Exciplex-forming cohost systems with highly twisted hexaphenylbenzene-core donors for high-efficiency orange and red OLEDs

Two hexaphenylbenzene (HPB)-based donors (D), CzTT and DDT-HPB, are blended with the triazine-centered acceptors (A) CN-T2T and PO-T2T to examine exciplex formation. The highly twisted conformation of the HPB core allows CzTT and DDT-HPB to exhibit high triplet states and excellent thermal stability. The red-shifted emission together with delayed fluorescence observed by transient photoluminescence analyses reveal effective exciplex formation with thermally activated delayed fluorescence (TADF) features for the CzTT : PO-T2T, DDT-HPB : CN-T2T and DDT-HPB : PO-T2T blends. The exciplex emission wavelength can be modulated by energy level alignment at the donor and acceptor interface. The device with the CzTT : PO-T2T (1 : 1) blend as the emission layer (EML) gives sky-blue electroluminescence (EL) with a lambda(max) of 480 nm and external quantum efficiency (EQE) of 9.8%, while the orange device employing the DDT-HPB : PO-T2T (1 : 1) blend exhibits an EL lambda(max) of 580 nm and EQE of 5.2%, revealing the influence of the donor on the device characteristics. Importantly, reducing the acceptor strength from PO-T2T to CN-T2T, the yellow device employing the DDT-HPB : CN-T2T (2 : 1) blend as the EML exhibits an EL lambda(max) of 550 nm and EQE of 8.9%, which maintains an EQE of up to 8.8% at 1000 nits. To further harness the excitons formed in the exciplex-forming blends, two D-A-D-type fluorescence emitters, NZDSF and DPyCN, were introduced as dopants. The good spectral overlap ensures efficient Forster energy transfer (FRET) between the exciplex-forming systems and the fluorescent dopants. The orange-red device employing the EML composed of 3 wt% NZDSF doped in the CzTT : PO-T2T (1 : 1) cohost exhibits an EL lambda(max) of 615 nm and EQE of 7.3%, while the deep-red device with 3 wt% DPyCN doped in the DDT-HPB : CN-T2T (2 : 1) cohost as the EML affords an EL lambda(max) of 667 nm and EQE of 5.5%. Our study shows that the combination of an exciplex cohost and a judiciously selected D-A-D-type fluorescence emitter is an effective strategy for developing high-efficiency OLED devices with all fluorescent materials.

Automated summary

This study examines exciplex formation and photoluminescence characteristics by blending hexaphenylbenzene-based donors and triazine-centered acceptors. The results show that effective exciplex formation and delayed fluorescence can be achieved through energy level alignment and the incorporation of fluorescent dopants. The combination of an exciplex cohost and a D-A-D-type fluorescence emitter is an effective strategy for developing high-efficiency OLED devices with all fluorescent materials.