Lithium/Silver-Doped Cu$_{2}$ZnSnS$_{4}$ with Tunable Band Gaps and Phase Structures: a First-Principles Study

Doping is an effective approach for improving the photovoltaic performance of Cu 2 ZnSnS 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 2x Cu2(1 − x) ZnSnS 4 and Ag 2x Cu2(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 2x Cu 2(1 − x) ZnSnS 4 as x is larger than 0.9. The tunable band gaps of Li 2x Cu 2(1 − x) ZnSnS 4 and Ag 2x Cu2(1 − x) ZnSnS 4 make them beneficial for achieving band-gap-graded solar cells.