The present invention relates to solid-state flat-panel OLED devices, and more particularly to temperature measurement of the OLED device.
Solid-state organic light emitting diode (OLED) image display devices are of great interest as a superior flat-panel digital display device and in solid-state lighting applications. These OLED devices utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material.
The OLED devices are not perfectly efficient and produce heat as a by-product of the energy conversion from current to light. As is well known, OLED materials degrade more rapidly in the presence of heat. Moreover, the thin-film transistors typically used to control OLED devices are sensitive to heat. Efficiency of the OLED materials can also be affected by temperature. Hence, there is a need to understand the temperature of an OLED device so that appropriate controls or corrections can be implemented to maximize the performance of an OLED device.
Thin-film temperature sensors are known in the prior art. For example, U.S. Pat. No. 6,774,883 entitled “Electro-optical display device with temperature detection and voltage correction” issued 20040810 describes a display device provided with a thin-film digital thermometer for sensing the temperature of the supporting plate of an electro-optical medium, for example, a liquid crystal display medium. U.S. Pat. No. 5,775,811 entitled “Temperature sensor system using a micro-crystalline semiconductor thin film” issued 19980707 describes a method for manufacturing a thin-film temperature-sensitive device. It is also known to apply such sensors to displays and LCDs. US20040150590 entitled “OLED display with aging compensation” by Cok et al published 20040805 describes an OLED display that includes a plurality of light emitting elements and a temperature measuring device located on the substrate.
While these methods are useful, the disclosures only describe measuring the temperature of a device at a single location. Applicants have demonstrated that OLED devices in operation can have a variable temperature across the substrate so that a measurement at a single location on the substrate may not correspond to the temperature elsewhere or to the average temperature. Indeed, given that the sensor is likely to be located at the periphery of the display, it is likely to be cooler than other locations on the display. Moreover, as described an additional circuit must be provided, reducing yields and potentially complicating the design layout of the OLED display.
There is a need therefore for an improved OLED device and method for the detection of temperature within an OLED device.
In accordance with one embodiment, the present invention is directed towards an OLED device, comprising: a) a substrate; b) one or more OLED element(s) formed on the substrate and having a common electrical connection for passing current through the OLED element(s); c) one or more transistor circuit(s) for controlling current passing through the OLED element(s); and d) a controller for measuring a current passing through the common electrical connection when the OLED element(s) and/or transistor circuit(s) are at an unknown temperature, for controlling the transistor circuit(s), and for comparing the current measured to an established current response determined under known OLED element(s) and/or transistor circuit(s) temperature conditions to determine the temperature of the OLED element(s) and/or transistor circuit(s).
In accordance with another embodiment, the present invention is directed towards a method for the detection of temperature of an OLED device, comprising: a) providing an OLED device by forming one or more OLED element(s) on a substrate and one or more transistor circuit(s) for controlling current passing through the OLED element(s), the OLED element(s) having a common electrical connection for passing current through the OLED element(s); b) providing a controller for measuring the current passing through the common electrical connection and for controlling the transistor circuit(s); c) driving the OLED element(s) with a known drive signal, and measuring a current passing through the common electrical connection; and d) comparing the measured current to an established OLED device current response determined under known OLED element(s) and/or transistor circuit(s) temperature conditions to determine the temperature of the OLED device.
The advantages of this invention are an OLED device with improved temperature detection and simplified construction.
Referring to
The OLED device 10 may have one or more OLED elements 14 and the transistor circuit 18 may be integrated on the substrate 12 locally to an OLED element 14 to form a pixel 15 of an active-matrix OLED device. In this case, a plurality of pixels 15 may be distributed regularly over the surface of the substrate 12 as shown in
Silicon transistors and circuit elements are known to behave differently at different temperatures. Likewise, the behavior of an OLED element changes at different temperatures, although much less significantly. Applicants have determined that, for a given control signal, when the transistor circuit(s) 18 and OLED element(s) 14 are operated at different temperatures a consistent, proportional and measurable change in current passing through the OLED results and may be measured through a common electrical connection to the OLED element(s), for example power signal-26 or signal 16. Silicon devices including polysilicon, amorphous silicon, continuous grain silicon, micro-crystalline silicon, or crystalline silicon circuits may respond in this way. Thin-film devices are known and may be employed for transistor circuit 18.
In a particular embodiment, the OLED device may comprise a display device, and the plurality of OLED elements 14 may define or be part of a display area. The present invention has the great advantage of not requiring any additional circuitry on the substrate 12 and of integrating the response due to temperature over the entire display area. The common electrical connections of the present invention may be the CV ground signal 16 typically connected to the cathode of the OLED element(s), or Vdd signal 26 that provides power to the OLED element(s) through drive transistor 28. Although signal 26 is not directly connected to OLED element(s) 14, it is considered a common electrical connection for the OLED element(s) 14 according to the present invention as it supplies power to all of the OLED elements in common. All these electrical connections are present in conventional OLED devices. The only additional circuitry required is a measurement circuit 22 for measuring the current used by the OLED element(s). Such a measurement circuit is readily integrated into a conventional controller using conventional designs and manufacturing processes known in the art. The construction of an OLED device is also known in the art. The present invention may be applied to a variety of transistor circuit designs, including both constant current source and constant voltage source transistor circuits.
According to the present invention, a method for the detection of temperature of an OLED device may include the steps of providing an OLED device 10 by forming one or more OLED element(s) 14 on a substrate 12 and one or more transistor circuit(s) 18 for controlling current passing through the OLED element(s) 14 having a common electrical connection 16 (and/or 26) for passing current through the OLED element(s) 14; providing a controller 20 for measuring the current passing through the common electrical connection and for controlling the transistor circuit(s) 18; driving the OLED element(s) 14 with a known drive signal, and measuring a current passing through the common electrical connection; and comparing the measured current to an established OLED device current response determined under known OLED element(s) and/or transistor circuit(s) temperature conditions to determine the temperature of the OLED device 10.
In operation, the OLED device 10 may be first calibrated to establish an OLED device current response by operating the device in a controlled environment with a pre-determined known test signal where the temperature of the device is directly measured with a temperature sensor (e.g., a thermocouple secured directly to the device). The device will output light in accordance with the known test signal, and will consume a varying amount of current dependent upon the device temperature. By changing the temperature of the controlled environment while monitoring the actual temperature of the device and measuring the current consumed, a current response for a variety of device temperatures may be established. The OLED device 10 may then be used for an application at an unknown device temperature. The OLED device is operated with the known test signal and a current measured. This measured current may be different from the current measured during calibration because of the effect of the temperature on the OLED element(s) 14 and transistor circuit(s) 18. The measured currents are compared in accordance with the established current response to determine the unknown temperature of the OLED device 10. The current measured by the current measuring device 22 will include all of the current from all of the OLED element(s) having a common electrical connection 16. Typically, all of the OLED elements on the OLED device are connected in common and are spread across most of the device substrate, thereby providing a great deal of current and a sensitive means to detect the average temperature of the OLED device.
As shown in
Although the current measurement used to determine device temperature must be made with a known drive signal, the signal may be very short and may not be noticeable to a user. For example, a single frame of a video signal ( 1/30 or 1/60 seconds) may be employed. Moreover, the known signal may be dark and gray so as to be as unobtrusive as possible. The signal may be a flat-field or may be colored. The signal may be a part of a user interface, for example a start-up or shut-down screen, or may have icons that represent marketing information such as advertisements or corporate logos. Alternatively, if the overall characteristics of a drive signal over time are known (but a specific, instantaneous signal is unknown) the current may be measured as an average over time for such drive signal. In this case, no special test signal need be employed. This may be effective if the OLED device is employed to view video sequences. The average color and brightness of a video or sequence of still images is typically an 18% gray. If it is known that the drive signal is a video or still image sequence, such known average brightness and content (over time) for such signal may be used as an estimate, and a repeatable current measurement over time may be made. In yet another alternative, a specific known test signal may be employed by the device as part of its user interface, for example as part of a start-up or shut-down process splash screen or logo. In these cases, no additional known test signal need be employed.
Referring to
The construction of a lookup table may be performed as part of a factory calibration of an OLED device and may include determining the entire current response to a variety of temperatures by using multiple measurements or a single point on the curve determined and a typical curve employed to create the table. Alternatively, if the variability in OLED device performance is small enough to meet the requirements of a particular application, the established OLED device current response may be determined on a single device and the associated conversion table may be used for all subsequent similar devices. In this case, a functional transformation (possibly implemented with a lookup table) using the pre-established response may be provided in the controllers shipped with the OLED devices. Therefore, the calibration of the established OLED device current response to temperature may be measured for each device or for an exemplary device. Complete response curves may be determined for each OLED device or family of OLED devices, or single values used and typical performance curves employed.
Applicants have also determined that the ambient illumination incident on an OLED device can affect the current and brightness of an OLED device. This effect can be discounted by ensuring that the temperature is measured in a dark surround and/or by covering the OLED device to prevent the incidence of ambient light upon the transistor circuit(s) of the OLED device while driving the OLED element(s) with the known drive signal and measuring the current passing through the common electrical connection. Note that color filters, electrodes, and black matrix materials sometimes used with OLED devices may also be effective at obscuring ambient light.
It is important to note that OLED devices exhibit a significant amount of self-heating. Temperature readings taken at the same location within a few minutes of each other can be very different depending on the operation of the OLED device. This effect can be used to distinguish the ambient temperature and the operating temperature. For example, an initial reading taken shortly after a device is turned on, before the OLED device has been emitting large quantities of light, and using a dark known control signal can be presumed to be operating at ambient temperature, while a reading taken after the OLED device has been operating normally for some time may be presumed to represent the operating temperature of the device which may be significantly different from the surrounding ambient environment.
The transistor circuit can be manufactured using thin-film technology using silicon or organic semiconductors as is known in the art. The present invention may be used in both top- and bottom- emitting OLED structures. The use of top-emitting OLED devices may be particularly applicable to the present invention, as such devices typically employ opaque bottom electrodes which shield the transistor circuitry from ambient light, thereby minimizing the effect of ambient light on current measurements. While the invention is described above primarily in connection with OLED devices comprising a plurality of pixels, in an extreme case, an OLED device, e.g., a lamp, may comprise a single pixel, and the present invention is applicable thereto.
As noted above, OLED materials degrade more rapidly in the presence of heat, and the thin-film transistors typically used to control OLED devices are sensitive to heat. The present invention enables monitoring of the temperature of an OLED device so that appropriate controls or corrections can be implemented to maximize the performance of an OLED device. In a particular embodiment, aging of the OLED elements during their lifetimes may be conveniently estimated in response to the OLED device temperatures determined in accordance with the present invention.
In a preferred embodiment, the invention is employed in a device that includes Organic Light Emitting Diodes (OLEDs) which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al. Many combinations and variations of organic light emitting displays can be used to fabricate such a device.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.