Enhanced open-circuit voltages and efficiencies: the role of oxidation state of molybdenum oxide buffer layer in polymer solar cells

Abstract

Molybdenum oxide (MoO_x) is widely used as a buffer layer in optoelectronic devices to improve the charge extraction efficiency. The oxidation state of MoO_x plays an important role in determining its electrical properties. However, there are few studies on the oxidation state to further guide the optimization of the MoO_x buffer layer. In this work, inverted-structured polymer solar cells (PSCs) with a MoO_x buffer layer were fabricated. Post-air annealing was used to control the cation valence state in MoO_x. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM) and transient photocurrent (TPC) were employed to study the valence state, energy level, morphology of the MoO_x layers and the photovoltaic property and charge transfer efficiency of the devices. It was found that the oxidation state was effectively improved by the post-annealing process. As a result, the work function of MoO_x was raised and the hole mobility was improved. The open-circuit voltages and the efficiencies of PTB7-Th:PC₇₁BM based PSCs were enhanced from 0.77 V and 8.66% to 0.81 V and 10.01%, respectively. The results show that high oxidation state MoO_x provides optimized energy level alignment, reduced defects and better charge transfer efficiency, which is more in line with the requirement of buffer layer materials for optoelectronic applications.