Problems and Possible Solutions for Antimony Selenide Interfaces

Anisotropic materials such as antimony selenide (Sb₂Se₃) are emerging as excellent candidates for thin film application, due to the unique optical and electrical properties related to its structure. The material consists of quasi-one dimensional (Q-1D) ribbons with strong covalent bonding in one direction, and weak van der Waals forces in the other two. This Q-1D structure confers to Sb₂Se₃ anisotropic characteristics, i.e., their optical and electrical properties strongly depend on the orientation of the ribbons. Some of these properties are of special interest for photovoltaic applications, including the preferential carrier transport in the [001] direction, that can be relatively easily tuned during the synthesis process. In addition, Sb₂Se₃ exhibit other interesting advantages such as the relatively low synthesis temperature (320°C) which allows to fabricate efficient devices in substrate and superstrate configuration, an optimum band-gap between 1.2 and 1.3 eV and a high absorption coefficient (&gt;10⁴ cm^-1). The current record efficiency of 9.2% was reported in a device with substrate configuration, and is still far from its theoretical upper limit of about 33.3%.<br/>Even though the last progresses achieved with this material, the substrate configuration (Mo/Sb₂Se₃/CdS/i-ZnO/ITO) inspired in high efficiency Cu(In,Ga)(S,Se)₂, and accordingly to recent simulations, exhibit fundamental problems at the interfaces, reflected in the major detrimental effect on the Voc deficit of the material. This work focuses on elucidating the possible interfaces issues on Sb₂Se₃ substrate solar cells, by investigating the intermixing between the absorber and the selective contacts, the possible presence of secondary phases in the absorber itself or due to the presence of unsatisfied dangling bonds (non-passivated interfaces).<br/>To do so, Sb₂Se₃ was synthesized onto Mo-coated soda lime glass substrates, using a sequential process based on the thermal evaporation of Sb, followed by a reactive annealing under Se atmosphere. Then, different strategies for surface management were employed, namely: i. analysis of different selective surface etchings including KCN, CS₂, HCl, HNO₃,-H₃ PO₄, H₂SO₄ -KMnO₄, (NH₄)₂S, Br; and ii. the development of ultrathin oxides for surface stabilization and passivation by atomic layer deposition, by testing oxides likeTiO₂, Al₂O₃, and HfO₂. We demonstrate that the presence of oxides stabilizes the upper most surface through the formation of robust interfaces, enhancing the resilience and stability of the devices. In addition, KCN selective etching exhibit a remarkable improvement on the devices properties, mainly on the fill factor (FF), and reporting a FF record of 63% and a conversion efficiency over 5%. Based on these results, and through a complete characterization including: photothermal deflection spectroscopy, Raman spectroscopy, x-ray photoemission spectroscopy, x-ray diffraction, x-ray fluorescence, scanning electron microscopy; combined with a complete device’s characterization including spectral response and J-V analysis in light, dark and with temperature; the possible contributions of the different surface recombination origins will be thoroughly discussed.