Crystalline Molybdenum Oxide Layers as Efficient and Stable Hole Contacts in Organic Photovoltaic Devices

High work function metal oxides such as molybdenum oxide (MoOx) have demonstrated good hole contact properties in organic photovoltaic (OPV) devices in the past years and have replaced the otherwise conventionally used PEDOT:PSS. In this work, we introduce new crystalline MoOx layers that outperform the otherwise commonly used thermally grown MoOx layers in OPV devices. These hole contact layers are developed from superoxidized MoO3.2 films grown by reactive sputtering, followed by postannealing in high vacuum to induce crystallization of the otherwise amorphous MoOx films. Standard configuration organic solar cell devices based on DBP as electron donor and C-70 as electron acceptor were developed on top of the sputtered MoOx, and a large increase in power conversion efficiency as a function of the MoOx annealing temperature was observed, which is in clear contrast to devices grown on thermally deposited MoOx. The crystallization of the sputtered MoOx at elevated temperatures is shown to lead to high work function films with improved conductivity, resulting in the appealing device properties. Importantly, long-term stability investigations revealed that devices based on these crystalline MoOx films exhibit superior stability as compared to devices based on thermally grown MoOx. These characteristics show that crystalline MoOx prepared by postannealing sputter deposited films forms a superior hole contact layer material for future air-stable organic optoelectronic devices.