Control of the phase evolution of kesterite by tuning of the selenium partial pressure for solar cells with 13.8% certified efficiency

The control of the phase evolution during the selenization of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) is crucial for efficient solar cells. Here, we regulate the phase-evolution kinetics of Ag-alloyed CZTSSe by applying a positive pressure in the reaction chamber at the initial stage of the annealing process. The partial pressure of Se decreases, reducing the collision probability between selenium molecules and the kesterite precursor during the initial formation of the crystals. This results in the precursor transforming into CZTSSe in a single step, without the formation of secondary phases. CZTSSe forms at relatively higher temperature than conventional methods, leading to high-crystallinity kesterite films with fewer defects. We demonstrate solar cells with a total area efficiency of 14.1% and a certified total area efficiency of 13.8%. This work provides insights into the selenization mechanism and phase evolution of kesterite absorbers, enabling efficient solar cells. Secondary phases or multi-step phase formation lead to poorly crystallized and defective kesterite films. Now Zhou et al. convert precursors into kesterite in a single step, using low partial pressure of selenium, and achieve solar cells with 13.8% certified efficiency.