Out-of-plane dipole in graphene p-n junctions boosts charge separation and transport in photovoltaic cells

Vertically stacked graphene/semiconductor/graphene van der Waals heterostructures are promising candidates for flexible photovoltaic devices due to the strong light absorption of active semiconductors and high carrier mobility of graphene electrodes. However, the separation and extraction of photogenerated carriers by graphene electrodes depends on different electrostatic doping between the graphene layers, which is disadvantageous for practical applications. Here, we propose that introducing a vertical graphene p-n junction as an electrode is an effective solution to address this issue. Taking B-doping graphene/MoSe2/N-doping graphene (GrB/MoSe2/GrN) heterostructures as examples, we systematically investigated their intrinsic electronic properties and dynamics of photoexcited carriers by performing first principles calculations and non-adiabatic molecular dynamics simulations. Our results demonstrate that the presence of out-of-plane dipole between the heteroatom-doped graphene sheets not only separates the photogenerated carriers produced from the MoSe2 monolayer, but also extracts electrons and holes onto the GrN and GrB electrodes, respectively. More interestingly, a type-II band alignment can be formed in the GrB/MoSe2 bilayer/GrN system since the energy levels of the MoSe2 bilayer are split by the built-in electric field, which prolongs the lifetimes of photogenerated carriers from 4.09 to 7.24 ns. This efficient separation and extraction of photoexcited charges are further confirmed by the GrB/CsPbBr3/GrN heterostructure, illustrating the great potential for photovoltaic devices with high photoresponsivity.