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CZTS Raman spectra beyond kesterite: a first-principles study

Cu$_2$ZnSnS$_4$ is an earth-abundant photovoltaic absorber material predicted to provide a sustainable solution for commercial solar applications. One of the main limitations restricting its commercialization is the issue of cation disorder. Raman spectroscopy has been a sought after technique to characterize disorder in CZTS while a clear consensus between theoretical and experimental results is yet to be achieved. In the present study, via the virtual crystal approximation, we take into account the progressive nature of Cu-Zn disorder in CZTS: we obtain the phonon frequencies at zone-center within the density functional perturbation theory formalism, and further compute the Raman spectra for the disordered phases, achieving a consensus between theory and experiment. These calculations confirm the presence of complete disorder in Cu-Zn 2$a$, 2$c$ and 2$d$ Wyckoff sites. They also show that the Raman intensities of two prominent $A$ phonon modes characterized by motion of S atoms, also known to be experimentally significant, play a key role in understanding the nature of disorder in CZTS.