Cu2ZnSnSe4: sonochemistry synthesis and characterization
University of Delaware
With the development of solar cell industry, research on the synthesis of new photovoltaic materials has been developing rapidly. The materials attracting much attention today are the Cu(In,Ga)(S,Se)2 (CIGSe) materials due to their direct band gap and high absorption coefficient. However, In and Ga in this type of materials are not earth abundant elements. This has made large-scale production of devices based on CIGSe materials very costly. As a substitute, the Cu2ZnSn(S,Se)4 (CZTSe) materials are more environmentally friendly and cheaper compared to CIGSe. CZTSe materials also have direct band gaps and are suitable for solar cell application. These materials have delivered a rather satisfactory performance on devices so far as well. Additionally, Zn and Sn are much cheaper and have more abundant sources. This research conducts an investigation on the reaction path for the synthesis of the CZTSe particles using sonochemsitry. Sonochemistry is a method of using ultrasound irradiation to induce rapid chemical reactions. In this research, the stoichiometrically mixed elemental precursors were first ultrasonicated in the organic solvent 2-cyanopyridine and then dried on the hot plate to be transferred into furnace for annealing in Ar atmosphere. A rapid reaction between Cu and Se was obtained directly after sonication according to x-ray diffraction (XRD) analysis. The effective heat of formation (EHF) model was applied to predict the phase formation sequence. The results obtained from the synthesis matched well with the model. Moreover, to study the formation paths, a crystallographic model was applied to determine the reaction potential between different phases. First the Bravais-Niggli-Donna-Harker (BNDH) law was used to predict the most probable facets for the reactant phases. These phases are then compared with each other to check for epitaxial relations by looking for common structures in their most probable crystal planes. Those phases containing structural similarities in at least one of the most probable facets are considered suitable for epitaxy. Epitaxial relations between reactants have been shown to be able to remarkably enhance reaction rates by minimizing the diffusion required for the formation of product lattices. Based on this model, Hergert et al proposed two formation paths for the CZTSe synthesis. The two paths are confirmed by the experiment results in this research. XRD is the major technique used for phase identification. The quaternary CZTSe phase obtained with sonication and post annealing was further confirmed by Raman spectroscopy and UV-Vis results.