Investigation into complex defect properties of near-stoichiometric Cu₂ZnSnSe₄ thin film

In this work, a systematic investigation of the effects of point defects on the efficiency of a Cu₂ZnSnSe₄ (CZTSe) thin film solar cell was conducted using temperature-/power-dependent photoluminescence (PL) and time-resolved photo-luminescence (TRPL). The studied stoichiometric CZTSe absorber layer for a CZTSe solar cell with an efficiency of 4.4 % was prepared using the low-toxicity selenization process. The fitting to the Arrhenius plot and the stretched tail at the long-wavelength side of PL spectrum obtained at 10 K indicated a high concentration of zinc on copper (Cu_Zn) point defects. The dispersive-photon-energy-dependent TRPL at 10 K exhibited non-exponential stretched PL decay at all photon energies and, in particular, revealed the highly fluctuating potential of CZTSe that caused carrier localization. This effect was observed to be caused by [2Cu_Zn⁻ + Sn_Zn²⁺] defect clusters, which produce a significant conduction/valence band-edge shift in their vicinity in CZTSe and can cause shunt leakage and an increase in series resistance. This systematic investigation presents the experimental characterization of [2Cu_Zn⁻ + Sn_Zn²⁺] defect clusters and provides perspective to help clarify the performance deterioration of CZTSe solar cells.