Resumen de Crystalline and vibrational properties of kesterites
Mirjana Dimitrievska
The main objective of this thesis is to deepen the knowledge of fundamental properties of kesterite materials (Cu2ZnSnS4 and Cu2ZnSnSe4) and their solid solutions Cu2ZnSn(S,Se)4, which are used as absorber layers in solar cells. This principally included full characterization of structural and vibrational properties of these materials mainly using various Raman spectroscopy techniques. Special focus is put on the investigation of defect dynamics in kesterites, especially on the experimental identification of defects and their effect on the optoelectronic properties and thus the performance of solar cells devices. Additionally, among the objectives of the thesis was development of Raman based methodologies for the compositional assessment of these materials, as well as obtaining more information regarding the fundamental properties of ZnSSe secondary phase. The results were shown in the series of articles which have been published in high impact peer-review journals. In the first part of the thesis, a complete analysis of all Raman active modes of the stoichiometric Cu2ZnSnS4, Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 compounds was made using six different excitation wavelengths from near infrared to ultraviolet. Based on the first principle simulations, normal displacements of each Raman mode were calculated and provided insightful knowledge about the involvement of atoms in vibrations corresponding to different modes. Additionally defect dynamics in kesterite materials was explored. Asymmetry in the shape of the low frequency region of the main Raman modes has been observed for Cu2ZnSnS4 samples with different crystal quality. This has been attributed to phonon confinement effects which are arising from the loss of translational symmetry in the crystal caused by a high density of defects. Based on this model, a simple methodology independent of measuring conditions is proposed for the quantitative assessment of crystal quality through the correlation length. After this, the effect of specific defect clusters on the Raman spectra and optoelectronic properties was investigated. Combinatorial Cu2ZnSnSe4 thin films with lateral compositional gradients were synthesized and made into solar cell devices (around 200 cells per sample), in order to study the correlation between the optoelectronic properties and absorber composition. Furthermore, detailed analysis of the Raman spectra has allowed investigation of the changes in the relative intensity of Raman peaks in relation to the occurrence of different kinds of defect clusters. Finally, the influence of point defects and secondary phases on the performance of Cu2ZnSnSe4 devices was presented. Last part of the thesis was dedicated to development of a methodology for the quantitative assessment of the anion composition of Cu2ZnSn(S,Se)4 solid solutions using Raman spectroscopy. The methodology is based on the analysis of the integral intensity ratio of Raman bands sensitive to anion vibrations. Finally, a fundamental study, based on experimental and theoretical investigation, of the Raman resonance effects in ZnSSe solid solutions was presented. The Raman scattering intensities of LO ZnS-like and ZnSe-like phonon modes, corresponding to pure S and Se vibrations, respectively, were revealed to be significantly enhanced when excited with 325 nm excitation in the case of S vibrations, and with 455 nm in the case of Se vibrations. This behavior was explained by the interaction of the excitation photons with the corresponding S or Se electronic states in the conduction band, and further confirmed by first principles simulations. In conclusion, the works presented in this thesis are a significant contribution to the study of fundamental properties of materials in general, and kesterites materials in particular. Additionally, due to the unique approach of utilizing Raman spectroscopy with other characterization techniques, these methods could prove to be very successful in structure/function studies of other multinary compounds which are gaining increasing interest for electronic applications.
