Heat-field-stimulated decomposition reaction in Cu2ZnSnS4

Thermal stability is essential for the potential solar cell material Cu2ZnSnS4 (CZTS) in achieving a satisfactory photovoltaic device performance. Although the loss of Sn from CZTS has been reported, the basic decomposition mechanism of a CZTS system has not been well established yet, especially with regard to the role of active Cu1+ ions. This paper not only provides a deeper understanding of the change of Sn species, which includes an equimolar–isobaric vaporization mode transition and a solid–vapor phase transition in a self-generated atmosphere, but also reveals the oxidation state alternation (Cu1+/Cu2+) and transfer mechanism of Cu species through carefully designed experiments and a reaction kinetic study. Cu ions are unexpectedly found to be active in affecting the degradation reaction by valance alternation and ion movement upon the application of a heat field to balance the derivation caused by a non-uniform temperature gradient. As a result, a Cu–Zn separation appears, with Cu accumulating near the hot area and Zn near the cold area. A decomposition reaction model of CZTS under a directional heat field is proposed to describe the elemental and electronic state change in atomic scale, and a perfect match is obtained between the model and the experimental results. This paper paves a way to solve the thermal stability issue of Cu2ZnSnS4.