Abstract: White organic light-emitting diodes (WOLEDs) are considered alternative light sources to traditional white light sources. They are efficient solid-state light sources, and their electro-optical conversion efficiency has exceeded that of incandescent lamps. Recently, they have made great progress in device structure and the synthesis of new materials. Here, we give a brief introduction to the methods of obtaining white organic light-emitting diodes from organic light-emitting diodes (OLEDs), their advantages, disadvantages and recent progress, and also discuss the problems of device structure and related device design. 1. Introduction A huge amount of electricity is consumed worldwide every year. Of all electricity consumption, lighting accounts for 20% of the total electricity production. Fluorescent lamps and incandescent lamps are the most commonly used traditional lighting sources, consuming 40% of the electricity used for lighting. Incandescent lamps convert 90% of the electrical energy into heat energy, while fluorescent lamps perform better, converting 70% of the consumed electrical energy into light energy. The typical luminous efficiencies of incandescent lamps and fluorescent lamps are 13-20 lm/W and 90 lm/W, respectively [1]. So, in order to save energy in the world, one way is to find alternatives to traditional light sources. Researchers have spent more than a decade studying semiconductor light-emitting diodes with better performance. Red, green, blue and other colors of light-emitting diodes made of inorganic materials have long been available on the market and are widely used in traffic lights, car taillights and other small applications. Inorganic white light-emitting diodes have also appeared on the market, but their prices are still relatively high compared to ordinary lighting. Now a new competitor for lighting sources has also entered the market, namely light-emitting diodes based on organic semiconductor materials. In the past decade, OLEDs have shown strong competitiveness in the field of display technology, comparable to liquid crystals. Since the discovery of efficient electroluminescence in tris (8-hydroxyquinoline) aluminum (Alq3)[2] in 1987 and in poly (p-phenylene vinylene) (PPV)[3] in 1990, OLEDs have become the most attractive display technology. They have advantages such as simple fabrication, short response time, high brightness, wide viewing angle, low driving voltage, and the most likely application on flexible substrates and full-color display. OLED displays offer advantages such as durability, high efficiency, and the ability to be fabricated on flexible substrates, such as plastics and paper, allowing the displays to be bent or rolled up. Unlike liquid crystals, OLEDs are self-emissive and require no backlight, enabling thinner and lighter displays. OLEDs are multilayer devices consisting of an active charge transport layer and an emissive layer sandwiched between two thin-film electrodes, at least one of which is transparent. Generally, indium tin oxide (ITO), with high work function (~4.8 eV), low surface resistivity (~20 Ω/□), and transparency to visible light, is used as the anode. The cathode is typically a low work function metal, such as Ca, Ma, Al, or their alloys Ma:Ag, Li:Al. An organic layer with good electron transport and hole blocking properties is placed between the cathode and the emissive layer. Similarly, a hole transport layer and an electron blocking layer are used between the anode and the emissive layer. When an external bias voltage is applied, electrons and holes are injected from the cathode and anode of the OLED, respectively. Under the influence of an external electric field, electrons and holes migrate in opposite directions, recombine in the light-emitting region to form excitons, which decay and emit light outwards. The migration dynamics and properties of excitons are not discussed here. White OLED technology has attracted considerable attention due to its applications in general solid-state lighting and as a liquid crystal backlight in flat panel displays. In the fabrication of full-color displays, the three primary colors are equally important, but white light emission has received more attention because any desired color gamut can be obtained by filtering white light. The first white OLED device was fabricated in 1993 by Kido and his colleagues. This device contained compounds that could emit red, green, and blue light, which together produced white light. However, this also presented some problems. The device efficiency was less than 1 lm/W, the device required a large driving voltage, and it burned out quickly. But now the efficiency of these devices has improved rapidly. The annual progress in efficiency of traditional LEDs, nitride LEDs, and white OLEDs is shown in Figure 1. [align=center]Figure 1. Annual Table of Light Emitting Diode Efficiency Progress[/align] 2. Pathways for Generating White Light from OLEDs White light for lighting should have a good color rendering index (>75) and a good color coordinate position (close to the (0.33, 0.33) point on the chromaticity diagram of the International Commission on Illumination). Generating white light from OLEDs can be broadly categorized into two pathways: (a) Wavelength Conversion: Blue or ultraviolet light emitted from the OLED is used to excite several phosphorescent materials. The different colors emitted by each material are mixed together to obtain white light with a rich wavelength range. This technique is called phosphorescent downconversion. (b) Color Mixing: This method uses multiple emitting layers in a single device. White light is generated by mixing the different colors emitted by different emitting layers. White light can be obtained by mixing two complementary colors (blue and orange) or three primary colors (red, green, and blue). Typical methods for generating multi-colored light through multilayer structures and mixing various colors to obtain white light include: (a) multilayer structures containing red, green, and blue emitting layers; (b) Förster/Dexter energy conversion; (c) microcavity structures; (d) obtaining white light through vertical/horizontal stacked structures; and (e) mixing or doping different emitting materials into a mixed layer. In color mixing technology, since no phosphorescent materials are used, the loss caused by wavelength conversion does not occur, and this technology has the potential to achieve higher efficiency. The various methods are discussed in detail below. Obtaining high-quality white light does not require a breakthrough, but obtaining stable white light remains a hot research and development topic. 2.1 Color Mixing 2.1.1 Multilayer Film Device Structure: This method of obtaining white light utilizes the mixing of light emitted simultaneously from two or more emitting layers. This technology is based on continuous deposition or co-evaporation of different materials and control of exciton recombination regions. This structure contains many organic-inorganic interfaces, and the energy barriers at the interfaces increase the difficulty of carrier injection and generate Joule heating. Therefore, in order to reduce the charge injection barrier and Joule heating at the organic-inorganic interface, the selection principle of the luminescent material is that the highest occupied molecular orbital and the lowest vacant molecular orbital of the neighboring luminescent material need to be matched. The luminescence of the device depends on the composition and film thickness of each layer, and the composition and film thickness of the luminescent layer need to be precisely controlled to achieve color balance. The exciton recombination region is controlled by adding a blocking layer that blocks only one type of charge carrier between the hole transport layer and the electron transport layer. This allows the recombination region to occur in two or three different luminescent layers. The result is that light is emitted in different luminescent layers (Figure 2). [align=center] Figure 2 Schematic diagram of multilayer white OLED structure[/align] By controlling the recombination current in different organic layers, the light emitted from the red, green and blue luminescent layers is balanced to obtain the desired purity of white light. Deshpande et al. [4] obtained white light by performing continuous energy level conversion in different layers. The fabricated device structure is ITO / α-NPD / α NPD:DCM2 (0.6–8 wt%) / BCP / Alq3 / Mg:Ag (20:1) / Ag. Here, 4,4-bis[N-(1-napthyl-N-phenyl-amino)]biphenyl (α NPD) is used as the hole injection layer. α NPD: DCM2 (2,4-(dicyanomethylene)-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5Hbenzo[I,j]quinolizin-8-yl)vinyl]-4H-pyran) was used as the hole transport layer and the light-emitting layer. The purpose of depositing the 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) layer was to block holes. Alq3 was used as the green light-emitting layer and the electron transport layer. Mg:Ag alloy and the following thick Ag layer were used as the anode. The maximum reported brightness of this device was 13500 cd m-2, the maximum external quantum efficiency was greater than 0.5%, and the luminous efficiency was 0.3 lm/W. Recently, Wu et al. [5] reported an OLED device with dual light-emitting layers, and the device was studied with and without the blocking layer. In addition to exhibiting better performance, devices with barrier layers also achieve an external quantum efficiency of 3.86%. The color of the light emitted by these devices is strongly dependent on the thickness of the light-emitting layer and the applied voltage. The disadvantage of this technology is that the fabrication process is complex and there is a large amount of waste of organic materials, resulting in relatively high manufacturing costs. Another way to obtain white light excitation from multilayer OLED devices is to use a multi-quantum-well structure [6] (Figure 3), which includes two or more light-emitting layers separated by barrier layers. Electrons and holes tunnel through the barrier layer and are uniformly distributed into different quantum wells to emit light. The energy level matching requirements of different organic materials in this system are not very strict. Excitons are formed and decayed in different wells, emitting different colors of light in their own wells. The confinement of charge carriers by the quantum wells increases the possibility of exciton formation, preventing excitons from moving to other regions or transferring their energy to other regions. However, this method is very complex and requires optimization of the thickness of various light-emitting and barrier layers. Due to the thickness of many layers, this multilayer structure requires a relatively high operating voltage. [align=center] Figure 3 Schematic diagram of white OLED structure with multilayer quantum well[/align] 2.1.2 Donor-acceptor system By doping a narrow bandgap acceptor molecule into a wide bandgap donor material, the excitation energy can tunnel from the high-energy donor material to the low-energy acceptor material. In such a system, if the doping concentration is maintained at a certain value, the emission of the donor can be ignored, while the emission of the acceptor is dominant. In this emission, all the energy of the donor is transferred to the acceptor. Another possible emission mode of this system is that the energy is not completely transferred, in which the light emission comes from both the donor and acceptor regions. By making the light emitted by the donor and acceptor reach a suitable ratio, white light can be emitted. The donor-acceptor system that produces white light can be a single dopant in a single layer or multiple layers[7], or multiple impurity dopants in a single layer or multiple layers[8]. In order to obtain stable white light, the doping concentration needs to be precisely controlled. Dopant materials can be naturally occurring phosphorescent or fluorescent materials. Doping sites can be directly excited or excited through energy/charge transport from donor molecules. 2.1.3 White Light Emission from a Single-Emitting-Layer Structure The fabrication process and luminescence of the devices discussed above are very complex. To obtain better color rendering and high luminous efficiency, many parameters need to be optimized. Due to the stacking of several substrates used to perform specific functions, the device thickness increases, requiring a high driving voltage. In white organic light-emitting diodes, to reduce the driving voltage, the device thickness must be reduced. These complex multilayer structures can be solved by single-layer luminescence. Single-layer white light-emitting diode devices contain only one organic luminescent layer. Doping different dyes or mixing two or more polymers in an organic layer containing blue light emission to obtain white light has been reported by many. The biggest advantage of OLED devices with only one luminescent region compared to other OLED devices is that the emitted light has better color stability. However, a drawback of this method is that different dopants have different energy transfer rates, eventually leading to color imbalance. The high-energy portion (blue light) can easily transfer energy to the green and red emitters, and the green emitter can transfer energy to the red emitter. If the concentrations of the three colors of emitters are the same, red light will eventually dominate. Therefore, the doping ratio must be blue > green > red, and a good balance needs to be achieved. Recently, Shao et al. [9] demonstrated that white OLEDs using a uniform donor single emitting layer have high color stability. D'Andrade et al. [7] reported a white OLED with only one emitting layer. The emitting layer contains three organometallic phosphorescent dopants: tris(2-phenylpyridine)iridium(III)[Ir(ppy)3] as the green emitter, iridium(III)bis(2-phenylquinolyl-N,C2-)(acetylacetonate)[PQIr] as the red emitter, and iridium(III)bis(4-,6-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate[FIr6] as the blue emitter. These three materials are co-doped in the wide-bandgap donor material p-bis(triphenylsilyly)benzene (UGH2). This white OLED device achieves a peak efficiency of 42 lm/W, a color rendering index of 80, and a maximum external quantum efficiency of 12%. To obtain white light, dyes are not required; mixtures of polymers can also be used. Recently, Gong et al. [10] successfully fabricated a white OLED device using a mixture of {PFO-ETM and PFO-F (1%)} and [Ir(HFP)3] as the emitting layer. The device achieved a luminous intensity of 10,000 cd at 25V. 2.1.4 Microcavity Structure A microcavity is a pair of mirrors with a high reflectivity and a spacing of micrometers. In 1994, Dodabalapur and his collaborators at Bell Labs fabricated an electronic device by placing Alq and an inert material into two reflective surfaces to form a microcavity. In traditional structures, light can escape in all directions. In a microcavity structure, light can only escape from one end of the microcavity structure, thereby improving the efficiency of the device. By changing the thickness of the layer, unwanted light can be filtered out, and light of any desired wavelength can be obtained. LEDs with microcavity structures have higher efficiency, use less current, and have a longer lifespan. Microcavity structures can be used to optimize the color of light, and white OLEDs using microcavity structures are an example of light color optimization. Microcavity resonators are one of the most effective methods to enhance the brightness of monochromatic LEDs [11,12]. The use of microcavity structures in OLEDs to narrow the emission spectrum and enhance the emission intensity has been reported [13]. However, since the emitted light is single-frequency after passing through it, it is ineffective for white OLEDs. Dodabalapur et al. [14] achieved control over OLED emission by using resonators with multiple modes, which can mix different light emitted by the material through different resonant modes to obtain white light. In a microcavity structure, the light-emitting layer is embedded between two metal mirrors or between a metal mirror and a partially reflective bottom mirror containing distributed Bragg reflections [15]. Microcavity structures can strongly modulate the spectrum. Distributed Bragg structures contain two mirror layers with different reflectance coefficients, which provide tunable luminous efficiency in a specific wavelength range. When the device is working, standing waves are generated, and the wavelength of the standing waves depends on the length between the mirrors in the microcavity structure and the reflectivity of the mirrors. Shiga et al. [16] fabricated a modified Fabry-Perot resonant cavity, which contains two microcavity structures with different distances. (As shown in Figure 4, MM, DM, EML and FL represent metal mirror, insulator mirror, light-emitting layer and filter layer, respectively). The light emitted from the mixed microcavity structure can produce white light. The disadvantage of this method is that the color of the light changes with the viewing angle. This disadvantage limits the application of microcavity structures in white OLEDs. [align=center] Figure 4 Concept of microcavity (a) Ordinary microcavity structure (b) Multi-wavelength resonant microcavity[/align] 2.1.5 Using vertical/horizontal stacked structures to generate white light This technology is similar to liquid crystal flat panel display technology. The three primary color pixels are arranged independently in a pattern in a horizontal or vertical manner (as shown in Figure 5). In the horizontal stacked pattern, individual color emitting pixels are deposited in the form of dots, squares, circles, thin lines or strips. The light of the desired wavelength range is mixed to obtain white light. Since each color component is deposited in an independent position, different color pixels can reduce the aging rate by changing the operating current. Each pixel can be optimized to achieve the highest efficiency at the minimum operating voltage. Similarly, the device lifetime can be extended to the maximum by reducing the area of ​​the pixels. Stacked OLED devices are a good candidate for light sources because stacked devices can achieve 2 to 3 times the current efficiency of single-emitting-layer OLED devices. Matsumoto et al. [17] reported that a stacked structure of red and blue light devices produced pink light. This stacked structure can produce light of various colors. It is expected that stacked white OLEDs can achieve higher brightness and efficiency than conventional white OLEDs. Recently, Sun et al. [18] reported a high-efficiency stacked white OLED in which an anode-cathode layer was added between the blue light emitting layer and the red light emitting layer. The anode and cathode layers were used as an intermediate electrode. By adjusting the bias voltage applied to the two light-emitting layers to a suitable ratio, white light could be obtained. The device they reported had a maximum emission of 40,000 cd m-2 at 26V, which was located at (0.32, 0.38) in the color coordinates. The luminous efficiency at 28 mA cm-2 was 11.6 cd A-1. [align=center] Figure 5 (a) Schematic diagram of white OLED with horizontal (b) vertical stacked structure[/align] Kido found that the brightness of the stacked structure device composed of N light-emitting layers is N times that of the single light-emitting layer device. This rule is very attractive for white OLEDs with high efficiency. Recently, Chang et al. [19] prepared two white OLEDs with stacked structures and a comparison device for comparison, in which the stacked structure device used an Ag:Alq 3 /WO 3 connecting layer in the middle. In these devices, white light was obtained by mixing blue light and yellow light. Device 1 is a stacked structure consisting of a yellow emitting layer and a blue emitting layer. Device 2 is two Device 1 structures stacked together. Compared to Device 1 and the control device, Device 2 exhibits better performance. A noteworthy amplification effect was observed in Device 2, which achieved the highest efficiency of 22 cdA⁻¹, almost three times that of the control device. This is a result of the microcavity effect, which enhances the amount of light propagating forward. Therefore, connecting two devices improves the device efficiency. It was also found that the driving voltage increases with the number of emitting layers. Device 2 is the most unstable, while the control device exhibits the longest half-life. This is because Device 2 is subjected to a higher driving voltage than Device 1 and the control device. Since the connecting layers in the stacked devices are non-ohmic contacts, thermal breakdown may occur in the stacked structure devices. At 100 cdm⁻², the half-life of Device 2 is expected to reach 80,000 hours. In these stacked structure devices, luminous intensity and color are also highly dependent on the viewing angle. This dependence of luminous intensity and color on viewing angle is due to the microcavity effect. Therefore, better optical design for stacked structure devices is crucial. 2.2 Wavelength Conversion Among the various methods for achieving white light emission in OLEDs, poor color stability due to the varying lifespans of different types of emitting layers is a common problem. Employing wavelength down-conversion using phosphorescent materials to emit white light may be a viable option. In this technology, a blue OLED is connected to one or more phosphorescent material layers, one of which contains inorganic light-scattering particles. A phosphorescent material layer is coated on the back of the blue OLED. A portion of the emitted blue light passes directly through the phosphorescent layer without wavelength down-conversion, while the remaining portion is used to excite the phosphorescent materials. Exciting different materials produces different colors of light, which mix with the unconverted blue light to obtain the broadest and most wavelength-rich spectrum. Here, only the blue emitting layer conducts charge and is the only directly excited active layer. Once excitons are generated, they excite other phosphorescent materials, obtaining the complementary colors needed to produce white light. Due to the aging of the blue light-emitting layer, the emitted blue light also attenuates, and the light emitted from the associated phosphorescent material also attenuates proportionally. This is because the intensity of the light emitted from the phosphorescent material is directly related to the blue light-emitting layer. Therefore, in wavelength down-conversion technology, there is no differential color aging problem. The color of the emitted light can be adjusted by changing the doping concentration and thickness of the phosphorescent layer. Figure 6 is a schematic diagram of the wavelength down-conversion mechanism of phosphorescent materials. White light emission can also be obtained by combining ultraviolet light with red, green, and blue phosphorescent materials. In this case, ultraviolet light excites several phosphorescent materials, each of which emits light of different wavelengths, and the mixture of these lights produces white light. This technology has good color stability, but the reduction in efficiency caused by the wavelength down-conversion process is the main drawback of this technology. [align=center] Figure 6 Schematic diagram of the working principle of a device that achieves white light using the wavelength down-conversion method[/align] Professor Junji Kido of Yamagata University in Japan is an expert in OLEDs. He and his colleagues have done a lot of work on white OLEDs, achieving significant improvements in device efficiency, half-life, and device characteristics. The greatest contribution of the Yamagata and Forrest groups was the realization of the transition from fluorescent to phosphorescent materials. In phosphorescent materials, both singlet and triplet excitons can recombine to emit light, resulting in higher device efficiency. 3. Summary To achieve luminous efficiency comparable to existing lighting resources, much work remains to be done. Carrier imbalance in the emissive layer is the main reason for low device luminous efficiency and high operating voltage. Carrier injection, thickness, and material transparency are crucial for maximizing exciton recombination efficiency in the emissive layer and enhancing device efficiency. The aging time of different emissive materials is another concern for white OLEDs. Different emissive materials have different aging times, and the white light quality of the device tends towards the most stable color as the device operates longer. This problem can be addressed through approaches such as designing devices with materials exhibiting uniform aging rates, ensuring consistent aging effects across the entire device, or designing a single-layer emissive layer capable of emitting white light. Single-layer devices also deserve attention, as they offer advantages in large-area, low-thickness, low-cost, and flexible substrate fabrication. Currently, the surface area of ​​OLED devices is only on the order of a few square inches. For applications in lighting, flat panel displays need to be on the order of several square feet. Cost is another important factor in the acceptance of white OLEDs in the general lighting market. However, it is difficult to fabricate devices using vacuum deposition to reduce costs. Therefore, the focus should be on fabricating white OLED devices using spin coating or inkjet printing techniques, which can significantly reduce costs. The main problems with this solution are carrier injection imbalance and insufficient polymer purity, resulting in a maximum device efficiency of less than 15 lm/W. Dendrimers may be a way to solve the fabrication method of white OLEDs. Dendrimers are a mixture of small organic molecules and polymers, combining the advantages of both: efficient phosphorescent recombination centers of small molecules, and the ability to be fabricated by spin coating. This could potentially reduce the manufacturing cost of white OLEDs by using printing technology. However, so far, we have not seen any reports on the application of dendrimers in white light. Currently, white OLEDs still face many challenges, and much research is needed in fabrication technology to achieve the desired results. Whether white OLEDs can ultimately replace fluorescent tubes and whether the manufacturing cost can be low enough remains to be seen. There is growing interest in white OLED research, and its performance is gradually improving. White OLED is sure to have a bright future.