Presentation Information

[22p-13M-2]Highly efficient carbazole-based host material to improve molecular orientation for organic light-emitting diodes

〇(D)Bhagya Madushani1, Masashi Mamada2, Chihaya Adachi1 (1.Kyushu Univ., 2.Kyoto Univ.)


outcoupling efficiency,horizontal orientation,organic light-emitting diodes

The small energy gap between the singlet and triplet energies (ΔEST) in thermally activated delayed fluorescence (TADF) emitters makes it easier for the triplets to upconvert into singlets by reverse intersystem crossing (RISC). Although 100% internal quantum efficiency (IQE) is achievable in TADF organic light-emitting diodes (OLEDs), the low outcoupling efficiency limits the external quantum efficiency (EQE) to 20-30%. The dipole orientation and the device construction both affect the outcoupling efficiency. Experimental evidence indicates that thin-film OLEDs with host materials or light-emitting materials that often align horizontally to the substrate often exhibit significant outcoupling efficiency, and hence, result in a higher EQE. Here, we introduced a new host material consisting of six carbazole (Cz) units attached to a center phenyl ring (6CzPh), which has comparable triplet energy with the well-known host mCBP. Regarding the PL measurements, the new host showed low performance with the green emitter 4CzIPN compared to the mCBP host with the same emitter. In contrast to the mCBP host with 4CzIPN, the new host's device performance was better with 4CzIPN. With angular-dependent measurements, we were able to verify that the 6CzPh host was able to provide a favorable horizontal orientation with the emitter than mCBP. In addition, we enhanced the carrier balance by using a mixed host system, which improved the device lifetime and EQE.