO3 used as anode buffer layer.

M. Ghasemi Varnamkhasti et al. / Solar Energy Materials & Solar Cells 98 (2012) 379384380with about 15O/& sheet resistance (purchased from Sigma-Aldrich)were cleaned with detergent, acetone, ethanol and iso-propanol in anultrasonic cleaner and then rinsed with de-ionized water and weredried with a nitrogen ow. The organic materials copper phthalocya-nine (CuPc 99.9%) as donor and fullerene (C60 99.9%) as acceptor werepurchased from Sigma-Aldrich and used as received without furtherpurication. Different materials such as MoO3 and WO3 were used asanode buffer layer and Ag was deposited as cathode. In the presentwork, bathocuproine (BCP) was used as exciton blocking layer.Deposition rates and thickness of thin lms were estimated andmonitored in situ with a quartz oscillator crystal. All layers inphotovoltaic devices were deposited under a base pressure of3105 mbar. In order to obtain an optimal thickness of the MoO3and WO3 as anode buffer layers, we changed their thicknesses in therange between 2 and 8 nm. The MoO3 and WO3 layers werethermally evaporated at deposition rate of 1 A/S over the anode. A40 nm thick CuPc layer as the electron donor and a 40 nm thick C60 asthe electron acceptor were thermally evaporated at the depositionrate of 0.8 A/S. Thereafter, an 8 nm thick exciton blocking layer of BCPwith deposition rate of 0.5 A/S was evaporated to complete theorganic multilayer. Finally a 100 nm thick Ag cathode with depositionrate of 1 A/S was deposited. Optical measurements of the sampleswere done in the wavelength range from 300 to 800 nm with adouble-beam spectrophotometer (Shimadzu UV 3100) by recordingthe UVvisible optical transmission and absorption spectra. Thecurrentvoltage (IV) characteristics were measured with a Keithley2400 source meter in dark and under 1 sun global AM 1.5 simulatedsolar illumination. The electrical measurements were performed inair without encapsulation at room temperature. The interfacemorphologies were also studied using atomic force microscope(AFM) to determine any changes with lm deposition. The crystalstructure of MoO3, WO3 and CuPc coated ITO were characterizedusing XRD technique with a D8 Advanced Bruker X-ray diffractometerat room temperature, with monochromated CuKa(l1.54 A1) in thescan range of 2y between 51 to 901 with a step size of 0.01 (2y/s).

Fig. 1. Schematic structure of the organic photovoltaic cells under study.Measurements were taken under beam-acceleration conditions of40 kV/35mA.

3. Results and discussions

Organic photovoltaic cells with a structure of ITO/buffer layer(x nm)/ CuPc (40 nm)/ C60 (40 nm)/BCP (8 nm)/Ag are investigated.The thickness of each anode buffer layer changed from 2 to 8 nm.

Here, we want to stress that we do not intend to achieve thebest overall cell performance. We demonstrate that metal oxides(MoO3 and WO3) can be used as effective layers in organicphotovoltaic cells based on CuPc/C60 heterojunctions.

Fig. 2 shows the dependence of the current densityvoltage (JV)characteristics of cells with different thickness of anodic bufferlayer. Short circuit current (Jsc), open circuit voltage (Voc), ll factorincrease in Jsc, Voc and FF. Then further increase of anode bufferlayer thickness causes a decrease in the above parameters. The Jsc islimited by various factors such as light absorption efciency,exciton dissociation efciency and charge carrier collection ef-ciency. Since in this research anode buffer layer has low thickness, itis anticipated that there is no remarkable change in light absorptionefciency and exciton dissociation efciency [21].

The optical absorption curve of active materials (CuPc/C60) andthe transmittance spectra of the anode buffer (MoO3 andWO3) foroptimum thickness are shown in Fig. 3. It can be seen from Fig. 3bthat the optical transmission of anode buffer layers in visiblewavelength region is very high (95%). This transparency ismatched with the absorption region for active layer. Also, thereis no difference between the absorption of CuPc/C60 devices andthat of CuPc/C60/ anode buffer layer (MoO3 and WO3). On theother hand 4 nm MoO3 and 6 nm WO3 do not signicantly

ses

M. Ghasemi Varnamkhasti et al. / Solar Energy Materials & Solar Cells 98 (2012) 379384 381Table 1Parameters of organic photovoltaic devices with different MoO3 and WO3 thicknes

Anode buffer layer MoO3

Thickness (nm) 2 4 6 8

Jsc(mA/cm2) 2.03 7.98 6.93 5.54

Voc(V) 0.35 0.44 0.42 0.37FF 0.47 0.45 0.43 0.35contribute to the absorption of photovoltaic device (see Fig. 3).Thus the increase in Jsc can be attributed to an increased chargecarrier collecting efciency due to the anode buffer layer. Thehighest occupied molecular orbital (HOMO) energy level of CuPcand the valence band of MoO3 and WO3 are 5.2, 5.3 and 5.1 eV,respectively. The valence bands of MoO3 and WO3 are near toFermi level of ITO and the HOMO of CuPc. The lowest unoccupiedmolecular orbital (LUMO) energy level of CuPc and the conductionbands of MoO3 and WO3 are 3.5, 2.3 and 1.6 eV, respectively.

Z(%) 0.33 1.58 1.25 0.71(Ocm2)RsA 41.1 15.2 21.6 30.2

Fig. 3. Absorption spectra of (a) CuPc, C60, MoO3, WO3 lms and (b) CuPc/C60,CuPc/C60/ MoO3(4 nm) and CuPc/C60/WO3(6 nm). Inset: Transmission spectra of

optimized anode buffer layers.as determined from JV characterization.

WO3 Without

buffer layer

2 4 6 8 0

2.9 5.18 5.69 5.08 1.20.28 0.37 0.43 0.32 0.150.33 0.4 0.41 0.35 0.250.26 0.76 1.01 0.56 0.045

53.8 22.0 16.8 26.2 127.5Therefore, MoO3 and WO3 are effective in blocking electrons andalso increasing hole collection from the CuPc layer. The Voc valuesof the organic photovoltaic cells with optimized anode bufferlayer are 0.29 V higher than those of photovoltaic devices withoutthe anode buffer layer. According to the literature, it has beenstated that the Voc depends on the work function difference ofelectrodes or the difference of the HOMO energy level of donorand the LUMO energy level of acceptor [22]. The enhancement ofVoc may be described by a variation in interface energeticsbetween the ITO anode and active layer due to the MoO3 andWO3 buffer layers.

Dark current densityvoltage plots of various cells are pre-sented in Fig. 4. It can be found that the MoO3 and WO3 anodebuffer layers are useful for charge carrier injection, leading to anincrease in dark current density at 1 V by factors of 56 and 41 forMoO3 and WO3, respectively. This exhibits a good interfacecontact between CuPc and anode. Also it can be found that theintroduction of MoO3 and WO3 signicantly decreases the seriesresistance. As shown in Table 1, series resistance (RSA) of organicphotovoltaic cells decreased from 127.5 to 15.2 Ocm2 and16.8 Ocm2 when we used 4 nm thick MoO3 and 6 nm thickWO3 as buffer layer, respectively. The RSA includes both contactresistances and bulk resistance. The contact resistance emergesfrom the interface between the active layer and electrodes, andbulk resistance consists of the resistance of organic layers andelectrodes. Since all the devices were prepared under the sameconditions the difference of ensemble resistance originates fromthe decrease of contact resistance between organic layer and ITOelectrode due to the introduction of MoO3 and WO3 buffer layer.Also, JV characteristics of the optimized devices were studied tounderstand the effects of anode buffer layer on carrier transport

Fig. 4. JV characteristics of photovoltaic devices without anode buffer layer andwith optimum thickness of MoO3 and WO3 as anode buffer layer in dark.

and to illustrate transport limited mechanism. We can nd fromthe log JlogV plots (at VS1) that the both types of photovoltaicdevices with MoO3 and WO3 buffer layer follow the space chargelimited current (SCLC). In SCLC model almost all traps are lledand therefore the relation between current and voltage obeysChilds low [23]:

j 98ee3m

V2

d31

Where e3 is the permittivity of vacuum, e the relative dielectricconstant, m the effective charge carrier mobility, d the active layerthickness and V the applied voltage. Assuming e3, the effectivecharge carrier mobility in active layer was calculated using Eq. (1).The hole mobility values of the devices with MoO3 and WO3 bufferlayer are (1.070.1)104 cm2/V.S and (6.470.2)105 cm2/V.S,respectively. Higher value of carrier mobility of the device withMoO3 is in agreement with its better performance than that of thedevice with WO3. Improvement in carrier mobility is useful fororganic photovoltaic cells performance. It is because decreasing

bias-dependent carrier recombination effectively increases themean free path of carriers [24] or decreases the space chargebuild-up [25].

In order to analyze the inuence of thin buffer layer on theinterface between donor/anode and the surface of active layers,AFM images of the bare ITO surface, ITO/MoO3 (4 nm), ITO/WO3(6 nm), ITO/MoO3 (4 nm)/CuPc (40 nm) and ITO/MoO3 (4 nm)/CuPc (40 nm)/C60 (40 nm) were shown in Fig. 5(ae). The surfaceroot-mean-square (RMS) roughness and the average grain orcluster size of various lms were presented in Table 2. Bydepositing each layer on top of underlying layer in photovoltaicdevice the average cluster size was increased. Also, it can be saidthat the surface roughness of ITO lms is polished by MoO3 andWO3 layers. The RMS value of ITO covered with MoO3 layer issmaller than bare ITO and ITO coated with WO3 layer. Afterthermally evaporated 40 nm of CuPc over the ITO/MoO3 surface,the roughness of the CuPc surface was increased. The subsequent40 nm of C60 layer is thermally evaporated on top of CuPc layer.The deposited C60 lm typically has a morphology that is

O3

M. Ghasemi Varnamkhasti et al. / Solar Energy Materials & Solar Cells 98 (2012) 379384382Fig. 5. Atomic force microscope (AFM) images of the surface of (a) ITO, (b) ITO/Mo

(4 nm)/CuPc (40 nm)/C60 (40 nm).(4 nm), (c) ITO/WO3 (6 nm), (d) ITO/MoO3 (4 nm)/CuPc (40 nm) and (e) ITO/MoO3

ITO/MoO3 (4 nm)/CuPc (40 nm)/C60(40 nm)

2.149 10278.1

M. Ghasemi Varnamkhasti et al. / Solar Energy Materials & Solar Cells 98 (2012) 379384 383Table 2Surface root-mean-square (RMS) roughness and the average grain or cluster size

of various structures.

Structure RMS roughness

(nm)

Average grain or

cluster size (nm)

ITO 2.258 43.276.3ITO/MoO3 (4 nm) 0.357 76.375.2ITO/WO3 (6 nm) 1.091 85.777.6ITO/MoO3 (4 nm)/CuPc (40 nm) 1.778 93.276.4governed by the underlying surface, resulting in rougher surface.It is known that a at surface is very useful to decrease leakagecurrent and contact resistance [26]. By comparing these resultswith device performance, we conclude that the smoothed anodesurface may effectively decrease series resistance.

Fig. 6 shows the x-ray diffraction (XRD) pattern of MoO3, WO3and CuPc lms grown on ITO glass. The line at 2y31.51corresponds to the reection from the (222) plane. This peak isclose to the position of the strongest line of the reference indiumoxide pattern. The other peaks are due to the reection from the(211), (400), (431), (440) and (622) planes. As shown in Fig. 6(a),the characteristic peak at 2y6.951 is observed, which corre-sponds to the (200) plane of a-CuPc phase. No crystalline peaksare observed for MoO3 and WO3 layers in Figs. 6(b) and (c),respectively. Thus MoO3 and WO3 lms under study have beenamorphous. Being amorphous can be considered as an advantagebecause it has been reported that it reduces spatial non-unifor-mity, which is favorable for large area device fabrication [27].

4. Conclusions

In summary, the inuence of metal oxides (MoO3 and WO3) asanode buffer layer with various thicknesses on the performance ofCuPc/C60 heterojunctions organic photovoltaic cells were investi-gated and compared. It was found that all Jsc, Voc, FF, and Z can besignicantly increased by inserting an optimum thickness ofMoO3 or WO3 as buffer layer between the active layer and ITOanode compared to devices without anode buffer layer. Thisimprovement of performance mainly results from the

polymer photovoltaic cells by using an ethanol-soluble conjugated polyuor-

molecular architecture and intermixing on the photovoltaic, morphologicaland spectroscopic properties of CuPcC60 heterojunctions, Solar Energy

Fig. 6. X-ray diffraction patterns of (a) ITO/CuPc, (b) ITO/MoO3 and (c) ITO/WO3.Materials & Solar Cells 83 (2004) 229245.[12] Z.R. Hong, Z.H. Huang, X.T. Zeng, Investigation into effects of electron

transporting materials on organic solar cells with copper phthalocyanine/C60 heterojunctions, Chemical Physics Letters 425 (2006) 6265.

[13] V. Tripathi, D. Datta, G.S. Samal, A. Awasthi, S. Kumar, Role of excitonblocking layers in improving efciency of copper phthalocyanine basedorganic solar cells, Journal of Non-Crystalline Solids 354 (2008) 29012904.

[14] Y. Lare, B. Kouskoussa, K. Benchouk, S.O. Djobo, L. Cattin, M. Morsli, F.R. Diaz,M. Gacitua, Inuence of the exciton blocking layer on the stability of layeredorganic solar cells, Journal of Physics and Chemistry of Solids. 72 (2011)97103.

[15] J.C. Bernede, L. Cattin, M. Morsli, Y. Berredjem, Ultra thin metal layerpassivation of the transparent conductive anode in organic solar cells, SolarEnergy Materials & Solar Cells 92 (2008) 15081515.

[16] M. Jrgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solarcells, Solar Energy Materials & Solar Cells 92 (2008) 686714.

[17] K.S. Kang, H.K. Lim, K.J. Han, J. Kim, Durability of PEDOT:PSS-pentaceneSchottky diode, Journal of Physics D 41 (2008) 012003-1012003-4.

[18] M.D. Irwin, B. Buchholz, A.W. Hains, R.P.H. Chang, T.J. Marks, p-Typeene as cathode buffer layer, Solar Energy Materials & Solar Cells 93 (2009)604608.

[9] S.Y. Park, H.R. Kim, Y.J. Kang, D.H. Kim, J.W. Kang, Organic solar cellsemploying magnetron sputtered p-type nickel oxide thin lm as the anodebuffer layer, Solar Energy Materials & Solar Cells 94 (2010) 23322336.

[10] J. Dai, X. Jiang, H. Wang, D. Yan, Organic photovoltaic cell employing organicheterojunction as buffer layer, Thin Solid Films. 516 (2008) 33203323.

[11] S. Heutz, P. Sullivan, B.M. Sanderson, S.M. Schultes, T.S. Jones, Inuence ofenhancement of hole-collecting and electron-blocking by MoO3and WO3 layers. Higher charge injection, and lower series resis-tance were seen with introducing optimal thickness of MoO3 orWO3 layer. The AFM images exhibit that the insertion of MoO3and WO3 as anode buffer layers will decrease the roughness ofITO surface, which is desirable to improve the device perfor-mance. MoO3 and WO3 layers with optimum thickness play amajor role in creating good interface contact between donor andanode. The results demonstrate that the best performance isobtained with a 4 nm MoO3. In comparison to device with WO3,the better performance of device with MoO3 was attributed tofurther smoothing of ITO roughness and higher carrier mobility.For the optimized device with a MoO3, power conversion ef-ciency is 0.5% higher than that of the optimized device with WO3.

Acknowledgment

The authors would like to thank the Graduate Ofce of IsfahanUniversity for their support. The authors are greatly indebted to theIran Nanotechnology initiative council for their nancial support.

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M. Ghasemi Varnamkhasti et al. / Solar Energy Materials & Solar Cells 98 (2012) 379384384

Comparison of metal oxides as anode buffer layer for small molecule organic photovoltaic cellsIntroductionExperimental detailsResults and discussionsConclusionsAcknowledgmentReferences