Ultrafast interphase exciton funneling in alternating-cation 2D perovskites through Förster-type energy transfer

Efficient exciton transport is central to the development of low-dimensional semiconductors for optoelectronic applications. Layered metal halide perovskites provide a versatile platform for regulating excitonic interactions through spacer-layer engineering. Here, we demonstrate that alternating-cation-in-the-interlayer space (ACI) perovskites exhibit markedly rapid interphase exciton funneling compared with conventional Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) architectures. Using femtosecond transient absorption and time-resolved photoluminescence spectroscopy, complemented by full-spectrum global analysis, we reveal rapid exciton transfer from lower-n toward higher-n quantum wells. The concomitant decay of lower-n excitonic signals and growth of higher-n features, together with the species-associated spectral evolution, are consistent with a predominantly Förster-type interphase energy-transfer mechanism. The characteristic early-time dynamics correlate with shorter interlayer spacing along the stacking direction, suggesting that spacer-layer architecture contributes to modulation of interphase excitonic coupling. While differences in phase distribution, spectral overlap, and domain morphology may additionally influence the measured dynamics, these results establish spacer-layer engineering as an effective strategy for regulating ultrafast energy flow in layered excitonic semiconductors.