Substrate and temperature dependence of the formation of the Earth abundant solar absorber Cu₂ZnSnS₄ by ex situ sulfidation of cosputtered Cu-Zn-Sn films

Copper zinc tin sulfide (CZTS) thin films were synthesized by ex situ sulfidation of Cu-Zn-Sn metal alloy precursor films cosputtered from Cu, Cu-Zn, and Cu-Sn targets onto five different substrate materials: single crystal quartz, fused quartz, sapphire, Pyrex, and soda lime glass (SLG). Cosputtered precursor films, which were found to consist of Cu, Zn, and Sn metals and Cu_6.26Sn₅ ordered alloys, were sulfidized between 100 and 600°C, corresponding to an S pressure range of 0.051-36 Torr. While CZTS forms at temperatures as low as 300°C on all substrates, the film's phase composition is dominated by binary metal sulfides between 300 and 400°C. Significant phase composition variations among films synthesized on different substrates begin to emerge at 400°C. Films grown on SLG are nearly phase pure CZTS by 500°C, with small amounts of ZnS. In contrast, films deposited on all other substrates persistently contain significant amounts of impurity phases such as SnS₂ and Cu₄Sn₇S₁₆ until the sulfidation temperature is increased to 600°C. Significant grain growth also begins between 500 and 600°C. At 600°C, CZTS films synthesized on SLG were found to have significantly larger grains than films grown on any of the other substrates. These results demonstrate that CZTS phase purity and grain size, properties that may affect solar cell performance, are affected by impurity diffusion from the SLG substrate, further emphasizing the importance of selecting appropriate substrates.