Electronic and chemical properties of ZnO in inverted organic photovoltaic devices Anirudh Sharma a , Joseph B. Franklin b,1 , Birendra Singh c , Gunther G. Andersson a , David A. Lewis a,⇑ a Flinders Centre for Nanoscale Science and Technology, Flinders University, PO Box 2100, Adelaide, SA 5001, Australia b Department of Materials and London Centre for Nanotechnology, Imperial College, London SW7 2BP, UK c CSIRO, Materials Science and Engineering, Bayview Avenue, Clayton, Victoria 3168, Australia article info Article history: Received 4 February 2015 Received in revised form 19 May 2015 Accepted 23 May 2015 Available online 26 May 2015 Keywords: Pulsed laser deposited ZnO Inverted OPVs Band structure of ZnO Workfunction XPS and UPS abstract Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Organic photovoltaic (OPV) technologies have attracted a lot of attention due to their potential to be a cheap source of clean energy. The activity has resulted in substantial progress in the power conversion efficiency (PCE) of OPVs [1] from 2.5% [2] in 2001 with Heliatek more recently reporting to have achieved a record PCE of 12% [3]. Though remarkable progress has been made in terms of device performance, the stability of devices; which is an essential prerequisite for commercializing this technology, still remains an area which needs further research advancements. Inverted device structures incorporating transparent oxides such as ZnO as a replacement of low workfunction metals have been shown to improve the stability of devices [4]. Inverted device architecture such as ITO/ZnO/P3HT:PCBM/MoO 3 /Ag, also have the potential to overcome the interfacial instability at the poly(3,4-e thylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ITO interface in conventional devices [5], which could otherwise lead to degradation of OPVs [6]. ZnO has been demonstrated as a promising electron acceptor material in hybrid OPVs or an electron selective layer in inverted OPVs [7–9] and it continues to attract significant research focus as an electron transport material due to its excellent electronic properties, transparency and ability to be synthesized using vari- ous methods [10–16]. Though ZnO buffer layers with a range of dif- ferent morphologies such as nanoparticles and nanorods have been used [17,18], the dependence of device performance on the mor- phology of the ZnO film is controversial with some reports suggest- ing a significant dependence of device performance on the ZnO film thickness [19], while other reports attributed the performance changes to the morphology of these thin films [20,21]. For sol– gel processed ZnO, Sharma et al. recently proposed a model where ZnO layer is porous due to the spherical particle morphology, resulting in some inter-diffusion at the ZnO–P3HT:PCBM interface and hence the thickness of the ZnO layer plays a crucial role [22]. However, it must be emphasized that the validity of such a model depends on the morphology and the porosity of the resultant ZnO films, which can be strongly dependent on the deposition method used. Consequently the optimal thickness of ZnO buffer layer for a device would also depend on the deposition methods and this has not been systematically investigated, resulting in a range of ZnO buffer layer thicknesses being reported in the literature [19,23]. Pulsed laser deposition (PLD) of ZnO films has been reported [24] to produce high quality oxide closed layers with controllable and scalable thickness while maintaining low surface roughness [24], unlike other vacuum deposition techniques such as mag- netron sputtering where layer thickness has been found to signifi- cantly influence the surface roughness [16]. Resultant morphology http://dx.doi.org/10.1016/j.orgel.2015.05.032 1566-1199/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: david.lewis@flinders.edu.au (D.A. Lewis). 1 Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University, 50 Nanyang Drive, Singapore. Organic Electronics 24 (2015) 131–136 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel