Earth-abundant chalcogenide thin-films with tunable bandgap for Photovoltaic applications

Identifying new low cost and low environmental impact energy sources is becoming an imperative need of civil society. Photovoltaic (PV) is the most mature alternative renewable energy technology, led by silicon and whereas chalcogenide-based thin-film solar cells have gain market section… commercialization. Terawatt generation Here we present the synthesis and the characterization of Earth-abundant chalcogenide thin-films with tunable bandgap, leading to well-defined phases of Cu2XYS4 (with X = Zn, Fe, Mn; Y = Sn, Ge). As an alternative to the physical deposition of the metal precursor, here, we propose a straightforward and very cheap synthesis method based on the sol-gel technique. The Cu2XYS4 (with X = Zn, Fe; Y = Sn, Ge) thin films are produced thanks to a direct drop-casting of the precursor solution, followed by a gelation process and heat treatment in argon atmosphere to generate the desired crystalline phase of the quaternary alloy.3 UV-Vis, μ-Raman and XRD have characterized the so-synthesized layers, and EDX spectroscopy measurements and their morphology was studied by SEM imaging, proving the excellent quality of the material. The optical bandgaps of the presented compounds range from 1.4 to 2.1 eV, depending on the combination of the chosen metal precursors, making them suitable to be employed as stand-alone devices or as top solar cells in tandem architecture with various bottom cells. Promising single-junction prototypes of working devices have been produced, even employing semi-transparent substrates.4-6 An exciting alternative to the most studied Cu2ZnSnS4 (CZTS) is Cu2MnSnS4 (CMTS), a p-type semiconductor with Earth-abundant and low-cost elements. Manganese is cheaper (2$/kg vs 2,7$/kg) and has a higher occurrence in the Earth's crust than zinc (440ppm mean vs 65ppm mean), leading to the more economical and environmental sustainability of CMTS in respect of CZTS.1,2 Here we report the CMTS thin films, grown by a two-step deposition process (sputtering, followed by high-temperature annealing in sulphur vapours), produced in collaboration with RSE S.p.A., the industrial partner in the research project. The CMTS-based solar cells reached a record efficiency of about 1% and an open-circuit voltage exceeding 400 mV. Photoluminescence measurements allowed us to identify the detrimental defects acting in working conditions, suggesting paths to improve the photovoltaic properties. 1. S. Marchionna, A. Le Donne, M. Merlini, S. Binetti, M. Acciarri and F. Cernuschi, Journal of Alloys and Compounds, 2017, 693, 95-102. 2. A. Le Donne, S. Marchionna, M. Acciarri, F. Cernuschi and S. Binetti, Solar Energy, 2017, 149, 125-131. 3. V. Trifiletti, S. Mostoni, F. Butrichi, M. Acciarri, S. Binetti and R. Scotti., ChemistrySelect, 2019, 4, 4905-4912.