Systematic Study via Controlling the Molecular Design for Reducing the Triplet Delayed Lifetime in Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs.

A strategy for developing high-performance thermally activated delayed fluorescence (TADF) emitters with shorter delayed lifetimes (τD) is highly sought after. To address this, we have systematically constructed three emitters based on open and fused molecular structures by opting for the same 3,6-di-tert-butyl-9H-carbazole (tCz) as the donor unit and varying the acceptor core. NP-tCz (fused), BPh-tCz (open), and PPy-tCz (N-substituted open) showed gradually decreased excited singlet-triplet energy gap (ΔEST) values of 0.48 eV, 0.18 eV, and 0.11 eV, respectively, in a 10 wt % emitter: 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) blend. NP-tCz displayed photoluminescence (PL) maxima at 463 nm, exhibiting both room temperature phosphorescence (RTP) and TADF signatures, owing to its large ΔEST and prolonged τD of 40 ms. In contrast, BPh-tCz showed a PL peak at 478 nm with typical TADF behavior and a τD of 456 μs, while PPy-tCz exhibited PL maxima at 498 nm, also characteristic of TADF, with the shortest τD of 116 μs. The emissive layer in the organic light-emitting diodes (OLEDs) comprised 10 wt % emitter blended in mCBP host. The OLEDs based on NP-tCz, BPh-tCz, and PPy-tCz exhibited electroluminescence peaks at 478, 484, and 499 nm, respectively, with maximum external quantum efficiency of 7.2%, 11.9%, and 26.9%. PPy-tCz (open) containing N-substitutional doping is the most efficient among the three emitters, offering the shortest τD and high efficiency. Deeper molecular-level insights into the excited-state deactivation processes of these emitters were obtained from reliable quantum-chemical calculations, which complemented and supported the experimental observations. These findings offer a pathway to control the delayed lifetime and improve efficiency by tailoring molecular structures through open and fused configurations.