Currently little is known about the atomic and electronic structure of Cu${}_2$ZnSnS${}_4$ (CZTS) surfaces, although the efficiency of kesterite-based solar cells has been increased to over 11$\%$. Through the first-principles calculations, we studied the possible surface structures of the frequently observed cation-terminated (112) and anion-terminated ($\overline{1}\overline{1}\overline{2}$) surfaces, and found that the polar surfaces are stabilized by the charge-compensating defects, such as vacancies (V${}_{\mathrm{Cu}}$, V${}_{\mathrm{Zn}}$), antisites (Zn${}_{\mathrm{Cu}}$, Zn${}_{\mathrm{Sn}}$, Sn${}_{\mathrm{Zn}}$), and defect clusters (Cu${}_{\mathrm{Zn}}$+Cu${}_{\mathrm{Sn}}$, 2Zn${}_{\mathrm{Cu}}$+V${}_{\mathrm{Sn}}$). In stoichiometric single-phase CZTS samples, Cu-enriched defects are favored on (112) surfaces and Cu-depleted defects are favored on ($\overline{1}\overline{1}\overline{2}$) surfaces, while in non-stoichiometric samples grown under Cu poor and Zn rich conditions both surfaces favor the Cu-depleted defects, which explains the observed Cu deficiency on the surfaces of the synthesized CZTS thin films. The electronic structure analysis shows that Cu-enriched surfaces produce detrimental states in the band gap, while Cu-depleted surfaces produce no gap states and are thus benign to the solar cell performance. The calculated surface properties are consistent with experimental observation that Cu-poor and Zn-rich CZTS solar cells have higher efficiency.

• Received 24 April 2013

DOI:https://doi.org/10.1103/PhysRevB.88.045427