Crystal-Growth-Controlled Exciton Funneling in BA2MAPb2I7 Ruddlesden-Popper Perovskite Thin Films.

We investigate BA2MAPb2I7 (BAMA) quasi-2D Ruddlesden-Popper perovskite thin films prepared through single-crystal-derived and conventional polycrystalline routes. Morphological and X-ray diffraction analyses reveal significant differences in film texture, crystallinity, and phase distribution. Steady-state and time-resolved optical spectroscopies show that polycrystalline films are mainly composed of n = 2 and n = 3 phases and exhibit limited interphase energy transfer. In contrast, single-crystal-derived films display a richer excitonic landscape characterized by the presence of higher-(n) domains. Transient photoluminescence and pump-probe measurements demonstrate that the n = 2 exciton acts as the primary donor state and, uniquely in the single-crystal-derived films, undergoes two distinct transfer processes on the same timescale. The correlation between the decay of the n = 2 exciton and the population of lower-energy excitonic states provides direct evidence of hierarchical exciton funneling. These findings highlight the crucial role of phase distribution and crystallinity in governing exciton migration and energy-transfer pathways in low-dimensional perovskite heterostructures.