Diffusivity and band offset analysis of a graphene interlayer at the back contact of a copper zinc tin sulphide solar cell

Cu₂ZnSnS₄ (CZTS) is a promising 3rd generation solar cell absorber based on earth-abundant and nontoxic elements. However, the formation of detrimental MoS₂ in the Mo/CZTS back contact interface hinders the overall performance due to poor band alignment in the back contact and a degraded interface. We propose that graphene can be a suitable candidate for the protective interlayer at the back contact to prevent the formation of amorphous MoS₂ by blocking S diffusion. Using first principles calculations, we investigated the kinetics processes of S atom diffusion on and across the graphene plane with various defects considered. It was found that while an S atom can easily diffuse on graphene with a diffusion barrier of 0.355 eV, it is hard to diffuse across the graphene plane with or without defects, with diffusion barriers ranging from 1.19 eV through a double vacancy to 8.66 eV across pristine graphene. In addition, a band offset calculation using a local potential alignment method was performed to understand the role of graphene in hole transport. We discover that the interpolation of Hartree potential data can largely improve the stability of the band offset algorithm, comparable to a core level alignment method. The band offset calculation results show that the Fermi level of graphene is 0.664 eV higher than the valence band maximum of CZTS. Therefore, graphene is a benign interlayer in the back contact that can facilitate hole collection. Our calculations suggest that graphene is a promising protective interlayer.