The effect of the electron-donor ability on the OLED efficiency of twisted donor-acceptor type emitters

Two twisted donor-acceptor (D-A) chemical structures, CCDMB and PCDMB, were developed as a new class of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs). Two emitters consist of 3-substituted carbazole as a first donor and trivalent boron as an electron acceptor in common, and carbazole and phenoxazine as second donors with different electron donor ability. While PCDMB with a strong phenoxazine donor decreased the lowest singlet excited state (S1) level and thus showed a small singlet-triplet energy difference (ΔEST) value of 0.13 eV, resulting in effective reverse intersystem crossing (RISC), however, CCDMB with a weak donor showed a large ΔEST value of 0.21 eV. Efficient triplet harvesting of PCDMB was confirmed by a delayed component in transient PL decay curves of 25 wt% PCDMB-doped bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) films. OLED devices with a CCDMB emitter showed deep-blue emission with Commission Internationale de l’Eclairage (CIE) of (0.16, 0.12) but a low maximum EQE of 5.5%, indicative of insufficient triplet harvesting. PCDMB-based devices showed green emission with CIE of (0.21, 0.45) and a high maximum EQE of 22.3%. Our study revealed the effect of the electron donor ability of structurally similar emitters on ΔEST values, triplet harvesting, and device efficiency.