Phase evolution regulation of CZTSSe absorbers via a ZnO blocking layer enables 14.45% efficient kesterite solar cell with a 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 reaction 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, the band tail states and non-radiative recombination are significantly suppressed, enhancing the charge transportation and extraction. Ultimately, we achieve a state-of-the-art CZTSSe device with an efficiency of 14.45% and a VOC of 572.6 mV, featuring the lowest VOC deficit reported in the kesterite solar cells (VOC/VOCSQ = 69.7%). This research offers valuable insights into the regulation of phase evolution and selenization process of CZTSSe absorbers, paving the way for more efficient solar cell designs.