Properties of phase transition and interfaces of Cu2ZnSnS4 with hetero-phase junctions

Over the past few years, quaternary Cu2ZnSnS4 (CZTS) has attracted a great deal of attention as the most promising photovoltaic absorber layer, because it is composed of abundant and non-toxic elements. CZTS has two fundamental crystal structures under conventional preparation conditions: kesterite (KS) and stannite (ST). Because of the extremely similar crystal structure and the extremely small crystal binding energy, the phase transition between these two crystal structures is very easy to occur. Therefore, in the actual preparation and application process, it is very likely that there is a phase junction between KS and ST in CZTS material. In this article, based on density functional theory calculations, we first discussed the phase transition process caused by stress in details, which is one of the common phase transitions in experiments. The transition pressure exists at around 31 GPa and the phase transition is most likely to appear along the [100] crystal orientation. On this basis, we calculated and analyzed the structure and properties of five kinds of hetero-phase junctions which are composed of low index crystal planes and have very little interfacial stress. The hetero-phase junctions that composed by (101), (110), and (100) crystal planes present the type-II band alignment, which is beneficial for the photoelectric conversion. While, the hetero-phase junctions that composed by (112) and (001) crystal planes present the type-I band alignment, which offer larger energy barrier for the photo-generated carriers and is disadvantageous for improving photoelectric conversion. These findings can contribute to research on the relevant properties of CZTS absorber layer in the future.