Investigation of Combinatorial Coevaporated Thin Film Cu2ZnSnS4. I. Temperature Effect, Crystalline Phases, Morphology, and Photoluminescence

Cu₂ZnSnS₄ is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325°C) and high (475°C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu₂ZnSnS ₄-Cu₂SnS₃ tie line in the quasi-ternary Cu ₂S-ZnS-SnS₂ phase system, and this effect is attributed to structural similarity between the Cu₂ZnSnS₄, Cu ₂SnS₃, and ZnS crystalline phases. At 475°C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling along the ZnS-Cu₂ZnSnS₄-Cu ₂SnS₃ tie line. Room-temperature photoluminescence imaging reveals a strong correlation for these samples between film composition and photoluminescence intensity, where film regions with Cu/Sn ratios greater than ∼2 show strong photoluminescence intensity, in comparison with much weaker photoluminescence in regions that received excess Sn flux during growth or subsequent processing. The observed photoluminescence quenching in regions that received excess Sn flux is attributed to the effects of Sn-related native point defects in Cu₂ZnSnS₄ on non-radiative recombination processes. Implications for processing and performance of Cu ₂ZnSnS₄ solar cells are discussed.