Patent Application
20130048961
The present application relates to organic light-emitting devices.
Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays; illumination, including lighting panels; and backlighting Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction.
More details on OLEDs, and the definitions described above, maybe found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.
For some applications of OLEDs, such as elements of lighting panels, it may be desirable that the light emitted by the OLED be highly uniform in both intensity and in color spectrum across an emitting surface of the device. The larger the area of the emitting surface the more difficult it may be to achieve this desired uniformity. One cause of non-uniform emission may be variations in electrical potential across a face of a device from which light is emitted. Achieving a more uniform potential across the face may result in a greater uniformity of light emission across the face.
A light emitting device with high light emission uniformity is disclosed. The device is comprised of a first electrically conductive layer having a positive polarity and an electrically conductive uniformity enhancement layer in contact with the first electrically conductive layer. The device is further comprised of a second electrically conductive layer having a negative polarity and a light-emitting structure situated between the first and the second electrically conductive layers. The light-emitting structure is comprised of an organic material in direct contact with the second electrically conductive layer. The uniformity enhancement layer transmits essentially all wavelengths of light emitted by the light-emitting structure.
Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton,” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.
The simple layered structure illustrated in
Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in
Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. patent application Ser. No. 10/233,470, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink-jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processibility than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing.
On the substrate 310 is a first electrically conductive layer 315. In the embodiment shown in
In contact with the first electrically conductive layer 315 is an electrically conducting uniformity enhancement layer 318, described in detail below. As used herein, the terms “uniformity enhancement layer” and “enhancement layer” refer to an electrically conducting uniformity enhancement layer, such as feature 318 of
The OLED 300 further contains a light-emitting structure 305 comprised of at least one organic layer. The following details of the light emitting structure 305 are not intended to be limiting. In the OLED embodiment 300 of
A second electrically conductive layer 360 may be in direct contact with an organic layer 345 of the light emitting structure 305. In the embodiment shown in
In the embodiment shown in
The enhancement layer 318 does not function as a microcavity, either by itself or in combination with other layers. A microcavity is an optically resonant structure designed to increase the external emission intensity of a light emitting device in a particular direction. Because of its resonant nature, a microcavity may significantly alter the spectrum of the light emitted by the device. Evidence is presented below to show that, in an embodiment reduced to actual practice, enhancement layer 318 does not act as, or give rise to, a microcavity. A light-emitting device employing a microcavity is described in U.S. Published Patent Application No. US2008/0067921.
In an alternative embodiment, a uniformity enhancement layer may be situated between two electrically conductive layers and in contact with both. A resulting sandwich-like structure may be configured as a composite anode.
Table 1 shows results of comparative measurements between two 5 cm.×5 cm. OLED panels emitting white light, one (Device B) having an enhancement layer as described above, the other (Device A) having the same layer structure as Device B but without an enhancement layer. Measurements were taken at a constant current density of 4 mA/cm2.
The enhancement layer in Device B is a 2 nanometer (nm) thick layer of calcium (Ca) situated as shown in
In a similar investigation, two 5 cm.×5 cm. blue-light emitting devices are compared, one with an enhancement layer, the other one without, but otherwise having the same layer structure. With no enhancement layer, luminance in the center of the emitting face of the device is 91.2% of the average luminance at the edge. With a 2 nm thick Ca enhancement layer, luminance in the center is 98.1% of the average luminance at the edge. Thus, the luminance of the emitted light varies less than 2% over a distance of at least 2.5 centimeters across an emitting face of the device. It is expected that similar improvement in luminance uniformity will be achieved in devices of dimensions significantly larger than 5 cm.×5 cm. It may not be necessary, however, to maintain such a high level of uniformity across the lighting panel at higher luminance levels and/or for much larger panel sizes. For example, luminance of the emitted light that varies less than 10%, or even as much as 20%, over a distance of 2.5 centimeters across an emitting face of the device may be adequate. Such uniformity would also be readily achievable using the methods and structures disclosed here.
OLEDs fabricated in accordance with the above embodiments may be incorporated into a wide variety of consumer products, including flat panel displays, computer monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads up displays, fully transparent displays, flexible displays, laser printers, telephones, cell phones, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control OLEDs fabricated in accordance with the above embodiments, including passive matrix and active matrix. Many of the devices are intended for use in a temperature range comfortable to humans, such as 18° C. to 30° C., and more preferably at room temperature (20-25° C.).
The materials and structures described herein may have applications in devices other than OLEDs. For example, other optoelectronic devices such as organic solar cells and organic photodetectors may employ the materials and structures. More generally, organic devices, such as organic transistors, may employ the materials and structures.
It is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the embodiments. The embodiments as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. It is understood that various theories as to why various embodiments work are not intended to be limiting.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2010/023034 | 2/3/2010 | WO | 00 | 10/18/2012 |
