First-principles investigation of structural, electronic, optical, and mechanical properties of a novel Cs₂YAgX₆ (X = F, Cl) double perovskite for UV photodetector

Lead-free halide double perovskites have emerged as promising materials for next-generation optoelectronic devices due to their environmental friendliness, structural stability, and tunable electronic properties. In this work, a comprehensive first-principles investigation of Cs₂YAgX₆ (X = F, Cl) double perovskites is carried out using density functional theory. Structural optimization and elastic constant analysis confirm the thermodynamic and mechanical stability of the compounds. Electronic band structure results reveal a wide bandgap semiconducting nature suitable for ultraviolet (UV) detection, with significant contributions from F-p states in the valence band and Ag-d/Y-d states in the conduction band. Optical analysis demonstrates strong absorption in the UV region, indicating excellent potential for photodetection applications. Mechanical parameters, including bulk, shear, and Young's moduli, further confirm the robustness and ductility of the material for device integration. Device-level simulations highlight superior performance of Cs₂YAgF₆, exhibiting high optical absorption (90–95%), strong carrier generation (∼10 28 m −3 s −1 ) and an internal electric field of approximately −90 kVcm −1 for efficient charge separation. The photodetector achieves an external quantum efficiency of 0.74 and responsivity of 0.33 AW −1 , along with microsecond-scale switching speed. These results establish Cs₂YAgF₆ as a highly promising candidate for efficient and stable UV–visible photodetectors.