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2 Solar Energy Materials & Solar Cells 19 (13) Contents lists available at SciVerse ScienceDirect Solar Energy Materials & Solar Cells journal homepage: Transparent and flexible PEDOT:PSS electrodes passivated by thin IZTO film using plasma-damage free linear facing target sputtering for flexible organic solar cells Ju-Hyun Lee a, Hyun-Su Shin a, Seok-In Na b, Han-Ki Kim a,n a Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, 1 Seocheon-dong, Yongin-si, Gyeonggi-do 6-71, Republic of Korea b Graduate School of Flexible and Printable Electronics, Chonbuk National University, 66-1 Deokjin-dong, Deokjin-ku, Jeonju-si, Jeollabuk-do , Republic of Korea article info Article history: Received 8 June 1 Received in revised form 13 October 1 Accepted 16 October 1 Keywords: In Zn Sn O (IZTO) PEDOT:PSS LFTS Flexibility Flexible organic solar cells abstract We have investigated the characteristics of transparent and flexible PEDOT:PSS electrodes passivated by thin InZnSnO (IZTO) film for flexible organic solar cells (FOSCs). Using damage-free linear facing target sputtering (LFTS), an IZTO passivation layer was deposited on the gravure printed PEDOT:PSS electrode without causing plasma damage. The thickness of the IZTO passivation layer was found to critically affect electrical and optical properties of the IZTO/PEDOT multilayer because the continuity and morphology of the thin IZTO passivation layer were greatly influenced by thickness. At optimized IZTO passivation thickness of nm, the IZTO/PEDOT:PSS multilayer electrode exhibited decreased sheet resistance of O/sq and optical transmittance of 83.9% without flexibility degradation. The FOSC with an optimized IZTO/PEDOT:PSS electrode showed better performances than the FOSC with PEDOT:PSS electrodes due to the effect of a stable IZTO passivation layer. This indicates that IZTO passivation on the PEDOT:PSS electrode using a LFTS is one of the key solutions for improving the properties and stability of flexible PEDOT:PSS electrodes for high performance FOSCs. & 1 Elsevier B.V. All rights reserved. 1. Introduction Extensive research on active organic materials and polymer solar cell structures has led to power conversion efficiencies (PCE) of 9% for flexible organic solar cells (FOSCs), which could accelerate FOSC mass production [1]. FOSCs have several advantages, including simple structure, light weight, flexibility, simple roll-to-roll-based fabrication and mass production, and large area. Because of these advantages, FOSCs have been considered for next-generation cost-efficient photovoltaics [ 5]. Flexible and transparent electrodes have been considered as key FOSC components, because they critically influence the performance and reliability of FOSCs. These electrodes also have the highest fabrication and materials cost of all FOSCs components. In particular, Sn-doped In O 3 (ITO) films, which are widely used in OSCs or FOSCs as anode electrodes, increased the cost of FOSCs, due to the high cost of indium elements and the high energy consumption of a vacuum-based coating process, as reported by Krebs et al. [6 9]. Furthermore, sputtered ITO electrodes in FOSCs have a negative impact on the energy payback time of FOSC since the vacuum sputtering process consumed fairly high energy n Corresponding author. Fax: þ address: imdlhkkim@khu.ac.kr (H.-K. Kim). [1,11]. Although ITO films have been employed as the main anode in OSCs due to the low resistance and high transparency, ITO films easily form and propagate cracks, when they are bent or curved. This is a critical drawback that prevents the use of ITO for flexible and transparent electrodes in FOSCs [1]. Several ITO-free anodes such as poly(3,-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), carbon nanotube, graphene, Ag grids, and Ag nanowire network, have been investigated to replace ITO anodes in FOSCs [13 17]. In particular, conductive PEDOT:PSS films have been employed in FOSCs as flexible and transparent electrodes due to their superior flexibility and roll-to-roll printing process. Na et al. reported that polymer solar cells with a highly conductive PEDOT:PSS anode showed similar performances to polymer solar cells with an ITO anode [13]. Kim et al. reported on a highly conductive PEDOT:PSS electrode for organic solar cells with optimized solvent and thermal post-annealing [18]. In our previous work, we reported that the gravure-printed PEDOT:PSS electrode possessed good flexibility for FOSCs [19]. Furthermore, Chang et al. reported on polymer solar cells with a conductive PEDOT anode prepared by in-situ EDOT polymerization []. Although printable PEDOT:PSS electrodes have been used as flexible anodes in FOSCs due to their excellent flexibility and solution-based printing process, they are unstable under humidity or ambient atmosphere. Jørgensen et al. reported that the PED- OT:PSS degradation is attributed to hydroscopic properties of the 97-8/$ - see front matter & 1 Elsevier B.V. All rights reserved.

3 J.-H. Lee et al. / Solar Energy Materials & Solar Cells 19 (13) PEDOT:PSS layer, which takes up water from the atmosphere and increased sheet resistance [11,1,]. Therefore, PEDOT:PSS electrodes must be protected from humidity and ambient atmosphere using a transparent inorganic passivation layer. The conventional sputtering process used to deposit a transparent inorganic passivation layer on the PEDOT:PSS electrode caused degradation of the PEDOT:PSS layer because energetic particles from high density plasma can break the chemical bonds of the PEDOT:PSS layer. Therefore, the development of inorganic layerpassivated flexible PEDOT:PSS electrodes by plasma-damage free sputtering technique is imperative to realize reliable PEDOT:PSS electrodes for FOSCs. In this work, we investigated the characteristics of flexible PEDOT:PSS electrodes passivated by a thin IZTO layer using damage free sputtering to improve the stability and reduce the sheet resistance of PEDOT:PSS films. Using specially designed linear facing target sputtering (LFTS), a thin IZTO passivation layer can be deposited on the PEDOT:PSS electrode without electrical or optical degradation, even though the electrode was exposed to high density plasma. Due to effective plasma confinement and substrate location the transparent IZTO passivation layer can be deposited on the PEDOT:PSS electrode. By optimizing the thickness of the IZTO passivation layer, we obtained a highly flexible IZTO/PEDOT:PSS anode with sheet resistance of O/sq and optical transmittance of 83.1%. We also compared the performance of FOSCs containing a single PEDOT:PSS anode with those containing a IZTO/PEDOT:PSS anode to verify the usefulness of a transparent IZTO passivation layer on the PEDOT:PSS electrode.. Experimental A 1 nm thick PEDOT:PSS electrode was printed on flexible PET substrate with a width of 3 mm and thickness of 175 mm using the commercial gravure printing system with a roll width of 5 mm using an aqueous solution [17]. The conductive PEDOT:PSS electrodes were printed by adding 7% dimethlysulfoxide (DMSO) to a recently developed PEDOT:PSS formulation (Clevios PH 51). The printing roll picks up PEDOT:PSS solution ink from the feed reservoir. The PEDOT:PSS ink is then transferred to the surface of the flexible PET substrate. Finally, transferred PEDOT:PSS film is dried at 13 1C. After printing PEDOT:PSS electrodes, the conductive IZTO passivation layer was sputtered on PEDOT:PSS films using a specially designed LFTS system equipped with rotatable facing targets. In our previous works [3 6], we reported the plasma damage free deposition of LFTS for OSCs. Fig. 1 shows a schematics diagram of the LFTS system used to deposit conductive IZTO passivation layers on the PED- OT:PSS electrode without plasma damage. Facing guns consisted of ladder type magnet arrays that possessed a uniform and strong magnet field between targets. The IZTO passivation layer was sputtered on PEDOT:PSS films using commercial ITO (1 wt% SnO -doped In O 3 ) and IZO (1 wt% ZnO-doped In O 3 ) targets at a constant Ar/O flow ratio of /.6 sccm, and a working pressure of. mtorr as a function of the IZTO thickness. Constant DC powers of 3 W and 3 W were simultaneously applied to each ITO and IZO target, respectively. By controlling the sputtering time, the thickness of the IZTO passivation layer could be adjusted. ITO and IZO targets were parallel and faced each other at a target-to-target distance of 6.5 mm, as shown in Fig. 1. The target to substrate distance was maintained at 5 mm during IZTO sputtering. The thicknesses of IZTO passivation layers were measured using a surface profiler. The electrical properties of the IZTO/PEDOT:PSS multilayer were measured by Hall measurements (HL55PC, Accent Optical Technology) at room temperature as a function of the IZTO thickness from 5 to nm. Linear Facing Target Sputtering Fig. 1. Schematics of linear facing target sputtering system with rotating facing guns equipped with ITO and IZO hetero targets. Because high density plasma is effectively confined between ITO and IZO targets, the IZTO film can be sputtered on the PEDOT:PSS electrode without plasma damage. The optical transmittance of the IZTO/PEDOT:PSS multilayer was measured using a UV/visible spectrometer with increasing the thickness of the IZTO passivation layer. X-ray diffraction (XRD) examination was employed to investigate the structure of the IZTO passivation layer. In addition, the surface morphology of IZTO/PEDOT:PSS films was investigated by atomic force microscopy (AFM: PUCOStation STD). The mechanical properties of the single PEDOT:PSS and IZTO/PEDOT:PSS films were investigated by a lab-made inner and outer bending test system. Furthermore, the dynamic fatigue bending of the single PEDOT:PSS and IZTO/ PEDOT:PSS films was examined by repeating the outer bending mode. The bending radius and frequency were approximately 1 mm and 1 Hz, respectively. To investigate the effect of transparent IZTO passivation layers on the performance of FOSCs, conventional bulk-heterojunction FOSCs were fabricated on single PEDOT:PSS and IZTO/PEDOT:PSS electrodes, respectively. Poly(3,-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS, Clevios PH51) was spincoated on the single PEDOT:PSS and IZTO/PEDOT:PSS electrodes and subsequently annealed at 1 1C for 1 min in air. A blend solution containing 5 mg of poly(3-hexylthiophene) (P3HT, Rieke Metal) and 5 mg of 1-(3-methoxycarbonyl)-propyl-1- phenyl-(6,6)c 61 (PCBM, Nano-C) in 1 ml of 1,-dichlorobenzene was then coated onto the PEDOT:PSS buffer layers in nitrogen ambient. A solvent-annealing treatment was then performed by keeping the active films inside a covered glass jar for h and then annealing further at 11 1C for 1 min to form an active layer with a thickness of 3 nm. The fabrication of a device with an island-type electrode design was completed by thermal evaporation of a Ca/Al ( nm/1 nm) metal top electrode using a metal mask with an area of.66 mm under vacuum at a pressure of 1 6 Torr. To compare the performance of FOSCs with different transparent electrodes, all layers on the PEDOT:PSS and IZTO/ PEDOT:PSS electrodes were simultaneously coated at the same conditions. Photocurrent density-voltage (J V) curves were measured using a Keithley source measurement unit. Cell performance was measured under illumination intensity of 1 mw/cm, which was generated using a 1 kw Oriel solar simulator with an AM 1.5 G filter in a N -filled glove box. To ensure accuracy, the light intensity was calibrated using a radiant power meter and a reference silicon solar cell certified by NREL (PVM188 with a KG5 color-filtered window).

4 19 J.-H. Lee et al. / Solar Energy Materials & Solar Cells 19 (13) Results and discussion Fig. shows the Hall measurement results of the conductive IZTO passivated PEDOT:PSS electrode as a function of IZTO thickness. Without the conductive IZTO passivation layer, single PEDOT:PSS showed a fairly high sheet resistance of 51 O/sq due to the thin thickness (1 nm) as depicted in Fig. (a). Even though deposition of the conductive IZTO layer, the IZTO/PED- OT:PSS electrode showed slightly increased sheet resistance below the IZTO thickness of nm. However, sheet resistance of the conductive IZTO passivated PEDOT:PSS electrode linearly decreased to 16.3 O/sq at IZTO thickness of nm. Fig. (c) shows the mobility and carrier concentration of IZTO/PEDOT:PSS samples as a function of IZTO thickness. As expected from Fig. (b), the low sheet resistance of PEDOT:PSS assivated by Sheet resistance [Ohm/square] Mobility [cm /V-s] Sheetresiistance resistivity IZTO thickness [nm] Mobility Concentration IZTO thickness [nm] Fig.. (a) In-situ measured sheet resistance of the PEDOT:PSS electrode without IZTO passivation layer, (b) sheet resistance, resistivity, and (c) mobility, and carrier concentration of the IZTO/PEDO:PSS electrode as a function of IZTO thickness. Resistivity [Ohm-cm] Carrier concentration [/cm 3 ], 3, and nm thick IZTO layers could be attributed to a higher carrier mobility than that of the single PEDOT:PSS. IZTO films less than 1 nm thick existed as a type of island on the PEDOT:PSS electrode while the IZTO films more than nm thick existed as a type of completely covered layer on the PEDOT:PSS electrode. Therefore, differences in sheet resistance of the IZTO/PEDOT:PSS sample with different IZTO thicknesses could be attributed the continuity of the IZTO film on PEDOT:PSS films. In addition, the IZTO passivated PEDOT:PSS electrode did not show degradation of its electrical properties. In general, sputtering of an inorganic layer on the organic layer results in significant degradation of electrical properties due to broken chemical bonds caused by bombardment of energetic particles coming from the high density plasma region [7,8]. During the IZTO sputtering, most energetic particles such as electrons and charged ions, spiral with the helix pitch along the direction of the high density magnetic field. The high density magnetic field is generated perpendicular to both the ITO and IZO target planes and oscillates between the ITO and IZO targets due to their negative voltage [3 6]. Oscillating electrons and charged ions can be effectively confined by the high density magnetic field applied perpendicular to both the ITO and IZO target planes, as shown in Fig. 1. Therefore, the effective confinement of energetic particles between the IZO and ITO targets makes it possible to deposit a conductive IZTO passivation layer on the PEDOT:PSS electrode without plasma-damage related electrical degradation. Fig. 3(a) shows optical transmittance of IZTO/PEDOT:PSS electrode as a function of IZTO thickness. The single PEDOT:PSS electrode shows a fairly high transmittance of 8% at a wavelength of 55 nm. The 5 and 1 nm thick IZTO passivated samples showed optical transmittance similar to that of the single PED- OT:PSS sample. However, further increases in the conductive IZTO layer on the PEDOT:PSS electrode led to a decrease in optical transmittance. This was particularly apparent in the wavelength region between and 7 nm. The upper pictures show the transparency of IZTO/PEDOT:PSS/PET samples as a function of IZTO thickness. Compared to the IZTO (5 nm)/pedot:pss/pet samples, the IZTO(3 and nm)/pedot:pss/pet samples showed lower transparency. Based on sheet resistance (R sh ) and optical transmittance (T), the figure of merit value (T 1 /R sh ) was calculated for the IZTO/PEDOT:PSS electrode as a function of IZTO thickness, as shown in Fig. 3(c) [9]. Considering the high sheet resistance of very thin IZTO passivated PEDOT:PSS electrodes and the lower transmittance of thicker IZTO passivated PEDOT:PSS, the nm thick IZTO passivated PEDOT:PSS electrode was considered an optimized sample. Fig. shows the XRD plots obtained from the transparent IZTO passivated PEDOT:PSS electrode as a function of IZTO thickness. All XRD plots of the flexible IZTO/PEDOT:PSS electrodes show only an intense PET substrate peak at the region of Due to low substrate temperature during the LFTS sputtering process, all IZTO layers on the PEDOT:PSS electrodes show a complete amorphous structure regardless of IZTO thickness. This indicates that substrate temperature was effectively maintained below 5 o C without an intentional cooling system in LFTS. We have previously reported that IZTO films grown by the LFTS technique showed a complete amorphous structure due to effectively confined plasma and substrate geometry located out of high density plasma, unlike conventional DC sputtering [3]. Fig. 5(a) shows AFM surface images of the transparent IZTO passivated PEDOT:PSS electrode as a function of IZTO thickness. The surface morphology of the IZTO/PEDOT:PSS electrode changed with increasing IZTO thickness. At an IZTO thickness of 5 and 1 nm, shown in Fig. 5(a), disconnected IZTO islands existed on the PEDOT:PSS electrodes, as confirmed by surface AFM images. The IZTO islands began to merge at a thickness of

5 J.-H. Lee et al. / Solar Energy Materials & Solar Cells 19 (13) nm 1nm nm 3nm nm Transmittance [%] Transmittance [%] PEDOT 5 nm 1 nm nm 3 nm nm Wavelength [nm] 1.x1-3 8.x1-6.x1 -.x1 -.x1 - Figure of merit [ohm -1 ] RMS roughness 5nm IZTO 3nm PEDOT:PSS Island growth 1nm IZTO nm Film growth nm 5nm 1nm nm 3nm nm IZTO thickness [nm] PEDOT:PSS nm Fig. 5. (a) Surface AFM images of IZTO/PEDO:PSS electrodes with increasing IZTO thickness. (b) RMS roughness of IZTO/PEDO:PSS electrodes as a function of IZTO thickness. Inset shows the IZTO growth mechanism at different growth regions. Non 5nm 1nm nm 3nm nm IZTO thickness [nm] Fig. 3. (a) Optical transmittance of IZTO/PEDO:PSS electrodes as a function of IZTO thickness. Upper panel showing the transparency of IZTO/PEDO:PSS electrodes. (b) Figure of merit of IZTO/PEDO:PSS electrodes calculated from sheet resistance (R sh ) and optical transmittance (T) as a function of IZTO thickness. Intensity [a.u.] PET. nm 3nm nm 1nm 5nm θ [Degree] Fig.. XRD plots of IZTO/PEDO:PSS electrodes as a function of IZTO thickness. nm and existed as a layer structure. All AFM images of IZTO/ PEDOT:PSS samples above an IZTO thickness of nm showed similar smooth surface morphology, indicating a complete covering of PEDOT:PSS by the conductive IZTO layer. Fig. 5(b) shows root mean square (RMS) roughness of the IZTO/PEDOT:PSS electrodes as a function of IZTO thickness with the inset of growth mechanism. Below an IZTO thickness of 1 nm, the IZTO/PEDOT:PSS electrodes showed fairly high RMS roughness of nm due to island growth of the IZTO film. However, above an IZTO thickness of nm, the IZTO/PEDOT:PSS electrodes showed low RMS roughness due to a smooth IZTO layer that completely covered the bottom PEDOT:PSS layer. Fig. 6(a) shows the outer bending test results of optimized IZTO ( nm)/pedot:pss electrodes with a decreasing outer bending radius. The upper panel shows outer bending steps by a lab-made bending test machine. The change in resistance of the flexible PEDOT:PSS and IZTO ( nm)/pedot:pss electrodes can be expressed as DR¼(R R )/R, where R is the initially measured resistance and R is the value measured after substrate bending. The outer bending test results demonstrate that the electrical resistance of bent PEDOT:PSS and IZTO ( nm)/ped- OT:PSS electrodes did not change until a bending radius of 5 mm. It is noteworthy that the DR/R value remained nearly constant below 1. even though the electrode was bent below 5 mm. The identical flexibility of PEDOT:PSS and IZTO ( nm)/pedot:pss electrodes against outer bending indicates that passivation of the transparent IZTO layer ( nm) does not affect the flexibility of the PEDOT:PSS electrode. Similar to the outer bending test, we also measured the change in resistance of the flexible PEDOT:PSS and IZTO ( nm)/pedot:pss electrodes with a decreasing inner bending radius. Fig. 5(b) shows the change in resistance of the

6 196 J.-H. Lee et al. / Solar Energy Materials & Solar Cells 19 (13) Resistance change [ΔR/R ] R PEDOT: PSS IZTO/PEDOT: PSS Outer bending radius [mm] it was bent below 5 mm. It was impossible to decrease the outer and inner bending radius below 5 mm using the lab-made test system due to contact with the Cu clamp. Fig. 7 shows the dynamic outer bending fatigue test results of the PEDOT:PSS and IZTO ( nm)/pedot:pss electrodes at a fixed inner bending radius of 1 mm. The inset panels show dynamic bending test steps for one cycle. The bending radius of 1 mm is an acceptable value to test the fatigue of the IZTO ( nm)/ PEDOT:PSS electrode. Both dynamic outer bending fatigue tests showed no change in resistance (DR) after 15 cycles due to the superior flexibility of the PEDOT:PSS and IZTO ( nm)/ped- OT:PSS electrodes. Due to the stability of the amorphous structure of the thin IZTO passivation layer, the IZTO/PEDOT:PSS electrode showed identical mechanical flexibility to the PEDOT:PSS electrode. Compared to previously reported flexible ITO film grown by roll-to-roll sputtering, the IZTO ( nm)/pedot:pss electrode showed better stability against repeated bending tests [1]. To investigate the potential of the IZTO ( nm)/pedot:pss electrode as a flexible electrode for FOSCs, we fabricated conventional bulk heterojunction FOSCs using the optimized IZTO ( nm)/pedot:pss electrode and single PEDOT:PSS electrodes. Fig. 8 shows the current density voltage (J V) curves and sample 5 PEDOT: PSS Resistance change [ΔR/R ] PEDOT:PSS IZTO/PEDOT:PSS Inner bending radius [mm] Fig. 6. (a) Outer and (b) inner bending test results of the single PEDOT:PSS and IZTO/PEDO:PSS electrodes as a function of the outer and inner bending radii. Inset panels show the picture of the outer and inner bending test steps with decreasing bending radius. flexible PEDOT:PSS and IZTO ( nm)/pedot:pss electrodes as a function of inner bending radius. The pictures in the upper panel show the inner bending test of the flexible IZTO ( nm)/ped- OT:PSS electrode coated on a PET substrate with a decreasing inner bending radius. As expected based on the outer bending test results, the IZTO ( nm)/pedot:pss electrode also showed constant resistance at the initial inner bending stage. The measured resistance of the IZTO ( nm)/pedot:pss electrode was constant until the sample was bent to an inner bending radius of 5 mm. Like the outer bending radius, the IZTO ( nm)/pedot:pss electrode showed a very small DR/R value (1.), even though R Resistance change [ΔR/R ] Resistance change [ΔR/R ] Outer bending cycles [Times] IZTO/PEDOT: PSS Outer bending cycles [Times] Fig. 7. Dynamic fatigue test results of the: (a) PEDOT:PSS and (b) IZTO/PEDO:PSS electrodes with increasing bending cycles. Inset panel shows the dynamic outer bending steps for one cycle.

7 J.-H. Lee et al. / Solar Energy Materials & Solar Cells 19 (13) Flexible OPV on PEDOT: PSS/PET Table 1 Performance comparison of FOSCs fabricated on single PEDOT:PSS and IZTO/ PEDOT electrodes. Data obtained from different cathode positions from 1 to as shown in the inset of Fig. 8. Current density [ma/cm ] Current density [ma/cm ] PEDOT-1 PEDOT- PEDOT-3 PEDOT Voltage [V] Flexible OPV on IZTO/PEDOT:PSS/PET 3 1 IZTO/PEDOT-1 IZTO/PEDOT- IZTO/PEDOT-3 IZTO/PEDOT Voltage [V] OSC series resistance critically affected the slope of the J V curve at J¼ ma/cm. Because the sheet resistance of transparent electrodes takes a part major portion of the series resistance, the slope of J V curve at J¼ ma/cm is closely related to sheet resistance and contact resistance of the PEDOT:PSS and IZTO/ PEDOT:PSS anodes. The FOSC with the IZTO/PEDOT:PSS anode has a higher slope than the FOSC with the PEDOT:PSS electrode, indicating that sheet resistance and contact resistance between the organic layer and anode was significantly reduced by passivation of a conductive IZTO layer. Detailed information regarding Z, J sc,v oc, and FF of the FOSC with single PEDOT:PSS electrodes and the IZTO/PEDOT:PSS electrode are presented in Table 1. As shown in Table 1, the OSC fabricated on conductive IZTO passivated PEDOT:PSS film showed better cell performance than the FOSC with a single PEDOT:PSS electrode. FF and J sc of the organic solar cells critically depended on series resistance and transmittance of the transparent electrode. The improved FF and J sc values of FOSCs with the IZTO/PEDOT:PSS electrode can be attributed to the low sheet resistance and high transparency of the IZTO passivation layer. In addition, the identical J V curves of the FOSC with IZTO/ PEDOT:PSS electrodes in Fig. 8(b) indicated that a conductive IZTO passivation layer acts as an effective current spreading layer on the PEDOT:PSS electrode due to its lower resistivity. As a result, the overall characteristics of the FOSC with the IZTO/PEDOT:PSS electrode were better than the FOSC with a single PEDOT:PSS electrode.. Conclusion FF (%) J sc (ma/cm ) V oc (V) PCE (%) PEDOT IZTO/PEDOT Fig. 8. J V curves of FOSCs fabricated on (a) the single PEDOT:PSS and (b) IZTO/ PEDO:PSS electrodes. The upper panel shows the flexibility of the FOSCs with a single PEDOT:PSS and IZTO/PEDOT:PSS electrodes. Inset picture shows the position of Ca:Al cathode and IZTO/PEDOT:PSS anode (rectangular shape) for the FOSCs. picture (inset) of FOSCs fabricated using the single PEDOT:PSS and IZTO/PEDOT:PSS electrodes. The inset panels show the superior flexibility of the FOSC with single PEDOT:PSS and IZTO/PED- OT:PSS electrodes. The FOSC with a single PEDOT:PSS electrode in Fig. 8(a) shows different J V curves depending on the position of the Ca:Al cathode due to the high sheet resistance of the gravure printed PEDOT:PSS electrode. With increasing distance between the Ca:Al cathode and PEDOT:PSS anode from number 1 to number, the FOSC shows decreased fill factors (FF) and short circuit current density. However, the FOSC fabricated on the IZTO/PEDOT:PSS electrode shows similar J V curves regardless of the cathode position due to decreased sheet resistance of the conductive IZTO passivation layer (Fig. 8(b)). It is well known that Transparent IZTO passivated PEDOT:PSS electrodes were investigated for their potential application as flexible electrodes for FOSC by examining their electrical, optical, structural mechanical, and surface properties. Using the LFTS technique, conductive IZTO passivation layers were successfully sputtered on gravure printed PEDOT:PSS electrodes without degrading electrical or optical properties. At an optimized IZTO thickness of nm, the IZTO/PEDOT:PSS electrode showed sheet resistance of O/sq and optical transmittance of 83.1% which are better than those of the single PEDOT:PSS electrode. Despite IZTO deposition, the IZTO/PEDOT:PSS electrode exhibited a mechanical flexibility identical to that of the single PEDOT:PSS electrode. This was due to the stable amorphous structure of IZTO with a thickness of nm. Moreover, the FOSC with an optimized IZTO/PEDOT:PSS electrode showed better performance than the FOSC with PEDOT:PSS electrodes due to the effect of the conductive IZTO passivation layer. This indicates that IZTO passivation is a key solution for improving the properties and stability of flexible PEDOT:PSS electrodes for high performance FOSCs.

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Angmo, F.C. Krebs, Solar cells with one-day energy payback for the factories of the future, Energy and Environmental Science 5 (1) [11] J.-S. Yu, I. Kim, J.-S. Kim, J. Jo, T.T. Larsen-Olsen, R.R. Sondergaard, M. Hosel, D. Angmo, M. Jorgensen, F.C. Krebs, Silver front electrode grids for ITO-free all printed polymer solar cells with embedded and raised topographies, prepared by thermal imprint, flexographic and inkjet roll-to-roll processes, Nanoscale (1) [1] K.-H. Choi, J.-A. Jeong, J.-W. Kang, D.-G. Kim, J.K. Kim, S.-I. Na, D.-Y. Kim, S.-S. Kim, H.-K. Kim, Characteristics of flexible indium tin oxide electrode grown by continuous roll-to-roll sputtering process for flexible organic solar cells, Solar Energy Materials and Solar Cells 93 (9) [13] S.-I. Na, S.-S. Kim, J. Jo, D.-Y. Kim, Efficient and flexible ITO-free organic solar cells using highly conductive polymer anodes, Advanced Materials (8) [1] G. Fanchini, S. Miller, B.B. Parekh, M. Chhowalla, Optical anisotropy in singlewalled carbon nanotube thin films: implications for transparent and conducting electrodes in organic photovoltaics, Nano Letters 8 (8) [15] Y.-Y. Choi, S.J. Kang, H.-K. Kim, W.M. Choi, S.-I. Na, Multilayer graphene films as transparent electrodes for organic photovoltaic devices, Solar Energy Materials and Solar Cells 96 (1) [16] M.-G. Kang, M.-S. Kim, J. Kim, L.J. Guo, Organic solar cells using nanoimprinted transparent metal electrodes, Advanced Materials (8) [17] J.-W. Lim, D.-Y. Cho, K. Eun, S.-H. Choa, S.-I. Na, J. Kim, H.-K. Kim, Mechanical integrity of flexible Ag nanowire network electrodes coated on colorless PI substrates for flexible organic solar cells, Solar Energy Materials and Solar Cells 15 (1) [18] Y.H. Kim, C. Sachse, M.L. Machala, C. May, L.M. Meskamp, K. Leo, Highly conductive PEDOT:PSS electrode with optimized solvent and thermal posttreatment for ITO-free organic solar cells, Advanced Functional Materials 1 (11) [19] C.-K. Cho, W.-J. Hwang, K. Eun, S.-H. Choa, S.-I. Na, H.-K. Kim, Mechanical flexibility of transparent PEDOT:PSS electrodes prepared by gravure printing for flexible organic solar cells, Solar Energy Materials and Solar Cells 95 (11) [] Y.-M. Chang, L. Wang, W.-F. Su, Polymer solar cells with poly(3,-ethylenedioxythiophene) as transparent anode, Organic Electronics 9 (8) [1] M. Jørgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Solar Energy Materials and Solar Cells 9 (8) [] M. Jørgensen, K. Norrman, S.A. Gevorgyan, T. Hromholt, B. Andreasen, F.C. Krebs, Stability of polymer solar cells, Advanced Materials (1) [3] J.-M. Moon, H.-K. Kim, sputtering of aluminum cathodes on OLEDs using linear facing target sputtering with ladder-type magnet arrays, Journal of the Electrochemical Society 155 (8) J187 J19. [] H.-K. Kim, K.-S. Lee, J.-H. Kwon, Transparent indium zinc oxide top cathode prepared by plasma damage-free sputtering for top-emitting organic lightemitting diodes, Applied Physics Letters 88 (6) 113. [5] K.-H. Choi, J.-A. Jeong, H.-K. Kim, Dependence of electrical, optical, and structural properties on the thickness of IZTO thin films grown by linear facing target sputtering for organic solar cells, Solar Energy Materials and Solar Cells 9 (1) [6] J.-A. Jeong, H.-K. Kim, Low resistance and highly transparent ITO-Ag-ITO multilayer electrode using surface plasmon resonance of Ag layer for bulk heterojunction organic solar cells, Solar Energy Materials and Solar Cells 93 (9) [7] T.-N. Chen, D.-S. Wuu, C.-Y. Lin, C.-C. Wu, R.-H. Horng, Thin Solid Films 517 (9) [8] H.-K. Kim, S.-W. Kim, K.-S. Lee, K.-H. Kim, Direct Al cathode layer sputtering on LiF/Alq3 using facing target sputtering with a mixture of Ar and Kr, Applied Physics Letters 88 (6) [9] C. Haacke, New figure of merit for transparent conductors, Journal of Applied Physics 7 (1976) [3] J.-A. Jeong, H.-S. Shin, K.-H. Choi, H.-K. Kim, Flexible Al-doped ZnO film grown on PET substrates using linear facing target sputtering for flexible OLEDs, Journal of Physics D: Applied Physics 3 (1) 653.