Photoelectrical properties of undoped and Na-doped Cu2ZnSnS4 single crystals measured by optical time-resolved spectroscopy

We have studied photocarrier recombination and relaxation dynamics in Cu₂ZnSnS₄ (CZTS) single crystals using a combination of various optical time-resolved spectroscopy. The band tails and deep defects govern primarily the photocarrier dynamics in CZTS. We pointed out the physical origins behind low power conversion efficiencies of CZTS-based solar cells. In addition, we found that the photocarrier lifetime increases as Na is doped in CZTS; this result implies that Na doping suppresses a part of nonradiative recombination centers in CZTS. Therefore, Na dopant is important to improve the photovoltaic performance of CZTS-based solar cells.