TADF versus TTA emission mechanisms in acridan and carbazole-substituted dibenzo[a,c]phenazines: Towards triplet harvesting emitters and hosts

Abstract

Carbazole or acridan-substituted dibenzo[a,c]phenazines (CzDbp and AcDbp, respectively) were synthesized and investigated exploiting the donor–acceptor-donor (D-A-D) architecture expecting thermally activated delayed fluorescence (TADF) in both cases. Unexpectedly, while experimental microseconds-lived TADF behaviour was observed for AcDbp efficiency of which was found to be dependent on environment, CzDbp exhibited nanosecond-lived fluorescence complemented by triplet–triplet annihilation (TTA). Theoretical calculations by means of the ωB97XD functional with optimally tuned range separation parameter ω, were performed for both molecules, supporting their experimentally established electrochemical, optical and photophysical properties. Using the same emitter and device structure, CzDbp as the bipolar host allowed to achieve by 12.5% better external quantum efficiency and better roll-off efficiency of organic light-emitting diodes (OLEDs) in comparison to that of OLEDs based on the commercial host 1,3-bis(N-carbazolyl)benzene (mCP). In optimized OLED structure, device based on CzDbp host showed higher external quantum efficiency reaching 15.9% and lower roll-off efficiency in comparison to that of reference devices containing commercial hosts. This achievement can be explained by both the fast TTA triplet harvesting enhancing the substantial fluorescence efficiency of CzDbp, and by relatively high charge mobilities exceeding 10-3 cm2/V·s for holes and 10-4 cm2/V·s for electrons. CzDbp and AcDbp as TADF/TTA emitters were used, respectively, external quantum efficiencies of 19.4% and 22.1% for doped yellow and orange devices were achieved. The detailed discussion on TADF mechanism is presented, and the new “dynamical” state-energy diagram is proposed as the means allowing to better understand the TADF mechanism and the experimental results.