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

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)PbBr4, 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.