Thin Solid Films 520 (2012) 6803–6806

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Thin Solid Films

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Transparent indium zinc oxide thin films used in photovoltaic cells based onpolymer blends☆

Cristina Besleaga a, L. Ion a, Veta Ghenescu a,b, G. Socol c, A. Radu a, Iulia Arghir a,d,Camelia Florica a,d, S. Antohe a,⁎a University of Bucharest, Faculty of Physics, 405 Atomistilor Street, PO Box MG-11, 077125, Magurele-Ilfov, Romaniab Institute for Space Sciences, 409 Atomistilor Street, PO Box MG-23, 077125, Magurele-Ilfov, Romaniac National Institute for Lasers, Plasma and Radiation Physics, PO Box MG-36, 077125, Magurele-Ilfov, Romaniad National Institute of Materials Physics, 105 bis Atomistilor, PO Box MG.7, 077125 Magurele-Ilfov, Romania

☆ All authors have equal contributions to this work.⁎ Corresponding author.

E-mail address: santohe@solid.fizica.unibuc.ro (S. An

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.07.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 July 2011Received in revised form 12 July 2012Accepted 12 July 2012Available online 20 July 2012

Keywords:Indium zinc oxideOrganic photovoltaicsTransparent conducting oxidePulsed laser deposition

Indium zinc oxide (IZO) thin films were obtained using pulsed laser deposition. The samples were prepared byablation of targets with In concentrations, In/(In+Zn), of 80 at.%, at low substrate temperatures under reactiveatmosphere. IZO films were used as transparent electrodes in polymer-based – poly(3-hexylthiophene) and1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 1:1 blend – photovoltaic cells. The action spectra measure-ments revealed that IZO-based photovoltaic structures have performances comparable with those using indi-um–tin–oxide as transparent electrode.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Indium zinc oxide (IZO) thin films are being increasingly used astransparent conductive oxides (TCO) in electronic or optoelectronicdevices, due to the well suited physical properties of this material[1–3]. Indium oxide (In2O3) films dopedwith zinc (Zn) exhibit a betterstability compared to tin doped indium oxide (ITO) films [4,5]. IZOfilms are transparent tomost of the solar spectrumused in photovolta-ic solar cells, and their sheet resistances are comparable to those of ITO.Due to the small difference between the covalent bond length of In\O(1.87 Å) and Zn\O (1.97 Å), only an unimportant deformation of thezinc oxide (ZnO) lattice occurs, even at high concentrations of In. Thisallows for a better control of the films' conductivity by proper doping[6].

In addition, polymer–fullerene bulk heterojunction solar cellshave shown promising perspectives [7–11]. Bulk heterojunction pho-tovoltaic structures based on poly(3-hexylthiophene) (P3HT) and1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) (1:1) blendwere intensively studied in the past decade, as one of themost promisingroute to low cost photovoltaicswith reasonably good conversion efficien-cy (about 4–5%), processed both on rigid and flexible substrates [12].

In this paperwe report on the fabrication and characterization of IZOthin films by pulsed laser deposition (PLD) and also on their ability to

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work as transparent electrodes in polymer-based bulk heterojunctionphotovoltaic cells. IZO samples show a large free carrier density, whiletheir mobility remains reasonably high.

2. Experimental procedures

2.1. TCO thin film synthesis

50 nm IZO thin filmswere grown in a typical PLD setup, using targetswith an In/(In+Zn) atomic ratio of 80%. A planetary ballmill was used toobtain a homogeneous mixing of the In2O3 (Aldrich, 99.9% purity) andZnO (Aldrich, 99.99% purity) powders. In order to obtain compact pellets,the ground powder was pressed at 5 MPa and sintered for 12 h at1100 °C in air. The targets were ablated using a KrF* laser source (λ=248 nm, 25 ns full width at half-maximum of the pulse) in an oxygenatmosphere of 1 Pa. The laser fluency onto the target surface and therepetition rate of the laser pulses were 5 J/cm2 and 10 Hz, respectively.The distance between the substrates and the targets was set at 5 cm.Prior to introduction in the deposition chamber the glass substrateswere successively cleaned in an ultrasonic bath of acetone, ethanol, anddeionizedwater for 15 min, and then blow-driedwith high-purity nitro-gen. All IZO depositions were performed at room temperature.

2.2. Photovoltaic cell preparation

IZOfilmswereused inpolymer-basedphotovoltaic cells in superstrateconfiguration. The P3HT:PCBM layers were prepared on top of IZO films

Fig. 1. Schematic diagram of our devices' structure.

Fig. 3. Typical AFM image (a) and EDXmapping (b) for an IZO thin film deposited by PLD.

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using commercially available raw materials (Aldrich, 99.9%) by spin-coating in two steps:

a) 70 s at an angular velocity of 1500 rot/s and an acceleration of1000 rot/s2;

b) 20 s at an angular velocity of 2000 rot/s and an acceleration of1000 rot/s2.

A 2 nm lithium fluoride (LiF) thin film was then deposited onto theactive layer of some of the structures by single source thermal vacuumevaporation (the source temperature was maintained at 450 °C, theresidual pressure in the deposition chamber was 0.021 Pa, and thesubstrate was kept at room temperature). LiF is commonly used inorganic photovoltaics as an interfacial layer between the active layerand the cathode to enhance electron injection and to reduce the contactresistance [13,14]. The multilayer thin film solar cells were completedby adding an aluminum top contact.

Two photovoltaic structures, IZO/PEDOT/P3HT:PCBM (1:1)/Al (iden-tified as S1; PEDOT stands for poly(3-4-ethylene dioxythiophene)–poly(styrene sulfonate)) and IZO/PEDOT/P3HT:PCBM (1:1)/LiF/Al(identified as S2) differing in the presence of a LiF buffer layerwere tested, for comparative purposes. Their schematic diagramsare presented in Fig. 1. The PEDOT (Aldrich, 99.9%) layer is used to fa-cilitate the hole transfer between the active layer and the transparentelectrode; its work function (5.2 eV, [15]) is close to the work functionof IZO (5.14 eV, [16]), and the HOMO levels of the P3HT organic semi-conductor. Thus, by using IZO films instead of ITO (with a work functionof 4.2–4.7 eV, [17]) the contact resistance at the transparent electrode isreduced, and this has a positive impact on the hole transport in our or-ganic photovoltaic cells.

2.3. Characterization methods

The crystallographic structure of the IZO sample was analyzed byX-ray diffraction (XRD), in symmetric geometry (θ–θ), on a BrukerD8 Discover diffractometer in parallel beam setting, with monochro-matic Cu Kα1 radiation (λ=1.5406 Å). The scattered intensity wasscanned in the range of 16–75° (2θ), with a step size of 0.08°.

The morphological features of the IZO films were examined withan A100 atomic force microscope (AFM) working in the non-contact

Fig. 2. XRD pattern of the IZO thin film deposited by PLD.

mode in air at a 325 kHz resonance frequency, and equipped with acommercial silicon cantilever. AFM micrographs were recorded fromdifferent regions of the samples, using sampling areas of 5 μm×5 μm.

Chemical composition analysis was performed by energy-dispersiveX-ray spectroscopy (EDX, FEI Inspect S) with a SiLi detector in order totest the uniformity of the elements' distribution inside the IZO thinfilms. The EDX spectra were recorded at an energy of 7 keV using acollection time of 593 s per measurement in order to have a suitablevisual representation of the cation species distribution. No conductivecoating was applied.

The electrical properties were investigated by using a Keithley 2400source meter and a Keithley 6517 electrometer at room temperature,contacting the probes with soft Ag wire in the four-point geometry.Absorption spectra were recorded at room temperature using a UV–Vis

Fig. 4. Transmission spectra of the IZO thin film (black squares) and of the glass substrate(gray circles). Near-threshold absorption spectrum is shown in the inset.

Table 1Electrical resistivity, charge concentration and mobility at room temperature of theIZO-PLD thin film deposited at room temperature.

Sample Thickness (nm) ρ (Ω·cm) n (cm−3) Mobility (cm2/V s)

IZO 50 4.8×10−4 5.1×1020 25.5ITO [22] 60 4.1×10−4 5.5×1020 27

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Perkin-Elmer Lambda 35 spectrophotometer. The action spectra of theexternal quantum efficiency (EQE) of the photovoltaic cells were mea-sured with a setup consisting of a Cornerstone130 monochromatorand a Keithley 2400 source meter, controlled by a computer.

Fig. 5. Photocurrent normalized by the power of the light source (black square), andthe corresponding absorption spectrum of the P3HT:PCBM (1:1) blend (gray line) forsample S1 (a) and sample S2 (b).

3. Results and discussion

The XRD pattern of the investigated IZO films (Fig. 2) does not showsharp peaks. IZO samples deposited by PLD in the above mentionedconditions are amorphous [18]. The broad feature at 20°–35° is due to asuperposition of broadened diffraction peaks, corresponding to amixtureof highly disordered ZnO+In2O3 nanocrystalline phases, (PDF2 36-1451,PDF2 06-0416).

AFM investigations have shown that the IZO films are smooth, with avalue of surface roughness as small as ~1 nm (Fig. 3a). Qualitatively, themulti-point EDX scanning (Fig. 3b) of IZO samples pointed out a uniformdistribution of In and Zn cations over the entire film, in relation to a goodchemical homogeneity of the deposit, which presented an average In/(In+Zn) atomic ratio of 57.0%.

The analyzed IZO thin films have an average transmittance ofabout 80% in the visible range, as demonstrated by the transmissionspectra displayed in Fig. 4. The bandgap value EgIZO=2.71 eV wasdetermined from the near-threshold absorption spectra, shown inthe inset in Fig. 4, in the usual form, corresponding to direct bandsemiconductors: α=A(ℏω−Eg)1/2/ℏω.

The electrical mobility and carrier concentration values for IZO thinfilms are comparable to those recorded in the corresponding crystallineform of this material [19,20]. The Hall mobility of IZO films deposited atroom temperature (Table 1) fits within the typical values reported forITO thin films [21,22].

Taking into account their optical and electrical properties (Fig. 4 andTable 1) we can state that IZO thin films can be used as TCO structuresin optoelectronic devices. To test this further, the polymer photovoltaiccells we have fabricated were characterized and the results aresummarized in Table 2. The action spectra of EQE of S1 and S2 cells,together with the absorption spectrum of the P3HT:PCBM (1:1) blendlayer, under illumination through the IZO electrode, are shown in Fig. 5.The spectral response of these structures was better than the resultsreported in the case of ITO/P3HT:PCBM-based cells prepared in similarconditions [23]. Also, one can observe that for the S2 structure (Fig. 5b)the quantum efficiency is significantly larger due to the presence of theLiF layer (2 nm), which improved the charge collection at the backelectrode [24]. Moreover, the action spectra of the blend structure showa symbatic response, suggesting that the photo-carriers are generated inthe entire volume of the P3HT:PCBM (1:1) blend, due to the appropriatevalue of the blend film's thickness.

The fourth quadrant of the I–V characteristics of cells S1 and S2,respectively, measured at illumination under AM 1.5, is shown in Fig. 6,

Table 2The external quantum efficiency (EQE) values of the S1 and S2 structures. The typicalparameters in photo-element regime: fill factor (FF), short circuit current density(JSC), and open circuit voltage (VOC) of the S1 and S2 structures.

Sample EQE (%) FF (%) JSC (mA/cm2) VOC (V)

S1 25 26 3 0.18S2 40 25 3 0.18

and the main photovoltaic parameters are shown in Table 2. For thesesamples, the open circuit voltage was 0.18 V, whereas the short circuitphotocurrent was 1.2 mA, with a fill factor of 26%, and 25%, respectively.All these parameters are comparable to the corresponding valuesobtained in the case of the ITO-based cells [25–27].Moreover, our prelim-inary studies have shown that the blend structures in which the ITO isreplaced with IZO as TCO were more stable in time.

4. Conclusions

Our results show that IZO films can be used as transparent electrodes,replacing ITO, in photovoltaic cells based on polymeric blends. Thephotovoltaic devices fabricated on transparent IZO cathodes exhibitedpromising EQE, VOC, JSC, and FF values of 40%, 0.18 V, 3 mA/cm2, and25%, respectively.

The versatility of ZnO to a doping process with the conservation of itshigh transparency and electronic mobility, even if the deposition isperformed by keeping the substrates at room temperature, opens upthe perspective of preparing efficient and low cost polymer-based photo-voltaic cells, on both rigid or flexible substrates.

Acknowledgments

C.B. is grateful for financial support from the European Social Fundthrough the POSDRU107/1.5/S/80765 project. G.S. acknowledges the fi-nancial support from the Romanian National Authority for ScientificResearch through the TE98 84/2010 project.

Fig. 6. The fourth quadrant I–V characteristic of S1 [IZO/PEDOT/P3HT:PCBM (1:1)/Al]device (a) and S2 [IZO/PEDOT/P3HT:PCBM (1:1)/LiF/Al] device (b), respectively.

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References

[1] K.K. Banger, Y. Yamashita, K. Mori, R.L. Peterson, T. Leedham, J. Rickard, H. Sirringhaus,Nat. Mater. 10 (2011) 45.

[2] B.J. Lee, H.J. Kim, W. Jeong, J.-J. Kim, Sol. Energy Mater. Sol. Cells 94 (2010) 542.[3] Q. Yao, S. Li, Q. Zhang, Semicond. Sci. Technol. 26 (2011) 085011.[4] H.-M. Kim, S.-K. Jung, J.-S. Ahn, Y.-J. Kang, K.-C. Je, Jpn. J. Appl. Phys. 42 (2003)

223.[5] D.-S. Liu, C.-S. Sheu, C.-T. Lee, C.-H. Lin, Thin Solid Films 516 (2008) 3196.[6] C.E. Benouis, M. Benhaliliba, A. Sanchez Juarez, M.S. Aida, F. Chami, F.

Yakuphanoglu, J. Alloys Compd. 490 (2010) 62.[7] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 85.[8] D. Chirvase, J. Parisi, J.C. Hummelen, V. Dyakonov, Nanotechnology 15 (2004)

1317.[9] G. Li, V. Shrotriya, Y. Yao, Y.J. Yang, J. Appl. Phys. 98 (2005) 043704.

[10] P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele, C.J. Brabec, Chem. Mater. 17(2005) 2175.

[11] Y. Kim, S.A. Choulis, J. Nelson, D.D.C. Bradley, S. Cook, J.R. Durrant, Appl. Phys. Lett.86 (2005) 063502.

[12] L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R.H. Friend, J.D.MacKenzie, Science 293 (2001) 1119.

[13] Y.Q. Li, M.K. Fung, Z.Y. Xie, S.T. Lee, L.S. Hung, J.M. Shi, Adv. Mater. 14 (2002) 1317.[14] D. Grozea, A. Turak, X.D. Feng, Z.H. Lu, D. Johnson, R. Wood, Appl. Phys. Lett. 81

(2002) 3173.[15] M. Girtan, M. Rusu, Sol. Energy Mater. Sol. Cells 94 (2010) 446.[16] J.-A. Jeong, K.-H. Choi, J.-H. Bae, J.-M. Moon, S.W. Jeong, I. Kim, H.-K. Kim, M.-S. Yi,

J. Electroceram. 23 (2009) 361.[17] R. Schlafa, H. Murata, Z.H. Kafafi, J. Electron. Spectrosc. Relat. Phenom. 120 (2001)

149.[18] G. Socol, D. Craciun, I.N.Mihailescu, N. Stefan, C. Besleaga, L. Ion, S. Antohe, K.W. Kim, D.

Norton, S.J. Pearton, A.C. Galca, V. Craciun, Thin Solid Films 520 (2011) 1274.[19] G. Gonçalves, E. Elangovan, P. Barquinha, L. Pereira, R. Martins, E. Fortunato, Thin

Solid Films 515 (2007) 8562.[20] D.-H. Shin, Y.-H. Kim, J.-W. Han, K.-M. Moon, R.-I. Murakami, Trans. Nonferrous

Met. Soc. China 19 (2009) 997.[21] K. Ellmer, R. Mientus, Thin Solid Films 516 (2008) 5829.[22] V. Craciun, D. Craciun, X. Wang, T.J. Andreson, R.K. Singh, J. Optoelectron. Adv.

Mater. 5 (2003) 401.[23] L. Magherusan, P. Skraba, C. Besleaga, S. Iftimie, N. Dina, M. Bulgariu, C.G. Bostan,

C. Tazlaoanu, A. Radu, L. Ion, M. Radu, A. Tanase, G. Bratina, S. Antohe, J.Optoelectron. Adv. Mater. 12 (2010) 212.

[24] M. Shin, H. Kim, Y. Kim, Mater. Sci. Eng. B-Adv. Funct. Solid-State Mater. 176 (2011)382.

[25] T. Hu, F. Zhang, Z. Xu, S. Zhao, X. Yue, G. Yuan, Synth. Met. 159 (2009) 754.[26] W.-H. Baek, H. Yang, T.-S. Yoon, C.J. Kang, H.H. Lee, Y.-S. Kim, Sol. Energy Mater.

Sol. Cells 93 (2009) 1263.[27] E.W. Okraku, M.C. Gupta, K.D. Wright, Sol. Energy Mater. Sol. Cells 94 (2010) 2013.