Patterning of polymer light emitting devices using...

Details Patterning of polymer light emitting devices using... Patterning of polymer light emitting devices using...

2000-04-11

2003-08-05

King, Roy (Department: 1742)

Electrolysis: processes, compositions used therein, and methods

Electrolytic coating

Forming nonmetal coating

C204S492000

Reexamination Certificate

active

06602395

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of patterning polymeric organic light emitters onto the substrate of an organic light emitting diode (OLED) using electrochemical polymerization, and to the resulting OLED device.
2. Description of the Related Art
OLEDs are known, and have been used for various types of displays.
FIG. 1
illustrates a general overview of a portion of an OLED
10
. Device
10
has a cathode electrode
12
that is spaced from a transparent anode electrode
14
deposited on a transparent substrate
18
which is typically comprised of glass or transparent plastic. Although only the anode electrode
14
is illustrated as transparent to allow light ho to pass through, the cathode electrode
12
or both the anode and cathode electrodes
14
and
12
may be transparent. An organic light emitter
20
, which is capable of electroluminescence, is sandwiched between the cathode and anode electrodes
12
and
14
. An encapsulating layer
23
may then be deposited on top of cathode electrode
12
to protect device
10
from the environment.
The organic emitter
20
has been comprised of electroluminescent thin films of either small discrete molecules such as aluminum tris(8-hydroxyquinoline) (Alq
3
) or dye-doped Alq
3
, or certain polymers. Different organic emitters emit different color light. For example, the Alq
3
molecule emits green light. The cathode electrode
12
is usually a low work function metal such as an alkaline earth metal or reactive metal alloy. Examples of cathode electrodes include calcium, magnesium/silver, and aluminum/lithium. Typically, the anode electrode
14
is a high work function thin film of transparent indium tin oxide (ITO). Other materials used may be polyaniline or fluorine-doped tin oxide. Semitransparent metal films have also been used, although they tend to be less transmissive at thicknesses that are suitably conductive for electrodes. The phrase “work function” refers to the energy difference, in electron volts (eV), between a free electron and an electron at the Fermi level of the material. The “Fermi level” is the energy level at which the probability that an energy state is occupied is equal to one-half.
Referring back to
FIG. 1
, forward biasing device
10
(placing a higher voltage on the anode electrode
14
than on cathode
12
) causes current to flow through the organic emitter
20
. This current flow enables the recombination of holes injected at the anode electrode
14
with electrons injected from the cathode electrode
12
within the organic emitter, generating light h&ugr; in all directions, that is, electroluminescence. The light transmitted out to the sides of the device is lost and the light that hits the cathode is reflected. The output light h&ugr; is transmitted through the transparent anode
14
and the substrate
18
.
FIGS. 2A and 2B
illustrate in detail the electrodes of the OLED shown in FIG.
1
.
FIG. 2A
illustrates a well-known pre-patterned transparent substrate
18
having parallel rows of transparent anode electrodes
14
deposited thereon.
FIG. 2B
illustrates parallel columns of deposited cathode electrodes
12
on a substrate
18
A. Although both the anode electrodes
14
of FIG.
2
A and the cathode electrodes.
12
of
FIG. 2B
are illustrated as straight strip patterns, other patterns may also be used. If rows and columns are used, then the electrodes are normally oriented orthogonal to each other. The intersection of an anode electrode
14
with a cathode electrode
12
defines a single pixel. All of the pixels together form a matrix from which images can be formed by illuminating desired pixel patterns. Respective leads
14
a
and
12
a
in a conventional matrix-addressing scheme electronically address the anodes and cathodes.
There are two common methods for the deposition (or patterning) of organic light emitters onto a substrate with a pre-patterned electrode, such as substrate
18
with pre-patterned anode electrodes
14
. Both methods are similar in that emitter
20
is deposited as a corresponding matrix of discrete pixel elements in registration with the pixels defined by the electrodes. The choice of deposition method however, will depend upon the type of organic emitter used.
As mentioned above, the two main groups of organic emitters in common use are discrete molecules and polymers. For discrete molecules such as Alq
3
or dye-doped Alq
3
, the preferred technique is vapor deposition through a mask (commonly referred to as masking). Ink jet printing is commonly used for polymeric organic emitters. Both methods are well known.
FIG. 3A
illustrates the deposition of organic emitters R, B, and G (Red, Blue, and Green) onto the electrodes
14
of substrate
18
using the masking method. Masking uses a metal plate
30
(illustrated in
FIG. 3B
) with patterned openings
31
which are commensurate in pattern and number with the specific pattern and number of pixels that are to be deposited; conventional OLED displays can have millions of pixels. For a full color spectrum, at least three different masks
30
R,
30
B, and
30
G (shown in
FIG. 3C
) may be used to deposit three different organic emitters, each with its own color emission characteristic, onto different sets of electrodes
14
. Discrete organic emitter molecules that emit the desired colors are deposited through the mask openings onto the desired pixel areas of the underlying electrodes
14
. Red emitter is deposited through openings
31
R in mask
30
R, blue emitter through openings
31
B in mask
30
B, and green emitter through openings
31
G in mask
30
G. The masks are placed as close to the substrate
18
as possible without touching it, to avoid disrupting the organic emitters previously deposited on the electrodes
14
.
A drawback of masking is that the deposition areas
33
(shown within dashed lines in
FIG. 3A
) tend to be larger than the actual mask openings
31
. Because the mask
30
does not touch the substrate
18
, molecules passing through its openings
31
are diffused sideways through the gap between the mask
30
and the substrate
18
, and are deposited on areas beyond the boundaries of the openings
31
. With this method, there is a lack of control over exactly where the individual organic emitter molecules are deposited. This imprecise deposition (or patterning) can create overlaps
34
of different organic emitter molecules having different color emission characteristics onto the same pixel.
To overcome this problem, efforts have been made to place the mask
30
as close to the substrate
18
as possible and to reduce the size of the mask openings
31
. However, since some gap is still required between the mask
30
and substrate
18
to avoid damage, spreading of the organic emitter molecules can still occur. While reducing the size of the mask openings
31
reduces the spreading problem, it also reduces the amount of light generated while increasing the spacing between pixels. Another proposal is to reduce the number of pixels and increase their size. While this could reduce or eliminate overlapping depositions, it would also reduce the resolution of the device.
FIG. 3D
is a magnified illustration of a pixel
35
(shown within dashed lines), defined by the intersection of the anode
14
and cathode
12
electrodes, which is deposited with an organic emitter using the masking method. Pure color emission might not be possible if there is an overlap
34
of two or more different molecules with different color emission characteristics on the pixel region
35
.
FIG. 4A
illustrates the deposition of polymeric organic emitters R, B, and G onto electrodes
14
of substrate
18
using the ink jet method. With ink jet printing the polymeric organic emitter with the desired color emission characteristic is first dissolved in a solvent such as xylene, and then dropped in a discrete non-continuous manner onto desired pixel locations on the electrodes
14
. The emitter in the solvent is in liquid form, and therefore the emitter drops spread
37
upon

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