Oxygen‐Assisted High‐Temperature Deposition of High‐Efficiency Trigonal Selenium Solar Cells

Abstract Selenium (Se), the oldest material used in photovoltaic devices, initiated the development of modern solar technology. Trigonal Se (t‐Se), the most thermodynamically stable and photoactive phase, exhibits remarkable photoelectric properties. However, achieving high‐quality, pure‐phase, and well‐oriented t‐Se films remains a significant challenge due to the high energy barrier for phase transformation and its inherent anisotropy. Here, an oxygen‐assisted high‐temperature deposition strategy is reported, wherein films are grown at temperatures exceeding the selenium re‐sublimation point and under elevated selenium vapor pressure. This approach effectively overcomes the intrinsic thermodynamic limitations, enabling the one‐step fabrication of single‐phase, highly crystalline, and vertically oriented polycrystalline t‐Se absorber layers. The resulting t‐Se absorbers exhibit improved carrier transport, reduced deep‐level defects, minimized surface potential fluctuations, and enhanced grain boundary properties. These advances significantly reduce electrical losses in t‐Se thin film solar cells, leading to a certified efficiency of 7.55%, the highest certified efficiency so far. This work provides a thermodynamically viable route to the growth and application of trigonal Se and other low‐dimensional materials.