PHOLED (Phosphorescent Organic Light Emitting Device) Display Technology-EEWORLD

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PHOLED (Phosphorescent Organic Light Emitting Device) is a type of organic light emitting diode (OLED) that can be up to 4 times more efficient than conventional fluorescent OLEDs. OLEDs are monolithic solid-state devices that are typically composed of a series of organic thin films sandwiched between two thin-film conductive electrodes. If power is applied to the OLED, conductive carriers (holes and electrons) are injected from the electrodes into the organic thin films. These carriers then migrate within the device under the influence of the electric field until they recombine to form excitons. Once formed, these excitons, or excited states, drop to lower energy levels by emitting light and/or generating heat.

With conventional fluorescence emission, only about 25% of the exciton energy is converted into light, while the remaining 75% is lost as heat. By using certain phosphorescent materials, UDC's 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

The discovery is a major breakthrough. It means that OLEDs can be four times more efficient than previously possible - putting them on par with LCDs today and, in the future, incandescent and fluorescent lighting.

Features and Performance
Table 1 highlights the excellent device performance data for several of our commercial PHOLED materials.
In addition to the colors in the table, there are a number of “developmental” red, green, orange, yellow, and blue emitting systems that also exhibit excellent performance.

Record-breaking energy efficiency
PHOLED technology and superior PHOLED materials are essential to achieving low power OLED displays and lighting. PHOLED's external quantum efficiency and luminous efficiency are four times that of fluorescent OLED materials, thereby reducing heat generation and increasing the choice of AMOLED backplanes. This provides an important advantage that enables OLEDs to compete with LCDs and traditional light sources. PHOLEDs can also operate at very low voltages, which further enhances their low power consumption characteristics.

Vivid colors
PHOLED colors can be deep red, bright green or even bright blue. As development continues, the variety of PHOLED colors is constantly increasing.

Long operating life
The operating life of PHOLED materials has made great progress in the past few years. Red materials are now used in a variety of commercial products, providing extremely long operating life under a wide range of operating conditions. UDC's green materials are undergoing commercial evaluation, and blue materials are continuing to improve towards commercialization.

Thermal stability for production
Many PHOLED materials have been tested on commercial production equipment, demonstrating the long cycle times required for high-volume production.

The versatile
PHOLED technology and materials produced can well meet the needs of a variety of production processes. Currently, PHOLED materials are usually 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/patterning processes, including the inkjet printing process currently under development.

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

The importance
of UniversalPHOLED technology in manufacturing OLEDs has been recognized by the industry, with OLEDs being particularly well suited for battery-powered mobile display applications. However, although less obvious, it also has the advantage of being able to meet large-area TV and lighting needs from a wall socket. To demonstrate this advantage, we modeled the power consumption of active-matrix OLEDs (AMOLEDs) using PHOLED and fluorescent OLED (FL-OLED) technologies.

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

Large-area OLED TVs can also achieve similar power savings. Although these applications can usually be powered by the mains, there is a growing demand for improving the energy efficiency of TVs through the U.S. Department of Energy's Energy Star program and other programs. In addition, improving energy efficiency is also the central idea of DOE's launch of solid-state lighting. To this end, PHOLED technology also realizes the possibility of high-efficiency white light illumination - creating new opportunities for the use of OLEDs in lighting applications.

Less noticeable temperature rise
Displays and lighting generally experience a temperature rise when operating, because electrical energy that is not converted into light energy is converted into heat. This temperature rise becomes particularly noticeable in large-size OLED TVs or lighting. PHOLED technology can significantly reduce this temperature rise. For example, the temperature rise in FL-OLED is about 30°C, while PHOLED technology reduces this value to 10-17°C (assuming a 40-inch diagonal AMOLED). Reducing the temperature rise is very important. It can extend the life of the OLED, because the speed of aging is related to temperature. It also relieves the pressure on air conditioning required to transfer the generated heat - this makes PHOLED technology an important element in any "green" or environmental protection strategy.

Backplane compatibility
Currently, amorphous silicon (a-Si) backplane technology is the mainstream technology, which has a mature and low-cost production base. Low-temperature polysilicon (LTPS) is a relatively new technology, which uses a more complex process and has a lower yield rate than a-Si. However, LTPS has higher performance - it provides higher carrier mobility, so the drive 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 high-current drive conditions of OLED. Before the advent of PHOLED, this idea was correct. 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 in whether a-Si backplane can be used in large-area displays. In the future, PHOLED may also promote the industry's adoption of low-cost organic TFTs.

P2OLED
P2OLED printable, phosphorescent OLED materials and technologies are advancing the integration of efficient PHOLED technology with low-cost printing equipment, such as inkjet printing, which has the potential to provide a lower-cost solution for large-area OLED displays. Significant progress has been made in the development of P2OLED printable, phosphorescent OLED materials using solutions-based manufacturing processes. These advances are due in part to the production joint development program, according to a joint paper presented by Seiko Epson Corporation (Epson) at the 2008 Society for Information Display Symposium in Los Angeles. Along with the development of high-efficiency PHOLED technology, the team has made significant progress in extending the operating life of red and green P2OLED materials in spin-coated, bottom-emitting devices by using a complete set of UDC-developed OLED materials, as shown in Table 2.