StatusThe thesis was presented on the 8 May, 2019
Approved by NCAA on the 9 July, 2019
Abstract– 2.52 Mb / in romanian
6.35 Mb /
The thesis has been written in Romanian language and consists of the introduction, 4 chapters, general conclusions and recommendations, and the list of 199 references. The thesis contains 144 pages, 68 figures, 19 tables and 83 formulas.
The goal: The main goal of the thesis is investigations of the optical and transport fundamental properties of Cu2Zn(Sn,Ge)(S,Se)4 compounds and solid solutions. This suggests determination of the material characteristic parameters depending on the composition and structure, for their optimal use in high quality, ecological and efficient photovoltaic and optoelectronic devices.
Research objectives: The measurement and the analysis of the temperature dependences of the resistivity and magnetoresistance of the Cu2ZnSn(Ge)S4 quaternary compounds and Cu2ZnSnxGe1-xS(Se)4 solid solutions; Determination of the main transport mechanisms, of the macroscopic and microscopic parameters, as well as the establishment of their dependence on the composition of the samples; The measurement and analysis of the ellipsometric spectra using three-phase model for the Cu2ZnSnS4 și Cu2ZnSn(SxSe1-x)4 polycrystals.
Scientific novelty and originality of the results: This work is the first comprehensive study of the resistivity and magnetoresistance within a broad temperature interval, in magnetic fields up to 20 T, of the Cu2ZnSn(Ge)S4 quaternary compounds, as well as of Cu2ZnSnxGe1-xS(Se)4 solid solutions with the composition x between 0.0 and 1.0. For the first time, the influence of the Sn and Ge cation interchange in the Cu2ZnSnxGe1-xS(Se)4 solid solutions on their electronic properties was analyzed. For the first time, the ellipsometric spectra of the Cu2ZnSnS4 and Cu2ZnSn(SxSe1-x)4 polycrystals were analyzed using the three-phase model. The latter provides a realistic description of the air-surface-bulk system and the spectral dependences of the absorption, extinction and normal-incidence reflection coefficients, as well as of the refractive index.
The main scientific problem solved in this thesis consists in the establishment of the main transport mechanisms, in the determination of an important set of characteristic parameters and investigations of their dependence on the composition of Cu2ZnSn(Ge)S4 quaternary semiconductors, along with the Cu2ZnSnxGe1-xS(Se)4 solid solutions, where x = 0.0 – 1.0; as well as investigations of the spectral dependences of the optical constants and the description of the main optical transitions of Cu2ZnSnS4 and Cu2ZnSn(SxSe1-x)4 polycrystals. This provides a better understanding and minimization (or even elimination) of the detrimental effects appearing during obtaining and industrial manipulation processes of the corresponding compounds, for their utilization in a next generation of cost-efficient optoelectronic devices.
Theoretical significance and applicative value of the obtained results, consists of a considerable extention of the knowledge base in the aria of the Cu2Zn(Sn,Ge)(S,Se)4 quaternary semiconductor fundamental properties. This is important for a better understanding and more efficient utilization of their potential in a next generation, of low-cost and ecologically friendly optoelectronic devices.
The implementation of the scientific results: All the obtained results in the frame of this thesis were used to successfully carry out several national and international projects. Also these data will be available for use by the scientific and industrial community for the development of optoelectronic devices based on quaternary semiconductors. Оптические и транспортные свойства твёрдых растворов Cu2Zn(Sn,Ge)(S,Se)4”