/ Graphene Heterostructures from First Principles*

Heterostructures involving lead halide perovskites have shown impressive potential for many applications, primarily for high efficiency photovoltaics. This work considers graphene as an interfacial layer in contact with CsPbI₃ and CsPbBr₃ perovskites. First principles calculations are used to model the electronic structure and band level alignment of CsPbI₃ / graphene and CsPbBr₃ / graphene heterostructures. Since this analysis requires the creation of large supercells, the effect of the structural mismatch and induced strain during supercell construction is examined. This examination concludes the strain necessary to form the supercells has minimal effects on the perovskite properties. The band level alignment of eight perovskite / graphene supercells with up to 560 atoms is predicted using spin-orbit coupled hybrid density functional theory. The graphene Dirac point consistently lies inside the perovskite bandgap, validating the utility of the graphene layer for charge extraction. Furthermore, the PbX₂ (X=Br, I) terminated (010) crystal plane in both perovskites shows stronger interaction with the graphene than the CsX terminated (010) plane, as evidenced by adsorption energy calculations and confirmed by the graphene Fermi velocity. The graphene Dirac point lies closer to the perovskite CBM for the PbX₂ termination and closer to the VBM for the CsX termination.