Artificial Sun based on Light-emitting Diode

Transcription

1 Artificial Sun based on Light-emitting Diode Siyu Chen, 22/05/2015 Department of Physics, Xiamen University 1. INTRODUCTION Electroluminescence as an optical phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs. After that many scientists devoted to important photonic technologies that are changing the world around us by enabling new applications or transforming existing ones. The Light Emitting Diode or LED as it is more commonly called, seems to be a typical and important product since then. Over the past forty years, advances in materials science have enabled these compound semiconductors rapidly becoming a brand new light source which emits a bright, efficient light that is giving cars, interior lighting and signage a radical new look. However, although LEDs break the restriction of traditional luminescent mechanism and has many advantages such as the high stability, the wide controllable output range as well as the long life, there exist some defects. One of serious defects is that compared with traditional halogen and xenon lamps which can cover a wide spectral bandwidth, the spectrum of LED is usually monochromatic, which is quite different from solar spectrum. In this article, I present an original approach to combine numerous multiple color LEDs to fabricate an artificial sun. It functioned well as a mini sun with the features of low cost and easy implementation. I believe that this light source that reproduces the solar spectrum and power will be vital for animal, plant and solar devices in the future. 2. THEORY AND FOUNDATION 2.1. Semiconductor material As a matter of fact, the popular LED is basically just a specialized type of diode made from a very thin layer of fairly heavily doped semiconductor material. In a semiconductor material, such as silicon, the atoms have 4 electrons in the outermost electron energy band, called the valence band. This leaves room for 4 more electrons in the valence band, and these available "electron-slots", often referred to as holes. By introducing atoms with different electron-configurations into the lattice we can create a doped semiconductor. If we introduce trace amounts of the boron atom to a silicon lattice we get a p-type semiconductor. Because boron only has three valence electrons, this will leave occasional holes in the valence bands in the lattice. At room temperature these holes will move quite freely around in the material effectively making it a positive charge carrier, hence the name "p-type" semiconductor. Similarly an n-type semiconductor can be formed by doping the same silicon lattice with arsenic. Arsenic has 5 valence electrons and will provide the material with easily movable electrons at room temperature, which is a negative charge carrier, hence the name "n-type" semiconductor.

2 Fig.1. (a) P-type semiconductor (b) N-type semiconductor 2.2. LED Inner workings When a piece of semiconductor material changes type from p-type to n-type over a cross-section it forms a P-N junction. At temperatures well above absolute zero, holes from the p-side p of the P-N junctionn wander into the n-side due to thermal vibrations. At the same time electrons from the n-side wander into the p-side. This leaves an area over the junction pretty much free of charge-carries holes in the depletion area. They will fill the holes and release energy in the form of a called the depletion area. When given an external applied voltage, continuous electrons encounter photon. Fig.2. Inner workingss of an LED, showing circuit (top) and band diagram (bottom) 3. EXPERIMENTS (a mere fiction) 3.1. Spectrum of sunlight As a first step, in order to obtain the main frequency ingredient of the uneven solar irradiation, Fourierr transform method was used to analyze solar spectrum and to calculatee the intensity distribution. Becausee

3 the Earth's atmosphere blocks out a broad range of the sun spectrum, only allowing a narrow band of light to reach the surface, most of the energy of radiation that sun emits is in the visible, near-infrared and near-ultraviolet region. Therefore, I focus on the wavelength range from ultraviolet light (330 nm) through visible light to infrared light (930 nm) and extract the 6 main component of the frequencies in sunlight spectrum They are infrared(710 nm), red(650 nm), yellow(580 nm), green(530 nm), blue(470 nm) and ultraviolet(390 nm) respectively Colors and I-V characteristic As the above theories in section 2 proved, the wavelength of photons emitted, and also the color of light, depends on the band gap energy and structure of the materials forming the P-N junction. According to Table 1 from Wikipedia, I chose and compounded 6 kinds of materials which have a direct band gap with energies corresponding to near-infrared, visible, and near-ultraviolet light used for the LED in the laboratory for Physics at Xiamen University. No matter what colors of LEDs are, they are operated from a low voltage DC supply with a series resistor used to limit the forward current to prevent destruction by overheating in my experiment. These LEDs used for artificial sun emit light when approximately 5mA current flow through it and can withstand 30mA or more current where a high brightness light output is needed. Each LED was applied its own forward voltage across the PN junction for a specified amount of forward conduction current (typically 20mA) and this parameter which is determined by the semiconductor material. Color Wavelength [nm] Semiconductor material Infrared λ > 760 Red 610 < λ < 760 Orange 590 < λ < 610 Yellow 570 < λ < 590 Green 500 < λ < 570 Blue 450 < λ < 500 Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs) Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Gallium arsenide phosphide (GaAsP) Gallium arsenide phosphide (GaAsP) Aluminium gallium phosphide (AlGaP) Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC)

4 Violet 400 < λ < 450 Indium gallium nitride (InGaN) Ultraviolet λ < 400 Diamond (C) Aluminium nitride (AlN) Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) Table 1 Available colors with wavelength range, and semiconductor material 3.3. Structure of LED Through advanced technology in microelectronics, I successfully make a large number of LED devices with ever-shorter wavelengths, emitting light in a variety of colors. The detailed construction of my Light Emitting Diode is very different from that of a normal diode. The PN junction of it is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell or body which protects the LED from both vibration and shock. LED chip as the core consists of a chip of semiconducting material doped with impurities. It is fixed on the column and powered by current from the positive lead frame to negative one. Although not directly labeled, the anvil and post act as anchors, to prevent the conductors from being forcefully pulled out from mechanical strain LED lattice and IRGBU system As we know, white light can be formed by mixing red, green and blue lights. Therefore, it is my belief that the invention of the blue LED made possible a simple and effective way to generate sunlight, combining red, yellow, green, blue, near-infrared, and near-ultraviolet LEDs. My arterial sun is born by utilizing this principle. It is constituted by IRGBU system I invented which is an extension of the RGB system. Each of IRGBU system consists of one near-infrared, one red, one yellow, one green, one blue, and one near-ultraviolet LED. These six LEDs with different wavelengths are capable of producing any color and this mixture of colored light will be perceived by humans as sunlight and can be used for general illumination even providing energy for plants to photovoltaics. But several key factors in my method including color stability, color rendering capability, and luminous efficacy need a strict adjustment because higher efficiency often means lower color rendering. After repeated attempts I finally presented a trade-off between the luminous efficiency and color rendering.

5 Fig.3. (a) illuminant schematic (b) Structure of LED lattice Inspired by structure of graphene, I set every 6 LEDs of different wavelengths on thee regular hexagon lattice constructed in a honeycomb. Initially, I arrange all LED on the same plane like display to ensure that incident light requires illuminating the whole object as flat as possible. The schematic illustration of the equipment is shown in Fig. 3. The equipment has 6 different wavelength chip-type LEDs, that colors are red, yellow, green, blue, infrared, ultraviolet. I assumed that these LEDs are put in the hexagon h as a unit, and the distance between the same colors is 12 mm and that between each LED is 4 mm. The example of LED arrangement is shown in Fig.3.(b). The theoretical value of luminous intensity is ass follows: Where I(x,y) is the irradiance at the measurement point, n is a numberr of LEDs, I ls is the irradiance of LED, θ and r are the angle and the distance from thee light source to the measurement point, shown in Fig.3.(a). The last step is to warp the plane LED lattice into a global as if processs graphene into a Bucky Ball. At the same time, a high-performance battery and a control system of circuit is hidden in this global to t drive the whole artificial sun. Fig.4. The progress of fabricated an artificial sun 4. RESULT AND DISCUSSION (a mere fiction) Finally, I tested this optical system of multi-leds so that ensure its spectral power distribution simulates s the sunlight accurately. Using spectrograph, the emission spectrum line of my artificiall sun is measured. From the figure below we can see that the curve of LED spectrum is close to the curve off real sunlight spectrum. As a result, artificial solar irradiation is confirmed.

6 Fig.5. Sun spectrum vs LED spectrum 5. CONCLUSION As everyone knows, solar energy as an essential power source is providing energy for everything from plants to photovoltaics. My method employs different LED arrays driven by constant current source and successfully makes the output irradiance reliable and closes to the sun. By adjusting the parameter of control system, we can achieve different distribution forms of output spectrum to meet the demands of spectral matching degree in different areas. Using cage structure light source of LEDs instead of traditional lamp will also greatly simplify the optical system and lower the manufacturing cost. In short, I successfully fabricated an artificial sun based on LEDs and the capability of this equipment was examined in the experiment, which means that I can illuminate an object like an actual sun. This simple system for emitting high-quality solar light will be useful for next-generation lighting systems. I believe that my artificial sun will play extensively an important role in solar thermal systems, photo catalysts, and plant factories. ACKNOWLEDGEMENT I express appreciation for fruitful discussions with my roommates Xi-hao Chen and Yi-nong Chen, and thanks for my specialized English teacher Tien-Mo Shih giving me a great deal of help and Wikipedia offering relevant data. REFERENCE Haitz's law". doi: /nphoton Retrieved en.wikiversity.org/wiki/pn_junction "The life and times of the LED: a 100-year history". Nature Photonics 1 (4): doi: /nphoton

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