OLED, or Organic Light-Emitting Diode, is a revolutionary display technology that doesn’t require a backlight. It offers high contrast, thin design, wide viewing angles, fast response times, and flexible panels, making it ideal for a variety of applications. With its excellent structural and process characteristics, OLED is considered one of the most promising next-generation flat panel display technologies. As a result, numerous manufacturers worldwide are investing heavily in R&D. In China’s mobile phone market, OLED products are already being adopted. These include 10 monochrome models, 15 regional color models, 8 models with 256 colors, and 3 full-color models (see Table 1). Additionally, domestic mobile phone design companies are currently developing 7 OLED models, along with SKD/CKD products and international branded options.
It is expected that by the end of this year, over 50 OLED mobile phones will be available in China’s market, and they are gaining popularity (see Table 2). According to data from Table 3, OLED is anticipated to compete with STN-LCD and TFT-LCD technologies in the future. This article aims to provide an overview of OLED technology, starting from its history to its working principle, types, and colorization methods.
**First, the history of OLED development**
OLEDs can be broadly categorized into two types: small molecule systems and polymer-based systems. The small molecule type, known as OLED, was first developed by Eastman Kodak in 1987. Dr. C.W. Tang, who was born in Hong Kong and graduated from National Taiwan University, played a key role in this breakthrough. On the other hand, the polymer-based system, referred to as PLED (Polymer Light-Emitting Diode) or LEP (Light-Emitting Polymer Device), was introduced by the University of Cambridge in 1990. By 1992, Cambridge established a company called CDT (Cambridge Display Technology) to commercialize PLED technology.
**Second, the principle of OLED lighting**
The basic principle of OLED illumination is similar to that of LEDs. When an external voltage is applied, holes and electrons move from the positive and negative electrodes toward the organic light-emitting layer, where they combine to produce light. The anode is typically made of ITO (indium tin oxide), while the cathode may consist of metals like Mg, Al, or Li. The structure is shown in Figure 4. The color emitted by the OLED depends on the material used in the light-emitting layer. Manufacturers can adjust the material to achieve different colors. In essence, OLED lighting occurs when electrons and light interact to generate luminescence. Depending on the energy levels of the electrons and holes, light of different wavelengths—i.e., different colors—is produced.
**Third, OLED vs. Polymer OLED (P-OLED)**
OLED is a self-luminous material that does not require a backlight. It offers wide viewing angles, uniform image quality, fast response times, easy color customization, and a simple driving circuit. Its structure is lightweight and thin, making it suitable for small and medium-sized panels. However, OLED requires a high driving voltage, which reduces energy efficiency. In contrast, PLED (Polymer OLED) does not need a vacuum process or complex thin-film equipment, resulting in a simpler two-layer structure and lower investment costs. However, PLED has inferior color performance compared to OLED, with varying decay constants and color deviations that require compensation. It also has a broader bandwidth, making color adjustment more difficult, which affects product lifespan. Currently, PLED is mainly used in large-sized panels, while OLED focuses on high-value, high-priced applications.
**Fourth, Passive OLED vs. Active OLED**
OLEDs can be classified into passive matrix (PM-OLED) and active matrix (AM-OLED) based on their driving method. PM-OLED uses current-driven operation, and its structure is simpler, making it suitable for small displays with good resolution and image quality. However, for larger screens, power consumption increases and lifespan decreases. AM-OLED, on the other hand, offers better current control, reduced leakage, and is more suitable for large-size OLEDs. When combined with low-temperature polysilicon (LTPS) TFT technology, AM-OLED can achieve low-profile, high-performance displays, meeting the demands of large-screen applications.
**Fifth, OLED colorization techniques**
There are three main methods of achieving color in OLEDs: RGB three-color light-emitting structure, color conversion structure (white light + color filter), and color filter film (blue light + color conversion layer). The RGB structure uses separate red, green, and blue materials, offering high efficiency without the need for additional filters or layers. This is the most commonly used method today. However, the shadow mask evaporation technique used in RGB processing results in lower resolution. The color conversion method involves using a blue light-emitting material with a thin film, leading to lower efficiency. The color filter method uses white light with a color filter, also resulting in lower efficiency. Currently, only a few manufacturers have successfully implemented white light technology.
Consumer Electronics
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