ffi ciency air stable organic photovoltaics with an aqueous inorganic contact †

Solar cells based on blends of organic semiconductors (such as conjugated polymers or small molecules) and fullerene derivatives are rapidly gaining acceptance as an alternate technology for renewable energy generation. For solution processed organic photovoltaics (OPVs), the bulk heterojunction (BHJ) architecture which consists of an interpenetrating network of the exciton generating organic donor semiconductor and a fullerene based acceptor is considered to be one of the most effective in terms of device performance. Often, these BHJ films are sandwiched between the ITO anode which has been modified using a poly (3,4-ethylene dioxythiophene): (polystyrene sulfonic acid) (PEDOT:PSS) layer and a reflecting cathode that is modified either with a metal oxide, metal oxide/graphene or polyelectrolytic interlayer to fabricate the standard OPV architecture. Whilst this device structure has led to highly efficient devices, it is not favourably looked upon due to the hygroscopic and acidic nature of PEDOT:PSS which degrades the ITO anode resulting in short-lived devices. While the utilisation of carbon nanotubes in place of PEDOT:PSS has been suggested for standard devices, another route towards alleviating this problem is through the use of the inverted device architecture. In fabricating inverted devices, the ITO electrode is modified to reduce its work function allowing electron extraction as opposed to hole extraction in the standard architecture. ZnO, a II–VI semiconducting material that can be applied using solution based techniques is widely regarded as the “universal material” for modifying the ITO contact. Such ZnO films can be deposited through coating of nanoparticle inks or sol–gel processing. While the low temperature processable nature of ZnO is attractive in terms of the requirements for roll-to-roll processing of OPVs, most techniques for deposition of ZnO such as sol–gel processing present several issues for implementation in roll-to-roll systems. For example, these are often based on hazardous and/ or toxic precursors as well as solvent systems that are unfavorable for roll-to-roll manufacturing. Furthermore, such sol–gel processes are strongly dependent on the ambient humidity levels, placing and additional control parameter during the deposition of films, and also possess a relatively low shelf life. All of the above factors clearly indicate a strong need for precursors that are moisture insensitive and preferably water based in order to apply to roll-to-roll manufacturing. In addition to the aspects highlighted above, another issue in using such metal oxides in inverted architectures is the occurrence of S shaped electrical curves due to charge trapping in defect states on the surface of the metal oxide which necessitates a light soaking treatment for efficient device operation through minimization of charge. In this work, we employ an inorganic ink based on the aqueous ammine–hydroxo complex [Zn(NHx)x](OH)2. This is based on the work reported by Meyers et al. where this precursor was used to fabricate ZnO thin film transistors. Following this work, Bai et al. reported a modified process of fabricating inverted devices based on blends of poly (3-hexylthiophene) (P3HT) and [6-6]-phenyl C61 butyric acid methyl ester (PC61BM) using the above ink to form the electron transport layer resulting in devices with power conversion efficiencies (PCEs) in excess of 4%. The notable feature of the work was the excellent performance of the †Electronic supplementary information (ESI) available: Experimental methods, performance of standard architecture, inverted architecture with TiOx interlayer, onset of s-curves for the standard architecture, photoluminescence of the ZnO film, field effect characteristics of the ZnO thin films, thermogravimetric analysis of ZnO. See DOI: 10.1039/c5nr01239b Advanced Technology Institute, University of Surrey, Guildford, Surrey, GU2 7XH, UK. E-mail: [email protected]; Fax: +44 (0)1483 68 6081; Tel: +44 (0)1483 68 9825 National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China. E-mail: [email protected]; Fax: +86 02134207734; Tel: +86 02134207734