Eliminating cracking morphology of solution-processed CZTSSe absorbers by Sn-rich composition engineering

Sol-gel method has attracted widespread attention for fabricating highly efficient Cu2ZnSn(S, Se)4 (CZTSSe) solar cells. However, the volume shrinkage of the gel during the baking process will cause severe cracking of CZTSSe films, which results in shunt paths and significantly deteriorates device performance. Herein, an Sn-rich CZTS precursor is proposed to produce high-quality CZTSSe films, which could relieve the shrinkage forces in CZTS precursor film and compensate for Sn loss of CZTSSe absorbers during selenization process. The effects of Sn content on CZTS precursor compactness and the post-selenization process on the crystallization quality of CZTSSe films are thoroughly investigated. Furthermore, the formation of the MoSe2 interfacial layer and the SnSe2 secondary phase can be regulated efficiently. As a result, the separation and collection of carriers are greatly improved as the depletion region width (Wd) of the CZTSSe solar cell is broadened. And the difference between the activation energy (Ea) and band gap (Eg) has decreased, indicating that the carrier recombination loss caused by the deviated Sn content in CZTSSe films has been effectively reduced. Consequently, the efficiency of the CZTSSe device increased from 7.6% to 9.3% through Sn-rich composition engineering. This study elucidates the role of Sn content in the fabrication of high-quality CZTSSe films and advances the development of ambient air-processed CZTSSe solar cells.