White organic light-emitting diodes based on a novel starburst fluorene derivative

A white organic light-emitting diode (WOLED) with an undoped structure based on a novel starburst fluorene derivative of 1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2-yl) benzene (HKEthFLYPh) as an energy transfer layer was fabricated by the thermal vacuum deposition method. N,N-di(naphthalen-2-yl)-N,N-diphenyl-benzene (NPB) and tris(8-hydroxyquinolinato) aluminum (Alq) were used as the blue and yellow-green emission layers, respectively. The structure of the device was indium-tin-oxide (ITO)/NPB/HKEthFLYPh/Alq/Mg:Ag. The results demonstrated that the triple-layer device had a white light emission with a maximum luminance of 8800 cd/cm2 at 15 V and a stable current efficiency from 1.8 to 1.4 cd/A at various bias voltages ranging from 5.5 to 15 V. The electroluminescent (EL) spectra overlaid from 400 to 700 nm with a full width at half maximum (FWHM) of 190 nm, which covered the whole visible wavelength region. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates showed (x=0.29,y=0.36) at 8 V and changed from (x=0.31,y=0.38) to (x=0.26,y=0.31) under bias voltage from 5 to 12 V.

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