Ritscher, Anna: Synthesis and Structural Characterization of Semiconductors based on Kesterites. , 2016
Technischen Universität Berlin
Open Access Version
The here-presented thesis provides detailed investigations on the structural characteristics of the chalcogenide Cu2ZnSnS4 (CZTS). This semiconductor material is a promising candidate for absorber layers in solar cells due to its desirable properties for thin film photovoltaic applications. Yet, compared to current used chalcopyrite-based devices, efficiencies are significantly lower. This could be attributed to structural effects. Fundamental understanding of the structural characteristics of potential thin film absorber compounds is crucial for proper materials design in the field of solar technology. Therefore, intensive research is necessary to obtain knowledge on so far unexplored structural features or to certify and extend current literature data. The main objective of the here-presented work was to deepen the understanding of the quaternary sulfide Cu2ZnSnS4. Primary, this implied the full characterization of structural properties of the compound using various diffraction techniques. As the synthesis of phase-pure kesterite powder is a challenging problem, one of the central points of this work was the development of a novel rapid and facile synthesis process leading to single phase material. A mechanochemical approach was successfully introduced, including the reaction of the corresponding binary sulfides in a planetary ball mill followed by an annealing procedure in H2S atmosphere, leading to highly crystalline powder samples. The crystallization of the as-milled powder during annealing was tracked by high temperature X-ray diffraction measurements. With this synthesis method it is nicely possible to control the composition of the synthesized powder. Therefore, it was feasible to prepare single phase stoichiometric as well as off-stoichiometric samples with desired compositions. Phase purity and composition were determined by means of electron microprobe analysis and X-ray absorption spectroscopy. First, structural analysis of a stoichiometric sample was performed using X-ray powder diffraction methods including Rietveld refinement. As copper and zinc are not distinguishable using conventional X-ray diffraction methods, only the Sn/(Cu/Zn) istribution could be disclosed, whereas Cu/Zn order remained unknown. For full cation distribution analysis, neutron diffraction measurements were performed, as copper and zinc show a significant difference in the neutron scattering length. It could be shown that the powder sample adopts the kesterite-type structure with a partial disorder of copper and zinc on the two Wyckoff positions 2c and 2d. Another key issue of the present thesis was the investigation of the order-disorder transition in Cu2ZnSnS4. For this purpose, a series of stoichiometric CZTS samples were synthesized according to the mechanochemical synthesis process and afterwards annealed at different temperatures in a range of 473 – 623 K. Again, neutron diffraction techniques were used to investigate the samples. Detailed structural analysis revealed a Landau-type second order transition from an ordered to a disordered kesterite-type structure at a critical temperature of 552±2 K. Additionally, a fully ordered Cu2ZnSnS4 powder sample (within the standard deviation) was successfully synthesized at 473 K. In a final step, special focus was put on the study of intrinsic point defects in offstoichiometric kesterite. For this purpose, B- and C-type off-stoichiometric samples were prepared and analyzed by means of X-ray and neutron diffraction measurements. It could be shown that it is possible to synthesize phase-pure kesterite samples with a composition far off the stoichiometric point.