Observation of triplet-assisted long-distance charge-transfer exciton transport in single organic cocrystal.

Charge-transfer (CT) states with long transport distances are highly desired for promoting the performance of organic optoelectronic devices in photoconversion and electroluminescence. However, due to the limited lifetime and small diffusivity, only nanoscale CT transport has been observed so far. Herein, taking a binary CT cocrystal (trans-1,2-diphenylethylene-1,2,4,5-tetracyanobenzene, named as T-T) with efficient thermally activated delayed fluorescence (TADF) as a model material, we report the direct observation of long-distance CT exciton transport by using modified time-resolved and photoluminescence-scanned imaging microscopy, which reveals a triplet-assisted CT transport mechanism. We demonstrate that, enabled by the long-lived and high-yield triplet state and efficient TADF, the average transport distance of over 80% of CT excitons in T-T can be significantly enhanced from intrinsic nanoscale (≤58 nm) to ~11.2 μm. Our findings provide an effective strategy for greatly promoting short-lived CT exciton transport, which is of great significance for optoelectronic material design and device optimization.