Organic electroluminescent devices ( OLEDs ) have all-solid-state, self-illuminating, low operating voltage, low power consumption, and can be used for flexible substrates. They can be used as backlights for flat panel displays (such as liquid crystal displays) or as planar illumination sources. More and more attention from the majority of scientific research workers. Following the inorganic light-emitting diode (LED), OLED lighting technology has become a research hotspot and focus in the field of semiconductor lighting. Compared with LED, OLED has advantages in lightness, flexibility, eye protection, etc. It is especially suitable for indoor large-area illumination, and has attractive application prospects in the future lighting field. In addition, white OLED combined with filter technology can achieve full color display. After nearly two decades of development, the performance and theoretical research of white OLED devices have made great progress, which is close to the luminous efficiency of fluorescent lamps, showing its huge application prospects, and is considered to be the most promising new generation of semiconductor illumination sources.
Advantages of white OLED lighting
Compared with other types of artificial light source technology, OLED technology has unique advantages and is the best illumination source so far. The biggest feature of OLED lighting is that the light source itself is surface-emitting. Existing lighting, including LED lighting, uses point and line sources to illuminate the space. When the surface illumination is required, a plurality of point light sources and line light sources are always arranged together, and a panel-shaped lamp cover is covered on the outside. The use of white OLED technology enables direct illumination of the entire surface to produce the best range of uniform illumination. In addition, the white light OLED is fabricated on a flexible substrate, which can realize a curved light source, can be flexed, and has the characteristics of being not easily broken, which will make the lighting products and application technologies new and beyond the existing imagination.
As people pay more and more attention to environmental protection, incandescent lamps and fluorescent lamps are gradually being replaced, LED lighting sources have become the main force, and OLED lighting is expected to become a new lighting technology due to its unique advantages. According to the estimation of Prof. Koji Yuji of the Institute of Science and Technology of Yamagata University in Japan, OLED lighting is expected to reduce 6.7 million tons and about 2.3% of carbon dioxide emissions by 2020. Since 2000, the US Department of Energy has invested $30 million annually in the development of OLED lighting technology. Philips, Philips, Osram and General Electric (GE), the world's top three lighting manufacturers, are also involved in OLED lighting applications. In 2010, Lumiotech also launched OLED lighting products. China also has Visionox, Nanjing Dijon Organic Luminescence, and Beijing BOE to actively invest in the R&D and industrialization of OLED lighting panels.
Structural classification of white OLED devices
Figure 1 1931CIE x, y color coordinate chart
According to the principle of colorimetry (as shown in Figure 1), white light is often achieved by mixing two complementary colors (such as blue and yellow) or three primary colors (red, green, and blue). As long as the connection of the color coordinates of the two complementary colors can pass through the white light region, or the triangle formed by the connection of the color coordinates of the three colors includes the white light region, then by adjusting the luminous intensity of the various colors, white light can be obtained by reasonable superposition. In addition, white light can also be obtained by a single compound. However, at present, such compounds are relatively few, and the prepared device has generally low luminance and luminous efficiency. White light for illumination should have a good color rendering index (Ra > 80) and a good color coordinate position, close to the (0. 33, 0.33) point of the International Lighting Association's chromaticity diagram. In order to obtain high efficiency, stable performance of white OLED, researchers have done a lot of research in material selection, device structure design and so on. Below we will classify white OLED devices from different perspectives.
White light OLED device classification based on luminescent materials
From the perspective of luminescent materials, white OLED devices can be classified into ordinary pure fluorescent devices, pure phosphorescent devices, fluorescent phosphor hybrid devices, and thermally activated delayed fluorescence (TADF). Devices, as well as excimer or exciplex devices.
1) Ordinary pure fluorescent device. Organic fluorescent materials are characterized by good stability and long device life. The first reported fluorescent white OLED device is the device structure of the double-emitting layer of Kodak Company in the United States, which is doped with a yellow fluorescent material (such as a derivative of rubrene) into a hole transport layer (such as NPB), plus high-efficiency sky blue light. Light-emitting layer. For devices made of ordinary pure fluorescent materials, only 1/4 of singlet excitons can be converted into photons, the upper limit of internal quantum efficiency can only reach 25%, and the luminous power is hard to exceed 20 lm /W, which can not meet the display and illumination. Practical application.
2) Pure phosphorescence devices. Phosphorescent materials have a theoretical value of quantum efficiency of 100% due to the heavy atom coupling effect, and the luminous efficiency can be four times that of the fluorescent material only OLED. In 2008, Professor Kido's research team published a light-colored stable white light device. An ultra-thin orange light-emitting layer of 0.25 nm on both sides is interposed between the two blue light-emitting layers, and the efficiency is 44 lm /W at a luminance of 1000 cd / m 2 , and the color coordinates are (0. 335, 0). 396) Efficient and stable luminescent emission [7]. The emergence of phosphorescent materials makes it possible to achieve efficient OLED technology.
3) Fluorescent phosphorescent hybrid (white) OLED devices. The lifetime of the fluorescent material is better, but the efficiency is lower; the phosphorescent material can achieve higher internal quantum efficiency, but the current life of the blue phosphorescent material is lower, which becomes a bottleneck for the performance improvement of the white OLED. The all-phosphorescent white OLED composed of red, green and blue tri-color pure phosphorescent materials has not been solved due to the stability of the blue phosphorescent material. After a period of time, the light color of the device will be reddish and the lifetime of the device will be affected.
Considering that the lifetime of the fluorescent blue light material has completely met the requirements of illumination use, a white light OLED with good efficiency, longevity and stability can be obtained by using a blue fluorescent material combined with a red, green or yellow phosphorescent material to form a hybrid system. Fluorescent/phosphorescent hybrid illumination is considered to be the best way to achieve OLED illumination applications. Hybrid white OLEDs have become a hot spot in organic lighting research.
In the early days, people directly superimposed the phosphor layer and the phosphor layer. Although white light emission can be obtained, the efficiency is relatively low, because the triplet energy level of the fluorescent material is usually lower than that of the phosphorescence, and the fluorescence directly contacts the phosphor layer, resulting in a large amount of phosphor layers. The triplet exciton energy is returned to the phosphor layer causing many triplet excitons to be in the non-emission mode [1]. In 2006, the Forrest Group of the United States proposed the idea of ​​adding an intermediate layer between the fluorescent and phosphorescent layers. This structure utilizes the different diffusion lengths of singlet and triplet excitons to achieve simultaneous emission of fluorescence and phosphorescence, making full use of all excitons. When the luminance of the device is 500 cd / m2, the external quantum efficiency is obtained ( 18.7). ± 0. 5) % , the power efficiency is 23. 8 lm / W, and the color coordinates at 100 mA / cm 2 are (0. 38, 0.40).
In 2014, the Ma Dongge research team proposed an efficient WOLED without intermediate layers. They dod the blue fluorescent material with higher triplet energy level into the bipolar blended host material. The bipolar blended body can effectively suppress the mutual quenching between fluorescence and phosphorescence, and the blue fluorescent material and phase The endothermic energy return between the adjacent phosphorescent materials reduces the triplet energy loss, so that higher luminous efficiency can be obtained without using the intermediate layer. The hybrid OLED structure without intermediate layer is relatively simple, not only has high luminous efficiency, but also has a very stable spectrum. This hybrid white OLED with high efficiency and spectral stability has good practical value in lighting applications.
4) Thermally activated delayed fluorescent luminescence (TADF) WOLED devices. Although phosphorescent materials are highly efficient, they are expensive because of the presence of rare metals, and the preparation cost is high. The researchers turned their attention to thermally activated delayed fluorescent materials that can achieve 100% of the theoretical internal quantum efficiency. The energy difference ( ΔEST ) between the S1 state and the T1 state of the molecule in the thermally activated delayed fluorescent material is small, and the T1 state can return to the S1 state after the thermal excitation, and then the radiation transition generates fluorescence. It is generally characterized by an increase in fluorescence efficiency with increasing temperature, which is clearly distinguishable from ordinary luminescent materials. In 2012, the Adachi Group of Kyushu University of Japan published an OLED technology with a thermally active delayed fluorescence effect, which uses a combination of highly dense electron donors (don, D) and electron acceptors ( acceptors, A) to increase orbital overlap and improve Luminous efficiency, a significant breakthrough in some systems (especially green light), achieving a device with an external quantum efficiency of 19.3%, close to the efficiency level of phosphorescent devices, has become a huge breakthrough in organic fluorescent devices. Subsequently, the research team designed a series of material systems with zero TS energy gap, and the device efficiency performed well [10-11]. At present, high-efficiency monochromatic luminescent heat-activated delayed fluorescent materials are under development, and there are few reports on the preparation of WOLEDs. In 2014, Qiu Yong's research team published a WOLED device using a blue heat-activated delayed fluorescent material to obtain an efficient and stable warm white light emission with an external quantum efficiency EQE of 22.5% and a power efficiency of 47.6 lm /W.
5) Excimer or exciplex WOLED devices. This is a relatively special luminescent material that does not utilize the intrinsic emission of the material, but uses an organic molecule to generate an exciplex with molecules of adjacent layers or to generate excimer luminescence using its own molecules. This type of excited state is lower energy, and can obtain a red-shifted and broad spectrum. It can obtain a wider white OLED spectrum with other color groups, and generally has a higher color rendering index. This is a promising approach to reduce the number and structural heterogeneity of dopants in multilayer devices. However, because the light-emitting composites have relatively low luminous efficiency, it is difficult to compare with the WOLED devices containing phosphorescent materials. In addition, the luminescence spectra of such devices vary greatly under different voltages, resulting in unstable performance, and attention is paid to such WOLED devices. There are not many research teams. However, the Adachi team recently found strong observations in exciplexes using mMTDATA as the donor material and 2,8-bis(biphenylphosphonium)dibenzo[b,d]thiophene (PPT) as the acceptor material. The delayed fluorescence yielded a quantum yield of PL of up to 28.5% [13]. Its working mechanism has once again received everyone's attention.
White light OLED device classification based on the number of light emitting units
From the perspective of the number of light-emitting units included in the device, white OLED devices can be divided into single-type, tandem and horizontal-striped structures.
1) Single WOLED, that is, the device only contains one light-emitting unit, which has a simple structure and is relatively easy to prepare. Most of the team's research work on OLEDs is carried out on a single device. However, to solve the lifetime problem of WOLED devices, the advantages of stacked structures are very obvious.
2) Multilayer white OLED. The stacked OLED structure was originally proposed by Professor Kido of Yamagata University in Japan. Compared with the traditional single-cell OLED device, it has higher current luminous power efficiency, and its luminous power efficiency can be increased with the number of series devices. The multiple is increased, and a large initial brightness can be obtained at a small driving current, which is very suitable for lighting use. Since the lifetime of an OLED is inversely proportional to the current density it passes through, a plurality of OLED units are stacked, and the current density passed in each OLED unit is reduced with the same brightness, thereby greatly increasing the lifetime of the OLED. In the device with stacked structure, as the number of superimposed layers increases, the microcavity effect will gradually increase, so that the illuminating color and other properties have great changes at different angles.
Tandem, also known as Multi-photo emission (MPE), is equivalent to connecting multiple independent OLED units in series. This structure requires a connection layer capable of generating charges. (CGL: charge generation layer) Connects one light unit. The connection layer is equivalent to an electrode shared by the upper and lower cells, and two carriers of electron holes can be simultaneously generated and injected into the light-emitting units on both sides. The choice of the tie layer material and thickness control are very critical techniques in the laminate structure.
In the 2008 SID, Prof. Kido proposed a 4-layer tandem white OLED with two layers of blue light in series with two layers of orange light. The efficiency is 20 lm /W at 5000 cd / m2. In SID 2009, LG Company used green and red light-doped phosphor layers to form a yellow light device and a fluorescent blue light device in series to form white light. When the brightness is 1000cd / m2, the efficiency is 30lm / W, and the color coordinates are (0 37,0. 39 ), half-life lifetime is 31000h [16]. Japan's Idemitsu Lucky Co. also published a novel red and green phosphorescent luminescent material at the conference, and the obtained device has an efficiency of 35. 2 lm / W at 1000 cd / m2, and the color coordinates are (0. 32, 0). 42), half-life lifetime is 9400h.
Laminated structure is a hot topic in OLED white light illumination. Because different design methods can be used according to different needs, plus light extraction technology, the lifetime at the same current density (especially high brightness) will have a more significant advantage than a single OLED device.
Figure 2 (a) a single white OLED; (b) a stacked white OLED;
3) Striped white OLED. As shown in Figure 2, on 2014 SID, Universal Display published a novel stripe white light structure. In the first two configurations, the lighting units are independent, but cannot be controlled separately. In the stripe structure, the RGB three-color illuminating area is strip-shaped and can be separately controlled, so that the illuminating color can be easily adjusted, and the 15cm × 15cm panel can be obtained, and the light color can be changed from 2700K to 4000K. A luminous efficiency of 41 lm /W is obtained without the light extraction technique, and 63 lm /W can be achieved with the light extraction technique.
Figure 3 (a) bottom emitting device; (b) top emitting device
The white light OLED devices based on the light exit direction are classified according to the direction in which the light exits from the device, and the OLEDs can be further divided into a bottom emitting device (Fig. 3(a)) and a top emitting device (Fig. 3(b)).
The top emission structure may also become an important technical development direction of white OLED for illumination due to its advantages in effective light-emitting area and improved efficiency, and the combination of other structures and top emission structures can develop a higher performance white OLED. In the top-emitting device, light exiting from the top of the device is not affected by the TFT, so the aperture ratio can be effectively increased, which facilitates the integration of the device and the circuit. Moreover, the top emitting device also has the advantages of improving device efficiency, narrowing the spectrum, and improving color purity. The top-emitting WOLED is particularly suitable for preparing large-size, high-definition, full-color active display devices because it can fully utilize the respective advantages of the top emission structure and the white light device.
Performance of high efficiency white OLED devices
Table 1 Current laboratory level of white OLED devices
High efficiency, long life and low cost are the key to the industrialization of white light OLED illumination sources. Among them, efficiency reflects the ability to convert electrical energy into light energy. Lifetime reflects its practicality, and cost is the premise of wide application in the market. Considering the choice of materials and the design of the device structure, the combination of fluorescent blue light material and yellow or red and green phosphorescent materials can be used to obtain high efficiency white light emission, combined with light extraction technology, greatly improve power efficiency, and utilize laminated structure. Increasing the stability of the device and obtaining a practical life is the best choice for WOLED. At the same time, thermally induced retardation materials, as a new generation of organic luminescent materials, are receiving attention and rapid follow-up by researchers. At present, the performance of white OLED devices in the world is changing with each passing day. The laboratory level of white OLED devices released in the past two years is shown in Table 1. In 2013, LG reported a stacked device at the SID (Society for Information Display) meeting with an efficiency of 80 lm/W and a long life. The all-phosphorescent WOLED device reported by Panasonic has an efficiency of over 100 lm/W. In 2014, Nanjing Dijon Organic Optoelectronics published a 3-cell laminated structure with an efficiency of 117 lm/W. The 3-cell laminate structure using external light extraction technology has been able to achieve mass production in a production line at 1000 cd / m2. With more than 80 lm/W and more than 60 lm/W at 3000 cd/m2, the product performance has reached the international leading level.
White OLED technology outlook
In addition to materials and device structures, there are some technologies that are critical to improving the efficiency of white OLEDs, namely light extraction and packaging. In addition, the biggest advantage of OLEDs is that flexible devices can be fabricated. At present, flexible OLED technology has become one of the most popular research topics.
1) Light extraction technology. The OLED device fabricated on a common transparent substrate has an optimized light output coupling efficiency of only about 20%, which means that about 80% of the light generated inside the device is limited or lost inside the film layer of the device, and is not utilized. . In order to obtain a highly efficient white OLED, the light extraction efficiency of the device must be greatly improved, so the development of light extraction technology is particularly important. A variety of device modification techniques have been developed to improve light extraction efficiency, which are mainly divided into external extraction scheme (EES) and internal extraction scheme (IES). EES is for the outer surface of the substrate, and IES is for the substrate and the transparent electrode. EES is relatively simple to prepare, and microlens technology, coated scattering layers, shaped substrate technology, nanopatterns, and nanoporous membranes have been used in practical mass production products. In contrast, IES has a higher degree of improvement in light extraction rate than EES, but because it is difficult to prepare and complicated in process, it is still only in the laboratory stage, inserting a low refractive index layer, and adopting lithography and the like. The device ITO / organic region is made into corrugated shapes, photonic crystals, etc. [31]. In addition, the addition of a properly designed microcavity to the OLED device can improve light extraction efficiency. So far, researchers have developed many light extraction technologies, but there are not many applications that can meet the application requirements. The most important reason is the cost problem caused by process complexity and the problem of large area.
2) Packaging technology. One of the key technologies related to OLED lifetime is packaging technology. The traditional OLED package uses a metal cover or a glass cover. The traditional OLED package technology is effective, but it is awkward and costly. Furthermore, it is apparent that such a cover plate is not suitable for the packaging of flexible devices. Further, a thin film encapsulation technique has appeared. The film package can be divided into inorganic film package, organic film package, inorganic/organic composite film package, etc. according to the packaging material.
3) Flexible technology. Flexible display technology has always been a dream of people and the most unique advantage of OLED technology. Research on flexible OLED devices has focused on improvements in substrate-side anodes and in flexible substrates. The traditional ITO process is not suitable for flexible devices based on plastic materials because of its high preparation process. And because of the lack of indium resources, it has become a research hotspot to find transparent anode materials that can replace ITO. At present, the main organic conductive film materials and carbon nanotubes. Both plastic and metal substrates such as PET, PES, and PEN can be used to fabricate flexible OLED devices.
Conclusion
With the deepening of research, the performance, lifetime and brightness of white OLEDs are gradually improving, and will be developed toward large area, high reliability, high efficiency and flexibility. On the other hand, the prelude to the industrialization of OLED lighting has been opened, and more high-quality white OLED products will appear, bringing us more comfortable and perfect enjoyment.
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