Device limitations and light-soaking effects in CZTSSe and CZTGeSSe

Advancements in thin film Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) solar cells have recently achieved power conversion efficiencies >;10%, indicating the potential of this low cost, earth abundant material system as a viable alternative to CIGS and CdTe absorbers [1]. Understanding the limitations in this material system is essential for further advancements in device performance. This work demonstrates the importance of the CdS/CZTSSe interface and conduction band alignment for improving the performance of CZTSSe solar cells. Through incorporation of germanium into CZTSSe, a material with tunable band gap and conduction band level can be achieved. Comparing Cu₂Zn(Sn_yGe_1-y)(S_xSe_1-x) (CZTGeSSe) with standard CZTSSe devices allows for characterization of device limitations as a function of band gap and the conduction band offset at the CdS/CZTSSe interface. This work characterizes the effect of Ge incorporation on recombination, series resistance, and quantum efficiency in CZTSSe.