Since their discovery more than 50 years ago, organic light-emitting diodes (OLEDs) have been recognized as having significant advantages, especially in display technology applications. OLEDs are fast becoming a viable option for the next generation of energy-efficient lighting. After a high-efficiency bilayer OLED technology was discovered in the mid-1980s, research on its device structure became more intensive [1-18]. In the late 1990s, OLED materials have been commercialized, which shows that OLED technology is considered as a possible next-generation lighting technology 1-281 Since 2000, technology development on OLED lighting has continued to increase.
To a large extent, traditional lighting companies such as Philips, Osram, General Electric (GE) and Panasonic, as well as technology providers such as General Display, Cambridge Display Technologies, Kodak and Nov ale d push their proprietary technologies outside the display field . Advances in science and technology and improvements in product development have led to an explosion in the number of patent applications and scientific and technical publications related to OLED lighting.
OLED-based solid-state lighting technology is an alternative to traditional incandescent and fluorescent lighting, which can greatly improve power efficiency, color quality, and prolong life at lower cost and less environmental pollution. The introduction of OLED lighting sources into the field of general lighting will solve problems such as environmental pollution, because it does not contain toxic substances or gases and reduces energy consumption, the wide application of OLEDs will likely be an example of a major transformation in the lighting industry.
- Reasons why white OLEDs have attracted much attention
Compared to solid-state inorganic light-emitting diodes (LEDs), which are composed of point light sources, OLEDs can be used for low-power large-area light sources or general lighting due to their revolutionary properties of being ultra-thin, flat, transparent, lightweight, and flexible. . Compared with traditional lighting sources, OLED can save 70% of energy, which means that OLED lighting can also help reduce energy consumption. It is therefore no surprise that OLEDs are increasingly seen as a prime candidate for next-generation lighting.
Recently, the European Union has set a goal of reducing CO emissions by at least 20% by 2020. Research by the European Union’s Joint Research Center (JRC) shows that there is great potential to achieve energy savings through higher energy efficiency. In this context, The lighting industry has gradually begun to see the replacement of traditional light sources with OLEDs as a promising option.
In the future, the development of a 6x6in OLED product with a light efficiency of 45lm/W (LPW), a working brightness of 1000cd/m2 and a life expectancy of more than 10000h will be widely commercialized and accepted. The device must have high illumination quality, that is, its color coordinate must be located near the blackbody locus (<0.01CIE unit), the color temperature range is between 2700 ~ 6500K, and the color rendering index (CRI) >90, therefore, the researchers’ future research The goal and direction is to develop and design new structures and materials to make white OLEDs at least 3 times more energy efficient than current designs.
Ultimately, the design should promise OLEDs with greater than 100 LPW efficacy and outstanding color and spectral performance at lower manufacturing costs. Achieving this ultimate goal (> 100LPW) requires a combination of physical control associated with device vapor deposition growth techniques, flexible material design, and possibly a wide range of processing conditions using polymeric materials.
- OLED light-emitting principle
An OLED is a heterostructure consisting of an organic light-emitting layer (EML) and an electron transport layer (ETL), sandwiched between an anode and a cathode. The anode is typically a transparent steel tin oxide (ITO), and the cathode is typically calcium, magnesium, or lithium-aluminum fluoride with low work function (see Figure 1). When a voltage is applied to the transparent electrode and the current passes through the organic layer, electrons and holes recombine and emit light in the light-emitting layer. Figure 2 shows the working principle of a multilayer OLED. Depending on the material selected for the organic light-emitting layer, OLEDs can emit light of various colors, including white light of various color temperatures. Combined with the advantages of high light efficiency and long life, OLEDs are generally considered to be the best choice for architectural lighting.
As a next-generation lighting application, the basic requirements for OLEDs must be high brightness, low cost and low power consumption. To achieve these functions, it can be achieved by selecting efficient driving circuits, reasonable and efficient device structures, and solid-state packaging and other technical processes.
As early as 1982, Tang and Van Slyke showed that the performance of bilayer devices by simply inserting a hole transport layer (HTL) into a single-layer device structure with poor performance at the early stage can be greatly improved . Around the same time, the Kodak Group’s research group also realized organic light-emitting devices with high power conversion efficiency by doping the light-emitting layer. Subsequently, the improvement is achieved by inserting a buffer layer between the anode and the hole transport layer, electron transport layer, hole blocking layer (HBL) in the heterostructure, or by inserting a multilayer structure such as a sandwich between the cathode and the electron transport layer. Device performance [2, 3], this multilayer structure generally leads to an increase in the driving voltage of OLEDs. Generally, the working voltage of high-brightness OLEDs is much higher than the thermodynamic limit, such as 2.4eV for green light devices, which can be significantly reduced by chemical doping of electron donors (electron transport materials) and electron acceptors (hole transport materials). voltages on these thin film layers.
Devices with multilayer structures with doped electron transport layers or hole transport layers have good device performance, but their operating voltages are still higher than their thermodynamic limits. After that, Leo and his research group proposed the concept of p-type doped hole transport layer and n-type doped electron transport layer [2-31]. These pin-structured devices have the advantages of high brightness and high efficiency at very low operating voltages. Indeed, all these devices with multilayer structures have the advantages of high current and high power efficiency, but the light-emitting layer is very thin. However, the thin light-emitting layers of pin PHOLEDs and the complex design structure of phosphorescent OLEDs do not hold a good fabrication prospect.
So far, the lighting application of OLED is hindered by two main factors: 0 Compared with the traditional high-brightness LED’s 20000~50000h lifespan, the OLED lifespan is relatively limited [calculated at 150 (50% light decay), it is about 5000h] ;@ Its light output is much lower than high brightness LEDs (roughly 251m vs. 100~1501m above). However, unlike traditional LEDs, OLEDs have certain advantages in future lighting technologies due to their unique physical properties, and thus will have a place in the next-generation solid-state lighting technology field.