Abstract
We have investigated white-emitting organic light-emitting
devices (WOLEDs) making use of both blue-phosphor-sensitized
orange-red fluorescence and the residual blue phosphorescence. By carefully
adjusting the concentrations the phosphor and the fluorophore in the emitting
layer and choosing the carrier-transport
layers in the device structure, WOLEDs containing a single phosphor-sensitized
emitting layer (type-I devices) can give colors close to the equal-energy
white (0.33, 0.33), CRI up to 75, and efficiencies up to (10%, 23 cd/A, 13.4 lm/W).
Furthermore, by doping a green phosphor into the poorly emitting electron-transport
layer (type-II devices) to recycle excitons formed there, the EL efficiencies
can be further enhanced up to (12.1%, 35.3 cd/A, 23.9 lm/W).
In both types of devices, the phosphor sensitization reduces population of
triplet excitons in the emitting region and substantially mitigates the efficiency
roll-off with the driving current or brightness that is often observed in
all-phosphor OLEDs. At the brightness of 1000 cd/m<sup>2</sup>, both types of devices retain
quantum and cadmium per ampere (cd/A) efficiencies similar to their peak values.
© 2007 IEEE
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