Improved stability of non-ITO stacked electrodes for large area flexible organic solar cells Mike Hambsch a , Hui Jin a , Andrew J. Clulow a , Andrew Nelson b , Norifumi L. Yamada c , Marappan Velusamy a , Qingyi Yang d , Furong Zhu d , Paul L. Burn a,n , Ian R. Gentle a , Paul Meredith a,n a Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, QLD 4072, Australia b The Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia c Neutron Science Division, High Energy Accelerator Research Organization (KEK), 203-1 Shirakata, Tokai, Naka, Ibaraki 319-1106, Japan d Department of Physics and Institute of Advanced Materials, Hong Kong Baptist University, 224 Waterloo Road, KowloonTong, Hong Kong article info Article history: Received 14 April 2014 Received in revised form 20 June 2014 Accepted 25 June 2014 Available online 26 July 2014 Keywords: Blade coating Organic photovoltaics Organic semiconductors Neutron reflectometry abstract We present ITO-free rigid and flexible monolithic organic solar cells with an active area of 25 cm 2 . The devices employ a transparent insulator/metal/insulator anode consisting of a molybdenum oxide/silver/ zinc sulfide stack that had a peak transmittance of 80% and a sheet resistance of 3.6 Ω/□. Neutron reflectometry showed that zinc sulfide formed a more stable capping layer at the air interface than molybdenum oxide. It was found that blade coating could be used to form good quality large area films on both rigid (glass) and flexible [poly(ethylene terephthalate)] substrates with much reduced material consumption. Cells with an active layer comprised of a blend of poly[N-9″-heptadecanyl-2,7-carbazole- alt-5,5-(4 0 ,7 0 -di-2-thienyl-2 0 ,1 0 ,3 0 -benzothiadiazole)] (PCDTBT) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 70 BM) gave a power conversion efficiency of 2.7%, which was 40% higher than a standard ITO- based device. No loss in efficiency was observed when a flexible substrate was used. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Organic solar cells have progressed in recent years and finally crossed the 10% efficiency barrier for single junction cells making them as efficient as solar cells made from amorphous silicon [1]. However, these efficiencies have only been achieved for devices with small active areas ( r1 cm 2 ). The next step on the way to becoming a commercial product is to transfer these efficiencies to large area devices ( Z10 cm 2 ), preferably in combination with mass production processes like printing or coating [2]. So far the reported large area solar cells produced by roll-to-roll processes have not exceeded efficiencies of 2% [3–8]. The main limiting factor for the efficiency of large area mono- lithic solar cells is the low conductivity of the commonly used indium tin oxide (ITO) electrode (sheet resistance 10–40 Ω/□ depending on thickness and substrate) [9]. Researchers have chosen two different approaches to address this issue. The first approach is to divide the overall junction area into smaller areas, usually stripes interconnected either in series or parallel [4–6]. The disadvantages of this approach are the loss of active area in comparison to the overall area of the module and the more complex fabrication process. The second approach is to replace the ITO electrode with a more conductive electrode with similar optical properties. For example, using a metal grid in combination with a transparent extraction layer can give rise to good conductivities, although at the expense of active area loss due to the shadow cast by the opaque metal grid [3,4,8]. Recently there have been a few reports of insulator/metal/insulator stacks being used as transparent conduct- ing electrodes [10–13]. By varying the thickness of each layer the optical (i.e. transmittance) and electrical properties (i.e. conductiv- ity) of the electrode can be tuned. Using this latter strategy we have previously reported a top absorbing 25 cm 2 monolithic solar cell with a molybdenum(VI) oxide/silver/molybdenum(VI) oxide (MAM) stack as the transparent conducting electrode with an efficiency of up to 3.2% under AM1.5G illumination [10]. In this manuscript we show that the environmental instability of the MAM stack can be overcome by replacing the molybdenum (VI) oxide in contact with air with a zinc sulfide layer (a MAZ stack), and at the same time the conductivity of the transparent conducting electrode can be improved. For sub-module monolithic 25 cm 2 cells in which the photoactive layer comprising a blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4 0 ,7 0 -di-2-thienyl- 2 0 ,1 0 ,3 0 -benzothiadiazole)] (PCDTBT) and [6,6]-phenyl-C 71 -butyric Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.06.032 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding authors. Tel.: þ61 733653778. E-mail addresses: p.burn2@uq.edu.au (P.L. Burn), meredith@physics.uq.edu.au (P. Meredith). Solar Energy Materials & Solar Cells 130 (2014) 182–190