Tuning Out-of-Plane Charge Transport via Spacer-Cation Positional Isomerism in 2D Layered Perovskites.

Two-dimensional layered perovskites offer enhanced stability for photovoltaic applications, yet how organic spacer chemistry affects out-of-plane charge transport remains poorly understood. Here, we combine ab initio molecular dynamics, projection-operator diabatization, and semiclassical Marcus theory to examine the effect of position-selective monofluorination of phenethylammonium (PEA) cations (ortho, meta, and para) on vertical charge transport. We show that fluorination position strongly influences interlayer charge transport, with meta- and para-fluorinated systems generally exhibiting enhanced charge transfer performance relative to that of pristine and ortho-substituted analogues. Among them, meta-fluorination promotes dense and ordered spacer packing through synergetic π-π stacking and intermolecular C-H···F interactions, which strengthens hydrogen bonding with the inorganic framework. This suppresses lattice fluctuations and significantly reduces the reorganization energy that enters the Marcus rate expression. Simultaneously, the reduced interlayer spacing enhances electronic coupling. These effects accelerate out-of-plane charge transport, yielding subnanosecond transfer rates. Our results establish positional isomerism as a key design parameter controlling spacer packing, dynamic disorder, and charge transport. The findings provide general design principles for optimizing charge transport in layered optoelectronic materials.