Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2000-05-24
2002-11-19
Kim, Robert H. (Department: 2882)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S504000
Reexamination Certificate
active
06483236
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to organic light-emitting devices and particularly to a low-voltage organic light-emitting device.
BACKGROUND OF THE INVENTION
In a normal configuration of an organic light-emitting device, the device structure comprises, in sequence, a glass support, a transparent conductive hole-injecting electrode, an organic hole-transporting layer, an organic light-emitting layer, an organic electron-transporting layer, and an electron-injecting electrode. In order to operate this device at desirably low-voltage, the electron-transporting material is required to have a high electron mobility. However, most effective electron-transporting materials possess a comparably large electron affinity whereas materials commonly used to form the light-emitting layer have a lower electron affinity, thus blocking or impeding electron transport from the electron-transporting layer into the light emission layer. Therefore, interfacial modification between the electron-transporting layer and the light-emitting layer is important to achieve improved electron transport from the electron-transporting layer into the light-emitting layer.
Furthermore, aluminum chelate (Alq) materials that can be used in forming the organic electron-transporting layer have a relatively low electron mobility reported to be as low as 1.4×10
−6
cm
2
/Vs at an electric field of 4×10
5
V/cm. Thus, even if both the hole-injecting electrode and the electron-injecting electrode were injecting respective electrical charge carriers perfectly into the hole-transporting and electron-transporting layers respectively, the low electron mobility of Alq becomes a limiting factor in an effort to reduction of the voltage applied between the two electrodes in order to provide efficient operation of the organic light-emitting device. In practical applications of such devices, it is very important to operate them at low-voltages and corresponding low power consumption. Moreover, the lowering of operating voltage and power consumption would also lead to improved device operational stability by reducing heat generation.
Various efforts have been made to develop improved electron-transporting materials. For example, Hamada et al. reported in
Appl. Phys. Lett
. 71, 3338 (1997) that bis(5-hydroxyflavonato)beryllium has a better electron transport feature than Alq. However, a voltage of 8V was still required to generate a luminance of 2000-3000 cd/m
2
in an organic light-emitting device using this material in an electron-transporting layer.
Kido and Matsumoto reported in
App. Phys. Lett
. 73, 2866 (1998) about a lithium-doped Alq layer as an electron-transporting layer having an increased electron conductivity compared to undoped Alq. However, it is generally believed that lithium diffuses readily in most organic materials and such lithium diffusion would create non-radiative recombination centers, thus shortening the device lifetime. This clearly indicates the importance of searching for an effective and stable electron transporting material to replace Alq in order to provide a low-voltage organic light-emitting device.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a low-voltage organic light-emitting device.
It is another object of the present invention to provide a low-voltage organic light-emitting device having an improved electron-transporting layer.
It is a further object of the present invention to provide a low-voltage organic light-emitting device having a bi-layer interfacial structure formed between the electron-transporting layer and an organic light-emitting layer so as to provide effective electron transport from the electron-transporting layer to the organic light-emitting layer.
These objects are achieved in a low-voltage organic light-emitting ice, comprising:
a) a substrate;
b) a hole-injecting electrode formed over the substrate;
c) an organic hole-transporting layer formed over the hole-injecting electrode;
d) an organic light-emitting layer formed over the hole-transporting layer, the organic light-emitting layer being an n-type semiconductor characterized by an electron affinity and an electron mobility;
e) a bi-layer interfacial structure formed over the organic light-emitting layer;
f) an electron-transporting layer formed over the bi-layer interfacial structure, the electron-transporting layer being an n-type semiconductor characterized by an electron affinity and an electron mobility which are higher than the electron affinity and the electron mobility of the organic light-emitting layer wherein the bi-layer interfacial structure provides effective electron transport from the electron-transporting layer to the organic light-emitting layer; and
g) an electron-injecting electrode formed over the electron-transporting layer.
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Eastman Kodak Company
Kim Robert H.
Owena Raymond L.
Yun Jurie
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