Sn-Doping Induced Lattice Distortion and Deep-Red Self-Trapped Exciton Emission in 2D Lead Halide Perovskites

Abstract

Two-dimensional (2D) Ruddlesden-Popper perovskites have emerged as a versatile platform for broad-band light-emitting applications. Although pristine (C6H5C2H4NH3)2PbI4 (PEPI), typically yields narrow-band free exciton (FE) emission, the introduction of ns2-metal ions like Sn2+ can trigger broad-band self-trapped exciton (STE) emission by softening the inorganic PbI4 framework. However, distinguishing intrinsic STE states from sub-gap defect transitions in doped systems remains a significant challenge. In this work, we demonstrate robust deep-red STE emission centered at 690 nm in Sn-doped PEPI, characterized by an exceptionally large Stokes shift of 574 meV and a broad emission linewidth. Temperature-dependent photoluminescence (PL) and excitation (PLE) spectroscopy reveal that Sn substitution enhances lattice distortion and generate a large Huang-Rhys factor of 15.14. Notably, two-photon absorption (2PA) PLE measurements show a distinct absence of resonant absorption at the emission wavelength, providing unambiguous evidence that the deep-red emission originates from an intrinsic lattice-relaxed state rather than sub-gap defects. As a proof of concept, we fabricated prototype deep-red light emitting diodes (LEDs) driven by a commercial blue LED chip, exhibiting high color saturation in the red spectral region. Our findings clarify the controversial origin of broad-band emission in ns2-metal doped perovskites and highlight its potential applications in optical communications.