U.S. patent application number 12/481119 was filed with the patent office on 2010-12-09 for electrically conductive polymers.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Zakya H. Kafafi, Woohong Kim, Gary P. Kushto.
Application Number | 20100307791 12/481119 |
Document ID | / |
Family ID | 43299933 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100307791 |
Kind Code |
A1 |
Kim; Woohong ; et
al. |
December 9, 2010 |
Electrically Conductive Polymers
Abstract
An electrically conductive film suited to use as a transparent
anode, a method of forming the film, and an electronic device
comprising the film are disclosed. The device includes a conductive
polymer electrode defining first and second surfaces and having an
electrical conductivity gradient between the first and second
surfaces. A second electrode is spaced from the second surface by
at least one organic material layer, such as a light emitting
layer.
Inventors: |
Kim; Woohong; (Lorton,
VA) ; Kushto; Gary P.; (Crofton, MD) ; Kafafi;
Zakya H.; (Alexandria, VA) |
Correspondence
Address: |
NAVAL RESEARCH LABORATORY;ASSOCIATE COUNSEL (PATENTS)
CODE 1008.2, 4555 OVERLOOK AVENUE, S.W.
WASHINGTON
DC
20375-5320
US
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Washington
DC
|
Family ID: |
43299933 |
Appl. No.: |
12/481119 |
Filed: |
June 9, 2009 |
Current U.S.
Class: |
174/126.1 ;
313/504; 427/595 |
Current CPC
Class: |
H01L 51/006 20130101;
H01L 2251/5346 20130101; H01L 51/0021 20130101; H01L 51/5215
20130101; H05B 33/28 20130101; H01L 51/0094 20130101; H01L 51/56
20130101; H01B 1/128 20130101; H01L 51/0037 20130101; Y02E 10/549
20130101; H01L 51/5206 20130101 |
Class at
Publication: |
174/126.1 ;
427/595; 313/504 |
International
Class: |
H01B 5/00 20060101
H01B005/00; B05D 5/12 20060101 B05D005/12; H01J 1/62 20060101
H01J001/62 |
Claims
1. An electronic device comprising: a conductive polymer electrode
defining first and second surfaces and having an electrical
conductivity gradient between the first and second surfaces; a
second electrode, the second electrode being spaced from the second
surface by at least one organic material layer.
2. The electronic device of claim 1, wherein the electrical
conductivity is higher in a first region than in a second region,
the first region being closer to the first surface than the second
region.
3. The electronic device of claim 1, wherein a ratio of the
conductivity in the first region to the conductivity in the second
region is at least 10:1.
4. The electronic device of claim 3, wherein the ratio of the
conductivity in the first region to the conductivity in the second
region is at least 100:1.
5. The electronic device of claim 1, further comprising a substrate
in contact with the first surface.
6. The electronic device of claim 1, wherein the at least one
organic material layer comprises a light emitting layer which emits
light when a power source is connected between the first electrode
and the second electrode.
7. The electronic device of claim 1, wherein the first electrode is
an anode.
8. The electronic device of claim 1, wherein the first electrode
includes a plurality of layers including a first layer and a second
layer, the second layer being intermediate the first layer and the
at least one organic layer, the first layer having a higher
electrical conductivity than the second layer.
9. The electronic device of claim 8, wherein the plurality of
layers includes at least one additional layer intermediate the
second layer and the at least one organic layer, the conductivity
of the plurality of layers decreasing towards the organic
layer.
10. The electronic device of claim 8, wherein the conductive
polymer is the same in each of the plurality of layers.
11. The electronic device of claim 10, wherein the conductive
polymer is conjugated with a dopant and wherein a ratio of
conductive polymer to dopant is higher in the first layer than in
the second layer.
12. The electronic device of claim 11, wherein the ratio of
conductive polymer to dopant is at least 1.2 times higher in the
first layer than in the second layer.
13. The electronic device of claim 1, wherein the first electrode
has a gradually varying conductivity between the first and second
surfaces.
14. The electronic device of claim 1, wherein the conductive
polymer is one which becomes water insoluble upon heating.
15. The electronic device of claim 1, wherein the conductive
polymer comprises a polythiophene.
16. The electronic device of claim 1, wherein the conductive
polymer comprises poly(3,4-ethylenedioxythiophene) (PEDOT).
17. The electronic device of claim 16, wherein the conductive
polymer comprises PEDOT doped with a dopant and wherein a ratio of
PEDOT:dopant is higher adjacent the first surface than adjacent the
second surface.
18. The electronic device of claim 1, wherein the at least one
organic material layer comprises a light emitting layer.
19. The electronic device of claim 1, wherein the device is
selected from the group consisting of a liquid crystal display, an
organic light-emitting device, a photovoltaic cell, and an organic
thin film transistor.
20. A method of forming the device of claim 1 comprising: forming
the first electrode comprising depositing a solution comprising a
conductive polymer and a dopant to form a first layer; and
thereafter performing at least one of: a) depositing a second
solution comprising a conductive polymer and a dopant on the first
layer to form a second layer with a different conductivity than the
first layer, and b) irradiating the first layer to produce a
conductivity gradient in the first layer; and forming at least one
organic material layer intermediate the first electrode and a
second electrode.
21. The method of claim 20, further comprising: prior to depositing
the second solution, heating the first layer to render the
conductive polymer insoluble in the second solution.
22. A method of forming an electrically conductive film comprising:
depositing a first solution comprising an electrically conductive
polymer and a dopant on a substrate to form a first conductive
layer; depositing a second solution comprising a conductive polymer
and a dopant on the first layer to form a second conductive layer
with a different conductivity than the first layer; and prior to
depositing the second solution, heating the first layer to lower a
solubility of the first layer's conductive polymer in the second
solution.
23. An electrically conductive film formed by the method of claim
22.
24. The method of claim 22, wherein the conductive polymer in the
first and second solutions is the same.
25. The method of claim 22, wherein the first solution has a
different conductive polymer to dopant ratio than the second
solution.
26. A method of forming an electrically conductive film comprising:
depositing a solution comprising an electrically conductive polymer
and optionally a dopant on a substrate to form a conductive layer;
irradiating the conductive layer to generate a conductivity
gradient in the conductive layer.
27. An electrically conductive film formed by the method of claim
26.
Description
BACKGROUND
[0001] This disclosure relates to organic conducting films. It
finds particular application in connection with an electrode formed
from one or more electrically-conducting polymers and having two or
more regions of different electrical conductivity. It is to be
appreciated that the conductive films may find application in a
variety of electronic devices, particularly opto-electronic
devices, as well as in antistatic coatings and electromagnetic
shielding applications.
[0002] Most opto-electronic devices, such as liquid crystal
displays (LCDs), organic light-emitting devices (OLEDs),
photovoltaic cells (PVs), and organic thin film transistors (OTFTs)
employ one or more electrically conductive and transparent
electrodes. Typically, various metal oxides, such as indium tin
oxide (ITO) are used as electrodes, since they are highly
conductive and transparent in the visible region. ITO, for example,
can be vacuum vapor-deposited, sputtered, or pulsed laser deposited
(PLD) onto glass or plastic substrates. ITO films with a surface
resistance of less than 100 .OMEGA./square and a high transparency
of >90% can be easily obtained using these deposition methods.
However, ITO has some disadvantages as an electrode material.
Deposition techniques are carried out under vacuum and can be
costly. Also, metal oxide films tend to be very brittle. They are
generally not suitable for use with a flexible substrate since they
tend to delaminate easily.
[0003] Recently, conducting polymers have attracted attention as a
potential replacement for ITO in many electronic devices,
especially for those using flexible substrates. This is primarily
due to their good mechanical strength and ability to maintain their
electrical and optical properties upon substrate flexing and
bending (see, R. Paetzold, K. Heuser, D. Henseler, S. Roeger, G.
Wittmann, and A. Winnacker, Appl. Phys. Lett., 82 (19), 3342,
(2003)). Conducting polymers are particularly suitable, since their
surface resistance does not seem to be affected either by sharp
bending and/or by repeated bending cycles.
[0004] The application of conducting polymers as the electrode in
polymer light-emitting devices (PLEDs) is disclosed, for example,
in U.S. Pat. No. 5,766,515 to Jonas, et al. OLEDs fabricated on
glass substrates using poly(3,4-ethylenedioxythiophene) (PEDOT) as
the conducting polymer electrode and methoxyethylhexyloxy
phenylenevinylene (MEH-PPV) as an emissive material are described.
Molecular organic light-emitting diodes (MOLEDs) using an anode
fabricated from a poly(3,4-ethylenedioxythiophene) and
poly(styrenesulfonic acid (PEDOT:PSS) conducting polymer and
tris(8-hydroxyquinolinolato)aluminum (III) (Alq.sub.3) as an
emissive layer have also been described (see W. H. Kim, A. J.
Makinen, N. Nikolov, R. Shashidhar, H. Kim, and Z. H. Kafafi, Appl.
Phys. Lett., 80, 3844, (2002); and W. H. Kim, G. P. Kushto, H. Kim,
and Z. H., Kafafi, J. Polym. Sci. Part B: Polym. Phys., 41, 2471,
(2003).) High external electroluminescence quantum efficiency and
high luminance MOLEDs using a low sheet resistance conducting
polymer anode based on high fluorescence and high electron mobility
silole derivatives have also been developed. (see W. H. Kim, L. C.
Palilis, M. Uchida, and Z. H. Kafafi, Polymer Electrodes for
Flexible Organic Light-Emitting Devices, Chem. Mat. 16, 4681
(2004).
[0005] These references suggest that conducting polymers are
promising candidates as anode materials and may eventually replace
the most widely used ITO electrodes in many opto-electronic and
other electronic devices, especially those fabricated on flexible
substrates. Conducting polymers provide another advantage in
applications such as OLEDs and OPVs due to their relatively high
work function. The energy barrier for hole injection may be lowered
due to the higher work function of the conducting polymers
(.about.5.0 eV) compared to that of ITO (.about.4.7 eV). The X-ray
and ultraviolet photoelectron spectroscopic (XPS and UPS) of
PEDOT:PSS films have been studied. (see, R. Schlaf, H. Murata, and
Z. H. Kafafi, J. Electron Spectrosc. Relat. Phenom. 120, 149
(2001).) The work function of the PEDOT:PSS films was measured to
be 5.0.+-.0.2 eV, regardless of the surface sheet resistance and
the presence of various additives such as surfactants, polyalcohols
and high boiling point solvents. This is significant for devices
where the conducting polymers are used as an electrode since the
device performance (such as driving voltage and efficiency) is
largely dependent on the subtle change in the work function of the
electrode. Therefore, the injection of holes from the electrode
(anode) is not a limiting factor since the energy barrier between
the anode and the organic layer is maintained.
[0006] However, the long-term stability of the conducting polymer
electrode remains a problem. Some chemical degradation or
electrical shorting is experienced under a high electric field,
especially at the interface between the conducting polymer
electrode and the organic layers. Therefore these devices tend to
have poor operational stability and show rather low brightness and
efficiency. Further, OLEDs fabricated using conducting polymer
electrodes show very high leakage current that results in a low
rectification ratio. A high ratio is beneficial for high resolution
matrix displays where cross-talk between adjacent lines has to be
avoided.
[0007] FIG. 1 shows the typical current density--voltage--luminance
characteristics of an OLED based on the hole transporter,
N,N'-Bis(naphthalene-1-yl)-N,N'-bis(phenyl)benzidine (.alpha.-NPB)
and Alq.sub.3 as the electron transporter/emitter, fabricated using
a conducting polymer as an anode and a Mg:Ag alloy as a cathode.
The device shows symmetric current-voltage characteristics in
forward and reverse bias and does not exhibit a diode behavior. A
low rectification ratio of about 1 over the range of .+-.5V is
observed. The device also shows a rather low luminance especially
at higher voltage. These are common characteristics for OLEDs using
the conducting polymer anode that are caused by the leakage
current.
[0008] There remains a need for a conductive polymer electrode
which addresses these problems.
INCORPORATION BY REFERENCE
[0009] U.S. Pat. No. 6,649,327, entitled METHOD OF PATTERNING
ELECTRICALLY CONDUCTIVE POLYMERS, to Kim, et al., is incorporated
herein by reference in its entirety. Disclosed therein is a method
of patterning electrically conductive polymers including forming a
surface of a conducting polymer on a substrate, applying a mask to
this surface, applying irradiation to form regions of exposed
conducting polymer and regions of unexposed conducting polymer,
removing the mask, and gently removing by non-chemically reactive
means, the regions of exposed conducting polymer.
BRIEF DESCRIPTION
[0010] In accordance with one aspect of the exemplary embodiment,
an electronic device includes a conductive polymer electrode
defining first and second surfaces. The electrode has an electrical
conductivity gradient between the first and second surfaces. A
second electrode is spaced from the second surface by at least one
organic material layer.
[0011] In accordance with another aspect, a method of forming an
electrically conductive film includes depositing a first solution
comprising an electrically conductive polymer and a dopant on a
substrate to form a first conductive layer, A second solution
comprising a conductive polymer and a dopant is deposited on the
first layer to form a second conductive layer with a different
conductivity than the first layer. Prior to depositing the second
solution, the method includes heating the first layer to lower a
solubility of the first layer's conductive polymer in the second
solution.
[0012] In accordance with another aspect, a method of forming an
electrically conductive film includes depositing a solution
comprising an electrically conductive polymer and optionally a
dopant on a substrate to form a conductive layer. The conductive
layer is irradiated to generate a conductivity gradient in the
conductive layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a plot of current density vs. voltage and
luminance vs. voltage in the range of -5V to +12V for a comparative
organic light emitting device with an electrically conducting
polymer layer as an anode;
[0014] FIG. 2 illustrates an exemplary electronic device in
accordance with one aspect of the exemplary embodiment;
[0015] FIG. 3 illustrates an exemplary electronic device in
accordance with another aspect of the exemplary embodiment;
[0016] FIG. 4 is an absorption spectrum showing absorbance as a
function of the wavelength of light irradiating a 150 nm thick film
of PEDOT:PSS on a quartz substrate;
[0017] FIG. 5 illustrates an exemplary electronic device in
accordance with another aspect of the exemplary embodiment;
[0018] FIG. 6 is a plot of current density and luminance as a
function of applied voltage for an OLED based on PET
substrate/two-layered PEDOT:PSS anode/.alpha.-NPD layer/2PSP
layer/PyPySPyPy layer/Mg:Ag cathode;
[0019] FIG. 7 is a plot of the relative luminance as a function of
applied voltage for a) an OLED based on PET/four layer-PEDOT:PSS
anode/.alpha.-NPD/Alq.sub.3/Mg:Ag and b) a comparative OLED based
on PET/gradientless single layer-PEDOT:PSS
anode/.alpha.-NPD/Alq.sub.3/Mg:Ag;
[0020] FIG. 8 is a plot of relative luminance as a function of
applied voltage for an OLED with a single layer gradient type anode
and a comparative OLED with a single layer anode with no
conductivity gradient.
[0021] FIG. 9 is a schematic view of an exemplary OPV device;
and
[0022] FIG. 10 illustrates current density vs. voltage for the
device of FIG. 9.
DETAILED DESCRIPTION
[0023] Aspects of the exemplary embodiment relate to an electronic
device comprising an electrically-conducting polymer film and to a
method of forming the device and film.
[0024] The electrically-conducting polymer film comprises an
electrically conductive organic polymer which renders the film
electrically conductive. For convenience, as used herein, the terms
"conductive" and "conductivity" refer to electrical conductivity,
unless otherwise noted. Electrical conductivity .sigma. is a
measure of a material's ability to conduct an electric current.
Conductivity can be expressed as Siemens per meter (Sm.sup.-1). The
conductivity .sigma. is defined as the ratio of the current density
to the electric field strength. Where the electric field strength
is a constant, current density (e.g., measured in amperes/unit
area) can be used as a proxy for conductivity. Conductivity is the
inverse of resistivity. Resistivity can be measured, for example,
in ohms centimeter (.OMEGA.-cm) or ohms meter (.OMEGA.-m).
[0025] The electrically-conducting polymer film has a conductivity
gradient. Specifically, conductivity in the electrically-conducting
polymer film varies, whereby a first region of the film has a first
conductivity and a second region of the film has a second
conductivity, different from the first conductivity. In the
exemplary embodiment, the conductivity varies across the film in a
direction perpendicular to the opposed major surfaces (top and
bottom) of the film. The conductivity differences may be provided
by varying the concentration of the electrically conducting
polymer, changing the type of electrically conducting polymer, or a
combination thereof. The conductivity may vary stepwise or
continuously.
[0026] In one embodiment, the electrically-conducting polymer film
includes two or more discrete layers, whereby a first of the layers
has a different conductivity from a second of the layers. The
regions of the film are thus defined by the layers and the
conductivity varies stepwise.
[0027] In another embodiment, the electrically-conducting polymer
film is in the form of a single layer in which the conductivity
continuously changes across the film. The regions are thus defined
by regions of different conductivity.
[0028] In another embodiment, a method of forming the film with a
stepwise varying conductivity includes applying successive layers
to a common substrate. The applied layers may each include a
conducting polymer and have a different conductivity. When formed,
the polymer film thereby includes at least two layers which differ
in their conductivity. In this embodiment, the conducting polymer
is one which becomes insoluble upon solidification, whereby layers
with different conductivity can be formed successively without
affecting the intrinsic properties of the previously applied
layer.
[0029] In another embodiment, a method of forming the film with a
continuously varying conductivity gradient includes applying UV
irradiation to a single layered conductive film, whereby a
conductivity gradient is generated.
[0030] The exemplary methods provide stable and efficient
conductive films suitable for many electronic, particularly
opto-electronic, applications. The gradual distribution of the high
electric field throughout the conductive films during the operation
of the electronic device result in a dramatic decrease in the
device degradation and hence an enhancement in the device stability
and lifetime. Device performance is also improved by reducing
current leakage.
[0031] The exemplary electrically-conducting polymer film is
suitable for many applications where a stable conducting film is
needed. As an example, it may be used as an electrode in organic
light-emitting devices and organic photovoltaic cells where a high
rectification ratio is desired to avoid cross-talk between adjacent
lines.
[0032] FIG. 2, by way of example, illustrates an exemplary organic
light emitting device (OLED) 10 in which the exemplary electrically
conducting film 12 is in the form of a multi-layer electrode (here,
the anode). The electrode 12 in FIG. 2 comprises multiple layers
14, 16, 18, 20 (four in the illustrated embodiment) which increase
in conductivity towards an underlying substrate 22 (layer 18 has
greater conductivity than layer 20, layer 16 greater than layer 18,
and layer 14, greater than layer 16). The conductivity thus
increases stepwise through the layer 12. The layer 14 has a
conductivity which is greater than the underlying substrate 22. One
or more organic material layers 24 space the anode 12 from a
cathode layer 26. In the exemplary embodiment, the uppermost layer
20 of the film 12 has the lowest conductivity, yet still has a
conductivity which is greater than that of the adjacent organic
layer 24. A voltage source 28 is connected with the cathode 26 and
anode 12 by electrical conductors.
[0033] The exemplary layers 14, 16, 18, 20 of the conductive film
electrode 12 are coextensive with each other and supported by the
underlying substrate 22. Specifically, layer 14 may be the first
layer to be formed and contacts the substrate 22, layer 16 is the
second layer, layer 18 is the third layer, and layer 20, the
fourth. As will be appreciated, the number of layers of different
conductivity in film 12 is not limited to four and can be, for
example, from two to ten, or more. The film 12 has a first (lower)
surface 30, defined by highest conductivity layer 14 and a second
(upper) surface 32, defined by the lowest conductivity layer 20.
The exemplary opposed surfaces 30, 32 are planar and parallel with
each other, although other configurations of spaced first and
second surfaces 30, 32 are contemplated. In the exemplary
embodiment, the surface 30 is contiguous with the substrate 22,
contacting the substrate along a predominant portion of its length.
The surface 32 is contiguous with one of the organic material
layers 24, and may contact the organic layer along a predominant
portion of its length.
[0034] Layers 14, 16, 18, 20 of the film may each have a thickness
of less than 200 nm, e.g., about 10-100 nm, and the entire anode 12
may have a thickness of up to 1 .mu.m, e.g., 50-200 nm.
[0035] FIG. 3 shows another embodiment of an OLED 10' in which
similar elements are accorded the same numerals. Device 10' may be
configured as for device 10, except as otherwise noted. In this
embodiment, the anode 12' comprises a single electrically
conductive film layer with a conductivity which decreases gradually
from its lower surface 30 towards its upper surface 32.
Accordingly, a region 34 of the layer 12', which is spaced from the
lower surface (i.e., closer to the organic layer(s) 24), has a
lower conductivity than a region 36, which is closer to the lower
surface 30 (i.e., further from the organic layer(s) 24).
[0036] Suitable substrates 22 for the exemplary devices 10, 10' are
transparent supports, such as glass, quartz, silicon wafer, or
films of plastic such as polyester, for example, polyethylene
terephthalate (PET) polyethylene naphthalate, polycarbonates,
polyacrylates, polysulfones, polyimide, polyetherketones, waxes,
polyesters, polyvinylacetates, polyolefins, polyethers, polyesters,
polyvinylmethylethers, polyvinylbutylethers, polyamides,
polyacrylamides, polyimides, polyketones, fluoropolymers, aromatic
hydrocarbon polymers, acrylate and acrylic acid polymers, phenolic
polymers, polyvinylalcohols, polyamines, polypeptides, siloxane
polymers, polyvinylchlorides, polyvinylbenzylchlorides,
polychlorostyrenes, polyvinylbutyrals, copolymers thereof, and
combinations thereof. The support 22 may be rigid or flexible.
[0037] The exemplary electrically conductive film 12, 12' as well
as other layers of the device 10, 10' can be formed by various
methods including spin coating, inkjet printing, spray printing,
screen printing, dip coating, combinations thereof, and others.
[0038] Exemplary conducting polymers which may be employed in the
layer 12, 12' include polythiophenes, polyanilines, polypyrroles,
and their derivatives, and combinations thereof. The conducting
polymer(s) may be in a neutral state or doped with various dopants.
The conducting polymer(s) may be soluble in various solvents or
dispersed in water or other solvents as a complex with a dopant. In
one embodiment, the conducting polymer comprises a complex of PEDOT
(poly(3,4-ethylenedioxythiophene)) and a polymeric dopant, PSS
(poly(styrenesulfonic acid), as shown below.
##STR00001##
[0039] As will be appreciated, the basic structure of the PEDOT
and/or PSS polymer can be modified through the addition of
substituent groups. Additionally a ratio of PEDOT:PSS can be
adjusted to provide various concentrations of the conducting
polymer, PEDOT, in the film.
[0040] In the embodiment of FIG. 2, the conductivity of the layers
14, 16, 18, 20 can be controlled by adjusting the ratio of
PEDOT:PSS (which is expressed in terms of a mole ratio throughout).
In the exemplary embodiment a ratio R=R.sub.1/R.sub.2 is at least
1.2:1, where R.sub.1 is the PEDOT:PSS ratio in one of layers 14,
16, 18, and R.sub.2 is the PEDOT:PSS ratio in a higher layer 16,
18, or 20. In some embodiments, the ratio R may be at least 1.5:1,
e.g., at least 2 or at least 4 (e.g., between the highest and
lowest conductivity layers) and can be up to 100 or more. The
ratios R.sub.1, R.sub.2, etc. for the various layers can be, for
example, from about 100:1 to 1:50. In general, the conducting
polymer concentration is highest in the layer 14 that is closest to
the substrate 22 (layer) and decreases with each successive layer.
Analogously for FIG. 3, spaced regions 36, 34 can have a ratio
R=R.sub.1/R.sub.2 which is at least 1.2:1, with the ratio varying
gradually between the two regions.
[0041] The conductivity of the layers 14, 16, 18, 20 or regions 34,
36 may range between about 1000 and 10.sup.-6 S cm.sup.-1. For
example, the highest conductivity layer 14/region 36 may have a
conductivity which is at least ten times the conductivity of the
lowest conductivity layer 20/region 34. In some embodiments the
ratio of highest:lowest conductivity may be at least 100:1 or at
least 1000:1.
[0042] In the exemplary embodiment, no layer/region of the anode
12, 12' is entirely lacking in conductive polymer. For example, the
conductive polymer may be present at .gtoreq.0.1% by weight in the
lowest conductivity layer 20. In some embodiments, the same
conductive polymer is used in all layers/regions of the anode 12,
12'. For example, each layer 14, 16, 18, 20/region 36, 34 includes
PEDOT as the conductive polymer. In some embodiments, the same
dopant is used in all layers/regions of the anode 12, 12'. For
example, each layer 14, 16, 18, 20/region 36, 34 includes PSS as
the dopant.
[0043] The total thickness t of the film 12, 12' can be adjusted by
controlling the thickness of each layer. In either embodiment, the
total thickness of the film 12, 12' is generally less than 1 .mu.m,
e.g., about 100-200 nm.
[0044] The cathode 26 (negative electrode) layer of the organic
electroluminescence device may comprise an active metal material
having a small work function (generally 4 eV or less) to inject an
electron into the organic material layer effectively. The cathode
may be formed from a conductive metal, such as an alloy of
magnesium and silver. Other materials suitable for the cathode
layer include metals, such as Al, Ti, In, Na, K, Mg, Li, Cs, Rb,
rare earth metals, and alloy compositions, such as Na--K alloy,
Mg--Cu alloy, Al--Li alloy. In other embodiments, the cathode layer
26 can be formed of the same materials as the anode layer 12,
12'.
[0045] The cathode layer 26 may have a thickness of 1 .mu.m or
less, e.g., 20-200 nm.
[0046] There are numerous configurations of the organic layers 24.
The essential requirements of an OLED are an anode, a cathode, and
an organic light-emitting layer located between the anode and
cathode. Additional layers may be employed as exemplified in the
typical device 10'' of FIG. 4, where layer 12 can be as described
for the embodiment of FIG. 2 or FIG. 3. In this embodiment, the
organic layers 24 include, in order, an optional (positive charge)
hole-injecting layer 40, an optional hole-transporting layer (HTL)
42, a light-emitting layer 44, an optional hole-blocking layer 46,
and an optional electron-transporting layer 48, which are
sequentially arranged between the anode 12 and the cathode 26. The
total combined thickness of the organic layers can be less than 500
nm.
[0047] In other embodiments (not shown) the substrate 22 may
alternatively be located adjacent to the cathode 26, or the
substrate 22 may actually constitute the anode 12 or cathode
26.
[0048] As shown in FIG. 5, a hole-injecting layer 40 may be
provided between anode 12 and hole-transporting layer 42. The
hole-injecting material can serve to improve the film formation
property of subsequent organic layers and to facilitate injection
of holes into the hole-transporting layer 42. Suitable materials
for use in the hole-injecting layer 40 include porphyrinic
compounds, as described in U.S. Pat. No. 4,720,432,
plasma-deposited fluorocarbon polymers as described in U.S. Pat.
No. 6,208,075, and some aromatic amines, for example,
4,4',4''-tris[(3-methylphenyl)phenylamino]triphenylamine (MTDATA).
The thickness of the hole-injection layer can be in the range of
0.2 to 200 nm and suitably in the range of 0.3 to 1.5 nm. The
positive hole-injecting layer is sometimes referred to as an anode
buffer layer.
[0049] The hole-transporting layer (HTL) 42 is an organic layer
that is readily able to transport the holes supplied by the anode
12. The hole-transporting layer 42 contains at least one
hole-transporting compound such as an aromatic tertiary amine,
which can be an arylamine, such as a monoarylamine, diarylamine,
triarylamine, or a polymeric arylamine, which is optionally
substituted with a vinyl group. Exemplary triarylamines are
described in U.S. Pat. Nos. 3,180,730; 3,567,450; 3,658,520;
4,720,432; and 5,061,569. In one exemplary embodiment, the organic
hole transport layer 42 includes a triarylamine, such as
naphthyl-substituted benzidine derivative, such as
4,4'-bis[N-(1-naptithyl-D-N-phenyl-amino]-biphenyl (.alpha.-NPD),
N,N'(3-methylphenyl)-1,1''-biphenyl-4,4'-diamine, (TPD), or a
mixture thereof. Layer 42 may have a thickness of 10-500 nm, e.g.,
50 nm. The structure of .alpha.-NPD is given below:
##STR00002##
[0050] The organic light emission layer 44 emits light when a
voltage is applied across the device. The light emission layer may
comprise any material commonly known the art for such use. In one
embodiment, the light emission layer comprises a light emitting
material, such as tris(8-hydroxyquinoline)aluminum (III)
(Alq.sub.3), a polyphenylene vinylene (PVV) derivative, a
polyfluorene derivative, or combination thereof, which may be used
alone or combined with a fluorescent dye. Other exemplary emitters
are silole derivatives having benzene rings in the skeleton, such
as 1,2-bis(1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane
(2PSP), which has the formula given below:
##STR00003##
[0051] The hole-blocking layer 46 (or multiple hole blocking
layers) may be located between the electron-transporting layer 48
and the light-emitting layer 44 to help confine the excitons or
recombination events to the light-emitting layer. Examples of
useful hole-blocking materials are bathocuproine (BCP),
bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (III)
(BAlq) and fac-iridium (III) tris(1-phenylpyrazolato-N,C.sup.2).
Other metal complexes known to block holes and excitons are
described in U.S. Pub. No. 2003/0068528. When a hole-blocking layer
is used, its thickness can be between 2 and 100 nm, such as between
5 and 10 nm.
[0052] The electron-transporting layer 48, where present, may
include one or more metal-chelated oxinoid compounds, including
chelates of oxine itself (also commonly referred to as 8-quinolinol
or 8-hydroxyquinoline). Such compounds help to inject and transport
electrons, exhibiting high levels of performance, and are readily
fabricated in the form of thin films. Other electron-transporting
materials suitable for use in the electron-transporting layer 48
include various butadiene derivatives as disclosed in U.S. Pat. No.
4,356,429 various heterocyclic optical brighteners as described in
U.S. Pat. No. 4,539,507, and silacyclopentadiene (silole)
derivatives. Exemplary compounds of this type include
2,5-bis-(2',2''-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadi-
ene (PyPySPyPy), which has the formula below:
##STR00004##
[0053] Layer 48 may have a thickness of 10-500 nm, e.g., less than
50 nm.
[0054] Two or more OLEDs, as disclosed herein, may be formed on a
common substrate 22. In another embodiment, a display such as a TV
screen includes an array of OLEDs as described herein.
[0055] The polymer film 12, 12' also finds application as an
electrically conductive and transparent electrode (e.g., anode) in
other electronic devices, such as liquid crystal displays (LCDs),
organic photovoltaic cells (OPVs), organic thin film transistors
(OTFTs), and the like.
[0056] Exemplary methods for forming the layer and devices
comprising it are described in general terms followed by several
specific examples.
[0057] The substrate 22 is initially cleaned thoroughly using
conventional methods, such as water, and dried, e.g. with a flow of
nitrogen gas. Oxygen plasma treatment and/or UV/ozone treatment may
be applied where high adhesion properties are desired.
[0058] The conducting polymer electrode 12, 12' can be formed in
two different ways: a) forming multiple layers of conducting
polymer films with different electrical conductivity (as for the
device of FIG. 2) and b) forming a single layer conducting polymer
film in which the electrical conductivity changes gradually across
the film (as for the device of FIG. 3). These methods have been
demonstrated to work well on several types of substrates including
glass, ITO on glass, various plastic films, and Si wafers. While
these substrates are generally non-conducting, ITO is a conducting
substrate. In this case, the conductive polymer layer acts as a
buffer layer.
[0059] The conducting polymer, PEDOT:PSS, is selected as an example
for describing the method. The multilayer film of the conducting
polymer may be wet processed by various casting and printing
techniques from solutions or dispersions. For example, the
conducting polymer may be spin-coated onto the substrate as an
aqueous dispersion. Exemplary spin coating parameters are speeds
between 1 00 to 5000 rpm for about 5-500 seconds. A small amount of
alcohol or surfactant may be added to the dispersion before spin
coating to increase the uniformity and adhesion property of the
film. A process, either chemical or physical, to promote the
adhesion properties can be applied prior to or together with the
coating. The film is subsequently heated, resulting in a
pinhole-free film with excellent optical qualities. An exemplary
heating temperature is between 50-300.degree. C. for 3-10
minutes.
[0060] In the case of the multilayer film 12 (FIG. 2), the film is
built up by coating and then heating each layer in turn, the
composition of each dispersion being different to achieve the
desired conductivity gradient. First, the PEDOT:PSS with the
highest conductivity (highest PEDOT:PSS ratio) is coated onto the
substrate. The film is subsequently annealed resulting in a
pinhole-free and insoluble layer 14 which can resist most organic
solvents and even hot water. The conducting polymer with the next
highest conductivity is coated and annealed in a similar way. This
process is repeated until the conducting polymer layer 20 with the
lowest conductivity is formed. The number of layers is equal to or
more than two. The multiple-layered conducting polymer film 12 now
can serve as a stable and efficient electrode for many
applications. The conducting polymer film can be patterned in any
step, if desired, e.g. to form multiple anodes on a common
substrate.
[0061] This method avoids many of the problems involved with
conventional methods for forming multi-layered films. Generally,
the formation of a multi-layered film is extremely difficult, since
the film in the bottom layer is dissolved during the formation of
the subsequent layer. The morphology and the properties of the
preceding film layer are thus disrupted. In the present method, the
annealing process applied to the film layers prior to applying a
subsequent layer creates a film layer which is insoluble in the
subsequently applied layer.
[0062] Conducting polymers are available as aqueous dispersions of
poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonic acid)
(PEDOT:PSS). For example, PEDOT:PSS dispersions are available from
Bayer, Germany under the trade name BAYTRON P and from Agfa-Gavaert
under the trade name ORGANCON. PEDOT:PSS has properties which make
it particularly suited to this application. Once it is solidified
by annealing the polymer film, at a suitable temperature, it is not
soluble in most of organic solvents and even in hot water.
Additionally the conductivity of the films is conveniently adjusted
by varying the ratio of poly(3,4-ethylenedioxythiophene) to
poly(styrenesulfonic acid). PEDOT:PSS dispersions are available at
different bulk resistivities, thereby providing a range of
conductivity for the layers. Alternatively or additionally, the
conductivity of the film can be controlled by diluting the
PEDOT:PSS dispersion with additional PSS. Generally, lower
conductivity is obtained from the higher ratio of PSS in PEDOT:PSS
composition. For example, the molar ratios of PEDOT:PSS for the
resistivities of 1, 100, and 100,000 Ohms-cm (i.e., conductivities
of 1, 0.01, and 0.00001) are 1:2.5, 1:6, and 1:20, respectively.
Thus, a large variation in conductivity across the film 12 can be
achieved by selection of appropriate PEDOT:PSS ratios for the
aqueous dispersions. Since the only difference in each conducting
polymer layer is the ratio of PEDOT:PSS, the intrinsic properties
of the film, such as work function and optical properties are not
affected.
[0063] For conductive polymers which do not become insoluble
through annealing, other options to form multi-layered films from
solution include the selection of two incompatible solvents for the
film formation of each layer so that it is not dissolved during the
formation of the next layer.
[0064] To form a single layer conducting polymer electrode 12' with
continuous change in conductivity (FIG. 3), a photo-irradiation
technique is used to decrease the conductivity in a gradual manner
across the film 12'. For example, the film is irradiated with light
from a UV light source having a wavelength in the range of 150-400
nm for a period of 1-10,000 seconds. (The term "light" is used
generally here to refer to radiation of all wavelengths below about
750 nm and is not limited to the visible region). The time of
exposure and the wavelength range of the light are selected to be
less than that which would result in loss of adhesion of the layer
to the underlying substrate.
[0065] It is proposed that the exposure of conducting polymers
based on conjugated polymers to UV light decreases the electrical
conductivity of the polymers due the decrease in conjugation length
caused by the oxidation of the aromatic rings. The change in
electrical properties is determined by the intensity and the
wavelength of UV light used. Since the penetration depth of the
light of a specific wavelength through the conducting polymer film
is primarily determined by the absorbance of the polymer film, the
conductivity across the film can be conveniently controlled by
selecting a suitable light source and adjusting the exposure time.
Highly absorbing light will mainly modify the properties of the
shallow portion of the films since most light will be absorbed near
the surface. On the other hand, the light with low absorbance will
penetrate to the deeper portion of the film and the degree of
deterioration will be quite even across the film thickness. Thus
the time of exposure and range of wavelengths used may be selected
to provide a desired conductivity gradient.
[0066] FIG. 4, for example, shows the absorption spectrum of a
PEDOT:PSS film of 150 nm thickness on a quartz substrate. The film
is formed from a dispersion. It is seen that the PEDOT:PSS film is
quite transparent in the visible region and starts to absorb light
in which the wavelength is shorter than 280 nm. Therefore, the
penetration depth of that portion of the light having a wavelength
shorter than 280 nm will be reduced compared to the portion having
wavelengths longer than 280 nm. The change in the conductivity due
to the reduced conjugation and chain scission caused by
photo-induced degradation is proportional to the amount of light
exposure. Most of the light below 240 nm will not penetrate into a
deeper layer due to the high absorbance of the PEDOT:PSS in the
upper region. Therefore, the conductivity will be the lowest near
the surface where photodegradation occurs most and will gradually
change and, will be the highest at the interface between the
substrate and the conducting polymers.
[0067] To form multiple anodes on a single substrate, the anode
layer 12, 12' formed by either method may be patterned. Patterning
may be performed by the method disclosed in U.S. Pat. No.
6,649,327. In particular, a photo-irradiation technique is used to
weaken the adhesion properties between the conducting polymer and
substrate 22. For example, it may be exposed to a UV light source
(wavelength<400 nm) through a patterned photomask on top of the
substrate. Patterning typically takes much longer than the UV
irradiation step, described above. For example, the length of
patterning is typically longer than 30 minutes and can be as long
as 10 hours, depending on the thickness of the film. The substrate
may be heated during light exposure to accelerate the
photoreaction. The organic materials are easily removed by rinsing
or sonicating the substrate in mild organic solvent and/or water
for about 1-20 seconds. The photo-irradiation of the film 12, 12'
reduces the adhesion properties of the conducting polymer allowing
it to be easily or gently removed from the substrate during
sonication.
[0068] One or more organic material layers 24 can be formed on the
optionally patterned anode layer 12, 12' by conventional
film-forming techniques. In the case of an OLED, these may include
an optional hole-injecting layer 40, an optional hole-transporting
layer 42, a light-emitting layer 44, an optional hole-blocking
layer 46, and an optional electron-transporting layer 48. A cathode
layer 26 can be formed on top of the organic layer(s).
[0069] As will be appreciated, the method may be performed in
essentially the reverse order by depositing a cathode layer 26 on a
substrate analogous to substrate 22, thereafter depositing one or
more organic material layers 24 and thereafter forming the anode 12
by laying down the least concentrated (lowest conductivity)
conductive polymer layer first on the organic layer(s) 24 and
laying down one or more subsequent conductive polymer layer(s), the
conductivity of each subsequent layer being higher than that of the
previously deposited layer.
[0070] The methods disclosed herein enable the formation of a
stable and efficient conducting polymer electrode 12, 12' that is
suitable for many electronic, e.g., opto-electronic applications.
The methods form a conducting polymer electrode in which the
conductivity of the film decreases gradually or stepwise across the
film thickness. The conductivity of the lowest conductive layer is
suggested to be greater than that of the active semiconductor layer
that is formed on top of it. This conducting polymer electrode can
enhance the device performance and operational stability since
device degradation caused by high electric field at the organic
electrode interface is greatly reduced. An even distribution of the
charges and attenuation of the electric field through the
multi-layered organic electrode with varying conductivity can
prevent the device failure or degradation caused by the
micro-shorts occasionally observed from single layered conducting
polymer electrode devices. The introduction of additional
layer/layers between the conducting polymer anode and the organic
layer can be used to smooth the surface of the conducting polymer
film and result in better interfacial properties. This effect can
be obtained by forming additional layer/layers of conducting
polymers with lower conductivity than the bottom conducting polymer
electrode layer. Since a direct contact of the organic
semiconductor layer to the conducting polymer electrode is
prevented, device failure caused by the micro-shorts in the
interface will be dramatically decreased.
[0071] Some advantages of the exemplary embodiments are that the
intrinsic properties of the conducting polymer electrode, such as
work function and optical properties, are not changed upon the
formation of the multi-layer/continuously graduated films. This is
because the conductivity of the final film in case of PEDOT:PSS is
determined, at least in part, by the ratio of the conducting
polymer (PEDOT) to the polymeric dopant (PSS). Maintaining a high
work function is desirable for many applications, such as organic
light-emitting devices (OLEDs) and organic photovoltaic cells
(OPVs). For example, a high work function in an OLED promotes
efficient hole-injection from the anode to the hole transporting
layer (HTL). It also increases the open circuit voltage of OPV
cells, which is of great advantage for high efficiency devices. A
decrease in the leakage current caused by planarization of the
conducting polymer electrode is also expected and this will result
in the improvement of the device performance such as efficiency and
rectification ratio.
[0072] The exemplary methods disclosed herein are able to form a
stable and efficient conducting polymer electrode, in which the
device performance is greatly enhanced due to the minimization of
the direct contact of the organic layer to the high electric field
and to the even distribution of the field across the conducting
polymer film.
[0073] Without intending to limit the scope of the exemplary
embodiment, the following examples describe the preparation of
electrodes and devices in accordance with the exemplary methods
disclosed herein.
EXAMPLE 1
Formation of a Multilayer Conducting Polymer Electrode
[0074] Substrates 22 (glass, plastic, or silicon wafer) are
thoroughly cleaned and dried with N.sub.2. They are briefly treated
with O.sub.2 plasma or UV/Ozone prior to the film formation.
Aqueous dispersions of conducting polymer,
poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT/PSS),
with various conductivities are obtainable from Agfa-Gevaert N.V.
Belgium and Bayer, Germany. For example, three different PEDOT/PSS
dispersions are available from Bayer under the tradename BAYTRON P
with a bulk resistivity of 1, 100, and 100,000 Ohms-cm,
respectively. PEDOT:PSS with lower resistivity (<0.01 Ohms-cm)
is obtainable from Afga-Gevaert under the tradename ORGANCON PEDOT
coating solution.
[0075] First, the lowest resistivity (highest conductivity)
PEDOT:PSS dispersion obtained from Agfa-Gevaert is spin coated onto
the substrates at speeds between 500 to 5000 rpm for 180 sec. After
spin coating, the film 14 is annealed at 20 to 200.degree. C. for 1
to 200 minutes. The annealing renders the conducting polymer
insoluble in the aqueous dispersion next applied. Then, PEDOT:PSS
with a resistivity of 1 Ohm-cm is coated and annealed in a similar
way. A similar procedure is repeated with the higher resistivity
PEDOT:PSS dispersions (100 and 100,000 Ohms-cm). A uniform and
pinhole free conducting polymer film 12 is formed and there is no
film deformation generally caused by the dissolution of the
sublayer upon multilayer formation. The total thickness of the film
is generally less than 1 .mu.m.
EXAMPLE 2
Formation of a Second Multilayer Conducting Polymer Electrode
[0076] Substrates are prepared as for Example 1. The PEDOT:PSS
dispersion with low resistivity (<0.01 Ohms-cm, from Agfa) is
further diluted with PSS to achieve four aqueous dispersions having
a PEDOT:PSS ratio between 1:3 and 1:20 and a total solid content of
.about.1%. First the lowest resistivity (highest conductivity)
PEDOT:PSS (the as received, undiluted <0.01 Ohms-cm) is spin
coated onto the substrates at speeds between 500 to 5000 rpm for
180 sec. After spin coating, the film is annealed at 20-200.degree.
C. for 1-200 minutes. Then, the PEDOT:PSS dispersion with a
PEDOT:PSS ratio of 1:3 is spin coated on top of the lowest
resistivity polymer film and annealed in a similar way. A similar
procedure is repeated with the PEDOT:PSS dispersions having
PEDOT:PSS ratios of 1:5, 1:10 and 1:20. A uniform and pinhole free
conducting polymer film 12 is formed. The total thickness of the
film is generally less than 1 .mu.m.
EXAMPLE 3
Formation of a Single Layer Conducting Polymer Film with a
Continuous Change in Resistivity Across the Film
[0077] Substrates 22 are prepared as for Example 1. An aqueous
dispersion of the low resistivity PEDOT/PSS conducting polymer
solution (<0.01 Ohms-cm, from Agfa), is spin coated onto the
substrates at speeds between 500 to 5000 rpm for 180 sec. After
spin coating, the film is annealed at 20-200.degree. C. for 1-200
minutes. The conducting polymer film is patterned using the method
described in U.S. Pat. No. 6,649,327. The active area of the
patterned film is irradiated using a 254nm UV lamp (.about.300
.mu.W/cm.sup.2 at a 15 cm distance) to create a conductivity
gradient. The total thickness of the film 12' is generally less
than 1 .mu.m.
EXAMPLE 4
Organic Light Emitting Device Fabrication Using a Two Layered
Conducting Polymer Anode
[0078] Molecular organic light-emitting devices (MOLEDs) 10 are
fabricated using a two-layer conducting polymer anode formed as
described in Example 1 (only the highest conductivity PEDOT:PSS and
the lowest conductivity PEDOT:PSS are used for the two-layered film
formation). PET is used as the substrate 22. The conducting polymer
film 12 is patterned using the method described in U.S. Pat. No.
6,649,327. The device configuration used for the experiment is as
shown in FIG. 5, except that layers 40 and 46 are omitted. For the
hole transporting layer 42,
N,N'-Bis(naphthalene-1-yl)-N,N'-bis(phenyl)benzidine (.alpha.-NPB)
was used as a hole transporter. For the light emitting layer 44,
1,2-bis(1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane
(2PSP) was used as an emitter. For the electron transporting layer
48,
2,5-bis-(2',2''-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadi-
ene (PyPySPyPy) was used as an electron transporter. Organic layers
comprising these materials are sequentially deposited on top of the
multi-layered conducting polymer anode 12. The total thickness of
the organic layers 42, 44, 48 is estimated to be .about.110 nm. A
150-nm thick Mg:Ag film, prepared by co-evaporation of Mg and Ag at
a weight ratio of 10 to 1, is deposited onto the organic layers
using a shadow mask and is used as the cathode material 26.
[0079] FIG. 6 shows a plot of the current density and the luminance
as a function of applied voltage for an OLED formed as described
(PET substrate/two-layered PEDOT:PSS
anode/.alpha.-NPD/2PSP/PyPySPyPy/Mg:Ag). An excellent diode
behavior with a rectification ratio of as high as about 10.sup.5
over the range .+-.5 V is observed from the device. The device
shows very high external electroluminescence (EL) quantum
efficiency (photons/electron) as well as luminous power efficiency,
measured as lumen W.sup.-1. For example, a peak external EL
efficiency of 2% and a peak luminous power efficiency of 4.7 lm/W
were measured. These values are much higher than measured for a
comparative device formed analogously, except that it is based on a
conducting single layer polymer anode of uniform conductivity
throughout the layer.
[0080] Table 1 compares the performance of OLEDs with a
four-layered conducting polymer anode device (Device A) according
to the present exemplary embodiment with that of a single layer
conducting polymer anode device (Device B). A dramatic enhancement
in the device performance is obtained from the OLED using the
multilayer conducting polymer anode with varying conductivity
across the film.
TABLE-US-00001 TABLE 1 Peak external Rectification Peak power
electroluminescence ratio efficiency (lm/W) efficiency (%) @ .+-.5
V Device A 4.9 2.0 51000 Device B 1.2 0.8 11 Percent 308 150 463500
improvement (%)
EXAMPLE 6
Organic Light Emitting Device Fabrication Using a 4-Layered
Conducting Polymer Anode
[0081] Molecular organic light-emitting devices (MOLEDs) are
fabricated using a four-layer conducting polymer anode as described
in Example 1. The conducting polymer film was patterned using the
method described in U.S. Pat. No. 6,649,327. Organic layers
including hole transporter .alpha.-NPB 42, and electron
transporter/emitter, Alq.sub.3 44 were sequentially deposited on
top of the four-layered conducting polymer anode 12. The total
thickness of the organic layers is estimated to be .about.110 nm. A
150-nm thick Mg:Ag film, prepared by co-evaporation of Mg and Ag at
a weight ratio of 10 to 1, is deposited onto the organic layers
using a shadow mask and is used as the cathode material 26. A
comparative device was formed in the same way but using only a
single-layered, conducting polymer anode of similar thickness but
uniform conductivity.
[0082] FIG. 7 shows a plot of the relative luminance as a function
of applied voltage for an OLED based on PET/four layer-PEDOT:PSS
anode/.alpha.-NPD/Alq.sub.3/Mg:Ag (Device C) compared with the one
fabricated on a single layer conducting polymer electrode without
modification with a similar device structure (Device D). Device D
is a control device using a single layer conducting polymer anode
fabricated from PEDOT:PSS obtained from Agfa and no treatment or
modification is made to vary the conductivity across the film. An
enhancement in the luminance of as much as 100% is obtained from
the OLED using the multilayer conducting polymer anode. For
example, luminances (brightness) of 860 cd/m.sup.2 and 425
cd/m.sup.2 at 14V are obtained from device C and device D,
respectively.
EXAMPLE 7
Organic Light Emitting Device Fabrication with the Conducting
Polymer Anode with Continuous Change in Conductivity
[0083] Molecular organic light-emitting devices (MOLEDs) are
fabricated using a gradient-type single layered conducting polymer
anode as described in Example 3. The conducting polymer film is
patterned using the method described in U.S. Pat. No. 6,649,327.
The hole transporter, .alpha.-NPB and the electron
transporter/emitter, Alq.sub.3 are sequentially deposited on top of
a single layer conducting polymer anode 12' with continuous change
in the conductivity obtained by UV irradiation (Device E). The
total thickness of the organic layers is estimated to be .about.110
nm. A 150-nm thick Mg:Ag film, prepared by co-evaporation of Mg and
Ag at a weight ratio of 10 to 1, is deposited onto the organic
layers using a shadow mask and is used as the cathode material. A
comparative device (Device F) was prepared in the same way except
that a single-layered conducting polymer anode without UV
irradiation was used.
[0084] FIG. 8 shows a plot of the relative luminance as a function
of applied voltage for an OLED based on PET substrate/single
layered PEDOT:PSS anode with continuous change in the conductivity
across the film obtained by UV
irradiation/.alpha.-NPD/Alq.sub.3/Mg:Ag (Device E) and a
comparative device fabricated on a single layer conducting polymer
electrode (without any treatment) using a similar device structure
(device F). An enhancement in the luminance of as much as 60% is
obtained from the OLED using the gradient conducting polymer anode
12. For example, luminances of 1840 cd/m.sup.2 and 1100 cd/m.sup.2
at 12V are obtained from the device E and device F,
respectively.
EXAMPLE 8
Organic Photovoltaic Cell Fabrication
[0085] Organic photovoltaic cells (OPVs) are fabricated as
illustrated in FIG. 9 using a plastic substrate 22 and a two-layer
conducting polymer anode 12 as described in Example 1 (only the
highest conductivity PEDOT:PSS and the lowest conductivity
PEDOT:PSS are used for the two-layered film formation). The
conducting polymer film is patterned using the method described in
U.S. Pat. No. 6,649,327. The hole transporting material .alpha.-NPD
was used as an electron-donor/hole transporter in layer 50 and
C.sub.60 as the electron-acceptor/transporter in layer 52. The
cathodes 26 for all devices are magnesium/silver alloy (.about.12:1
by mass) grown via codeposition of the metals from separate
sublimation sources. Mg:Ag cathodes are deposited through a shadow
mask producing devices having a 2 mm.times.2 mm (0.04 cm.sup.2)
active area.
[0086] FIG. 10 shows the current density/applied bias
characteristics of the device built on the plastic substrate under
1 sun, white light illumination (AM1.5, 97 mW/cm.sup.2) and varying
incident light intensities. Under 1sun illumination, these cells
exhibit substantial open circuit voltages (Voc) of about 0.85V and
surprisingly large power conversion efficiencies (1.1%) considering
these materials absorb very little light below 500 nm. An
enhancement in the power conversion efficiency of as much as 80% is
obtained from the OPV using the multilayer conducting polymer
anode. For example, power conversion efficiencies of 1.1% and 0.61%
are obtained from the device using two-layered conducting polymer
anode and single layered conducting polymer anode (without any
modification), respectively.
[0087] As the Examples above illustrate, the exemplary methods
provide stable and efficient conductive films for many electronic
and opto-electronic applications. Operational stability of the
device is greatly improved compared to a conventional device with
single layer conducting polymer anode. The device performance
characteristics, such as luminance and efficiency, are also greatly
improved. Since the device using the conducting polymer electrode
disclosed herein has very low leakage current and extremely high
rectification ratio, it can be used in high resolution matrix
display devices, which has not been feasible with conventional
polymer electrode devices.
[0088] It will be appreciated that various of the above-disclosed
and other features and functions, or alternatives thereof, may be
desirably combined into many other different systems or
applications. Also that various presently unforeseen or
unanticipated alternatives, modifications, variations or
improvements therein may be subsequently made by those skilled in
the art which are also intended to be encompassed by the following
claims.
* * * * *