Structural Reorganization Drives Exciton Relaxation Pathways in Layered 2D Ruddlesden-Popper (RP) Perovskite BA2PbI4

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 (BA2PbI4) 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 similar to 270 K, collectively corroborating strong exciton-phonon coupling. Transient absorption spectroscopy shows longer carrier lifetimes (similar to 1.7 ns) in the annealed film vs. the as-grown (similar to 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.