Research progress of OLED backlight technology

introduction

Liquid crystal display ( LCD ) is widely used in computer monitors, televisions and other display fields due to its advantages of flat panel, low power consumption, no electromagnetic radiation, high resolution, high contrast , digital interface , easy integration and light weight and portability . It is the mainstream in flat panel display . As a passive display device, LCD needs to be displayed through transmission or reflection of external light source because the liquid crystal itself does not emit light. The backlight is the provider of light source for TFT-LCD (thin film liquid crystal display). The performance of the backlight determines the visual sense of the LCD display, so the backlight is an indispensable and important component of the LCD display device.

At present, the backlight source of LCD TV is mainly cold cathode tube ( CCFL ) light source. Due to the defects of CCFL such as low light efficiency , short life, poor color gamut , small brightness adjustment range, high energy consumption, long response time, low safety factor, mercury-containing harmful gases and environmental pollution, the EU environmental regulations in 2006 will lead to the gradual discontinuation of the current mainstream LCD light source CCFL. Backlight accounts for 20% to 30% of the total material cost of LCD monitors. In order for LCD monitors to maintain their competitiveness in the future market, the development and design of new backlight technology has become an important issue.

High brightness White LED light source not only greatly reduces the thickness and weight of TV, but also has higher reliability and stability; in terms of energy saving , the brightness of LED can be actively adjusted according to the brightness of the picture, and compared with LCD TV , the effective energy saving can reach more than 30%; in terms of environmental protection, the advantage of LED backlight source is that it does not contain toxic and harmful substances such as lead and mercury, and is a truly green and environmentally friendly light source. The LCD industry is about to enter a revolutionary period in which LED replaces CCFL.

In addition to the above advantages of inorganic LEDs, organic white light OLEDs can easily obtain organic blue light compared to inorganic LEDs , and white light can be easily obtained by doping red and yellow dyes (dopant) in the organic light-emitting main layer, or by using multiple light-emitting layers to form white light. In addition, since OLED uses vacuum thermal evaporation and printing and spin coating to deposit thin films, the large-area process is simpler and the manufacturing cost is lower, making it easier to achieve the goal of reducing the finished product to compete with fluorescent lamps. Since OLED is a surface light source, it is more suitable for the backlight source of liquid crystal displays than the point light source of LEDs, so OLED has the opportunity to become the protagonist of the backlight source of liquid crystal displays in the new era.

1 Overview of Backlight Technology

Liquid crystal display (LCD) is a passive flat panel display device. Since the liquid crystal material itself does not have the property of emitting light, a light source must be added behind the LCD panel to achieve the display effect. The backlight module is the key component that provides the backlight source of the LCD display. A good LCD device backlight source should generally have high brightness, uniform light emission, low cost, adjustable, low power consumption, thin and not easy to damage, and preferably a surface light source.

When CCFL is used as the backlight source of LCD screen , its display quality has many shortcomings, such as: (1) Low display clarity: CCFL always illuminates the screen in a charged state, even if part of the displayed image should be black. The lack of deep black effect affects the visual clarity of the image. (2) Limited color saturation: Since the color of fluorescent objects is limited, the color saturation of the display is also limited. When LED is used as the backlight source of LCD screen, the driving circuit is complex, the overall structure of the backlight is complex, and the preparation cost is high.

2 Key technologies and progress of OLED backlight

OLED and LED are both semiconductor electro-optical conversion devices. The LED backlight technology developed above is also suitable for OLED backlight. There are also differences between the two: OLED is an organic surface light source, which is prepared by low-cost vacuum evaporation, spin coating and inkjet printing technologies; while LED is an inorganic point light source, which is prepared by expensive MOCVD technology.

OLED backlight is a reflective two-dimensional surface light source with the characteristics of high brightness, wide color gamut, stamping resistance, low voltage, light weight and low power consumption. Its cathode metal layer is a mirror reflection layer with high reflectivity. Therefore, OLED backlight does not need light guide plate , diffuser plate and other light guide and light uniformity auxiliary optical accessories. It can directly reflect the light emitted by the light emission layer to the LCD, which well meets the requirements of LCD for backlight.

Although the development of OLED backlight is less than 10 years old, it has made great progress. After the development in recent years, OLED backlight has overcome the initial problems of lifespan and light efficiency and entered the experimental use stage. Many related manufacturers in the world are already formulating plans to produce OLED backlight for LCD.

2.1 Improvement of the luminous efficiency of white light OLEDs After more than 10 years of research, the luminous efficiency of white light OLEDs has been continuously improved, especially in the past two years, the luminous efficiency of devices has made great progress.

In 2004, GE demonstrated a white OLED light source composed of 16 small panels. Each small panel was 50 mm*50 mm in size, with a color temperature of 4 000 K and a color rendering index (CRI) of 88; the power efficiency was 15 lm/W.

In 2006, Philips-Nova LED jointly developed a white light OLED with an efficiency of 32 lm/W, a color rendering index of 88 and a lifespan of 20,000 h at a brightness of 1000 cd/m2; Konica Minital demonstrated a white light OLED with a brightness of 1000 cd/m2, a power efficiency of 64 lm/W and a lifespan of 20,000 h.

In 2007, Universal Technology (UDC) of the United States produced a phosphorescent white OLED with an efficiency of 45 lm/W at a brightness of 1000 cd/m2; Novaled achieved a white OLED with an efficiency of 35 lm/W at a brightness of 1000 cd/m2, a color rendering index of 90, and a lifespan of 100,000 h.

In 2008, Philips-Nova led jointly developed a white light OLED with an efficiency of 50 lm/W and a life of 10,000 hours at a brightness of 1000 cd/m2; American Universal UDC developed a phosphorescent white light OLED with a luminous efficiency of 100 lm/W, a color rendering index of 70, a color temperature of 3900 K, and a life of 8000 hours at a brightness of 1000 cd/m2. This is the highest efficiency in white light technology at present, exceeding the efficiency of traditional fluorescent lamps, and heralding the rapid momentum of white light OLED entering the lighting market.

2.2 Improved lifespan and stability of white light OLED

Since the organic functional layer in OLED is very sensitive to water and oxygen, it is easy to react with the infiltrated water vapor or oxygen to form non-luminous black spots. In addition, the organic layer, especially the hole transport material, will also crystallize at room temperature, and accelerate with the increase of ambient temperature, resulting in a greatly shortened life of organic light-emitting devices at higher temperatures . Therefore, life is an important indicator for the practical application of white OLED light sources. The average life of incandescent lamps is 750~2500 hours, while the average life of fluorescent lamps is about 20000 hours. Short life has always been a major bottleneck in the development of OLED light sources, but in the past two years, the life of white OLED light sources has been greatly improved.

In 2008, UDC announced a full-phosphorescent white OLED device with a life of 20,000 h at an initial brightness of 1000 cd/m2. The long life of this white light device indicates that the blue phosphorescent material used is relatively stable. Idem itsu announced a full-fluorescent white OLED device with a life of 70,000 h at an initial brightness of 1000 cd/m2. Tsinghua University and Beijing Visionox have developed a full-fluorescent white OLED device with a life of more than 100,000 h at an initial brightness of 1000 cd/m2. Nova led has developed a stacked white OLED device with a life of more than 100,000 h at an initial brightness of 1000 cd/m2 using blue fluorescent and green and red phosphorescent materials.

2.3 Preparation of large-area OLEDs of dot matrix and panel types

Generally, organic thin films are deposited by vacuum thermal evaporation, inkjet printing, or spin coating. However, the thickness of OLED films is usually only about 100 nm. The organic films made in this way are prone to being non-dense and discontinuous, making it difficult to achieve uniform light emission over a large area of ​​OLED. How to achieve a large area of ​​uniform light source will be the first problem to be solved in the preparation of OLED backlight sources for LCDs. Currently, there are usually two methods for preparing large-area OLED devices: one is dot-matrix light emission, and the other is full-surface light emission.

Most of the existing OLED backlight sources for LCD use a full-surface light-emitting method. Since the anode of the OLED device generally uses a relatively large-resistance transparent electrode ITO, and the OLED itself is a current injection device, the current distribution of the larger area light source screen is uneven, resulting in poor uniformity of light emission of the entire screen, affecting the LCD display effect and the light emission stability of the OLED backlight source itself. Moreover, since the full-surface light-emitting OLED backlight source is always in a bright state during the LCD display process, this will inevitably lead to a relatively large power consumption of the OLED backlight source.

Dot matrix luminescence is to prepare many small luminous points with small gaps between each other, and these small luminous points emit light at the same time to achieve the effect of whole-surface luminescence. These small luminous points can be connected in parallel or in series. The advantage of connecting all the small luminous points in parallel is that it is easy to prepare, that is, it can be achieved by using the commonly used photolithography and cathode isolation column preparation methods. The disadvantage is that as long as one of all the small luminous points is short-circuited, the entire device will fail. The advantage of connecting all the small luminous points in series is that the short circuit of one or several of the small luminous points will not affect the luminescence of the entire device. The disadvantage is that there are great difficulties in the process to realize this preparation method. The overall luminescence performance of dot matrix light-emitting devices with the same effective luminous area is higher than that of whole-surface light-emitting devices.

The patented liquid crystal display and its backlight applied by Qiu Yong's team at Tsinghua University use a dot matrix OLED backlight. Its pixels are set to match the pixels in the liquid crystal panel . Reducing the power consumption of the backlight source, improving the uniformity and stability of the backlight source, and also helping to improve the display effect of the LCD device.

2.4 Inkjet printing for large-area OLEDs

Spin coating is the most commonly used technology for preparing PLED solution coatings. Although its production process is fast and simple, it has great limitations: the film thickness is not uniform enough and it is impossible to achieve precise positioning of RGB on the panel.

Professor Jacob of the University of Arizona in the United States developed the screen printing technology and has successfully developed the ink-jet printing (IJP) thin film preparation technology for full-color P LED displays . The PLED research groups currently developing this technology include CDT, Seiko-Epson, Philips, Covion, Litrex, Toshiba, and Taiwan Industrial Technology Research Institute.

By using PLED inkjet printing manufacturing equipment, adopting luminescent material blending technology, and adjusting the evaporation rate of the printing solution through real-time temperature control technology, the uniformity of the printed film can be adjusted to achieve high-speed and stable output of the printing solution. It can achieve continuous and stable printing of polymer solutions on large-area substrates and prepare large-area uniform OLED backlight panels. Cao Yong's team at South China University of Technology replaced the metal cathode with conductive silver glue and took the lead in the world in realizing the full printing preparation of OLED panels.

2.5 Polarized OLED Technology

OLED has the advantages of self-luminescence, lightness, thinness, and power saving. If polarized OLED materials and process technologies are developed, they can replace the existing LCD backlight and eliminate the use of the lower polarizer. Polymer OLED (PLED) has the advantages of easy process, easy mass production, and easy large-scale application. Polarized O/PLED devices can replace the backlight and a polarizer in LCD backlight applications, so they can achieve lower power consumption and higher brightness. They can reduce the number of polarizers, simplify the manufacturing process, and reduce costs. Their application prospects in the field of LCD backlight illumination are even more worth looking forward to. In the current OLED market, small molecule OLEDs are still the majority, while polymer OLEDs are the minority. However, in terms of polarization research, there are few studies on small molecules as polarized luminescent films.

The polymer molecules are arranged irregularly in the light-emitting layer of PLEDs, but if a certain method is used to arrange the polymer molecular chains in an orderly manner in a specific direction so that the light-emitting layer has dichroism, linear polarized light emission in this orientation direction can be obtained. There are many methods to achieve polarized luminescence, such as mechanical stretching, friction transfer and molecular self-assembly. International research on polarized PLEDs is not very in-depth. The method of producing polarized OLEDs requires that the light-emitting molecules are arranged in an inhomogeneous manner in one direction. At present, the development of organic light-emitting polarized OLED elements is mainly based on inhomogeneous film formation of polymer or polymer materials.

In 1995, Dyreklev first used the method of stretching molecular arrangement to grow poly (3- (4-octy lphenyl) -2, 2-bithiophene) (PTOPT) on polyethy lene (PE) film, stretched it to twice its original length, and then transferred the stretched PTOPT to the EL element to make an electroluminescent EL element that emits polarized light. The polarization ratio (the light intensity in the direction parallel to the molecular arrangement divided by the light intensity in the direction perpendicular to the molecular arrangement) is 2-4. However, this method has many disadvantages, such as complex film transfer methods and mechanical stretching control. Since then, research on polymer polarized luminescence has begun in some scientific research institutions.

Another method is the application of liquid crystal alignment film technology. MH Amaguch i et al. first used this friction alignment method on the conjugated polymer material alkoxy-substituted PPV , and made a polarized light OLED element including PrecPPV as a hole injection layer, alkoxy-substituted PPV as a friction-aligned light-emitting layer and electron transport material (2- (4-biphenyly l) -5- (4-tert-buty lpheny l) -1, 3,4-oxadiazo le, PBD) with a polarization ratio of 4. This method is currently the most ideal method for making polarized light OLED elements.

For polymer electroluminescent materials, preparing the light-emitting layer on the pre-rubbed oriented layer, and then annealing the luminescent molecules to arrange them in order along the direction of the oriented layer, is a simpler way to achieve polarized luminescence. D-Sa inova and MM isaki et al. achieved PFO blue light polarized electroluminescence through the light oriented layer and friction transfer method respectively. Compared with non-conductive polyimide, conductive polymers p-phenylene vinylene (PPV) and 3, 4-ethylenedioxythiophene: polystyrene sulfonic acid (PEDOT: PSS) have become the most commonly used oriented layer materials due to their own hole transport ability. KSW hitehead and P-Strohr iegl used rubbed PPV as the oriented layer to achieve high polarization luminescence of blue and green light respectively. Due to its water-soluble properties, it is not easily damaged by the luminescent layer solvent, so PEDOT: PSS is more widely used in PLED devices.

SH-Chen synthesized fluorene-based polymers of different wavelengths through molecular design and achieved polarized luminescence, but polymers with lower molecular weights must have lower stability in practical applications. DX-Zhu et al. first achieved polarized electroluminescence of a single white light polymer and used the FB dispersion model to study the optical constants of oriented films in different polarization directions.

So far, foreign research on polymer polarized luminescence has mostly focused on the friction orientation method, which will cause mechanical damage to the orientation layer to a certain extent; white light polymer polarization devices are realized by blending polymers with different luminous wavelengths, and phase separation is inevitable during the use of the blend.

2.6 OLED and LCD Matching Technology

Since OLED backlight devices are current-driven self-luminous bodies, small-sized OLEDs generally require a set of positive voltages (Vdd) and a set of negative voltages (Vss) for power supply, and the specifications of mobile phone power supplies are: Vdd voltage is about 2-5V, and Vss voltage range is -10~ -7 V. The input power supply of these two products is usually a lithium battery with a voltage range of about 3~ 4.2 V. Therefore, if the prepared OLED backlight device is to be well matched with the LCD screen, a backlight power supply solution must be designed.

Austrian Microelectronics has released the AS1343, a DC/DC boost converter that generates bias voltage for LCD or OLED displays from a low voltage input , thereby optimizing the overall performance of single-cell battery-powered applications. When powered by two or one AA batteries, the AS1343 can provide 24V and 40mA or 12V and 30 mA drive capabilities, respectively, and can provide an adjustable output voltage of 5.5 to 42 V. The AS1343 operates from a single input supply of 0.9 to 3.6 V, and the fixed switching frequency of 1MHz allows the use of tiny, ultra-thin inductors and capacitors to minimize the PCB footprint.

3 Analysis of the current status of OLED backlight industry

In 2003, the U.S. Department of Energy funded Osram with $4.6 million to develop OLED lighting technology. In February 2005, the European Union announced that it would spend 20 million euros to support the 45-month EU organic white light program. 14 university research institutes and electronics companies from EU member states participated in the program. The program is called the OLLA program, and Peter Visser of Philips is the program director of OLLA. This is the largest organic white light program ever seen.

Among the top ten applicants for white OLED-related patents abroad, eight are Japanese companies, and the other two are Korean and American companies. Japanese companies have a strong advantage in the field of white OLED, and the company with the most applications is Samsung SDI Co., Ltd. The patent technology topics of the top ten applicants are concentrated on electroluminescent light sources, organic optoelectronic devices, and luminescent materials. In terms of OLED lighting applications, lighting manufacturers such as Osram, Philips, and GE are mostly in the product research and development stage and have not yet officially launched white OLED lighting products.

In recent years, with the year-on-year improvement of OLED luminous efficiency and service life, before the advent of the new era of AMOLED display panels, OLED is becoming another best flat backlight technology for LCD panels with the help of mature panel preparation technology. In the OLED backlight market, Japan, Germany and Taiwan have taken the lead, but at this stage, OLED backlight is still in the stage of new technology and new market development.

In the development of OLED backlight sources for LCD, AMostery and others from Oxford University in the UK have done fruitful work. They have successively developed a green light and two blue light OLED backlight sources for LCD. General Motors of the United States has developed a white backlight source in the form of a large area surface light source. Universal Display recently developed a high-brightness and high-efficiency white phosphorescent backlight source. Recently, some people have been studying the production of OLED backlight sources with flexible substrates.

At the end of 2006, Japan's Tohoku Devices began to officially mass-produce 1.5-inch (37.5mm) OLED backlights that emit white light using two layers of orange and blue luminous materials. The brightness is 1000 cd/m2, the luminous life is more than 10,000 hours, and the thickness is 1.5 inches (37.5mm). It is used in mobile phones with STN-LCD panels as backlights, and will also begin to be supplied to BRICs (Brazil, Russia, India, and China). Because OLED backlights do not require light guide plates and diffusion films, their prices are close to those of LED backlight modules.

At CEATEC JAPAN 2007, Japan's Rohm Company demonstrated a backlight source that can emit three wavelengths of blue, green and red white light OLED panels. Its brightness is 5000 cd/m2, the average color rendering index Ra is 80, and the thickness is 1 mm. 20 pieces of white light OLED panels with an area of ​​40 mm2 are arranged like a puzzle inside the display panel to form a backlight source. In addition to being used in advertisements, public notice boards, etc., it can also be used in LCD panels.

In July 2008, Japan's Tohoku Device released a new ultra-thin OLED backlight product with a thickness of only 0.5 mm and mass-produced it. Traditional OLEDs require two encapsulation glass substrates, and the thinnest thickness is 1.13 mm. The new product is only attached to a glass substrate below and encapsulated with a film above. The amount of glass substrate is halved, the production cost is halved, and the panel weight is reduced by two-thirds. The weight of the 2.8-inch OLED backlight product is reduced from 6g to 2g. The original method of producing white light with orange and blue has also been upgraded to a mixed color method of red, blue and green.

Osram, a subsidiary of Siemens of Germany , announced in 2007 that it would stop producing OLED display panels and focus on lighting solutions using OLED technology. In terms of OLED lighting applications, Osram has achieved a transmittance of 55%, a 90m2 white light OLED with an emission ratio of 3.4, and a maximum luminous efficiency of 20 lm/W. It also announced that it would launch a limited edition OLED module table lamp in the spring of 2008. Taiwan OLED manufacturers such as CMEL, RiTdisplay and Univision are still focusing on the OLED display panel market, but have not made any achievements in OLED backlighting.

Due to the structure of OLED backlight, components such as light guide plate, light collecting plate and diffusion plate with low yield can be omitted. In addition, OLED backlight module does not need expensive LTPS TFT backplane or complex circuit process, only transparent conductive film substrate is needed, so its production cost can be greatly reduced. OLED technology is expected to make breakthroughs in technology and share the future backlight market, and even become the mainstream technology in the future backlight market. The adoption of low-cost molecular ink printing is expected to promote OLED to enter the backlight market on a large scale. It is estimated that the OLED backlight market will reach a scale of 1.1 billion US dollars in 2015.

4 Outlook of OLED backlight technology

Solid-state light source LED and OLED backlight are green and environmentally friendly products that do not contain mercury. They can improve color saturation, eliminate smearing, and increase contrast. The introduction of dynamic backlight technology has greatly improved the quality of LCD display images, fully meeting the modern society's demand for low power consumption and high quality.

In recent years, OELD backlight technology has made many breakthroughs, and its performance indicators such as luminous efficiency and service life have been steadily improved. At present, some OLED manufacturers have begun to ship OLED backlight modules to mobile phone manufacturers with small-size STN-LCD panels, but there are still some problems before large-scale application: 1. Preparation of large-area, uniform film thickness, uniform luminescence, and high-quality backlight sources; 2. OLED luminous efficiency needs to be improved; 3. The service life stability of OLED devices needs to be improved; 4. The performance of OLED backlight sources on flexible substrates needs to be improved; 5. The stability of color purity of white OLED backlight sources under voltage changes needs to be improved; 6. The integration technology of OLED backlight sources and LCDs needs to be improved; 7. Further reduce the production cost and market price of OLEDs.

Currently, OLED light source technology is catching up with LED light source technology in LCD backlight modules and solid-state lighting applications . Compared with LED point light sources, OLED also has the diversity of white light materials, simplicity of process and low cost, especially its surface light source properties. It can be predicted that in the application field of LCD backlight module, OLED backlight will have a brighter industrial prospect.

5 Conclusion

OLED backlight has become a research hotspot in recent years due to its advantages such as flat surface light source, easy large-area preparation and low preparation cost. After 20 years of development, OLED technology has developed from the initial single color to efficient white light, and has the conditions to be used as a white light backlight, and has been applied to small-sized backlights. However, in the long run, OLED backlights still need to continuously improve their luminous efficiency, stability and uniformity.