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 Li.Cu2-x,ZnSn(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.