Piezo-Phototronic Effect for Enhanced Flexible MoS2/WSe2 van der Waals Photodiodes

The recent discoveries of transition-metal dichalcogenides (TMDs) as novel 2D electronic materials hold great promise to a rich variety of artificial van der Waals (vdWs) heterojunctions and superlattices. Moreover, most of the monolayer TMDs become intrinsically piezoelectric due to the lack of structural centrosymmetry, which offers them a new degree of freedom to interact with external mechanical stimuli. Here, fabrication of flexible vdWs p-n diode by vertically stacking monolayer n-MoS2 and a few-layer p-WSe2 is achieved. Electrical measurement of the junction reveals excellent current rectification behavior with an ideality factor of 1.68 and photovoltaic response is realized. Performance modulation of the photodiode via piezo-phototronic effect is also demonstrated. The optimized photoresponsivity increases by 86% when introducing a -0.62% compressive strain along MoS2 armchair direction, which originates from realigned energy-band profile at MoS2/WSe2 interface under strain-induced piezoelectric polarization charges. This new coupling mode among piezoelectricity, semiconducting, and optical properties in 2D materials provides a new route to strain-tunable vdWs heterojunctions and may enable the development of novel ultrathin optoelectronics.