Rational understanding of substituent effects on multi carbazole thermally activated delayed fluorescence emitters

Abstract

Multi-carbazole thermally activated delayed fluorescence (TADF) molecules are promising emitters due to their fast reverse intersystem crossing (RISC) rate, and high device efficiency. They are composed of a benzonitrile acceptor and four or five carbazole donors or other functional groups that are connected to the acceptor. Since multiple moieties can be attached to benzonitrile, modification of the carbazole donors or other functional groups of multi-carbazole TADF molecules is facile. In addition, these modifications largely affect the emission spectrum, RISC rate, and device efficiency, which has led to the development of various multi-carbazole TADF molecules and OLEDs based on them. Still, the effect of the modification of functional groups attached to the acceptor core has not been clearly investigated due to the complicated excited states of the multi-carbazole TADF molecules. Herein, we report two novel multi-carbazole TADF molecules developed by ortho-biphenyl and ortho-indolocarbazole substitutions of a multi-carbazole TADF molecule based on a benzonitrile backbone. The new TADF molecules, 4CzBN-BP and 4CzBN-ICz, comprise four carbazole donors, a benzonitrile acceptor core, and an ortho-biphenyl/indolocarbazole triplet scaffold. The devices based on 4CzBN-BP and 4CzBN-ICz showed maximum external quantum efficiencies (EQEs) of 12.5% and 4.2%, which were improved and decreased values, respectively, from the 10.9% of the 4CzBN device. To investigate the effect of substitutions on the excited-state dynamics and photophysical properties of the TADF molecule, we performed various photoluminescence measurements along with time-dependent density functional theory (TD-DFT) calculations. Based on comprehensive experimental and theoretical investigations, we found that not only the local triplet state of the triplet scaffold but also the local singlet state and the energy transfer to the charge-transfer singlet state of the TADF molecule are critical to the TADF emission process.