Design Strategy for Influencing Hot Carrier Cooling Dynamics in Low‐Dimensional Perovskites

ABSTRACT The structural landscape of low‐dimensional perovskites, modulated by the organic spacer cation, governs the relaxation of hot carriers. Using transient absorption spectroscopy and molecular dynamics simulations, we show that spacer‐induced octahedral distortions and lattice rigidity define the balance between polaronic screening and phonon‐driven relaxation. We found that the phenethylammonium spacer induces significant distortions and stronger exciton‐phonon coupling, favoring polaron formation and delayed carrier cooling. In contrast, a rigid bithiophenethylammonium spacer suppresses distortions, stabilizes coherent phonons, and accelerates thermalization, while thiopheneethylammonium represents intermediate behavior. The structure‐phonon‐carrier interplay reconciles conflicting views on hot‐carrier dynamics, revealing that soft lattices stabilize polarons, whereas rigid heteroatom‐containing conjugated spacers activate efficient phonon relaxation pathways. This structural picture operates alongside a coupled electronic contribution from spacer‐derived electronic states within the conduction manifold, which provides an additional relaxation pathway, indicating that lattice rigidity and electronic coupling are intrinsically coupled rather than acting independently. These insights provide molecular design principles for tailoring carrier relaxation, guiding perovskite engineering toward optoelectronic devices and the efficient harnessing of hot carriers.