White color emitting organic electroluminescence (EL) device is a convenient means to realize full-color display because the three necessary primary colors could be obtained by color filters from white back light. In this paper a new three-layer device is presented. We used N

N'-bis-(lnaphhyl)-N

N'-diphenyl-1

1'-biphenyl-4

4'-diamine (NPB) as the hole transport layer

Alq as the electron transport layer

CuPc as the buffer layer

and TPBi as the blocking layer inserted between NPB and Alq.The 5

6

11

12-tetraphenyltetracene (Rubrene) as dopant was doped into the blocking layer.It was thought initially that if energy transfer existed between the host and dopant in photoluminescence (PL) of NPB doped with Rubrene

white emission might be obtained in a device such as ITO/CuPc/NPB:Rubrene/TPBi/Alq/Mg:Ag. Thus the PL of NPB:Rubrene system excited by NPB absorption band with different dopant concentrations were investigated in advance. There are two bands in the PL spectra

the blue one is emitted by NPB and the yellow one is by Rubrene. The ratio of these two bands can be adjusted by changing the Rubrene concentration. This shows that energy transfer does occur from NPB to Rubrene. Time-resolved spectra of this system

which will be given in another paper

confirm further that the energy transfer exists and is quite effective. White emission can be produced by adjusting the concentration of Rubrene in PL. However

devices of structure ITO/CuPc/NPB:Rubrene/TPBi/Alq/Mg:Ag were made according to these results with TPBi as the blocking layer

white emission can not be obtained by changing the Rubrene concentration.For another device of structure ITO/CuPc/NPB/TPBi:Rubrene/Alq/Mg:Ag

with a blocking layer TPBi doped by Rubrene concentration 1.5% inserted between NPB and Alq

white emission can be obtained. The highest luminance and maximum efficiency are 8635cd/m

2

and 1.39lm/W respectively

CIE coordinate

x

=0.31

y

=0.32. The color of the emitted light can still be optimized by adjusting the relative thickness of the relevant layers and dopant concentration.The electroluminescence mechanism of two kinds of cells was discussed.