At present, LEDs have so many applications at present because of the advantages that are mentioned above. However, there are still some issues need to be addressed. With the development of modern technology, the full potential of LEDs is waiting to be achieved. Hence, the next content of this paper will be given as follows:1) current status of LEDs; 2) issues that need to be addressed; 3) future directions in the R&D of LEDs.
The first commercial LED was invented in 1962.
In the period of 1960s to 1980s, the major application of LEDs were indicators and alpha-numeric displays. The first high-brightness LED bulb was invented in 1980s. From 1990s to today. LEDs have expanded their application in many fields and Table 1 gives some examples. In brief, as a light source, LEDs can be used for general lighting or as backlights in some products, such as TVs, cameras and mobile phones . As indicators and singes, LEDs are often be used for traffic lighting, exit lighting and emergency lighting.
Besides, LEDs can also find a place in communication and signalling, one typical application is fiber optics . Nowadays, science and technology keep moving forward, LEDs are gradually starting to be used in the machine vision system, medical service and agriculture [3-5].
In the past few years, research and development have yielded impressive improvements within this area. In medical fields, LEDs are usually be used for medical illumination, and there are still some reports about that the LEDs are suitable for the therapy of specific illness.
For example, the clinical results are encouraging due to minor incidences of adverse events when using LEDs on the treatment of dermatology [6-8]. For single ultraviolet LED, its light extraction efficiency has been improved a lot by using aluminum nitride (AIN) hybrid nanostructure . As for white LED, the aging problem of organic binder in the white LEDs was solved by introducing the ceramic phosphors . LEDs also play a role in the agriculture, most of the applications focus on plant growth lights, fishing lights and selective pest trap lights, etc. The crop yield and nutritional value are able to be improved when choosing proper LED light solution [11-13]. In addition, many material scientists put their efforts on the photoelectric conversion material and have great achievements. One of the typical material should be perovskite, and some new structure designs of LED have been proposed based on it. For instance, a new “insulator-perovskite-insulator ” structure was reported, the experimental results show that this design strategy can induce charge carriers into the perovskite crystal, and block the current leakage effectively . Organic-inorganic hybrid perovskite is also a good solution to overcome the electroluminescence efficiency limitations, and the current efficiency can go up to 42.9 candelas per ampere . Besides, there are still some improvements regarding the packaging material, heat management, system reliability analysis and manufacturing method, etc . all these research outcomes promote the development of LED industry and push different LED device go to the market.
Application fields Application examples
Lighting LED lamps for general lighting
Backlight (TVs, cameras, mobile phones)
Data communication and signalling Fiber optics
Others Machine vision system (barcode scanner)
Despite the rapid progress in the R&D of the LEDs that are mentioned above, they still have not yet achieved the full potential. Some issues remain to be addressed in these areas, such as degradation, light efficiency, system reliability, energy efficiency, design flexibility and cost, etc. In this section, two major issues, i.e. reliability and cost are given in detail below.
when it comes to reliability, it presents the ability of a system or component to perform its demanded functions under stated conditions for a specified period of time. This is the definition given by the Institute of Electrical and Electronic Engineers (IEEE). As a component, the performance of one LED is judged by the light output or change of the color. This can be distinguished by eyes. Sometimes, the stability of light source for LED is ignored in general lighting conditions. However, for some specific applications, like medical devices and spectrograph, which required very high stability. In other words, it asks for the LEDs to be more reliable.
Most of the time, the reliability is more like a systematic concept, because the LEDs are always cooperated with other components, like to be attached to a board or to be integrated into chips. Then it needs drivers to extract light out of the LED. For driver components, such as resistors and capacitors, which have to be mated to the LEDs. And then, there are secondary optics that are placed over these LEDs. Therefore, it is really complicated to estimate the reliability of LEDs because any of those factors could influence its performance. . Luckily, relative standards have already been built by some institutions, but there are still a lot of works need to be done regarding this issue.
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