Doping is an effective approach for improving the photovoltaic performance of Cu2ZnSnS4 (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 Li2𝑥Cu 2(1−𝑥) ZnSnS4 and Ag2𝑥Cu 2(1−𝑥) ZnSnS4 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 Li2𝑥Cu 2(1−𝑥) ZnSnS4 as 𝑥 is larger than 0.9. The tunable band gaps of Li2𝑥Cu 2(1−𝑥) ZnSnS4 and Ag2𝑥Cu 2(1−𝑥) ZnSnS4 make them beneficial for achieving band-gap-graded solar cells.

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