This work presents a complete analysis of all Raman active modes of Cu₂ZnSnS₄ measuring with six different excitation wavelengths from near infrared to ultraviolet. Simultaneous fitting of spectra allowed identification of 18 peaks from device grade layers with composition close to stoichiometry that are attributed to the 27 optical modes theoretically expected for this crystalline structure, including detection of 5 peaks not observed previously, but theoretically predicted. Resonance effects are assumed to explain the observed increase in intensity of weak modes for near infrared and ultraviolet excitations. These results are particularly relevant for experimental discrimination of Raman modes related to secondary phases.

Solid lithium and sodium closo-polyborate-based salts are capable of superionic conductivities surpassing even liquid electrolytes, but often only at above-ambient temperatures where their entropically driven disordered phases become stabilized. Here we show by X-ray diffraction, quasielastic neutron scattering, differential scanning calorimetry, NMR, and AC impedance measurements that by introducing “geometric frustration” via the mixing of two different closo-polyborate anions, namely, 1-CB₉H₁₀^– and CB₁₁H₁₂^–, to form solid-solution anion-alloy salts of lithium or sodium, we can successfully suppress the formation of possible ordered phases in favor of disordered, fast-ion-conducting alloy phases over a broad temperature range from subambient to high temperatures. This result exemplifies an important advancement for further improving on the remarkable conductive properties generally displayed by this class of materials and represents a practical strategy for creating tailored, ambient-temperature, solid, superionic conductors for a variety of upcoming all-solid-state energy devices of the future.

This work presents a detailed analysis of the impact of compositionally induced defects on the vibrational properties of Cu₂ZnSnSe₄ absorbers for kesterite based solar cells. Systematic changes in the intensity of the E and B modes located around the 170, 220, and 250 cm⁻¹ frequency regions, which involve mostly cation vibrations, were observed and analyzed in relation to the occurrence of different kinds of defect clusters involving V_Cu, Zn_Cu, Zn_Sn, Cu_Zn, and Sn_Zn point defects. Additional changes are also interpreted in terms of the appearance of SnSe, ZnSe, and CuSe-like contributions at the 185 and 250 cm⁻¹ spectral regions, respectively. The sensitivity of the Raman measurements to the presence of these kinds of defects corroborates the potential of Raman scattering for point defect assessment in these systems.

Polycrystalline kesterite Cu₂ZnSnSe₄ (CZTSe) semiconductors have drawn attention as promising absorber candidates for the next generation of thin film photovoltaics. However, the narrow tolerance to stoichiometry variations, which favours the formation of secondary phases and cluster defects, is a challenging drawback that restrains the device performance. In this context Raman spectroscopy has been demonstrated to be a powerful tool for the characterization of CZTSe, allowing the assessment of relevant parameters such as structure, crystal quality and secondary phases. In this work, multiwavelength Raman scattering measurements using nine excitation wavelengths (from 325 to 1064 nm) were performed on CZTSe polycrystalline thin films in order to give a detailed identification of all the active Raman modes expected in this crystalline structure. The experimental results compare well with the vibrational properties that have been computed by first-principles calculations based on density functional theory. Calculations of the phonon density of states (PDOS), as well as the simulation of the Raman spectra, have allowed a better understanding of the experimentally observed vibrational modes. A “non-band gap resonant” Raman effect was observed under 325 nm excitation conditions, making ultra-violet (UV) Raman spectroscopy a very promising characterization technique due to its high sensitivity to the surface region which provides information on the buffer/absorber interface. Strong intensity enhancement of spectral regions around 176 cm⁻¹ and 250 cm⁻¹ is observed. Calculated PDOS and combinatorial experiments on more than 200 samples with stoichiometries close to the high efficiency device compositions lead to the correlation of these intensities with the concentration of V_Cu and Zn_Sn point defects, respectively. Correlation with the optoelectronic properties of the devices allowed identification of the optimal concentration of V_Cu and Zn_Sn point defects for the highest performance devices. The application of the presented methodology has been confirmed by performing the same Raman analysis on the CZTSe absorber with 11% device efficiency. This work demonstrates that defect engineering in CZTSe is of the foremost importance for the improvement of solar cell performance, and that UV-based Raman spectroscopy is an effective technique for the non-destructive assessment of defects in kesterite materials.