Solar Energy Materials & Solar Cells 95 (2011) 3036–3040

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

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lgh1636

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

Spontaneous formation of nanoripples on the surface of ZnO thin filmsas hole-blocking layer of inverted organic solar cells

Dong Chan Lim a, Won Hyun Shim a,c, Kwang-Dae Kim b, Hyun Ook Seo b, Jae-Hong Lim a,Yongsoo Jeong a, Young Dok Kim b,n, Kyu Hwan Lee a,n

a Materials Processing Division, Korea Institute of Materials Science, Changwon 641-010, Koreab Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Koreac School of Materials Science and Engineering, Pusan National University, San 30 Jangjeon-dong Geumjeong-gu, Pusan 609-735, Korea

a r t i c l e i n f o

Article history:

Received 20 April 2011

Received in revised form

9 June 2011

Accepted 19 June 2011Available online 12 July 2011

Keywords:

Inverted organic solar cell

ZnO

Nanostructure

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.06.028

esponding authors. Tel.: þ82 55 280 3545; fa

ail addresses: ydkim91@skku.edu (Y.D. Kim),

@kims.re.kr (K.H. Lee).

a b s t r a c t

A simple method for spontaneous formation of nanoripples on ZnO thin films was developed, and these

nanostructured ZnO films were used as hole-blocking layer in inverted organic solar cells. Moreover,

the size (height) of nanoripples on ZnO surface could be controlled in the range of several tens of

nanometers. Among various ZnO films, surface structures with �70 nm-high nanoripples resulted in

the best photovoltaic performance of the organic solar cell consisting of a stack of indium tin oxide/

ZnO/ regioregular poly (3-hexyl thiophene), phenyl-C61-butyric acid methyl ester/Ag. The power

conversion efficiency of inverted organic solar cells consisting of with 70 nm-high ZnO nanoripples

(�3.2%) was higher than that of a relatively flat ZnO surface by a factor of �2. Existence of nanoripples

on ZnO results in a higher contact area between ZnO and active layer, leading to an enhanced

photovoltaic performance.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Use of solar light as energy source has been regarded as one ofthe environmental-friendly ways of producing energy. Amongvarious energy devices using light, organic solar cell (OSC) hasbeen attracting particular attention for the last decades due to itscost-effectiveness and potential application in flexible devices[1–5]. One of disadvantages of OSCs is its short life-time, which isdue to the fast (photo)-degradation of various interfaces existingin OSCs [6–8]. In order to increase stability of OSC, an invertedstructure of OSC (IOSC) has been developed: in a regular devicestructure, electron–hole pairs are created in polymer activelayers, holes and electrons are injected into transparent conduct-ing oxide and counter-electrode (e.g. Al), respectively. In contrast,in IOSC, electrons are injected into the transparent conductingoxide, i.e. the electrode alignment is reversed [9–20].

Various methods have been used for improving power con-version efficiency of OSCs. Different polymers were used as light-harvesting layer for enhancing power conversion efficiency ofOSCs [21,22]. Carrier mobility has been increased by dopinghighly-conducting nanostructures such as carbon nanotubes andmetal nanoparticles in active and buffer layers existing in OSCs[23,24]. Methods for increasing light absorption and more

ll rights reserved.

x: þ82 55 280 3570.

efficiently separating electron–hole pairs have also been devel-oped using various intermediate layers and modifiers [25–29].

Among various nanomaterials used as electrodes and bufferlayers in OSCs, nanorods and wires with a high conductivity andincreased contact area between electrodes and active layers haveattracted much attention [5,30–37]. Synthesis of nanowires canbe achieved using electrochemical or hydrothermal methods[34–39]. In the present work, a simpler strategy for the formationof ZnO nanoripple structure, whose dimension is comparable tothat of nanowires, is presented. Surface structures comparable tothe nanowire-incorporated thin films can spontaneously form byproperly heating spin-coated ZnO films. For spin-coating of ZnO,various mixtures of ZnO nanoparticles and zinc acetate solutionwere used. It is demonstrated that by adjusting the amount ofZnO naonaprticles added in zinc acetate solution nanoripple sizecan be controlled for the best device performance of IOSCconsisting of these ZnO thin films on transparent conductingoxide as hole-blocking and electron-colleting layer.

2. Experimental

The IOSCs studied in the present work consisted of a stack of150 nm thick-indium tin oxide film on glass/ZnO/active layer/Ag asshown in Fig. 1. For the preparation of diverse ZnO thin films, ZnOsol–gel solutions with various amounts of ZnO nanoparticles werespin-coated and heated differently (Fig. 2). The mean thickness ofZnO film was about 70 nm, and the detailed structure of each ZnO

Fig. 1. Left: Scanning electron microscopy (SEM) images of and IOSC device fabricated in the present work. Right: The band diagram of IOSCs consisting of active layer

(P3HT), hole-blocking and electron accepting layers (P3HT and ZnO) and both electrodes (indium tin oxide and Ag) are displayed [41,42].

Fig. 2. AFM images of various ZnO film surfaces on indium tin oxide used as hole-blocking layers in IOSCs. (a) ZnO film was spin-coated using bare zinc acetate solution,

and heated to 350 1C with a heating rate of 23 1C/min. (b–f) ZnO films were spin-coated using mixtures of ZnO nanoparticles and zinc acetate solutions. Then, the films

were heated to 350 1C with a heating rate of 23 1C/min. From b to f, the amount of ZnO nanoparticles added was increased. Amount of ZnO nanoparticles added to 21 ml of

zinc acetate solution was 0.25, 1, 3, 5 and 10 mg for b–f, respectively.

D.C. Lim et al. / Solar Energy Materials & Solar Cells 95 (2011) 3036–3040 3037

film is shown in Fig. 2. For the preparation of ZnO sol–gel solutionszinc acetate [Zn(CH3COO)2.2H2O] was dissolved in 2-methoxyetha-nol solution containing ethanolamine as a stabilizer. Concentrationof zinc acetate was 0.75 M. This solution was stirred at 60 1C for30 min to yield a clear and homogeneous solution, which served asthe coating solution. For the preparation of active layer, a mixtureof regioregular poly (3-hexyl thiophene) (RR-P3HT) (30 mg/ml)and phenyl-C61-butyric acid methyl ester (PCBM) (21 mg/ml) weredissolved in 1,2-dichlorobenzene under vigorous stirring overnight.Then, this solution was spin-coated on ZnO films deposited onindium tin oxide under atmospheric pressure at room temperature(600 rpm, 40 s). The thickness of active layer was 380 nm. Topelectrode (Ag) was deposited by thermal evaporation method witha thickness of 100 nm under 5�10–6 torr condition. The active-area of the device with metal mask was 0.38 cm2.

The structures of ZnO films on indium tin oxide were analyzedby atomic force microscopy (AFM, Vecco; MMAFM-2), and their

optical properties were measured using UV/Vis transmittance spec-trometer (Verian Cary 5000), respectively. Hall effects measure-ments were performed using ECOPIA NMS-3000 for determiningcarrier mobility. The current density–voltage (J–V) characteristics ofthe OSC devices were measured under AM 1.5 simulated illumina-tion with an intensity of 100 mW/cm2 (Pecell Technologies Inc., PEC-L11 model 13). The intensity of sunlight illumination was calibratedusing a standard Si photodiode detector with a KG-5 filter. The J–V

curves were recorded automatically with a Keithley SMU 2400source meter by illuminating the OSCs.

3. Results and discussion

As shown in Fig. 1, the IOSCs fabricated and studied in thepresent work consisted of a stack of indium tin oxide-coatedglass, ZnO thin films, RR-P3HT:PCBM layer and Ag electrode. As it

D.C. Lim et al. / Solar Energy Materials & Solar Cells 95 (2011) 3036–30403038

has been already demonstrated by Sekine et al. surface of spin-coated ZnO thin films using zinc acetate solution on indium tinoxide could be converted into a ridge structure by the followingprocedures: the sample was heated from room temperature to350 1C with a heating rate of 11 1C/min, and the heating wasstopped when the final temperature was reached [14]. ZnOsurfaces underwent this preparation showed a ridge structurewith a mean ridge width of �1 um and height of �150 nm (datanot shown). When a faster heating rate (23 1C/min) was used forheat-treatment of the spin-coated ZnO films, formation of rippleswith lower heights and widths with respect to the aforemen-tioned ridge structure could be identified (Fig. 2a). It is notablethat size of the nanoripples in Fig. 2a is not homogeneous, butthere are two different types of nanoripples: ZnO- nanorippleswith a mean height of �100 nm, and �20 nm, respectively, canbe found. We used mixtures of zinc acetate solution and ZnOnanoparticles with a mean particle size of o100 nm (Aldrich)instead of pure zinc acetate solution for spin-coating of ZnO films,and the films were subsequently heated to 350 1C with a heatingrate of 23 1C/min. When 0.25 mg of ZnO nanoparticles was addedto 21 ml of zinc acetate solution for the fabrication of ZnO thinfilm, a more homogeneous nanoripple structure was formed(Fig. 2b). In this case, the mean height of nanoripples was70 nm, which is significantly lower than that of Fig. 2a. Withincreasing amount of ZnO nanoparticles in the zinc acetatesolution, the ripples became narrower in width and lower inheight (Fig. 2b–f). The mean height of the ripples becameo30 nm, when 10 mg of ZnO was added to 21 ml of zinc acetatesolution for the ZnO film preparation (Fig. 2f). Previously, it wasfound that the size and morphology of ZnO particles prepared bythe sol–gel method can be sensitive to the annealing temperatureand rate, since decomposition of Zn precursor and evaporation ofsolvent take place with various rates at different heating condi-tions [40]. At lower temperatures, solvent is not completelyvaporized and therefore, ZnO crystal growth with a preferredorientation is disturbed by solvent. This could be one of thedriving forces for the formation of three-dimensional roughstructures of ZnO surfaces upon slow heating. In addition, it islikely that ZnO nanoparticles acted as seeds of the ZnO nanorippleformation during heating. By changing the amount of ZnOparticles added, density of nucleation centers can be controlled,which can have direct influence on the ripple size and density.

Fig. 3. (a) Photovoltaic performances of IOSCs consisting of ZnO thin films with various

short-circuit current of each device is summarized.

Without using additional nanowire-synthesis process (e.g. hydro-thermal synthesis after seed formation), surfaces correspondingto the nanowired-structures can spontaneously form.

The ZnO thin films on indium tin oxide demonstrated in Fig. 2were used for fabricating IOSCs. In Fig. 3, photovoltaic performancesof various IOSCs prepared in the present work are summarized.When ripple-structure of ZnO films was prepared using a bare zincacetate solution in Fig. 2a was used as buffer layer of IOSC, a powerconversion efficiency of 2.78% could be reached. For comparison,IOSCs with two different ZnO films reported in previous works,namely without any ripple, or broader ridges, were fabricated, andtheir photovoltaic performances were tested. Both devices showedpower conversion efficiency of less than 1.8%, indicating thatformation of nanoripples in ZnO in Fig. 2a resulted in a significantimprovement in photovoltaic performance of IOSCs with respect tothe IOSCs fabricated using conventional ZnO films (data not shown).As the amount of added ZnO nanoparticles in zinc acetate solutionincreased, the power conversion efficiency of IOSC first increasedand then decreased, i.e. there is an optimum value of the amount ofZnO nanoparticles added in zinc acetate solution for yielding thebest photovoltaic performance of IOSC. Using ZnO thin films with aripple height of �70 nm (Fig. 2b), a power conversion efficiency of3.2% could be reached. It is remarkable that this power conversionefficiency value could be obtained without any additional oxidelayers (e.g. NiO and V2O5) between Ag and P3HT as hole-collectionlayer [34]. In the case of fill-factor (FF) and short-circuit current (Jsc),the generally observed trend is similar to that of the powerconversion efficiency, i.e. as a function of the amount of ZnOnanoparticles added to the zinc acetate solution, the device perfor-mance first increased and then decreased (Fig. 3).

In order to shed light on the origin of the superior photovoltaicperformances of IOSCs consisting of ZnO nanoripples with a meanheight of �70 nm, carrier mobility and transmittance of variousZnO thin films in Fig. 2 were measured (Fig. 4). For carriermobility, no significant variation could be observed as s functionof surface morphology. Regarding transmittance of light, also, nochange could be identified as the ZnO surface structure changed.This result implies that both electronic and optical properties ofZnO thin films were not much changed by the addition of ZnOnanoparticles in zinc acetate solution for spin-coating ZnO films.Thus, it is more reasonable to conclude that the morphologicalchange of ZnO film surface should have a large influence on

surface structures in Fig. 2(a–f). On the right, power conversion effcieincy, FF and

(a)(b)(c) ZnO NR 15 min with NP (1.00 mg)(d) ZnO NR 15 min with NP (3.00 mg)(e)

Tra

nsm

ittan

ce (%

)

54

52

50

48

46Car

rier

Mob

ility

(Cm

2 /Vs)

(f) ZnO NR 15 min with NP (10.00 mg)

(a) 400

80

60

Samples Wavelength (nm)

ZnO NR 15 min with NP (0.25 mg)ZnO NR 15 min

ZnO NR 15 min with NP (5.00 mg)

500 600 700 800(b) (c) (d) (e) (f) (g) (h)

Fig. 4. (a) Carrier mobility and (b) transmittance of ZnO thin films in Fig. 2a–f are measured and compared.

D.C. Lim et al. / Solar Energy Materials & Solar Cells 95 (2011) 3036–3040 3039

photovoltaic performance. Addition of ZnO nanoparticles in zincacetate solution made the size of nanoripples more homogeneous,which resulted in an enhanced photovoltaic performance of therespective IOSC. On one hand, the surface area of the sample inFig. 2b with 0.25 mg of ZnO particles could have been increasedcomparing to that of Fig. 2a, by increasing the surface contact areabetween ZnO and active layers. Electrons and holes created in theactive layers during absorption of light can more efficiently beseparated by increased contact area between buffer and activelayers. On the other hand, wetting of the active layer on ZnO couldhave been better on more homogeneous nanoripple structure inFig. 2b than that in Fig. 2a. With further increasing amount of ZnOnaonparticles added, ZnO surface became more flat, resulting in areduced contact area between ZnO and active layer, and respec-tive photovoltaic efficiencies of IOSCs.

4. Conclusion

A simple route for formation of various ZnO nanoripples on ZnOthin films was developed. Structure of ZnO surfaces with nanor-ipples can be controlled by various amounts of ZnO naoparticlesadded to zinc acetate solution for spin-coating of ZnO thin films.Photovoltaic performances of the IOSC consisting of these ZnO thinfilms depended on the surface structure of ZnO, i.e. the photo-voltaic performance could be optimized by adjusting the amount ofZnO nanoparticles used for the fabrication of ZnO films. The bestpower conversion efficiency value of the IOSC in the present workwas 3.2%, which was obtained using ZnO nanoripples with a meanheight of 70 nm. The power conversion efficiency of IOSC of ZnOfilms with nanoripples was higher than that of relatively flat ZnOthin films by a factor of 2, indicating that our method can be usefulfor fabricating highly efficient IOSCs.

Acknowledgements

This study was supported by the Korea Institute of MaterialsScience (KIMS) and the New and Renewable Energy of the KoreaInstitute of Energy Technology Evaluation and Planning (KETEP)Grant (No. 20103020010050) funded by the Ministry of theKnowledge Economy, Republic of Korea.

References

[1] J. Peet, A.J. Heeger, G.C. Bazan, ‘‘Plastic’’ solar cells: self-assembly of bulkheterojunction nanomaterials by spontaneous phase separation, Acc. Chem.Res. 2009 (42) (2009) 1700–1708.

[2] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Polymer photovoltaic cells-enhanced efficiencies via a network of internal donor–acceptor heterojunc-tions, Science 270 (1995) 1789–1791.

[3] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.Q. Nguyen, M. Dante, A.J. Heeger,Efficient tandem polymer solar cells fabricated by all-solution processing,Science 317 (2007) 222–225.

[4] F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printingand coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394–412.

[5] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Hybrid nanorod-polymer solar cells,Science 295 (2002) 2425–2427.

[6] J. Alstrup, K. Norrman, M. Jorgensen, F.C. Krebs, Lifetimes of organic photovoltaics:design and synthesis of single oligomer molecules in order to study chemicaldegradation mechanisms, Sol. Energy Mater. Sol. Cells 90 (2006) 2777–2792.

[7] K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D.D.C. Bradley, J.R. Durrant,Degradation of organic solar cells due to air exposure, Sol. Energy Mater. Sol.Cells 90 (2006) 3520–3530.

[8] R. Pacios, A.J. Chatten, K. Kawano, J.R. Durrant, D.D.C. Bradley, J. Nelson,Effects of photo-oxidation on the performance of poly[2-methoxy-5-(30 ,70-dimethyloctyloxy)-1,4-phenylene vinylene]:[6,6]-phenyl C-61-butyric acidmethyl ester solar cells, Adv. Funct. Mater. 16 (2006) 2117–2126.

[9] M. Glatthaar, M. Niggemann, B. Zimmermann, P. Lewer, M. Riede, A. Hinsch,J. Luther, Organic solar cells using inverted layer sequence, Thin Solid Films491 (2005) 298–300.

[10] J. Kim, D.Y. Khang, J.H. Kim, H.H. Lee, The surface engineering of top electrodein inverted polymer bulk-heterojunction solar cells, Appl. Phys. Lett. 92(2008) 133307-1–133307-3.

[11] T. Kuwabara, T. Nakayama, K. Uozumi, T. Yamaguchi, K. Takahashi, Highlydurable inverted-type organic solar cell using amorphous titanium oxide aselectron collection electrode inserted between ITO and organic layer, Sol.Energy Mater. Sol. Cells 92 (2008) 1476–1482.

[12] A.K.K. Kyaw, X.W. Sun, C.Y. Jiang, G.Q. Lo, D.W. Zhao, D.L. Kwong, An invertedorganic solar cell employing a sol–gel derived ZnO electron selective layerand thermal evaporated MoO3 hole selective layer, Appl. Phys. Lett. 93 (2008)221107-1–221107-3.

[13] S.K. Hau, H.L. Yip, N.S. Baek, J.Y. Zou, K. O’Malley, A.K.Y. Jen, Air-stableinverted flexible polymer solar cells using zinc oxide nanoparticles as anelectron selective layer, Appl. Phys. Lett. 92 (2008) 253301-1–253301-3.

[14] N. Sekine, C.H. Chou, W.L. Kwan, Y. Yang, ZnO nano-ridge structure and itsapplication in inverted polymer solar cell, Org. Electron. 10 (2009) 1473–1477.

[15] Y.J. Cheng, C.H. Hsieh, Y.J. He, C.S. Hsu, Y.F. Li, Combination of indene-C60 bis-adduct and cross-linked fullerene interlayer leading to highly efficientinverted polymer solar cells, J. Am. Chem. Soc. 132 (2010) 17381–17383.

[16] K. Norrman, M.V. Madsen, S.A. Gevorgyan, F.C. Krebs, Degradation patterns inwater and oxygen of an inverted polymer solar cell, J. Am. Chem. Soc. 132(2010) 16883–16892.

[17] R. Søndergaard, M. Helgesen, M. Jørgensen, F.C. Krebs, Fabrication of polymersolar cells using aqueous processing for all layers including the metal backelectrode, Adv. Energy Mater. 1 (2011) 68–71.

[18] J. Alstrup, M. Jorgensen, A.J. Medford, F.C. Krebs, Ultra fast and parsimoniousmaterials screening for polymer solar cells using differentially pumped slot-die coating, ACS Appl. Mater. Interfaces 2 (2010) 2819–2827.

[19] F.C. Krebs, T. Tromholt, M. Jorgensen, Upscaling of polymer solar cellfabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886.

[20] F.C. Krebs, S.A. Gevorgyan, J. Alstrup, A roll-to-roll process to flexible polymersolar cells: model studies, manufacture and operational stability studies,J. Mater. Chem. 19 (2009) 5442–5451.

[21] F. Huang, K.S. Chen, H.L. Yip, S.K. Hau, O. Acton, Y. Zhang, J.D. Luo, A.K.Y. Jen,Development of new conjugated polymers with donor-p-bridge-acceptorside chains for high performance solar cells, J. Am. Chem. Soc. 131 (2009)13387–13886.

[22] B. Lim, J. Jo, D. Khim, H.G. Jeong, B.K. Yu, J. Kim, D.Y. Kim, Synthesis of analternating thienylenevinylene-benzothiadiazole copolymer with high holemobility for use in organic solar cells, Org. Electron. 11 (2010) 1772–1778.

[23] A.J. Miller, R.A. Hatton, S.R.P. Silva, Interpenetrating multiwall carbonnanotube electrodes for organic solar cells, Appl. Phys. Lett. 89 (2006)133117-1–133117-3.

[24] S.Y. Park, W.D. Kim, D.G. Kim, J.K. Kim, Y.S. Jeong, J.H. Kim, J.K. Lee, S.H. Kim,J.W. Kang, Effect of hybrid carbon nanotubes-bimetallic composite particles

D.C. Lim et al. / Solar Energy Materials & Solar Cells 95 (2011) 3036–30403040

on the performance of polymer solar cells, Sol. Energy Mater. Sol. Cells 94(2010) 750–754.

[25] P. de Bruyn, D.J.D. Moet, P.W.M. Blom, A facile route to inverted polymersolar cells usin precursor based zinc oxide electron transport layer, Org.Electron. 11 (2010) 1419–1422.

[26] X.W. Sun, D.W. Zhao, L. Ke, A.K.K. Kyaw, G.Q. Lo, D.L. Kwong, Inverted tandemorganic solar cells with a MoO3/Ag/Al/Ca intermediate layer, Appl. Phys. Lett.97 (2009) 053303-1–053303-3.

[27] D.W. Zhao, P. Liu, X.W. Sun, S.T. Tan, L. Ke, A.K.K. Kyaw, An inverted organicsolar cell with an ultrathin Ca electron-transporting layer and MoO3 hole-transporting layer, Appl. Phys. Lett. 95 (2009) 153304-1–153304-3.

[28] D. Cheyns, B.P. Rand, P. Heremans, Organic tandem solar cells with com-plementary absorbing layers and a high open-circuit voltage, Appl. Phys. Lett.97 (2010) 033301-1–033301-3.

[29] Y.G. Wei, C. Xu, S. Xu, C. Li, W.Z. Wu, Z.L. Wang, planar waveguide-nanowireintegrated three-dimensional dye-sensitized solar cells, Nano Lett. 10 (2010)2092–2096.

[30] D.C. Olson, Y.J. Lee, M.S. White, N. Kopidakis, S.E. Shaheen, D.S. Ginley,J.A. Voigt, J.W.P. Hsu, Effect of polymer processing on the performance ofpoly(3-hexylthiophene)/ZnO nanorod photovoltaic devices, J. Phys. Chem. C.111 (2007) 16640–16645.

[31] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455–459.

[32] L.E. Greene, M. Law, B.D. Yuhas, P.D. Yang, ZnO–TiO2 core-shell nanorod/P3HT solar cells, J. Phys. Chem. C 111 (2007) 18451–18456.

[33] B. Pradhan, S.K. Batabyal, A.J. Pal, Vertically aligned ZnO nanowire arrays inRose Bengal-based dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 91(2007) 769–773.

[34] J.S. Huang, C.Y. Chou, M.Y. Liu, K.H. Tsai, W.H. Lin, C.F. Lin, Solution-processedvanadium oxide as an anode interlayer for inverted polymer solar cellshybridized with ZnO nanorods, Org. Electron. 10 (2009) 1060–1065.

[35] K. Takanezawa, K. Hirota, Q.S. Wei, K. Tajima, K. Hashimoto, Efficient chargecollection with ZnO nanorod array in hybrid photovoltaic devices, J. Phys.Chem. C 111 (2007) 7218–7223.

[36] D.C. Olson, S.E. Shaheen, R.T. Collins, D.S. Ginley, The effect of atmosphere andZnO morphology on the performance of hybrid poly(3-hexylthiophene)/ZnOnanofiber photovoltaic devices, J. Phys. Chem. C 111 (2007) 16670–16678.

[37] J.P. Liu, S.S. Wang, Z.Q. Bian, M. Shan, C.H. Huang, Organic/inorganic hybridsolar cells with vertically oriented ZnO nanowires, Appl. Phys. Lett. 94 (2009)173107-1–173107-3.

[38] A.L. Briseno, T.W. Holcombe, A.I. Boukai, E.C. Garnett, S.W. Shelton,J.J.M. Frechet, P.D. Yang, Oligo- and polythiophene/ZnO hybrid nanowiresolar cells, Nano Lett. 10 (2010) 334–340.

[39] F. Xu, M. Dai, Y.N. Lu, L.T. Sun, Hierarchical ZnO nanowire-nanosheetarchitectures for high power conversion efficiency in dye-sensitized solarcells, J. Phys. Chem. C 114 (2010) 2776–2782.

[40] Y.S. Kim, W.P. Tai, S.J. Shu, Effect of preheating temperature on structural andoptical properties of ZnO thin films by sol–gel process, Thin Solid Films 491(2005) 153–160.

[41] J.-C. Wang, W.-T. Weng, M.-Y. Tsai, M.-K. Lee, S.-F. Horng, T.-P. Perng,C.-C. Kei, C.-C. Yu, H.-F. Meng, Highly efficient flexible inverted organic solarcells using atomic layer deposited ZnO as electron selective layer, J. Mater.Chem. 20 (2010) 862–866.

[42] T. Kuwabara, H. Sugiyama, T. Yamaguchi, K. Takahashi, Inverted type bulk-heterojunction organic solar cell using electrodeposited titanium oxide thinfilms as electron collector electrode, Thin Solid Films 517 (2009) 3766–3769.