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.