Monolithic Interface Contact Engineering in 2D Semiconductor Photovoltaic Heterojunctions

In optoelectronic devices based on two-dimensional (2D) semiconductor heterojunctions, the charge transport across the interface is a critical factor to determine the device performance. Even though much effort has been recently made to improve the generation and dissociation of excitons in 2D semiconductors, effective strategies that can enhance charge extraction at the contact interface have rarely been explored. Here, we report an unexplored approach to boost the optoelectronic device performances of 2D heterojunctions via the phase-transition-induced modulation of the interface band alignment. In the proposed device, the atomically thin WO_x layer, which is monolithically formed by layer-by-layer oxidation of WSe₂, is used as a hole transport layer. When the WO_x interlayer was introduced at the semiconductor/electrode interface, the power conversion efficiency of the WSe₂-MoS₂ <i>p-n</i> junction devices increased by about an order of magnitudes, from 0.7 to 5.0%, maintaining the response time. The enhanced characteristics can be understood by the formation of the low Schottky barrier and favorable interface band alignment resulting from the monolithic phase transition, as confirmed by band alignment analyses and supported by first-principle calculations. By further optimizing the electron transport of <i>p-n</i> junction devices, we achieved high <i>V</i>_OC of ~0.8 V approaching to the bandgap and high PCE of ~7 %. Our work suggests a new route to achieve band engineering in the heterostructures toward realizing high-performance 2D optoelectronics.