High quality sustainable Cu₂ZnSnSe₄ (CZTSe) absorber layers in highly efficient CZTSe solar cells

Polycrystalline Cu₂ZnSnSe₄ (CZTSe) thin films were directly deposited on Mo-coated glass substrates by evaporation and following selenization. Single-phase CZTSe films were formed in the temperature range of 480-540°C, with a selenization step of 30 min. X-ray diffraction and Raman spectroscopy revealed that these thin films exhibited high crystallinity and strong preferential orientation along the (112) direction, confirming the presence of the kesterite CZTSe phase. The films prepared at temperatures above 520°C showed many voids at the bottom of the CZTSe absorber layer, due to Sn loss during high-temperature growth, as confirmed by scanning electron microscopy and energy dispersive X-ray spectroscopy analyses. The band gaps (E_g) of the CZTSe thin films, which were obtained by photoluminescence spectroscopy, varied from 0.88 to 0.93 eV, depending on the SnSe_x loss during selenization. The solar cell fabricated with the CZTSe film grown at 500°C showed the best conversion efficiency of 7.18%, with an open-circuit voltage of 0.38 V, a short-circuit current density of 42.34 mA cm^-2, and a fill factor of 44%. Further improvements in the microstructure and Sn loss of CZTSe films may increase the efficiency of the CZTSe solar cells.