Raman scattering crystalline assessment of polycrystalline Cu2ZnSnS4 thin films for sustainable photovoltaic technologies: Phonon confinement model

Phonon confinement effects in the Raman scattering spectra from polycrystalline Cu2ZnSnS4 thin films synthesized with different crystalline qualities – including photovoltaic grade layers – are investigated in terms of a phonon confinement model. Interpretation of the experimentally obtained spectra required the development of a modified model that includes additional features such as lattice strain. The model has been systematically applied for the simultaneous fitting of the two main A symmetry Raman peaks in the experimental spectra. The experimental data show that the line shape of the Raman peaks is determined mainly by phonon confinement effects, while their frequency is also sensitive to the presence of strain in the lattice. Simultaneous modeling of the A dominant peaks allows a quantitative estimation of the frequency strain shift that has been correlated with the strain measured by X-ray diffraction. Estimation of the correlation length also provides a quantitative indicator of the crystalline quality of the films that is found to correlate with the grain size assessed by scanning electron microscopy. Based on this model, a general and simple methodology for the assessment of the crystalline quality of Cu2ZnSnS4 samples independently of the experimental measurement conditions is proposed through the use of an asymmetry parameter.