Organic light emitting diodes(OLEDs) based on aluminum tris(8-hydroxyquinoline)(Alq

3

) as an active luminescent material have shown tremendous growth since its inception.Indeed

numerous experiments have provided an insight into mechanisms and processes such as the formation of the metal/organic interface

injection processes

charge transport

the effects of doping

transport

and recombination near organic/organic interfaces.However

even with a relatively good knowledge of these particular processes

it is still not an easy task to predict the properties of a multiplayer organic device without recourse to experiment.We therefore focused our effort on the detailed study of the luminescence characteristics of Alq

3

-based OLED.The OLEDs with the structure of ITO/TPD/Alq

3

/LiF/Al were fabricated in the in-line 5-chamber deposition system.The

J-V-B

characteristics were tested in N

2

atmosphere glove-box and in the air using the home-made system.The

J-V

characteristics in the forward direction

when ITO is positively biased

appears to be trapped-charged limited current(TCLC) which can be described by power laws J∝V

m

.The reverse current may be governed by tunneling under high electric field.For low reverse bias

the current may be the leakage caused by pinhole.At high current densities

the brightness-current relationship tends to a linear function.The luminescence efficiency rises as the voltage rising at the low bias voltage region and decrease as the voltage rising at the high bias voltage region.In case of this typical bilayer structure

which is often called "model device"

a strong influence of ambient atmosphere on the electrical properties was observed.The

J-V

characteristics in vacuum exhibited anomalous behaviour

including regions with negative differential resistance(NDR).In air

the anomalous behaviour disappeared.Such

J-V

characteristics were also observed by other researchers in some OLEDs based on polymers and dye doped Alq

3

during the last few years. But no reasonable explanation has been provided for this phenomenon.Especially

the NDR of the "model device" at the low voltage region was not discussed.The test results showed that the negative differential resistance is observed at the voltage of 4 V.Futher analysis indicates that this phenomenon may be attributed to the current filament caused by the pinhole.