Novel Chalcogenides for Photovoltaics—Crystal Growth, Structure and Properties

Chalcogenides are becoming increasingly important materials that combine ideal properties as solar absorbers with earth-abundant and low-toxic components. Quaternary 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.<br/>The properties of materials are specifically influenced by the combination of elements and intrinsic point defects formed due to off-stoichiometric compositions. The latter is especially important in quaternary adamantines (e. g. in Cu₂ZnSnSe₄, CZTS and Cu₂ZnSnS₄, CZTSe). A substitution of Cu⁺ and Zn²⁺ by a three valent cation results in the formation of a defect adamtine like Cu■GaSnS₄ (the symbol ■ indicates a cation vacancy) [1]. Nowadays this class of materials undergoes a renaissance because of their potential for photovoltaic applications.<br/>Using barium with its larger ionic radius (1.42 Å, 8-fold coordination) instead of the smaller zinc in CZTS or CZTSe (0.60 Å and 4-fold coordination), a quaternary alkaline earth metal chalcogenide, e.g. Cu₂BaMX₄ with M=Sn, Ge and X=S, Se, can be derived. This new class of wide bandgap Cu⁺-based quaternary chalcogenides are p-type semiconductors due to Cu deficiency, suggesting that Cu vacancies are the intrinsic hole producers [2].<br/>In parallel, ternary ABS₃chalcogenide perovskites, such as BaZrS₃, as well as binary chalcogenides, e.g. Sb₂S₃, play an important role in the search for new photovoltaic materials [3,4].<br/>Here we report on the single crystal growth of quaternary, ternary and binary chalcogenides, their structural and optical properties.<br/>Single crystals of Cu■GaSnS₄, Cu₂BaGeS₄, BaZrS₃ and Sb₂S₃ were grown by chemical vapor transport with iodine as transport agent. The crystal structure of the grown crystals was determined by X-ray diffraction. The chemical composition, measured by XRF, has a significant influence on the atomic structure and lattice parameters of these semiconductors. Depending on the growth conditions, the composition can vary widely for selected materials. The band gap energy of the synthesized semiconductors was determined from diffuse reflectance measured by UV-VIS spectroscopy. The presentation will show the relations between chemical composition, off-stoichiometry, atomic structure and band gap energy in these different new photovoltaic materials.<br/>References:<br/>[1] Y. Tomm, D.M. Többens, G. Gurieva, S. Schorr, <i>Crystals</i> 13 (2023)1545<br/>[2] J. Ge, Y. Yu and Y. Yan, <i>ACS Energy Lett.</i> 1 (2016) 583<br/>[3] S.P. Ramanandan, A. Giunto. E.Z. Stutz, B. Reynier, I.T.F.M. Lefevre, M. Rusu, S.Schorr, T. Unold, A. Fontcuberta I. Morral, J.A. Márquez, M. Dimitrievska, <i>J. Phys. Energy</i> 5 (2023) 014013<br/>[4] R. Kondrotas, C. Chen, J. Tang, <i>Joule</i> 2 (2018) 857