Discussion on the bottleneck of LED promotion and application

introduction

As one of the new high-efficiency solid-state light sources (SSL), LED has significant advantages such as long life, energy saving and environmental protection. It is another leap in the history of human lighting following the advent of incandescent lamps, fluorescent lamps and high-pressure gas discharge lamps. [1] Especially in the early 1990s, Japanese researcher Nakamura Shuji successfully developed a Mg-doped homojunction GaN blue LED, which made the white LED have practical and effective technical solutions, semiconductor illumination source and solid state. The field of lighting has also become a more attractive hot spot in the field of optoelectronics research at home and abroad. With the rapid development of material growth and fabrication technology, the luminous efficiency of LEDs has been significantly improved. LED devices have also evolved from early indicator LEDs (constant current 20mA) to power LEDs (constant current 350mA).

With the continuous improvement of LED performance, its application fields continue to expand, from the initial state representation to the current signal display, street lighting and automotive lighting. Although the application field of LED lighting is expanding, there are still many problems in the popularization of lighting applications. This article will explore the bottlenecks in the promotion and application of LEDs, focusing on the analysis of luminous efficiency, heat dissipation, drive circuits, non-imaging optical design, cost and standards.

1 Technical level

1.1 Luminous efficiency

At present, the luminous flux of a single LED needs to be improved, one of the ways is to improve the luminous efficiency of its LED under the same power conditions. In recent decades of research, LED light efficiency has achieved remarkable achievements. The LED developed by the laboratory has achieved a luminous efficiency of 150 lm/W. The LED light performance of the commercial mass production specification of the industry more than 1 year ago has reached 50 lm/w, and recently it has increased to 70 lm/W, even higher. Report. The level of light achieved by it has created a powerful challenge for traditional light sources.

In general, the improvement of LED luminous efficiency is mainly two basic ways, namely to improve its internal quantum efficiency and improve its external quantum efficiency. The internal quantum efficiency is the ratio of the number of photons generated by radiation recombination per second to the total number of electron-hole pairs recombined per second in the active region. The external quantum efficiency is the photon emitted by the device per second. The ratio of the number and the number of electrons passing through the LED per second [2]. The key to improving the internal quantum efficiency is to improve the epitaxial process of the crystal, reduce the misalignment of the crystal, and improve the quantum well structure by optimizing the quantum well width and other measures [3], thereby further improving the crystal quality and improving the device performance. In order to improve the external quantum efficiency, the future will be mainly from the chip technology point of view, through the optimization of the chip structure design, such as optimized substrate stripping technology, surface roughening technology and the use of photonic crystal structure, can simultaneously improve the internal and external quantum efficiency of the chip [4 ].

Based on current technical conditions and research and development levels, Philips Lumileds proposed in 2008 that the efficiency parameters will be improved in the future:

Table 1 [5] List of efficiency parameters when the drive current reaches 2A

The phosphor conversion efficiency in the table will reach 240 lm/Optical Watt in the future. For cool white LEDs, this is a achievable numerical level, but for warm white LEDs (CCT around 3000 °C), this efficiency will be reduced by 10% to 20%. Therefore, the research on more efficient LED phosphors will be of great significance for the further improvement of LED light efficiency.

1.2 Thermal issues

The 70% of the electric energy added by the LED operation, even a higher proportion of electric energy is converted into heat, and unlike the conventional lighting device, the white LED does not contain the infrared part in the luminescence spectrum, so its heat cannot be released by radiation, due to temperature. LED has emission characteristics of the material greatly affect the total luminous flux output by the LED with its own temperature increases rapidly lowered, the heat dissipation problem LED development becomes great care must be an important factor.