Letter Transparent Cu nanowire mesh electrode on flexible substrates fabricated by transfer printing and its application in organic solar cells Myung-Gyu Kang a , Hui Joon Park b , Se Hyun Ahn c , L. Jay Guo a,n a Department of Electrical Engineering and Computer Science, The University of Michigan, 1301 Beal Ave., Ann Arbor, MI 48109, USA b Macromolecular Science and Engineering, The University of Michigan, 2300 Hayward St., Ann Arbor, MI 48109, USA c Department of Mechanical Engineering, The University of Michigan, 2350 Hayward St., Ann Arbor, MI 48109, USA article info Article history: Received 10 January 2010 Received in revised form 23 February 2010 Accepted 25 February 2010 Available online 24 March 2010 Keywords: Transparent metal electrode PDMS Nanoimprint lithography (NIL) Roll-to-roll nanoimprint lithography (R2RNIL) Organic solar cell abstract We report the flexible transparent Cu nanowire mesh electrode fabricated by simple transfer printing from flexible PDMS stamp as a potential replacement for the conventional ITO electrode in organic solar cell applications. Fabricated Cu mesh electrode shows a greater flexibility than conventional ITO electrode deposited on plastic substrates, and exhibits high optical transmittance and electrical conductance. It can be bent to 3 mm radius of curvature with no degradation of the conductance. Large area nanoscale metal electrodes on flexible substrates are demonstrated using a roll-to-roll process. The organic solar cell made with the transparent Cu electrode performs as good as the one with ITO electrode, which indicates that such electrode has the potential to replace conventional ITO electrode for low-cost, large-area flexible organic solar cell applications. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Organic solar cells (OSCs) offer a promising alternative to inorganic solar cells due to their low cost, easy fabrication, and compatibility with flexible substrates over large areas [1–5]. In particular, flexible OSCs [6] have potentials to apply to packaging, clothing, flexible screens, and recharging small mobile electronics. Recently, significant advances have been made for the realization of low-cost and large-area flexible OSCs by focusing on the materials and processing methods [7,8]. The possibility of mass production of flexible OSCs using roll-to-roll fabrication has also been demonstrated by Krebs et al. [9]. Most of the organic solar cells have been made on indium tin oxide (ITO) substrate because ITO offers transparency in the visible range of the electromagnetic spectrum as well as electrical conductivity. Therefore, current high performance organic solar cells are mostly fabricated on ITO electrode [10–15]. However, ITO is not the best choice for low- cost and high-performance flexible OSC applications, because the high quality ITO, especially high conductivity, requires high temperature annealing, which is incompatible with plastic-based flexible substrates. The poor conductivity of the ITO film on flexible substrates reduces the fill-factor (FF) of the device resulting in low power conversion efficiency of large area OSCs [16]. The rather brittle ITO film is also not sufficient for flexible applications. In fact, the poor mechanical stability of ITO can cause device failure when the ITO-coated flexible substrate is bent [16–18]. Moreover, the price of ITO drastically increases due to the limited supply of the indium element and the increasing demand from the rapidly expanding display market. These aspects of ITO potentially prevent the realization of low-cost and high-performance large scale OSC fabrication. Several alter- native materials as transparent conductive electrode (TCE) includ- ing carbon nanotube networks [18–21], conductive polymers [22–24], and random Ag nanowire mesh [25] have been recently investigated and showed adequate performance. However, they suffer from either low conductivity or high surface roughness, which cause reduced FF and power conversion efficiency. We have recently developed another type of TCE based on periodic metallic nanostructures on glass substrates using nanoimprint lithography (NIL) and demonstrated that high optical transmit- tance and high electrical conductivity could be achieved at the same time by controlling the line-width and the thickness of the metallic nanostructures [26]. Specifically, it has been shown that conductivity of such transparent metal electrode was enhanced by a factor of three at the cost of a small decrease in optical transmittance by just doubling the metal thickness. The high conductivity of TCE is one of the most important parameters for high-performance large area organic solar cell applications. It is ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.039 n Corresponding author. Tel.: + 1 734 647 7718; fax: + 1 734 763 9324. E-mail address: guo@eecs.umich.edu (L. Jay Guo). Solar Energy Materials & Solar Cells 94 (2010) 1179–1184