Understanding defects and band tailing characteristics and their impact on the device performance of Cu2ZnSn(S,Se)4 solar cells

Abstract

The main causes of the large open-circuit voltage (V_oc)-deficit in kesterite-based thin-film solar cells (TFSCs) are the high concentration of defects, related defect clusters, and poor band tailing characteristics. We report thorough investigations of defects, defect clusters, and band tailing characteristics in Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films as well as their impact on device performance. Through detailed analyses, we find that (i) the relative concentration of the Zn_Cu and Sn_Cu defects and B-type [2Zn_Cu + Zn_Sn] and C-type [2Cu_Zn + Sn_Zn] defect clusters plays a critical role in determining the kesterite device parameters such as the V_oc, fill factor, and power conversion efficiency (PCE) under Cu-poor and Zn-rich conditions; in particular, we observed that the higher Zn content has minimal impact on V_Cu formation; though, Zn_Cu shallow donors over the optimal concentration cause the electron sink/electron localization to facilitate carrier recombination and induce V_oc loss in the device. (ii) the trend in short circuit current densities (J_sc) is independent of defects and defect cluster concentrations; (iii) band tailing characteristics calculated from the energy difference between bandgap energy (E_g) from external quantum efficiency (EQE) and photoluminescence (PL) spectra can also be affected by the existence of minor secondary phases on the kesterite surface. This work offers new insights and correlations between defects, defect clusters, band tailing characteristics, and device performance, which will help further improve the performance of kesterite-based TFSCs.