Phosphorescence-TADF complimentary-color WOLEDs with high performance fulfilled by optimizing optoelectronic behavior of the local-doped single emission layer

In this research, blue phosphorescent emitter FIrpic and orange thermally activated delayed fluorescent (TADF) XN-S3-I have been employed to fabricate complimentary-color white organic light-emitter diodes (WOLEDs). In order to furnish low driving voltage and reduce consuming electricity, exciplex composed of 1,3-Bis( N -carbazolyl)benzene (mCP) and 4,6-Bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine (B4PYMPM) as host of emission layer have been adopted. Owing to the bipolar charge-carrier transporting of the mCP:B4PYMPM exciplex host and the expansion of exciton recombination region of the emission layer, all WOLEDs can be turned on at low voltage ca. 3.0 V. However, in the emission layer, the direct connection of orange-emission zone and blue-emission zone will increase energy-transfer channels, facilitating energy transfer from FIrpic to XN-S3-I and result in an increase in energy exchange losses. All these aspects have brought low electroluminescent (EL) efficiencies and variable white EL spectra at different voltages. In order to cope with these problems, the orange-emission zone and blue-emission zone in the emission layer of the optimized WOLEDs have been separated by a thin zone of exciplex host, which can adjust the distribution of excitons in the emission layer, block Dexter energy transfer channel and facilitate full utilization of excitons for the emitters to avoid energy loss. Hence, the optimized WOLEDs can achieve impressive EL performance: maximum η _ext of 16.3%, maximum η _L of 39.1 cd A⁻¹ and maximum η _P of 41.0 lm W⁻¹ and CRI of 79. Furthermore, CIE coordinates just shift very slightly Δ x ≈ 0.02 and Δ y ≈ 0.01 in the wide luminance range from 1000 to 20,000 cd m⁻², demonstrating excellent EL stability.