Patent Application
20130293082
This disclosure relates to organic light-emitting diodes (OLEDs) and their operation in OLED devices.
OLEDs have shown promise for use in various electronics display and lighting applications, due in part to their low deposition temperatures, mechanical flexibility, and low material cost. Being made partly with organic materials, OLEDs can be more sensitive to degradation by certain environmental factors than other types of LEDs. For example, exposure to oxygen, water, or other substances can negatively affect some OLED materials, rendering them non-functional or reducing their longevity as light-producing materials. OLED devices have been constructed to encapsulate OLEDs to protect them from the environment.
According to one embodiment, there is provided an OLED device. The device includes an OLED with an organic layer arranged between first and second electrodes. The device also includes a substrate supporting one side of the OLED and a cover layer at an opposite side of the OLED. A thermal junction included between the OLED and the cover layer has a thermal resistance of 0.2 m2·K/W or less.
In accordance with another embodiment, there is provided an OLED device. The device includes an OLED with an organic layer arranged between first and second electrodes. The device also includes a substrate supporting one side of the OLED and a cover layer at an opposite side of the OLED. A thermal junction included between the OLED and the cover layer is sized so that the OLED reaches a steady-state temperature within 10° C. of an ambient temperature at a luminance of 3000 cd/m2.
In accordance with another embodiment, there is provided a method of making an OLED device. The method includes the steps of: (a) forming an OLED on a substrate; and (b) disposing a cover layer over the substrate to form a thermal junction between opposing surfaces of the OLED and the cover layer. The thermal junction has a thermal resistance of 0.2 m2·K/W or less.
In accordance with another embodiment, there is provided an electrically powered 3000 cd/m2 light source with an encapsulated electroluminescent element that operates at a steady-state temperature within 1° C. of an ambient temperature.
Preferred exemplary embodiments will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:
As will become clear from the following disclosure, it is possible to construct an electrically powered light source with an electroluminescent element that can operate essentially at room temperature while providing a useful amount of light. Certain previously unrecognized characteristics of encapsulation structures can be used to manage the operating temperature of the encapsulated element. For example, a thermal junction may be formed within an OLED device and can be configured to have a thermal resistance sufficiently low to prevent thermal energy from being trapped within the device, thus lowering the operating temperature of the device and increasing its useful life.
Referring to
OLED 14 includes an organic layer 32 arranged between first and second electrodes 34, 36. Organic layer 32 is the electroluminescent element of the device 10 and includes one or more layers of organic material that together produce light when a voltage is applied via electrodes 34, 36. Various types of materials that can be powered to produce light at desired wavelengths are known and can be used to form the organic layer 32. Electrodes 34, 36 are electrically conductive, and one or both may be formed from a material that is at least partly transparent. Indium tin oxide (ITO) is one example of an electrode material that is sufficiently transparent to allow light produced in the organic layer 32 to pass through to be emitted by the device 10. For example, the first electrode 34, located between the substrate 12 and the organic layer 32, may be at least partly transparent to form a bottom-emitting OLED that emits light toward the substrate 12. In another example, the second electrode 36, located between the cover layer 16 and the organic layer 32, is at least partly transparent to form a top-emitting OLED that emits light toward the cover layer 16. Or both of the electrodes 34, 36 can be at least partly transparent to allow emission of light in both directions. In one embodiment, one of the electrodes 34, 36 is at least partly transparent and the other of the electrodes is opaque and/or reflective to absorb or reflect light produced in the organic layer 32. For example, the non-transparent electrode may be a layer of metal such as aluminum that is sufficiently conductive to provide voltage to the organic layer 32 and sufficiently reflective to direct light back through the transparent electrode.
The illustrated OLED 14 is only one example of an OLED and could include various other elements or layers of material. For example, the OLED may include reflective layers separate from the electrodes or intervening layers that are located between the electrodes 34, 36 and the substrate 12 or cover layer 16 of the assembled device. Each of the illustrated OLED components typically range in thickness from about 0.1 μm to about 1.0 μm but are not limited to this range. Each OLED layer or element can be deposited over the substrate 12 by known techniques including vapor deposition, vapor jet printing, vapor thermal evaporation, or other techniques. Each of the substrate 12 and the cover layer 16 are provided in any material sufficient to help encase the OLED and provide other optional characteristics, such as transparency, opacity, rigidity, flexibility, etc. In one embodiment, the substrate and/or the cover layer is constructed from a suitable glass material, such as borosilicate glass, which may be useful in applications where rigidity is desired. In another embodiment, the substrate and/or the cover layer is constructed from a polymer material, which may be useful in applications where flexibility is desired.
Certain characteristics of the thermal junction 18 can predictably affect the operating temperature of the OLED in the device 10. For example, the size and the material composition of the thermal junction 18 can be varied to affect the operating temperature. The thermal junction 18 has a thermal resistance across the junction that is equal to the distance across the junction divided by the thermal conductivity of the material at the junction:
where R is the thermal resistance, T is the distance across the junction, and λ is thermal conductivity. Thermal conductivity is a material property that is known for many materials. Thermal junction 18 may include a layer of such a material or it may include a combination of different materials so that the value for thermal conductivity is a composite or effective value.
In one embodiment, the thermal junction 18 is constructed to have a thermal resistance of 0.2 m2·K/W or less. A thermal resistance in this range can be achieved with countless combinations of material types and layer thicknesses. For example, the thermal junction 18 may include a layer of material between the opposing surfaces 24, 26 that is 2 mm in thickness and has a thermal conductivity of 0.1 W/m·K. As indicated by Equation 1, thermal resistance is proportional to the distance across the thermal junction 18 and inversely proportional to the thermal conductivity of the material at the junction 18. In one embodiment, the thermal junction 18 includes a layer of material layer having a thermal conductivity of 0.1 W/m·K or greater. In another embodiment, opposing surfaces 24, 26 are spaced apart by 2 mm or less at the thermal junction. Greater material layer thickness or lower thermal conductivity at the junction 18 is also possible in certain combinations.
Where distance T is greater than zero, the junction may include one or more layers of any type of material, whether a solid, liquid, gas, or a phase change material. In one particular embodiment, the thermal junction includes a non-liquid material, such as a gas or a solid, which in some cases is easier to handle in a manufacturing environment compared to liquids. For example, solid material layers maintain their shape during handling or can be deposited using similar equipment as used with the OLED layers. A gas material layer can be formed by assembling the cover layer 16 over the OLED 14 and substrate in a gaseous environment to entrap the gas between the OLED 14 and cover layer 16. Gaseous thermal junction layers can also provide transparency to make top-emitting OLEDs possible. In one embodiment, the non-liquid material is air. In another embodiment, the non-liquid material is an inert gas such as argon.
A recently developed transmission matrix method for calculating heat transfer characteristics was used to model various OLED device constructions to determine the degree to which each of the device layers contributes to the operating temperature of the device. This modeling confirmed that, with a gas layer such as air at the thermal junction, the distance T between the cover layer and the OLED has the greatest effect on heat transfer away from the OLED compared to other layer thicknesses. This modeling also indicates that with a thermal junction where T=0 or T is close to zero, the OLED can operate at or very near the ambient temperature outside the device.
Modeling was performed using device constructions layered as in
The simulated devices were operated at 114 W/m2 of input thermal power, which corresponds to a luminance of 3000 cd/m2, and the ambient temperature in which the simulated devices operated was 23.5° C. A steady-state temperature was determined for the OLED of each device according to a maximum temperature that each OLED asymptotically approached. As shown in
OLED devices were also modeled by adjusting the thickness of the other layers, but none of these adjustments had a substantial effect on the steady-state temperature. Practical thicknesses for the OLED layers are too thin to substantially affect the heat transfer properties of the device, while increased glass thickness merely increased the time required for the device to reach steady-state without altering the steady-state temperature.
It is to be understood that the foregoing description is of one or more embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to the disclosed embodiment(s) and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art.
As used in this specification and claims, the terms “e.g.,” “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.
