Influence of flexible substrates on inverted organic solar cells using sputtered ZnO as cathode interfacial layer Youssef Jouane a , Silviu Colis b , Guy Schmerber b , Aziz Dinia b , Patrick Lévêque a , Thomas Heiser a , Yves-André Chapuis a,⇑ a ICube Laboratory, Department of Electronic of Solid, Systems and Photonic (D-ESSP), CNRS and University of Strasbourg, 23 rue du Loess, BP 20, F-67037 Strasbourg Cedex 2, France b Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS and University of Strasbourg (UDS-EPCM), 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France article info Article history: Received 9 January 2013 Received in revised form 9 April 2013 Accepted 9 April 2013 Available online 29 April 2013 Keywords: Organic solar cell Interfacial engineering ZnO Sputtering Flexible substrate abstract Zinc oxide (ZnO) has recently shown to be of considerable interest for the development of interfacial buffer layers in inverted organic solar cells (OSCs). High quality ZnO thin films can indeed be prepared on large-area ITO-coated flexible substrates, using low tempera- ture deposition techniques such as sputtering, a compatible technique with roll-to-roll pro- cess. However, further studies are still needed for a better understanding of the influence of the flexible substrate properties on the photovoltaic performances of those devices. In this work, ZnO films have been sputtered on ITO-coated flexible (PEN) substrates and annealed at different temperatures. The role of the surface morphology and the crystalline quality of ZnO films has been investigated. In the window of flexible compatible process, we found that moderate annealing temperatures of ZnO (6180 °C) lead to improved structural prop- erties and performances. Interestingly, we achieve optimal performances for an annealing temperature of 160 °C, resulting in power conversion efficiency (PCE) equivalent to the highest performances usually achieved on rigid cells. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Because of their high mechanical flexibility, simple fab- rication process, low cost and light weight, flexible organic solar cells (OSCs) give an attractive alternative to Si based flexible thin film solar cells [1]. Flexibility means that the solar cells can be easily mass-produced using roll-to-roll processing. However, in order to achieve high device effi- ciencies on flexible substrates, further studies on the appropriate materials and processes used in the OSCs are still needed. This is especially true for inverse OSCs, which have attracted a growing attention since their initial devel- opment [2]. With respect to conventional structures, in- verted devices have many advantages, such as the absence of the indium tin oxide/poly(3,4-ethylene dioxy- thiophene):poly(styrenesulfonaten) (ITO/PEDOT:PSS) interface [3], and the possibility to use air-stable metal as anode for collecting holes (e.g. gold or silver) [4]. It was also reported that the phenomenon of vertical phase segre- gation between poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) makes the inverted structure a better choice for OSCs [5]. Several studies have shown that inverted devices can exhi- bit high power conversion efficiencies (PCEs) using P3HT:PCBM or newly developed donors [6]. However, up to now, research has been mainly focused on efficiency and stability, while the effects of the flexible substrate on OSC performances have not been considered. These effects are something far more important so that recent progress in OCSs has been achieved using semiconducting metal oxides as interfacial layers [7]. As a consequence of this, further restrictive processes that can affect flexible devices 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.04.024 ⇑ Corresponding author. Tel.: +33 0388106538. E-mail address: ya.chapuis@unistra.fr (Y.-A. Chapuis). Organic Electronics 14 (2013) 1861–1868 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel