Highly Efficient Blue OLEDs Based on Metal-Assisted Delayed Fluorescence Pd(II) Complexes

A series of metal-assisted delayed fluorescence (MADF) palladium(II) complexes, PdN1N, PdN1N-dm, and PdN6N, are studied. Their 77 K photoluminescence emission spectra show a narrow primary emission peak at 464, 466, and 470 nm, respectively, with small vibronic peaks characteristic of many phosphorescent emitters. At room temperature (300 K), the dominant emission peaks for PdN1N, PdN1N-dm, and PdN6N are slightly redshifted to 472, 470, and 476 nm, respectively. In the meantime, a broad sideband is also shown between 410 and 450 nm due to the MADF process. A blue device employing PdN1N-dm demonstrates an electroluminescence emission peak at 476 nm and the maximum external quantum efficiency (EQE) of 25.1%. The high maximum EQE of the blue organic light-emitting diode (OLED) device indicates that both the phosphorescence and the delayed fluorescence are very efficient. In addition, a stable OLED device of PdN1N-dm gives a maximum EQE of 9.8% and the estimated operational lifetime LT70 of 350 h at the practical luminance of 100 cd m(-2).