Large Surface Photovoltage of WS₂/MoS₂and MoS₂/WS₂Vertical Hetero-bilayers

Hetero-bilayers, composed of transition metal dichalcogenide (TMD) monolayers of MoS2 (1L-MoS2) and 1L-WS2, are grown on Al2O3 substrates through sulfurization of pre-deposited metal thin films. A very smooth surface morphology and the uniform contact potential difference (CPD) maps show that high-quality homogeneous TMD layers are obtained. The light-induced CPD change, that is, surface photovoltage (SPV), of the samples is investigated. A record high SPV is measured under 532 nm illumination with a power density of 13 mW/cm2 from the hetero-bilayers, with values of 800 mV for 1L-WS2/1L-MoS2 and -790 mV for 1L-MoS2/1L-WS2. Band diagrams are proposed to explain the observed CPD and SPV characteristics. The power dependence of the SPV demonstrates that the measured values represent the expected open-circuit voltage of the hetero-bilayers. The obtained results suggest that optimal growth and proper contact formation with the TMD vertical hetero-bilayers will enable high-efficiency ultrathin photovoltaic devices.