Highly-Miscible and Bandgap-Tunable (Li_xCu_1−x)₂ZnSnS₄ Alloys

Doping or alloying lithium into Cu₂ZnSnS₄ (CZTS) has drawn extensive attention recently because it is crucial for fabricating high-performance Cu₂ZnSnS₄ thin film solar cells. Experiments showed that lithium can be alloyed into CZTS with a high content and even completely substitute copper to form a new compound Li₂ZnSnS₄ (LZTS). Using the first-principles calculations, we found that Li₂ZnSnS₄ adopts wurtzite-kesterite as ground state structure with a band gap around 3.29 eV. The valence band offset between Li₂ZnSnS₄ and Cu₂ZnSnS₄ is 2.20 eV. As the Li content increases, the band gap of (Li_xCu_1−x)₂ZnSnS₄ alloy increases rapidly with a big band gap bowing. The mixing enthalpies of (Li_xCu_1−x)₂ZnSnS₄ alloys in the kersterite and wurtzite-kesterite structures are both small, suggesting that the (Li_xCu_1−x)₂ZnSnS₄ alloys can easily form at a low growth temperature. These results demonstrate that the lithium alloying may be an effective way for adjusting the band structure and the optoelectronic properties of CZTS.