Kesterite thin-films deposited by sol-gel techniques with tunable bandgap as absorbers for photovoltaic applications

Currently, the PV research focuses on finding low cost and easily processable materials. Here, we propose a simple chemical procedure for synthesizing Earth-abundant chalcogenide thin-films with tunable bandgap, leading to well-defined phases of Cu2XYS4 (with X = Zn, Fe; Y = Sn, Ge). The deposition process is straightforward and very cheap, based on the sol-gel technique, where the 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. Metal acetate precursors were proved to have a primary role in creating a network, during the sol-gel transition, by coordinating and pre-organizing the metals in solution, together with thiourea (as the only source of Sulphur) and DMSO as solvent. Moreover, the addition of dopant amounts of KCl into the precursor solution was experimentally demonstrated to be beneficial for the grain growth and material quality, both crucial for the final solar device performance. XRD, μ-Raman, UV-Vis, and EDX spectroscopy measurements have supported the characterization of the so-synthesized layers (CZTS, Fe-substituted CZTS, Ge-substituted CZTS); their morphology was studied by SEM imaging, confirming the thin-film good quality. The bandgap, obtained from transmittance measurements, ranges from 1.4 to 2.1 eV depending on the combination of the chosen metal precursors, suggesting this class of materials as a suitable candidate as a top absorber in a tandem device architecture and promising single-junction prototypes of working solar-devices have been produced, even on semi-transparent substrate. [1 - 4] [1] V. Trifiletti et al. Chemistry Select 4 (2019) 4905. [2] V. Trifiletti et al. Materials 13 (2020) 1471. [3] G. Tseberlidis et al. Solar Energy 208 (2020) 532. [4] G. Tseberlidis et al. Solar Energy 224 (2021) 134.