Exciton Recycling as a Strategy for Controlling Energy Transfer in Doped Perovskite Nanocrystals

Controlling interfacial energy transfer is central to hybrid optoelectronic and light-harvesting systems but is often limited by the short lifetime of donor excitations. Here, we show that dopant-induced exciton reservoirs regulate energy transfer. In Mn2+-doped CsPbX3 (X = Br, Cl) nanocrystals, vibrationally assisted delayed fluorescence (VADF) enables exciton capture, storage, and repopulation. This exciton recycling sustains energy transfer to molecular acceptors through Förster resonance energy transfer (FRET). Ultrafast transient absorption and time-resolved photoluminescence measurements reveal a nonmonotonic dependence of FRET efficiency on Mn concentration, consistent with an optimal regime of Mn-mediated exciton recycling. By tuning halide composition and temperature, we establish a correlation between VADF strength and energy-transfer efficiency, achieving up to a 4-fold enhancement in FRET. These findings identify exciton availability as a key control parameter for interfacial energy transfer and highlight exciton-reservoir engineering as a strategy for controlling energy flow in semiconductor nanostructures.