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

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This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite type CZTSe by means of neutron powder diffraction. We revealed the existence of copper vacancies (VCu), various cation anti site defects (CuZn, ZnCu, ZnSn, SnZn and CuZn) as well as interstitials (Cui, Zni) in a wide range of off-stoichiometric polycrystalline powder samples synthesized by solid state reaction. The results show, that the point defects present in off-stoichiometric CZTSe agree with the off-stoichiometry type model, assuming certain cation substitutions accounting for charge balance. Additional to the known off-stoichiometry types A to H new types (I to L) have been introduced. For the very first time a correlation between the chemical composition of the CZTSe kesterite type phase and the occurring intrinsic point defects is presented. Additional to the offstoichiometry type specific defects Cu/Zn disorder is always present in the CZTSe phase. In Cupoor/Zn-rich CZTSe, a composition considered as the one that delivers the best photovoltaic performance, mainly copper vacancies, ZnCu and ZnSn anti sites are present. Also this compositional region shows the lowest degree of Cu/Zn disorder.

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In this work, Raman spectroscopy and X-ray diffraction were applied together to evaluate the crystal structure and the phonon modes of photovoltaic grade Cu2ZnSn(SxSe1−x)4 thin films, leading to a complete characterization of their structural and vibrational properties. Vibrational characterization has been based on Raman scattering measurements performed with different excitation wavelengths and polarization configurations. Analysis of the experimental spectra has permitted identification of 19 peaks, which positions are in good accord with theoretical predictions. Besides, the observation of Cu2ZnSnS4-like A symmetry peaks related to S vibrations and Cu2ZnSnSe4-like A symmetry peaks related to Se vibrations, additional Raman peaks, characteristic of the solid solution and previously not reported, are observed, and are attributed to vibrations involving both S and Se anions.

Low-temperature cation ordering is emerging as a critical factor limiting the efficiency of CZTSSe kesterite photovoltaic materials. By means of direct determination of the site occupancies from anomalous X-ray powder diffraction data at the Cu-and Zn-absorption edges, the ordering of Cu þ and Zn 2þ in B-type Cu 2 ZnSnSe 4 (CZTSe) kesterite upon annealing at temperatures below 203 (6) 8C is demonstrated. Anomalous X-ray diffraction on the Cu-and Zn-K absorption edges allows determination of the distribution of isoelectric Cu 1þ and Zn 2þ over the crystallographic sites in a B-type CZTSe kesterite (Cu 1.949(20) Zn 1.059(10) Sn 0.983(10) Se 4 ) powder. By Rietveld refinement the quantitative determination of both Cu-and Zn-occupancy for all relevant sites is achieved. From this, the temperature dependency of a structure-based, quantitative order parameter is determined. The critical temperature of the phase transition is confirmed at 203 (6) 8C. The ordering mechanism is in agreement with a transition from disordered to ordered kesterite. The photoluminescence band maximum shows a closely related temperature dependency, directly demonstrating the effect of cation ordering on the optical properties of CZTSe.

The earth-abundant ternary compound BaZrS3, which crystallizes in the perovskite-type structure, has come into view as a promising candidate for photovoltaic applications. We present the synthesis and characterization of polycrystalline perovskite-type BaZrS3 thin films. BaZrO3 precursor layers were deposited by pulsed laser deposition and sulfurized at various temperatures in an argon-diluted H2S atmosphere. We observe increasing incorporation of sulfur for higher annealing temperatures, accompanied by a red shift of the absorption edge, with a bandgap of E g = 1.99 eV and a large absorption strength >105 cm–1 obtained for sulfurization temperatures of 1000 °C. X-ray diffraction analysis and SEM indicate enhanced crystallization at the higher annealing temperatures, but no evidence for a crystalline solid solution between the BaZrO3 and BaZrS3 phases is found. The charge carrier sum mobility estimated from optical-pump–terahertz-probe spectroscopy indicates increasing mobilities with increasing sulfurization temperature, reaching maximum values of up to ∼2 cm2 V–1 s–1.

The dielectric functions of Cu 2 ZnGeS 4 bulk crystals grown by the Bridgman method were measured over the energy range 1.4 to 4.7 eV at room temperature using variable angle spectroscopic ellipsometry. The observed structures in the dielectric functions were adjusted using the Adachi's model and attributed to interband transitions E 0 , E 1A , and E 1B at ⌫:͑000͒, N͑A͒ :2 / a͑0.5 0.5 0.5͒, and T͑Z͒ :2 / a͑0 0 0.5͒ points of the first Brillouin zone, respectively. The model parameters ͑threshold energy, strength, and broadening͒ have been determined using the simulated annealing algorithm. The decrease in the first gap, E 0 , has been attributed to a higher Ge-S hybridization. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived.