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 Cu 2 ZnSn(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.