Tuning the Band Gap of Cu2ZnSn(S,Se)4 Thin Films via Lithium Alloying

Alkali metal doping plays a crucial role in fabricating high-performance Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this study, we report the first experimental observation and characterizations of the alloyed LixCu2–xZnSn(S,Se)4 thin films. It is found that Cu+ ions in Cu2ZnSn(S,Se)4 thin films can be substituted with Li+ ions, forming homogeneous LixCu2–xZnSn(S,Se)4 (0 ≤ x ≤ 0.29) alloyed thin films. Consequently, the band gap, conduction band minimum, and valence band maximum of LixCu2–xZnSn(S,Se)4 thin films are profoundly affected by Li/Cu ratios. The band alignment at the LixCu2–xZnSn(S,Se)4/CdS interface can be tuned by changing the Li/Cu ratio. We found that the photovoltaic parameters of the LixCu2–xZnSn(S,Se)4 solar cell devices are strongly influenced by the Li/Cu ratios. Besides, the lattice constant, carrier concentration, and crystal growth of LixCu2–xZnSn(S,Se)4 thin films were studied in detail.