U.S. patent application number 11/222205 was filed with the patent office on 2006-01-12 for active matrix addressed polymer led display.
Invention is credited to Mark R. Hueschen, Ronald L. Moon, James R. Sheats.
Application Number | 20060007076 11/222205 |
Document ID | / |
Family ID | 35266400 |
Filed Date | 2006-01-12 |
United States Patent
Application |
20060007076 |
Kind Code |
A1 |
Sheats; James R. ; et
al. |
January 12, 2006 |
Active matrix addressed polymer LED display
Abstract
A display having a plurality of light emitting pixels. Each
pixels includes an isolation transistor, a driving circuit, and an
organic light emitting diode (OLED). The driving circuit storing a
value that determines the magnitude of the light emitted by that
pixels, the driving circuit placing the OLED in a conducting path
between the first and second power terminals. The driving circuit
is programmed through the isolation transistor. In one embodiment
of the present invention, the driving circuit includes a storage
capacitor and a driving transistor. The OLEDs are part of an array
of OLEDs. The array of OLEDs is constructed on a flexible sheet
having first and second surfaces, the flexible sheet being
transparent to light of a first wavelength. A transparent first
electrode layer is in contact with the first surface. A light
emitting layer including an organic polymer is in contact with the
first electrode layer. A plurality of second electrodes, one such
second electrode corresponding to each of the OLEDs, is in contact
with the light emitting layer. Each second electrode has an
isolated conducting area. The driving transistor are part of a
transistor array having a plurality of connection points disposed
on a surface, each connection point corresponding to one of the
second electrodes in the array of OLEDs. The connection points are
arranged such that each second electrode overlies the corresponding
connection point when the array of OLEDs is properly aligned with
the transistor array. The connection points are bonded to the
corresponding second electrodes by a bonding layer.
Inventors: |
Sheats; James R.; (Palo
Alto, CA) ; Hueschen; Mark R.; (Palo Alto, CA)
; Moon; Ronald L.; (Atherton, CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.;Intellectual Property Administration
Legal Department, DL429
P. O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
35266400 |
Appl. No.: |
11/222205 |
Filed: |
September 7, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09098190 |
Jun 16, 1998 |
6965361 |
|
|
11222205 |
Sep 7, 2005 |
|
|
|
Current U.S.
Class: |
345/76 |
Current CPC
Class: |
H01L 2251/5338 20130101;
H01L 27/3251 20130101; H01L 27/322 20130101 |
Class at
Publication: |
345/076 |
International
Class: |
G09G 3/30 20060101
G09G003/30 |
Claims
1-2. (canceled)
3. A display comprising a plurality of light emitting pixels, each
pixel comprising an isolation transistor, a driving circuit, and an
organic light emitting diode (OLED), said driving circuit storing a
value that determines the magnitude of the light emitted by that
pixel, said driving circuit placing said OLED in a conducting path
between first and second power terminals, said isolation transistor
connecting said driving circuit to a bit line when said isolation
transistor is placed in a conducting state by the application of a
logic signal to a word line, wherein said OLEDs are part of a
flexible array of OLEDs, said array of OLEDs comprising: a flexible
sheet having first and second surfaces, said first and second
surfaces being parallel to one another, said flexible sheet being
transparent to light of a first wavelength; a first electrode
comprising a first electrode layer in contact with said first
surface, said first electrode layer being transparent to light of
said first wavelength; a light emitting layer comprising an organic
polymer in electrical contact with said first electrode layer; and
a plurality of second electrodes, one such second electrode
corresponding to each OLEDS, each of said second electrodes
comprising an isolated conducting area in electrical contact with
said light emitting layer, said light emitting layer generating
light of said first wavelength in a region adjacent to said second
electrode when a potential difference is applied across said first
and second electrodes, and wherein said isolation transistors are
located on a substrate that is separate from said flexible array of
OLEDs, said substrate being bonded to said flexible array of OLEDs
by an adhesive layer.
4-12. (canceled)
13. A method for fabricating a display having a plurality of
pixels, said method comprising: fabricating an array of OLEDs on a
flexible first substrate; fabricating an array of isolation
transistors, and driving circuits on a second substrate, each
driving circuit storing a value that determines the magnitude of
the light emitted by a corresponding one of said pixels in said
display, said driving circuit placing said OLED in a conducting
path between first and second power terminals, said isolation
transistor connecting said driving circuit to a bit line when said
isolation transistor is placed in a conducting state by the
application of a logic signal to a word line; and bonding said
first and second substrates together.
14. The method of claim 13 wherein said first and second substrates
are bonded together by a layer of adhesive.
15. The method of claim 14 wherein said adhesive is an anisotropic
conductive adhesive.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to display devices, and more
particularly, to information displays using organic
electroluminescent materials and thin film field effect
transistors.
BACKGROUND OF THE INVENTION
[0002] Active matrix liquid crystal displays for computers and the
like provide significantly improved performance; however, the cost
of such displays is also a significant factor in the overall price
of portable computers using the same. In addition, active matrix
displays require substantially more power than passive displays
since this type of display utilizes backlighting.
[0003] An active matrix liquid crystal display is constructed from
two components that are separately constructed and then bonded
together. The first component consists of an array of thin film
transistors (TFTs) fabricated on a glass substrate, one per pixel.
Each TFT has an associated capacitor which stores a potential value
which is then applied to a liquid crystal element which passes a
fraction of the light incident thereon, depending on the applied
voltage. The TFTs are connected by transparent row and column
electrodes, which are used to select each pixel. The liquid crystal
layer together with color filters are fabricated on a second glass
substrate. An adhesive is screen-printed around the edge of one
glass plate, and an optical aligner is then used to bring the two
together with the TFTs aligned to the filters. The adhesive is then
UV (ultraviolet) cured and the liquid crystal material introduced
by capillary action. The polarizers and a backlight are attached to
the outside of the display.
[0004] Displays based on organic light emitting devices (OLEDs)
have the potential for substantially reducing the cost of computer
displays. OLEDs are emissive displays that provide an alternative
to other types of light emission, such as vacuum fluorescence,
plasma, inorganic electroluminescense, and inorganic light emitting
diodes. Since a pixel only draws power when "on", a display based
on OLEDs requires less power than a backlight-based display.
[0005] OLEDs are constructed on a transparent substrate coated with
a transparent conducting material, such as Indium Tin oxide (ITO),
one or more organic layers and a cathode made by evaporating or
sputtering a metal of low work function characteristics, such as Ca
or Mg. The organic layers are chosen so as to provide charge
injection and transport from both electrodes to the
electroluminescent organic layer (EL) where the charges recombine
emitting light. Often there is at least one or two organic hole
transport layers (HTL) between the ITO and the EL, as well as at
least one or two electron injection and transporting layers (EL)
between the cathode and the EL.
[0006] While OLEDs have the potential for reducing display costs,
prior art methods for fabricating OLEDs have several disadvantages
when applied to active matrix displays. In principle, a set of TFTs
and conductors on a substrate can also be utilized for constructing
an active matrix display based on OLEDs. After constructing the
TFTs the pixels are defined by depositing a patterned conductor
corresponding to either the cathode or anode of the OLED. The
organic light emitting layers and associated transport layers are
then deposited, followed by a uniform layer of the other OLED
electrode (anode or cathode). Such structures are described in
detail in Eugene Y. Ma, et al., Soc. Inform. Display Conf. Proc.
Sep. 15-19, 1997 (Toronto, Canada), p. L78.
[0007] While such a process has the potential to produce useful
displays which may be competitive with AMLCDs, it has several
disadvantages. First, the processing of the organic LED must be
done on the same substrate as used for the TFTs, typically glass.
As a result, the large cost advantages associated with fabricating
polymer LEDs on flexible plastic substrates with roll-to-roll
equipment cannot be realized. Ma, et al., attempt to overcome this
limitation by using stainless steel foil as a substrate. In this
method, the TFTs and OLEDs are constructed as described in the
previous paragraph, using stainless steel foil in place of glass.
However, since this substrate is opaque, the top electrode of the
OLED must be transparent. The preferred transparent electrode
material is indium tin oxide (ITO) which is deposited by
sputtering. Hence, the method suggested by Ma, et al. requires the
active organic layers to be subjected to the hostile conditions
associated with the sputtering of the ITO. This leads to damage of
the active polymer layers. In addition, this architecture requires
the organic material to be deposited on top of the cathode.
However, the cathode must be constructed from a low work function
material such as Ca or Mg which is easily oxidized.
[0008] Broadly, it is the object of the present invention to
provide an improved OLED-based active matrix display.
[0009] It is a further object of the present invention to provide
an OLED-based active matrix display that can be fabricated
utilizing roll-to-roll processing techniques on polymer films.
[0010] It is a still further object of the present invention to
provide an OLED-based active matrix display which does not require
the deposition of the transparent electrode onto the already
deposited organic layers.
[0011] These and other objects of the present invention will become
apparent to those skilled in the art from the following detailed
description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
[0012] The present invention is a display having a plurality of
light emitting pixels. Each pixel includes an isolation transistor,
a driving circuit, and an organic light emitting diode (OLED). The
driving circuit stores a value that determines the magnitude of the
light emitted by that pixel, the driving circuit placing the OLED
in a conducting path between first and second power terminals. The
driving circuit is programmed through the isolation transistor. The
isolation transistor connects the driving circuit to a bit line
when the isolation transistor is placed in a conducting state by
the application of a logic signal to a word line. In one embodiment
of the present invention, the driving circuit includes a storage
capacitor and a driving transistor, the storage capacitor storing a
charge that determines the magnitude of the light emitted by the
pixel. The gate of the driving transistor is connected to the
storage capacitor, the driving transistor connecting the OLED
between the first and second power terminals. In this embodiment,
the isolation transistor connects the storage capacitor to the bit
line when the isolation transistor is placed in the conducting
state by the application of the logic signal to the word line. The
OLEDs are part of an array of OLEDs. The array of OLEDs is
constructed on a flexible sheet having first and second surfaces,
the first and second surfaces being parallel to one another, the
flexible sheet being transparent to light of a first wavelength. A
first electrode layer is in contact with the first surface, the
first electrode layer being transparent to light of the first
wavelength. A light emitting layer including an organic polymer is
in electrical contact with the electrode layer. A plurality of
second electrodes, one such second electrode corresponding to each
of the OLEDs, is in electrical contact with the light emitting
layer. Each second electrode has an isolated conducting area in
electrical contact with the light emitting layer, the light
emitting layer generating light of the first wavelength in a region
adjacent to the second electrode when a potential difference is
applied across the first and second electrodes. The driving
transistors are part of a transistor array having a plurality of
connection points disposed on a surface, each connection point
corresponding to one of the second electrodes in the array of
OLEDs. The connection points are arranged such that each second
electrode overlies the corresponding connection point when the
array of OLEDs is properly aligned with the transistor array. The
display includes a bonding layer located between the transistor
array and the array of transistors, the bonding layer electrically
connecting each of the second electrodes to that second electrodes
corresponding connection point.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic drawing of a portion of a display
according to the present invention.
[0014] FIG. 2 is a top view of an OLED array according to the
present invention.
[0015] FIG. 3 is a cross-sectional view of the array shown in FIG.
2 through line 51-52.
[0016] FIG. 4 is a cross-sectional view of a portion of a display
according to the present invention illustrating the connection of
the OLEDs to the driving transistors.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention overcomes the problems associated with
the above described prior art methods by utilizing a laminating
technique that allows an OLED-based sheet of light emitting diodes
formed on a flexible plastic substrate to be laminated onto a solid
TFT substrate. In this manner, the cost advantages associated with
OLEDs are preserved.
[0018] Refer now to FIG. 1 which is a schematic drawing of a
portion of a display 10 according to the present invention. The
portion of display 10 shown in FIG. 1 consists of four pixels 20,
25, 30, and 35. Each pixel consists of two TFT transistors, an
OLED, and a capacitor for storing the intensity to be displayed by
the OLED. Referring to pixel 20, the current flowing through OLED
24 is determined by the voltage on the gate of TFT 22 and the
supply voltage provided on bus 15. The voltage on the gate of TFT
22 is set by applying the desired voltage to a bit line 14 while
placing gate TFT 21 in a conducting state. The voltage programs
capacitor 23 with the voltage on the bit line.
[0019] The pixels are written in "words". All of the pixels
connected to a particular word line are written in parallel by
applying the desired programming voltages to the bit lines. Pixels
20 and 30 are programmed through bit line 14, and pixels 25 and 35
are programmed through bit line 16. The word selected for
programming is selected by applying an appropriate logic voltage on
a corresponding word line. Pixels 20 and 25 are selected via word
line 12, pixels 30 and 35 are selected via word line 13.
[0020] A display according to the present invention is constructed
by bonding an array of OLEDs to an array of TFTs. To simplify the
following discussion, the fabrication of an array of OLEDs for a
monochrome display will be described first. The manner in which a
color display is fabricated will be discussed in more detail
below.
[0021] Refer now to FIGS. 2 and 3 which illustrate an array of
OLEDs 50 according to the present invention. FIG. 2 is a top view
of array 50, and FIG. 3 is a cross-sectional view of array 50
through line 51-52 shown in FIG. 2. To clarify the various layers
that make up array 50, the layers have been shown displaced from
one another at the edges of the array. An array is constructed on a
thin plastic substrate 61 constructed from a transparent polymer
that is capable of withstanding the processing temperatures
involved in depositing the various layers. The processing
temperatures are typically less than 150.degree. C. Substrates
constructed from poly(ethylene terephthalate), or PET may be used.
A transparent conducting layer 62 is deposited on the substrate.
The preferred conducting material is ITO. Layer 62 becomes a common
anode electrode which is shared by all of the OLEDs. The various
polymer layers are then deposited on the anode electrode. These
include any hole and electron transport layers as well as the
electroluminescent layer. To simplify the drawing, these layers are
shown as a single layer 63. Finally, a cathode layer is deposited
and patterned to form individual cathode electrodes 64. By applying
a voltage between a cathode electrode and the common anode
electrode, the portion of the light emitting layer 62 under that
cathode electrode is caused to emit light that escapes through the
transparent plastic substrate 61.
[0022] Array 50 is bonded to a substrate on which the TFTs and
associated capacitors have already been fabricated utilizing an
anisotropic conductive adhesive (ACA), which serves both to hold
the two devices together and to make the necessary electrical
contacts. ACAs suitable for this purpose are well known and are
manufactured by Hitachi, 3M Corp., and Alpha metals, Inc. These
adhesives are described in "Flip Chip Technologies", ed. John Lau
(McGraw-Hill, 1996). Referring to FIG. 4, which is a
cross-sectional view of a portion of a display 80 containing one
pixel that is controlled by a TFT 71. TFT 71 is constructed on a
glass substrate 76 having a SiN insulating gate oxide layer 73
which isolates gate 72 from a polysilicon layer 74. The drain and
source of TFT 71 are shown at 75 and 77, respectively. The source
electrode 77 is covered by an insulating layer 78.
[0023] TFT 71 is connected to the corresponding OLED by a layer of
ACA 81 which bonds the drain 75 to the cathode 64 of the OLED. The
electrical conduction is provided by compressible conducting
particles 82 in an electrically insulating glue material. The array
of OLEDs is pressed onto the array of TFTs to form the various
contacts. Hence, it is advantageous to have an array of OLEDs that
is constructed on a flexible substrate, as this allows sufficient
flexing of the OLED array to assure that each OLED is connected to
its corresponding TFT. Alignment of the two substrates is
accomplished by an optical aligner of the type that is currently
used for aligning the standard liquid crystal cells described
above. The TFT substrate provides a solid barrier against
permeation of water or oxygen into the OLED, leaving a requirement
of only a single barrier 88.
[0024] The above described embodiment of the present invention was
a monochromatic display. That is, all of the OLEDs generate the
same wavelength light. Color displays require that pixels generate
different color spectra. Typically, a color pixel is constructed
from three pixels having red, green, and blue emitters.
[0025] Multicolor pixels may be provided by two approaches. The
first approach consists of depositing emitting materials with
different spectral characteristics. Each of these materials
(typically one material with red emission, one for green and one
for blue) is deposited separately in different parts of the
substrate to achieve "full color" (RGB) pixels by separately
powering the three color pixels. This technique requires the
patterning of the electroluminescent layer 63 discussed above.
Methods for patterning such layers are known in the art, and hence,
will not be discussed in detail here.
[0026] The second approach consists of depositing a single emissive
material and using (RGB) filters, resonant cavities and/or
photoluminescent materials that can absorb the light from the
emissive material and re-emit light at longer wavelengths (green
and red). This approach requires the emissive material to have a
substantial emission in the blue. Such wavelength converters are
known in the art, and hence, will not be discussed in detail. The
reader is directed to U.S. Pat. Nos. 5,121,214 and 5,294,820 for a
discussion of such materials.
[0027] Devices based on color conversion are preferred because it
is much easier to pattern the wavelength converters than
electroluminescent materials. In addition, these devices do not
require that the electroluminescent characteristics of three
different emissive materials be balanced. In general, the
voltage-current characteristics and quantum efficiencies are
usually very different for each material. The lifetimes of the
electroluminescent materials are also quite difficult to equalize
between different materials.
[0028] In the preferred embodiment of the present invention, the
color conversion materials are deposited on the opposite side of
the plastic substrate from the ITO layer. Such a light converter is
shown in FIG. 4 at 89.
[0029] While the above described embodiments of the present
invention utilize a layer of an anisotropic conductive adhesive to
bond the array of OLEDs to the array of TFTs, other bonding
arrangements may be utilized. For example, a patterned conductive
adhesive can be applied using screen printing or precision stencil
printing techniques. Such adhesives are described in D. Durand, et
al, "Electrically Conductive Cement Containing Agglomerate, Flake
and Powder Metal Fillers", U.S. Pat. No. 5,180,523 and in K.
Gilleo, "Polymer Bonding Systems Offer Alternatives to Soldering",
Electronic Packaging and Production, December 1992, pp. 52-54.
These techniques are used in printed circuit fabrication to apply
conductive adhesives, and hence will not be described in detail
here. The anisotropic conductive adhesive described above is
preferred because no patterning or mask step is needed.
[0030] Various modifications to the present invention will become
apparent to those skilled in the art from the foregoing description
and accompanying drawings. Accordingly, the present invention is to
be limited solely by the scope of the following claims.
* * * * *