Manipulating peripheral non-conjugated substituents in carbazole/oxadiazole hybrid TADF emitters towards high-efficiency OLEDs

The chemical modification of existing thermally activated delayed fluorescence (TADF) materials is a simple strategy to increase the luminescence efficiency and is also favorable for a deeper understanding of the structure-property relationship. In this work, based on a previous multi-functional carbazole/oxadiazole hybrid light-blue TADF emitter 2-(2,3,4,5,6-penta(9H-carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole (5CzOXD), a series of 5CzOXD derivatives have been designed by rationally introducing non-conjugated electron-donating tert-butyl (t-Bu) units at the 3- and 6-positions of carbazole and/or electron-withdrawing trifluoromethyl (CF3) units at the para- or meta-positions of the terminal phenyl ring in the diphenyl-1,3,4-oxadiazole group. The TADF emission colors were slightly affected by non-conjugated peripheral substituents, while a singlet-triplet band gap (Delta E-ST) value could be easily tuned from 0.24 to similar to 0 eV. It is interesting that through step-by-step chemical modification, progressively increased photoluminescence (22-88%) and electroluminescence (2.3-13.7%) quantum efficiencies could be achieved for the TADF emitters in doped films of the o-CzOXD host matrix. Similar trends were also obtained for 26DCzPPy-hosted TADF OLEDs. For example, bare 5CzOXD showed a maximum external quantum efficiency (EQE) of 8.7%, the value of t-Bu-modified 5tCzOXD was increased to 14.2%, and even higher EQE values in the range of 20-22% were obtained for t-Bu and single CF3-substituted mCF(3)5tCzOXD and pCF(3)5tCzOXD. The best EQE value of 23.3% was achieved for dCF(3)5tCzOXD by synergistical t-Bu and double CF3-substitution due to its similar to 0 eV of Delta E-ST for a significantly increased reverse intersystem crossing rate and a decreased triplet non-radiative decay rate. This work provides an effective strategy for achieving high-efficiency TADF OLEDs through the modification of currently less efficient TADF emitters.

Automated summary

Chemical modification of existing TADF materials has been shown to increase luminescence efficiency and adjust emission properties by introducing different peripheral substituents. Tuning the singlet-triplet band gap allowed for achieving higher quantum efficiencies in TADF emitters, highlighting the potential of this strategy for high-efficiency OLEDs.