White organic light-emitting diodes based on 2,7-bis(2,2-diphenylvinyl)-9,9 '-spirobifluorene: Improvement in operational lifetime

Abstract

Very bright white organic light-emitting diodes were fabricated using 2,7-bis(2,2-diphenylvinyl)-9,9(')-spirobifluorene (DPVSBF) doped with [4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran)] (DCJTB) as the emission layer. With a device configuration of ITO/NPB(40 nm)/DPVSBF:0.2%DCJTB(11 nm)/Alq(30 nm)/LiF(1 nm)/Al(150 nm), a brightness of 1575 cd/m(2) with an external quantum efficiency of 3.31% and luminous efficiency of 8.00 cd/A, a power efficiency of 5.35 lm/W was achieved at a driving current of 20 mA/cm(2) (4.7 V). The brightness reached 66 000 cd/m(2) at 15 V. The Commission Internationale de l'Eclairage coordinates stayed nearly constant, changed from (0.35, 0.36) to (0.32, 0.34) when the voltage increased from 6 to 12 V. The relative operational lifetime of the device increased by a factor of similar to20 compared with a similar device based on 4,4(')-bis-(2,2-diphenylvinyl)-1,1(')-biphenyl as the source of the blue emission. The much extended half-lifetime was attributed to the higher morphological stability of the DPVSBF. (C) 2004 American Institute of Physics.