Multiple Charge Transfer Processes Enable Blue Emitter for Highly Efficient OLEDs

Solely through-space charge transfer (TSCT)-type thermally activated delayed fluorescent (TADF) emitters exhibit low radiative decay rates (krs$k_{\rm{r}}<^>{\rm{s}}$), while solely through-bond charge transfer (TBCT) type TADF emitters generally suffer from low reverse intersystem crossing (RISC) rates (k(RISC)). Here, PhCzSpiroS-TRZ with versatile spiro-heterocyclic architecture and an extra donor of 9-phenyl-9H-carbazole are developed as emitter and ideal host for TADF- and TADF-sensitized fluorescence (TSF) organic light-emitting diodes (OLEDs) in detail. Abandoning solely TSCT or TBCT process, PhCzSpiroS-TRZ with multiple charge transfer characteristic exhibits photoluminescence quantum yield of 96.3% and krs$k_{\rm{r}}<^>{\rm{s}}$ of 2.2 x 10(7) s(-1), which is considerably higher among all the reported TSCT-TADF molecules. Noticeably, state-of-the-art blue OLEDs using PhCzSpiroS-TRZ as emitter and sensitizer show maximum external quantum efficiencies (EQEs) of 33.6% and 32.8%, ranking among TADF materials for achieving EQEs >30% in both TADF- and TSF-OLEDs.