Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2002-08-07
2004-02-17
Clinger, James (Department: 2821)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C315S169300
Reexamination Certificate
active
06693296
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to large area organic light emitting device (OLED) and to methods to reduce power consumption due to series resistance and to methods to reduce detrimental impact due to shorting defects.
BACKGROUND OF THE INVENTION
Organic light emitting devices (OLED) generally can have two formats known as small molecule devices such as disclosed in commonly-assigned U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device is typically a thin film structure comprising an organic EL element sandwiched between a cathode layer and an anode layer formed on a substrate such as soda-lime glass. The organic EL element can actually be constructed of several layers including a hole-injecting layer, a hole-transporting layer, a light-emitting layer, an electron-transporting layers, and an electron-injecting layer. Not all these layers, with the exception of the light-emitting layer, need to be present in a particular OLED device; on the other hand there may be additional functional layers in the device as well. The light-emitting layer may be selected from any of a multitude of fluorescent or phosphorescent organic materials. The light emitting-layer may also include multiple sub-layers. When a potential difference is applied between the anode and the cathode, negatively charged electrons move from the cathode layer into the OLED device. At the same time, positive charges, typically referred to as holes, move from the anode layer into the OLED device. When the positive and negative charges meet, they recombine and produce photons. The wavelength, and consequently the color, of the photons depend on the electronic properties of the organic material in which the photons are generated. In an OLED device either the cathode layer or the anode layer is transparent to the photons generated, allowing the light to emit from the device to the outside world.
An OLED device can also have a stacked structure as taught in U.S. Pat. No. 6,337,492. The OLED device having a stacked structure (a stacked OLED device) comprises a top electrode, a bottom electrode, and a plurality of individual light emitting devices vertically stacked between the top electrode and the bottom electrode. A pair of inter-device electrodes are also provided between the neighboring individual light emitting devices. These inter-device electrodes are to inject electrons and holes, respectively, to the individual light emitting devices above and below them, and to electrically connect these two individual light emitting devices. The individual light emitting devices in the stack are thereby connected in series. In operation, electricity is applied between the top electrode and the bottom electrode. The same current flows through all the individual light emitting devices in the stack and the applied voltage is divided among all the individual light emitting devices in the stack. The inter-device electrodes are commonly 0.1 to 15 nm thick, and include allegedly transparent metal alloys, metal oxides, and other well known inorganic electrode materials commonly used in OLED devices.
The OLED devices are low voltage, high current devices. A typical device operates at 3-10 volts of voltage and has about 1 to 10 Cd/A of light-generating efficiency. For many display or lighting applications, a brightness of about 1000 Cd/m
2
is requred. The operating current, therefore, has to be about 100 A/m
2
to 1000 A/m
2
. These characteristics are ideal for small devices such as those for portable applications that require device areas less than about 0.01 m
2
. When device area increases, however, these characteristics lead to practical problems. For example, some lighting applications may require devices with area as large as 1 m
2
. The operating current in these devices can be as high as 100 A to 1000 A. Since the anode and cathode layers are thin-films having limited electrical conductivity, they are not able to carry these high currents without substantial energy loss due to series resistance. This problem is accentuated since one of the electrode layers also has to be optically transparent to allow emitted light to get through. If a stacked OLED device is used, the situation is somewhat improved. If a stacked OLED and a non-stacked OLED device are operated at the same light output level, the operating current of the stacked OLED device equals I/N where I is the current of the non-stacked OLED device and N is the number of individual light emitting elements in the stacked OLED device. The lowered operating current results in lowered power loss due to series resistance. However, since the total number of cells in the stack is limited by practical factors, A stacked OLD device is still a relative low voltage, high current device and the energy loss due to series resistance is still a serious problem.
Another common problem encountered in making large area OLED devices is failure due to shorting defects. Since OLED devices use very thin layers, pinholes, dust particles, and many other kinds of defects can cause shorting between the anode and the cathode. Applied electricity will go through the shorting defect instead of the light-emitting device. A single shorting defect can cause an entire OLED device to fail. Even with the best efforts practiced in manufacturing, it is difficult to eliminate all shorting defects in large area thin-film electrical devices. Assuming the defects are randomly distributed, the probability of finding X defects in a device of area A with a defect density of N
d
can be expressed by
P
(
X, A, N
d
)=[(
A·N
d
)
x
exp(−
A·N
d
)]/
X!
Thus the probability of having a defect free device of area A is
P
(0,
A, N
d
)=exp(−
A·N
d
).
The probability decreases exponentially with increasing area. For example, even if the defect density is as low as 0.001/cm
2
, the probability of having a defect free 1 m
2
device is only 36.8%. Thus for making large area OLED devices practical, it is imperative to find solution to the shorting defect problem.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved large-area OLED apparatus with reduced detrimental impacts due to series resistance and shorting defects.
This object is achieved by providing an OLED apparatus comprising:
(a) a substrate;
(b) a plurality of OLED devices including spaced apart bottom electrodes disposed over the substrate;
(c) each one of the plurality of OLED devices including at least one organic layer extending over an edge of its corresponding spaced apart bottom electrode; and
(d) each one of the plurality of OLED devices including a top electrode spaced apart from the top electrodes of other OLED devices and extending into electrical contact with the spaced apart bottom electrode of a neighboring OLED device so that a series connection of OLED devices is provided and current flows between the spaced apart top and bottom electrodes of each OLED device and from the spaced apart bottom electrode of such OLED device to the spaced apart top electrode of the next OLED device which reduces power loss due to series resistance.
An advantage of the present invention is a reduced energy loss due to series resistance. Another advantage of apparatus made in accordance with this invention is a reduced impact due to shorting defects. A further advantage of the apparatus made in accordance with this invention is that it can be designed to have tunable color. Another further advantage of the apparatus is that it can use stacked cells to further improve its performance. A still further advantage of the present invention is that the apparatus can be manufactured at low cost. The present invention is particularly suitable for forming large-area OLED apparatus.
Additional objects and advantages of the invention are set forth, in part, in the description which follows, and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
REF
Clinger James
Owens Raymond L.
Tran Chuc
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