A band-gap-graded CZTSSe solar cell with 12.3% efficiency

Although Cu₂ZnSn(S,Se)₄ (CZTSSe) has attracted attention as an alternative to CuInGaSe₂ (CIGS) as an absorber material in solar cells, its low efficiency is a serious shortcoming preventing its commercialization. To realize a high-efficiency CZTSSe solar cell, improved grain crystallinity, inhibited secondary-phase formation, controlled defect generation, adequate Na content, and band gap grading are required in the absorber layer. Few studies have focused specifically on band gap grading. In this study, a method of using SeS₂, a new potential chalcogenization source material, to control the S and Se contents in a CZTSSe absorber and its effects were investigated. Using an appropriate SeS₂/Se weight ratio, band gap grading was realized within the depletion region. By increasing the value of V_OC through band gap grading in the depletion region, a record V_OC deficit of 0.576 V was achieved. Furthermore, the possibility of enhancing J_SC through the formation of a type-inverted n-type phase at the absorber surface in response to an appropriate alignment of the conduction-band minimum energy level and the Fermi energy pinning level is discussed. By introducing the chalcogenization source material SeS₂ during the annealing process, CZTSSe solar cells with a maximum efficiency of 12.3% were obtained.