Influence of The Growth Method of Sb₂Se₃ Absorber Layer for Thin-Film Solar Cells

In recent years, photovoltaic devices have gained considerable attention due to their potential to address the environmental problems associated with traditional energy resources, offering a clean and sustainable solution to meet the growing global energy demand through the direct conversion of solar radiation into electricity. Thin film solar cells (TFSC) have emerged as a suitable alternative to the traditional silicon technology. The best results in TFSC have been produced using CIGS and CdTe, but they use materials that are either toxic or scarce in the earth crust, limiting the future implementation of this technology. Other promising materials such as perovskite present toxicity and stability problems and the complexity of the growth control of CZTSSe kesterite-type material limits the device performance.<br/>This is why there is a need for simple and stable compounds, with a low toxicity and earth-abundant materials. Among the many available candidates, binary Sb chalcogenides have emerged as promising alternatives due to their optoelectronic properties and good stability. Among them, Sb₂Se₃ has attracted interest not only as an absorber for photovoltaics, where it has already reached efficiencies above 10% [1], but also in battery and photochemical applications. Sb₂Se₃ presents a high absorption coefficient, which allows to reduce the absorber film thickness to 100-500 nm, a band gap energy of 1.2 eV, p-type conductivity, a high stability, and a low melting point, requiring low processing temperatures. In addition, this material presents a quasi-1D crystal topology, which could be one of its main advantages for high performance PV. This chain-structure provokes anisotropy in electrical conductivity, edge absorption and carrier transport. For this, the control of the fabrication process is crucial to obtain high efficiency solar cells.<br/>In this work, we propose two different methods to prepare Sb₂Se₃ films to be used as absorber in a photovoltaic solar cell. The first one consists in the evaporation of a Sb film on top of Mo/SLG and SLG substrates followed by a thermal treatment in the presence of elemental Se under Ar atmosphere. The second method is the direct evaporation of Sb₂Se₃ into the substrate using vapor transport deposition (VTD). Both methods have been studied to prepare Sb₂Se₃ thin films of different thicknesses in the range 400-1000 nm. The goal of this work is to investigate and compare the properties of Sb₂Se₃ thin films grown by the two deposition methods. For that, the effect of the different fabrication parameters, such as deposition time, maximum temperature, amount of Se added or the temperature difference between source and substrate, on the orientation, morphology, structural, vibrational and optoelectronic properties of Sb₂Se₃ are investigated. The properties of the absorber layer are studied by GIXRD, SEM, AFM, Raman spectroscopy and transmittance measurements. In the case of the VTD process, the variation of substrate temperature is critical for the formation of a compact layer, observing a nanoribbon-type structure when temperature increases up to 400°C. In addition, there is a significant difference on the structure of Sb₂Se₃ when Mo is selenized before the deposition of the absorber layer. However, the selenization of evaporated Sb film leads to a much more compact and uniform morphology. First SLG/Mo/Sb₂Se3/CdS/iZnO/ITO substrate configuration photovoltaic devices are fabricated, achieving efficiencies of 3.8% for the thinnest active layer of 400 nm. Further studies are being performed to optimize the different growth processes that result in an enhanced solar cell efficiency.<br/><br/>[1] Z. Duan et al., Adv. Mater. 2022, 34, 2202969.