Purely organic materials for extremely simple all-TADF white OLEDs: a new carbazole/oxadiazole hybrid material as a dual-role non-doped light blue emitter and highly efficient orange host

Abstract

White organic light-emitting diodes (WOLEDs) based on thermally activated delayed fluorescence (TADF) emitters are highly attractive owing to TADF materials providing a favorable low-cost alternative approach to realize high-efficiency electroluminescence. In this work, an extremely simple all TADF WOLED with a single-emissive-layer singe-host/dopant system is demonstrated, by the design of a new TADF material as a dual-role non-doped light-blue emitter and efficient host for a yellow TADF dopant. The carbazole/oxadiazole hybrid compound 2-(2,3,4,5,6-penta(9H-carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole (5CzOXD) shows bright light-blue TADF emission with a photoluminescence (PL) peak at 496 nm, and a PL quantum yield of 8.6 and 58.2% under ambient conditions and under degassed toluene, respectively. 5CzOXD also exhibits a delayed decay time of 13 μs in a PMMA doped film as well as a narrow singlet–triplet bandgap (ΔE_ST) of 0.22 eV. A maximum current efficiency (η_c) and external quantum efficiency (η_ext) of 19.3 cd A⁻¹ and 9.3% or 12 cd A⁻¹ and 3.6% are achieved in light blue TADF OLEDs by using 5CzOXD as a doped or non-doped emitter, respectively. A moderately high triplet energy level of 2.58 eV guarantees the application of 5CzOXD as an efficient organic host material for green and yellow TADF OLEDs, with maximum η_ext as high as 16.2 and 17.1%, respectively. In addition, a two-color WOLED with the extremely simple device architecture of ITO/MoO₃ (8 nm)/TAPC (40 nm)/5CzOXD:4CzPNPh (0.6 wt%, 10 nm)/BmPyPB (40 nm)/LiF (1 nm)/Al is subsequently fabricated, which shows maximum η_c and η_ext of 21.5 cd A⁻¹ and 7.2%, respectively, with CIE coordinates of (0.35, 0.44). This simple strategy provides a bright future for constructing low-cost efficient vacuum-deposited WOLEDs based on all organic TADF materials for solid state lighting.