Defect engineering of solution-processed ZnO:Li window layers towards high-efficiency and low-cost kesterite photovoltaics

Abstract

Advances in the development of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells are urgently needed to further exploit their high-efficiency and low-cost nature. However, almost all the CZTSSe photovoltaics are presently fabricated using physically sputter-deposited i-ZnO as a window layer, which often results in self-doping with detrimental oxygen vacancies (V_O) and zinc interstitial (Zn_i) defects, leading to ineffective charge collection and enlarged interface recombination losses. Herein, a solution-processed ZnO:Li nanoparticle (NP) window layer is demonstrated via the construction of a high-quality ZnO:Li/CdS/CZTSSe heterojunction to enhance the charge collection and minimize the interface recombination. The interstitial Li doping at a certain content significantly enhances the electrical conductivity of ZnO and quasi-Fermi level splitting at the p–n junction interface and reduces the concentration of V_O and Zn_i defects. These electric benefits finally improve the conversion efficiency from 11.31% up to 12.60% with significant gains in FF and V_oc. Our findings suggest that developing a ZnO:Li NP window layer is an effective method to manage electrical conductivity, contact resistance and interface band offset at the p–n junction, boosting the development of high-efficiency and low-cost kesterite solar cells to a higher level.