Extremely High Photovoltage (3.16 V) Achieved in Vacuum-Ultraviolet-Oriented van der Waals Photovoltaics

Open-circuit voltage is one of the most important indicators to judge the performance of photovoltaic devices (such as solar cells and photovoltaic detectors), which makes its improvement greatly needed and pursued. Heterostacking based on van der Waals forces opens up a new way for devices which aim to achieve high photovoltage through realizing perfect heterojunctions. Here we propose a method to enhance the quasi-Fermi level splitting of AlN-based van der Waals heterojunction photovoltaic devices by adjusting the Fermi level of graphene as a hole transport layer through chemical doping. Based on this method, the open-circuit voltage, short circuit current and power conversion efficiency of the device are significantly improved by 28.5%, 11.2%, and 74.5%, respectively (under 185 nm VUV irradiation), of which the 3.16 V open-circuit voltage is the highest value reported nowadays of heterojunction devices. The mechanism of this significant increase in open-circuit voltage can be explained by a combined effect, including the effective regulation of the Fermi surface of graphene by chemical doping and the enhanced Fermi level splitting achieved by photoinjection of doped graphene. The device design and processing strategies proposed in this work provide a reference for improving the performance of other wide-bandgap semiconductor-based photovoltaic devices.