Organic light-emitting diode (OLED) has attracted considerable attention due to their unique image quality and market potentials in display field. Efficient blue

green and red emitters with excellent color purity are required for full-color display application

so far great improvements have been achieved for the life time and efficiency of the red and green materials. However

the blue emitting material is still a barrier to commercial applications of OLEDs.Using vacuum thermal evaporation method

blue OLEDs were fabricated by inserting a bathocuproine (BCP) as hole blocking layer (HBL) into conventional double layer (DL) device at the interface of N

N-bis-(1-naphthyl)-N

N-biphenyl-1

1-biphenyl-4

4-diamine (NPB)/8-hydroxyquinoline aluminum (Alq

3

). Device structure is indium tin oxide (ITO)/NPB/BCP/Alq/Mg:Ag

where NPB acts as both hole transporting layer (HTL) and emitting layer (EML)

and Alq

3

as electron transport layer (ETL). The active emissive area of each device defined by the overlap of ITO and Mg:Ag was 4mm×4mm. Measurements of luminance-voltage (L-V) and current density-voltage (J-V) characteristics were carried out with a Keithley 4200 Semiconductor Characterization System

photoluminescence (PL) and electroluminescence (EL) spectra were measured with an OPT-2000 system. The luminance-voltage and current density-voltage characteristics of devices were investigated

and dif-ferences between their performances were discussed. The results demonstrate that the maximum luminance and luminance efficiency of the BCP device are 2900 cd/m

2

and 0.55 lm/W

respectively. The peak of electroluminescence (EL) spectrum locates at 445nm and the Commissions Internationale d′Eclairage (CIE) coordinates are (

x

=0.16

y

=0.09)

which are independent on the variation of bias voltage. Energy level diagrams were employed to analyze the diversity of device performance with different structures. We find that the high EL performance of the BCP device is attributed to better balance of carriers in the NPB layer by inserting BCP as hole blocking layer (HBL). Optimization of device structure and performance

via adjusting thickness of organic layers

is under investigation.