Light-blue thermally activated delayed fluorescent emitters realizing a high external quantum efficiency of 25% and unprecedented low drive voltages in OLEDs

Thermally activated delayed fluorescent (TADF) emitters are one of the most promising candidates for low-cost and high efficiency organic light-emitting devices (OLEDs) to realize an internal quantum efficiency of unity. However, the power efficiency (η_p), which is inversely related to the drive voltage, is significantly lower than that of the phosphorescent counterparts, especially for blue devices. Here, we developed a series of TADF emitters, 2-functionalized-4,6-bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]pyrimidine called Ac-RPM. We introduced a phenylacridine moiety into the 4,6-position of the pyrimidine core to induce a twisted structure leading to a high photoluminescence quantum yield of ∼80%, and a small singlet and triplet excited energy difference of <0.20 eV. The optimized device realized an η_p of 62 lm W^-1, a high external quantum efficiency of 25%, light-blue emissions with the Commission Internationale de l'Eclairage chromaticity coordinates of (0.19, 0.37) and a low turn-on voltage of <3.0 V.