New Insight of Li-Doped CuZnSn(S,Se) Thin Films: Li-Induced Na Diffusion from Soda Lime Glass by a Cation-Exchange Reaction.

In our recent report (ACS Appl. Mater. Interfaces 2016, 8, 5308), Li ions had been successfully incorporated into the lattice of the selenized CuZnSn(S,Se) thin film on a quartz substrate by substituting equivalent Cu ions, and Li ions was also found to have the little effect on the crystal growth and defect passivation. To further improve the cell performance of Li-doped CZTSSe devices, we conducted the same experiments on the sodium-rich soda-lime glass (SLG) substrate in this study, instead of sodium-free quartz substrate. Surprisingly, only trace amounts of Li (Li/Cu molar ratio ∼1 × 10) were detected in the final CZTSSe thin films; meanwhile, a large amount of sodium was present on the surface and at the grain boundaries of the selenized thin films. A Li/Na exchange mechanism is used to explain this phenomenon. Only on the sodium-free substrate can Li ions enter the CZTSSe host lattice, and doping Li ions on the SLG substrate are nearly identical to doping Na ions.