Na Ion Migration in NaF-Doped Cu₂ZnSn(S,Se)₄ Thin–Film Solar Cells on Flexible Mo Foil

Cu₂ZnSn(S,Se)₄ (CZTSSe) materials are the most popular alternatives to CIGS-based materials and have been studied for several years. However, so far, the efficiency of CIGS-based thin film solar cells (TFSCs) has not been kept up. The low V_oc compared to CIGS is one of the main reasons for the low conversion efficiency of CZTSSe TFSCs. The causes of low V_oc are as follows: i) secondary phases such as Cu2Sn(S,Se)₃, Cu_x(S,Se), and Zn(S,Se), ii) defects, iii) problems at interface, and iv) thick Mo(S,Se)₂ at the rear interface.[1] To solve these problems, NaF is doped into the precursor during the CZTSSe solar cell process. Na plays a role in alleviating the above-mentioned problems by improving the quality of the absorber layer and the interface in the process of forming the CZTSSe absorber layer. Although much work has been done on the best method to incorporate Na and optimized concentrations, little is known on the migration properties of Na⁺ ions once they are introduced into the absorber. Being charged particles, Na⁺ ion distribution throughout the cell post device fabrication will be determined by a contribution of both built-in-potential and diffusion forces. In this work, we present evidence that Na as well as several ionic species (Se and Zn), do not remain stationary after device fabrication, but in fact migrate under electrical biasing. Furthermore, this ionic migration can be manipulated at room temperature by exposing the device to an external electric forming filed. We outline a novel procedure that can effectively control and adjust ionic movement and associated local distribution in fully fabricated devices. Our results show that this simple treatment leads to favorable improved device performance and provides insight into light-induced reduction in performance which may be partially reversible.<br/>[1] J. Mater. Chem. A, 2020, <b>8</b>, 21547-21584