Improving the external quantum efficiency and minimizing the efficiency roll-off in OLEDs: a study on the optimization of donor linkage and acceptor nitrogen atom positions in TADF emitters†

Abstract

The position and linkage of the donor and acceptor units in thermally activated delayed fluorescence (TADF) emitters play significant roles in determining the device efficiency and efficiency roll-off. Herein, we designed and synthesized three organic emitters, namely, 3BPy-mDPXZ, 3BPy-mPXZ, and 3BPy-pPXZ, by changing the number and linkage position of the donor units. All three emitters showed negligible emission in solution and less than 0.2 eV of singlet–triplet energy barrier (ΔE_ST) in their CBP host-doped films. Unlike the general conception, the use of multiple donor units led to a reduction in PLQY (8.9%) in 3BPy-mDPXZ, resulting in a poor EQE_max of 1.5%. By reducing one donor unit, PLQY doubled to 18.1% and EQE_max increased to 2.7% in 3BPy-mPXZ. However, when the donor unit's position was shifted from meta to para, 3BPy-pPXZ showed 4 times enhancement in PLQY (73.8%) and 6 times enhancement in EQE_max (17.3%). Along with the EQE enhancement, the emitter also showed a drop in the efficiency roll-off from 22% in 3BPy-mPXZ to 10.9% in 3BPy-pPXZ. The critical luminance (L₉₀) for the OLED device of 3BPy-pPXZ was calculated to be 831 cd m⁻². Furthermore, by changing the pyridine nitrogen position in the acceptor unit from 3^rd to 4^th (4BPy-pPXZ), EQE_max further increased to 23.7%. This work highlights the importance of donor–acceptor linkage and hetero atom position in determining the device performance and efficiency roll-off of OLEDs.