Self‐Trapped Excitons in 2D Quaternary Metal Selenophosphate AgInP 2 Se 6 Crystal With Intrinsic Structural Distortion

ABSTRACT 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, AgInP 2 Se 6 , 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 [InSe 6 ] 9− , [AgSe 6 ] 11− , and [P 2 Se 6 ] 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 AgInP 2 Se 6 ‐based photodetectors to achieve high responsivity (1.45 A W −1 ), high external quantum efficiency (401%), and high specific detectivity (1.41 × 10 12 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.