PHOLED (phosphorescent organic light emitting device) display technology

The birth of PHOLED
PHOLED (Phosphorescent Organic Light Emitting Device) is a type of organic light emitting diode (OLED), and its efficiency can be as high as 4 times that of traditional fluorescent OLED. OLED is a monolithic solid-state device, usually composed of a series of organic thin films sandwiched by two thin-film conductive electrodes. If power is applied to the OLED, conductive carriers (holes and electrons) will be injected from the electrode into the organic thin film. Then, under the action of an electric field, these carriers migrate inside the device until they recombine to form excitons. Once formed, these excitons or excited states will fall to a lower energy level by means of light emission and / or heat generation.

With traditional fluorescence emission, only about 25% of the exciton energy will be converted into light, while the remaining 75% will be lost in the form of heat. By using certain phosphorescent materials, UDC partners at Princeton University and the University of Southern California discovered in the late 1990s that up to 100% of the exciton energy can be converted into light.

Table 1 Device performance data of Universal PHOLED materials

This discovery is a major breakthrough. This means that the efficiency of OLED can reach 4 times the efficiency that can be achieved in the past-this makes the current OLED comparable to LCD, and in the future can also compete with incandescent lighting and fluorescent lighting.

Features and performance Table 1 highlights the excellent device performance data of several of our commercial PHOLED materials.
In addition to the colors in the table, there are many red, green, orange, yellow, and blue lighting systems in the "development stage", which also have excellent performance.

Record-breaking energy efficiency
PHOLED technology and excellent PHOLED materials are essential to achieve low-power OLED display and lighting. PHOLED's external quantum efficiency and luminous efficiency are four times that of fluorescent OLED materials, so it also reduces the heat generated and increases the choice of AMOLED backplanes. This provides an important advantage that enables OLEDs to compete with LCDs and traditional light sources. PHOLED can also work at very low voltage, which further enhances their low power consumption characteristics.

Bright colors
PHOLED color can be dark red, bright green or even bright blue. As development continues, PHOLED color types continue to increase.

Long working life
The working life of PHOLED materials has made great progress in the past few years. Now, the red material has been used in a variety of commercial products, providing a long working life under various working conditions. UDC's green materials are being evaluated for commercial use, while blue materials are continuing to improve towards commercial targets.

Thermal stability for production Many PHOLED materials have been tested on commercial production equipment, demonstrating the long cycle time required for mass production.

Versatility in production
PHOLED technology and materials can well meet the needs of various production processes. Currently, PHOLED materials are commonly used in vacuum thermal evaporation (VTE) equipment, but it can also be used for OVPD organic vapor deposition, laser transfer (LITI) and other new deposition / pattern forming processes, including inkjets currently under development Printing process.

Environmental friendly
HOLED may be a very good "green" display and lighting solution. PHOLED reduces the demand for electrical energy and non-renewable fossil fuels by increasing energy efficiency. Less energy consumption also means less environmental pressure. At the same time, the OLED adopts a thin-film structure and has a small volume, which can significantly reduce the waste and its removal problems that are common in CRT and fluorescent lamps.

Power consumption
The importance of UniversalPHOLED technology in the manufacture of OLEDs has been recognized by the industry, and OLEDs are particularly suitable for battery-powered mobile display applications. However, although it is not obvious, it also has the advantage of being able to meet the needs of large-area TVs and lighting through wall sockets. To demonstrate this advantage, we modeled the power consumption of active matrix OLED (AMOLED) using PHOLED and fluorescent OLED (FL-OLED) technologies.

The luminous efficiency of PHOLED is higher than that of FL-OLED, up to 4 times of the latter. This means that to achieve the same pixel brightness as FL-OLED, PHOLED only needs a much smaller current. In AMOLED, PHOLED can reduce power consumption by both OLED and thin film transistor (TFT) backplane.
Consider the following example: In full-color AMOLED, 30% of the pixels are lit. The luminous efficiency of PHOLED that we can currently achieve can save 50% of power consumption compared with FL-OLED, and it is similar to current LCD Than can save 40% power consumption. Through further optimization of PHOLED, this advantage will be further expanded.

Large-area OLED TVs can also save similar power consumption. Although these applications can usually be powered by city power, the need to increase the energy efficiency of televisions through the US Department of Energy's Energy Star program and other programs is also increasing. In addition, improving energy efficiency is also the central idea of ​​DOE starting solid-state lighting. For this reason, PHOLED technology has also realized the possibility of high-efficiency white light illumination-creating new opportunities for the use of OLEDs in lighting applications.

The temperature increase is not obvious because the electrical energy that has not been converted into light energy will be converted into heat, so the display and lighting usually have a temperature increase phenomenon when working. In large-size OLED TVs or lighting, this increase in temperature becomes particularly noticeable. PHOLED technology can significantly reduce this heating. For example, the temperature in FL-OLED will increase by about 30 ℃, and PHOLED technology will reduce this value to 10 ~ 17 ℃ (assuming that the diagonal is 40 inches AMOLED). It is very important to lower the value of temperature rise. It can extend the life of OLED, because the speed of aging is related to temperature. It also relieves the pressure of air conditioning needed to transfer the generated heat-this makes PHOLED technology an important element in all "green" or environmental protection strategies.

Backplane compatibility At present, amorphous silicon (a-Si) backplane technology is the mainstream technology, which has a mature, low-cost production basis. Low temperature polysilicon (LTPS) is a relatively new technology. It uses a more complex process and the yield is lower than a-Si. However, the performance of LTPS is higher-it provides higher carrier mobility, so the driver circuit can be directly integrated on the substrate to reduce costs, especially for small area displays. In the past, people also believed that the higher mobility provided by LTPS was necessary to meet the OLED high-current driving conditions. Before PHOLED came out, this idea was right. The low current drive of PHOLED reduces the power consumption of the TFT backplane, thereby reducing the need for mobility. Therefore, PHOLED technology has become a key factor for whether a-Si backplane can be applied in large-area display. In the future, PHOLED may also facilitate the adoption of low-cost organic TFTs in the industry.

P2OLED
P2OLED printable, phosphorescent OLED materials and technology are promoting the combination of efficient PHOLED technology and low-cost printing equipment. Such as inkjet printing, it is possible to provide a lower-cost solution for large-area OLED displays. In the use and solution-based manufacturing process, P2OLED can be printed, and phosphorescent OLED material development has made significant progress. Seiko Epson Co., Ltd. (Epson) believed in a joint document published at the 2008 Academic Seminar of the Information Display Society in Los Angeles that these developments benefited in part from the joint production development plan. With the application and development of high-efficiency PHOLED technology, through the use of a complete set of UDC-developed OLED materials, the team has made significant progress in extending the working life of the red and green materials P2OLED in spin-coating and bottom-emitting devices ,As shown in table 2.

Table 2 Using a complete set of UDC OLED materials, the change of working life at room temperature

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