Self-Trapped Excitons in 2D Quaternary Metal Selenophosphate AgInP₂Se₆ Crystal With Intrinsic Structural Distortion

The search for materials exhibiting intrinsic self-trapped excitons (STEs) is of great importance for advancing optoelectronic technologies. In this work, we report a two-dimensional quaternary metal selenophosphate crystal, AgInP₂Se₆, which exhibits luminescence characteristics diagnostic of STE formation in the long-wavelength near-infrared region. The emission is characterized by a broad band with a full width at half maximum (FWHM) of up to 523 nm and a Stokes shift of approximately 0.47 eV, associated with structural distortions in the [InSe₆]^9-, [AgSe₆]^11-, and [P₂Se₆]^4- octahedral units. Variable-temperature photoluminescence (PL) and transient absorption (TA) spectra reveal a large Huang-Rhys factor (S ≈ 10.97), an ultrafast formation time of ∼160 fs for the self-trapped state, and a long carrier lifetime exceeding 3 ns. The prolonged carrier lifetime, enabled by the carrier trapping and slow dissociation processes observed under bias, allows AgInP₂Se₆-based photodetectors to achieve high responsivity (1.45 A W^-1), high external quantum efficiency (401%), and high specific detectivity (1.41 × 10¹² Jones). This study provides new insights into the design of materials with intrinsic STEs and offers practical guidance for their application in high-performance photodetectors.