Boron Trioxide‐Assisted Post‐Annealing Enables Vertical Oriented Recrystallization of Sb ₂ Se ₃ Thin Film for High‐Efficiency Solar Cells

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Crystallization process is critical for enhancing the crystallinity, regulating the crystal orientation of polycrystalline thin films, as well as repairing defects within the films. For quasi-1D Sb₂Se₃ photovoltaic materials, the preparation of Sb₂Se₃ thin films still faces great challenges in adjusting orientation and defect properties, which limits the device performance. In this study, a novel post-treatment strategy is developed that uses a low melting point B₂O₃ coating layer as a flux to drive the recrystallization of Sb₂Se₃, thereby regulating the micro-orientation of thermal evaporation-derived Sb₂Se₃ films and optimizing their electrical properties. Mechanistic investigations show that B₂O₃ exhibits stronger adsorption with (hk1) planes of Sb₂Se₃ to induce a vertical orientation growth of the film, while blocking the volatilization channels of Se and inhibiting Se vacancy defects by interacting with Sb₂Se₃. The Sb₂Se₃ film with [hk1] preferential orientation and suppressed deep-level defects promotes the effective transport of charge carriers in solar cells. As a result, the B₂O₃-treated device delivers a champion efficiency of 9.37% without MgF₂ anti-reflection coating, which is currently the highest efficiency in Sb₂Se₃ solar cells achieved by thermal evaporation method. This study provides a new method and mechanism for regulating optical and electrical properties of low-dimensional inorganic thin films.