The efficiency of kesterite-based solar cells is limited by various non-ideal recombination paths, amongst others by a high density of defect states and by the presence of binary or ternary secondary phases within the absorber layer. Pronounced compositional variations and secondary phase segregation are indeed typical features of non-stoichiometric kesterite materials. Certainly kesterite-based thin film solar cells with an offstoichiometric absorber layer composition, especially Cu-poor/Zn-rich, achieved the highest efficiencies, but deviations from the stoichiometric composition lead to the formation of intrinsic point defects (vacancies, anti-sites, and interstitials) in the kesterite-type material. In addition, a non-stoichiometric composition is usually associated with the formation of an undesirable side phase (secondary phases). Thus the correlation between off-stoichiometry and intrinsic point defects as well as the identification and quantification of secondary phases and compositional fluctuations in non-stoichiometric kesterite materials is of great importance for the understanding and rational design of solar cell devices. This paper summarizes the latest achievements in the investigation of identification and quantification of intrinsic point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterite-type materials.This work focuses on structural variations in kesterite-type compound semiconductors, in particular Cu/Zn disorder and intrinsic point defects, as well as on compositional variations, in particular stoichiometry deviations in the kesterite-type phase and the segregation of related binary and ternary phases.This review provides the vital approaches by discussing results and trends concerning intrinsic point defects and structural disorder, compositional fluctuations, and secondary phases on a macroscopic and microscopic scale and even on the nano-scale. Various analytical methods have been used in these studies.The review comprises five parts:A. Crystal structure, structural disorder, and intrinsic point defects in kesterites B. Raman spectroscopy investigations on kesterites OPEN ACCESS RECEIVED

UV-based resonant Raman spectroscopy is an effective technique for a non-destructive assessment of defects in kesterite materials.

This work presents a complete analysis of Raman active modes of Sb2Se3 measured by six different excitation wavelengths from NIR to UV, under different polarization configurations and at low temperature. Simultaneous fitting of spectra allowed the deconvolution and identification of the 28 Raman peaks obtained in monocrystalline Sb2Se3 sample from the 30 modes predicted by the group theory analysis for this crystalline structure. Analysis of the spectra measured under different polarization configurations yielded the preliminary assignment of the peaks symmetry, while the measurements under low temperature resulted in a fine resolution of the peaks in Raman spectra. Additionally, evaluation of the spectra of the most probable secondary phases under different excitation wavelengths allowed to define the most appropriate measurement conditions for experimental discrimination of their Raman peaks in the spectra of Sb2Se3 based thin films solar cells. The combination of different wavelength allows a non-destructive methodology for high sensitivity detection of main secondary phases of the Sb2Se3.