Stability and electronic structure of Cu2ZnSnS4 surfaces: First-principles study

Currently little is known about the atomic and electronic structure of Cu2ZnSnS4 (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 ((1) over bar(1) over bar(2) over bar) surfaces, and found that the polar surfaces are stabilized by the charge-compensating defects, such as vacancies (V-Cu, V-Zn), antisites (Zn-Cu, Zn-Sn, Sn-Zn), and defect clusters (Cu-Zn + Cu-Sn, 2Zn(Cu) + V-Sn). In stoichiometric single-phase CZTS samples, Cu-enriched defects are favored on (112) surfaces and Cu-depleted defects are favored on ((1) over bar(1) over bar(2) over bar) 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.