Note: Descriptions are shown in the official language in which they were submitted.
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ENCAPSULATION OF ORGANIC ELECTRONIC DEVICES
FIELD OF THE INVENTION
This invention relates to organic polymer-based electronic devices such as
diodes, for example light-emitting diodes and light-detecting diodes. More
specifically, this invention relates to fabrication processes and structures
for such
devices which lead to high device efficiencies and which promote commercially
acceptable, long operating lives.
BACKGROUND OF THE INVENTION
to Solid state electronic devices fabricated with conjugated organic polymer
layers have attracted attention. Conjugated polymer-based diodes and
particularly
light-emitting diodes (LEDs) and light-detecting diodes are especially
attractive
due to their potential for use in display and sensor technology. These
references
as well as all additional articles, patents and patent applications referenced
herein
are incorporated by reference.
This class of devices have a structure which includes a layer or film of an
electrophotoactive conjugated organic polymer bounded on opposite sides by
electrodes (anode and cathode) and carried on a solid substrate.
Generally, materials for use as active layers in polymer diodes and
2o particularly LEDs include semiconducting conjugated polymers, such as
semiconducting conjugated polymers which exhibit photoluminescence. In
certain preferred settings, the polymers are semiconducting conjugated
polymers
which exhibit photoluminescence and which are soluble and processible from
solution into uniform thin films.
The anodes of these organic polymer-based electronic devices are
conventionally constructed of a relatively high work function metals and
transparent no'nstoichiometric semiconductors such as indium/tin-oxide. This
anode serves to inject holes into the otherwise filled pi-band of the
semiconducting, luminescent polymer.
Relatively low work function metals such as barium or calcium are
preferred as the cathode material in many structures. Ultrathin layers of such
low
work function metals and their oxides are preferred. This low work function
cathode serves to inject electrons into the otherwise empty pi*-band of the
semiconducting, luminescent polymer. The holes injected at the anode and the
electrons injected at the cathode recombine radiatively within the active
layer and
light is emitted.
Unfortunately, although the use of low work function materials is required
for efficient injection of electrons from the cathode and for satisfactory
device
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performance, low work function metals such as calcium, barium and strontium,
and their oxidesare typically chemically reactive. They readily react with
oxygen
and water vapor at room temperature and even more vigorously at elevated
temperatures. These reactions destroy their required low work function
property
and degrade the critical interface between the cathode material and the
luminescent semiconducting polymer. This is a persistent problem which leads
to
fast decay of the device efficiency (and light output) during storage and
during
stress, especially at elevated temperature.
Other organic polymer-based solid state devices present similar stability
to problems. The construction of, and materials used in, photodetecting
devices and
arrays of devices are very similar to those found in polymer-based LEDs. The
main differences between polymer-based LEDs and photodetectors are that
extremely reactive low work function electrodes need not be used, and that the
electrical polarity of the electrodes is often reversed. Nevertheless,
moisture and
15 oxygen react with the components of these devices and again lead to a
decrease in
device performance over time.
One approach to minimizing the deleterious effects of atmospheric
exposure has involved enclosing the devices in a barrier to separate the
active
materials from oxygen and moisture. This approach has had some success but it
2o does not always adequately address the problems caused by even those small
amounts of moisture trapped within the enclosure or diffusing into the
enclosure
over time.
Kawami, et al in U.S. Patent 5,882,761 discloses a method for packaging
light emitting devices fabricated using thin films of luminescent organic
25 molecules as the active layer that seeks to address the problem of water
contamination. That patent describes the placement of a water-reactive solid
compound such as sodium oxide within the enclosure for the device. This
reactive
compound covalently reacts with water in the enclosure and converts it into a
solid product. As an example, the sodium oxide just noted reacts with water to
3o yield solid sodium hydroxide. This patent describes that it employs these
water-reactive compounds to remove water in order that the moisture is
retained at
high temperatures. Kawami et al. note that materials which physically absorb
moisture cannot be used since the moisture will be discharged at high
temperatures (for example, at 85°C).
35 The solid compounds with which the water react in the Kawami patent are
themselves very reactive and lead to reaction products which are likewise very
reactive. Thus, any accidental contact between these compounds or reaction
products with.other components of the device or the device enclosure can be
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WO 01/19142 CA 02381230 2002-O1-29 pCT/US00/24126
deleterious. Thus, there is a need for methods of encapsulation of organic
polymer-based solid state electronic devices, said encapsulation being
sufficient to
prevent water vapor and oxygen from diffusing into the device and thereby
limiting the useful lifetime.
In addition, many of the known processes for achieving a hermetic
encapsulation of electronic devices require that the devices be heated to
temperatures in excess of 300°C during the encapsulation process. Most
polymer-based light-emitting devices are not compatible with such high
temperatures.
1o SUMMARY OF THE INVENTION
The present invention relates to an electronic device containing a polymer
electronic device including a pair of electrodes opposed to each other and an
active polymer layer interposed between the electrodes; an airtight enclosure
having an inner surface adjacent to the polymer electronic device and an
opposing
15 outer surface adjacent to an external atmosphere; a drying agent adjacent
to the
inner surface, the drying agent having a porous structure and being capable of
trapping water by physically absorbing it into its porous structure; wherein
the
airtight enclosure encapsulates the polymer electronic device, to isolate the
polymer electronic device and the drying agent from the external atmosphere.
The
2o present invention also relates to a method of fabricating a polymer
electronic
device with improved lifetime; by encapsulating the polymer electronic device
iin
an airtight enclosures with a solidy drying agent.
In a preferred embodiment the drying agent is incorporated into one or
more layers) of a substrate supporting the polymer electronic device.
25 As used herein, the phrase "adjacent to" does not necessarily mean that
one layer is immediately next to another layer, but rather to denote a
location
closer to a first surface (e.g., the drying agent is closer to the inner
surface) when
compared to a second surface (e.g., outer surface) opposing the first surface.
3o BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic cross-sectional diagram of a representative
device of the present invention;
Figure 2 is a graph showing the effect of various desiccant materials on
encapsulated device lifetime is compared at 85°C under ambient humidity
35 conditions;
Figure 3 is a series of graphs comparing the effectiveness of water removal
according to the present invention with water removal using the materials and
methods of the prior art; and
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WO 01/19142 PCT/US00/24126
Figure 4 is a graph comparing the stability of water removal of the method
of the present invention with the method of the prior art.
DESCRIPTION OF PREFERRED EMBODIMENTS
As best seen in Figure 1, an electronic device 100 of the present invention
includes a polymer electronic device 110 made up of the anode 112 and cathode
114 with electrical attaching leads 116, 118, the layer of electrically active
organic
polymer 120, and, in this preferred embodiment, a substrate 122. The device
110
also includes an encapsulating enclosure 124 isolating the electronic device
from
to the atmosphere. This enclosure is made up of the substate 122 as a base
with a
cover or lid 126 affixed to the base 122 with a bonding agent 128. A drying
agent
130 is encapsulated within the enclosure 124, preferably affixed to an inner
surface 132 of the enclosure with a bonding agent 134.
The C»hetrata
The substrate 122 is typically impermeable to gases and moisture. In a
preferred
embodiment the substrate is glass. In a second preferred embodiment, the
substrate is silicon. In a third preferred embodiment, the substrate is a
flexible
substrate such as an impermeable plastic or composite material comprising a
combination of inorganic and plastic materials. Examples of useful flexible
2o substrate include a sheet, or a multilayer laminate, of flexible material
such as an
impermeable plastic such as polyester, for example polyethylene terephthalate,
or
a composite material made up of a combination of plastic sheet with optional
metallic or inorganic dielectric layers deposited thereupon. In a preferred
embodiment, the substrate is transparent (or semitransparent) to enable light
to
enter into the encapsulated region or to enable light to be emitted from the
encapsulated region through it.
The Enclosure
The airtight enclosure 126 isolates the polymer electronic device 110 from
the atmosphere. How the airtight enclosure is formed is not crucial, so long
as the
3o process steps do not adversely affect the components of the polymer
electronic
device 110. For example, the airtight enclosure 126 may be formed of multiple
pieces that are bonded together with a bonding agent. In a preferred
embodiment
the airtight enclosure includes a lid 126 bonded to a base. As best seen in
Figure
1, a preferred base 122 is the substrate of the polymer electronic device 110.
The material used to form the airtight enclosure 126 should be
impermeable to gases and moisture. In one embodiment, the lid is made from
metal. In another embodiment, the lid is made from glass or from a ceramic
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WO 01/19142 PCT/US00/24126
material. Plastics that are air-impermeable and water-impermeable can also be
used.
The thickness of the lid 126 is not crucial to the present invention, so long
as the lid 126 is thick enough to be a continuous barrier (with no voids or
pin-
s holes). Preferably, the lid 126 has a thickness of between about 10 and
about
1000 pm. Where the base is not the substrate of the polymer electronic device
(not shown), it is understood that the base can be made of the same material
as the
lid. As best seen in Figure l, the lid 126 is sealed to the substrate 122 with
a
bonding agent 128. This bonding agent should cure at a temperature below the
to decomposition temperature of the active layer 120, such as below
75°C and
preferably below 50°C and preferably at ambient temperature or only
moderately
elevated temperatures. This is advantageous as it eliminates exposure to high
temperatures common in the art which can often damage or degrade the
electronic
device 110. Preferred bonding agents include epoxies, either cured by exposure
to
15 ultraviolet light or by exposure to moderately elevated temperatures as
just noted
(or both). Various primer materials (not shown) may be used to assist in the
bonding process. As best seen in Figure 1, electrical leads 116, 118 emanate
from
the device. These leads 116, 118 should be sealed as wellm such as by the
bonding agent 128. Alternative but functionally equivalent lead configurations
2o can be used.
The Solid Drvin~ Agent
Prior to sealing the lid 126 onto the substrate 122 and enclosing the
electronic device 110, a solid drying agent (desiccant material) 130 is
inserted.
The form in which the desiccant is included is not important. For example, the
25 drying agent 130 can be in the form of a powder in a porous packet, a
pressed
pellet, a solid contained within a gel, a solid contained within a cross-
linked
polymer, and/or a film. The drying agent can be placed within the enclosure
124
in a variety of ways. For example, the drying agent 130 can be incorporated in
a
coating on the substrate or on an inner surface of the lid (not shown), or, as
best
3o seen in Figure 1, provided by affixing the drying agent 130 an inner
surface 132 of
the enclosurel24 with a bonding agent 134. Alternatively (not shown), the
drying
agent can be incorporated into a flexible substrate of the electronic device
or one
or more of the layers of a a multilayered or laminated substrate.
The nature of the solid drying agent is important. It is a porous solid, most
35 commonly an inorganic solid having a controlled pore structure into which
water
molecules can travel but in which the water molecules undergo physical
absorption so as to be trapped and not released into the environment inside
the
enclosure. Molecular sieves are one such material. In a preferred embodiment,
WO 01/19142 CA 02381230 2002-O1-29 pCT~S00/24126
the drying agent encapsulated into the sealed package is a zeolite. The
zeolites are
well known materials and are commercially available. In general, any zeolite
suitable for trapping water may be used. The zeolites are known to consist of
aluminum and silicon oxides in approximately equal amounts with sodium as the
counter ion. The zeolite materials absorb moisture by physical absorption
rather
than by chemical reaction. Physical absorption is preferred.
In a still more preferred embodiment, the drying agent 130 encapsulated
into the enclosure 124 is a zeolite material known as Tri-Sorb (available from
Siid-Chemie Performance Packaging, a member of the Siid-Chemie Group, a
to division of United Catalysts Inc., located in Belen, New Mexico). The
structure
of Tri-Sorb consists of aluminum and silicon oxides in approximately equal
amounts with sodium as the counter ion. Tri-Sorb absorbs moisture by physical
absorption. The remarkable improvement in stability and lifetime of the
polymer
LEDs when encapsulated with the methods described in this invention is
15 illustrated in the Examples. In particular, encapsulation with the
physically
absorbing zeolite material as desiccant significantly outperforms barium-oxide
as
desiccant; said barium oxide absorbs moisture by chemical absorption.
The amount of drying agent to be added should be determined to assure
that it provides adequate capacity to absorb the moisture trapped within the
2o enclosure when it is sealed shut. The water uptake capacity of the drying
agent is
a known property. The volume of the interior of the device and the humidity of
the air in the enclosure can be readily determined. Taking these factors into
account an adequate weight of drying agent can be determined and incorporated.
In a preferred embodiment, drying agent in excess of the calculated
25 amount can be added to compensate for any residual flux of water vapor into
the
active device area via imperfact edge seals and/or residual permeabilityof
water
vapor through the substrate.
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The Active La,
Among the promising materials for use as the active layers 120 in the
electronic devices protected by the present invention, such as polymer LEDs,
are
poly(phenylene vinylene), PPV, and soluble derivatives of PPV such as, for
example, poly(2-methyoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene),
MEH-PPV, a semiconducting polymer with an energy gap e.g. of > 2.1 eV. This
material is described in more detail in United States Patent No. 5,189,136.
Another material described as useful in this application is
poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), BCHA-PPV, a
to semiconducting polymer with an energy gap e.g. of > 2.2 eV. This material
is
described in more detail in United States Patent Application Serial
No. 07/800,555. Other suitable polymers include, for example, the
poly(3-alkylthiophenes) as described by D. Braun, G. Gustafsson, D. McBranch
and A.J. Heeger, J. Appl. Phys. 72, 564 (1992) and related derivatives as
described
by M. Berggren, O. Inganas, G. Gustafsson, J. Rasmusson, M.R. Andersson,
T. Hjertberg and O. Wennerstrom; poly(paraphenylene) as described by G. Grem,
G. Leditzky, B. Ullrich, and G. Leising, Adv. Mater. 4, 36 (1992), and its
soluble
derivatives as described by Z. Yang, I. Sokolik, F.E. Karasz in
Macromolecules,
26, 1188 (1993), polyquinoline as described by LD. Parker J. Appl. Phys, Appl.
2o Phys. Lett. 65, 1272 (1994). Blends of conjugated semiconducting polymers
in
non-conjugated host polymers are also useful as the active layers in polymer
LEDs as described by C. Zhang, H. von Seggern, K. Pakbaz, B. Kraabel,
H.W. Schmidt and A.J. Heeger, Synth. Met., 62, 35 (1994). Also useful are
blends
comprising two or more conjugated polymers as described by H. Nishino, G. Yu,
T-A. Chen, R.D. Rieke and A.J. Heeger, Synth. Met.,48, 243 (1995). Generally,
materials for use as active layers in polymer LEDs include semiconducting
conjugated polymers, more specifically semiconducting conjugated polymers
which exhibit photoluminescence, and still more specifically semiconducting
conjugated polymers which exhibit photoluminescence and which are soluble and
3o processible from solution into uniform thin films.
The High Work Function Anodes
Suitable relatively high work function metals for use as anode materials
112 are transparent conducting thin films of indium/tin-oxide [H. Burroughs,
D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns,
and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A.J. Heeger, Appl.
Phys.
Lett. 58, 1982 (1991)]. Alternatively, thin films of conducting polymers can
be
used as demonstrated by G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter,
N. Colaneri, arid A.J. Heeger, Nature, 357, 477 (1992), by Y. Yang and
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A.J. Heeger, Appl. Phys. Lett 64, 1245 (1994) and U.S. Patent Application
Serial
No. 08/205,519, by Y. Yang, E. Westerweele, C. Zhang, P. Smith and
A.J. Heeger, J. Appl. Phys. 77, 694 (1995), by J. Gao, A.J. Heeger, J.Y Lee
and
C.Y Kim, Synth. Met., 82,221 (1996) and by Y. Cao, G. Yu, C. Zhang, R. Menon
and A.J. Heeger, Appl. Phys. Lett. 70, 3191, (1997). Bilayer anodes comprising
a
thin film of indium/tin-oxide and a thin film of polyaniline in the conducting
emeraldine salt form are preferred because, as transparent electrodes, both
materials enable the emitted light from the LED to radiate from the device in
useful levels.
The Low Work Function Cathodes
Suitable relatively low work function metals for use as cathode materials
114 are the alkaline earth metals such as calcium, barium, strontium and rare
earth
metals such as ytterbium. Alloys of low work function metals, such as for
example alloys of magnesium in silver and alloys of lithium in aluminum, are
also
known in prior art (US Patent No. 5,047,687;5,059,862 and 5,408,109). The
thickness of the electron injection cathode layer has ranged from 200-5000 A
as
demonstrated in the prior art (US Patent 5,151,629, US Patent 5,247,190, US
Patent 5,317,169 and J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett., 67
(1995)
2281). A lower limit of 200-500 Angstrom units (~) is required in order to
form a
continuous film (full coverage) for cathode layer (US Patent 5,512,654; J.C.
Scott,
J.H. Kaufman, P.J. Brock, R. DiPietro, J. Salem and J.A. Goitia, J. Appl.
Phys., 79
(1996) 2745; LD. Parker, H.H. Kim, Appl. Phys. Lett., 64 (1994) 1774). In
addition to good coverage, thicker cathode layers were believed to provide
self encapsulation to keep oxygen and water vapor away from the active parts
of
the device.
Electron-injecting cathodes comprising ultra-thin layers of alkaline earth
metals, calcium, strontium and barium, have been described for polymer
light-emitting diodes with high brightness and high efficiency. Compared to
conventional cathodes fabricated from the same metals (and other low work
function metals) as films with thickness greater than 200, cathodes comprising
ultra-thin layer alkaline earth metals with thicknesses less than 100 provide
significant improvements in stability and operating life to polymer light
emitting
diodes (Y. Cao and G. Yu, U.S Patent Application 08/872,657).
Electron-injecting cathodes comprising ultra-thin layers of the oxides of
the alkaline earth metals, calcium, strontium and barium, have also been
described
for polymer light-emitting diodes with high brightness and high efficiency
(Y. Cao et al. PCT Application No. US99/23775, filed October 12, 1999)
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The construction of, and materials used in, photodetecting devices and
arrays of devices are very similar to the fabrication of polymer-based LEDs.
The
main differences between polymer-based LEDs and photodetectors is that
reactive
low work function electrodes need not be used, and that the electrical
polarity of
the electrodes is reversed. Nevertheless, hermetically sealed packaging is
required
for long lifetime of photodetecting devices fabricated from conducting
polymers.
Thus, the encapsulating enclosure of the present invention is also useful for
such
devices, said encapsulation being sufficient to prevent water vapor and oxygen
from diffusing into the device and thereby limiting the useful lifetime.
1o This invention will be further described with reference being made to the
following examples. These examples are provided solely to illustrate various
modes for practicing this invention and are not to be construed as limiting
its
scope.
EXAMPLE 1
A zeolite-based desiccant (Tri-Sorb) was used as the drying agent or
desiccant. As an example of a polymer-based electronic device, a polymer light-
emitting diode (LED) array was used.
An air- and water-impermeable lid made of glass, containing a desiccating
tablet composed of zeolite (available from Siid-Chemie Performance Packaging,
a
member of the Siid-Chemie Group, a division of United Catalysts Inc., located
in
Belen, New Mexico), was used to encapsulate the LED array and thereby isolate
it
from the atmosphere.
The drying agent was enclosed in the package by fixing the drying agent
on the internal surface of the impermeable lid by use of a thermal curing
epoxy
resin (Araldite 2014, Ciba Specialty Chemicals Corp., East Lansing, Michigan)
as
a bonding agent.
The drying agent was in the form of a compressed pellet of powder. The
impermeable lid was attached to the substrate using a bonding agent. The
completed device had the structure 100 shown in Figure 1. The lid was sealed
to a
3o substrate made of glass, using Araldite 2014 as a bonding agent.
Immediately after sealing the package, the dimensions of the light-emitting
pixels were measured. The packaged devices were then placed for an extended
period in an 85°C oven with ambient humidity. At fifty (50) hour
intervals, the
devices were removed from the oven and the dimensions of the light-emitting
pixels were re-measured. Degradation of the polymer electronic devices due to
moisture and oxygen was quantified by the loss in the active area. In this
particular example, the loss of light-emitting area for a pixellated LED
display
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was measured. As can be seen from Figure 2, the Tri-Sorb drying agent resulted
in
less than a 2% loss in light-emitting area after 300 hours storage at
85°C.
Also, as can be seen from Figure 2, the zeolite-based desiccant (in this case
a specific example going under the trade-name of Tri-Sorb) considerably
outperformed the other examples, notably Ba0 and CaS04 (which are desiccant
materials previously known is the art as useful desiccant materials (U.S.
Patent 5,882,761). This example shows that zeolite-based drying agents can be
very effective drying agents even at high temperatures.
EXAMPLE 2
l0 The experiments in Example 1 were repeated except that the storage
conditions were modified to include high humidity, i.e. 85°C/85%
relative
humidity. As can be seen from Figure 3, polymer LED arrays showed less than
5% loss of emissive area after 300 hours.
Also seen from Figure 3, the zeolite system is superior to many other
15 drying agents including Ba0 and Ca0 (which are desiccant materials
previously
patented as effective desiccant materials (U.S. Patent 5,882,761).
This example shows that zeolite-based drying agents are very effective
drying agents even at high temperatures in high humidity environments.
EXAMPLE 3
2o The experiments in Example 1 were repeated except the form of the drying
agent was a powder contained in a porous packet which was fixed on the
internal
surface of the impermeable lid by use of a bonding agent. The loss of emissive
area was comparable to the data shown in Figures 2 and 3.
This example shows that the particular physical form of the drying agent is
25 not important.
EXAMPLE 4
Thermogravimetric weight-loss studies were performed on Tri-Sorb and
Ba0 were compared for their performance in permanently removing water from
an electronic device enclosure. Standard, calibrated thermogravimetric
equipment
30 was used. Tablets of Tri-Sorb and Ba0 were heated (from room temperature to
400°C) in a dry atmosphere, while the mass of the tablets were
continually
monitored. No hysteresis was observed.
The results are shown in Figure 4. At room temperature both samples
have absorbed moisture. As they are heated, they both released this water due
to
35 thermodynamic processes and the sample weight decreases. However, as can be
seen, the Tri-Sorb releases less moisture. At 85°C,
the Tri-Sorb sample has released three times less water than the Ba0 sample.
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This example shows that Tri-Sorb has better water retention properties at
high temperature than does Ba0 (which was patented by Pioneer as a good drying
agent at 85°C).
As seen by the description above, the invention provides a technique for
- encapsulating polymeric light-emitting devices at the lowest possible method
temperatures. The method of encapsulation advantageously offers a hermetic
seal
between the device and the ambient air with its harmful moisture and oxygen.
In addition, the present method for encapsulation provides an overall
thickness of
the device is not significantly increased by the encapsulation of the device.
to Furthermore, the present encapsulation method requires fewer individual
process
steps than methods known to the art.
11
