Raman and X-ray Photoemission Identification of Colloidal Metal Sulfides as Potential Secondary Phases in Nanocrystalline Cu₂ZnSnS₄ Photovoltaic Absorbers

The aim of this study is to establish reliable spectroscopic fingerprints of compounds that may form as secondary phases in Cu2ZnSnS4 (CZTS) nanocrystals (NCs) synthesized by “green” colloidal chemistry directly in aqueous solutions or during post-processing of NC films for photovoltaic applications. For this purpose, we investigated a series of binary and ternary compound NCs synthesized under the same conditions as the quaternary CZTS NCs. The capabilities of combined Raman and X-ray photoemission (XPS) spectroscopy are used to identify these compounds formed separately and define spectral fingerprints for distinguishing them as possible secondary phases in the spectra of CZTS NCs. Besides the conventional analysis of element ratios and chemical shifts of the core-level peaks in the XPS spectra, the careful analysis of Auger lines and modified Auger parameters is applied to distinguish otherwise similar spectral contributions of different compounds. In the case of CuxS NCs, the binding energy separation between the Cu2p3/2 and S2p3/2 core-levels is used as the additional fingerprint. As a criterion of a certain crystal structure in Raman spectroscopy, we rely not only on frequency positions of particular phonon modes but also on selective probing of different compounds at different (resonant) excitation wavelengths. The reasons of controversial previous reports on Raman spectra of CuxS are revealed, and characteristic Raman spectra of Sn-poor Cu–Sn–S and Sn-poor Zn–Sn–S are proposed. For Cu–Zn–S, a mixture of CuxS and ZnS is formed under the given mild conditions rather than ternary compounds or alloys.