Phase evolution regulation of CZTSSe absorbers via a ZnO blocking layer enables 14.45% efficient kesterite solar cells with low VOC deficit

Abstract

The deep defects and secondary phases arising from complex phase evolution pathways pose a significant challenge that hinders the efficiency of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. Here, we regulate the phase-evolution pathways of CZTSSe by implementing a novel ZnO blocking layer on the kesterite precursor surface. The ZnO layer, which remains stable at low temperatures, effectively prevents contact and reactions between the precursor and low-concentration selenium (Se), thereby suppressing complex phase evolution pathways. As the temperature rises, the gradual diffusion and eventual disappearance of the ZnO layer facilitate a one-step phase transition between the precursor and high-concentration Se at elevated temperatures, leading to the direct formation of CZTSSe. This method yields high-quality CZTSSe films characterized by high crystallinity, absence of secondary phases, and fewer defects. Consequently, band tail states and non-radiative recombination are significantly suppressed, enhancing charge transportation and extraction. Ultimately, we achieve a state-of-the-art CZTSSe device with an efficiency of 14.45% and a V_OC of 572.6 mV, featuring the lowest V_OC deficit reported in kesterite solar cells (V_OC/V^SQ_OC = 69.7%). This study offers valuable insights into the regulation of phase evolution and selenization process of CZTSSe absorbers, paving the way for more efficient solar cell designs.