An Investigation of Phase Stability in the Cu-Zn-Sn-X (X=S, Se) Photovoltaic Systems

The research on new energy sources, in particular renewable and environmentally clean ones, started recently because of the limited availability and the pollution produced by fossil fuels. The sun is one of the cleanest and most abundant energy sources. The photovoltaic industry is mostly based on silicon technologies. Silicon is an indirect band gap material and is needed to produce perfect mono-crystals in order to obtain high efficiency solar modules, resulting in an increase in the costs [1]. As a consequence, the research on thin film technology is being developed and is based on direct bad gap materials such as CIS (CuInSe2), CIGS (CuInxGa1-xSe2 with 0<x<1) and CdTe [1]. Although the efficiency of these cells is around 20% [2], their use is limited because of the toxicity of and the scarce availability of the involved elements. Firstly, the Cu2ZnSnS4 (CZTS) phase was proposed as a non-toxic, low cost and easily available semiconductor, and then the Cu2ZnSnSe4 (CZTSe) phase was proposed to replace the sulfide in order to improve the efficiency of the cells. During the investigation of the bulk properties of the Cu-Zn-Sn-S and the Cu-Zn-Sn-Se systems, the existence of a new quaternary phase was observed. Samples with different compositions along the ZnS-Cu2SnX3 (X = S or Se) section were prepared from the powders of the pure elements and the compacts were subjected to different thermal treatments. The Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) allowed us to detect the phases: CZT(S,Se), Cu2SnX3, SnX and ZnX solid solution already known [3]. In addition, we observed a potentially new quaternary phase having Cu2-2yZn6+3ySn1-yX9 (0<y<4.1) composition. The stability and the composition of this phase is strongly influenced by heat treatments. References: [1] M. P. Suryawanshi et al. Materials Technology, 28 (2013) 98. [2] M. A. Green et al. Prog. Photovolt. Res. Appl., 22 (2014) 1. [3] I. D. Olekseyuk et al., JALCOM, 368 (2004) 135.