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

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.