New Insight of Li-Doped Cu2ZnSn(S,Se)4 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 Cu2ZnSn(S,Se)4 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 × 10-4) 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.