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

Molybdenum oxide (MoOx) is widely used as a buffer layer in optoelectronic devices to improve the charge extraction efficiency. The oxidation state of MoOx 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 MoOx buffer layer. In this work, inverted-structured polymer solar cells (PSCs) with a MoOx buffer layer were fabricated. Post-air annealing was used to control the cation valence state in MoOx. 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 MoOx 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 MoOx 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 MoOx 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.