Higher efficiency CuZnSnS (CZTS) based solar cells have been based on absorbers synthesized under Cu-poor and Zn-rich compositions in order to avoid the formation of Cu-S and Cu-Sn-S secondary phases, and favor V vacancy defects. Raman spectroscopy has demonstrated its potential as a characterization tool for CZTS, allowing the detection of the main secondary phases expected in these conditions. However, these non-stoichiometric conditions also have a strong impact on the crystalline quality of the CZTS films, and can lead to structural disorder. This in turn has a potentially significant impact on the spectral features of the main peaks in the Raman spectra. However, evaluation of these effects in CZTS requires a deeper knowledge of the vibrational properties of this material.

In order to deepen in the understanding of the CZTS vibrational properties, this work presents a detailed analysis of all Raman active modes of CZTS prepared by sulfurization of metallic precursors deposited by DC-magnetron sputtering onto soda-lime glass substrates. This process has led to device efficiencies around 5.5%. The surface Raman spectra have been measured using six different excitation wavelengths from NIR to UV, which include non-resonant and resonant conditions with Γ and Γ points of the electronic band diagram.

Combining the information of all spectra, 18 peaks have been resolved, 5 of which were not observed previously, and have been assigned with the 27 optical modes theoretically expected for this crystalline structure allowing the complete characterization of the vibrational properties of the CZTS kesterite structure.

To analyze the impact of the defects in the Raman spectra CZTS samples of different crystalline quality have been prepared using the same methodology as for the reference samples, but with different annealing times (from 0 to 300 minutes). Variation of the Raman shift and shape of the two main peaks at 287 and 338 cm assigned both to A symmetry modes has been observed. These variations are assigned to confinement effects due to the crystalline lattice periodicity breaking determined by the presences of the defects in the crystalline structure, which has been modeled and quantified using a phonon confinement model.

Simultaneous fitting of the two dominant peaks in the spectra is used to determine both the correlation length and stress induced shift components, that have been correlated with the grain size and crystalline quality assessed by complementary techniques (XRD and SEM). The estimated correlation length is shown to constitute a suitable quantitative indicator of the crystalline quality for these materials. Finally, a simple experimental methodology based on these non-destructive optical measurements is proposed for the structural assessment of these films. This methodology is compatible with micro-scale analysis configurations and can also be extended to CuZnSnSe compounds.

Victor Izquierdo-Roca

Irec, Catalonia Institute for Energy Research

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