Free-carrier dynamics and band tails in Cu2ZnSn(SxSe1-x)4: Evaluation of factors determining solar cell efficiency

We investigated the composition-dependent photocarrier dynamics in Cu2ZnSn(SxSe1-x)4 (CZTSSe) single crystals using various types of steady-state and time-resolved optical spectroscopy. Photoluminescence spectroscopy shows that the band-tail states formed below the band edge decrease monotonically with increasing Se content. THz time-resolved spectroscopy clarifies that an increase in the Se content leads to a shorter lifetime of the free photocarriers. A trade-off between the composition-dependent band-tail density and the free-carrier lifetime occurs in CZTSSe single crystals. Our experimental results provide insights into the physics behind the low and composition-dependent conversion efficiency of CZTSSe-based solar cells.