We investigated the effects of sodium doping on the photocarrier dynamics in Cu2ZnSnS4 (CZTS) single crystals using optical pump-THz probe transient reflectivity (THz-TR) and time-resolved photoluminescence (PL) spectroscopy. The THz-TR and PL decay dynamics are influenced by sodium doping, and their sodium-induced changes are consistent with each other. These time-resolved measurements revealed that the lifetime of photocarriers increases with sodium doping. This result indicates that a part of defects is suppressed by doping sodium into CZTS and implies that sodium doping improves the charge transport properties of CZTS, leading to an improvement in the performance of CZTS-based solar cells.
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