Lithium/Silver-Doped Cu_2ZnSnS_4 with Tunable Band Gaps and Phase Structures: a First-Principles Study

Doping is an effective approach for improving the photovoltaic performance of Cu_2ZnSnS_4 (CZTS). The doping by substitution of Cu atoms in CZTS with Li and Ag atoms is investigated using density functional theory. The results show that the band gaps of Li_2xCu_2(1-x)ZnSnS_4 and Ag_2xCu_2(1-x)ZnSnS_4 can be tuned in the ranges of 1.30–3.43 and 1.30–1.63 eV, respectively. The calculation also reveals a phase transition from kesterite to wurtzite-kesterite for Li_2xCu_2(1-x)ZnSnS_4 as x is larger than 0.9. The tunable band gaps of Li_2xCu_2(1-x)ZnSnS_4 and Ag_2xCu_2(1-x)ZnSnS_4 make them beneficial for achieving band-gap-graded solar cells.