The present and future of automotive LEDs

1 Introduction to LED

With the development of science and technology, people have higher and higher requirements on automobile light sources. LED (Light Emitting Diode) has gradually attracted the attention of technicians due to its excellent performance.

LED is made of semiconductors such as III-IV compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide phosphide), etc., and its core is PN junction. Therefore, it has the I-N characteristics of the general P-N junction, that is, forward conduction, reverse cutoff and breakdown characteristics. In addition, under certain conditions, it also has the characteristic of luminescence. Under forward voltage, electrons are injected from the N region into the P region, and holes are injected from the P region into the N region. A part of the minority carriers (minority carriers) entering the other region recombine with the majority carriers (majority carriers) to emit light, as shown in Figure 1.

Assuming that the luminescence occurs in the P region, the injected electrons directly recombine with the valence band holes to emit light, or are first captured by the luminescent center and then recombine with the holes to emit light. In addition to this luminescent recombination, some electrons are captured by non-luminescent centers (this center is between the conduction band and the intermediate band), and then recombine with holes. The energy released each time is not large and cannot form visible light. The greater the ratio of the luminescent recombination amount to the non-luminescent recombination amount, the higher the light quantum efficiency. Since the recombination is luminescent in the minority carrier diffusion region, light is only generated within a few μm close to the PN junction surface.

Theoretical and practical results show that the peak wavelength λ of light is related to the bandgap width Eg of the semiconductor material in the light-emitting area, namely:

The unit of Eg is electron volt (eV). If visible light (wavelength between 380 nm purple light and 780 nm red light) can be generated, the Eg of the semiconductor material should be between 3.26 and 1.63 eV. Light with a wavelength longer than red light is infrared light. There are now light-emitting diodes in infrared, red, yellow, green and blue colors, but due to reasons such as materials and manufacturing, the costs are somewhat different.

LED was introduced as early as the early 1960s. Although it has a short excitation response time, a long effective life, and low power consumption, due to the limitations of manufacturing technology, its luminous brightness is small and only red is available. For a long time, it was only used in limited fields such as small indicator light sources and alphanumeric displays. In 1986, the high-mounted brake lights on the Nissan 300ZX used 72 5 mm LEDs, marking the gradual entry of LEDs into the automotive industry. At present, with the development of science and technology, the performance of LEDs has gradually improved. For example, the increase in its luminous brightness has greatly reduced the number of LEDs required in each high-mounted brake light, thereby reducing costs and making it widely used. In addition, with the development of materials, brighter and more colorful LEDs will be used in other lamps.

2 Advantages of LED

2.1 Lighting Effect

The light efficiency is reflected in the luminous flux generated by the light source for every 1 W of electrical energy consumed. Figure 1 Schematic diagram of LED light-emitting principle (lumens). With the continuous development of semiconductor material technology, the light efficiency of LED has been continuously improved. By the mid-1990s, with the use of the new material AlInGaP, the light efficiency of red and amber LEDs reached 10-20 lm/W. According to Lumileds' technical progress, the light efficiency of red, orange and yellow high-brightness light-emitting diodes in 2005 was about 50 lm/W, which is higher than the light efficiency of incandescent lamps.

2.2 Lifespan

LEDs have an advantage in terms of lifespan. The lifespan of LEDs can be as long as tens of thousands of hours, which is much longer than the thousands of hours of incandescent lamps. Therefore, LEDs usually do not need to be replaced during the lifespan of a car.

2.3 Color

LED has good monochromaticity and its radiation spectrum is narrow-band, so no filter is needed and most of the light flux is used; while incandescent lamps radiate a continuous spectrum and filters are needed to obtain the desired color, which loses a lot of light flux in the process. Studies have shown that considering the advantages of LED's good monochromaticity and long service life, the economic benefits of using LED are not very bad.

2.4 Response Time

LED has a very short response time. This is conducive to improving the safety of the car, which is one of the reasons why LED is widely used in high-mounted brake lights. Studies have shown that the response time of LED is about 100 ns, which is much shorter than the 200 ms of incandescent lamps. If the speed is calculated at 100 km/h, using LED as the light source of the brake signal light can leave about 6 meters of braking distance for the road users behind.

2.5 Heat

LEDs generate much less heat than traditional automotive light sources, which is very important for headlamp design. Plastic can be used in the parts of LED headlamp systems that are connected to the light source, which provides designers with greater freedom in configuration design. At the same time, designers can use plastic light guides, which is difficult to achieve with current headlamp systems. This allows designers to use different methods to achieve aesthetic requirements according to different conditions, thereby making the front profile of the car more reasonable.

3 Problems in the application of LED in automobiles

3.1 Price

In terms of price, a set of ordinary light bulbs costs less than 10 yuan, while LED lights generally cost tens of yuan. Although considering that models using ordinary lights need to replace bulbs, the overall price difference is not much. However, due to the cost control of vehicle manufacturers, the promotion of LED lights is still subject to certain restrictions.

3.2 Brightness

The maximum brightness of today's LEDs is 4 cd/mm2, which is much lower than the brightness of halogen lamps (20-25 cd/mm2) and even lower than that of HID headlights, which is only a dozen times lower. Therefore, in order to design a suitable size of automotive headlight system, the output brightness of a single LED must be increased by 2-3 times.

4 Current Applications of LED in Automobiles

4.1 Interior lighting

LEDs were first used in cars as indicator lights on dashboards. For example, Mercedes-Benz and Audi use white, BMW uses orange, and Saab uses green LEDs as indicator lights for instruments and control units.

The color of LED is not only in harmony with the interior layout of the car, but also represents the culture of the company. In recent years, due to the reduction of product prices, the application field of LED has expanded to in-car reading lights, footlights, etc. For example, Gentex Corporation of the United States combines white LED with car taillights as in-car reading lights. Currently, car manufacturers including General Motors, Ford, Chrysler and others are using such products. Toyota's new minivan Alphard uses white light LED as in-car reading lights, and Audi A6 and other models also have similar designs.

4.2 Traffic lights

LEDs have been used in high-mounted brake lights for nearly 20 years. So far, the global market share of LED high-mounted brake lights is about 40%. In 2000, Cadillac launched the DeVille model, which first applied LEDs to the taillight group, opening up a new application of LEDs in the light source of the car's exterior. The taillight group launched by Cadillac consists of 68 LEDs, of which 16 are used as turn signals and warning lights, 4 are used as side marker lights, and 48 are used as brake lights. Although Cadillac was the first manufacturer to apply LEDs to the taillight group, other manufacturers applied LEDs to the rear light source of the car, such as brake lights and turn signals, long before Cadillac. In recent years, the trend of using LEDs as the light source of the rear end of the car has tended to expand. Many European and Japanese manufacturers have launched related products, including Audi, Alfa Romeo, BMW, Mercedes-Benz in Europe, Toyota, Mitsubishi and other manufacturers in Japan, and its application has developed from high-priced cars to civilian cars, and from the use of a single light source to the use of LEDs in the entire taillight group.

4.3 Headlights

Headlamp light source is the part with higher requirements in automobile light source. Due to the problems of insufficient unit brightness and high price of LED, as well as the strong competitive advantage of HID lamp in this market, LED is still limited to the application of future concept cars, such as Cadillac concept car (Sixteen), Audi concept car (Nuvolari), Mitsubishi concept car (CZ 2), Ford concept car (Model U), etc., which all use headlamp modules made of dozens of white light LEDs. However, judging from the historical experience of HID lamps in the development of automobile headlamps and the technological progress of LEDs, it is expected that commercial headlamp LEDs will appear in the next few years.

5 Application prospects

The main functions of automotive light sources are lighting and indication, which have a great impact on the safety of automobile driving. Therefore, all countries have formulated relevant standards to regulate automotive light sources. The speed and connotation of the formulation of standards for high-brightness LED, an emerging automotive light source, will also affect the speed and scope of the development of the automotive LED market.

Overall, automotive LEDs are still in their infancy in the market, but due to their excellent characteristics, their application prospects in automobiles are very bright. According to surveys, the production and matching of LED central high-mounted brake lights in China are already relatively common, while LED combination taillights, which have a larger market scale and broader prospects, are still in their infancy. Domestically, the bulk production of automotive LED combination taillights is mainly carried out by Sino-foreign joint ventures such as Koito, Hella, Valeo, and Stanley, and some other domestic car light companies have begun research and development of automotive LED combination taillights. In the next few years, automotive LED combination taillights will be one of the hot spots in the automotive market. It is predicted that by 2008, the matching volume of automotive LED combination taillights in the original equipment market in my country will exceed 1 million sets, and the average annual growth rate of the market will exceed 80% in the next few years.