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

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 s in a PMMA doped film as well as a narrow singlet-triplet bandgap (E-ST) of 0.22 eV. A maximum current efficiency ((c)) and external quantum efficiency ((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 (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 (c) and (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.