[Special] Research progress and development trend of white organic light-emitting devices for illumination

1 Introduction

Organic Light-Emitting Devices (OLEDs) have the advantages of ultra-thin, light weight, high luminous efficiency, low driving voltage, fast response, rich colors, wide viewing angle, etc., and have a very broad display and illumination field. The application prospects have set off a huge research boom in the world in recent years [1-3]. In 1987, Dr. CW Tang, a Chinese scientist at Kodak Company, prepared a low-voltage-driven high-efficiency green OLED based on the small molecule fluorescent material Alq3 by vacuum thermal evaporation, which opened the first step in practical OLED research. [4]. In 1993, Prof. Junji Kido of Yamagata University of Japan developed the first white organic electroluminescent device. At that time, the luminous efficiency of white OLEDs reported was only 1 lm/W, the external quantum efficiency was less than 1%, and the lifetime was less than one day [5]. Nowadays, research on white organic light-emitting devices has made great progress. It is possible to use the next generation of revolutionary lighting technology in daily life after fire, incandescent lamps and LEDs, as shown in Figure 1.

Figure 1 Changes in lighting technology and a new lifestyle brought about by OLED lighting (Photo courtesy of Lumiotec, Japan, 2013, http://)

Under normal circumstances, the brightness of the light source used for illumination needs to meet 3000-5000 cd/m2, and the luminous efficiency of the currently commercially available fluorescent tube can reach 70 lm/W, and the service life is over 10,000 hours. In view of the current situation, the next generation of lighting sources must have higher brightness and luminous efficiency, longer life, more realistic color reproduction and safer and more environmentally friendly performance. In the case of OLEDs, the emission of luminescent materials emitting different colors can produce white light with a high color rendering index and a suitable color temperature [6]. From an environmental point of view, white OLEDs are mercury-free light sources that meet the requirements of EU WEEE and RoHS. From the point of view of energy consumption and safety, a large part of the energy of traditional light sources becomes heat energy. For example, when the incandescent lamp emits light, the surface temperature can reach 90 ° C, and the fluorescent lamp is 60 ° C. There is a fire hazard, which requires a new light source. It must have high energy conversion efficiency and low operating voltage to maintain the normal temperature state of its surface. OLEDs called "green cold light sources" can maintain a surface temperature of around 30 °C, much lower than traditional light sources [7]. Not only that, OLEDs are unique in that they can achieve novel lighting methods, such as flexible transparent panel lighting and illuminating wallpaper [8,9], which can be used in many fields such as exhibition performance, home decoration, car interior, landscape layout, etc. Aspects have been applied.

The unique OLEDs are destined to shine in the field of artificial lighting. Many countries and enterprises have focused on and actively participated in the research boom of OLEDs. On the national front, the US Department of Energy's "Solid State Lighting Program (SSL)" promotes the rapid development of the OLED industry with national power [10]; the EU "Rainbow Program" and "OLED 100.eu Program" give priority to the development of OLED lighting; the Korean government The "New Light-Emitting Diodes for Effective Lighting Solutions" program (NoveLELS), the "National Semiconductor Lighting Project" in China and the "New Century Lighting Source Development Program" in Taiwan all strongly support the development of high-efficiency energy-saving lamps. Help promote the rapid development of OLEDs in the field of lighting [11].

On the enterprise side, Osram, a subsidiary of Siemens, announced the development of OLED technology with an illumination efficiency of 87 lm/W as early as 2011. In 2012, Panasonic and Toshiba researchers independently produced white OLED panels with a brightness of 1000 cd/m2 and a luminous efficiency of 90 lm/W. Researchers at Panasonic have increased the lifetime of laminated white OLEDs to 100,000 hours (with a brightness of 1000 cd/m2) [12, 13]. Only one year later, Panasonic used a high-index material-based optically coupled substrate to achieve a 114 lm/W ultra-high efficacy OLED panel at 1000 cd/m2 [14]. As shown in Figure 2, the established companies Panasonic, Toshiba and Lumitec, which are owned by the rookie company Mitsubishi, all disclosed their independently developed OLED lighting products in 2013. Hitachi also entered the OLED lighting business with its own coating technology [15]. Philips and BASF formed a strategic alliance to jointly develop transparent OLED lighting products, targeting the high-end dome lamp market. In addition, LG Chem also announced that it will manufacture a 300×300mm2 white OLED panel by 2015, with a brightness of over 3,000 cd/m2, a luminous efficiency of not less than 135 lm/W, and a lifetime of more than 40,000 hours [16]. . In 2008, Visionox Co., Ltd., which was established by Tsinghua University, successfully built China's first OLED panel scale production line in Kunshan to realize mass production of small-size OLED products [17]. Nanjing Dijon Organic Optoelectronics Co., Ltd., which has a number of OLED patents, successfully produced an internal light extraction white IES-OLED lighting device with a luminous efficiency of 73lm/W in 2012. The technical indicators have set a new world record and plan to achieve mass production this year [16] . While companies are stepping up OLED technology innovation and patent layout [18], well-known display panel manufacturers such as Samsung, LG, AUO, BOE, Chi Mei, Tianma, etc. are also expanding the industrial layout of OLED panels, striving for this display. And the industrialization of new lighting technologies has won the top spot.

Figure 2 Toshiba's wireless-powered OLED lighting panel (left) and Lumiotec's high color rendering OLED panel (right)

In this paper, after introducing the related technical principles of white organic electroluminescent devices, the current research status of white OLEDs for illumination is reviewed. From the perspective of OLED materials, the following three key technologies are elaborated.

(1) Low working voltage technology: Since the operating voltage and energy consumption of OLEDs are directly proportional, it is necessary to reduce the operating voltage of the OLED to achieve "economical" illumination;

(2) Phosphorescent OLED technology: high internal quantum efficiency (electron-photon conversion rate) can be achieved by using phosphorescent materials;

(3) Multi-Photon Emission (MPE) technology: Under normal circumstances, when the luminance of the luminescence is high, the OLEDs will be greatly reduced in luminous efficiency and lifetime, and the stacked OLED multiphoton illuminating device is adopted. High light efficiency and long life at high brightness.

2 Luminescence mechanism and structure of organic electroluminescent devices OLEDs

2.1 Luminescence mechanism of organic electroluminescent devices

The phenomenon of organic electroluminescence refers to the phenomenon that an organic semiconductor luminescent material is excited and radiated by an electric field [19]. Figure 3 shows a schematic diagram of the OLEDs' illuminating process [20]. Under forward voltage driving, the illuminating process has the following five steps:

(1) Carrier injection by an electrode: carriers (holes and electrons) overcome the interface barrier formed by the energy level mismatch between the electrode and the organic material, and then implant the device, and the interface barrier is too large. Blocking the injection of carriers, affecting the operating voltage and current density of the device;

(2) Transport of carriers: Carriers migrate or diffuse in a leaping manner in the functional layer and gradually approach the luminescent layer. During the jump, holes or electrons are easily trapped by impurities or defects, and current density is suppressed;

(3) Recombination of carriers: Holes and electrons meet and recombine in the luminescent layer. For small molecule OLEDs, the process can directly illuminate. For high molecular polymer OLEDs, holes and electrons are mutually captured by Coulomb forces to form a transient steady-state hole-electron pair (exciton);

(4) Exciton formation: According to the computational structure of quantum spin theory, the ratio of singlet excitons and triplet excitons is 1:3, ie 25% singlet excitons, 75% Triplet excitons [21];

(5) The diffusion, recombination and release of photons (Photon liberation): under the action of concentration gradient, excitons diffuse. Some of the excitons are attenuated by relaxation, and the singlet excitons and triplet excitons radiate fluorescence and phosphorescence [22].

Figure 3 Schematic diagram of organic electroluminescence process

2.2 Structure of organic electroluminescent device

OLEDs are carrier-injection type light-emitting devices, which usually have a multi-layer structure. In Figure 4, most of the current OLEDs adopt a "sandwich" structure. The structure is composed of a plurality of functional layers having different functions, namely an anode layer of a transparent metal oxide (Anode, usually indium tin oxide ITO), a hole transport layer (HTL), and an illuminating layer (Emissive Layer, EML), an electron transport layer (ETL), and a metal cathode layer (Cathode, usually a low work function metal Li, Ca, Al, Mg, Ag, etc.). An organic layer is interposed between the yin and yang electrodes, and the radiant light is emitted from the transparent conductive substrate on the side. There are organic/organic and organic/metal interfaces between the layers, and the properties of the interface layer materials have a great influence on the performance of the OLEDs. A more complicated structure is to provide a carrier blocking layer between the transport layer and the light-emitting layer to reduce the diffusion current ratio of the electrode quenching and improve the light efficiency. In addition, public research reports also include OLED devices using superlattice and quantum well structures [23].

Figure 4 Schematic diagram of OLEDs

2.3 Implementation method of white organic electroluminescent device

In order to obtain white light with good chromaticity and quality, white luminescent materials are generally not used directly, but a method of mixing white light is used. Organic blue light can be obtained relatively easily, so the white organic electroluminescent device is mainly realized by doping red or green luminescent materials in the organic luminescent main layer or combining white light with multiple luminescent layers of different colors. It is generally believed that the luminous efficiency of OLEDs exceeds 100 lm/W to replace general illumination.

3 low working voltage technology

The reduction of power consumption of OLEDs has important practical significance, and low operating voltage is an important way to achieve power consumption reduction. Meerheim et al. theoretically calculated the thermodynamic limit of the OLED operating voltage according to the principle of blackbody radiation [24]. For example, the thermodynamic limit voltage of green OLEDs at a luminance of 100 cd/m 2 is 1.95V. The lowest voltage value reported so far is that in 2010, Su et al. used Ir(ppy)3 as the material, and the voltage value at 100cd/m2 was 2.40V [25], which is still 0.45V compared with the theoretical value. .

Table 1 Abbreviation, English name, chemical structure and appearance position of the materials described in this paper (I)

The reduction of the operating voltage can be achieved in several ways: (1) introducing a buffer layer, reducing the injection barrier, (2) using a material with a high mobility, (3) chemical doping, and (4) a suitable luminescent structure such as reduction The effective thickness of the device. The more common methods (1), (2) and (3) are usually carried out at the interface layer between the ETL and the metal cathode layer. Currently, there are mainly two types of implementation methods. One type is to insert an inorganic-based electron-injection layer (EIL) at the interface of the ETL/cathode layer to promote electrons to be more efficiently injected into the organic light-emitting layer from the cathode. Ultra-thin LiF is a commonly used EIL. In 1997, Hung et al. reported that NPD/Alq-based OLEDs using LiF as EIL have a lower operating voltage of 7 V (current density of 100 mA/cm2) than Mg-Ag cathode OLEDs [26]. In addition, Lian et al. used Cs2CO3 [27], Ahn et al. used LiF/Yb bilayer structure [28], Qiu Yong et al. used pyrolysis Li3N [29], Cui Guoyu et al [30] used conventional Li3N and other inorganic salts to achieve operating voltage. Reduced.

The other type is to insert an ultra-thin metal layer or a metal-doped organic layer at the ETL/cathode layer interface. Fukase et al. reported a Li/Ag-based device in 1993, and the use of ultra-thin metal Li resulted in superior performance over conventional Mg-Ag cathode layers [31]. Metal doping is chemical doping. Usually, a free radical anion is used as an internal electron carrier to effectively reduce the barrier of electron injection and improve the conductivity of the doped layer. In 1998, Kido et al. reported a device with a Li-doped Alq/Al layer, which achieved a luminance of up to 30,000 cd/m2 at 10.5 V, while the undoped Alq/Al layer had a luminance of only 3400 cd/m2 at 14 V [ 32]. In 2002, Pfeiffer et al. reported an Ir(ppy)3-based device with a Cs-doped Bphen/Al layer that achieved a luminance of 1000 cd/m2 at a voltage of 3.0 V [33]. In 2012, Kim et al. used a method of co-deposition of Mg and Alq3 to obtain a luminance of 60 cd/m 2 at a current density of 11 mA/cm 2 [34]. In 2013, Schwab et al. greatly improved the luminous efficiency of transparent OLED-based OLEDs by inserting a 2 nm ultra-thin Au layer [35].

Table 2 Abbreviations, English names, chemical structures and appearance positions of the materials described in this article (II)

Li and Cs are active alkali metals that are highly oxidizable and difficult to handle in oxygen and water vapor environments. As an alternative, Kido et al. use an alkali metal complex such as lithium quinolate (Liq), sodium 8-hydroxyquinolate (Naq), lithium acetylacetonate (Liacac), lithium di-tert-amyl methoxide (Lidpm). Etc. as an organic/cathode interface layer. These metal composites can be evaporated at relatively low temperatures (200-300 ° C) and are easy to handle at ambient temperatures. The EIL is formed by co-depositing Alq and Liq, and electron injection from the cathode Al to the Alq layer is more efficient. The possible mechanism is that the thermal reduction of Al to Li+ occurs at the Liq/Alq interface to achieve efficient doping of Li in Alq [36].

Following Liq and Naq, Csq was also reported by the Qiu team in 2008 as an organic/cathode interface layer [37]. Other materials that have been reported are: LiPBO [38] reported by Kim et al. in 1999, lithium oxadiazole lithium complex reported by Wang et al. in 2003 [39], and quinoxaline derived by Li Yang et al. Metal complexes [40] and the like. The latest reports include the air-stable metallocene complex reported by Qi et al. in 2012 [41], the imidazoline salt compound o-MeO-DMBI-I reported by Wei et al. [42] and the 2013 Wetzelaer et al. The solution of bismuth stearate [43] was reported. The application of these metal composite materials has promoted the rapid development of OLED low-voltage technology and contributed greatly to the commercialization of high-efficiency white OLEDs.

3.1 Phosphorescent OLED technology

The rapid development of material chemistry has greatly promoted the improvement of the luminous efficiency of white OLEDs. After combining phosphorescence technology and optical coupling technology, its light efficiency can exceed that of fluorescent tubes [44]. Phosphorescent OLED technology is of great significance for energy-saving lighting. When the phosphorescent materials Ir(ppy)3 and FIrpic are excited, the lifetime of triplet excitons can be significantly reduced by spin-orbit coupling, and the probability of intersystem crossing is increased, thereby realizing singlet and triplet excitons. Hybrid phosphorescence radiation transitions ensure that all excitons in singlet and triplet states are converted into photons, with internal efficiencies approaching 100%, and their efficacy is four times that of OLEDs using only fluorescent materials [23, 45]. The phosphorescent material (guest material) is usually uniformly doped into a stable host material to overcome the problem of concentration quenching due to density unevenness, thereby affecting the luminescence quantum efficiency (ηPL). When the host material has a higher triplet energy (ET) than the phosphorescent material, a higher ηPL [46] can be obtained. For example, in a blue phosphorescent OLED, the ET of the host material must be greater than 2.75 eV to obtain effective illumination. Polyvinylcarbazole is currently the most widely used host material, and its triplet energy reaches 3.0 eV, which is one of the few polymer materials that can be used as blue phosphorescence [47]. Tokito et al.'s research shows that when FIrpic is used as a phosphorescent material and carbazoles CBP and CDBP are used as host materials, the ET values ​​of the materials have a great influence on the luminous efficiency. When using CBP as the host material, the luminous efficiency of FIrpic phosphorescent OLED is only 6.3 lm/W, but when CDBP with high ET value is used, its light efficiency is improved to 10.5 lm/W [48]. Sasabe et al. studied four 3-3'-biscarbazole host materials and constructed a light-emitting layer through a series of 3-3'-biscarbazole derivatives/FIrpic layers, which was achieved at a very low driving voltage of 3.1V. High-efficiency luminescence of 46 lm/W (luminous brightness 100 cd/m2) [49].

Table 3 Abbreviations, English names, chemical structures and appearance positions of the materials described in this paper (III)

In general, holes and electrons are combined near the HTL/EML or EML/ETL interface. Since the energy barrier of HTL or ETL to EML is large, in order to achieve higher efficiency, HTL and ETL with high ET values ​​are also used. Very important. In 2005, Kido et al. constructed a FIrpic-based OLED light-emitting device that uses high ET values ​​such as 4CzPBP, mTPPP, and 3DTAPBP to achieve luminous efficiencies as high as 37 lm/W [50]. In this device, the high ET material used is used both as a host material and as an ETL and HTL. In 2008, Sasabe et al. implemented wide-band B3PyPB as ETL to realize FIrpic-based high-efficiency OLEDs, and obtained luminescent quantum efficiency of more than 60 lm/W [51]. The device uses high ET HTL and ETL to transport the carrier on the one hand and triplet excitons on the other, greatly reducing the efficiency loss of the HTL/EML and EML/ETL interfaces.

Recent studies have shown that the recombination step of electrons and holes in EML is the key to phosphorescence excitation of OLEDs [52], so the carrier balance of electrons and holes in EML becomes an important factor for efficient OLEDs. In order to obtain the best carrier balance, the host material and the Frontier Molecular Orbitals (FMO) of HTL (ETL) need to be matched [53]. The chemical structure of the different materials makes it have different electro-optical properties, so each emissive material must match the matching material system to achieve its maximum potential [54].

There are two ways to construct white OLEDs using phosphorescent materials. One is to mix a blue fluorescent material with phosphors of other colors to form white light. In hybrid white OLEDs, the blue fluorescent material needs to have a higher ET value and a higher ηPL value than other phosphorescent materials. A representative example is that in 2012 Lee et al. reported a hybrid white OLED device with DADBT as a blue fluorescent emissive material and Ir(2-phq)3 as an orange-yellow phosphorescent material. The energy efficiencies at 100 cd/m 2 (ηp, 100) and 100 cd/m 2 (ηp, 1000) were 50 lm/W and 34 lm/W, respectively. The hybrid device and all OLED devices based on phosphorescent material emit light equivalent, opening the first high-efficiency hybrid white OLED device [55]. The shortcoming is that at this stage, the problem of large difference in electroluminescence spectrum at different current densities occurs in hybrid white OLED devices, which needs to be improved by studying new material systems and developing finer structures.

Another way to build a white OLED device is to use only phosphorescent materials. Universal Display reported a phosphorescent white OLED device based on light outcoupling enhancement techniques that achieved a light efficiency of 102 lm/W at 1000 cd/m2 brightness. Unfortunately, the materials and device structures used in this OLED have not been reported in detail [56]. In other published reports, the white OLED device reported by Reineke et al. combines RGB phosphorescence emission and lenticular (×2.7) luminescence enhancement technology with a luminous efficiency of 81 lm/W (1000 cd/m 2 ) and a color rendering index Ra. 80 [57]. In addition, Su et al. reported a highly efficient two-color white OLED that achieves a light efficiency of 44 lm/W (ηP, 1000) without the use of optical coupling enhancement [58]. Although the light effect is considerable, its color rendering index is only 68, so it cannot be used as daily lighting. In 2012, Adamovich et al. used a three-color phosphorescent laminate to form a high-efficiency white OLED, achieving a light efficiency of 54-56 lm/W at a luminance of 3000 cd/m2, a color rendering index of CRI of 82-83, and an chromatic product reaching the Energy Star. "Standard [59].

Table 4 Abbreviations, English names, chemical structures and appearance positions of the materials described in this paper (IV)

The CRI enhancement of white OLEDs can be achieved by adding a pure blue luminescent material with a wavelength of 450 nm, such as Ir(dbfmi) material, which has a phosphorescence lifetime (τp) of only 19.6 μs, which is about 12 times that of FIrpic [60]. White OLEDs based on this material exhibit a luminous efficacy of 59.9 lm/W (ηP, max) and 43.3 lm/W (ηP, 1000), and CRI can reach 80 or more. Due to the wide range of options for blue phosphors, the increase in CRI has also become simple. The performance of white OLEDs is largely determined by the blue light-emitting units within them, so the development of high-efficiency blue phosphorescent materials and related devices for OLEDs is particularly important [61].

OLEDs prepared by vacuum thermal evaporation deposition techniques typically use amorphous small molecule materials, and the alignment of the molecules is not oriented. Recently, researchers have discovered that intramolecular and intermolecular interactions can be utilized to improve device performance through sophisticated molecular design. A common method is to use a weak hydrogen bond between H(C) and N [62]. Ichikawa et al. reported Bpy-OXD, which uses its weak H(C) and N-hydrogen bond interactions with pyrimidine molecules to enhance luminescence of OLEDs. Compared with OXD7, Bpy-OXD in NPD/Alq devices exhibits superior performance due to the presence of H(C) and N hydrogen bonds, and the driving voltage is lower [63].

3.2 multiphoton luminescence technology

Traditional OLED devices have significantly reduced light efficiency and lifetime during lighting operations, as shown in Figure 5. When multi-photon emission (MPE) technology is used, the light efficiency and lifetime of OLEDs can be greatly improved, so this technology is regarded as the core technology for OLED devices to realize lighting applications.

Figure 5 shows and OLEDs performance requirements for lighting applications

As shown in Fig. 6, in simple terms, the MPE technology is to laminate the OLEDs, and when the low current is passed, the laminated OLEDs emit light together to produce high brightness [64]. Unlike the multi-emissive layer structure, each cell in the MPE is a separate unit with carrier (electron and hole) injection/transport and luminescent layers, which has no direct effect on each other. Each of the light-emitting units is connected by a charge generation layer (CGL) composed of a transparent inorganic or organic electroactive material, and the connection layer is equivalent to an electrode shared by the two side units, and at the same time, carriers can be generated under the electric field and injected separately. Into the two lighting units on both sides. The biggest feature of MPE technology is that the positive and negative charges injected into the device pass through multiple light-emitting units, which greatly improves the probability of compounding excitons and improves light efficiency.

Fig. 6 Schematic diagram of multi-photon illuminating device composed of two units

In an MPE apparatus including N transmitting units, the operating current remains unchanged when the operating voltage is increased by N times, and N times of luminous intensity can be obtained. In other words, when the luminous intensity is the same as that of a single OLED, such an MPE device having N cells has an operating current of only 1/N. In general, the lifetime of an OLED is inversely proportional to the current density through which it passes. Therefore, in a multiphoton light-emitting device, the number of light-emitting cells is sacrificed to obtain a low current density, which significantly increases the lifetime of the OLED. Moreover, in phosphorescent OLEDs, the problem of OLED efficiency reduction caused by triplet-triplet annihilation at high current densities is also solved. The more cells stacked in an MPE device, the less likely it is that a short circuit and circuit breakdown will occur.

Table 5 Abbreviations, English names, chemical structures and appearance positions of materials described in this paper (V)

The transparent ITO electrode is a common component in the charge generation layer, but the ITO electrode has an adverse effect on the sputtering of the organic layer, and its high electrical conductivity is also liable to cause crosstalk problems. Therefore, an organic insulating material is appropriately added in the CGL. Such as F4TCNQ and HATCN, can effectively solve the above problems [65]. The basic principle is that, as an electron-accepting organic compound, F4TCNQ and HATCN are driven by voltage, and the lowest unoccupied Molecular Orbital (LUMO) can accept electrons from nearby host materials (such as NPD). Effective conduction of electrons reduces the conductivity of CGL and avoids crosstalk problems. Liao et al. reported a fluorescent MPE device using a HATCN/NPD layer as a CGL. Their research indicates that the HATCN layer can effectively reduce the operating voltage and improve stability and energy efficiency [66]. Recently, Chiba et al. reported an ultra-efficient I(ppy)3 and HATCN MPE device. There are three units in the MPE device, which can achieve ultra-high current efficiency of 244 cd/A at 1000 cd/m2, and the internal quantum efficiency is significantly improved [67]. Recently, Wang Jin et al. constructed a novel charge-generating layer structure of PiN white organic laminated electroluminescent device. BCP: 5wt.% Cs2CO3/NPB: 20wt.% MoO3 was used as CGL to prepare two light-emitting units. In orange OLEDs, the maximum current efficiency is 2.5 times that of a single device, and complementary white light near the energy point of white light is obtained [68].

4 Prospects of white organic electroluminescent devices

In terms of basic research, the current mainstream white OLED panel has a luminous efficacy of up to 90 lm/W at 1000 cd/m2, which is superior to fluorescent tubes, but still far from the theoretical limit of 248 lm/W [69], so in the light There is still a lot of room for improvement. The energy consumption of the illumination source is proportional to the operating voltage, and the operating voltage can be reduced by using sensitive materials and new device technology. At the same time, improving internal quantum efficiency is also a very important and challenging task that can be achieved by developing multifunctional and sensitive materials. For basic OLED researchers, it is necessary to clarify the relationship between the chemical structure, physical properties and device performance of organic molecules and to summarize the laws to maximize the potential of materials and equipment.

At high brightness of 3000-5000 cd/m2, standard current-driven OLEDs experience significant degradation in efficiency and lifetime. The core to solve this problem is the use of multiphoton illuminating (MPE) devices. However, even with the most advanced MPE equipment, the operating voltage is still much higher than the theoretical limit. Therefore, it is particularly important to develop new materials for charge generation layer (CGL) and to study the luminescence mechanism on both sides.

In terms of industrialization, white OLEDs need to make breakthroughs from the following points:

(1) Product yield is low. Although the preparation process of OLED is simpler than LCD, the current preparation method and process are not mature, and the yield of OLED panel has not been effectively improved. The increased production cost has become a bottleneck that seriously restricts the development of OLEDs technology;

(2) The input cost is too large. OLEDs are a technology- and capital-intensive industry with high barriers to entry. In the early stage, a large amount of cost investment was required, which inhibited the investment enthusias of most enterprises and restricted the promotion and application of OLEDs technology;

(3) The technology monopoly is serious. The core technology patents of OLEDs are in the hands of several large companies and R&D teams. The degree of transparency in technology and production processes is low, the industry chain is not perfect, and industry standards are not yet unified.

5 Conclusion

After more than 20 years of development, white organic organic electroluminescent devices have made great progress, especially the development of three technologies: low working voltage technology, phosphorescent OLED technology and multiphoton luminescence technology, which promotes luminous efficiency and service life of white OLEDs. The improvement. Compared with the display field, the application prospect of white OLEDs in the field of lighting is more optimistic. More and more scientific research teams and enterprises have joined this camp to promote the rapid development of white OLEDs, and some products have also been released. Subject to process factors, input costs and key patent technologies, the current price of white OLEDs products is still too high, and it is still difficult to enter thousands of households. However, breakthroughs in science, great advances in technology, and urgent demand in the market will continue to broaden the commercialization of OLEDs. I believe that in the near future, OLEDs will replace traditional light sources and appear in every corner of the world. Humans bring light.

Edit: Cedar