Exciplex Cohosts Employing Nonconjugated Linked Dicarbazole Donors for Highly Efficient Thermally Activated Delayed Fluorescence-Based Organic Light-Emitting Diodes

Two new nonconjugated linked dicarbazole materials, dCzPSi and dCzPSO(2), with high triplet energy were synthesized and characterized. dCzPSi and dCzPSO(2) were adopted as unipolar host materials for the green thermally activated delayed fluorescence (TADF) emitter (4CzIPN) to achieve high-efficiency organic light emitting diodes (OLEDs). The electron-transporting acceptor, PO-T2T, was introduced to mix with dCzPSi and dCzPSO(2) to give two new exciplex-forming systems that can improve the exciton formation propensity in the emitting layer. The relevant properties of these new exciplexes were characterized, and they were suggested as promising cohosts for the green TADF emitter, 4CzIPN. The characteristics of the devices employing single hosts (dCzPSi and dCzPSO(2)) and exciplex-forming cohosts (dCzPSi:PO-T2T and dCzPSO(2):PO-T2T) were explored. The obtained results indicate that the Si-bridged dicarbazole compound dCzPSi outperforms its counterpart dCzPSO(2) in which two carbazole groups are linked by an SO2 group. The device employed with the dCzPSi:PO-T2T cohost with 10 wt % 4CzIPN achieved a low V-on of 2.2 V and maximum efficiencies of 21.1% (external quantum efficiency), 56.4 cd A(-1) (current efficiency), 59.1 lm W-1 (power efficiency), as compared to those (18.7%, 56.6 cd A(-1), and 68.5 lm W-1) of the dCzPSO(2):PO-T2T-hosted device. This work verifies the advantages of using a cohost that can form an exciplex for boosting the device efficiency with reduced efficiency roll-off of TADF-based OLEDs.