Effect of Geometrical Dimensions on the Structural and Excited State Dynamics in Halide Perovskites

The crystalline structure of semiconductor materials significantly alters their photophysical properties. Halide perovskites have rapidly become a cornerstone in optoelectronics due to their exceptional optoelectronic properties, including high absorption coefficients, long carrier diffusion lengths, and remarkable defect tolerance. The geometrical dimensionality of halide perovskites plays a crucial role in determining their optoelectronic properties, particularly in both steady-state and excited-state dynamics. However, the influence of dimensional tuning on the hot carrier dynamics and recombination pathways in methylammonium lead bromide (MAPbBr₃) perovskites remains insufficiently explored. In this study, we systematically investigate the impact of geometric dimensionality on the structural and excited-state properties of MAPbBr₃, ranging from bulk single crystals to nanocrystalline forms. Our results show that reducing the size from single crystals to nanocrystals (NCs) leads to a significant bandgap widening, from 2.16 eV to 2.74 eV, accompanied by a decrease in crystallite size. Ultrafast transient absorption spectroscopy reveals that hot carrier relaxation occurs more rapidly in NCs (13 ps) compared to polycrystalline thin films (39 ps). Furthermore, the carrier recombination lifetime is extended in bulk forms, which we attribute to band dispersion effects resulting from enhanced energy level overlaps as the material transitions from nanoscale to bulk dimensions. These findings provide critical insights into the role of dimensionality in tuning the photophysical behavior of halide perovskites, offering valuable guidance for their application in next-generation optoelectronic devices.