Substitution of Elements—From Ternary Chalcopyrite-Type CuInS₂ to Quaternary Adamantines CuBCX₄ with B= Al, Ga, C= Ge, Sn, X= S, Se

Adamantine-type compounds, including kesterites, are currently the most promising material for fully inorganic thin-film photovoltaic technology that is free of critical raw materials and thus offers sustainable solutions. To avoid indium and to find new compounds for absorber layers as well as window materials in thin film solar cells, the I-III-VI₂ chalcopyrite compound family is extended by chemical substitution to quaternary "defect adamantines" such as I-○-III-IV-VI₄ compounds.<br/>Coming from the chalcopyrite crystal structure and replacing one monovalent and one trivalent cation by one IV-valent cation by keeping the tetrahedral coordination, a quaternary compound with cation vacancies can be derived, the so called “defect adamantine”.<br/>Starting from the ternary chalcopyrites, we studied the group of defect adamantines with the general formula ABCX₄ with A = Cu, B = Al, Ga, C = Ge, Sn and X = S, Se. With band gap energies ranging from 1.42eV for CuGaSnSe₄[1] to 2.34eV for CuAlGeSe₄[1], these defect adamantines are interesting materials for photovoltaic applications.<br/>Here we report on the growth of single crystals of quaternary compounds CuBCX₄ with B = Al, Ga, C = Ge, Sn and X = S, Se such as CuAlGeSe₄, CuGaGeS₄ and CuGaSnSe₄ and their structural as well as optoelectronic properties. The single crystals were grown by chemical vapor transport using iodine as a transport agent.<br/>For chemical vapor transport, the growth conditions such as the temperature of the source, the temperature gradient and the concentration of the transport agent are of significant relevance for the formation of gaseous species, the transport of the same and the growth of single crystals. The main challenge here is to control the composition of the gas phase and to optimize the temperature field during growth.<br/>The structure of the grown crystals and single-phase materials were determined by X-ray diffraction. A model of the distribution of cations and vacancies in the chalcopyrite-type structure was determined from the cation site occupancies obtained by X-ray diffraction and from the chemical composition measured by XRF. The band gap energy was determined from the diffuse reflectance measured by UV-VIS spectroscopy.<br/>Reference<br/>[1] J.C. Woolley etal. Jpn.J.Appl.Phys. 19 (1980) Suppl. 19-3, 145-148