Research Article Indium-Free PTB7/PC 71 BM Polymer Solar Cells with Solution-Processed Al:ZnO Electrodes on PET Substrates P. Fuchs, 1 A. Paracchino, 2 H. Hagendorfer, 1 L. Kranz, 1 T. Geiger, 2 Y. E. Romanyuk, 1 A. N. Tiwari, 1 and F. Nüesch 2 1 Laboratory for Tin Films and Photovoltaics, Swiss Federal Laboratories for Material Science and Technology (Empa), Ueberlandstrasse 129, 8600 D¨ ubendorf, Switzerland 2 Laboratory for Functional Polymers, Swiss Federal Laboratories for Material Science and Technology (Empa), Ueberlandstrasse 129, 8600 D¨ ubendorf, Switzerland Correspondence should be addressed to P. Fuchs; peter.fuchs@empa.ch Received 30 October 2015; Revised 3 February 2016; Accepted 10 February 2016 Academic Editor: Giuseppe Portale Copyright © 2016 P. Fuchs et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Inverted PTB7/PC 71 BM polymer solar cells are prepared on solution-processed Al:ZnO transparent contacts on PET substrates. Al:ZnO is deposited by a low temperature chemical bath deposition route (T < 100 ∘ C at any step) to comply with the temperature sensitive substrate. A maximum conversion efciency of 6.4% and 6.9% is achieved for the indium-free solar cells on PET and glass substrates, respectively. Te devices are relatively stable in air whereby an initial efciency loss in the order of 15% afer storage for 15 days can be fully recovered by light soaking. 1. Introduction Major improvements have been made in the feld of organic photovoltaics (OPV) since the frst works in 1995 [1, 2]. Molecular engineering brought up a series of push-pull copolymers with narrow band gaps in the range of 1-2 eV and low-lying Highest Occupied Molecular Orbital (HOMO) level for increased light harvesting and open-circuit potential [3]. Te efciency of organic solar cells (OSCs) strongly depends on the morphology of the polymer-fullerene network, which in turn depends on thermal treatments and on the presence of additives in the polymer blend solution [4, 5]. Likewise interfacial bufer layers between the organic layer and the metal electrodes, notably metal oxides, self-assembled monolayers (SAM), and conjugated polyelectrolytes (CPE) improve the efciency and stability of OSCs [6, 7]. In case of metal oxides wide-bandgap interfacial layers (e.g., MoO 3 , WO 3 , ZnO, and TiO 2 ) provide ohmic contacts for electrons or holes, being blocking layers for the charge carriers of opposite sign. Further they bolster device stability, for example, by stopping difusion of metal atoms from the electrodes into the organic layer and fnally work as optical spacers to enhance the light absorption in the active layer [8, 9]. In this respect, it would be of interest if such oxide layers can also act as barrier layers for air and moisture, to signifcantly increase stability and ease the handling and production of OSC based solar cell devices. A typical OSC consists of an electron-donor polymer and an electron-acceptor fullerene-derivative blend as absorber sandwiched between a transparent conducting window layer and hole extraction layer with work functions carefully matched (Figures 1(a) and 1(b)). In the case of OSCs in the inverted geometry, the window layer is commonly a stack of indium tin oxide (ITO) and ZnO or TiO 2 to combine good conductivity (from the ITO layer) and appropriate band gap matching (from the ZnO or TiO 2 layer) [6]. With respect to the limited indium supply, the ITO layer would ideally be replaced by a doped ZnO layer. Doped ZnO thin flms can be synthesized by sol-gel based methods but to achieve suitable electronic conductivity an annealing step at 400 ∘ C or higher is typically required [10], which is not compatible with most polymer substrates (preferably processing temperatures < 120 ∘ C for low cost polymers such as PET). Sol-gel procedures at lower process temperatures (140 ∘ C–260 ∘ C) result in lower electrical performances and thus require the use of ITO to compensate for the insufcient Hindawi Publishing Corporation International Journal of Photoenergy Volume 2016, Article ID 2047591, 6 pages http://dx.doi.org/10.1155/2016/2047591