Optimized grain growth for efficient solution-processed Bi-doped Cu2ZnSn(S,Se)4 thin film solar cells via spin-coated layers adjustment and two-step selenization

Crystallization growth occupies a crucially important role in terms of impacting the film microstructure and device characteristics of the kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell. Previous studies frequently observed that the selenized CZTSSe film consists of a multiple-layer structure with a large-grained top layer and a fine-grained bottom layer. Herein, a novel strategy is proposed for adjusting spin-coated layers and conducting two-step selenization to promote the fine-grain layers growth of Bi-doped CZTSSe (CZTBiSSe) films for high-performance solar cells. This study systematically investigates the effects of spin-coated layers and the selenization process on growth kinetics of the CZTBiSSe films. By comparison, the optimal spin-coated layer can be identified as "7 + 3", the CZTBiSSe film has a bilayer large grain structure from up to down, which reduces the grain boundaries, thus reducing the charge recombination loss, and is more conducive to charge carrier transfer. Finally, a CZTBiSSe solar cell with the highest efficiency of 7.27 % was obtained, which increased by 36 % more than one prepared using traditional methods (5.34 %).