Organic light emitting diodes(OLEDs) have been extensively investigated and significantly improved over the past two decades.The mechanism for electroluminescent(EL) phenomenon involves in the injection of holes from anode and electrons from cathode under forward bias voltage

then the hole-electron forming the exciton

and light emission through the radiative recombination of excitons.Due to the superior achieved performance

OLEDs are potential candidate not only for next generation displays

but also for solid state lighting application which requires high efficiency and low operating voltage.Through proper material design/choice and device fabrication

various colors of bright luminance have been developed for use in singleor full-color application.Among various colors

white organic light-emitting diodes(WOLEDs) draw particular attention as their highly potential application in blacking

full color application

as well as in lighting purposes.So far

a variety of methods have been proposed to achieve a WOLED

such as(Ⅰ) a multilayer device with blue

green

and red emissive layers

(Ⅱ) a doped device with a host material and blue

green

and red fluorescence dyes

(Ⅲ) a single-emission-layer device with white emission material

(IV) excimer and exciplex emission

and(Ⅴ) stacked structure.About IV

white-light emission can be obtained from a bilayer device using two blue-emitting polymers

in which an exciplex emission at the bilayer interface occurs in addition to an emission from one of the emitter.In addition

finding a simple fabricating way to realize full-color OLEDs is also an important and imperative approach to reduce manufacturing cost in industrial production.In this work

OLEDs were fabricated with a simple structure of indium-tin-oxide(ITO)/poly(N-vinylcarbazole)(PVK):N

N'-bis-(1-naphthyl)-N

N'-diphenyl-1

1'-diphenyl-4

4'-diamine(NPB)/2

9-dimenthyl-4

7-diphenyl-1

10-phenanthroline(BCP)/Mg:Ag by spin coating method

which used a blend of two blue-emitting materials of PVK and NPB with different weight ratios.The luminance-voltage and current density-voltage characteristics of devices were investigated

and differences between their performances were discussed.The results demonstrate that optimum device performance could be obtained by choosing the concentration proportion of PVK:NPB at 1:1.In EL spectra of the device

except for the emission of NPB

a new emission peak at 640 nm proposed to be electroplex that formed between PVK and NPB was observed.Also

the relative emission intensity of electroplex increased with enhanced electric field.The Commissions Internationale De L'Eclairage(CIE) coordinates of Device Care x=0.30

y=0.28 at 12 V

which obviously belongs to white light emission.