Crystalline molybdenum oxide layers as efficient and stable hole contacts in organic photovoltaic devices

High work function metal oxides such as molybdenum oxide (MoO_x) 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 MoO_x layers that outperform the otherwise commonly used thermally grown MoO_x layers in OPV devices. These hole contact layers are developed from superoxidized MoO_3.2 films grown by reactive sputtering, followed by postannealing in high vacuum to induce crystallization of the otherwise amorphous MoO_x films. Standard configuration organic solar cell devices based on DBP as electron donor and C₇₀ as electron acceptor were developed on top of the sputtered MoO_x, and a large increase in power conversion efficiency as a function of the MoO_x annealing temperature was observed, which is in clear contrast to devices grown on thermally deposited MoO_x. The crystallization of the sputtered MoO_x 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 MoO_x films exhibit superior stability as compared to devices based on thermally grown MoO_x. These characteristics show that crystalline MoO_x prepared by postannealing sputter deposited films forms a superior hole contact layer material for future air-stable organic optoelectronic devices.