| Organic Light-Emitting Diode (OLED) panel |
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 The organic light-emitting diode (OLED) display panel has been attracting considerable attention as the next-generation display panel superceding the liquid crystal display (LCD) panel and plasma display panel (PDP). In fact, the OLED panel is already seeing application in small-sized display screens for cellular phones and handheld music players. With the further reduction in thickness compared to LCD and plasma display panels, and the self-luminous capability similar to PDP panels, as well as its higher contrast and enhance viewability, the OLED panel is thus becoming known as the "ultimate display panel".
This section introduces you to the basic structure and characteristics of OLED panels, and also the process of manufacturing them.
Structure of OLED panels
The term "EL" in OLED panels stands for "electroluminescence," which generally refers to the phenomenon when a material emits light in response to an applied voltage. The organic EL panel is based on the phenomenon called injection electroluminescence in which a material emits light based on the recombination of electron-positive hole pairs. Due to the similar light-emitting principle to that of an light-emitting diode (LED), the organic EL panel is also known as an Organic Light-Emitting Diode (OLED) panel. The organic electroluminescence (EL) or organic light-emitting diode (OLED) panel features an elemental structure based on a simple design in which an organic material is caused to emit light by electric current flowing through the material which is sandwiched between electrodes. Since the organic material itself emits light, the OLED panel provides clearer and sharper images compared to an LCD panel that utilizes backlighting for light emission, and the OLED panel allows for further reduction in thickness owing to the elimination of any backlighting components and structure.
Process of manufacturing OLED panel (Low-molecular type and High-polymer type)
The organic light-emitting diode (OLED) panel is composed of a light-emitting layer of organic material sandwiched between electrodes and other layers for increasing luminous efficiency. Among these components, the organic material plays an important role in governing the luminous efficiency and useful life of the display panel. The organic material, serving as a light-emitting layer, is available in two types according to the molecular weight of the organic compound used: low-molecular type and high-polymer type. The two types of organic material offer no distinct difference in the principle of light emission but involve some differences in terms of manufacturing the EL element.
The principle of light emission was discovered earlier in time for the low-molecular type of organic materials, which have been placed into practical application based on the earlier advanced development. The prevailing process of manufacturing is so-called"vacuum deposition method", in which a thin film layer of organic material is formed on the required areas by using a shadow mask combined with a vacuum deposition process.
In practice, however, this method requires higher temperatures for deposition to convert the low-molecular type organic material to a gaseous state, during which the metallic shadow mask will expand, creating a potential problem of an unevenly formed organic film layer. The temperature influence becomes even more significant for larger panel sizes (requiring larger shadow mask sizes). Therefore, at present, upsizing EL panels using low-molecular type organic materials is considered difficult to implement.
In the future, however, upsizing and production of larger scale panels is expected to become feasible due to advancement in the development of vacuum deposition systems.
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On the other hand, for the high-polymer type of organic materials, the mainstream manufacturing process of EL panels includes the ink jet method, in which the materials are applied in a manner similar to as used in ink jet printers associated with personal computers, taking advantage of the high degree of solubility of these organic materials in liquid. This ink jet method applies the organic materials to the areas requiring pixels, precisely allowing for micro-level control of the formation of the film layers. The method features higher efficiency of material consumption and is expected to be more suitable for upsizing and cost reduction compared to the methods used for the low-molecular type of organic materials.
We at Toshiba Matsushita Display Technology Co., Ltd. are equipped to utilize both manufacturing methods as deemed appropriate, depending on the OLED panel size needed.
Drive system for OLED panel (passive matrix type and active matrix type)
The drive system for an organic light-emitting diode (OLED) display panel is available in two types, similar to as found in the case of liquid crystal display (LCD) panels: passive matrix type and active matrix type.
In the passive matrix type OLED panel, current-carrying conductor wires are laid in a horizontal direction (X electrodes) and a vertical direction (Y electrodes), through which an electric current is fed with fixed timing causing the pixels at the intersections to illuminate. The passive matrix type, also known as simple matrix type, features a relatively simple structure allowing for panel production at lower costs, though it also has the disadvantage that the emission of light is limited to the period of time when the electric current is fed and thus a larger amount of current must be supplied to maintain a certain level of brightness. For these reasons, increased power consumption, shortened useful life of the organic material and other problems will result. This means that larger screen sizes and increased number of trace wires cause the duration of light emission from each pixel to become significantly reduced, thereby leading to problems, including the inability to assure required useful life. However, panels of smaller sizes and lower resolutions (smaller number of scanning lines) would provide an adequate degree of image quality even when based on the passive matrix type of drive system.
The active matrix type of OLED panel installs thin film transistors (TFTs) as switching elements at the intersections in the passive matrix type to control each individual EL pixel element. This active matrix type requires formation of complicated circuits, though it provides the advantages of allowing faster response time and higher resolution, and also of reducing drive voltages and energy consumption even in larger screen sizes compared to the passive matrix type, and moreover it provides for longer useful life.
TMD has been manufacturing active matrix type organic light-emitting diode panels based on our long-time developed and refined low-temperature poly-silicon (LTPS) technology. (See p-Si (low-temperature poly-silicon: LTPS) for details.)
Challenges and future development of OLED panels
Organic light-emitting diode (OLED) panels have the potential of becoming the "ultimate display panel" for their high definition, ultra-thin thickness, low power consumption, high contrast, wide viewing angle, and applicability to small- to large-sized screens. In practice, however, future mass production of OLED panels must combat issues concerning "useful life" and "cost".
Issues concerning useful life
The organic light-emitting diode is a self-luminous material capable of increasing the brightness in direct correlation to a larger amount of drive current, which, could decrease the useful life of the organic light-emitting diode. In the future, in order to achieve an adequate length of useful life for a wide range of applications including cellular phones and television sets, it is imperative to develop organic materials that provide higher luminous efficiencies that attain a satisfactory brightness with smaller amounts of electric current. As a high-technology manufacturer of display panels, TMD has successfully achieved higher luminous efficiency and longer useful life of OLED panels through the improvement of process design for the EL element structure and electrode materials, and also through the refinement of display driving technologies.
Issues concerning cost
The passive matrix type of OLED panel, which is proceeding toward practical utilization, may be manufactured at relatively low costs due to its simple structure. In contrast to this, the active matrix type, which allows higher resolution and larger screen sizes, involves complicated manufacturing processes for the formation of transistors at individual pixels, thereby increasing the costs at this present point in time. However, as the active matrix type of OLED panels become produced on a larger scale and with advanced development of manufacturing processes and organic materials, the issues associated with high costs will be overcome. TMD is involved in improving the structure of will be overcome. TMD is improving EL element and manufacturing processes and equipment aimed to achieve such cost reductions.
As these issues are resolved, practical utilization of organic light-emitting diode panels will be further expanded. In reality, due to the reduced thickness and size and the decreased power consumption, OLED panels are already being applied to the main display of cellular phones and handheld music players. With further achievement of high resolution, high contrast, upsizing and other features, then OLED panels would find application in mobile PCs, TV sets and other various fields. In addition, by taking advantage of their self-luminous feature, OLED panels may be applied for bendable displays (flexible displays), and therefore OLED panels are expected to become the next generation of flat panel displays.
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