Evolution of Cu₂ZnSnS₄ during Non-Equilibrium Annealing with Quasi-in Situ Monitoring of Sulfur Partial Pressure

Chalcogen-based materials like Cu 2 ZnSnS 4 (CZTS) have attracted extensive attention for applications such as photovoltaics and water splitting. However, an inability to monitor the sulfur partial pressure ( P S 2 ) during the non-equilibrium annealing process at high temperatures complicates the synthesis of CZTS with controlled optoelectronic properties. Here we demonstrate that P S 2 can be monitored by investigating the Sn–S phase transformation. We showed that P S 2 drops considerably over the annealing time, causing gradual alterations in CZTS: (i) a change in defect type and (ii) evolution of ZnS and Sn x S y phases. With additional ordering treatment, we observed that the low room-temperature photoluminescence energy usually seen in CZTS can result from insufficient P S 2 during annealing. It is proven that remarkable V oc beyond 700 mV for solar cells with nonoptimal CdS buffer can be repeatedly achieved when CZTS is prepared under a sufficiently high P S 2 . An ordering treatment before CdS deposition can further improve V oc to 783 mV.