Ferroelectric Phonon‐Dressed Exciton Polarons Enable Internal Charge Separation in Strongly Confined 2D Perovskites

ABSTRACT Strongly confined two‐dimensional semiconductors exhibit intense excitonic absorption, but their large exciton binding energies generally suppress the internal separation of charge carriers. Here we show that the order‐parameter phonons of improper ferroelectricity provide an excited‐state pathway to overcome this limitation. In the n = 1 improper‐ferroelectric perovskite (Mpda)PbBr 4 , time‐ and spin‐resolved optical spectroscopy, coherent phonon analysis and first‐principles calculations reveal that the initially generated strongly bound intralayer exciton couples to two low‐frequency order‐parameter phonons associated with the ferroelectric lattice distortion. This coupling drives relaxation along the structural coordinates that generate improper ferroelectricity, transforming the strongly bound exciton into a charge‐transfer‐like ferroelectric exciton polaron beyond conventional Fröhlich coupling to polar longitudinal optical phonons. The resulting extended polarization response and layer‐asymmetric lattice relaxation dynamically screen the electron‐hole Coulomb interaction, reducing the effective binding energy from ∼540 to ∼16.5 meV. The weakly bound exciton polaron is then separated by the out‐of‐plane ferroelectric polarization field into long‐lived carriers, yielding a zero‐bias photocurrent ∼56 times larger than that of a nonferroelectric analogue. These findings identify improper‐ferroelectric order‐parameter phonons as active excited‐state structural coordinates for controlling exciton binding and charge separation in low‐dimensional semiconductors.