Efficiency enhancement in fluorescent deep-blue OLEDs by boosting singlet exciton generation through triplet fusion and charge recombination rate

We report fluorescent deep-blue organic light emitting diodes (OLEDs) with significantly high efficiency by enhancing singlet exciton generation via triplet-triplet annihilation (TTA). Here, electron transport layers (ETL) and exciton blocking layers (EBL) with different characteristics (especially mobility) are applied to increase singlet harvesting (using TTA process) through appropriate charge carrier injection in the emissive layer. OLED with high electron mobility ETL, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB) and high hole mobility EBL, 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine (PCzAc) exhibit relatively high external quantum efficiency of 9.18% with deep-blue color coordinates of (0.135, 0.115) at the luminance of 1000 cd/m² and low driving voltage (4.5 V). The transient electroluminescence measurements are conducted to investigate the relation between prompt fluorescence from the initial singlet excitons and delayed fluorescence from the TTA process. This analysis shows significant enhancement in the singlet exciton generation by TTA process and charge recombination rate.