Density Functional Theory Studies of Protective Interlayer of Graphene for the Back Contact of Copper Zinc Tin Sulphur Thin Film Solar Cell

CZTS(Se) based solar cell is one of the promising candidates in the development of the 3rd generation solar cell absorbers. However, the efficiency of CZTS is insufficient for commercial product. One of the major causes of low performance is poor back contact-absorber interface quality. During annealing process of CZTS, S atom diffuses into Mo back contact layer and form a detrimental secondary phase of MoS₂. Interlayer engineering has been proposed. However, most of the proposed interlayers are based on insulating systems. Here, we propose a new protective material of graphene as an effective diffusion barrier to block any S atom diffusion. Using Climbing Image Nudged Elastic Band Method based on Density Functional Theory calculation to find diffusion barrier and minimum energy path, we can show that the choice of our protective layer can effectively block S atoms diffusion under fabrication and operation environment. We also calculated the band alignment between the proposed protective interlayer and CZTS absorption layer. The results suggest the protective interlayer we proposed will not significantly degrade the performance of the solar cell.