Note: Descriptions are shown in the official language in which they were submitted.
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TRANSPARENT SUPPORT FOR ORGANIC LIGHT EMITTING DEVICE
TECHNICAL FIELD
This invention relates to organic light emitting diode (OLED) devices and
methods for their production; the invention is more especially concerned with
oxygen and water vapor impermeable flexible substrates for such devices and
methods of producing such flexible substrates.
BACKGROUND ART
An organic light emitting diode (OLED) device is an emissive display in which
a
transparent - substrate is coated . with a transparent conducting material,
for
example, indium-tin oxide (ITO) which forms a hole-injecting electrode as the
lowest layer of a light emitting diode. The remaining layers of the diode
commencing with the layer adjacent the ITO layer comprise a hole transporting
layer (HTL), an electron transporting layer (ETL) and an electron-injecting
electrode.
The hole-transporting layer is essentially a p-type semi-conductor and the
electron-transporting layer is essentially an n-type semi-conductor. These are
organic layers and in particular are conjugated organics or conjugated
polymers,
the latter are poor conductors without dopants but are doped to conduct holes
(p-
type) or electrons (n-type).
The electron-injecting electrode is typically a metal such as calcium,
lithium or magnesium.
When a voltage is applied to the diode, electrons flow towards the hole-
transporting layer and holes flow towards the electron-transporting layer.
This
produces electron-hole recombinations which release energy as light.
Collectively
the hole-transporting layer (HTL) and the electron transporting layer (ELT)
form
the electroluminescent layer (EL) of the diode.
Such OLEDs. provide a new generation of active organic displays of high
efficiency, large view angle, excellent color definition and contrast,
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and most important of low cost. In those displays, high-quality images are
created by a matrix of the light emitting diodes encapsulated in transparent
materials.
The diodes are patterned to form a pixel matrix, where a single-pixel
junction or EL emits light of a given color . All organic displays, designed
so far, contain oxygen- and moisture-sensitive components, namely organic
semiconductors and electron-injecting metals.
Consequently, the diodes require protection by means of an
impermeable layer forming a barrier to oxygen and water vapor, which
impermeable layer envelops the layers of the diode, and a substrate
supporting the enveloped diode, of high transparency, and which is
impermeable providing a barrier to oxygen and water vapor.
Thus far glass plate has been the supporting substrate of choice,
since it has excellent barrier and transparency properties. On the other
hand, glass plate has the drawbacks of brittleness, high-weight, and rigidity.
A strong demand exists for plastic-film substrate material that may
bring flexibility, high impact resistance, low weight, and, most of all,
which may enable roll-to-roll processing, as opposed to batch processing
which has been used thus far. Such plastic film substrate material should,
of course, be essentially impermeable, displaying low oxygen and water
vapor transmission rates.
Although one may expect some further improvement in oxygen and
moisture resistance of organic semiconductors employed in the diodes,
extremely water-sensitive electron-injecting metals such as Ca, Li and Mg
seem to be irreplaceable until a major breakthrough is made in solid state
physics or in display design, both rather unlikely in the predictable future.
Other properties, which the substrate material for organic displays
should present, such as thermal resistance, low roughness, and low costs,
are listed in [J. K. Mahon et al., Society of Vacuum Coaters, Proceedings of
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the 42nd Annual Technical Conference, Boston 1999, p. 496]. Organic
photovoltaic devices also require similar, flexible, barrier materials, so do
liquid crystal flexible displays, where barrier requirements are, however,
less demanding.
Organic displays are proposed for such equipment as high-resolution
computer displays, television screens, cell-phones and advanced
telecommunication devices etc., which require m-scale precision
manufacturing, vacuum operations and lithography. In other words:
technologies similar to those at present used in microelectronics. Other
applications include large scale displays for advertising and entertainment,
and various communication devices. These latter applications may require
lower precision in manufacturing, processing in inert-dry atmospheres, roll-
to-roll operations, inexpensive methods of patterning, for example,
stamping or ink jet printing. In other words: low-cost technologies, perhaps
similar to those at present used in special quality graphic-printing.
The problem is thus to develop flexible polymer films as supporting
substrates which are essentially barriers to oxygen and water vapor and
which can be produced at low thickness sufficient for their support
functions and such that they can be readily employed in commercial
manufacture of the organic devices in roll-to-roll processing.
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WO 00/65670 illustrates a prior attempt to develop flexible organic
electronic devices with improved resistance to oxygen and moisture
degradation.
In order to satisfy market requirements relative to existing
competitive organic displays having glass plate substrates or the more
conventional inorganic light emitting devices, a polymer film substrate for
an OLED would need to prevent oxygen and water molecules from
reaching the diode components for a period of years and typically for a life
of at least 10,000 hours.
It is known in the flexible packaging art, to coat polymer films or
sheets with thin inorganic coatings, for example, metal oxide coatings, to
render the polymer films or sheets essentially impermeable to oxygen and
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water vapor. In practice it is impossible in commercial manufacture to
produce such coatings without some pinholes or other defects which permit
passage of oxygen and water molecules through the otherwise impermeable
coating. This may not be a serious problem in the flexible packaging art
where the packaging is typically protecting a food product of limited shelf
life. However, the levels of permeability that may be acceptable in the
short working life of flexible packaging in the food and other industries will
certainly not meet the more exacting requirements for organic displays
based on organic light emitting diodes which must have a life of years
rather than the days or weeks which represent the typical useful or working
life of flexible packaging.
DISCLOSURE OF THE INVENTION
This this invention seeks to provide an organic light emitting diode
device having a flexible film substrate of enhanced impermeability to
oxygen and water vapor.
This invention also seeks to provide such a device in which the
flexible film substrate comprises an organic polymer film having an
impermeable barrier coating thereon and in which loss of impermeability
arising from pinholes and other types of defects in the coating is reduced.
Still further, this invention seeks to provide a flexible film barrier
support substrate for an OLED device.
This invention also seeks to provide a method of producing a flexible
film barrier support substrate for an OLED device.
Still further, this invention seeks to provide a method of producing
an OLED device having a flexible film barrier support substrate.
Still further, this invention seeks to provide an OLED device in
which the diodes are encased in an impermeable barrier casing comprising a
flexible film substrate for the diodes and an impermable covering, at least
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one of the substrate and the covering being of enhanced impermeability to
oxygen and water vapor.
In accordance with one aspect of the invention there is provided in
an organic light emitting diode device, in which light emitting organic
5 diodes are encased in a barrier envelope comprising a transparent substrate
supporting said diodes and a barrier covering, said substrate and covering
being impermeable to oxygen and water vapor, the improvement wherein at
least one of said substrate and said covering comprises: i) ' an organic
polymer support film, and ii) a composite layer on said support film and
disposed intermediate said support film and said light emitting diodes, said
composite layer comprising first and second discrete coating layers bonded
together in opposed facing relationship, each of said first and second
coating layers being impermeable to oxygen and water vapor when formed
as a continuous coating, each of said first and second coating layers having
inadvertent discontinuities therein such that said coating layers exhibit
discontinuity-controlled permeation of oxygen and water vapor
therethrough.
In accordance with another aspect of the invention there is provided
in a method of manufacturing an organic light emitting device in which
light emitting diodes are formed on a transparent substrate impermeable to
oxygen and water vapor, the improvement wherein the transparent substrate
is as defined hereinbefore.
In accordance with still another aspect of the invention there is
provided a transparent support substrate for an organic light emitting diode
device comprising: i) an organic polymer support film, and ii) a composite
layer on said support film, said composite layer being adapted to be
disposed between said support film and light emitting organic diodes of an
organic light emitting device, said composite layer comprising first and
second discrete coating layers bonded together in opposed facing
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relationship, each of said first and second coating layers being impermeable
to oxygen and water vapor when formed as a continuous coating, each of
said first and second coating layers having inadvertent discontinuities
therein such that said coating layers exhibit discontinuity-controlled
permeation of oxygen and water vapor therethrough.
In accordance with yet another aspect of the invention there is
provided a method of producing a transparent support substrate for an
organic light emitting device comprising: a) coating a first transparent
organic polymer film surface with a first coating layer, b) coating a second
transparent organic polymer film surface with a second coating layer, c)
bonding said coating layers together in opposed facing relationship to form
a composite layer between said first and second polymer film surfaces, each
of said first and second coating layers being impermeable to oxygen and
water vapor when formed as a continuous coating; each of said first and
second coating layers having inadvertent discontinuities therein such that
said coating layers exhibit discontinuity-controlled permeation of oxygen
and water vapor therethrough.
DESCRIPTION OF PREFERRED EMBODIMENTS
i) OLED
Organic light emitting diode devices rely on electroluminesce, their
general structure is well established and is not the subject of this
invention.
Such devices employ component layers which are sensitive to oxygen and
water molecules and must thus be effectively sealed from ingress of oxygen
and water vapor while maintaining transparency to light and different
desired physical characteristics.
In general an OLED comprises a plurality of light emitting diodes
mounted on a support substrate. The support substrate must have high
transparency to light, and present a barrier to oxygen and water vapor. The
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diodes placed on the support substrate are covered by a barriercovering,
also impermeable to oxygen and water vapor. The support substrate and
covering together form a barrier envelope encasing the diodes.
ii) Substrate
The supporting substrate of the invention comprises an organic
polymer support film having a composite layer thereon. The composite
layer comprises a pair of coatings bonded together in opposed facing
relationship.
The substrate may comprise a polymer film with the composite layer
thereon or it may comprise a pair of polymer films in opposed facing
relationship with the composite layer sandwiched therebetween such as to
form an intermediate layer between the polymer films.
The substrate may suitably have a thickness of 5 m to 10 mm, more
typically 25 m to 1000 gm.
a) Polymer Films
The support film should be transparent and of any suitable organic
polymer, including homopolymers, copolymers and terpolymers which can
be fabricated as a suitably thin film having the necessary and desirable
physical characteristics to form a barrier support substrate for the diodes,
physical characteristics of particular importance are strength and flexibility
at desired film thickness for the OLED device.
While the polymer films do not need to be and generally will not be
impermeable to oxygen and water vapor, polymer films which are of lesser
permeability to oxygen and water vapor will generally be preferred to those
of higher permeability.
Suitable polymers for the polymer film include, by way of example,
polyolefins, for example, polyethylene and polypropylene;
cyclopolyolefins, for example, polynorbornenes; polycarbonates;
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polyesters; polyarylates, polyacrylates, polyethyleneterephthalate;
polyethylenenaphthalate; polystyrene; polyamides; polyimides;
polyethersulfone, and polyorganosilicones, as well as other transparent
polymers and copolymers including other high Tg polymers. The polymer
film may include one or more layered polymer components.
Preferred polymer films are chosen from high Tg polymers, for
example cyclopolyolefins, polyethersulfones, polyarylates, and from
polyethyleneterephthalate and polyethylenenaphthalate.
The polymer films will have a thickness to achieve the desired
substrate thickness.
Where the support substrate comprises a pair of polymer films with
the coating layers therebetween, the polymer films may be of the same or
different polymers.
b) Impermeable Composite
The composite layer provides a barrier to oxygen and water vapor
and is composed of a pair of discrete coating layers which when formed as
continuous coatings are impermeable to oxygen and water vapor.
On the other hand, since discontinuities are inevitable in the discrete
coating layers, each coating layer will exhibit discontinuity-controlled
permeation of oxygen and water molecules therethrough.
As it will be described below, each discrete coating layer may be
formed as a single coating or may be composed of a plurality of single
coatings preferably of different materials, which also show the presence of
inadvertent discontinuities. The plurality of single coatings are each formed
as discrete coatings of the coating layer.
These inadvertent discontinuities described more fully below are
essentially pinholes which form during deposition of the coating layer and
are inherent in the coating techniques available, and other types of defects
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which arise from external factors during or subsequent to deposition of the
coating layers.
The coating layers may be the same or different; by way of example,
suitable coating layers may be formed from transparent materials such as
oxides, nitrides, mixed compositions, and salts; for example, SiOX, SiOXCy,
SixNy, SiXNyCZ, SiOXNy, TiOX, A1XOy, SnOy, indium-tin oxide, magnesium
fluoride, magnesium oxyfluoride, calcium fluoride, tantalum. oxide, yttrium
oxide, zirconium oxide, barium oxide, magnesium oxide, and mixtures
thereof, wherein x is from 1 to 3, y is 0.01 to 5 and z is 0.01 to 5.
Particular
examples include silica, alumina and titania; other examples include
amorphous carbon, borosilicate, sodium and potassium glass.
Preferred coating layers are stoichiometric or non-stoichiometric
silicon dioxide deposited by plasma, stoichiometric or non-stoichiometric
silicon nitride deposited by plasma; and multilayer structures including
discrete coatings of one or both of silicon dioxide and silicon nitride, and
polymer coatings, for example polyacrylates or organic plasma polymers
obtained from organosilicones, hydrocarbons or acrylates.
Each coating layer suitably has a thickness of 10 mu to 10 m,
preferably 60 nm to 5 m and more preferably 100 nm to 2 m.
In a particular embodiment, one or both coating layers may be
composed of a plurality of single coatings, for example alternate inorganic-
and organic coatings; in such case, the coatings of the plurality are formed
as discrete coatings in separate coating operations, and there are at least
two
such discrete coatings in a coating layer, such as those described in the
literature [J. This is most desirable especially in the case of coating layers
thicker than 150-200 nm, where single coatings of different materials
provide necessary mechanical stability and barrier properties.
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The barrier substrate must be transparent to light and suitably will
have a transparency greater than 65% and preferably greater than 85%,
measured according to ASTM D 1746-97.
Each coating layer which forms a barrier to oxygen and water vapor
5 should suitably display an oxygen transmission rater lower then 1
cm3/(m2dayatm), and preferably lower than 0.01 cm3/(m2dayatm) and
more preferably lower than 0.005 cm3/(m2dayatm) measured according to
ASTM F 1927 or D 3958; and a water vapor transmission rate (WVTR)
lower than 0.01 g/(m2dayatm), preferably lower than 0.005 g/(m2dayatm)
10 and more preferably lower than 0.00 1 g/(m2dayatm) as measured according
to ASTM F 1249.
c) Adhesives
The pair of coating layers are suitably bonded or laminated together
with an adhesive.
In one embodiment of the method of producing coating layers, each
layer is independently formed as a discrete (single or multiple layer) coating
on a film substrate to form a pair of coated film substrates which are then
bonded together, coating layer to coating layer. In this way the coating
layers forma composite layer sandwiched between a pair of film substrates.
In a first embodiment this assembly forms the support substrate.
In a second embodiment one of the film substrates is composed of
first and second polymer films having release properties at the interface
therebetween for ready removal of the first polymer film from the substrate
after the bonding of the two film substrates in coating layer to coating layer
contact. In this way the coating layers form a composite layer on the
second polymer film as the support substrate, and the first polymer film
functions as a thin temporary protective film on the side of the support
substrate on which the diodes are to be placed.
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The release coating may be formed from the same classes of
adhesive employed for bonding the coating layers together, but with
appropriate adjustment, as well known in the adhesive art, to introduce the
required release characteristics.
Adhesives that may be employed for bonding the coating layers and
for the release coating include thermoplastic-and elasto-plastic polymers;
polymers which are curable by radiation, for example, ultraviolet or
electron-beam; heat, by chemical initiators or by combinations thereof;
organic or organic-containing adhesives, such organics being, for example,
acrylics, urethanes, epoxides, polyolefins, organosilicones and others;
composite ceramic materials, composite organic/ceramic materials; and
products of plasma-polymerization, oligomerization, or curing of organic-,
organosilicon and other organometallic compounds, either volatile or
deposited by other means such as spraying, casting or dip-coating.
Particular examples of adhesives include elastomer based adhesives,
for example; synthetic organic adhesives, for example, phenolic resins,
acrylic resins, polyvinyl acetals, epoxies, polyamides or silicone adhesives;
and inorganic polymer adhesives, for example, soluble silicates such as may
be prepared by fusion of silica and alkali metal carbonates.
The adhesive forms an adhesive layer bonding the two coating
layers. The adhesive layer may suitably have a thickness of 50 nm to 10,
m, preferably 100 nm to 2 m. Especially advantageously the adhesive
layer has a thickness less than the dimensions of thediscontinuities.
In preferred embodiments the adhesive exhibits a scavenging effect
towards oxygen, water or both and such scavenging may be, for example,
by adsorption, absorption or chemical reaction. In this way the adhesive
layer provides an additional barrier.
iii) Method of producing Support Substrate
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The method of producing the support substrate essentially involves
coating each of a pair of organic polymer films, as described hereinbefore,
with a coating layer, as described hereinbefore.
It will be understood that the pair of films may be derived from a
single sheet or roll of film, with the coated film being cut to provide two
separate coated films. However, in a continuous or continual manufacture,
the coating layers will preferably be formed independently on separate
films.
The coating layers may be applied by various coating techniques, but
preferably by physical vapor deposition (PVD), for example, evaporation or
sputtering or by chemical vapor deposition (CVD), for example, plasma
enhanced chemical vapor deposition (PECVD) or organic vapor phase
deposition (OVPD). These methods are capable of producing very thin
coatings, which are stable and flexible but of satisfactory hardness, and
which exhibit low oxygen and water vapor permeations. PVD and PECVD
are carried out under vacuum. In this way a pair of film substrates is
obtained each having a coating layer.
The substrates are then bonded to, or adhered, together coating layer
to coating layer, thereby forming a supporting substrate which comprises
the pair of film substrates in opposed facing relationship with the coating
layers forming a composite layer sandwiched between the film substrates.
In another embodiment the second of the pair of film substrates is
itself a composite substrate comprising a pair of film layers bonded together
with an adhesive whereby the film layers can be readily separated. After
bonding the coating layers together the outermost of the film layers of the
second film substrate provides a temporary protective layer which is
removed when the diodes are to be mounted on the support substrate. In
this case the diodes are supported in contact with the remaining film layer
of the second film substrate.
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v) Applications of Support Substrate
In one embodiment the support substrate forms the front, transparent
support of an OLED device, the diodes of the device being encapsulated on
the other side by a suitable non-transparentbarrier covering, also
impermeable to oxygen and water vapor, thus providing the OLED device
having one-side light emission. A suitable nontransparent barrier covering
material may be of a metal can, plate, foil or an evaporated film, as is well
known in the OLED art.
In another embodiment the support substrate forms the front,
transparent background of an OLED device, and the barrier covering on the
other side is also formed of a transparent support substrate of the invention,
thus providing an OLED device that is transparent and emits light on both
sides. A support substrate according to the invention thus encapsulates the
diodes both as the front support and as the rear barrier covering, together
forming a barrier envelope.
In still another embodiment, the support substrate of the invention
forms the barrier covering and the front support of the OLED device is of
another material, for example glass, as known in the OLED art.
The support substrate according to the invention may be used also in
other types of devices, such as liquid crystal displays or in organic
photovoltaic devices, which are known in prior art to require transparent
materials impermeable to oxygen and water vapor.
vi) Manufacture of OLED
The OLED is suitably formed under vacuum conditions to minimize
.25 introduction of contaminants which may chemically or physically damage
the OLED or alter its characteristics. Small molecule diode components,
sensitive to oxygen and water molecules, are deposited onto the support
substrate by vacuum evaporation. One particular type of organic light
emitting diode, namely polymeric light emitting diodes (PLED) may, for
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example, be deposited onto the support substrate for example from a
solution in a suitable organic solvent in an inert atmosphere.The support
substrate is produced as outlined hereinbefore. Thereafter, in a vacuum
process a transparent conductive layer, for example, indium-tin oxide, is
deposited on the support substrate.
The transparent conductive layer is patterned to form the lower
electrode of the diode, which is the hole-injecting layer. On the hole-
injecting layer there is deposited, successively, the hole-transporting layer
and the electron-transporting layer, both of which are organic layers, and
thereafter the electron-injecting layer which forms the upper electrode, and
which may be, for example, of calcium, lithium, magnesium or aluminium,
or suitable metal alloys.
The afore-mentioned layers may be deposited by vacuum
evaporation, well known in the OLED art.
Instead of vacuum evaporation, the organic layers and the upper
electrode may also be deposited by printing, for example ink jet printing,
stamping or other transfer techniques in an inert atmosphere, as well
known in the PLED art
BRIEF DESCRIPTION OF DRAWINGS
FIG. I illustrates schematically a portion of an OLED of the
invention in a first embodiment;
FIG. 2 illustrates schematically a portion of an OLED of the
invention in a second embodiment;
FIG. 3 illustrates schematically the OLED of FIG. 1 partly exploded;
FIG. 4 illustrates schematically the low permeation in a support
substrate of the invention;
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FIG. 5 demonstrates graphically the calculated permeation through
the support substrate of FIG. 4 relative to horizontal distance between
defect centres;
FIG. 6 illustrates schematically a support substrate of the invention;
5 FIG. 7 is a plot demonstrating the probability (PR) of matching
random defects face-to-face vs defect number density (DN) in the coating
layers of a support substrate of the invention; and
FIG. 8 is a schematic representation of a system for producing the
support substrate of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
WITH REFERENCE TO DRAWINGS
With further reference to FIG. 1, there is illustrated a portion of an
OLED 10 having a plurality of diodes 12 on a support substrate 14 and an
impermeable envelope 16.
Support substrate 14 comprises polymer films 18 and 20 with a
composite layer 22 therebetween.
Composite layer 22 comprises a coating layer 24 formed on polymer
film 18 and a coating layer 26 formed on polymer film 20. Coating layers
24 and 26 are bonded together by an adhesive layer 28.
FIG. 1 includes an exploded view of a diode 12. Each diode 12 is of
the same form. Diode 12 comprises, in a stack, a hole-injecting layer 30, a
hole-transporting layer 32, an electron-transporting layer 34 and an
electron-injecting layer 36.
In the manufacture, coating layer 24 is first formed as a discrete
layer on polymer film 18 and separately coating layer 26 is formed as a
discrete layer on polymer film 20.
With reference to FIG. 2, there is shown an OLED 200 which differs
from OLED 10 in the structure of support substrate 214.
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The manufacture of support substrate 214 is essentially the same as
that for support substrate 14 except that polymer film 18 comprises a pair of
support films 38 and 42 (film 42 is not shown) which are bonded together
in opposed facing relationship by a release coating (not shown). Prior to
assembly of OLED 200, support film 42 is removed to expose support film
38 for contact with the diodes 12.
With further reference to FIG. 3, there is shown an OLED of FIG. 1,
partly exploded so that the arrangement of diodes in the OLED 10 can be
observed.
With further reference to FIGS. 4and 6, there is further illustrated
composite layer 22 of the OLED 10 of FIGS. 1 and 3.
Coating layer 24 has inadvertent defects 50 and 52 and coating layer
26 has inadvertent defects 54, 56 and 58, which defects are of essentially
the same size as the thickness of adhesive layer 28. Since such inadvertent
defects are random and thus not necessarily aligned in the opposed coating
layers 24 and 26, a tortuous path for passage of oxygen and water vapor is
provided as illustrated by the arrows. In this way a diminished defect
controlled permeation is exhibited.
In FIG. 6 the composite layer 22 is shown in conjunction with the
polymer films 18 and 20 in the support substrate 14.
With further reference to FIG. 8, there is illustrated schematically a
system for manufacture of the support substrate 14 of FIG. 6. The system
comprises an assembly 60 having a roll 62 of polymer film 18 and a roll 64
of polymer film 20.
Assembly 60 further includes degassing roll 66, a coating drum 68
for polymer film 18 and a coating drum 70 for polymer film 20 and an
adhesive deposition zone 72.
Assembly 60 further includes laminator 74, a rewind roll 76 and a
support roll 78.
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In operation polymer films 18 and 20 are continuously fed from rolls
62 and 64 respectively over degassing rolls 66 to coating drums 68 and 70
respectively.
An adhesive layer 25 is applied to coating layer 24 on one face of
polymer film 18 in adhesive deposition zone 72 and adhesive layer 25' is
applied to coating layer 26 on one face of polymer film 20 in adhesive
deposition zone 72, in such a way that coating layers 24 and 26, applied to
the faces of polymer films 18 and 20, are in facing relationship.
The thus coated polymer films 18 and 20 are fed through laminator
74 where lamination occurs with coating layers 24 and 26 in opposed facing
relationship. Lamination involves application of an adhesive between the
coating layers 24 and 26 and cure of the adhesive, the nature of which
depends on the adhesive employed.
The resulting laminate which is support substrate 14 is fed onto
rewind roll 76.
The support substrate 14 may be fed directly to a station for OLED
manufacture or may be stored and subsequently transported to a station for
OLED manufacture.
The manufacture is suitably carried out under vacuum which
minimizes the risk of defects in coating layers 24 and 26 as a result of dust
particles or other contaminants. After deposition, the coating layers 24 and
26, which are the working impermeable layers providing a barrier to
oxygen and water vapor, are shielded or protected by the polymer films 18
and.20.
Experimental and Explanations
Discontinuities
a) Nature of Defects.
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The coating layers in the composite layer of the barrier support
substrate each contain inadvertent discontinuities which permit permeation
of oxygen and water vapor through the otherwise impermeable layers.
The decrease in permeation of oxygen and water vapor through a
polymer film substrate resulting from inorganic, impermeable coatings is
usually characterized by a Barrier Improvement Factor (BIF), namely the
measured ratio of permeation of the bare substrate to that of the coated film.
Such inorganic coatings may decrease permeation of oxygen and water
vapor through polymer films by several orders of magnitude, thus showing
a BIF of 103, 104 or more for thin polymer film substrates. However, the
barrier properties of simple coated polymer films are still at least two
orders
of magnitude higher than those required for an OLED.
The permeation of gases through silica-coated plastic films is a
discontinuity or defect-controlled phenomenon [A. S. da Silva Sobrinho,
G. Czeremuszkin, M. Latreche, G. Dennler and M. R. Wertheimer, Surf.
and Coat. Technol. 116-119, 1204 (1999), H. Chatham, Surf. Coat.
Technol. 78, 1 (1996)]. This means that the residual permeation is due to
the presence of micrometer- and submicrometer-size defects in the coatings.
Size and shape of defects, their number density, and thickness of the plastic
substrate film, are found to be important parameters that determine
permeation through coated films.
Three main types of inadvertent defects in. the coatings may be
distinguished, namely:
- submicrometer-size pinholes, characteristic to the method of deposition
and related to surface micro-roughness of the polymer film,
- sub-micrometer to multimicrometer-size defects due to dust and micro-
particles,
- cracks and scratches created during production, handling and converting.
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All of those defects are practically inevitable in real industrial-scale
(roll-to-roll) manufacture operations.
Evenly distributed, sub-micrometer-size pinholes contribute to
overall permeation, and lead to a quasi one-dimensional diffusion front at
the back face of the coated film. Permeation through those small pinholes
decreases the lifetime of an organic display, causing its uniform
deterioration. On the other hand, large defects, for example, related to dust
particles, will form zones of localized diffusion, where the rapid local
deterioration of the device will be observed as "dark spots" in the display.
The presence of a single, dust-related defect in the barrier encapsulation
may cause failure of the display much before its expected lifetime, thus
justifying the rejection of the final product by quality control. Therefore,
technology which provides barrier-coated films, practically free of large
defects, will be of very high value to the organic display industry.
Decreasing the number density of defects in inorganic barrier-coated
films produced by physical vapor deposition (PVD) and plasma enhanced
chemical vapor deposition (PECVD) is a very difficult task, since some
requirements are in mutual contradiction. For example, the required very
low permeability (low defect density) calls for thick inorganic barrier
coatings [A. S. da Silva Sobrinho, G. Czeremuszkin, M. Latreche, and M.
R. Wertheimer, J. Vac. Sci. Technol. A 18, 149 (2000).], but these thick
barrier coatings are usually brittle, and show high internal stresses, low
flexibility and low stretchability. Polymer films, even of optical grade,
show certain surface micro-roughness, which, as described above, is an
important source of submicron-size pinholes in the coatings deposited on
such films.
b) Defect-Controlled Permeation
In order to characterize sub-micrometer-size defects in transparent
barrier coatings deposited on transparent polymer films, a special detection
CA 02452323 2003-12-29
technique based on plasma etching has been developed. Defects are thereby
rendered visible in optical microscopy (OM), their patterns may be
observed by simple visual inspection, and their number densities and size
distributions can be evaluated with relative ease. In addition a simple model
5 of permeation through defects in otherwise impermeable coatings has been
developed, which allows a better understanding and analysis of the
mechanism of permeation through barrier-coated polymer films [G.
Czeremuszkin, M. Latreche, A. S. da Silva Sobrinho, and M. R.
Wertheimer, SVC, Proc 42' Annual Tech. Conf., 176 (1999).]. Using this
10 model, it has now been determined that only a very thin surface layer of
the
polymer film substrate, in the immediate vicinity of the barrier-coating,
plays a significant role in the overall barrier performance. This has
permitted an evaluation of the maximum BIF for a pair of discrete coating
layers bonded together on a polymer film, displaying defect-controlled
15 permeation and devoid of large pinholes, cracks or scratches, which is
merely twice that of the same polymer film with a single coating layer. The
upper-limit of applicability of the model, corresponding to BIF =1,
expected for very thick substrate films was identified. It was found that the
lower-limit of the applicability of derived equations corresponds to a
20 substrate polymer film with a thickness close to the size of typical
defects
in the coating. Below this lower-limit thickness value, the model cannot be
used.
A special technique for detection of micrometer-size defects in
transparent barrier coatings deposited on transparent polymer films has
been developed. The technique is based on plasma etching, and renders
defects and their patterns visible in optical microscopy (OM) and detectable
by simple visual inspection. A simple model of permeation through defects
in otherwise impermeable coatings, allows a better understanding of
mechanisms of permeation through barrier-coated polymer films.
CA 02452323 2003-12-29
21
For purposes of this invention the model of permeation was modified
by adopting the theory of heat transport in thin plates, for diffusion of
permeant through thin films. As the conclusion from theoretical
considerations, the invention deposits a barrier coating onto each of two
independent polymer films and bringing them together, coated face to
coated face in permanent contact, for example by laminating, using a layer
of adhesive, preferably having low permeation to water vapor and oxygen.
Four critical parameters that determine permeation through the proposed
structure have been identified:
A. Size of defects in the component barrier-coatings;
B. Number density of defects in the coatings;
C. Thickness of the adhesive layer, which should preferably be lower
than the size of defects.
D. The material of the adhesive layer, which should preferably show
low permeability to the permeants.
As confirmed by experimental results, the overall permeation through
transparent barrier-coated polymer films is lowered, according to the
present invention, by at least an additional two orders of magnitude or
more. For large defects in a single coating, for example, those due to dust
particles, this translates to an extremely low or negligible probability of
finding a "dark spot" in a large organic display having a barrier support
substrate of the invention.
The invention thus relies on previously unforeseen observations and
conclusions from a novel approach to very thin, double coated layers on
polymer film support substrates in which the coating layers have
micrometer and sub-micrometer-size defects. In order to calculate the
permeation through coated, micrometer-thick polymer films, the theory of
heat diffusion across thin plates was adapted (it is known that the heat
CA 02452323 2003-12-29
22
transport and the transport of permeant through thin plates are described by
the same types of diffusion equations).
On this basis the permeation P through a pair of coating layers of a
composite layer of the invention can be defined by:
P 2iD0oL
= (I)
d2
acosh 2R -1
0
where each coating layer contains randomly distributed defects of the same
circular shape and the same diameter 2R0
Typically, 2R0 = 0.5 - 2 m for micrometer-size pinholes, and 2R, =
10 m for dust-related defects, where the number density of those defects
may reach several thousands per cm2, and not more than a few per cm2,
respectively.
This situation is equivalent to permeation through the layer with a
defect controlled permeation. The plastic film of small thickness, L, and of
diffusion coefficient of the permeant, D, has the coatings on both sides,
which contain the defects of radius R0. Those defects, in both coatings, are
separated by a horizontal (projected) distance d. The film is then exposed
to the permeant only from one side, concentration of which at the surface
zones on both sides is then 1 o and 0, respectively.
This is equivalent to permeation through a transparent substrate of
the invention comprising a pair of transparent polymer films having the
composite layer therebetween, with a defect-controlled permeation. It is
assumed that the polymer films are of thickness L, and of diffusion
coefficient of the permeant D, and each coating layer has defects of radius
CA 02452323 2003-12-29
23
R0, and the defects in both coatings are separated by an overall horizontal
distance d. The transparent substrate is then exposed to the permeant only
from one side, concentration of which at the surface zone or both sides is
then 4). and 0, respectively.
FIG. 4 presents schematically the situation when the thickness of
composite layer 22 is comparable with the average size of defects 50, 52,
54, 56 and 58 in the coating layers 24 and 26. As explained below,
permeation of gases through such a structure may be very low, and BIF >>
2 may be expected. This is due to the tortuous path of permeant molecules,
which will encounter a much longer effective path of diffusion in the bulk
of the composite layer, when transported from one defect 50 in the coating
layer 24 to an adjacent defect 54 in the coating layer 26.
In particular, defects 50 and 52 in coating layer 24 and defects 54, 56
and 58 in coating layer 26 occur randomly so that on a statistical probability
basis defects 50 and 52 are remote from, and in non-opposed relationship
with defects 54, 56 and 58.
FIG. 5 shows the calculated permeation through a support substrate
of the invention in which the coating layers contain a single (circular)
defect of radius Ro each, vs. the horizontal (projected) distanced between
centers of those defects.
In the case of d<R4i the defects partly overlap, and the permeation
may only be 2x lower than the permeation through a single coating. In the
case of d>>Ro, permeation decreases rapidly, which, however, strongly
depends on the thickness of the polymer film. It can be seen from FIG. 5
that permeation can be two orders of magnitude lower than permeation
through a polymer film having a single coating layer. This may be
achieved when the polymer film is very thin, and when the polymer
material possesses low permeability of the permeant in question.
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24
The principle of this invention may be further explained by reference
to FIG. 6. FIG. 6 .shows a support substrate 14 of the invention comprising
polymer films 18 and 20 each having been previously coated with
transparent coating layers 24 and 26 respectively, each containing random
defects 50, 52 and 54, 56 and 58, respectively, and then brought together in
permanent contact by means of adhesive lamination with adhesive layer
28. Overall permeation through this support substrate 14 film will be much
lower than permeation through each distinct coated film (film 18 with
coating layer 24 or film 20 with coating layer 26). The support substrate 14,
according to the invention, may exhibit super-barrier properties. (OTR <
0.001 cm3/(m2dayatm), and WVTR < 0.000001 g/(m2dayatm), even if both
component films show much poorer barrier performance. More important,
however, the substrate 14 according to the invention practically eliminates
harmful effects of permeation through large defects in the coating layers,
such as those created by dust particles.
FIG. 7 presents in two cases (a and b) the probability of matching
random defects face-to-face in two coating layers brought together in
accordance with the invention. A simplified statistical analysis used here,
based on so-called Bernoulli trials, leads to overestimation of the
probability. In the analysis, the circular "defect" of a given -radius
(representing a pinhole in the second coating layer) is randomly placed on
the surface representing the first coating layer, which contains a given
number of identical, circular defects. The probability of defects overlapping
is then calculated. The procedure is repeated, so that the total number of
trials corresponds to the number of defects in the second coating layer.
-Obviously, in the Bernoulli Scheme, the defect may be randomly placed in
the position already occupied by another defect (and previously taken in the
CA 02452323 2003-12-29
trials), which gives rise to the source of the overestimation. Therefore the
real probability of defects matching is even lower than the one calculated.
As can be seen from the presented analysis, and this is confirmed by
experimental results, the overall permeation through transparent barrier-
5 coated plastic films may be lowered, in accordance with this invention, by
an additional two orders of magnitude with probability exceeding 99.9%.
In the case of large defects, such as those due to dust particles, the number
density of which is not high in the original coating layers, this translates
into an extremely low or negligible probability of finding a "dark spot" in a
10 large organic display produced using the support substrate according to the
invention.
EXAMPLES .
Polymer films of polyethyleneterephthalate and polypropylene were
15 "cleaned" of large dust particles by blowing compressed nitrogen over their
surfaces. A controlled number of sodium chloride micro-crystals were
deposited onto the surface -of the polymer films using an aerosol of salt
solution in water, created ultrasonically. The deliberately contaminated
films were then coated with 36 or 70 nm of silica using the PECVD
20 method, from HMDSO/02/Ar precursor mixture. The films were then
washed with flowing water (DI/RO, 18 MO), which dissolved the micro-
crystals, and were then dried with dry nitrogen. Since the polymer films
were not silica coated in places previously occupied by the crystals, the
above procedure provided numerous well-characterized defects in the
25 coating. The sodium chloride crystals deposited onto the polymer films, and
corresponding defects in the coating, were clearly visualized, the latter
using the defect detection technique described above. In order to control the
number of defects and to avoid false, dust-related pinholes, the
cleaning/washing operation was performed in a clean-room. Two such
CA 02452323 2003-12-29
26
coated films were laminated face-to-face using a thin layer of an UV-
curable adhesive, which shows lower oxygen permeation than adhesives
typically used in bonding of polymer films. Lamination was performed both
in a vacuum and in air. Up to two orders of magnitude improvement of the
barrier (vs. single-side coated film with defects) was observed.
In another experiment, two typical silica-coated polymer films were
laminated using a -3 m thickness of soft adhesive, of similar permeation
to those typically used in bonding of plastic films. The measured OTR
value was only -'2x lower than the one determined for the single component
film.
The results obtained in this experiment part are summarized in Table
I.
The experiments confirm the theoretical findings, and they show
that:
- the thickness of the adhesive, more precisely, the distance between
the two coating layers containing defects, should be preferably
smaller than the size of the defects in the coatings;
- the material of an adhesive between the two coating layers should
preferably display low permeability to the permeant.
In accordance with the invention it has been determined that there
are four parameters that particularly determine permeation through the
support substrate of the invention.
A. The size of defects in the component transparent barrier-coating
layers, which are responsible for permeation through such
component barriers;
B. The number density of defects in the component transparent
barrier-coating layers, which determines permeation through
such component barriers;
CA 02452323 2003-12-29
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C. The distance between the coating layers of the two component
transparent barrier-coatings containing defects (equivalent to the
thickness of the laminating or adhesive layer), which, according
to the invention, should preferably be less than the typical size
of defects mentioned in A above.
D. The material(s) filling the gap between the two coating layers
(equivalent to the material of an adhesive layer), which,
according to this invention, should preferably show low
permeability to the permeant.
Support substrates . prepared. according to the invention show
substantial improvement in barrier properties.. When the parameters shown
in A-D above are properly chosen, this improvement may reach or exceed
two orders of magnitude.
Advantages of the support substrates of the invention, are the
following:
- practically total elimination of permeation through large-scale, dust-
related defects, thus preventing "dark spot" appearance in organic
displays;
- significant improvement of barrier properties, thus allowing one to
reach the super-barrier level with coatings, which themselves do not
show sufficiently low permeation;
- excellent flexibility of the film in handling and converting, due to the
position of the coatings close to the neutral plane for bending;
- protection of the coating layers against mechanical damage, as a
result of their being sandwiched between polymer films.
