Structural Reorganization Drives Exciton Relaxation Pathways in Layered 2D Ruddlesden–Popper (RP) Perovskite BA ₂ PbI ₄

ABSTRACT 2D halide perovskites are promising materials for optoelectronics due to their strong excitonic effects and soft, dynamically active lattices. Synthesis conditions, particularly thermal annealing, play a critical role in tuning their structural and excitonic properties by influencing lattice vibrations and defect states. The impact of structural reorganization in 2D Ruddlesden–Popper (RP) n‐butyl ammonium lead iodide (BA ₂ PbI ₄ ) has been systematically characterized using various state‐of‐the‐art experimental techniques, such as temperature‐dependent X‐ray diffraction (XRD), temperature‐dependent photoluminescence (TDPL), temperature‐dependent resonance Raman spectroscopy, terahertz time‐domain spectroscopy (THz‐TDS), transient absorption spectroscopy (TAS), and further supported by first‐principles DFT calculations, reveals a direct link between thermal processing and structural dynamics. Raman spectra show broadened low‐frequency modes in the annealed sample, indicative of enhanced lattice anharmonicity. THz‐TDS reveals stronger phonon absorption near 2 THz, aligning with Raman‐active modes and confirming increased lattice anharmonicity. The 2 THz phonon mode in the annealed film exhibits a nearly threefold increase in oscillator strength (OS), calculated by integrating the real part of the optical conductivity between 0.2 and 2.5 THz, increasing from 39.01 S m ^{− 1} THz in the as‐grown film to 146.19 S m ^{− 1} THz after annealing, indicating enhanced exciton–phonon coupling; this is further complemented by TDPL measurements, which show more pronounced self‐trapped exciton (STE) emission in the annealed film below ∼270 K, collectively corroborating strong exciton–phonon coupling. Transient absorption spectroscopy shows longer carrier lifetimes (∼1.7 ns) in the annealed film vs. the as‐grown (∼1.1 ns), consistent with increased exciton localization. Thermal annealing boosts lattice dynamics and exciton–phonon coupling, offering a strategy for future low‐dimension material design.