OLED shows strong competitiveness in the field of display technology

LED --Alternative to Traditional Light Sources

White light organic light-emitting diodes are considered to be an alternative to traditional white light sources. They are highly efficient solid-state light sources, with electro-optical conversion efficiencies exceeding those of incandescent lamps, and great progress has been made recently in device structure and synthesis of new materials.

The world consumes a huge amount of electricity every year . Of all the electricity consumed, lighting accounts for 20% of the total electricity production. Fluorescent 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 electricity into heat energy, while fluorescent lamps perform better, converting 70% of the consumed electricity into light energy. The typical luminous efficiency of incandescent and fluorescent lamps is 13-20lm/W and 90lm/W respectively. 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 developing semiconductor LEDs with better performance. Red, green, blue and other color LEDs made of inorganic materials have long been on the market, and they are widely used in traffic lights , car taillights and other small applications. Inorganic white light LEDs have also appeared on the market, but their prices are still relatively high for general lighting use. Now a new competitor to lighting sources has also entered the market, and it is LEDs based on organic semiconductor materials.

OLED shows strong competitiveness in the field of display technology

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) aluminium (Alq3)[2] in 1987 and poly(p-phenylene vinylene) (P PV )[3] ​​in 1990, OLEDs have become the most attractive display technology. It has the advantages of simple preparation, short response time, high brightness , wide viewing angle, low driving voltage, the most likely application on flexible substrates and full color display. OLED displays are durable, efficient and can be prepared on flexible substrates such as plastic and paper surfaces, and the prepared displays can be bent or rolled up. Unlike liquid crystals, OLEDs are self-luminous and do not require a backlight, which makes OLED displays thinner and lighter.

OLED is a multilayer device consisting of an active charge transport layer and a light-emitting layer sandwiched between two thin film electrodes, at least one of which is transparent. Generally, indium tin oxide (ITO) with high work function (~4.8eV), low surface resistance (~20 Ω / □) and transparent to visible light is used as the anode, and the cathode is generally made of metal with low work function, 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 light-emitting layer.

Similarly, the hole transport layer and electron blocking layer are used between the anode and the light-emitting layer. When a bias is applied externally, electrons and holes are injected from the cathode and anode of the OLED respectively. Under the action of the external electric field, the electrons and holes migrate in opposite directions and recombine in the light-emitting area to form excitons, which decay and radiate light outward. The migration mechanics 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 in flat panel displays as a backlight source for liquid crystals. In the preparation of full-color displays, the three primary colors are equally important, but white light emission has gained more attention because any desired color range can be obtained by filtering white light.

The first white OLED device was made by Kido and his colleagues in 1993. The device contained compounds that emit red, green and blue light, which together produce white light. But there were some problems. The efficiency of the device was less than 1lm/W, the device required a large drive voltage, and it burned out quickly. But now the efficiency of these devices is improving rapidly. The annual efficiency improvement in traditional LEDs, nitride LEDs, and white OLEDs is shown in Figure 1.

Figure 1: Annual progress in light-emitting diode efficiency

OLED's way to generate white light

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 of the International Commission on Illumination's chromaticity diagram). There are two ways to generate white light from OLEDs. (a) Wavelength conversion. The blue light

or ultraviolet light emitted from the OLED is used to excite several phosphorescent materials. The different colors of light emitted by each material are mixed together to obtain white light with a rich wavelength range. This technology is called phosphorescent down-conversion.

(b) Color mixing. This method uses multiple light-emitting layers in a device and mixes the different colors of light emitted by different light-emitting layers to produce white light. White light can be obtained by mixing two complementary colors (blue and orange) or three primary colors (red, green, and blue). Typical methods of generating multiple colors of light through a multi-layer structure and mixing various colors to obtain white light mainly include:
(1) Multi-layer structure containing red, green, and blue light-emitting layers
(2) F¨orster/Dexter energy conversion
(3) Microcavity structure
(4) Obtaining white light through vertical/horizontal stacking structure
(5) Mixing or doping different light-emitting materials into a mixed layer.
In color mixing technology, since no phosphorescent material is used, the loss caused by wavelength conversion will not occur. This technology has the potential to achieve higher efficiency. At present, obtaining stable quality white light is still a hot topic in research and development.