First-principle study on phase stability of kesterite Cu2ZnSnS4 for thin film solar cells with off-stoichiometric composition

Cu2ZnSnS4 is a promising and noticeable material for the absorber layer of thin film solar cells. Suitable off-stoichiometic composition of pure kesterite phase is profitable for the photoelectron properties, but such compositions tend to cause the appearance of secondary phases. The stable composition range of pure kesterite phase is difficult to confirm by experiment. In this paper, calculation models of single kesterite phase and other possible phases in Cu-Zn-Sn-S alloy system were constructed and the formation energies E were calculated based on first-principle method. The stability of single kesterite phase with off-stoichiometric compositions was deduced by comparing its E with that of different phase combinations in Cu-Zn-Sn-S alloy system. Following conclusions were drawn. When Cu/(Zn+Sn) ≤ 0.778, single kesterite phase is not as stable as any of phase combinations because of its smallest E, so secondary phases definitely appear. When Cu/(Zn+Sn) = 0.778–0.882, the secondary phases should also appear on equilibrium condition, while they may not appear on non-equilibrium condition because the E difference is relative small. When Cu/(Zn+Sn) = 1.000–0.882, single kesterite phase should be the stablest phase and the secondary phases should not appear on equilibrium condition. All experimental results we could find are in accordance with the calculated kesterite phase stable range. Such a range can provide a profitable guidance for the composition design of Cu-Zn-Sn-S alloys for thin film solar cells. The calculation model and method can be further extended for the prediction of the phase stability in other alloy systems.