A Vapor Pressure‐Mediated Annealing Strategy for Vertically Oriented Trigonal Selenium Films Enabling Efficient Solar Cells

ABSTRACT Selenium (Se), as the oldest photovoltaic material, has regained research interest recently due to the wide bandgap, high stability, and non‐toxicity of its trigonal phase ( t ‐Se). However, its device performance is fundamentally limited by the strong anisotropic carrier transport inherent in its typical low‐dimensional crystal orientation, where Se chains lie parallel to the substrate. Herein, we report a vapor pressure‐mediated annealing strategy, namely dubbed dual‐sheath annealing (DSA), to overcome this bottleneck. By recrystallizing amorphous Se films under a precisely controlled high Se vapor pressure, we direct a surface re‐evaporation and re‐deposition process that induces a novel top‐down growth mode. This method successfully fabricates t‐Se films with a dominant vertical [101] orientation, a texture that facilitates efficient longitudinal charge transport. The resulting films feature large, smooth grains, which further enable robust coupling with a 2D MXene hole‐transport layer via Coulomb force. The constructed FTO/TiO 2 /t‐Se/MXene solar cells achieve a champion power conversion efficiency of 6.5%, which surpasses the performance of devices from traditional annealing by 33.74% and outperforms reference devices with Au electrodes by 11.11%. This work establishes vapor pressure control as a powerful and general thermodynamic lever for tailoring crystal orientation in low‐dimensional semiconductor films for high‐performance optoelectronics.