Brush-painted flexible organic solar cells using highly transparent and flexible Ag nanowire network electrodes

Solar Energy Materials & Solar Cells 122 (2014) 152–157

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Letter

Brush-painted flexible organic solar cells using highly transparent and flexible Ag nanowire network electrodes Sin-Bi Kang a, Yong-Jin Noh b, 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, Gyeonggi-do 446-701, Republic of Korea b Graduate School of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-ku, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 7 September 2013 Received in revised form 18 November 2013 Accepted 28 November 2013

We report on the highly flexible and cost-efficient brush painting of flexible organic solar cells (FOSCs). Brush painting was applied for all of the solution-based layers of FOSCs—an Ag nanowire (NW) anode, a poly(3,4ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) hole transporting layer, a poly(3-hexylthiophene) (P3HT)- and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)-based active layer, and a ZnO electron transporting layer. By the simple and fast brush painting of the Ag NW network with a low sheet resistance of 26.4 Ω/square, a high transmittance of 81.8%, and superior flexibility, we can avoid the use of typical sputtered Sn-doped In2O3, which is used as anode electrodes in typical FOSCs. The P3HT:PCBM-based FOSCs fabricated by brush-painting processes showed a short circuit current of 7.276 mA/cm2, an open circuit voltage of 0.574 V, a fill factor of 49.20%, and a power conversion efficiency of 2.055%. The successful operation of brush-painted FOSCs indicated that brush painting is a promising ultra-low-cost and fast coating process for the fabrication of solution-based cost-efficient FOSCs. & 2013 Elsevier B.V. All rights reserved.

Keywords: Flexible organic solar cell Brush painting Ag nanowire ITO

1. Introduction Organic solar cells have been extensively investigated due to their potential for use as flexible solar cells fabricated by solutionbased and cost-effective processes [1–3]. In particular, the rapid advance of organic active materials and printing-based coating technologies and the continuous increase in power conversion efficiency (8–10%) allow us to anticipate the mass production of low-cost flexible organic solar cells (FOSCs) in the near future [4,5]. Although the organic active layers and buffer layers in FOSCs have been fabricated by solution-based coating processes, such as spin-coating, spray coating, gravure printing, inkjet printing, and flexography printing, the transparent anode and metal cathode films in OSCs have still been fabricated by vacuum-based sputtering and evaporation due to the absence of printable transparent electrodes and low work function metal ink [6–10]. Krebs et al. reported that all-printable FOSCs with PEDOT:PSS electrodes and an Ag grid had a PCE of 1.92%, which is much lower than OSCs with sputtered ITO anodes and evaporated metal cathodes [11,12]. However, considering the cost effectiveness of FOSCs, they should be fabricated by an all-printing process under atmospheric ambient conditions without a vacuum-based coating process. Although

n

Corresponding author. E-mail address: [email protected] (H.-K. Kim).

0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.11.036

spin coating is the most widely employed coating technique for the simple fabrication of FOSCs in academic and industrial research, further development of ultra low-cost printing technology, which is simultaneously applied in the coating of the organic and electrode layers, is urgently needed for the realization of allprintable FOSCs. As an ultra-low-cost coating method, a brush painting technique was recently suggested for the production of cost-efficient OSCs. Kim et al., investigating brush-paintable OSCs with a high PCE of 3.6%, suggested that the organic active layer could be coated by simple brush painting due to the shear stress of the paint brush [13,14]. In our previous work, we also reported the brush-painting of Ag nanowire (NW) networks and PEDOT:PSS electrodes for FOSCs and showed the potential of brush painting as an ultra-low-cost coating process [15,16]. Recently, Qi et al., showed the possibilities of brush painting by demonstrating an all-brush-painted top gate organic thin film transistor with a maximum mobility of 0.14 cm2/V s [17]. Although organic layers and electrodes have been fabricated by brush painting, there have been no reports on the creation of all-brush-painted FOSCs including brush-painted transparent electrodes, PEDOT:PSS, active layers, and ZnO. In this letter, we report on highly cost-effective FOSCs fabricated by a simple and fast brush-painting method. All of the solution-based layers of solar cells—the Ag NW anode, PEDOT:PSS hole transporting layer, P3HT:PCBM active layer, and ZnO electron transporting layer—were brush-painted under atmospheric

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PEDOT:PSS layer was brushed onto the Ag NW electrodes without intentionally heating the PET substrate. Then, the PEDOT:PSS layer was dried at 120 1C for 10 min under ambient conditions. After the brush painting of the PEDOT:PSS, the P3HT:PCBM active layer was brush-painted on the PEDOT:PSS layer using a blend solution containing 50 mg poly(3-hexylthiophene) (P3HT, Rieke Metal) and 50 mg of 1-(3-methoxycarbonyl) -propyl-1-phenyl-(6,6)C61 (PCBM, Nano-C) in 2 ml of 1, 2-dichlorobenzene. After the brush painting of the P3HT:PCBM active layer, a solvent-annealing treatment was performed by keeping the active films inside a covered glass jar for 120 min. Next, a ZnO buffer layer was brushpainted using a 0.75 M zinc acetate solution in 4% ethanolamine and 96% 2-methoxyethanol, followed by annealing at 150 1C for 30 min [18]. Finally, an Al top electrode (
100 nm) with an area of 4.66 mm2 was deposited using thermal evaporation under
10  6 Torr. Due to the absence of Al metal ink for brush painting, the Al cathode was deposited by an evaporation process. The inset picture in Fig. 1(b) shows the brush-painted FOSC including a brush-painted Ag NW anode, PEDOT:PSS, P3HT:PCBM, and ZnO buffer layer. The active area of the brush-painted FOSCs was defined by the shadow mask. The photocurrent density–voltage (J–V) curves were measured using a Keithley 1200 source measurement unit, and its photovoltaic performance was measured under an illumination intensity of 100 mW/cm2 generated using a 150 W Oriel solar simulator at AM 1.5G conditions. To ensure its accuracy, a reference Si solar cell certified by the International System of Units (SI) (SRC 1000 TC KG5 N, VLSI Standards, Inc.) was

ambient conditions. In addition, we investigated the electrical, optical, structural, and mechanical properties of the brush-painted Ag NW network for its application in brush-painted FOSCs.

2. Experimental The brushed-painted FOSCs with a structure consisting of Al cathode/ZnO buffer/P3HT:PCBM/PEDOT:PSS/Ag NW anode/PET were fabricated by simple brush painting under atmospheric ambient conditions without vacuum-based sputtering. Using a general paintbrush made of nylon fibrils, the brush painting of all of the components in the FOSCs except the Al cathode was carried out [15,16]. First, the Ag NW network electrode was coated onto an 188 μm thick PET substrate by brush painting using Ag NW inks. As shown in Fig. 1(a), prior to the Ag NW brush painting, the surface of the PET substrate was treated with atmospheric plasma to increase the adhesion of the Ag NW and the wettability of the Ag NW ink. Fig. 1(b) shows the schematic brush painting process for the fabrication of cost-efficient FOSCs. After atmospheric plasma treatment, the Ag NWs were coated onto the PET substrate by simple brush painting at a constant speed of 2 cm/s. Then, the brush-painted Ag NWs electrodes were annealed at 150 1C for 5 min to remove the solvent. After brush painting the Ag NW anodes, a PEDOT:PSS layer was directly brush-painted on the Ag NW anode using a commercial PEDOT:PSS ink (Clevios PH510). After dipping the paintbrush in the PEDOT:PSS ink for 1 min, the

Ag NW (Bottom)

PEDOT:PSS

P3HT:PCBM

Brush-painted OPVs Al electrode (Top) ZnO Buffer

Fig. 1. (a) Picture of the brush painting process of the Ag NWs on the PET substrate. After surface treatment of the PET substrate (left side picture), the Ag NWs were coated on the PET substrate by simple brush painting (middle picture). The right side picture shows the high transparency and flexibility of the brush-painted Ag NW network on the PET substrate. (b) Schematics of the brush painting process to prepare the brush-painted FOSCs, with the inset picture showing the flexibility of the brush-painted FOSCs.

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used. The microstructure of the brush painted Ag NWs electrodes was investigated by synchrotron x-ray diffraction (XRD) examinations at a beam line 5A of the Pohang light source.

3. Results and discussion Fig. 2(a) shows the surface FESEM images of the brush-painted Ag NW network anode with different brush cycles. The Ag NW network fabricated by a one-brush cycle showed a lower Ag NW density than the Ag NW network fabricated by three brush cycles. As we previously reported, the density of the Ag NW was critically dependent on the brush cycles [15,16]. Due to the shear stress of the paintbrush, the Ag NW network anode had a well-connected wire–wire junction and a smooth surface unlike the forest-like Ag NW structures observed in the transfer-printed Ag NWs [19]. Fig. 2(b) exhibits the sheet resistance of the brush painted Ag NW network as a function of the number of brushing cycles.

1

Sheet Resistance [Ohm/sq.]

1

45 40 35 500 nm

30 25 20 15 10

500 nm

1 brush

2 brush

3 brush

Number of brush painting 100 Reference ITO

90

Ag Nanowire

70

1.2

60

Absorbance [a. u.]

Transmittance [%]

80

50 40 30

1.0 0.8 0.6 0.4

20

0.2

10

0.0 300

0 300

400

500

600

700

800

Wavelength [nm]

400

500

600

700

800

Wavelength [nm] Fig. 2. (a) FESEM surface image of the one-brush- and three-brush-painted Ag NW networks on the PET substrate. (b) Sheet resistance of the brush-painted Ag NW network as a function of brush cycles with the inset picture showing the different connectivity of the Ag NWs. The letters A and B refer to the images obtained from regions A and B in (a). (c) Optical transmittance of the Ag NW and the reference ITO anode on the PET substrate. The inset shows the absorption of the brush-painted P3HT:PCBM active layer.

The one-brush Ag NW network showed a fairly high sheet resistance of 38.7 Ω/square due to the low density of Ag NW and the randomly unconnected wire–wire contacts as shown in the inset of the enlarged FESEM image. The inset image is the enlarged image of region A in Fig. 2(a). However, it is evident that with increasing numbers of brush painting cycles, the sheet resistance was significantly decreased from 38.7 Ω/square to 14.0 Ω/square. The decreased sheet resistance of the brush-painted Ag NW network with increasing numbers of brush cycles can be attributed to the increased Ag NW density and improved wire–wire contact caused by the shear stress of the brush paint as shown in the inset FESEM images. Fig. 2(c) shows the optical transmittances of the brush-painted Ag NW and reference ITO electrode. Due to a broad surface plasmon resonance band in the UV region, the brush-painted Ag NW network electrode showed a lower transmittance than the ITO film at a wavelength region of 400 nm. Considering the diameter (25–30 nm) and length (10–20 μm) of the brush painted Ag NW, the abruptly reduced transmittance in the UV region is closely related to the surface plasmon resonance [20–22]. However, considering the absorption of P3HT:PCBM in the inset of Fig. 2(c), the transmittance of the Ag NW network is enough for use as an anode for P3HT:PCBM-based FOSCs. At a wavelength of 550 nm, the Ag NW network showed a transmittance of 81.8% which is similar to the reference ITO film. However, as shown in Fig. 1(a), the Ag NW network showed a slightly hazylooking color. The hazy-looking Ag NW electrode would be beneficial in FOSCs because the large scattering of light in the network Ag NWs could enhance the absorption of light into the active layer. Fig. 3(a) shows enlarged cross-sectional TEM images obtained from the brush-painted Ag NW network anode on the PET substrate. The cross-aligned Ag NWs showed good wire–wire junctions in the Ag NW networks. Due to the shear stress of the paint brush, the Ag NWs were well-aligned parallel to the PET substrate with well-connected wire junctions. It was shown that the Ag NWs terminated by the (111) plane were connected to the (100) side surface plane of the Ag NWs. The randomly connected Ag NW provides a main conduction path for the carrier. Fig. 3(b) shows the synchrotron x-ray diffraction (XRD) results obtained from the brush-painted Ag NW network electrodes. The XRD plot showed a strong (111) peak at 38.141 in addition to two broad PET substrate peaks, indicating that the brush-painted Ag NW electrodes on the PET substrate had a crystalline structure with a strongly (111) preferred orientation as expected from the TEM results. To investigate the feasibility of using the brush-painted Ag NW network as a flexible electrode, we carried out outer bending tests as shown in Fig. 3(c). The upper panels show an outer bending step of the Ag NW network with decreasing bending radius using a lab-made bending test system. The change in the resistance of the Ag NW or ITO reference electrodes can be expressed as R¼(R R0)/ R0, where R0 is the initially measured resistance and R is the value measured after the substrate bending. The outer bending test results demonstrate that the resistance of the bent Ag NW network did not change until it was bent to a bending radius of 5 mm, as shown in the upper panel pictures. However, the reference ITO film showed an abruptly increased resistance change at a bending radius of 16 mm due to rapid crack formation and propagation [23]. The robustness of the brush-painted Ag NW network electrode against the extreme outer bending indicates that the brush-painted Ag NW network electrode is desirable as a flexible electrode for FOSCs. Fig. 4(a) shows the current density (J)–voltage (V) curves of the brush-painted FOSCs with energy band diagrams. For comparison, reference FOSCs were also fabricated on the amorphous ITO-coated PET substrate. As shown in Fig. 4(a), the FOSCs with the reference ITO

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Current density [mA/cm2]

4 2

Reference Brush painted FOSC

0 -2 -4 -6 -8 -10 -0.4

-0.2

0.0

0.2

0.4

0.6

30

35

PET PET

Intensity [a.u.]

Ag (111)

Voltage [V]

40

45

50

55

60

65

2theta [deg.] Fig. 4. (a) J–V curves obtained from the brush-painted FOSCs and the reference FOSCs fabricated on an ITO/PET substrate. The inset shows the energy band diagram of the brush-painted FOSCs. (b) Superior flexibility of the brush-painted FOSCs.

Resistance change [ΔR/R0]

1.0 0.8 0.6

2 brushed Ag NW Reference ITO

0.4 0.2 0.0 30

25

20

15

10

5

Outer bending radius [mm] Fig. 3. (a) Cross-sectional TEM image of the Ag NWs that is well-connected with each other. (b) Synchrotron x-ray diffraction result obtained from the brushpainted Ag NW network on the PET substrate. (c) Change of resistance of the twobrush-painted Ag NW and reference ITO anode as a function of the outer bending radius. Upper panel shows the outer bending step of the brush-painted Ag NW network with decreasing bending radius.

electrode (Rsh: 30 Ω/square and T: 83%) had a fill factor (FF) of 56.07%, a short-circuit current density (Jsc) of 8.480 mA/cm2, and an opencircuit voltage (Voc) of 0.592 V, resulting in a power conversion efficiency (PCE) of 2.817%. Due to the fairly high sheet resistance of the amorphous ITO anode, the reference FOSC showed a lower PCE than the conventional OSC fabricated on the crystalline ITO anode [15]. More importantly, the brush-painted FOSC also showed its successful operation with a FF of 49.20%, a Jsc of 7.276 mA/cm2, a Voc of 0.574 V, and a PCE of 2.055%. The lower PCE value in the brush-painted FOSC was attributed to the lower FF and Jsc values. Compared with the control ITO-based device, the low FF and Jsc in the brush-painted FOSC could be due to the higher series resistance and a lower transmittance of the Ag NW, which can be confirmed by the lower slope near the Voc region of the J–V curve in Fig. 4(a) and by the transmittance data of the Ag NW film shown in Fig. 2. Although the brush-painted FOSC showed a slightly lower PCE than that of the control FOSC with sputtered ITO, the successful operation of the brush-painted FOSCs indicates their

feasibility without using the spin-coating method, offering a potential cost-efficient replacement. Because we removed the need for the brittle ITO anode, the brush painted FOSC showed superior flexibility, as shown in Fig. 4(b). To investigate the interface between the brush-painted Ag NW electrode and the buffer layers in the brush-painted FOSC, HRTEM examination was carried out. Fig. 5(a) shows a cross-sectional TEM image of a brush-painted FOSC fabricated on a brush-painted Ag NW anode/PET sample. The cross-sectional TEM image revealed that all components of the FSOSs such as the Ag NW anodes, PEDOT:PSS, P3HT:PCBM active layers, and ZnO, were uniformly coated on the PET substrate without protrusion. Even though all of the solution-based layers were coated by a simple brush paint, the cross-sectional TEM image showed a very well-defined interface and the smooth morphology of each layer was similar to the previously reported spin-coated OSCs [24]. Fig. 5(b) shows the HRTEM image obtained from the interface region between the brush-painted Ag NW anode and the PEDOT:PSS buffer layer. It was shown that the interface between the parallel-aligned Ag NW and the PEDOT:PSS layer were well defined. Due to the shear stress of the brush, the Ag NWs were parallel-aligned to the PET substrate unlike the forest-like Ag NWs which were prepared by the transfer method or spin coating [13,14]. A 150-nm thick brushpainted PEDOT:PSS buffer layer uniformly existed on the Ag NW network electrode. Even though we coated the PEDOT:PSS by brush coating, we could control the thickness of the PEDOT: PSS by optimizing the painting speed. The enlarged image in Fig. 5(c) exhibits the amorphous PEDOT:PSS layer that was wellapplied to the brush-painted Ag NWs. In addition, the Ag NW with an (111) end-plane was also well-applied to the (200) side plane of the Ag NWs. Considering the morphology and porosity of the Ag NW network, the brush-painted PEDOT:PSS layer was desirable to fill out the porous region of the Ag NW network. There was no interfacial layer between the Ag NW and PEDOT:PSS layer even

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Fig. 5. (a) Cross-sectional TEM image of the brush-painted FOSC. (b) Cross-sectional TEM image obtained from the Ag NW anode and PEDOT:PSS buffer layer. (c) HRTEM image of the interface region between the brush-painted PEDOT:PSS and the Ag NW anode.

though the PEDOT:PSS layer was painted by a brush under atmospheric ambient conditions. The stable interface and good contact between the brush-painted Ag NWs and PEDOT:PSS layers indicate that the hole carriers generated at the P3HT:PCBM active layer are easily extracted to the Ag NW network.

4. Conclusion In summary, we fabricated brush-painted FOSCs on brushpainted transparent Ag NWs electrodes and showed the successful operation of these FOSCs which had a PCE of 2.055%. Due to the low sheet resistance, high transmittance, and superior flexibility of the brush-painted Ag NW network electrode, we can completely remove the need for the sputtered ITO anode, which have been typically used as flexible electrodes in FOSCs. Even though all of the components were prepared by brush painting, they showed stable interfaces and well-defined layers similar to FOSCs prepared by conventional spin-coating. Although the brush-painted FOSCs showed a slightly lower PCE of 2.055% as compared to that of the FOSCs with a sputtered ITO (2.81%), the removal of the vacuumbased high-cost coating process points to the potential of the brush-painting method as an ultra-low-cost coating process for printing-based cost-efficient FOSCs.

Acknowledgments This work was supported by New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (MKE) (20113010010030)

References [1] G. Yu, J. Gao, J.C. Hemmelen, F. Wudl, A.J. Heeger, Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor–acceptor heterojunctions, Science 270 (1995) 1789–1791. [2] C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanchez, J.C. Hummelen, Origin of the open circuit voltage of plastic solar cells, Adv. Funct. Mater. 11 (2001) 374–380. [3] K. Norman, M.V. Madsen, S.A. Gevorgyan, F.C. Krebs, Degradation patterns in water and oxygen of an inverted polymer solar cell, J. Am. Chem. Soc. 132 (2010) 16883–16892. [4] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Y. Yang, A polymer tandem solar cell with 10.6% power conversion efficiency, Nat. Commun. 4 (2013) 1–10. [5] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, Solar cell efficiency tables (Version 38), Prog. Photovolt.: Res. Appl. 19 (2011) 565–572. [6] K. Norrman, A. Ghanbari-Siahkali, N.B. Larsen, 6 studies of spin-coated polymer films, Annu. Rep. Prog. Chem. Sect. C 101 (2005) 174–201. [7] C. Girotto, B.P. Rand, J. Genoe, P. Heremans, Exploring spray coating as a deposition technique for the fabrication of solution-processed solar cells, Sol. Energy Mater. Sol. Cells 93 (2009) 454–458. [8] P. Kopola, T. Aernouts, R. Sliz, S. Guillerez, M. Ylikunnari, D. Cheyns, M. Valimaki, M. Tuomikoski, J. Hast, G. Jabbour, R. Myllyla, A. Maaninen, Gravure printed flexible organic photovoltaic modules, Sol. Energy Mater. Sol. Cells 95 (2011) 1344–1347. [9] J. Kim, S.-I. Na, H.-K. Kim, Inkjet printing of transparent InZnSnO conducting electrodes from nano-particle ink for printable organic photovoltaics, Sol. Energy Mater. Sol. Cells 98 (2012) 424–432. [10] J.E. Carle, T.R. Andersen, M. Helgesen, E. Bundgaard, M. Jørgensen, F.C. Krebs, A laboratory scale approach to polymer solar cells using one coating/printing machine, flexible substrates, no ITO, no vacuum and no spin coating, Sol. Energy Mater. Sol. Cells 108 (2013) 126–128. [11] F.C. Krebs, R. Søndergaard, M. Jørgensen, Printed metal back electrodes for R2R fabricated polymer solar cells studied using the LBIC technique, Sol. Energy Mater. Sol. Cells 95 (2011) 1348–1353. [12] J.-S. Yu, I. Kim, J.-S. Kim, J. Jo, T.T. Larsen-Olsen, R.R. Søndergaard, M. H€osel, D. Angmo, M. Jørgensen, 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, Nano Scale 4 (2012) 6032–6040. [13] S.-S. Kim, S.-I. Na, J. Jo, G. Tae, D.-Y. Kim, Efficient polymer solar cells fabricated by simple brush painting, Adv. Mater. 19 (2007) 4410–4415.

S.-B. Kang et al. / Solar Energy Materials & Solar Cells 122 (2014) 152–157

[14] S.-S. Kim, S.-I. Na, S.-J. Kang, D.-Y. Kim, Annealing-free fabrication of P3HT:PCBM solar cells via simple brush painting, Sol. Energy Mater. Sol. Cells 94 (2010) 171–175. [15] J.-H. Lee, H.-S. Shin, Y.-J. Noh, S.-I. Na, H.-K. Kim, Brush painting of transparent PEDOT/Ag nanowire/PEDOT multilayer electrodes for flexible organic solar cells, Sol. Energy Mater. Sol. Cells 114 (2013) 15–23. [16] J.-W. Lim, D.-Y. Cho, J. Kim, S.-I. Na, H.-K. Kim, Simple brush-painting of flexible and transparent Ag nanowire network electrodes as an alternative ITO anode for cost-efficient flexible organic solar cells, Sol. Energy Mater. Sol. Cells 107 (2012) 348–354. [17] Z. Qi, F. Zhang, C.-A. Di, J. Wang, D. Zhu, All-brush-painted top-gate organic thin-film transistors, J. Mater. Chem. C 1 (2013) 3072–3077. [18] Y.-J. Noh, S.-I. Na, S.-S. Kim, Inverted polymer solar cells including ZnO electron transport layer fabricated by facile spray pyrolysis, Sol. Energy Mater. Sol. Cells 117 (2013) 139–144. [19] W. Gaynor, G.F. Burkhard, M.D. McGehee, P. Peumans, Smooth nanowire/ polymer composite transparent electrodes, Adv. Mater. 23 (2011) 2905–2910.

157

[20] Y. Sung, Y. Yin, B.T. Mayers, T. Herricks, Y. Xia, Uniform silver nanowire synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly(vinyl pyrrolidone), Chem. Mater. 14 (2002) 4736–4745. [21] A. Graff, D. Wagner, H. Ditlbacher, U. Kreibig, Silver nanowires, Eur. Phys. J. D 34 (2005) 263–269. [22] H. Ditlbacher, A. Hohenau, D. Wagner, U. Kreibig, M. Rogers, F. Hofer, F.R. Aussenegg, J.R. Krenn, Silver nanowires as surface plasmon resonators, Phys. Rev. Lett. 95 (2005) 257403-1–257403-4. [23] D.R. Cairns, R.P. WitteII, D.K. Sparacin, S.M. Sachsman, D.C. Paine, G.P. Crawford, Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates, Appl. Phys. Lett. 76 (2000) 1425–1427. [24] H.-M. Lee, S.-B. Kang, K.-B. Chung, H.-K. Kim, Transparent and flexible amorphous In–Si–O films for flexible organic solar Cells, Appl. Phys. Lett. 102 (2013) 021914-1–021914-5.