Exploring the optimal two-step selenization process to enhance the performance of ionic liquid solution based Cu2ZnSn(S,Se)4 thin film solar cells

Two-step selenization process has been proved to control Cu2ZnSn(S,Se)4 (CZTSSe) crystallization growth, helping to fabricate a homogeneous large-grain structure of CZTSSe absorber. The two-step selenization process is usually composed of the low-temperature nucleation (LT) and high-temperature grain growth reaction (HT) processes. Here, an exploration of optimizing the annealing conditions at each step in the two-step process is reported to obtain the excellent structure of ionic liquid solution based Cu2ZnSn(S,Se)4 absorber, boosting performance of the solar cell. Based on the basic requirement of an absorber candidate for solar cells (the compact films), three effective kinds of two-step process are confirmed (LT400–10&HT540–10, LT400–10&HT550–10, and LT400–10&HT560–10). We further explore the other properties of sample prepared by one-step and these three kinds of two-step process, respectively, and the optimal two-step process can be identified as LT400–10&HT550–10, which also confirms a homogeneous large-grain structure of CZTSSe absorber. Finally, CZTSSe solar cell with the highest efficiency of 9.24% can be achieved, which can be improved 67% than one by one-step annealing process (5.54%). •The detail optimization of annealing conditions at each step in the two-step selenization process is explored.•Homogeneous large-grain structure of Cu2ZnSn(S,Se)4 absorber can be fabricated at the optimal two-step selenization condition.•The best efficiency of corresponding device can be obtained to be 9.24%, increased by 67% compared with one-step process.