| Owing to the advantages such as abundance, nontoxicity, adjustable band gaps, high absorption coefficient(?104cm-1), Cu2ZnSnS4(CZTS) compound becomes one of the most promising photovoltaic absorbing materials. However, presently, there is still a wide gap between the actual conversion efficiency(about 12%) and theoretical value(32.3%). The fundamental reason is that the preparation techniques of phase-pure and high-quality films have not been mastered. Therefore, it is especially significant to clarify the synthesis mechanism of CZTS films and explore the relations between technological parameters, microstructures and properties.As a kind of traditional technology, electrodeposition method has many advantages for the preparation of metallic films, such as low cost, low operation temperature(room temperature), simple craft and good compatibility with largescale industrial production. In this thesis, we prepared the precursors of Mo/Cu/Sn/Zn by layer-by-layer electrodeposition, synthesized phase-pure CZTS films through heating treatment at S atmosphere, clarified the kinetic process of electrodepostion and microstructural variation of precusors, revealed the electrodeposition mechanism of each metal, discussed the influence of heating treatment conditions on the microstructures of CZTS films, and revealed the sulphurization mechanism. The main contents and results are as follows:1. Nanometer-sized dense Cu films with the thickness of about 250 nm on Mo substrates were prepared by electrodeposition in the aqueous solution of copper sulfate(CuSO4) and trisodium citrate(C6H5Na3O7). Through measuring cyclic voltammetry(CV), polar curves of steady state and transient current-time curves by means of electrochemical analysis equipment and characterizing the film microsturcutres by SEM and XRD, we found that the diffusion process of copper ion is the determined step of deposition velocity and the growth of copper nuclei obeys a 3D nucleation mode. Based on the Scharifker-Hills model, the nucleation mechanism is revealed to be instantaneous nucleation. The potential, concentrations of copper sulfate solution and additive PEG-6000 affect the microstructures of Cu films.2. Sn films have been successfully prepared on Mo substrates covered with Cu by the electrodeposition method from the electrolyte solutions of tin sulfate(SnSO4) and trisodium citrate(C6H5Na3O7). The electrodeposition velocity is mainly controlled by the diffusion process of tin ions. Based on the Scharifker-Hills model and microstructural analysis, the Sn deposition mechanism is clarified to be instantaneous nucleation, and the formation process of Sn film includes nuclei germination, nuclei coalescence, crystallization and densification. The potentials and solution concentrations of SnSO4 affect the microstructures of Sn films.3. Dense Zn films have been prepared on Mo substrates by electrodeposition from the electrolyte solutions of zinc chloride(ZnCl2), potassium chloride(KCl) and boric acid(H3BO3). The nucleation growth of zinc obeys a 3D diffusion kinetics. Based on the Scharifker-Hills model, the involved nucleation mechanism of zinc is dependent on the growth conditions such as main salt ZnCl2 concentrations and organic additives. The additive PEG-6000 doesn’t affect the complex of zinc ion and chloridion, but changes the orientation of Zn grains, leading to the change of the nucleation mechanism from progressive to instantaneous.4. Utilizing the above optimal depositon conditions of Cu, Sn and Zn films, the precursors Mo/Cu/Sn/Zn have been prepared by layer-by-layer electrodeposition, and then the phase-pure CZTS films were synthesized by heating treatment at S atmosphere. The phase compostions of films were affected by heat treatment temperature and the ratio of metal elements. With increasing the temperarure, binary metal sulfides were firstly formed, then CuSn2S3 was obtained, and finally CZTS was synthesized through reacting wiht ZnS. XRD and Raman spectra show that the phase-pure CZTS films were obtained. |
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