Transparent electron-injecting electrode for use in an electroluminescent device

An electroluminescent device containing a transparent electron-injecting electrode is disclosed. The electrode includes a thin nonconductive layer contacting the electroluminescent layer, a conductive transparent overcoat layer, and the thickness of the nonconductive layer being selected so that the bilayer acts as an electron injecting contact, the bilayer providing stability against atmospheric corrosion.

CROSS REFERENCE TO RELATED APPLICATION 
Reference is made to commonly assigned U.S. Ser. No. 08/681,680, filed 
concurrently and entitled "Bilayer Electron-Injecting Electrode For Use in 
an Electroluminescent Device" to Hung et al, and commonly-assigned U.S. 
Ser. No. 08/681,565, filed concurrently and entitled "Bilayer Electrode on 
a N-type Semiconductor" by Hung et al, which is now U.S. Pat. No. 
5,677,572 the disclosures of which are incorporated by reference. 
FIELD OF THE INVENTION 
The present invention relates to transparent electron-injecting electrodes 
which are particularly effective for use with organic LED devices used in 
electroluminescent structures. 
BACKGROUND OF THE INVENTION 
There is increasing interest in the fabrication of organic 
electroluminescent (EL) devices which are capable of emitting light from 
the top surface. One important application is to achieve monolithic 
integration with an array of organic EL devices on an active semiconductor 
substrate where driver electronics and pixel switching elements are 
incorporated. 
In a normal configuration, the organic EL device is constructed on a 
transparent substrate such as a piece of glass through which the light 
emitted by the device is viewed. Thus, the EL structure comprises, in 
sequence, a glass support, a transparent conductive hole-injecting 
electrode, organic EL layers, and an electron-injecting electrode. The 
electron-injecting electrode, which constitutes the top layer of the EL 
structure, is generally not required to be light-transmissive. However, 
when a semiconductor wafer such as Si is used as the substrate, the light 
emission through the substrate is blocked. It is therefore necessary that 
the electron-injecting cathode is made light-transmissive, so that the EL 
light can exit through this layer. The configuration is commonly known as 
surface-emitting EL device. 
It is the object of the invention to provide a bilayer structure which is 
highly transmissive and acts as an effective electron-injecting electrode 
in an organic EL device. The structure consists of a thin layer of metal 
fluorides or oxides and a layer of electrically conductive films. 
It is another object of this invention to provide an effective electron 
injecting contact which is stable and resistant to atmosphere oxidation or 
corrosion. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a transparent 
electron-injecting electrode for use with an electroluminescent device. 
This object is achieved in an electroluminescent device containing a 
transparent electron-injecting electrode, the electrode comprising: 
a) a thin nonconductive layer contacting the electroluminescent layer, 
b) a conductive transparent overcoat layer; and 
c) the thickness of the nonconductive layer being selected so that the 
bilayer acts as an electron injecting contact, the bilayer providing 
stability against atmospheric corrosion. 
Specifically, when the invention is embodied in an organic 
electroluminescent device, the device has a conductive hole-injecting 
anode, an organic electroluminescent layer, and a top transparent 
electron-injecting electrode in contact with the organic 
electroluminescent layer, the electrode comprising: 
a) a fluoride or a oxide layer contacting the organic layer; 
b) a transparent conductive overcoat layer over the fluoride or oxide 
layer; and 
c) the thickness of the fluoride or oxide layer being selected so that the 
bilayer acts as an electron injecting contact, the bilayer providing 
stability against atmospheric corrosion. 
ADVANTAGES OF THE INVENTION 
There are a number of metal oxides having high electrical conductivity and 
low absorbance to visible light, however, these materials generally have a 
high work function, thus precluding their use for electron injection. The 
disclosed structures combine those oxides with a thin layer of alkali or 
alkaline earth fluorides or oxides to form a desirable transparent 
electron injector. 
In general, one can invert the structure to make a surface emitting diode, 
where the device comprises in order: a substrate, an electron injector, an 
organic single layer or multilayer structure for electroluminescence and 
carrier confinement, and a layer of high-work-function material as a hole 
injector which is transmissive to optical radiation. However, the inverted 
structures may cause problems, such as structure incompatibility and 
degraded device performance. In this invention, the surface-emitting diode 
without layer reversal is disclosed.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, an electroluminescence device 10 of the invention has, 
in order, a substrate 11, a bottom hole-injecting electrode 13, an organic 
layer structure 15, a top electron-injecting electrode 17. The top 
electrode includes a fluoride or fluoride layer 17a and a conductive 
overlayer 17b. 
Substrate 11 is a single crystal semiconductor substrate selected from the 
group consisting of Si, Ge, GaAs, GaP, GaN, GaSb, InAs, InP, InSb, or 
Al.sub.x Ga.sub.1-x As, where x is from 0 to 1. Substrate 11 can be either 
undoped, lightly doped, or heavily doped. Substrate 11 is either bare or 
covered with a layer of dielectric material such as Si oxides or Si 
nitrides. In some applications, part of the semiconductor can be used as 
substrate 11 for electroluminescent device 10, while the remainder of the 
semiconductor wafer can be processed to form drivers, switches, or other 
electronic circuitry. 
Bottom hole-injecting electrode layer 13 acts as a hole injector having a 
high work function with a value greater than 4.2 eV and good stability in 
ambient. This layer 13 is either a conductive oxide or a metal layer. 
Suitable metal oxides include indium-tin-oxide, aluminum- or indium-doped 
zinc oxide, tin oxide, magnesium-indium-oxide, nickel-tungsten oxide, and 
cadmium-tin-oxide. Suitable metals include gold, silver, nickel, 
palladium, and platinum. The desired metal oxides and metals can be 
deposited by evaporation, sputtering, laser ablation, and chemical vapor 
deposition. A thickness ranging from 10 to 1000 nm is useful, and the 
preferred range is from 30-500 nm. 
Organic layer structure 15 either has a single layer acting as a light 
emitter or a multilayer structure, including a light emitter and 
carrier-confinement layers. For instance, a useful structure includes an 
undoped and doped Alq layer as the emitter and a diamine layer for 
hole-transporting, as described in U.S. Pat. Nos. 5,294,869, and 
5,151,629. Other materials suitable for use as light emitters include 
poly(paraphenylene vinylene) (PPV), PPV copolymers and derivatives, 
polyaniline, poly(3-alkylthiophene), poly(3-octylthiophene), 
poly(paraphenylene), and fluorescent dyes and pigments. Organic layer 
structure 15 can be prepared by thermal evaporation or spin-coating from a 
solution. 
Top electron-injecting electrode 17 acts as an transmissive electron 
injector with a good stability against atmospheric oxidation. The 
electrode is a bilayer having a thin fluoride layer 17a and a thick 
conductive overlayer 17b. The inner layer, that is the layer in contact 
with the organic EL layer, is an essential part in this invention. The 
materials must have a low electron affinity or a strong dipole character. 
These are critical characteristics of the possible material choices. 
Besides fluorides and oxides of alkali and alkaline earth metals, other 
candidates may include their mixture, chlorides, iodides, and tellurides. 
The outer layer is also important to this invention. The layer must be 
conductive and transparent. The materials can be selected from the groups 
of oxides, nitrides, and sulfides. Suitable metal oxides include 
indium-tin-oxide, aluminum- or indium-doped zinc oxide, tin oxide, 
magnesium-indium-oxide, nickel-tungsten oxide, and cadmium-tin-oxide. 
Suitable nitrides include gallium nitride, indium nitride, or mixtures of 
Group III sulfide includes Group II sulfides such as ZnS. In some cases, a 
thin metal layer may be used as the outer layer to form a semi-transparent 
cathode. Suitable metals include gold, silver, aluminum, nickel, 
palladium, and platinum. When a thin metal layer is used for 
electron-injecting, a transparent encapsulating layer may be needed to 
protect the organic layered structure from moisture attack. 
In accordance with this invention, the thickness of the nonconductive layer 
should be from 0.3 to 5.0 nm, preferably 0.5 to 1.0 nm. When the thickness 
is below 0.3 nm, the layer can not fully cover its underlying organic 
layer. When the thickness is above 5.0 nm, the applied current can not 
pass through the bilayer into the organic layer. A useful range of the 
conductive layer thickness is from 10 to 1000 nm, preferably 50-500 nm. 
Electrode 17 can be deposited by many conventional means, such as 
evaporation, sputtering, laser ablation, and chemical vapor deposition. 
The following examples are presented for a further understanding of the 
invention. 
EXAMPLE 1 
An organic EL device satisfying the requirements of the invention was 
constructed in the following manner: 
a) a transparent anode of indium tin oxide coated glass was ultrasonicated 
in a commercial detergent, rinsed in deionized water, degreased in toluene 
vapor, and contacted a strong oxidizing agent; 
b) a 15 nm-thick CuPc layer was deposited on the anode; 
c) a 60 nm-thick hole transporting NPB layer was deposited on the CuPc 
layer; 
d) a 75 nm-thick electron transporting Alq layer was deposited on the NPB 
layer; 
e) a 0.5 nm-thick lithium fluoride layer was deposited on the Alq layer; 
and 
f) a 120 nm-thick aluminum layer was deposited on the LiF layer. 
All the materials were prepared by thermal evaporation from tantalum boats. 
EXAMPLE 2 
The same materials and processing procedures were employed as described in 
Example 1, except that the lithium fluoride layer was replaced by a 
magnesium fluoride, a calcium fluoride, a lithium oxide, or a magnesium 
oxide layer. 
EXAMPLE 3 
The same materials and processing procedures were employed as described in 
Example 1, except that the lithium fluoride layer was replaced by a 
germanium di-oxide or a silicon di-oxide layer. 
EXAMPLE 4 
An organic EL device was constructed in the following manner: 
a) a transparent anode of indium tin oxide coated glass was ultrasonicated 
in a commercial detergent, rinsed in deionized water, degreased in toluene 
vapor, and contacted a strong oxidizing agent; 
b) a 15 nm-thick CuPc layer was deposited on the anode; 
c) a 60 nm-thick hole transporting NPB layer was deposited on the CuPc 
layer; 
d) a 75 nm-thick electron transporting Alq layer was deposited on the NPB 
layer; and 
e) a 120 nm- thick aluminum layer or a 200 nm-thick Mg.sub.0.9 Ag.sub.0.1 
layer was deposited on the Alq layer. 
All the materials were prepared by thermal evaporation from tantalum boats. 
All the devices were evaluated with a positive potential applied to the 
anode and the cathode attached to ground to determine the characteristics 
of voltage-current and current-light emission, and the results are 
summarized in FIG. 2. In the plot, the horizontal axis shows the drive 
voltage to generate a light output of 0.1 mW/cm.sup.2, and the vertical 
axis shows the electroluminescence efficiency. The device with an aluminum 
cathode requires a drive voltage of approximately 12 V to generate a light 
output of 0.1 mW /cm.sup.2, which is substantially higher than that of the 
device with a MgAg cathode. Occurring with the higher drive voltage is a 
lower EL efficiency. The difference is attributed to a higher work 
function of Al (4.3 eV) than that of Mg (3.7 eV). It is surprising, 
however, that the device performance with an Al cathode can be 
dramatically improved by interposing an one- to two- monolayer of LiF, 
MgF.sub.2, CaF.sub.2, Li.sub.2 O, or MgO between Alq and Al. For instance, 
with a bilayer athode (Al/LiF) the drive voltage is reduced to 7.4 V, and 
the EL efficiency is increased to 0.028 mW/cm.sup.2. The results are much 
better than that with a MgAg cathode. 
EXAMPLE 5 
An organic EL device was constructed in the following manner: 
a) a transparent anode of indium tin oxide coated glass was ultrasonicated 
in a commercial detergent, rinsed in deionized water, degreased in toluene 
vapor, and contacted a strong oxidizing agent; 
b) a 0.5 nm-thick lithium fluoride layer was deposited on the ITO; 
c) a 75 nm-thick electron transporting Alq layer was deposited on the LiF 
layer; 
d) a 60 nm-thick hole transporting NPB layer was deposited on the Alq 
layer; 
e) a 15 nm-thick CuPc layer was deposited on the NPB layer; and 
f) a 50 nm- thick silver layer was deposited on the CuPc layer. 
EXAMPLE 6 
An organic EL device was constructed in the following manner: 
a) a transparent anode of indium tin oxide coated glass was ultrasonicated 
in a commercial detergent, rinsed in deionized water, degreased in toluene 
vapor, and contacted a strong oxidizing agent; 
b) a 75 nm-thick electron transporting Alq layer was deposited on the ITO; 
c) a 60 nm-thick hole transporting NPB layer was deposited on the Alq 
layer; 
e) a 15 nm-thick CuPc layer was deposited on the NPB layer; and 
f) a 50 nm- thick silver layer was deposited on the CuPc layer. 
The two devices were evaluated with a positive potential applied to the top 
electrode (Ag in Example 5 and Au in Example 6) and the bottom electrode 
(LiF/ITO in Example 5 and ITO in Example 6) attached to ground to 
determine the characteristics of voltage-current and current-light 
emission, and the results are summarized in FIG. 3. The device current in 
Example 6 was quite low although a high bias was applied, and no light 
output was detected. The device performance with an ITO cathode was found 
to be dramatically improved by interposing an one- to two-monolayer of LiF 
between Alq and ITO. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention. 
______________________________________ 
Parts List 
______________________________________ 
10 electroluminescent device 
11 substrate 
13 bottom hole-injecting electrode 
15 organic layer structure 
17 top electron-injecting electrode 
17a fluoride 
17b conductive overlayer 
______________________________________