In this work, TiB₂ thin films have been employed as intermediate layer between absorber and back contact in Cu₂ZnSnS₄ (CZTS) thin film solar cells for interface optimization. It is found that the TiB₂ intermediate layer can significantly inhibit the formation of MoS₂ layer at absorber/back contact interface region, greatly reduces the series resistance and thereby increases the device efficiency by short current density (Jsc) and fill factor boost. However, introducing TiB₂ degrades the crystal quality of absorber, which is detrimental to device performance especially Voc. The careful control of the thickness of TiB₂ intermediate layer is required to ensure both MoS₂ with minimal thickness and CZTS absorber with large grain microstructure according to the absorber growth process.

Alkaline metals doping is one of the approaches for achieving high efficiency Cu(In,Ga)Se₂ (CIGS) solar cell. Recently, potassium doping helps to break the record efficiency of CIGS solar cell doped with sodium. In this paper, we have investigated how incorporation of potassium can influence the properties of Cu₂ZnSnS₄ (CZTS) thin film and the performance of resulting solar cell. Our results showed that K doping can enhance the (112) preferred orientation, increase the grain size and reduce the second phase ZnS of the CZTS thin films. After K doping, despite of some drop of V_oc for CZTS thin film solar cells, the R_s is decreased and the J_sc is improved markedly, and the solar cell efficiency is boosted.