Electron Transport Layer-Free Inverted Organic Solar CellsFabricated with Highly Transparent Low-Resistance IndiumGallium Zinc Oxide/Ag/Indium Gallium Zinc Oxide MultilayerElectrode

JUN HO KIM,1 SUNG-NAM KWON,2 SEOK-IN NA,2 SUN-KYUNG KIM,3

YOUNG-ZO YOO,4 HYEONG-SEOP IM,1 and TAE-YEON SEONG1,5

1.—Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea.2.—Professional Graduate School of Flexible and Printable Electronics, Chonbuk NationalUniversity, Jeonju-si, Jeollabuk-do 54896, Korea. 3.—Department of Applied Physics, Kyung HeeUniversity, Yongin-si, Gyeonggi-do 17104, Korea. 4.—Duksan Hi-Metal Co. Ltd., Yeonam-dong,Buk-gu, Ulsan 44252, Korea. 5.—e-mail: tyseong@korea.ac.kr

Inverted organic solar cells (OSCs) have been fabricated with conventional Sn-doped indium oxide (ITO) and amorphous indium gallium zinc oxide (a-IGZO)/Ag/a-IGZO (39 nm/19 nm/39 nm) (a-IAI) electrodes and their electrical char-acteristics characterized. The ITO and optimized a-IAI electrodes showed hightransmittance of 96% and 88% at 500 nm, respectively. The carrier concen-tration and sheet resistance of the ITO and a-IAI films were 8.46 9 1020 cm�3

and 7.96 9 1021 cm�3 and 14.18 X/sq and 4.24 X/sq, respectively. Electrontransport layer (ETL)-free OSCs with the a-IAI electrode exhibited powerconversion efficiency (PCE) of 2.66%, similar to that of ZnO ETL-based OSCswith ITO electrode (3.27%). However, the ETL-free OSCs with the a-IAIelectrode showed much higher PCE than the ETL-free OSCs with the ITOelectrode (0.84%). Ultraviolet photoelectron spectroscopy results showed thatthe work function of the a-IAI electrode was 4.15 eV. This improved perfor-mance was attributed to the various roles of the a-IAI electrode, e.g., as aneffective ETL and a hole blocking layer.

Key words: Indium gallium zinc oxide, Ag, multilayer, transparentconducting electrode, organic solar cell

INTRODUCTION

Organic solar cells (OSCs) have attracted a greatdeal of interest because of their various advantagesover conventional Si-based thin-film solar cells,including light weight, flexibility, and simple, cost-effective, solution-based fabrication processes. Bulkheterojunction OSCs fabricated with a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 buty-ric acid methyl ester (PCBM) active layer wereshown to have power conversion efficiency (PCE) of3% to 5%.1–3 To further improve the PCE, new

active materials and buffer layers for increasedexciton formation and hole/electron extraction effi-ciencies4–7 as well as new device structures (e.g.,tandem or inverted OSCs)8,9 have been developed.In particular, to increase the electron and holecollection efficiency, carrier injection and/or trans-port layers can be inserted at the interface betweenactive layers and electrodes. Poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonic acid) (PED-OT:PSS) is a common anode interlayer used inconventional OSCs. However, the acidic nature ofthe PEDOT:PSS layer is known to cause the indiumtin oxide (ITO) electrode to be etched away andindium to diffuse into the organic active layer,degrading the OSC output performance.10 To solve(Received June 23, 2016; accepted November 14, 2016;

published online November 30, 2016)

Journal of ELECTRONIC MATERIALS, Vol. 46, No. 4, 2017

DOI: 10.1007/s11664-016-5147-4� 2016 The Minerals, Metals & Materials Society

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these problems, inverted device structures wereintroduced, where a novel metal with high workfunction (e.g., Ag or Au) is used as a hole-collectingback electrode with ZnO as an electron-collectinglayer (e.g., electron transport layer, ETL).11,12 Itwas also shown that cesium carbonate (Cs2CO3)13

and TiO214 films can be used as ETLs to improve

electron transport in such inverted OSCs. Amongthe components of an OSC, including the metalcathode, organic active layer, buffer layer, andtransparent conducting oxide (TCO) electrode, thesheet resistance of the TCO primarily contributes tothe series resistance.15–17 Furthermore, the trans-mittance of the TCO plays an important role inincreasing the exciton formation efficiency, becauselight arrives at the active layer through the TCOelectrode. Thus, to develop high-quality conductingelectrodes, TCO-only films18–23 and TCO filmssandwiching a thin metal film, i.e., dielectric/metal/dielectric (D/M/D) multilayers,24–31 have beenextensively investigated; For example, Ihn et al.20

investigated the electrical properties of a gallium-doped ZnO (GZO) layer to develop cathodes forinverted P3HT:PCBM bulk heterojunction (BHJ)OSCs. Inverted P3HT:PCBM BHJ OSCs fabricatedwith GZO cathode were shown to have PCE of 3.4%,comparable to that of the same OSCs with conven-tional ITO/ZnO cathode. Sio et al.23 showed thatinverted OSCs with ZnO:Al electrode and MoO3

buffer layer (ETL) exhibited PCE of 3.06%, compa-rable to that of the same OSCs with ITO electrode.Hajj et al.28 reported that ZnO/Ag/ZnO trilayerelectrodes exhibited transmittance of 76.5% at550 nm and sheet resistance of 8 X/sq, similar tovalues for ITO. OSCs fabricated with optimizedZnO/Ag/ZnO (30 nm/14 nm/30 nm) electrode gavefill factor of 57% and PCE of 2.7%. It was reportedthat amorphous indium gallium zinc oxide (a-IGZO)can be used as an efficient ETL in organic devicesbecause a-IGZO has energy levels of lowest occupiedmolecular orbital (LUMO) of �4.2 eV and highestoccupied molecular orbital (HOMO) of �7.5 eV,bandgap of 3.0 eV to 3.2 eV,32 high transmittance(above 80%) in the visible spectrum,33 and amor-phous structure. In addition, a-IGZO has muchhigher electron mobility than TiOx and a stronglylocalized valence band,34 which enables preventionof hole injection. IGZO was shown to be a goodcontact material in liquid-crystal display (LCD)backplanes, enabling reduced masking steps in thefabrication process.35 In previous study,31 we inves-tigated the electrical, optical, and bending charac-teristics of a-IGZO/Ag/IGZO (39 nm/19 nm/39 nm)multilayer films deposited on polyethylene tereph-thalate (PET) substrate at room temperature. At500 nm, the IGZO/Ag/IGZO multilayer film trans-mitted 88.8%, being comparable to ITO film (91.3%).IGZO/Ag/IGZO film showed mobility of 18.17 cm2/V-s and sheet resistance of 5.09 X/sq. IGZO/Ag/IGZOfilm exhibited much higher Haacke’s figure of merit(FOM) of 45.02 9 10�3 X�1 compared with the value

of 1.94 9 10�3 X�1 for ITO film. In this study, wefabricated buffer layer-free OSCs with optimized a-IGZO/Ag/IGZO multilayer films as electrode insteadof ITO, and compared their electrical performancewith that of ITO-based OSCs. Fabrication of bufferlayer-free OSCs is beneficial because it simplifiesthe fabrication process, reduces the fabricationcosts, and obviates the need to anneal theelectrodes.

EXPERIMENTAL PROCEDURES

A radiofrequency (RF) magnetron sputtering sys-tem was used to sequentially deposit a-IGZO/Ag/a-IGZO (a-IAI) multilayer thin films on CorningEAGLE XG glass substrate. A ceramic IGZO target(In2O3:Ga2O3:ZnO = 1:1:1 at.%, 99.99% purity) andpure Ag target (99.99% purity) were used at roomtemperature at base pressure below 1 9 10�6 Torr.Before being loaded into the sputtering chamber,the glass substrate (1.5 cm 9 1.5 cm) was cleanedwith acetone, methanol, and deionized water for15 min per cleaning agent in an ultrasonic bath,and finally dried in a N2 stream. Prior to deposition,both the IGZO and Ag targets were presputtered for30 min to remove contaminants. IGZO and Ag weredeposited using RF power of 90 W and 30 W,respectively. During sputtering, the glass substratewas constantly rotated at speed of 12 rpm for IGZOand 24 rpm for Ag. The thickness of the a-IGZO andAg layers were previously optimized to provide hightransmittance and low resistance.36 Hall measure-ments using the van der Pauw technique werecarried out in magnetic field of 0.55 T (HMS 3000,Ecopia). The four-point probe technique was usedfor measuring sheet resistance. The transmittanceof the multilayers was measured using an ultravi-olet–visible (UV–Vis) spectrometer (UV-1800, Shi-madzu). The crystal structure of the multilayerswas determined by x-ray diffraction (XRD) analysis(ATX-G, Rigaku). Ultraviolet photoelectron spec-troscopy (UPS) data were obtained from a-IAI filmat room temperature using a modified PHI 5000(Ulvac-PHI) system to examine the work function ofthe film. UPS measurements were performed at biasof �9 V using a HeI (21.2 eV) source.

Inverted P3HT:PCBM-based BHJ OSCs were fab-ricated with optimized a-IAI (39 nm/19 nm/39 nm)electrodes and reference ITO (150 nm) films, asschematically illustrated in Fig. 1a and b. Prior toOSC fabrication, a-IAI- and ITO-coated glass sam-ples were cleaned using Mucasol, acetone, iso-propanol, and deionized water. A ZnO (0.75 M) ETLlayer was then spin-coated on the ITO-coated glasssubstrate, then annealed at 150�C for 30 min in air. Ablend solution of P3HT (30 mg, Rieke Metals) andPCBM (15 mg, Nano-C) in chlorobenzene (2 ml) wasspin-coated on the a-IAI and ITO in N2 ambient,followed by annealing at 110�C for 10 min. APEDOT:PSS (Clevios P VPAI 4083) hole transportlayer was then spin-coated on the P3HT:PCBM

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active layer, followed by annealing at 110�C for10 min. All coating processes were carried out simul-taneously in the same glovebox and chamber.Finally, a 100-nm-thick Ag anode (4.64 mm2) wasdeposited on the PEDOT:PSS layer by thermalevaporation. The Ag anode was patterned using ametal shadow mask. The photocurrent density–volt-age (J–V) characteristic of the OSCs fabricated witha-IAI and reference ITO electrodes were measuredusing a Keithley 2400 source measure unit under100 mW/cm2 illumination at AM1.5G conditions.

RESULTS AND DISCUSSION

Figure 2 shows the XRD patterns obtained fromconventional ITO (150 nm thick) and a-IAI (39 nm/19 nm/39 nm) multilayer films deposited on glasssubstrates. The ITO layer shows peaks at2h = 21.4�, 30.5�, 35.4�, 37.6�, 41.8�, 45.6�, 51.0�,55.9�, and 62.1� that correspond to (211), (222),(400), (411), (332), (431), (440), (611), and (622)planes, respectively [Joint Committee on PowderDiffraction Standards (JCPDS) no. 71-2195]. Notethat ITO exhibits very strong (222) peak. This isattributed to restoration of the ITO stoichiometry.37

For the as-deposited IGZO sample, there is a peakat 2h = 38.3�, corresponding to (111) plane of Ag(JCPDS no. 87-0720). The IGZO appears to beamorphous, as shown in the inset (top right).

Figure 3 shows the transmittance spectraobtained from conventional ITO and a-IAI films.Both samples show high transmittance at 500 nm to600 nm, although the ITO exhibits higher trans-mittance; For example, the transmittance of theITO and a-IAI film was 96% and 88% at 500 nm,respectively. To confirm the measured transmit-tance spectra of the a-IAI multilayer film, finite-difference time-domain (FDTD) simulations38 wereperformed. In the FDTD simulation, the thicknessof the Ag and a-IGZO layer was fixed at 19 nm and39 nm, respectively. More details can be foundelsewhere.36 The FDTD results showed that, forboth samples, the transmittance reaches an overallmaximum then gradually decreases with increasingwavelength. Unlike the measured results, however,

the simulated results showed the maximum at�450 nm, which can be attributed to the fact thatthe optical constants of our deposited materialsslightly differ from those of reported materials.

The electrical properties obtained using Hallmeasurements are summarized in Table I. Thecarrier concentration and mobility were measuredto be 8.46 9 1020 cm�3 and 44.97 cm2/V-s for theITO film and 7.96 9 1021 cm�3 and 19.22 cm2/V-sfor the a-IAI film. The sheet resistance and resis-tivity were measured to be 14.18 X/sq and1.63 9 10�4 X cm�1 for the ITO film and 4.24 X/sqand 4.08 9 10�5 X cm�1 for the a-IAI film. Theresistivity is inversely proportional to the mobilityand the carrier concentration.39 The a-IAI filmshowed lower mobility (by a factor of �2) than theITO film, and the a-IAI film exhibited higher carrierconcentration (by one order of magnitude) than theITO film. This implies that the resistivity is dom-inantly affected by the carrier concentration.

The figure of merit (FOM;uTC) of the ITO and a-IAIfilms was calculated using the equation defined by

Fig. 1. Schematic diagrams of inverted P3HT:PCBM-based BHJ OSCs fabricated with (a) ZnO buffer and reference ITO (150 nm) layers, and(b) buffer-free a-IGZO/Ag/IGZO (39 nm/19 nm/39 nm) (a-IAI) electrode.

Fig. 2. XRD patterns obtained from conventional ITO (150 nm thick)and optimized IGZO/Ag/IGZO (39 nm/19 nm/39 nm) multilayer filmsdeposited on glass substrates.

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Haacke,40 uTC = Tav10/Rs, where Rs is the sheet resis-

tance andTav is the average optical transmittance (inthe range of 450 nm to 750 nm). Tav can be estimatedusing the relation Tav ¼

RVðkÞTðkÞ dk=

RVðkÞdk,

where T(k) is the transmittance and V(k) is thephotopic luminous efficiency function defining thestandard observer for photometry.41 The ITO anda-IAI samples gave FOM values of 43 9 10�3 X�1

and 49 9 10�3 X�1. The a-IAI films had higher FOM,which can be attributed to the dominant contributionof the sheet resistance (Table I).

Figure 4 illustrates the surface morphology of thea-IAI (39 nm/19 nm/39 nm) film. The AFM imagereveals a uniform surface without cracks or pits.The surface is smooth with root-mean-square (RMS)roughness of 0.63 nm (10 lm 9 10 lm). For OSCs,the organic layers (active layer, P3HT:PCBM) arespin-coated onto the electrode, so the smoothness ofthe electrode is critical. In other words, a smoothsurface of the underlying electrode is vital foruniform deposition of the organic layers, becauseOSC performance is sensitively affected by theuniformity of the electrode.

Figure 5 shows the current density–voltage (J–V)curves obtained from the inverted OSCs fabricatedwith conventional ITO and a-IAI electrodes. Notethat the buffer-free OSC with the a-IAI electrodeexhibits performance similar to that of the OSCwith the conventional ITO electrode. On the otherhand, the buffer-free OSC with the a-IAI electrode

exhibits better performance than the buffer-freeOSC with the ITO electrode. The short-circuitcurrent density (JSC), open-circuit voltage (VOC), fillfactor (FF), and PCE of the OSCs6 were measuredand are summarized in Table II. The buffer-free

Fig. 3. Transmittance spectra obtained from ITO and optimized a-IAI(39 nm/19 nm/39 nm), and FDTD simulation of a-IAI film.

Table I. Resistivity, carrier concentration, and Hall mobility of conventional ITO and optimized a-IAImultilayer films

ElectrodeCarrier

concentration (cm23)Mobility(cm2/V-s)

Resistivity(X cm)

Sheet resistance(X/sq)

Reference ITO 8.46 9 1020 44.97 1.63 9 10�4 14.18Optimized a-IAI 7.96 9 1021 19.22 4.08 9 10�5 4.24

Fig. 4. AFM image obtained from IGZO/Ag/IGZO (39 nm/19 nm/39 nm) multilayer film.

Fig. 5. J–V curves obtained from inverted OSCs fabricated with ITOand a-IAI electrodes. The inset shows OSCs fabricated with a-IAIelectrodes.

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ITO-based OSCs showed PCE of 0.84%, whereas thebuffer-free a-IAI-based OSCs gave PCE of 2.66%. Itis noteworthy that the buffer-free a-IAI-based OSCsshowed approximately 300% higher PCE than thebuffer-free ITO-based OSCs.

Figure 6 shows schematic diagrams of the energyband structure of inverted OSCs fabricated withITO and a-IAI electrodes. The ITO-based OSCs witha ZnO buffer layer showed much better performancethan the ZnO buffer-free OSCs (Fig. 5). This betterperformance can be explained as follows: First, theimprovement can be related to the fact that the ZnObuffer layer serves not only as an ETL but also as ahole blocking layer,42 because the conduction bandand valence band edges of ZnO (with wide bandgapof �3.4 eV) are about 4.3 eV and 7.7 eV, respec-tively.43 Second, this may be due to the preventionof direct contact between the active and ITO layers,where high densities of carrier traps or unfavorableinterface dipoles may hinder efficient charge collec-tion. Third, it could be associated with the formationof ohmic contacts between the ITO and activelayers, maximizing VOC.44 Furthermore, the a-IAI-based buffer-free OSCs showed better performancethan the ITO-based ZnO buffer-free OSCs (Fig. 5).For the ITO-based buffer-free OSCs, the ITO that is

Table II. Short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) of inverted OSCsfabricated with ITO, buffer-free ITO, and buffer-free a-IAI electrodes

Electrode ETL VOC (V) JSC (mA/cm2) FF (%) PCE (%)

ITO ZnO 0.66 7.84 63.09 3.27ITO No buffer 0.31 7.38 36.23 0.84IAI No buffer 0.65 6.37 64.47 2.66

Fig. 7. Work function (U) of optimized a-IAI (39 nm/19 nm/39 nm)film estimated using UPS.

Fig. 6. Schematic diagrams of energy band structure of invertedOSCs fabricated with ITO and a-IAI electrodes.

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in direct contact with the active layer showsnonohmic behavior because the work function ofITO (�4.8 eV) is much higher than the LUMO ofPCBM (3.7 eV).45 This results in formation of alarge energy barrier at the ITO–active layer inter-face, increasing the series resistance. The workfunction (U) of the optimized a-IAI films wasmeasured using UPS, as shown in Fig. 7. The workfunction (U) associated with the Fermi level (EF) isdetermined by the relation U = (photon energy of21.2 eV) — (binding energy of the secondary cutoffin the UPS spectrum).46 As shown in Fig. 7, the Uvalue of the a-IAI film was estimated to be 4.15 eV,consistent with previous observations by othergroups.47,48 Yao et al.47 showed that the U value ofa-IGZO films varied from 4.0 eV to 5.0 eV, depend-ing on the stoichiometry of the IGZO. As shown inFig. 6, the better performance could therefore alsobe related to the fact that the a-IAI electrode canserve as an effective channel for electron transferand as a hole blocking layer.

CONCLUSIONS

We fabricated inverted organic solar cells (OSCs)using conventional ITO and optimized a-IGZO/Ag/IGZO (a-IAI) electrodes. Both the ITO and a-IAIfilms showed high transmittance at 500 nm. The a-IAI films exhibited higher carrier concentration andlower sheet resistance than the ITO. The bufferlayer-free OSCs fabricated with the a-IAI electrodeshowed much higher PCE than the buffer layer-freeOSCs with the ITO electrode. This better perfor-mance can be explained in terms of the roles of thea-IAI electrode, serving as an ETL and a holeblocking layer. Use of such an integrated ETL/cath-ode a-IAI film simplifies the fabrication process,reduces the fabrication costs, and obviates the needto anneal the electrodes. The results imply thatsuch a-IAI films could be considered as promisingtransparent electrodes for cost-effective fabricationof OSCs using a simple process.

ACKNOWLEDGEMENTS

This work was supported by the Korea EvaluationInstitute of Industrial Technology (Grant No.10049601) and the World Class 300 Project (GrantNo. S2317456) through the Korea Institute forAdvancement of Technology (KIAT), which is fundedby the Small and Medium Business Administration.

REFERENCES

1. C.J. Brabec, A. Cravino, D. Meisser, N.S. Sariciftci, T.Fromherz, M.T. Rispens, L. Sanchez, and J.C. Hummelen,Adv. Funct. Mater. 11, 374 (2001).

2. A.A. Bakulin, A. Rao, V.G. Pavelyev, P.H.M. van Loos-drecht, M.S. Pshenichnikov, D. Niedzialek, J. Cornil, D.Beljonne, and R.H. Friend, Science 332, 1340 (2011).

3. G. Yu, J. Gao, J.C. Hummelen, F. Wudl, and A.J. Heeger,Science 270, 1789 (1995).

4. J.-S. Huang, T. Goh, X. Li, M.Y. Sfeir, E.A. Bielinski, S.Tomasulo, M.L. Lee, N. Hazari, and A.D. Taylor, Nat.Photon. 7, 479 (2013).

5. C.W. Tang, Appl. Phys. Lett. 48, 183 (1986).6. A. Luque and S. Hegedus, Handbook of Photovoltaics Sci-

ence and Engineering (New York: Wiley, 2003).7. G. Li, R. Zhu, and Y. Yang, Nat. Photon. 6, 153 (2012).8. L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T.

Moriarty, K. Emery, G. Li, and Y. Yang, Nat. Photon. 6, 180(2012).

9. Y.M. Nam, J. Huh, and W.H. Jo, Sol. Energy Mater. Sol.Cells 95, 1095 (2011).

10. A.W. Hains and T.J. Marks, Appl. Phys. Lett. 92, 023504(2008).

11. G. Li, C.-W. Chu, V. Shrotriya, J. Huang, and Y. Yang,Appl. Phys. Lett. 88, 253503 (2006).

12. S.K. Hau, H. Yip, N.S. Baek, J. Zou, K. O’Malley, and A.K.-Y. Jen, Appl. Phys. Lett. 92, 253301 (2008).

13. H.H. Liao, L.M. Chen, Z. Xu, G. Li, and Y. Yang, Appl.Phys. Lett. 92, 173303 (2008).

14. C. Tao, S. Ruan, X. Zhang, G. Xie, L. Shen, X. Kong, W. Dong,C. Liu, and W. Chen, Appl. Phys. Lett. 93, 193307 (2008).

15. P. Kuang, J.-M. Park, W. Leung, R.C. Mahadevapuram,K.S. Nalwa, T.-G. Kim, S. Chaudhary, K.-M. Ho, and K.Constant, Adv. Mater. 23, 2469 (2011).

16. V. Kumar and H. Wang, Sol. Energy Mater. Sol. Cells 113,179 (2013).

17. D. Apaydın, D. Yıldız, A. Cirpan, and L. Toppare, Sol.Energy Mater. Sol. Cells 113, 100 (2013).

18. H. Kim, J.S. Horwitz, G. Kushto, S. Qadri, and Z. Kafafi,Appl. Phys. Lett. 78, 1050 (2001).

19. Q. Wan, E.N. Dattoli, and W. Lu, Appl. Phys. Lett. 90,222107 (2007).

20. S.-G. Ihn, K.-S. Shin, M.-J. Jin, X. Bulliard, S. Yun, Y.Choi, Y. Kim, J.-H. Park, M. Sim, M. Kim, K. Cho, T. Kim,D. Choi, J.-Y. Choi, W. Choi, and S.-W. Kim, Sol. EnergyMater. Sol. Cells 95, 1610 (2011).

21. S.X. Zhang, S. Dhar, W. Yu, H. Xu, S. Ogale, and T.Venkatesan, Appl. Phys. Lett. 91, 112113 (2007).

22. O.D. Coskun and S. Demirela, Appl. Surf. Sci. 277, 35 (2013).23. A.D. Sio, K. Chakanga, O. Sergeev, K.V. Maydell, J. Parisi,

and E.V. Hauff, Sol. Energy Mater. Sol. Cells 98, 52 (2012).24. S. Yu, W. Zhang, L. Li, D. Xu, H. Dong, and Y. Jin, Thin

Solid Films 552, 150 (2014).25. A. Dhar and T.L. Alford, J. Appl. Phys. 112, 103113 (2012).26. K.-H. Choi, Y.-Y. Choi, J.-A. Jeong, H.-K. Kim, and S. Jeon,

Electrochem. Solid State Lett. 14, H152 (2011).27. J.-W. Lim, S.-I. Oh, K. Eun, S.-H. Choa, H.-W. Koo, T.-W.

Kim, and H.-K. Kim, Jpn. J. Appl. Phys. 51, 115801 (2012).28. A.E. Hajj, T. Kraft, B. Lucas, M. Bonnans, B. Ratier, and P.

Torchio, J. Appl. Phys. 115, 033103 (2014).29. J.H. Kim, T.-W. Kang, S.-I. Na, Y.-Z. Yoo, and T.-Y. Seong,

Curr. Appl. Phys. 15, 829 (2015).30. J.H. Kim, J.-Y. Na, S.-K. Kim, Y.-Z. Yoo, and T.-Y. Seong,

J. Electron. Mater. 44, 3967 (2015).31. J.H. Kim, D.-S. Kim, S.-K. Kim, Y.-Z. Yoo, J.H. Lee, S.-W.

Kim, and T.-Y. Seong, J. Electron. Mater. 45, 4265 (2016).32. T.-C. Fung, C.-S. Chuang, C. Chen, K. Abe, R. Cottle, M.

Townsend, H. Kumomi, and J. Kanicki, J. Appl. Phys. 106,084511 (2009).

33. K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, andH. Hosono, Nature 432, 488 (2004).

34. T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv.Mater. 11, 044305 (2010).

35. J.-Y. Yang, S. Lee, S.-J. Cho, M.-C. Jun, I.-B. Kang, S. Yeo,and J.-H. Park, SID Symp. Dig. 45, 469 (2014).

36. J.H. Kim, H. Lee, J.-Y. Na, S.-K. Kim, Y.-Z. Yoo, and T.-Y.Seong, Curr. Appl. Phys. 15, 452 (2015).

37. P. Thilakan, C. Minarini, S. Loreti, and E. Terzini, ThinSolid Films 388, 34 (2001).

38. S.-K. Kim, X. Zhang, D.J. Hill, K.-D. Song, J.-S. Park, H.-G. Park, and J. Cahoon, Nano Lett. 15, 753 (2015).

39. H. Han, J.W. Mayer, and T.L. Alford, J. Appl. Phys. 100,083715 (2006).

40. G. Haacke, J. Appl. Phys. 47, 4086 (1976).41. W.G. Driscoll and W. Vaughan, Handbook of Optics (New

York: McGraw-Hill, 1978).

Electron Transport Layer-Free Inverted Organic Solar Cells Fabricated with Highly TransparentLow-Resistance Indium Gallium Zinc Oxide/Ag/Indium Gallium Zinc Oxide Multilayer Electrode

2145

42. M.J. Tan, S. Zhong, J. Li, Z. Chen, and W. Chen, ACS Appl.Mater. Interfaces 5, 4696 (2013).

43. W. Qin, G. Ding, X. Xu, L. Yang, and S. Yin, J. Mater. Sci.Technol. 30, 197 (2014).

44. E. Polydorou, E. Makarona, A. Soultati, D.G. Georgiadou,T. Kyrasta, T. Speliotis, C. Tsamis, N. Papanikolaou, P.Argitis, I. Kostis, A. Kokkosis, D. Davazoglou, and M. Va-silopoulou, Microelectron. Eng. 119, 100 (2014).

45. C. Chu, V. Shrotriya, G. Li, and Y. Yang, Appl. Phys. Lett.88, 153504 (2006).

46. Y. Park, V. Choong, Y. Gao, B. Hseih, and C. Tang, Appl.Phys. Lett. 68, 2699 (1996).

47. J. Yao, L. Gong, L. Xie, and S. Zhang, Thin Solid Films527, 21 (2013).

48. F.-L. Kuo, Y. Li, M. Solomon, J. Du, and N. Shepherd, J.Phys. D Appl. Phys. 45, 065301 (2012).

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