Efficient and stable single-doped white OLEDs using a palladium-based phosphorescent excimer

Abstract

A tetradentate Pd(ii) complex, Pd3O3, which exhibits highly efficient excimer emission is synthesized and characterized. Pd3O3 can achieve blue emission despite using phenyl-pyridine emissive ligands which have been a mainstay of stable green and red phosphorescent emitter designs, making Pd3O3 a good candidate for stable blue or white OLEDs. Pd3O3 exhibits strong and efficient phosphorescent excimer emission expanding the excimer based white OLEDs beyond the sole class of Pt complexes. Devices of Pd3O3 demonstrate peak external quantum efficiencies as high as 24.2% and power efficiencies of 67.9 Lm per W for warm white devices. Furthermore, Pd3O3 devices in a carefully designed stable structure achieved a device operational lifetime of nearly 3000 h at 1000 cd m^-2 without any outcoupling enhancement while simultaneously achieving peak external quantum efficiencies of 27.3% and power efficiencies over 81 Lm per W.

Fig. 1. Photoluminescent spectra of Pd3O3, PtOO3, and fac-Ir(ppy)₃ at 77 K in 2-methyl-THF and molecular structures of select tetradentate excimer emitters and phenyl-pyridine based emitters.

Fig. 2. The normalized absorption spectrum of Pd3O3 in DCM (squares) and PL spectrum of a 1% Pd3O3 doped 26mCPy thin film (circles) and PL spectrum of a dilute DCM solution (triangles).

Fig. 3. External quantum efficiency versus luminance and electroluminescent spectra (inset) for Pd3O3 devices with 5% (squares) and 10% (circles) dopant concentrations in the structure: ITO/HATCN/NPD/TAPC/x% Pd3O3:26mCPy/DPPS/BmPyPB/LiF/Al.

Fig. 4. (a) Electroluminescent spectra, (b)external quantum efficiency versus luminance, (c) power efficiency versus luminance and (d) operational lifetime for Pd3O3 in Device 1 (red), Device 2 (green), and Device 3 (blue). The device operational lifetime was measured at a constant drive current of 20 mA cm^–2.