U.S. patent application number 11/211045 was filed with the patent office on 2006-06-29 for electrode for an electronic device.
This patent application is currently assigned to Osram Opto Semiconducts GmbH. Invention is credited to Pierre-Marc Allemand, Reza Stegamat.
Application Number | 20060138656 11/211045 |
Document ID | / |
Family ID | 33451394 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060138656 |
Kind Code |
A1 |
Stegamat; Reza ; et
al. |
June 29, 2006 |
Electrode for an electronic device
Abstract
An embodiment of the present invention pertains to an electrode
that includes a metal oxide layer, and a conductive layer on that
metal oxide layer. The metal oxide layer is an alkali metal oxide
or an alkaline earth metal oxide that is formed by: (1) decomposing
a compound that includes (a) oxygen and (b) an alkali metal or an
alkaline earth metal, or (2) thermally reacting at least two
compounds where one of the at least two compounds includes the
alkali metal or the alkaline earth metal, and another one of the at
least two compounds includes oxygen. The metal oxide layer can also
be formed by thermally reacting at least two compounds where one of
those compounds includes (a) oxygen and (b) an alkali metal or an
alkaline earth metal.
Inventors: |
Stegamat; Reza; (Milpitas,
CA) ; Allemand; Pierre-Marc; (San Jose, CA) |
Correspondence
Address: |
SIEMENS CORPORATION;INTELLECTUAL PROPERTY DEPARTMENT
170 WOOD AVENUE SOUTH
ISELIN
NJ
08830
US
|
Assignee: |
Osram Opto Semiconducts
GmbH
|
Family ID: |
33451394 |
Appl. No.: |
11/211045 |
Filed: |
August 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10447988 |
May 29, 2003 |
6946319 |
|
|
11211045 |
Aug 23, 2005 |
|
|
|
Current U.S.
Class: |
257/734 |
Current CPC
Class: |
H01L 51/5092
20130101 |
Class at
Publication: |
257/734 |
International
Class: |
H01L 23/48 20060101
H01L023/48 |
Claims
1-44. (canceled)
45. An electrode, comprising: a metal oxide layer; and a conductive
layer on said metal oxide layer, wherein said metal oxide layer is
an alkali metal oxide or an alkaline earth metal oxide that is
formed by: (1) decomposing a compound that includes (a) oxygen and
(b) an alkali metal or an alkaline earth metal, or (2) thermally
reacting at least two compounds where one of said at least two
compounds includes said alkali metal or said alkaline earth metal,
and another one of said at least two compounds includes said
oxygen.
46. The electrode of claim 45 wherein said compound that includes
(a) said oxygen and (b) said alkali metal or said alkaline earth
metal is a salt that includes (a) said oxygen and (b) said alkali
metal or said alkaline earth metal; and said one of said at least
two compounds that includes said alkali metal or said alkaline
earth metal is a salt that includes said alkali metal or said
alkaline earth metal.
47. The electrode of claim 46 wherein said salt that includes (a)
said oxygen and (b) said alkali metal or said alkaline earth metal
is any one of: formates, acetates, carbonates, bicarbonates,
sulphates, nitrates, or oxalates of said alkali metal or said
alkaline earth metal; and said salt that includes said alkali metal
or said alkaline earth metal is any one of: formates, acetates,
carbonates, bicarbonates, sulphates, nitrates, or oxalates of said
alkali metal or said alkaline earth metal.
48. The electrode of claim 45 wherein said metal oxide layer is a
cesium oxide layer; said compound that includes (a) said oxygen and
(b) said alkali metal or said alkaline earth metal is a compound
that includes (a) said oxygen and (b) cesium; and said one of said
at least two compounds includes said cesium.
49. The electrode of claim 48 wherein said compound that includes
(a) said oxygen and (b) said cesium is a cesium salt.
50. The electrode of claim 49 wherein said cesium salt is any one
of: a cesium carbonate, a cesium bicarbonate, a cesium acetate, a
cesium oxalate, or a cesium formate.
51. The electrode of claim 45 wherein said metal oxide layer is a
cesium oxide layer; said compound that includes (a) said oxygen and
(b) said alkali metal or said alkaline earth metal is cesium
carbonate; said one of said at least two compounds is cesium
sulfate; and said other one of said at least two compounds is
barium oxide.
52. The electrode of claim 51 wherein said cesium oxide layer is
any one of: CsO, Cs.sub.2O, or CSO.sub.2.
53. The electrode of claim 51 wherein said cesium oxide layer has a
thickness from about 0.2 nm to about 10 nm.
54. The electrode of claim 45 wherein decomposing said compound
that includes (a) said oxygen and (b) said alkali metal or said
alkaline earth metal includes heating said compound, and thermally
reacting said two compounds includes physically mixing said two
compounds and then heating said mixture.
55. The electrode of claim 45 wherein said metal oxide layer has a
thickness from 0.1 nm to 0.4 nm.
56. An electrode, comprising: a metal oxide layer; and a conductive
layer on said metal oxide layer, wherein said metal oxide layer is
an alkali metal oxide or an alkaline earth metal oxide that is
formed by thermally reacting at least two compounds where one of
said at least two compounds includes (1) oxygen and (2) an alkali
metal or an alkaline earth metal.
57. The electrode of claim 56 wherein said one of said at least two
compounds that includes (1) said oxygen and (2) said alkali metal
or said alkaline earth metal is a salt that includes (1) said
oxygen and (2) said alkali metal or said alkaline earth metal.
58. The electrode of claim 57 wherein said salt that includes (1)
said oxygen and (2) said alkali metal or said alkaline earth metal
is any one of: formates, acetates, carbonates, bicarbonates,
sulphates, nitrates, or oxalates of said alkali metal or said
alkaline earth metal.
59. The electrode of claim 56 wherein said metal oxide layer is a
cesium oxide layer; and said alkali metal or said alkaline earth
metal is cesium, wherein said cesium oxide layer is any one of:
CsO, Cs.sub.2O, or CsO.sub.2.
60. The electrode of claim 56 wherein thermally reacting said at
least two compounds includes physically mixing said at least two
compounds and then heating said mixture.
61. The electrode of claim 56 wherein said metal oxide layer has a
thickness from 0.1 nm to 0.4 nm.
62. An electronic device, comprising: a substrate; a first
electrode on said substrate; at least one semiconductive layer on
said first electrode; and a second electrode on said at least one
semiconductive layer, wherein a particular one of said first
electrode or said second electrode includes a metal oxide layer;
and a conductive layer on said metal oxide layer, wherein said
metal oxide layer is an alkali metal oxide or an alkaline earth
metal oxide that is formed by: (1) decomposing a compound that
includes (a) oxygen and (b) an alkali metal or an alkaline earth
metal, or (2) thermally reacting at least two compounds where a
first one of said at least two compounds includes said alkali metal
or said alkaline earth metal, and a second one of said at least two
compounds includes said oxygen, or (3) thermally reacting at least
two compounds where a particular one of said at least two compounds
includes (1) oxygen and (2) an alkali metal or an alkaline earth
metal.
63. The electronic device of claim 62 wherein said compound that
includes (a) said oxygen and (b) said alkali metal or said alkaline
earth metal is a salt that includes (a) said oxygen and (b) said
alkali metal or said alkaline earth metal; said first one of said
at least two compounds that includes said alkali metal or said
alkaline earth metal is a salt that includes said alkali metal or
said alkaline earth metal; and said particular one of said at least
two compounds that includes (1) said oxygen and (2) said alkali
metal or said alkaline earth metal is a salt that includes (1) said
oxygen and (2) said alkali metal or said alkaline earth metal.
64. The electronic device of claim 63 wherein said salt that
includes (a) said oxygen and (b) said alkali metal or said alkaline
earth metal is any one of: formates, acetates, carbonates,
bicarbonates, sulphates, nitrates, or oxalates of said alkali metal
or said alkaline earth metal; said salt that includes said alkali
metal or said alkaline earth metal is any one of: formates,
acetates, carbonates, bicarbonates, sulphates, nitrates, or
oxalates of said alkali metal or said alkaline earth metal; and
said salt that includes (1) said oxygen and (2) said alkali metal
or said alkaline earth metal is any one of: formates, acetates,
carbonates, bicarbonates, sulphates, nitrates, or oxalates of said
alkali metal or said alkaline earth metal.
65. The electronic device of claim 62 wherein said electronic
device is an organic light emitting diode and said metal oxide
layer is a cesium oxide layer; said compound that includes (a) said
oxygen and (b) said alkali metal or said alkaline earth metal is a
compound that includes (a) said oxygen and (b) cesium; said first
one of said at least two compounds includes said cesium; and said
particular one of said at least two compounds includes (1) oxygen
and (2) cesium.
66. The electronic device of claim 62 wherein said electronic
device is an organic light emitting diode and said metal oxide
layer is a cesium oxide layer; said compound that includes (a) said
oxygen and (b) said alkali metal or said alkaline earth metal is
cesium carbonate; said first one of said at least two compounds is
cesium sulfate; said second one of said at least two compounds is
barium oxide; and said particular one of said at least two
compounds includes (1) oxygen and (2) cesium.
67. The electronic device of claim 66 wherein said cesium oxide
layer has a thickness from about 0.2 nm to about 10 nm.
68. The electronic device of claim 62 wherein decomposing said
compound includes heating said compound, and thermally reacting
said at least two compounds includes physically mixing said at
least two compounds and then heating said mixture.
69. The electronic device of claim 66 wherein said at least one
semiconductive layer includes an organic emissive polymer layer and
said cesium oxide layer is on said organic emissive polymer layer,
and when said organic light emitting diode is activated, said
organic emissive polymer layer efficiently emits any one of the
following colors: (1) red, (2) green, (3) blue, or (4) white.
70. The electronic device of claim 62 wherein said metal oxide
layer has a thickness from 0.1 nm to 0.4 nm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of applicant's co-pending
application having application Ser. No. 10/447,988, filed May 29,
2003 and entitled "Electrode for an Electronic Device."
BACKGROUND OF THE INVENTION
[0002] Electronic devices need an electrode that provides a low
contact resistance with a semiconductive material. Example of
electronic devices include organic and inorganic electronic
devices. Examples of organic electronic devices include organic
light emitting diodes ("OLEDs") (the OLED can be used in, for
example, a display or as a light source element of a light source
used for general purpose lighting), organic solar cells, organic
transistors, organic detectors, and organic lasers. Such devices
typically include a pair of electrodes (e.g., an anode and a
cathode) with at least one semiconductive layer between the
electrodes.
[0003] In the particular case of the OLED, the OLED is typically
comprised of two or more thin organic layers (e.g., a conducting
polymer layer and an emissive polymer layer where the emissive
polymer layer emits light) separating its anode and cathode. Under
an applied potential, the anode injects holes into the conducting
polymer layer, while the cathode injects electrons into the
emissive polymer layer. The injected holes and electrons each
migrate toward the oppositely charged electrode and produce an
electroluminescent emission upon recombination in the emissive
polymer layer.
[0004] The OLED's cathode is typically a multilayer structure that
includes, generally, a thin electron injecting layer that has a low
work function, and also a thick conductive layer such as, for
example, aluminum, silver, magnesium, gold, copper, or a mixture
thereof. The electron injecting layer with the low work function
provides an electrically conductive path for current flow as well
as a way to efficiently inject electrons into the adjacent emissive
polymer layer. One problem with low work function metals is that
they readily react with the environment (e.g., oxygen and
moisture). For example, a low work function calcium cathode
survives only a short time in air due to rapid device degradation
caused by atmospheric moisture and oxygen. It would be desirable to
have an electron injecting layer that has a low work function and
is less likely to react with the environment.
[0005] In addition, the electron injecting layer can be difficult
to deposit on the semiconductive layer of the electronic device
because, for example, some materials have a high melting point. It
would be desirable to have an electron injecting layer that can be
easily deposited on the semiconductive layer.
[0006] In the OLED, typically, different electron injecting
materials are used depending on the type of emissive polymer layer
on which it is placed (different types of emissive polymer layers
are used depending on, for example, the desired color that is to be
emitted by the layer). For example, a first electron injecting
layer placed on an emissive polymer layer that emits the color blue
can provide a lower drive voltage than a second electron injecting
layer comprised of a different material. Specifically, in terms of
overall performance, barium and calcium are the most common and
effective materials for use as the electron injecting layer in OLED
devices that emit the color green or yellow. For an OLED that emits
the color blue, lithium fluoride is the most common material for
use as the electron injecting layer in this type of OLED (the
lithium fluoride is typically capped by a calcium layer). When
fabricating full color displays, it is desirable to have one
electron injecting layer that can be effectively used with emissive
polymer layers that emit any of the colors.
[0007] For the foregoing reasons, there exists a need for an
electron injecting layer that has a low work function, is less
likely to react with the environment, can be easily deposited, and
in the case of an organic light emitting diode, effectively
interfaces with different emissive polymer layers that emit
different colors.
SUMMARY
[0008] A first embodiment of an electrode is described. This
electrode includes a metal oxide layer, and a conductive layer on
the metal oxide layer. The metal oxide layer is an alkali metal
oxide or an alkaline earth metal oxide that is formed by: (1)
decomposing a compound that includes (a) oxygen and (b) an alkali
metal or an alkaline earth metal, or (2) thermally reacting at
least two compounds where one of the at least two compounds
includes the alkali metal or the alkaline earth metal, and another
one of the at least two compounds includes oxygen.
[0009] A second embodiment of an electrode is described. This
electrode includes a metal oxide layer, and a conductive layer on
the metal oxide layer. The metal oxide layer is an alkali metal
oxide or an alkaline earth metal oxide that is formed by thermally
reacting at least two compounds where one of the at least two
compounds includes (1) oxygen and (2) an alkali metal or an
alkaline earth metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows an embodiment of an electrode according to the
present invention.
[0011] FIG. 2 shows an embodiment of an electronic device according
to the present invention.
[0012] FIG. 3 shows an embodiment of an OLED according to the
present invention.
[0013] FIG. 4 shows the current density versus voltage curve, and
the luminance versus voltage curve for an OLED with barium as the
electron injecting layer.
[0014] FIG. 5 shows the current density versus voltage curve, and
the luminance versus voltage curve for an OLED with cesium oxide as
the electron injecting layer.
[0015] FIG. 6 is a table that compares the efficiency and luminance
of OLEDs in which the electron injecting layer is barium and cesium
oxide.
[0016] FIG. 7 shows the average luminance versus voltage graph for
four different sets of displays.
[0017] FIG. 8 is a table that compares the average luminance and
the average half-life of three different sets of displays with
different cesium oxide layer thicknesses.
DETAILED DESCRIPTION
[0018] FIG. 1 shows an embodiment of an electrode 118 according to
the present invention. In FIG. 1, the electrode 118 is on a
semiconductor 115. As used within the specification and the claims,
the term "on" includes when layers are in physical contact and when
layers are separated by one or more intervening layers. The
semiconductor 115 includes inorganic materials such as, for
example, cadium sulfide, gallium sulfide, zinc sulfide, or a
compound made of elements from groups III-V of the periodic table.
The semiconductor 115 also includes organic materials such as, for
example, polyphenylenevinylene ("PPV"), PPV derivatives,
polyfluorene ("PF"), PF derivatives, or small molecules such as
metal chelate compounds such as, for example,
aluminumhydroxyquinolate ("Alq3").
[0019] The electrode 118 includes: (1) an electron injecting layer
comprised of a metal oxide layer 118a , and (2) a conductive layer
118b . The conductive layer 118b is comprised of a metallic layer
such as, for example, silver, aluminum, magnesium, gold, copper, or
a mixture thereof. The metal oxide layer 118a is either an alkali
metal oxide or an alkaline earth metal oxide and is formed by: (1)
decomposing a compound that includes (a) oxygen and (b) an alkali
metal or an alkaline earth metal; (2) thermally reacting at least
two compounds where one of the compounds includes an alkali metal
or an alkaline earth metal, and another one of the compounds
includes oxygen; or (3) thermally reacting at least two compounds
where one of those compounds includes (a) oxygen and (b) an alkali
metal or an alkaline earth metal.
[0020] The term "compound", as used herein, includes salts. For
example, for (1) in the previous paragraph, the compound that
includes (a) oxygen and (b) an alkali metal or an alkaline earth
metal can be a salt that includes (a) oxygen and (b) an alkali
metal or an alkaline earth metal. The salt can be, for example,
formates, acetates, carbonates, bicarbonates, sulphates, nitrates,
or oxalates of the an alkali metal or the alkaline earth metal. In
addition, for (2) in the previous paragraph, the one of the
compounds that includes the alkali metal or the alkaline earth
metal can be a salt that includes the alkali metal or the alkaline
earth metal. For (3) in the previous paragraph, the particular one
of the compounds that includes (a) oxygen and (b) an alkali metal
or an alkaline earth metal can be a salt that includes oxygen and
an alkali metal or an alkaline earth metal.
[0021] In one configuration of this embodiment, the decomposing of
the compound occurs by heating it so that the metal oxide (i.e.,
the alkali metal oxide or the alkaline earth metal oxide) is
evaporated onto the semiconductor 115 while residual gasses are
removed. Thermally reacting the at least two compounds includes
physically mixing the compounds and then heating the mixture so
that the metal oxide is evaporated onto the semiconductor 115 while
the residual gasses are removed.
[0022] The term "alkali metal" is used in the conventional sense to
refer to elements of Group IA of the periodic table. Preferred
alkali metals include lithium (i.e., Li), sodium (i.e., Na),
potassium (i.e., K), rubidium (i.e., Rb), or cesium (i.e., Cs). The
term "alkali metal oxide" is used in the conventional sense to
refer to compounds of one or more alkali metals and oxygen. For
convenience, alkali metal oxides are referred to herein by the
chemical formula of the corresponding simple oxide (e.g.,
Li.sub.2O, Na.sub.2O, K.sub.2O, R.sub.2O, or Cs.sub.2O); however,
this reference to the simple oxide is intended to encompass other
oxides, including mixed oxides and non-stoichiometric oxides (e.g.,
Li.sub.xO, Na.sub.xO, K.sub.xO, Rb.sub.xO, or Cs.sub.xO, where x is
from about 0.1 to about 2).
[0023] The term "alkaline earth metal" is used in the conventional
sense to refer to elements of Group IIA of the periodic table.
Preferred alkaline earth metals include magnesium (i.e., Mg),
calcium (i.e., Ca), strontium (i.e., Sr), or barium (i.e., Ba). The
term "alkaline earth metal oxide" is used in the conventional sense
to refer to compounds of one or more alkaline earth metals and
oxygen. For convenience, alkaline earth metal oxides are referred
to by the chemical formula of the corresponding simple oxide (e.g.,
MgO, BaO, CaO, SrO, or BaO); however, this reference to the simple
oxide is intended to encompass other oxides, including mixed oxides
and non-stoichiometric oxides (e.g., Mg.sub.xO, Ba.sub.xO,
Ca.sub.xO, Sr.sub.xO, or Ba.sub.xO, where x is from about 0.1 to
about 1).
[0024] The range of thickness of the metal oxide layer 118a is such
that the metal oxide layer 118a forms a continuous film but not too
thick that the flow of electrons from the conductive layer 118b to
the semiconductor 115 is substantially reduced. Specifically, the
metal oxide layer 118a is a thin layer and the range of thickness
of the metal oxide layer 118a is from about 0.1 nanometers ("nm")
to about 20 nm; preferably, is from about 0.1 nm to about 10 nm;
and more preferably, is from about 0.3 nm to about 1 nm.
[0025] The alkali metal oxide and the alkaline earth metal oxide
have a low work function and reduce or eliminate the barrier height
for electron injection from the conductive layer 118b to the lowest
unoccupied molecular orbital ("LUMO") of the semiconductor 115.
[0026] Preferably, the metal oxide layer 118a is a cesium oxide
layer that is formed by: (1) decomposing a compound that includes
(a) the oxygen and (b) the cesium; (2) thermally reacting at least
two compounds where one of the compounds includes the cesium, and
another one of the compounds includes the oxygen; or (3) thermally
reacting at least two compounds where one of those compounds
includes (a) the oxygen and (b) the cesium. For (1), the compound
that includes (a) the oxygen and (b) the cesium can be a cesium
salt such as, for example, a cesium carbonate, a cesium
bicarbonate, a cesium acetate, a cesium oxalate, or a cesium
formate.
[0027] More preferably, the metal oxide layer 118a is a cesium
oxide layer that is formed by: (1) decomposing cesium carbonate; or
(2) thermally reacting cesium sulfate and barium oxide. For (1),
the cesium carbonate is decomposed by applying heat to the cesium
carbonate as shown in the following reaction:
Cs.sub.2CO.sub.3(s)+heat.fwdarw.Cs.sub.2O(s)+CO.sub.2(g) In one
configuration of this embodiment, the heating of the cesium
carbonate (which is in a solid state) occurs in a vacuum chamber,
and after sufficient heat is applied, the resulting cesium oxide
(which is in a solid state) is evaporated onto the semiconductor
115 while the resulting carbon dioxide (which is in a gaseous
state) is removed from the vacuum chamber.
[0028] For (2), the cesium sulfate (which is in a solid state) and
the barium oxide (which is in a solid state) are physically mixed
and this mixture is heated as shown in the following reaction:
Cs.sub.2SO.sub.4(s)+BaO(s)+heat.fwdarw.Cs.sub.2O(s)+BaSO.sub.4(g)
In one configuration of this embodiment, the heating of the mixture
occurs in a vacuum chamber and after sufficient heat is applied,
the resulting cesium oxide (which is in a solid state) is
evaporated onto the semiconductor 115 while the resulting barium
sulfate (which is in a gaseous state) is removed from the vacuum
chamber.
[0029] The cesium oxide layer can be any one of the following: CsO,
Cs.sub.2O, or CsO.sub.2. The range of thickness of the cesium oxide
layer (e.g., the metal oxide layer 118a ) is such that the cesium
oxide layer forms a continuous film but not too thick that the flow
of electrons from the conductive layer 118b to the semiconductor
115 is substantially reduced. Specifically, the range of thickness
of the cesium oxide layer is from about 0.1 nm to about 10 nm;
preferably, is from about 0.3 nm to about 1 nm; more preferably,
from about 0.3 nm to about 0.5 nm; and most preferably, about 0.3
nm.
[0030] The cesium oxide layer has a low work function of, for
example, 0.7 eV thus reducing or eliminating the barrier height for
electron injection from the conductive layer 118b to the lowest
unoccupied molecular orbital ("LUMO") of the semiconductor 115.
Metal oxides are also more stable than metals in their pure form.
For example, cesium oxide is more stable in air than, for example,
cesium, barium, or calcium metals. A thin layer of cesium is hard
and only slightly hydroscopic when exposed to air. Typically,
alkali metals and alkali earth metals almost instantaneously react
with air to form oxides or hydroxides. The cesium oxide layer is
less likely to react with the environment than other electron
injecting materials such as barium or calcium since cesium oxide is
already the oxidation product resulting from the oxidation of
metallic cesium by oxygen.
[0031] FIG. 2 shows an embodiment of an electronic device 305
according to the present invention. The electronic device 305
includes a substrate 308 and a first electrode 311 on the substrate
308. The first electrode 311 may be patterned for pixilated
applications or unpatterned for backlight applications. If the
electronic device 305 is a transistor, then the first electrode may
be, for example, the source and drain contacts of that transistor.
The electronic device 305 also includes one or more semiconductor
layers 314 on the first electrode 311. The semiconductor layers 314
can be comprised of organic or inorganic materials. The electronic
device 305 includes a second electrode 317 on the one or more
semiconductor layers 314. If the electronic device 305 is a
transistor, then the second electrode may be, for example, the gate
contact of that transistor. The second electrode 317 includes: (1)
an electron injecting layer comprised of a metal oxide layer 317a ,
and (2) a conductive layer 317b . The conductive layer 317b is
comprised of a metallic layer such as, for example, silver,
aluminum, magnesium, gold, copper, or a mixture thereof.
[0032] The metal oxide layer 317a is either an alkali metal oxide
or an alkaline earth metal oxide and is formed by: (1) decomposing
a compound that includes (a) oxygen and (b) an alkali metal or an
alkaline earth metal; (2) thermally reacting at least two compounds
where one of the compounds includes an alkali metal or an alkaline
earth metal, and another one of the compounds includes oxygen; or
(3) thermally reacting at least two compounds where one of those
compounds includes (a) oxygen and (b) an alkali metal or an
alkaline earth metal.
[0033] Preferably, the metal oxide layer 317a is a cesium oxide
layer that is formed by: (1) decomposing a compound that includes
(a) the oxygen and (b) the cesium; (2) thermally reacting at least
two compounds where one of the compounds includes the cesium, and
another one of the compounds includes the oxygen; or (3) thermally
reacting at least two compounds where one of those compounds
includes (a) the oxygen and (b) the cesium. More preferably, the
metal oxide layer 317a is a cesium oxide layer that is formed by:
(1) decomposing cesium carbonate; or (2) thermally reacting cesium
sulfate and barium oxide.
[0034] Other layers than that shown in FIG. 2 may be added
including insulating layers between the first electrode 311 and the
one or more semiconductor layers 314, and/or between the one or
more semiconductor layers 314 and the second electrode 317.
[0035] A specific example of an electronic device is an OLED. FIG.
3 shows an embodiment of an OLED 353 according to the present
invention. The OLED 353 includes a flexible or rigid substrate 356
that may be comprised of, for example, glass or plastic. The OLED
353 also includes a first electrode such as an anode layer 359 that
is deposited on the substrate 356. The anode layer 359 may be, for
example, indium tin oxide ("ITO"). The OLED 353 also includes at
least one semiconductor layer, preferably, two organic layers: a
conducting polymer layer 362 that is deposited on the anode layer
359, and an emissive polymer layer 365 that is deposited on the
conducting polymer layer 362. The conducting polymer layer 362
assists in injecting and transporting holes. The emissive polymer
layer 365 assists in injecting and transporting electrons. In one
configuration of this embodiment, the emissive polymer layer 365
emits light. In another configuration, another separate layer is
deposited that emits light. The OLED 353 includes a second
electrode that is a cathode layer 368 that is deposited on the
emissive polymer layer 365. The cathode layer 368 includes: (1) an
electron injecting layer comprised of a metal oxide layer 368a ,
and (2) a conductive layer 368b . The conductive layer 368b is
comprised of a metallic layer such as, for example, silver,
aluminum, magnesium, copper, gold, or a mixture thereof. The metal
oxide layer 118a is a thin layer comprised of either an alkali
metal oxide or an alkaline earth metal oxide.
[0036] Alternatively, in another embodiment of the OLED, the
cathode layer, rather than the anode layer, is deposited on the
substrate. The emissive polymer layer is deposited on the cathode
layer and the conducting polymer layer is deposited on the emissive
polymer layer. The anode layer is deposited on the conducting
polymer layer.
[0037] The OLED layers mentioned earlier are discussed in greater
detail below:
Substrate 356:
[0038] The substrate 356 can be any material, which can support the
layers, and is transparent or semi-transparent to the wavelength of
light generated in the device. By modifying or filtering the
wavelength of light which can pass through the substrate, the color
of light emitted by the device can be changed. Preferable substrate
materials include glass, quartz, silicon, and plastic, preferably,
thin, flexible glass. The preferred thickness of the substrate 356
depends on the material used and on the application of the device.
The substrate 356 can be in the form of a sheet or continuous film,
such as preferably used for roll-to-roll manufacturing processes,
which are particularly suited for plastic, metal, and metallized
plastic foils.
Anode Layer 359:
[0039] The anode layer 359 is a conductive layer which serves as a
hole-injecting layer and which comprises a material with work
function greater than about 4.5 eV. Typical anode materials include
metals (such as aluminum, silver, platinum, gold, palladium,
tungsten, indium, copper, iron, nickel, zinc, lead, and the like);
metal oxides (such as lead oxide, tin oxide, ITO, and the like);
graphite; doped inorganic semiconductors (such as silicon,
germanium, gallium arsenide, and the like); and doped conducting
polymers (such as polyaniline, polypyrrole, polythiophene, and the
like). When metals such as those listed above are used, the anode
layer 359 is typically sufficiently thin so as to be
semi-transparent to the light emitted from the emissive layer.
Metal oxides such as ITO and conducting polymers such as
polyaniline and polypyrrole are typically semi-transparent in the
visible portion of the spectrum.
[0040] The anode layer 359 can typically be fabricated using any of
the techniques known in the art for deposition of thin films,
including, for example, vacuum evaporation, sputtering, electron
beam deposition, or chemical vapor deposition, using for example,
pure metals or alloys, or other film precursors. Typically, the
anode layer 359 has a thickness of about 30 nm to about 300 nm.
Conducting Polymer Layer 362:
[0041] The conducting polymer layer 362 is used to enhance the hole
yield of the OLED in relation to the electric potential applied.
Preferred conductive polymers include, but are not limited to
polyethylenedioxythiophene-polystyrenesulfonic acid ("PEDOT:PSS")
and polyaniline ("PANI").
[0042] Preferably, the thickness of the conducting polymer layer
362 is from about 5 to about 1000 nm, more preferably from about 20
to about 500 nm, and most preferably from about 50 to about 250
nm.
[0043] The conducting polymer layer 362 is usually applied in the
form of a solution. Many application methods are known to those of
ordinary skill in the art. Examples include, but are not limited
to, spin coating, dip coating, roll coating, spray-coating, blade
coating, or thermal evaporation onto the anode layer 359. Printing
techniques including, but not limited to, screen-printing,
flexographic printing, and ink-jet printing (drop-on-demand,
continuous, or semi-continuous) may also be used to apply the
conducting polymer layer 362.
Emissive Polymer Layer 365:
[0044] For OLEDs, the emissive polymer layer 365 comprises an
electroluminescent, semiconductor, organic material. Examples of
the emissive polymer layer 365 include: [0045] (i) poly(p-phenylene
vinylene) and its derivatives substituted at various positions on
the phenylene moiety; [0046] (ii) poly(p-phenylene vinylene) and
its derivatives substituted at various positions on the vinylene
moiety; [0047] (iii) poly(p-phenylene vinylene) and its derivatives
substituted at various positions on the phenylene moiety and also
substituted at various positions on the vinylene moiety; [0048]
(iv) poly(arylene vinylene), where the arylene may be such moieties
as naphthalene, anthracene, furylene, thienylene, oxadiazole, and
the like; [0049] (v) derivatives of poly(arylene vinylene), where
the arylene may be as in (iv) above, and additionally have
substituents at various positions on the arylene; [0050] (vi)
derivatives of poly(arylene vinylene), where the arylene may be as
in (iv) above, and additionally have substituents at various
positions on the vinylene; [0051] (vii) derivatives of poly(arylene
vinylene), where the arylene may be as in (iv) above, and
additionally have substituents at various positions on the arylene
and substituents at various positions on the vinylene; [0052]
(viii) co-polymers of arylene vinylene oligomers, such as those in
(iv), (v), (vi), and (vii) with non-conjugated oligomers; and
[0053] (ix) polyp-phenylene-and its derivatives substituted at
various positions on the phenylene moiety, including ladder polymer
derivatives such as poly(9,9-dialkylfluorene) and the like; [0054]
(x) poly(arylenes) where the arylene may be such moieties as
naphthalene, anthracene, furylene, thienylene, oxadiazole, and the
like; and their derivatives substituted at various positions on the
arylene moiety; [0055] (xi) co-polymers of oligoarylenes such as
those in (x) with non-conjugated oligomers; [0056] (xii)
polyquinoline and its derivatives; [0057] (xiii) co-polymers of
polyquinoline with p-phenylene substituted on the phenylene with,
for example, alkyl or alkoxy groups to provide solubility; and
[0058] (xiv) rigid rod polymers such as
poly(p-phenylene-2,6-benzobisthiazole),
poly(p-phenylene-2,6-benzobisoxazole),
polyp-phenylene-2,6-benzimidazole), and their derivatives.
[0059] A preferred polymeric emitting material that emits
yellow-light and includes polyphenelenevinylene derivatives is
available as "Super Yellow" from Covion Organic Semiconductors
GmbH, Industrial park Hoechst, Frankfurt, Germany. Another
especially preferred polymeric emitting material that emits
green-light and includes fluorene-copolymers is available as
LUMATION polymers from Dow Chemical, Midland, Mich.
[0060] Alternatively, rather than polymers, small organic molecules
that emit by fluorescence or by phosphorescence can serve as the
emissive layer. Examples of small-molecule organic emitting
materials include: (i) tris(8-hydroxyquinolinato)aluminum (Alq);
(ii) 1,3-bis(N,N-dimethylaminophenyl)-1,3,4-oxidazole (OXD-8);
(iii) -oxo-bis(2-methyl-8-quinolinato)aluminum; (iv)
bis(2-methyl-8-hydroxyquinolinato)aluminum; (v)
bis(hydroxybenzoquinolinato) beryllium (BeQ.sub.2); (vi)
bis(diphenylvinyl)biphenylene (DPVBI); and (vii)
arylamine-substituted distyrylarylene (DSA amine).
[0061] Such polymeric and small-molecule materials are well known
in the art and are described in, for example, U.S. Pat. No.
5,047,687 issued to VanSlyke, and Bredas, J.-L., Silbey, R., eds.,
Conjugated Polymers, Kluwer Academic Press, Dordrecht (1991).
Metal Oxide Layer 368a:
[0062] The metal oxide layer 368a is either-an alkali metal-oxide
or an alkaline earth metal oxide and is formed by: (1) decomposing
a compound that includes (a) oxygen and (b) an alkali metal or an
alkaline earth metal; (2) thermally reacting at least two compounds
where one of the compounds includes an alkali metal or an alkaline
earth metal, and another one of the compounds includes oxygen; or
(3) thermally reacting at least two compounds where one of those
compounds includes (a) oxygen and (b) an alkali metal or an
alkaline earth metal.
[0063] Preferably, the metal oxide layer 368a is a cesium oxide
layer that is formed by: (1) decomposing a compound that includes
(a) the oxygen and (b) the cesium; (2) thermally reacting at least
two compounds where one of the compounds includes the cesium, and
another one of the compounds includes the oxygen; or (3) thermally
reacting at least two compounds where one of those compounds
includes (a) the oxygen and (b) the cesium.
[0064] More preferably, the metal oxide layer 368a is a cesium
oxide layer that is formed by: (1) decomposing cesium carbonate; or
(2) thermally reacting cesium sulfate and barium oxide. The range
of thickness of the cesium oxide layer is such that the cesium
oxide layer forms a continuous film but not too thick that the flow
of electrons from the conductive layer 368b to the emissive polymer
layer 365 is substantially reduced. Specifically, the range of
thickness of the cesium oxide layer is from about 0.1 nm to about
20 nm; preferably, is from about 0.3 nm to about 1 nm; more
preferably, from about 0.3 nm to about 0.5 nm; and most preferably,
about 0.3 nm.
[0065] The cesium oxide layer reduces the barrier to electron
injection from the conductive layer 368b to the semiconductive
organic emissive polymer layer 365. The work function of the cesium
oxide layer 368a is closer to the LUMO level of the emissive
polymer layer 365 than the work function of the conductive layer
368b . By bringing the work function closer to the LUMO level of
the emissive polymer layer 365, the barrier to electron injection
is reduced or eliminated thus increasing the efficiency of the OLED
353. The conductive layer 368b provides the electrons to the cesium
oxide layer that are injected to the emissive polymer layer
365.
[0066] Due, in part, to its low work function, the cesium oxide
layer can be efficiently used with various emissive polymer layers
with different LUMO levels. For example, the cesium oxide layer can
be the electron injecting layer in a common cathode that is used
with different emissive polymer layersthat emit, e.g. any of the
following colors red, green, blue, or white.
Conductive Layer 368b:
[0067] The conductive layer 368b is comprised of a metallic layer
such as, for example, aluminum, silver, gold, magnesium, copper, or
a mixture thereof, or alloys thereof. The thickness of the
conductive layer 368b is from about 10 nm to about 1000 nm, more
preferably from about 50 nm to about 500 nm, and most preferably
from about 100 nm to about 300 nm. While many methods are known to
those of ordinary skill in the art by which the conductive layer
can be deposited, vacuum deposition methods are preferred.
EXAMPLES
[0068] The following examples are presented for a further
understanding of the invention and should not be construed as
limiting the scope of the appended claims or their equivalents.
Example 1
OLED with a Prior Art Electron Injection Layer
[0069] An OLED was fabricated in the following manner: [0070] (1)
for the conducting polymer layer, a 70 nm layer of a hole
conducting polymer polyethylenedioxythiophene-polystyrenesulfonic
acid ("PEDOT:PSS") was deposited on an anode layer comprised of
about 120 nm of indium tin oxide ("ITO"). This conducting polymer
is commercially available from H. C. Starck, located in Goslar,
Germany. [0071] (2) for the emissive polymer layer, a 70 nm layer
of a substituted polyparaphenylenevinylene was deposited on the 70
nm layer of the conducting polymer layer. The emissive polymer
layer is known by its commercial name "Super Yellow" and is
commercially available from Covion Organic Semiconductors. [0072]
(3) for the electron injecting layer, a barium layer was deposited
on the emissive layer by resistive-heating in a vacuum evaporator
such that the barium layer was evaporated and deposited onto the
emissive layer. The thickness of the barium layer is 3.0 nm. [0073]
(4) for the conductive cathode layer, a 250 nm layer of aluminum
was deposited on the barium layer.
[0074] FIG. 4 shows the current density versus voltage curve, and
the luminance versus voltage curve for the OLED fabricated as
described earlier with barium as the electron injecting layer.
Example 2
[0075] An OLED was fabricated in the following manner: [0076] (1)
for the conducting polymer layer, a 70 nm layer of a hole
conducting polymer PEDOT:PSS was deposited on an anode layer
comprised of about 120 nm of ITO. [0077] (2) for the emissive
polymer layer, a 70 nm layer of Super Yellow was deposited on the
70 nm layer of PEDOT:PSS. [0078] (3) for the electron injecting
layer, a cesium oxide layer was deposited on the Super Yellow
layer. The thickness of the cesium oxide layer is 0.2 nm. The
cesium oxide layer was formed by the resistive heating of the
cesium carbonate, Cs.sub.2CO.sub.3, in a vacuum evaporator
resulting in a thermal decomposition of the cesium carbonate. The
thermal decomposition of the cesium carbonate produced two
compounds: the cesium oxide and the carbon dioxide. The produced
cesium oxide was deposited on the emissive layer and the carbon
dioxide was removed by the vacuum. The cesium carbonate is
commercially available from Aldrich Chemical Company located in
Milwaukee, Wis. [0079] (4) for the conductive cathode layer, a 250
nm layer of aluminum was deposited on the cesium oxide layer.
[0080] FIG. 5 shows the current density versus voltage curve, and
the luminance versus voltage curve for the OLED fabricated as
described earlier with cesium oxide as the electron injecting
layer.
[0081] FIG. 6 is a table that compares the efficiency and luminance
of OLEDs fabricated as described earlier in which the electron
injecting layer is barium and cesium oxide.
[0082] As FIGS. 4, 5, and 6 show, the cesium oxide material
provides better performance (current density, luminance, and
efficiency) as the electron injecting layer than the barium
material. For example, at 5.8 volts, there is 100% more current in
the OLED with cesium oxide as the electron injecting layer than
with barium. Also, the luminance increases by about 120% when
cesium oxide is used as the electron injecting layer rather than
barium.
Example 3
[0083] An OLED was fabricated in the following manner: [0084] (1)
for the conducting polymer layer, a 70 nm layer of a hole
conducting polymer PEDOT:PSS was deposited on an anode layer
comprised of about 120 nm of ITO. [0085] (2) for the emissive
polymer layer, a 70 nm layer of Super Yellow was deposited on the
70 nm layer of PEDOT:PSS. [0086] (3) for the electron injecting
layer, the cesium oxide layer was deposited on the Super Yellow
layer. The cesium oxide layer was formed by the resistive heating
of the cesium carbonate, Cs.sub.2CO.sub.3, in the vacuum evaporator
resulting in the thermal decomposition of the cesium carbonate. The
thermal decomposition of the cesium carbonate produced two
compounds: the cesium oxide and the carbon dioxide. The produced
cesium oxide was deposited on the emissive layer and the carbon
dioxide was removed by the vacuum. [0087] (4) for the conductive
cathode layer, a 250 nm layer of aluminum was deposited on the
cesium oxide layer.
[0088] One set of OLED displays were prepared in which the cesium
oxide layer had a thickness of 0.3 nm, a second set of displays
were prepared in which the cesium oxide layer had a thickness of
0.5 nm, and a third set of displays were prepared in which the
cesium oxide layer had a thickness of 1.0 nm. For comparison
purposes, a fourth set of displays were prepared as described
earlier in Example 1 in which the charge injecting layer is barium
with a thickness of 3 nm. Each set had at least four OLED displays
with each display having sixteen pixels (i.e., OLEDs).
[0089] FIG. 7 shows the average luminance versus voltage graph for
the four different sets of displays. FIG. 8 is a table that
compares the average luminance and the average half-life of the
three different sets of displays with different cesium oxide layer
thicknesses. As shown in FIGS. 7 and 8, the performance (luminance
and half-life) of the OLEDs is inversely proportional to the
thickness of the cesium oxide layer. This is due; in part to the
insulating nature of cesium oxide which becomes a more effective
insulator at larger thicknesses thus limiting the electron
injection and lowering the luminance. In one example, the figures
show that for the cesium oxide layer with a thickness of 0.3 nm, at
6 volts, the average luminance is 9700 cd/m2. The half-life of the
0.3 nm cesium oxide layer at 80.degree. C. and 250 cd/m2 is 10
hours corresponding to 5000 hours at room temperature. As shown by
FIG. 7, the turn-on voltage of the OLEDs with the cesium oxide
layer is lower than the turn-on voltage of the OLEDs with the
barium layer. In addition, the shape of the curves show that the
OLEDs using cesium oxide have the desirable characteristic of
rapidly reaching the peak luminance given a small increase in
voltage and this rapid increase occurs at a low voltage. For
example, between the low voltages of 2 to 3 volts, the luminance
increases from approximately 0.02 cd/m2 at 2 volts for the OLEDs
using cesium oxide to approximately 1000 cd/m2 at 3 volts. These
results indicate, for example, that (1) in terms of performance,
the cesium oxide is an effective electron injecting layer, and (2)
the decomposition product of the cesium carbonate is the low work
function cesium oxide.
[0090] While the embodiments of the electrode that includes the
metal oxide layer (i.e., the alkali metal oxide or the alkaline
earth metal oxide) are illustrated in which it is primarily
incorporated within an OLED, almost any type of electronic device
that uses an electrode may include these embodiments. In
particular, embodiments of the electrode of the present invention
may also be included in a solar cell, a phototransistor, a laser, a
photodetector, or an opto-coupler. The OLED described earlier can
be used within displays in applications such as, for example,
computer displays, information displays in vehicles, television
monitors, telephones, printers, and illuminated signs.
[0091] As any person of ordinary skill in the art of light-emitting
device fabrication will recognize from the description, figures,
and examples that modifications and changes can be made to the
embodiments of the invention without departing from the scope of
the invention defined by the following claims.
* * * * *