Harvesting Delayed Fluorescence in Perovskite Nanocrystals Using Spin-Forbidden Mn d States

Optoelectronic devices based on all-inorganic perovskite systems are an energy-efficient source of lighting due to their high photoluminescence quantum yield (QY). However, dominant surface trapping continues to plague the field, despite their high defect tolerance, as evidenced by the several-fold improvements in the external quantum efficiency of perovskite nanocrystals (NCs) upon appropriate surface passivation or physical confinement between high-band-gap materials. Here, we introduce the concept of drip-feeding of photoexcited electrons from an impurity-induced spin-forbidden state to address this major shortcoming. An increased and delayed (about several milliseconds) excitonic QY, Raman spectroscopy demonstrating specific vibrational modes of the PbX6 octahedra, and density functional theory establish the electron back-transfer, signifying an efficient recombination. We term this electron back-transfer from Mn2+ to the host conduction band in this prototypical example of Mn-doped CsPbX3 (X = Cl, Br) NCs through vibrational coupling as vibrationally assisted delayed fluorescence (VADF).