▎ 摘　　要

Graphene is an attractive material for optoelectronic applications due to its superior charge transport properties and strong and broadband light-matter interaction. In photodetector or photovoltaic devices with graphene as the active material, light absorption generates electron-hole pairs in graphene, which give rise to photoconductivity or photovoltaics. However, the lifetime of photocarriers in graphene is only a few picoseconds, which has been a major obstacle for high-performance graphene-based devices. Here we show that by interfacing graphene with two other semiconducting monolayers, unipolar optical doping of graphene can be achieved, which increases its photocarrier lifetime by about two orders of magnitude. In this scheme, by band-alignment engineering, photoexcited electrons and holes are separated in different layers, which suppresses their recombination. Since the efficiency of photodetectors and photovoltaic devices depends strongly on photocarrier lifetime, these new van der Waals materials can significantly improve the performance of graphene-based optoelectronic devices.