What are the problems with OLED lighting technology so far that need to be improved?

The 0LED lighting industry saw us see its first commercial product, but it was very expensive. While companies have made significant progress in OLED performance, expensive materials and manufacturing costs still keep OLED prices high. However, OLED lighting applications seem to have become the first choice for OLED manufacturers. In the last few years, most manufacturers are transitioning to lighting applications. The technical problems of OLED lighting are shown in Figure 1.

Figure 1 - OLED lighting technical issues
Figure 1 – OLED lighting technical issues

The technical requirements of OLED in lighting applications are simpler than those of display technology and well-established lighting technologies such as light bulbs and fluorescent tubes, and are easier to replace these traditional lighting technologies. OLED lighting technology is actually quite simple, i.e. a light can be one large pixel, while a display has thousands of pixels and may require an active matrix backplane. In addition, in some countries, OLED lighting has received substantial government funding for research and development (R&D).

A big news in 2009 was the introduction of the first OLED lighting products. Osram has launched the world’s first “functional table lamp” based on OLED technology, albeit in small quantities. However, it brings clarity and clarity to the OLED market that is possible. The “Big Three” lighting companies (GE, Osram, and Philips) set the stage for OLED lighting development, define acceptable performance levels, and introduce lighting panels with that performance that designers can design for inspiration .

Compared with existing light sources, the challenges faced by OLED lighting are:
1) High brightness: At least 1000cd/m2 brightness is required for lighting applications.
2) Long life: Long working and long life is necessary.
3) High efficiency: White light must have at least 30lm/W efficiency.
4) Good uniformity: especially in large area applications.
5) High color rendering under high brightness: >80, direct lighting application.
6) Very low cost.

Currently, almost all OLEDs use expensive rare metals in the organic layers and indium tin oxide transparent electrodes, but the focus is on how to incorporate rare elements in the organic layers to improve performance and replace steel with common materials. For example, polythiophene transparent electrodes have been used successfully. The continuous improvement of gravure printing drive circuits and printing technology is particularly promising.

  1. Luminous efficacy

Luminous efficacy describes the ratio of the luminous flux emitted by a luminaire to the electrical power consumed; the higher the efficacy, the less energy is lost. The efficiency of the interior and exterior is related to the efficient generation and extraction of photons, the less heat generated by electron-hole pair recombination, the greater the number of photons generated.
For energy-saving OLED lighting, a big step forward is its efficiency exceeding 60lm/W. OLEDs exceeding 60lm/W not only meet the international standard color requirements for white light, but also meet the color requirements of the International Energy Star SSL standard. Luminous efficiencies of this scale have been achieved before. However, as of now, the color values ​​of OLEDs are not within the acceptable range of color coordinates around the Planck curve, which is also defined by the ENERGY STAR SSL standard. The color values ​​of the new OLEDs remain within the white light color range for varying degrees of luminous intensities.

A brightness level of 100cd/m2 is necessary if the entire ceiling is illuminated (if the room is large, you will also get 100cd/m2 at tabletop level). For partially luminous ceilings (eg in a common office) the required brightness is around 1000cd/m2. However, the lighting industry generally does not accept brightness greater than 850cd/m2 due to glare. For a brightness of 850cd/m2, approximately 12% of the area of ​​the ceiling is required for lighting. The brightness level of 850cd/m2 determines the target for efficiency and stability calculations.

The short-term performance target is greater than 100lm/W. To achieve this, it is estimated that the efficiencies of R, G, and B need to be improved by a factor of 2, 3, and 4, respectively.

  1. Low drive voltage

The driving voltage is also a critical parameter for the efficiency of the OLED. However, pin technology has allowed the drive voltage to be relatively close to the thermodynamic limit. In solid-state lighting applications, phosphorescence has great potential for producing efficient white light [94] (Nakayama et al., 2007, Proceedings of society for information display, “Unpublished”, pl018).

A light-emitting device utilizing a blue-emitting fluorescent complex doped with wide-bandgap impurities has been reported [95], with a driving voltage as low as 4.8 V and a practical brightness of 1000 cd/m2, CIEx,y, coordinates (0.16, 0.35) . Noval ed company has proved that the working voltage of fluorescent white OLED is 3.05V, the brightness reaches 1000cd/m2, and the corresponding luminous efficacy is 17.5lm/w [96]. Polymeter has demonstrated a commercial white OLED lighting system and its electronic driver [97]. The set reportedly has a power consumption as low as 5.3V, 125mA (663mW).

  1. Color characteristics

White light with the same CIEx,y, chromaticity coordinates can be achieved by mixing two, three or more colors of light. It is important to find the best spectral mixture to obtain the proper CIE, chromaticity coordinates and color rendering. Fundamental material properties and fabrication processes can significantly influence and control the color and CRI of the emission. Therefore, to achieve a CRI for a specified white light, the material must have sufficient stability and a suitable emission spectrum.

  1. Longevity

High efficiency, low voltage, stable performance, cheap and reliable devices can be obtained by adjusting some parameters related to materials. Therefore, it is necessary to (1) understand the degradation and failure mechanisms to extend the actual lifetime of devices so that they have the most favorable lifetime cost possible, and (2) address key material performance challenges in OLED general lighting applications.

For general lighting applications based on OLEDs, the establishment of high efficiency, low voltage and stable material properties must simultaneously ensure that: substantially all electrons and holes entering the device form excitons; high probability of exciton radiative recombination; for a given The current density must have a minimum driving voltage; materials and devices have better stability under continuous working conditions. For OLED lighting, at 850cd/m2 brightness, at least 10000h of operation, the brightness loss of all colors is a maximum of 20%.

The Orbe os panel released by Osram is the first OLED product on the market. Ore o s has no delay at the moment of switching, and the brightness is continuously adjustable. Unlike LED lights, its thermal management is very simple.

Under ideal working conditions with input power less than 1W, the brightness level is usually 1000cd/m2, and the lifetime is about 5000h. For OLED lighting products, 5000h is an acceptable lifetime.

  1. Cost

The development of technology and manufacturing process determines the cost of OLED lamps. Therefore, it is necessary for the development of OLED technology to meet the performance requirements at an acceptable cost. The price per square meter of OLED lighting panels is still very high. The short-term goal is to reduce its price to less than or equal to $200 by 2012 and to less than or equal to $50 by 2020 [98]. The near-term price target is around $200/m2. Simplified and improved packaging and deposition techniques with advances in base materials may be some of the options for reducing manufacturing costs.

  1. Packaging

Moisture intrusion into the luminaire can damage or destroy the organic materials of the OLED. Organic materials and cathode metal films are sensitive to the presence of water vapor and oxygen. As a result, long lifetime has become a major and debatable issue for OLEDs. The presence of oxygen and water vapor in excess of parts per million (ppm) can degrade OLED device performance. Organic materials oxidize under oxygen and water vapor. Metal films used for cathodes are extremely susceptible to delamination in oxygen and water vapor environments. The end result is a dramatic reduction in brightness, with non-luminous “dark spots” forming and expanding. To achieve a minimum lifetime of 10000h, the water vapor transmission rate (WVTR) in the OLED encapsulation material is 10-6g/(m2·d). Accordingly, an improved OLED encapsulation process has been developed for reducing the ingress of moisture and increasing device lifetime.

Encapsulation technology is one of the most expensive parts of white OLED fabrication, both in terms of cost and processing time. OLED packaging for lighting applications shall meet the following standards:

1) Thermal management and cooling technology (not serious).
2) Use an encapsulant to achieve a stable device.
3) Down conversion material to maximize lumen output.

Therefore, it is necessary to design products that meet actual packaging requirements in order to meet marketing and production goals. Various efforts have been made to develop a new barrier material for significantly extending the lifetime of OLEDs. Regarded as a state-of-the-art performance, Huntsman has developed a thin barrier coating and was recently integrated in the world’s first flexible OLED system for a racing car.

6.1 Substrate packaging method

Manufacturers now seal displays in an inert atmosphere or vacuum. The glass cover is glued to the top of the display substrate, and the powder inside the display is used to absorb the moisture escaping through the adhesive. These seals are expensive and require labor-intensive assembly.

The easiest way to do this is to use glass as the top cover of the nitrogen-filled chamber. A schematic diagram of the structure is shown in Figure 2. The configuration uses epoxy, an encapsulant widely used in electronic packaging. In addition, a layer of a-Si N x is deposited to reduce the infiltration of humidity and oxygen from the side. The sealing process is carried out using pressure and heating elements.

Figure 2-a) The principle of substrate packaging of OLED devices Figure 1, b) The principle of substrate packaging of OLED devices Figure 2.
Figure 2-a) The principle of substrate packaging of OLED devices Figure 1, b) The principle of substrate packaging of OLED devices Figure 2.

6.2 Thin Film Encapsulation Method

Thin film encapsulation solutions are based on silicon carbide/silicon nitride/oxynitride sealant coatings. When these coatings are applied to OLED lighting devices that use glass and plastic, they will help achieve greater efficiency, longer lifetimes, and lower costs than existing products. More recently, thin-film device protection technologies are being developed to prevent degradation from airborne moisture.

The researchers used an advanced ion beam-assisted deposition technique, which uses reactive ions to deposit a high-density, non-porous, thin silicon oxynitride (Si ON) film on the OLED surface. Figure 4.26 is a schematic diagram of thin film packaging. Ideally, the film should be as thin as possible, but if it is too thin, pinholes or other defects can appear and cause problems. The results show that the film thickness between 50 and 200 nm is the most perfect. During testing, the Si ON encapsulated OLED showed no signs of degradation after 7 months in the open air, while the uncoated OLED completely degraded in less than 2 weeks under the same conditions. In the environmental accelerated aging test, keeping the temperature at 50°C and 50% relative humidity for at least 2 weeks, the Si ON thin film encapsulation of OLEDs showed little degradation. However, unencapsulated OLEDs decompose immediately. On the outside of the multi-layer material, epoxy resin and thin metal boxes can be added to enhance its effectiveness against water and oxygen penetration.

6.3 Packaging method

The encapsulation technology of flexible OLEDs using polydimethylsiloxane (PDMS) is a novel encapsulation method. In this method, the use of polycarbonate films, silicon dioxide and PDMS can improve the lifetime of OLEDs in air. Optical measurements of the calcium protective film encapsulated with PDMS show that the permeation rates of water and oxygen encapsulated by PDMS are reduced to 0.57 g/m2d. PDMS coatings have good application prospects for flexible OLEDs. A good moisture barrier and encapsulation technology is a key factor in the transition of flexible electronics from the laboratory to the market.

Considering the low production cost, the candidate technology must be compatible with roll-to-roll manufacturing processes. As a result, roll-to-roll plasma-enhanced chemical vapor deposition (PECVD) deposition tools for barrier packaging will be installed alongside existing roll-to-roll facilities.

Read more: What are LED materials and their evolution?