Afterglow OLEDs incorporating bright closely stacked molecular dimers with ultra-long thermally activated delayed fluorescence

Purely organic luminescent materials exhibiting afterglow emission after photoexcitation have been developed recently and hold great promise for future applications, whereas the afterglow from electroexcitation remains rare and inefficient. Here, afterglow electroluminescence is achieved based on an organic dimer system with high photoluminescence quantum yield and unprecedented long-lived thermally activated delayed fluorescence (TADF) characteristics. The strong π-π interactions and aggregation structures can suppress nonradiative transition and promote fluorescence emission. In addition, the molecular stacking also renders a moderate singlet-triplet energy gap, enabling a long-lived TADF afterglow. In combination with host-guest interaction optimization, efficient organic light-emitting diodes (OLEDs) based on the dimer system achieve high external quantum efficiency of 14.7% along with bright afterglow emission with lifetime of 157 ms. This work not only extends the scope of purely organic aggregates for novel optoelectronic properties and applications but also opens up new pathways for achieving efficient afterglow OLEDs with TADF mechanism.