Halide Engineering in Low‐Dimensional Perovskites (BA) 2 PbX 4 (X═Cl, Br, I): Insights Into Structural and Optical Properties for Optoelectronic Applications

ABSTRACT 2D Ruddlesden‐Popper perovskites exhibit strong excitonic behavior, but their performance is simultaneously governed by exciton‐phonon coupling and lattice dynamics. Here, we investigate the halide‐dependent lattice dynamics and its influence on exciton behavior in butylammonium‐based low‐dimensional perovskites, BA 2 PbX 4 (X ═ Cl, Br, I) thin films. Structural studies reveal systematic lattice expansion and reduced octahedral tilting from Cl to I, indicating increasing lattice softness. Temperature‐dependent XRD shows enhanced rigidity in chloride and bromide systems, while the iodide analog exhibits a phase transition. Optical measurements demonstrate a redshift in excitonic absorption and narrowing of the optical bandgap across the series. Temperature‐dependent photoluminescence reveals strong exciton‐phonon coupling in (BA) 2 PbI 4 and (BA) 2 PbCl 4 , leading to self‐trapped excitonic emission. These results reveal a direct correlation between lattice dynamics and exciton‐phonon coupling. Using the same set of materials, photodetector devices have been fabricated with (BA) 2 PbI 4 devices exhibiting the highest detectivity and responsivity.