U.S. patent application number 12/376110 was filed with the patent office on 2009-10-22 for methods of manufacturing opto-electrical devices.
This patent application is currently assigned to CAMBRIDGE DISPLAY TECHNOLOGY LIMITED. Invention is credited to Simon Goddard, Peter Lyon, Paul Wallace.
Application Number | 20090263567 12/376110 |
Document ID | / |
Family ID | 38691684 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090263567 |
Kind Code |
A1 |
Wallace; Paul ; et
al. |
October 22, 2009 |
Methods of Manufacturing Opto-Electrical Devices
Abstract
A method of fabricating an opto-electrical device, the method
comprising the steps: depositing, on a substrate comprising a first
electrode for injecting charge carriers of a first polarity, a
composition comprising a conductive or semi-conductive organic
material, a solvent, and a first additive; and, depositing a second
electrode for injecting charge carriers of a second polarity
opposite to the first polarity, wherein the first additive is a
basic additive.
Inventors: |
Wallace; Paul; (Cambridge,
GB) ; Goddard; Simon; (Impington, GB) ; Lyon;
Peter; (Histon, GB) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 SOUTH WACKER DRIVE, 6300 SEARS TOWER
CHICAGO
IL
60606-6357
US
|
Assignee: |
CAMBRIDGE DISPLAY TECHNOLOGY
LIMITED
Cambridgeshire
GB
|
Family ID: |
38691684 |
Appl. No.: |
12/376110 |
Filed: |
August 1, 2007 |
PCT Filed: |
August 1, 2007 |
PCT NO: |
PCT/GB07/02917 |
371 Date: |
May 21, 2009 |
Current U.S.
Class: |
427/67 |
Current CPC
Class: |
H01L 51/5088 20130101;
H01L 51/0037 20130101; H01L 51/0005 20130101; H01L 27/3246
20130101; H01L 51/0007 20130101; H01L 27/3283 20130101 |
Class at
Publication: |
427/67 |
International
Class: |
B05D 5/06 20060101
B05D005/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 1, 2006 |
GB |
0615278.9 |
Aug 1, 2006 |
GB |
0615279.7 |
May 16, 2007 |
GB |
0709430.3 |
Claims
1. A method of fabricating an opto-electrical device, the method
comprising: depositing, on a substrate comprising a first electrode
for injecting charge carriers of a first polarity, a composition
comprising a conductive or semi-conductive organic material, a
solvent, and a first additive; and depositing a second electrode
for injecting charge carriers of a second polarity opposite to the
first polarity, wherein the first additive is a basic additive.
2. A method according to claim 1, wherein the composition is a
dispersion.
3. A method according to claim 1, wherein the composition is a
solution.
4. A method according to claim 1, wherein the basic additive is an
amine.
5. A method according to claim 4, wherein the amine is an organic
amine.
6. A method according to claim 5, wherein the organic amine is
selected from the group consisting of triethanolamine,
diethanolamine, ethanolamine, ethylamine, 4-amino-1-butanol,
4-amino-2-butanol, 6-amino-1-hexanol, 5-amino-1-pentanol, and
ethylenediamine.
7. A method according to claim 1, wherein the basic additive has a
boiling point greater than 100.degree. C.
8. A method according to claim 1, wherein the basic additive is
provided in an amount in the range 1 to 40% by volume.
9. A method according to claim 1, wherein composition is an aqueous
composition.
10. A method according to claim 1, wherein the organic material is
polymeric.
11. A method according to claim 1 wherein the composition comprises
a conductive organic material.
12. A method according to claim 1, wherein the organic material
comprises acidic groups.
13. A method according to claim 1, wherein the composition has a pH
of at least 8.
14. A method according to claim 1, wherein the organic material
comprises a hole injecting material.
15. A method according to claim 14, wherein the hole injecting
material comprises PEDOT with a charge-balancing counterion.
16. A method according to claim 15, wherein the ratio of
PEDOT:counterion is between 1:6 and 1:30.
17. A method according to 15, wherein the counterion is PSS.
18. A method according to claim 1, comprising depositing the
composition by ink-jet printing.
19. A method according to claim 1, comprising depositing a
composition comprising a conductive organic material over the first
electrode and depositing a composition comprising a semi-conductive
organic material thereover, at least one of the conductive and
semi-conductive compositions comprising the basic additive.
20. A method according to claim 1, wherein the substrate comprises
a bank structure defining a plurality of wells and comprising
depositing the composition is deposited into the plurality of wells
by ink jet printing to form a display.
21. A method according to claim 20, comprising printing the display
in swathes.
22. A method according to claim 21, comprising successively
printing a first swathe and a second swathe adjacent to each other,
the print rate being such that the first swathe does not
significantly dry prior to completing printing of the second
swathe.
23. (canceled)
24. A method according to claim 1, wherein the basic additive is
provided in an amount in the range 5 to 30% by volume.
25. A method according to claim 1, wherein the basic additive is
provided in an amount in the range 10 to 20% by volume.
26. A method according to 16, wherein the counterion is PSS.
Description
FIELD OF INVENTION
[0001] This invention relates to a method of fabricating an
opto-electrical device, and the use of compositions in methods of
manufacturing opto-electrical devices.
BACKGROUND OF INVENTION
[0002] One class of opto-electrical devices is that using an
organic material for light emission (or detection in the case of
photovoltaic cells and the like). The basic structure of these
devices is a light emissive organic layer, for instance a film of a
poly (p-phenylenevinylene) ("PPV") or polyfluorene, sandwiched
between a cathode for injecting negative charge carriers
(electrons) and an anode for injecting positive charge carriers
(holes) into the organic layer. The electrons and holes combine in
the organic layer generating excitons which then undergoes
radiative decay to give light (in light detecting devices this
process essentially runs in reverse). In WO90/13148 the organic
light-emissive material is a polymer. In U.S. Pat. No. 4,539,507
the organic light-emissive material is of the class known as small
molecule materials, such as (8-hydroxyquinoline) aluminium
("Alq3"). In a practical device one of the electrodes is
transparent, to allow the photons to escape the device.
[0003] A typical organic light-emissive device ("OLED") is
fabricated on a glass or plastic substrate coated with a
transparent anode such as indium-tin-oxide ("ITO"). A layer of a
thin film of at least one electroluminescent organic material
(which here includes organometallic material) covers the first
electrode. Finally, a cathode covers the layer of
electroluminescent organic material. The cathode is typically a
metal or alloy and may comprise a single layer, such as aluminium,
or a plurality of layers such as calcium and aluminium.
[0004] A multicoloured display may be constructed using groups of
red, green, and blue emitting pixels. So-called active matrix
displays have a memory element, typically a storage capacitor and a
transistor, associated with each pixel whilst passive matrix
displays have no such memory element and instead are repetitively
scanned to give the impression of a steady image.
[0005] FIG. 1 shows a vertical cross section through an example of
an OLED device 100. In an active matrix display, part of the area
of a pixel is occupied by associated drive circuitry (not shown in
FIG. 1). The structure of the device is somewhat simplified for the
purposes of illustration.
[0006] The OLED 100 comprises a substrate 102, typically 0.7 mm or
1.1 mm glass but optionally clear plastic, on which an anode layer
106 has been deposited. The anode layer typically comprises around
150 nm thickness of ITO (indium tin oxide), over which is provided
a metal contact layer, typically around 500 nm of aluminium,
sometimes referred to as anode metal. Glass substrates coated with
ITO and contact metal may be purchased from Corning, USA. The
contact metal (and optionally the ITO) is patterned as desired so
that it does not obscure the display, by a conventional process of
photolithography followed by etching.
[0007] A substantially transparent hole injection layer 108a is
provided over the anode metal, followed by an electroluminescent
layer 108b. Banks 112 may be formed on the substrate, for example
from positive or negative photoresist material, to define wells 114
into which these active organic layers may be selectively
deposited, for example by a droplet deposition or inkjet printing
technique. The wells thus define light emitting areas or pixels of
the display.
[0008] A cathode layer 110 is then applied by, say, physical vapour
deposition. The cathode layer typically comprises a low work
function metal such as calcium or barium covered with a thicker,
capping layer of aluminium and optionally including an additional
layer immediately adjacent the electroluminescent layer, such as a
layer of lithium fluoride, for improved electron energy level
matching. The cathode may be transparent. This is particularly
preferred for active matrix devices wherein emission through the
substrate is partially blocked by drive circuitry located
underneath the emissive pixels. In the case of a transparent
cathode device, it will be appreciated that the anode is not
necessarily transparent. In the case of passive matrix displays,
mutual electrical isolation of cathode lines may achieved through
the use of cathode separators (element 302 of FIG. 3b). Typically a
number of displays are fabricated on a single substrate and at the
end of the fabrication process the substrate is scribed, and the
displays separated. An encapsulant such as a glass sheet or a metal
can is utilized to inhibit oxidation and moisture ingress.
[0009] Organic LEDs of this general type may be fabricated using a
range of materials including polymers, dendrimers, and so-called
small molecules, to emit over a range of wavelengths at varying
drive voltages and efficiencies. Examples of polymer-based OLED
materials are described in WO90/13148, WO95/06400 and WO99/48160;
examples of dendrimer-based materials are described in WO 99/21935
and WO 02/067343; and examples of small molecule OLED materials are
described in U.S. Pat. No. 4,539,507. The aforementioned polymers,
dendrimers and small molecules emit light by radiative decay of
singlet excitons (fluorescence). However, up to 75% of excitons are
triplet excitons which normally undergo non-radiative decay.
Electroluminescence by radiative decay of triplet excitons
(phosphorescence) is disclosed in, for example, "Very
high-efficiency green organic light-emitting devices based on
electrophosphorescence" M. A. Baldo, S. Lamansky, P. E. Burrows, M.
E. Thompson, and S. R. Forrest Applied Physics Letters, Vol. 75(1)
pp. 4-6, Jul. 5, 1999. In the case of a polymer-based OLED, layers
108 comprise a hole injection layer 108a and a light emitting
polymer (LEP) electroluminescent layer 108b. The electroluminescent
layer may comprise, for example, around 70 nm (dry) thickness of
PPV (poly(p-phenylenevinylene)) and the hole injection layer, which
helps match the hole energy levels of the anode layer and of the
electroluminescent layer, may comprise, for example, around 50-200
nm, preferably around 150 nm (dry) thickness of PEDOT:PSS
(polystyrene-sulphonate-doped polyethylene-dioxythiophene).
[0010] The deposition of material for organic light emitting diodes
(OLEDs) using ink jet printing techniques are described in a number
of documents including, for example: EP 0880303 and "Ink-Jet
Printing of Polymer Light-Emitting Devices", Paul C. Duineveld,
Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H.
Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den
Biggelaar, Jan-Eric J. M. Rubingh and Eliav I. Haskal, Organic
Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor,
Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be
used to deposit materials for any form of soluble organic material,
including both small molecule and polymer LEDs.
[0011] FIG. 2 shows a view from above (that is, not through the
substrate) of a portion of a three-colour active matrix pixellated
OLED display 200 after deposition of one of the active colour
layers. The figure shows an array of banks 112 and wells 114
defining pixels of the display.
[0012] FIG. 3a shows a view from above of a substrate 300 for
inkjet printing a passive matrix OLED display. FIG. 3b shows a
cross-section through the substrate of FIG. 3a along line Y-Y'.
[0013] Referring to FIGS. 3a and 3b, the substrate is provided with
a plurality of cathode undercut separators 302 to separate adjacent
cathode lines (which will be deposited in regions 304). A plurality
of wells 308 is defined by banks 310, constructed around the
perimeter of each well 308 and leaving an anode layer 306 exposed
at the base of the well. The edges or faces of the banks are
tapered onto the surface of the substrate as shown, heretofore at
an angle of between 10 and 40 degrees. The banks present a
hydrophobic surface in order that they are not wetted by the
solution of deposited organic material and thus assist in
containing the deposited material within a well. This is achieved
by treatment of a bank material such as polyimide with an
O.sub.2/CF.sub.4 plasma as disclosed in EP 0989778. Alternatively,
the plasma treatment step may be avoided by use of a fluorinated
material such as a fluorinated polyimide as disclosed in WO
03/083960.
[0014] As previously mentioned, the bank and separator structures
may be formed from resist material, for example using a positive
(or negative) resist for the banks and a negative (or positive)
resist for the separators; both these resists may be based upon
polyimide and spin coated onto the substrate, or a fluorinated or
fluorinated-like photoresist may be employed. In the example shown
the cathode separators are around 5 .mu.m in height and
approximately 20 .mu.m wide. Banks are generally between 20 .mu.m
and 100 .mu.m in width and in the example shown have a 4 .mu.m
taper at each edge (so that the banks are around 1 .mu.m in
height). The pixels of FIG. 3a are approximately 300 .mu.m square
but, as described later, the size of a pixel can vary considerably,
depending upon the intended application.
[0015] These devices have great potential for displays and
lighting. However, there are several significant problems. One is
to make the device efficient, particularly as measured by its
external power efficiency and its external quantum efficiency.
Another is to optimise (e.g. to reduce) the voltage at which peak
efficiency is obtained. Another is to stabilise the voltage
characteristics of the device over time. Another is to increase the
lifetime of the device.
[0016] To this end, numerous modifications have been made to the
basic device structure described above in order to solve one or
more of these problems.
[0017] One such modification is the provision of a layer of
conductive polymer between the light-emissive organic layer and one
of the electrodes. It has been found that the provision of such a
conductive polymer layer can improve the turn-on voltage, the
brightness of the device at low voltage, the efficiency, the
lifetime and the stability of the device. Examples of such
conductive polymers include polythiophene derivatives such as
poly(ethylene dioxythiophene), or polyaniline derivatives. It may
be advantageous in some device arrangements to not have too high a
conductivity of the conductive polymer. For example, if a plurality
of electrodes are provided in a device but only one continuous
layer of conductive polymer extending over all the electrodes, then
too high a conductivity can lead to lateral conduction (known as
"cross-talk"). Furthermore, if the conductive polymer is not
covered by the overlying layer(s) of organic material between the
conductive polymer and the cathode then there is a risk of shorting
between the conductive polymer and the cathode.
[0018] The conductive polymer layer may also be selected to have a
suitable workfunction so as to aid in hole or electron injection
and/or to block holes or electrons. There are thus two key
electrical features: the overall conductivity of the conductive
polymer composition; and the workfunction of the conductive polymer
composition. The stability of the composition and reactivity with
other components in a device will also be critical in providing an
acceptable lifetime for a practical device. The processability of
the composition will be critical for ease of manufacture.
[0019] Conductive polymer formulations are discussed in the
applicant's earlier application WO2006/123167. There is an ongoing
need to optimise the organic formulations used in these devices
both in the light emitting layer and the conductive polymer layer,
in particular to improve inkjet performance and wetting properties
of these compositions.
[0020] Furthermore, devices comprising acidic conductive polymers
such as PEDOT/PSS suffer from corrosion of layers adjacent to
PEDOT/PSS, in particular the anode layer.
[0021] A problem associated with ink jet printing of materials for
organic opto-electrical devices is that the printing process
involves printing stripes (or swathes) of ink (corresponding to the
ink jet head width) which results in an inbuilt asymmetry in the
drying environment. Specifically, at a swathe edge more drying
occurs on the unprinted side since the solvent concentration in the
atmosphere above the substrate is less than the printed side. With
more evaporation taking place on the unprinted side more solute is
deposited on this side and the film profile becomes asymmetric,
resulting in visible non-uniformities in the resultant display.
[0022] Another problem associated with ink jet printing of organic
opto-electrical devices such as those discussed above is that in
the resultant device, the organic hole injecting layer can extend
beyond the overlying organic semi-conductive layer providing a
shorting path between the cathode and the anode at an edge of the
well. This problem is exacerbated if the contact angle of the
conductive organic composition with the bank material is too low.
This problem is further exacerbated if the conductivity of the
organic hole injecting layer is too high. One solution to this
problem is to modify the bank structure. However, providing a more
complex bank structure is expensive and increases the complexity of
the manufacturing method for the device.
[0023] In addition to the aforementioned problems of depositing
prior art compositions using ink jet printing, it has also been
found that some compositions comprising conductive and
semi-conductive organic material are also difficult, or indeed
impossible, to deposit by other methods such as spin-coating. As
such, it is an aim of the present invention to provide compositions
which are easier to deposit by any solution processing method
including, for example, spin-coating as well as ink jet
printing.
[0024] The present applicant seeks to solve, or at least reduce,
the problems outlined above by adapting compositions, in particular
compositions for ink jet printing, comprising conductive or
semi-conductive organic material. These adapted compositions are of
particular use in the manufacture of light-emissive devices.
[0025] WO 2004/029128 discloses compositions of PEDOT with
counterions such as fluorinated polyacids as an alternative to
counterions such as PSS, and teaches that the pH of these
formulations may be modified using an ion-exchange resin. pH of
formulations modified in this way may be further modified by
addition of aqueous basic salts such as sodium hydroxide. Example
21 of this publication teaches that modification of pH of PEDOT/PSS
using an ion exchange resin significantly degrades device
performance.
[0026] WO 2005/034261 discloses a method for preserving an acidic
organic material having a 2 wt % concentration by increasing its
pH.
[0027] WO 2004/063277 discloses addition of co-solvents to aqueous
PEDOT/PSS in order to increase the conductivity of films formed
from the aqueous solution.
SUMMARY OF THE PRESENT INVENTION
[0028] According to a first aspect of the present invention there
is provided a method of fabricating an opto-electrical device, the
method comprising the steps: depositing, on a substrate comprising
a first electrode for injecting charge carriers of a first
polarity, a composition comprising a conductive or semi-conductive
organic material, a solvent, and a first additive; and depositing a
second electrode for injecting charge carriers of a second polarity
opposite to the first polarity, wherein the first additive is a
basic additive.
[0029] The solvent may dissolve the conductive or semiconductive
organic material, or the solvent and conductive or semi-conductive
organic material may together form a dispersion. For example, an
aqueous composition of PEDOT/PSS is in the form of a dispersion.
Preferably, the composition is a dispersion. Preferably, the
solvent is an aqueous solvent. Preferably, the organic material is
conductive.
[0030] In the case where the composition comprises a conductive
organic material, this material preferably comprises a polycation
and a charge balancing polyanion, for example PEDOT with a
polyanion such as PSS. Another example is polythienothiophene with
a polyanion.
[0031] It has surprisingly been found that the addition of a basic
additive to a composition comprising a conductive or
semi-conductive organic material in a solvent, in particular an
acidic composition, results in a composition that forms much
smoother films as compared to compositions without a basic
additive.
[0032] It has also been found that a basic additive can be used to
tune the conductivity of a film formed from the composition. For
example, the base can form a salt with an acidic conductive organic
material, in particular a conductive organic material comprising a
polyacid such as poly(styrene sulfonic acid), neutralizing acid
groups and increasing resistivity. Neutralisation of acidic
compositions in this way has the added benefit of making the
composition less hazardous, and also less likely to corrode an
inkjet head during inkjet printing. Thus, the organic material
preferably comprises acidic groups, and the composition according
to the invention preferably has a pH greater than or equal to 7,
more preferably greater than or equal to 8, most preferably in the
range 8-10.
[0033] It has further been found that a formulation comprising a
basic additive is much less corrosive than formulations without
such an additive. Without wishing to be bound by any theory, this
effect is attributed to the higher pH of formulations comprising a
basic additive. Thus, corrosion of metal tracking for electrodes
such as gold tracking or MoCr stacks is significantly or completely
reduced. Moreover, in the case of inkjet printed devices, the
present inventors have found that the formulations according to the
invention do not corrode the nozzle plate of the inkjet head,
unlike the higher pH formulations of the prior art.
[0034] It will be appreciated that any basic additive will serve to
neutralise acidic compositions. However, not all basic additives
result in an increase in resistivity of a film formed from the
composition. In particular, unsubstituted primary amines, in
particular primary alkylamines such as ethylamine and
ethylenediamine, have been found to give a large increase in
resistivity. In contrast, basic additives comprising hydroxy
substituents, for example hydroxy amines such as ethanolamine, tend
to either reduce or have little effect on resistivity. Thus,
appropriate choice of basic additives and mixtures thereof can be
used to tune the resistivity of the compositions.
[0035] Preferably, the basic additive can be evaporated from the
formulation upon drying. It will therefore be appreciated that the
additive is preferably not an ionic base such as sodium hydroxide
or similar salts. Preferably, the additive is a non-ionic organic
base. The provision of a basic additive, in particular a basic
additive with a high boiling point, can increase the drying time of
the composition. Thus, during ink jet printing, the amount of
evaporation occurring in the time between deposition of adjacent
swathes is reduced leading to a greater uniformity of drying and a
more symmetric film formation around a swathe join.
[0036] The basic additive may also serve to solubilise the
conductive or semiconductive organic material.
[0037] Typically, there will only be a few seconds until the next
swathe is printed when ink jet printing. However, due to the high
surface to volume ratio of an ink, drying times are in the order of
seconds. As a result significant drying can occur prior to
deposition of an adjacent swathe.
[0038] As explained above, a basic additive can serve to neutralise
an acidic composition. The inventors have found that the amount of
amine required for this purpose is very low, and can be less than
2% or even less than 1% by volume. The inventors have also found
that high boiling point amines can be used in excess of the amount
required to neutralise the composition. By using basic additives,
the amount of evaporation occurring in this time can be reduced.
Once adjacent swathes have been deposited the drying environment
becomes symmetrical resulting in symmetric layer profiles around
the swathe join.
[0039] The amount and type of basic additive to be added to a
composition will be dependent on how much of a reduction in drying
time is desired. This will be dependent on the time taken to print
adjacent swathes. Thus, for slower printing times, a slower drying
composition is desirable and a larger volume and/or higher boiling
point basic additive will be required.
[0040] The basic additive may be added to a composition from a
solution in a solvent, however in the case of non-ionic organic
bases, it is preferred that the additive is added in neat form to
avoid unnecessary dilution of the composition.
[0041] The addition of a basic additive is preferably the primary
means, preferably the only means, by which the pH of a composition
is modified.
[0042] The amount and/or type of solvent to be used will depend on
the speed of ink jet printing (how much time it takes to print
successive swathes). The amount and/or type of solvent will also
depend on the surface to volume ratio of the ink droplet. For
larger ink droplets, evaporation will be slower and for a given
print speed, a lower boiling point basic additive may be required
when compared to an arrangement utilizing smaller droplets. One key
feature of embodiments of the present invention is that the print
speed, the droplet size/well size, and the boiling point of the
basic additive are selected such that when a first swathe and a
second swathe are successively printed adjacent to each other, the
print rate is such that the first swathe does not significantly dry
prior to completing printing of the second swathe.
[0043] One problem found with adding high boiling point solvents
such as glycerol is that there is a large increase in the
conductivity of the composition resulting in problems due to
shorting between electrodes. Accordingly, it is required to add a
conductivity modifier in order to reduce the conductivity of the
composition. The conductivity modifier may be, for example, a large
excess of PSS in a PEDOT:PSS formulation. However, a problem with
this is that the composition becomes very acidic and device
lifetime may be poor. In contrast, as the basic additives of the
present invention do not produce a large increase in conductivity,
no conductivity modifier such as excess PSS is required, thus
improving device lifetime.
[0044] Following on from the above, a particular problem in organic
opto-electrical devices is that the conductive organic hole
injecting layer may extend beyond the overlying organic
semi-conductive layer providing a shorting path between the cathode
deposited thereover and the underlying anode. This problem is
exacerbated if the conductivity of the organic hole injecting layer
is high, which can be the case when using high boiling point
solvents such as glycerol. In contrast, compositions comprising a
basic additive have lower conductivity thus reducing the shorting
problem.
[0045] Asymmetric drying at the swathe join can also lead to
shorting paths being created at the swathe join. Accordingly, the
use of a basic additive which alleviates asymmetric drying will
also reduce the shorting problem caused by poor film morphologies.
The present applicant has found that in some cases addition of a
high boiling point solvent increases shorting at the swathe joins.
This has been found to be due to an increase in the conductivity of
the conductive polymer film. This problem is avoided by using a
basic additive.
[0046] The base in the additive may remain in a film formed using
the composition or may be volatile and evaporate from the film
during fabrication. In either case, the base may affect the charge
balance in the film or act as a surface modifier in order to
increase the lifetime of a device.
[0047] Preferably, the basic additive has a boiling point greater
than 100.degree. C. In some instances, vacuum drying and baking is
believed to break down any salt formed by the base and the organic
material releasing the base and acid groups on the organic
material.
[0048] Preferably, the basic additive is an amine, most preferably
an organic amine. It has been found that amines provide
particularly good compositions for use in fabricating organic
opto-electrical devices. Examples of classes of amines include
primary, secondary and tertiary alkyl amines; primary or secondary
aryl amines; diamines; pyridines; pyrimidines and quinolines. The
amine may optionally be substituted. In particular, alkylamines may
be substituted with one or more hydroxy, thio or amino groups.
Specific examples of substituted amines include alkylamines with
one or more hydroxy groups such as triethanolamine, diethanolamine,
ethanolamine, ethylamine, 4-amino-1-butanol, 4-amino-2-butanol,
6-amino-1-hexanol, 5-amino-1-pentanol, and ethylenediamine.
[0049] Because little or none of the amine additive may remain in
the resultant film formed by compositions of the present invention
if a volatile amine is utilized, the additive can be provided in
the composition in relatively large amounts or as a minor additive
component in the composition. Preferably, the additive is provided
in an amount in the range 1 to 40% by volume, more preferably 5 to
30%, most preferably 10-20%. If the additive is provided at lower
or higher concentrations, the solution processability of the
composition is not as good. In the case where an acidic conductive
organic material is used, up to about 2% v/v, such as around 0.1-2
or 1-2% v/v of the basic additive is typically required for
neutralisation of the conductive organic material.
[0050] The solubility, processability and functional properties of
the organic material may be very sensitive to changes in solvent.
Accordingly, it is advantageous to retain a portion of solvent in
which the organic material is stable. Water may be a suitable
solvent for some organic materials, particularly charged conductive
organic materials such as doped PEDOT, which forms a dispersion
with water. As such, the solvent will typically be the usual
solvent used for the organic material for achieving good
solubility, processability and conductance characteristics.
[0051] Suitable solvents for non-polar organic materials include
mono- or poly-alkylated benzenes, for example xylene.
[0052] The solid content of the composition may be between 0.5% and
6%, 1% and 4%, 1.5% and 3%, and in some cases is preferably 2%. The
solid content also affects the form of the film after drying. If
the solid content is too high then the film forms a dome shape
whereas if the solid content is too low then an excessive coffee
ring effect occurs. It has been found that the provision of a basic
additive allows the use of a higher solid content when compared
with high boiling point additives such as glycerol, which allows an
increase in film thickness over previous compositions.
[0053] A light-emitting layer may be deposited as a composition
comprising a semi-conductive organic material in a composition
according to the present invention. Preferably, the organic
material comprises a polymer and most preferably the polymer is
either fully or partially conjugated.
[0054] A charge injecting layer may be deposited as a composition
comprising a conductive organic material in a composition according
to the present invention. Preferably, the organic material
comprises a polymer and most preferably the organic material
comprises PEDOT with a suitable polyanion, for example PSS.
[0055] Embodiments of the present invention relate to new PEDOT ink
formulations for improved film uniformity within pixels and across
swathe joins. Slower drying inks have been formulated which do not
compromise other aspects of the inks performance. This provides an
alternative to interlacing which is very slow.
[0056] The present applicant has found that the problem of film
non-uniformity in PEDOT is very important to device performance.
The device performance may not be directly affected significantly
by the thickness of the PEDOT film. However, the uniformity of the
PEDOT film affects the uniformity of the overlying
electroluminescent layer. The EL layer is very sensitive to changes
in thickness. Accordingly, the present applicant has found that it
is paramount that uniform films of PEDOT profiles are achieved in
order to achieve uniform EL profiles.
[0057] Organic amines such as ethanolamine, when added to PEDOT
compositions act to neutralise acid groups on PEDOT/PSS giving a
large increase in resistivity. Thus amine salts are formed with
--SO.sub.3H groups on the PSS polymer. Vacuum drying and baking is
believed to break down the salt releasing volatile amine and free
--SO.sub.3H groups.
[0058] When added in excess of the amount required to neutralise
the conductive organic material (>.about.1%->20%) the amine
acts as a high boiling point solvent reducing evaporation rate of
the composition on drying and eliminating (physical) swath joins. A
solids content of 2% allows an increase in film thickness as
compared to a formulation not comprising a basic additive. The
result is an ink jet printing formulation that has the correct
resistivity to avoid electrical swath joins and a component that
reduces evaporation rate thus eliminating physical swaths.
[0059] In the case where the composition of the invention is inkjet
printed, it preferably has a surface tension of at least 35 mN m to
avoid leakage of the composition from the inkjet print head.
[0060] The quantity of counterion present in a PEDOT:counterion
composition is at least sufficient to balance the charge on PEDOT,
and the PEDOT:counterion ratio may be in the range 1:10 to 1:30,
more preferably in the range 1:15 to 1:20. Preferably, the
counterion is a polymeric acid such as a polysulfonic acid (for
example PSS or Nafion) or poly acrylic acid. Most preferably, the
counterion is PSS.
[0061] The compositions of the present invention may be deposited
by any solution processing method, for example ink-jet printing,
spin-coating, dip-coating, roll-printing, flexographic printing or
screen printing. The viscosity of the composition for inkjet
printing is preferably in the range 2 to 30 mPa, 2 to 20 mPa, 4 to
12 mPa, more preferably 6 to 8 mPa, and most preferably
approximately 8 mPa, at 20 degrees centigrade. Higher viscosities
may be suitable for other solution processing methods.
[0062] It will be appreciated from the above that the basic
additive can provide a number of advantages including film
smoothness, improved jeftability, resistivity modification, pH
control and improved device performance.
[0063] The compositions of the present invention may comprise more
than one basic additive in order to optimise the properties of the
composition. For example, a high-boiling basic additive such as
triethanolamine may be used to improve jetting characteristics,
whereas a low boiling basic additive such as ethylamine may be used
to enhance resistivity. Similarly, more than one basic additive may
be used to tune the resistivity of the composition.
[0064] Other additives may also be included, for example alcohol
ether additives such as butoxyethanol that may serve to improve
jetting properties and wetting of the composition; sufoxides such
as dimethylsulfoxide; and amides such and dimethylformamide.
[0065] Furthermore, the basic additive of the present invention may
be used in combination with other, non-basic, additives in order to
tune the properties of the composition. Examples of other additives
include polyacids, for example a polysulfonic acid such as PSS or
Nafion.RTM., or poly acrylic acid; and alcohols, in particular
polyols such as ethylene glycol.
[0066] According to one embodiment of the present invention there
is provided a method of manufacturing an organic light-emissive
display comprising: providing a substrate comprising a first
electrode layer and a bank structure defining a plurality of wells;
depositing a conductive polymer layer over the first electrode;
depositing an organic light-emissive layer over the conductive
polymer layer; and depositing a second electrode over the organic
light-emissive layer, wherein at least one of the conductive
polymer layer and the organic light-emissive layer is deposited by
ink jet printing a composition as described herein into the
plurality of wells.
BRIEF SUMMARY OF THE DRAWINGS
[0067] Embodiments of the present invention will now be described,
by way of example only, with reference to the accompanying drawings
in which:
[0068] FIG. 1 shows a vertical cross section through an example of
an OLED device;
[0069] FIG. 2 shows a view from above of a portion of a three
colour pixelated OLED display;
[0070] FIGS. 3a and 3b show a view from above and a cross-sectional
view respectively of a passive matrix OLED display;
[0071] FIG. 4 shows a titration curve for a composition according
to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0072] The general device architecture is illustrated in FIG. 1 and
has been described previously.
[0073] The device is preferably encapsulated with an encapsulant
(not shown) to prevent ingress of moisture and oxygen. Suitable
encapsulants include a sheet of glass, films having suitable
barrier properties such as alternating stacks of polymer and
dielectric as disclosed in, for example, WO 01/81649 or an airtight
container as disclosed in, for example, WO 01/19142. A getter
material for absorption of any atmospheric moisture and/or oxygen
that may permeate through the substrate or encapsulant may be
disposed between the substrate and the encapsulant.
[0074] Suitable polymers for charge transport and emission may
comprise a first repeat unit selected from arylene repeat units, in
particular: 1,4-phenylene repeat units as disclosed in J. Appl.
Phys. 1996, 79, 934; fluorene repeat units as disclosed in EP
0842208; indenofluorene repeat units as disclosed in, for example,
Macromolecules 2000, 33(6), 2016-2020; and spirofluorene repeat
units as disclosed in, for example EP 0707020. Each of these repeat
units is optionally substituted. Examples of substituents include
solubilising groups such as C.sub.1-20 alkyl or alkoxy; electron
withdrawing groups such as fluorine, nitro or cyano; and
substituents for increasing glass transition temperature (Tg) of
the polymer.
[0075] Particularly preferred polymers comprise optionally
substituted, 2,7-linked fluorenes, most preferably first repeat
units of formula:
##STR00001## [0076] wherein R.sup.1 and R.sup.2 are independently
selected from hydrogen or optionally substituted alkyl, alkoxy,
aryl, arylalkyl, heteroaryl and heteroarylalkyl. More preferably,
at least one of R.sup.1 and R.sup.2 comprises an optionally
substituted C.sub.4-C.sub.20 alkyl or aryl group.
[0077] A polymer comprising the first repeat unit may provide one
or more of the functions of hole transport, electron transport and
emission depending on which layer of the device it is used in and
the nature of co-repeat units.
[0078] In particular:
[0079] a homopolymer of the first repeat unit, such as a
homopolymer of 9,9-dialkylfluoren-2,7-diyl, may be utilised to
provide electron transport;
[0080] a copolymer comprising a first repeat unit and a
triarylamine repeat unit utilised to provide hole transport and/or
emission; or
[0081] a copolymer comprising a first repeat unit and heteroarylene
repeat unit may be utilised for charge transport or emission.
[0082] Particularly preferred triarylamine repeat units are
selected from optionally substituted repeat units of formulae
1-6:
##STR00002##
[0083] wherein X, Y, A, B, C and D are independently selected from
H or a substituent group. More preferably, one or more of X, Y, A,
B, C and D is independently selected from the group consisting of
optionally substituted, branched or linear alkyl, aryl,
perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl and
arylalkyl groups. Most preferably, X, Y, A and B are C.sub.1-10
alkyl. The aromatic rings in the backbone of the polymer may be
linked by a direct bond or a bridging group or bridging atom, in
particular a bridging heteroatom such as oxygen.
[0084] Also particularly preferred as the triarylamine repeat unit
is an optionally substituted repeat unit of formula 6a:
##STR00003##
[0085] wherein Het represents a heteroaryl group.
[0086] Another preferred hole transporting material comprises the
repeat unit of general formula (6aa):
##STR00004##
[0087] where Ar.sub.1, Ar.sub.2, Ar.sub.3, Ar.sub.4 and Ar.sub.5
each independently represent an aryl or heteroaryl ring or a fused
derivative thereof; and X represents an optional spacer group.
[0088] Preferred heteroarylene repeat units are selected from
formulae 7-21:
##STR00005##
[0089] wherein R.sub.6 and R.sub.7 are the same or different and
are each independently hydrogen or a substituent group, preferably
alkyl, aryl, perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl,
alkylaryl or arylalkyl. For ease of manufacture, R.sub.6 and
R.sub.7 are preferably the same. More preferably, they are the same
and are each a phenyl group.
##STR00006## ##STR00007##
[0090] Electroluminescent copolymers may comprise an
electroluminescent region and at least one of a hole transporting
region and an electron transporting region as disclosed in, for
example, WO 00/55927 and U.S. Pat. No. 6,353,083. If only one of a
hole transporting region and electron transporting region is
provided then the electroluminescent region may also provide the
other of hole transport and electron transport functionality.
[0091] The different regions within such a polymer may be provided
along the polymer backbone, as per U.S. Pat. No. 6,353,083, or as
groups pendant from the polymer backbone as per WO 01/62869.
[0092] Preferred methods for preparation of these polymers are
Suzuki polymerisation as described in, for example, WO 00/53656 and
Yamamoto polymerisation as described in, for example, T. Yamamoto,
"Electrically Conducting And Thermally Stable
.quadrature.-Conjugated Poly(arylene)s Prepared by Organometallic
Processes", Progress in Polymer Science 1993, 17, 1153-1205. These
polymerisation techniques both operate via a "metal insertion"
wherein the metal atom of a metal complex catalyst is inserted
between an aryl group and a leaving group of a monomer. In the case
of Yamamoto polymerisation, a nickel complex catalyst is used; in
the case of Suzuki polymerisation, a palladium complex catalyst is
used.
[0093] For example, in the synthesis of a linear polymer by
Yamamoto polymerisation, a monomer having two reactive halogen
groups is used. Similarly, according to the method of Suzuki
polymerisation, at least one reactive group is a boron derivative
group such as a boronic acid or boronic ester and the other
reactive group is a halogen. Preferred halogens are chlorine,
bromine and iodine, most preferably bromine.
[0094] It will therefore be appreciated that repeat units and end
groups comprising aryl groups as illustrated throughout this
application may be derived from a monomer carrying a suitable
leaving group.
[0095] Suzuki polymerisation may be used to prepare regioregular,
block and random copolymers. In particular, homopolymers or random
copolymers may be prepared when one reactive group is a halogen and
the other reactive group is a boron derivative group.
Alternatively, block or regioregular, in particular AB, copolymers
may be prepared when both reactive groups of a first monomer are
boron and both reactive groups of a second monomer are halogen.
[0096] As alternatives to halides, other leaving groups capable of
participating in metal insertion include groups include tosylate,
mesylate and triflate.
[0097] A single polymer or a plurality of polymers may be deposited
from solution to form layer 5. Suitable solvents for polyarylenes,
in particular polyfluorenes, include mono- or poly-alkylbenzenes
such as toluene and xylene. Particularly preferred solution
deposition techniques are spin-coating and inkjet printing.
[0098] Spin-coating is particularly suitable for devices wherein
patterning of the electroluminescent material is unnecessary--for
example for lighting applications or simple monochrome segmented
displays.
[0099] Inkjet printing is particularly suitable for high
information content displays, in particular full colour displays.
Inkjet printing of OLEDs is described in, for example, EP
0880303.
[0100] In some cases, distinct layers of the device may be formed
by different methods, for example a hole injection and/or transport
layer may be formed by spin-coating and an emissive layer may be
deposited by inkjet printing.
[0101] If multiple layers of the device are formed by solution
processing then the skilled person will be aware of techniques to
prevent intermixing of adjacent layers, for example by crosslinking
of one layer before deposition of a subsequent layer or selection
of materials for adjacent layers such that the material from which
the first of these layers is formed is not soluble in the solvent
used to deposit the second layer.
[0102] Numerous hosts are described in the prior art including
"small molecule" hosts such as 4,4'-bis(carbazol-9-yl)biphenyl),
known as CBP, and (4,4',4''-tris(carbazol-9-yl)triphenylamine),
known as TCTA, disclosed in Ikai et al. (Appl. Phys. Lett., 79 no.
2, 2001, 156); and triarylamines such as
tris-4-(N-3-methylphenyl-N-phenyl)phenylamine, known as MTDATA.
Polymers are also known as hosts, in particular homopolymers such
as poly(vinyl carbazole) disclosed in, for example, Appl. Phys.
Lett. 2000, 77(15), 2280; polyfluorenes in Synth. Met. 2001, 116,
379, Phys. Rev. B 2001, 63, 235206 and Appl. Phys. Lett. 2003,
82(7), 1006; poly[4-(N-4-vinyl benzyloxyethyl,
N-methylamino)-N-(2,5-di-tert-butylphenylnapthalimide] in Adv.
Mater. 1999, 11(4), 285; and poly(para-phenylenes) in J. Mater.
Chem. 2003, 13, 50-55. Copolymers are also known as hosts.
[0103] The emissive species may be metal complexes. The metal
complexes may comprise optionally substituted complexes of formula
(22):
ML.sup.1.sub.qL.sup.2.sub.rL.sup.3.sub.s (22)
[0104] wherein M is a metal; each of L.sup.1, L.sup.2 and L.sup.3
is a coordinating group; q is an integer; r and s are each
independently 0 or an integer; and the sum of (a. q)+(b. r)+(c.s)
is equal to the number of coordination sites available on M,
wherein a is the number of coordination sites on L.sup.1, b is the
number of coordination sites on L.sup.2 and c is the number of
coordination sites on L.sup.3.
[0105] Heavy elements M induce strong spin-orbit coupling to allow
rapid intersystem crossing and emission from triplet states
(phosphorescence). Suitable heavy metals M include:
[0106] lanthanide metals such as cerium, samarium, europium,
terbium, dysprosium, thulium, erbium and neodymium; and
[0107] d-block metals, in particular those in rows 2 and 3 i.e.
elements 39 to 48 and 72 to 80, in particular ruthenium, rhodium,
pallaidum, rhenium, osmium, iridium, platinum and gold.
[0108] Suitable coordinating groups for the f-block metals include
oxygen or nitrogen donor systems such as carboxylic acids,
1,3-diketonates, hydroxy carboxylic acids,
Schiff bases including acyl phenols and iminoacyl groups. As is
known, luminescent lanthanide metal complexes require sensitizing
group(s) which have the triplet excited energy level higher than
the first excited state of the metal ion. Emission is from an f-f
transition of the metal and so the emission colour is determined by
the choice of the metal. The sharp emission is generally narrow,
resulting in a pure colour emission useful for display
applications.
[0109] The d-block metals form organometallic complexes with carbon
or nitrogen donors such as porphyrin or bidentate ligands of
formula (VI):
##STR00008##
[0110] wherein Ar.sup.4 and Ar.sup.5 may be the same or different
and are independently selected from optionally substituted aryl or
heteroaryl; X.sup.1 and Y.sup.1 may be the same or different and
are independently selected from carbon or nitrogen; and Ar.sup.4
and Ar.sup.5 may be fused together. Ligands wherein X.sup.1 is
carbon and Y.sup.1 is nitrogen are particularly preferred.
[0111] Examples of bidentate ligands are illustrated below:
##STR00009##
[0112] Each of Ar.sup.4 and Ar.sup.5 may carry one or more
substituents. Particularly preferred substituents include fluorine
or trifluoromethyl which may be used to blue-shift the emission of
the complex as disclosed in WO 02/45466, WO 02/44189, US
2002-117662 and US 2002-182441; alkyl or alkoxy groups as disclosed
in JP 2002-324679; carbazole which may be used to assist hole
transport to the complex when used as an emissive material as
disclosed in WO 02/81448; bromine, chlorine or iodine which can
serve to functionalise the ligand for attachment of further groups
as disclosed in WO 02/68435 and EP 1245659; and dendrons which may
be used to obtain or enhance solution processability of the metal
complex as disclosed in WO 02/66552.
[0113] Other ligands suitable for use with d-block elements include
diketonates, in particular acetylacetonate (acac);
triarylphosphines and pyridine, each of which may be
substituted.
[0114] Main group metal complexes show ligand based, or charge
transfer emission. For these complexes, the emission colour is
determined by the choice of ligand as well as the metal.
[0115] The host material and metal complex may be combined in the
form of a physical blend. Alternatively, the metal complex may be
chemically bound to the host material. In the case of a polymeric
host, the metal complex may be chemically bound as a substituent
attached to the polymer backbone, incorporated as a repeat unit in
the polymer backbone or provided as an end-group of the polymer as
disclosed in, for example, EP 1245659, WO 02/31896, WO 03/18653 and
WO 03/22908.
[0116] A wide range of fluorescent low molecular weight metal
complexes are known and have been demonstrated in organic light
emitting devices [see, e.g., Macromol. Sym. 125 (1997) 1-48, U.S.
Pat. No. 5,150,006, U.S. Pat. No. 6,083,634 and U.S. Pat. No.
5,432,014]. Suitable ligands for di or trivalent metals include:
oxinoids, e.g. with oxygen-nitrogen or oxygen-oxygen donating
atoms, generally a ring nitrogen atom with a substituent oxygen
atom, or a substituent nitrogen atom or oxygen atom with a
substituent oxygen atom such as 8-hydroxyquinolate and
hydroxyquinoxalinol-10-hydroxybenzo (h) quinolinato (II),
benzazoles (III), schiff bases, azoindoles, chromone derivatives,
3-hydroxyflavone, and carboxylic acids such as salicylato amino
carboxylates and ester carboxylates. Optional substituents include
halogen, alkyl, alkoxy, haloalkyl, cyano, amino, amido, sulfonyl,
carbonyl, aryl or heteroaryl on the (hetero) aromatic rings which
may modify the emission colour.
Composition Formation Procedure
[0117] The composition used in the method according to the
invention may be prepared by simply blending the basic additive
with the conductive or semi-conductive organic material.
[0118] In the case of doped PEDOT, the basic additive may be
blended with a commercially available aqueous dispersion of doped
PEDOT, for example PEDOT:PSS sold under the name Baytron P.RTM. by
H C Starck of Leverkusen, Germany. Further solvents and/or
additives may also be added in order to optimise the properties of
the dispersion such as its jettability, drying characteristics and
resistivity, and/or in order to improve the performance of the end
device. Exemplary further additives include polymeric acids, for
example excess PSS, and exemplary further solvents include
alcohols, in particular polyols such as ethylene glycol.
[0119] An exemplary composition according to the present invention
comprises commercially available Baytron P VP A14083 to which is
added extra PSS, ethylene glycol and a basic amine to give a
formulation having a pH of 8.
Device Manufacturing Procedure
[0120] The procedure follows the steps outlined below:
[0121] 1) Depositing a PEDT/PSS composition according to the first
aspect of the invention onto indium tin oxide supported on a glass
substrate (available from Applied Films, Colorado, USA) by spin
coating.
[0122] 2) Depositing a layer of hole transporting polymer by spin
coating from xylene solution having a concentration of 2% w/v.
[0123] 3) Heating the layer of hole transport material in an inert
(nitrogen) environment.
[0124] 4) Optionally spin-rinsing the substrate in xylene to remove
any remaining soluble hole transport material.
[0125] 5) Depositing an organic light-emissive material by
spin-coating from xylene solution.
[0126] 6) Depositing a metal compound/conductive material bi-layer
cathode over the organic light-emissive material and encapsulating
the device using an airtight metal enclosure available from Saes
Getters SpA.
Full Colour Display Manufacturing Procedure
[0127] A full colour display can be formed according to the process
described in EP 0880303 by forming wells for red, green and blue
subpixels using standard lithographical techniques; inkjet printing
PEDT/PSS into each subpixel well; inkjet printing hole transport
material; and inkjet printing red, green and blue
electroluminescent materials into wells for red, green and blue
subpixels respectively.
Sample Experimental Results
1. Conductivity
[0128] PEDOT:PSS compositions comprising several different amines
were formulated and tested for their influence on resistivity. The
conductivity measurements were taken by spinning a film of
PEDOT-PSS onto an inter-digitated test structure to a thickness of
10 microns (as measured using DekTak apparatus). and using a
modified four-point probe in order to record the lateral
resistivity across the bottom of the PEDOT film. The resistivity of
the bulk PEDOT film is assumed to be the same.
TABLE-US-00001 Additive Resistivity (ohm cm) Ethylamine 8.769E+06
Ethylenediamine 1.836E+07 Ethanolamine 4.990E+04 None 5.207E+04
(comparison)
[0129] It can be seen that, unlike high boiling point additives
such as glycerol, the compositions do not shown a large increase in
conductivity when compared with a control composition with no
additive. The PEDOT:PSS composition comprising ethanol amine has a
resistivity closest to the control composition and has a favourable
boiling point to act as a high boiling point solvent. As such, this
amine is preferred.
2. Composition pH
[0130] FIG. 4 shows a titration curve for ethylenediamine added to
PEDOT/PSS. The pH increases sharply between around pH 2-7. The
present inventors have found that compositions in this pH range are
not processable because the solids present in the dispersion
aggregate. However, the dispersion is processable outside this pH
range at around pH 8 and above.
3. Film Profiles
[0131] The above composition comprising ethanolamine was ink-jet
printed onto a pixellated substrate comprising ink wells and film
thickness profiles were measured for each pixel across a row of the
substrate using a Zygo white light interferometer. Results
indicated that the films had a similar profile across the row, even
at a swath join.
[0132] The absence or presence of a change in film profile at a
swath join is demonstrated by plotting the centroid position of the
dry film profile in each pixel across the swath join as shown in
FIG. 6. FIG. 6 indicates how the centroid position varies for a
number of different PEDOT:PSS compositions. It can be seen that
many of the compositions have a large variation in the centroid
position around the swath position which is located at pixel
numbers 30/31. The composition comprising a basic additive
(ethanolamine) is labelled PC43 in FIG. 6. It can be seen that
there is little variation in the centroid position indicating that
film formation is substantially uniform across the substrate, even
at the swath position.
[0133] The film profile measurements were measured using a Zygo
interferometer. This can be used to scan either width or lengthways
over a pixel and the layer thickness recorded.
4. Device Results
[0134] Organic electroluminencent devices were manufactured
according to the previously described process. Devices were
manufactured with and without a basic additive in the PEDOT:PSS
composition.
[0135] Lifetime (in hours) of devices comprising an additive
according to the present invention are shown in the table
below.
TABLE-US-00002 PEDOT:PSS (1:16) + PEDOT:PSS PEDOT:PSS (1:40) +
glycerol + basic (1:16) glycerol additive Red 250 255 363 (4000
cd/m2) Green 250 138 248 (6000 cd/m2) Blue 130 80 142 (1800
cd/m2)
[0136] As can be seen, lifetime is improved as compared to devices
comprising no additive or the non-basic additive glycerol.
Moreover, device results such as drive voltage and luminescence are
comparable to controls indicating that the doped PEDOT:PSS
structure is unaffected as a result of neutralisation by the base
or is regenerated after baking. As such, compositions according to
the present invention improve film formation without detrimentally
affecting the opto-electrical properties of the device.
Furthermore, swath joins are eliminated, shorts are reduced and
lifetime of the devices is improved.
[0137] It has also surprisingly been found that the composition of
the invention provides a smoother film than a composition that does
not comprise a basic additive.
[0138] For example, inkjet printing of PEDOT/PSS with added amine
gave a film with a +/-2 nm variation in thickness (measured as root
mean square variation), whereas printing of PEDOT/PSS without added
amine gave a film with +/-5 nm variation
[0139] The compositions of the present invention has been described
for use in organic light emitting diodes, however it will be
appreciated that these compositions may similarly be applied to
other devices, for example organic photoresponsive devices
(including photodetectors and photovoltaic devices such as solar
cells); in organic switching devices, in particular organic thin
film transistors; and other plastic electronic devices.
* * * * *