Revealing Nanoscale Domains in Cu2ZnSnS4 Thin Films by Catalyzed Chemical Etching

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Abstract
The presence of ZnS precipitates is common in Cu2ZnSnS4 (CZTS) films due to the preferred Cu‐poor and Zn‐rich composition for high‐efficiency CZTS devices. It is important to identify their distribution in the film as it can serve as a fingerprint for the CZTS forming process. However, limited methods are viable to quantify their distribution as they are small in size and few in number. Herein, a fast and cost‐effective chemical treatment is presented that can reveal the nanoscale ZnS domains within CZTS films to be captured under a common scanning electron microscope (SEM). This method exploits a hole‐injection process driven by gold catalysis and the band alignment between CZTS and ZnS. ZnS precipitates as small as 10 nm in diameter can be revealed clearly. Using this approach, a characteristic lognormal size distribution of the ZnS precipitates close to the CZTS film surface with a median diameter of 80 nm and a density of around 2 in a 1 μm2 surface area is demonstrated. Energy‐dispersive X‐ray spectroscopy (EDS) mapping and scanning transmission electron microscopy (STEM) imaging further confirm the existence of ZnS precipitates in the film. This method provides a fast means for capturing fingerprints of ZnS precipitations in CZTS process.

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