Unveiling the emission mechanism in analog-doped carbazole-based organic afterglow materials.

Abstract

Afterglow from purely organic solids has drawn increasing attention over the past decade. In carbazole-based systems, recent studies revised earlier aggregation-state explanations and implicated trace carbazole analog impurities as the origin. However, the widely assumed charge-trapping mechanism cannot account for exponential decay kinetics, lacks robust experimental validation and leaves some photophysical features unexplained. Here we establish the mechanism by revisiting two representative series of carbazole-analog host-guest systems. Quantitative spectral decomposition and comparative photophysics reveal that the afterglow is dominated by guest phosphorescence, accompanied by an extremely weak power-law long-persistent luminescence. The analog guests are confirmed to act as deep triplet exciton traps and shallow hole traps, the latter of which can enhance hole-transport-type long-persistent luminescence. These insights clarify previously unresolved mechanisms and establish a unified framework for luminescence that integrates energy transfer, exciton diffusion, and three afterglow pathways (delayed fluorescence, room-temperature phosphorescence and long-persistent luminescence), offering design principles for brighter and more persistent purely organic afterglow materials.