Exciplex-forming co-host systems featuring highly twisted bis-(N-carbazolyl)benzene-based donors for high-efficiency orange OLEDs

Abstract

Two new carbazole-based donors (D), Ph2Cz and 2Ph2Cz, derived from the benchmark N,N′-dicarbazolyl-3,5-benzene (mCP) are blended with a triazine-centered acceptor (A), PO-T2T, to examine exciplex formation. The introduction of phenyl group(s) at C1 of the carbazole of mCP causes Ph2Cz and 2Ph2Cz to exhibit high-triplet states and excellent thermal stability. The red-shifted emission and delayed fluorescence observed in the Ph2Cz:PO-T2T and 2Ph2Cz:PO-T2T blends are a signature of exciplex formation with thermally activated delayed fluorescence (TADF) characteristics. The exciplex emission wavelength can be modulated by tuning the energy level at the D : A interface. The device with the Ph2Cz : PO-T2T (1 : 1) blend as the emissive layer (EML) exhibits an electroluminescence maximum wavelength (EL λ_max) of 492 nm and external quantum efficiency maximum (EQE_max) of 8.51%, while the device employing the 2Ph2Cz : PO-T2T (1 : 1) blend as the EML exhibits an EL λ_max of 504 nm and EQE_max of 8.51%. To further harness the excitons formed in the exciplex-forming blends, a π–A–π-type fluorescence emitter, NZDFT, was introduced as an emissive dopant. The good spectral overlap ensures efficient Förster energy transfer (FRET) between the exciplex-forming systems and the fluorescent dopant. The orange-red device employing the EML composed of 2 wt% NZDFT doped in the 2Ph2Cz : PO-T2T (1 : 2) co-host exhibits an EL λ_max of 619 nm and EQE_max of 11.30%. Our study shows that the combination of an exciplex-forming blend as a co-host and a judiciously selected fluorescent dopant is an effective strategy for developing high-efficiency OLED devices with all fluorescent materials.