Efficient and Stable Organic Light-Emitting Diodes Employing Indolo[2,3-b]indole-Based Thermally Activated Delayed Fluorescence Emitters

Triplet excitons can be effectively harvested in organic light-emitting diodes employing thermally activated delayed fluorescence (TADF) molecules as the emitter and host. A design strategy for blue and green emitters with small S1–T1 splitting (ΔEST) is to construct a donor-acceptor (D-A) type molecule with moieties combining a high T1 level with a strong electron-donating/withdrawing character. Here, we report a new kind of TADF emitter with an indolo[2,3-b]indole (IDID) donor. In comparison to other reported indolocarbazole and indoloindole donors, IDID has a higher T1 level, which is comparable to that of the classical donor 9,9-dimethyl-9,10-dihydroacridine (DMAC) for blue TADF emitters. The sky-blue and green TADF emitters based on the IDID donor and a phenyltriazine acceptor exhibit high photoluminescence quantum yields (0.78–0.92) and short TADF lifetimes (1.1–1.7 μs) in doped films. Devices employing these IDID-based emitters offer an external quantum efficiency of 19.2%, which is comparable to that obtained for a device employing an analogous compound with a DMAC donor, while the stability of the former is higher than that of the latter owing to the just-right D-A twisting angles (∼59°) in the IDID-based emitters leading to a balance between ΔEST and the fluorescence rate. The utilization of host materials with a similar polarity to the emitter is found to be an effective strategy to improve device stability.