Optical properties and secondary phase identification in PLD-grown Cu₂ZnSnS₄ for thin-film photovoltaics

One major hurdle to production of Cu₂ZnSnS₄ (CZTS) thin films for photovoltaic applications is the narrow phase diagram region in which CZTS is expected as a single phase at most temperatures of interest. Unwanted secondary phases such as ZnS, CuxSnSx+1 and SnxSy are thus likely to be included in CZTS films independently of the chosen deposition technique. Identification by standard X-ray diffraction (XRD) of some of those phases is challenging since their diffraction peaks overlap with CZTS peaks. In this study we employ Raman spectroscopy to determine which secondary phases are incorporated in CZTS films grown by pulsed laser deposition (PLD) for a range of laser energies and substrate temperatures. Film properties, such as absorption coefficient, refraction index and thickness are extracted from ellipsometry measurements. The same set of properties is evaluated for chemical-bath-deposited CdS due to its important use as a buffer layer in chalcogenide solar cells. The validity of the optical model used to derive optical constants by ellipsometry is discussed in relation to results from direct measurement methods such as UV-visible spectroscopy, Scanning Electron Microscopy (SEM) and profiling. Identification of secondary phases in CZTS films under different PLD process parameters and their effect on optical constants is an important factor in optimizing the deposition process for production of high-efficiency CZTS solar cells.