Artificial Sun based on Light-emitting Diode
Siyu Chen, 22/05/2015
493989910@qq.com
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.
Fig.1
1.
(a) P-ttype semicon
nductor
(b) N-type semico
onductor
2.2. LED Inner workinggs
When a piec
ce of semiconductor matterial change
es type from
m p-type to n-type over a cross-sectio
on it forms a
P-N junction
n. At tempera
atures well above
a
absolu
ute zero, hole
es from the p-side
p
of the P-N junction
n wander into
o
the n-side d
due to thermal vibrations
s. At the sam
me time electrons from the n-side wan
nder into the
e p-side. This
s
leaves an arrea over the junction pre
etty much fre
ee of charge--carries calle
ed the deplettion area. Wh
hen given an
n
external app
plied voltage
e, continuous electrons e
encounter ho
oles in the depletion are
ea. They will fill the holes
s
and release energy in th
he form of a photon.
Fig.2. Inn
ner workings
s of an LED, showing circ
cuit (top) and band diag ram (bottom
m)
3. EXPERIM
MENTS (a
a mere fiction)
3.1. Spectru
um of sunligh
ht
As a first sttep, in order to obtain the main fre
equency ingredient of the uneven ssolar irradiation, Fourierr
transform m
method was used to analyze solar sp
pectrum and
d to calculate
e the intens ity distributiion. Because
e
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.
3.2. 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]
Infrared
λ > 760
Semiconductor material
Gallium arsenide (GaAs)
Aluminium gallium arsenide (AlGaAs)
Aluminium gallium arsenide (AlGaAs)
Red
610 < λ < 760
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Gallium arsenide phosphide (GaAsP)
Orange
590 < λ < 610
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Gallium arsenide phosphide (GaAsP)
Yellow
570 < λ < 590
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Gallium(III) phosphide (GaP)
Green
500 < λ < 570
Aluminium gallium indium phosphide (AlGaInP)
Aluminium gallium phosphide (AlGaP)
Zinc selenide (ZnSe)
Blue
450 < λ < 500
Indium gallium nitride (InGaN)
Silicon carbide (SiC)
Violet
400 < λ < 450
Indium gallium nitride (InGaN)
Diamond (C)
Ultraviolet
λ < 400
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.
3.4. 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.
Fig.3
3.
(a) illuminant schem
matic
(b) Structure of LED la
attice
Inspired by structure of graphene, I set every 6 LEDs of diffe
erent wavele
engths on the
e regular hex
xagon lattice
e
constructed in a honeycomb. Initially, I arrange all LED on th
he same plan
ne like displa
ay to ensure that incidentt
light require
es illuminatin
ng the whole
e object as fla
at as possiblle. The schem
matic illustra
ation of the equipment
e
is
s
shown in Fig
g. 3. The equ
uipment has 6 different w
wavelength chip-type LED
Ds, that colorrs are red, ye
ellow, green,,
blue, infrare
ed, ultraviolet. I assume
ed that thesse LEDs are put in the hexagon
h
as a unit, and the distance
e
between the
e same colorrs is 12 mm and that be
etween each LED is 4 mm
m. The exam
mple of LED arrangement
a
t
is shown in Fig.3.(b).
The theorettical value off luminous in
ntensity is ass follows:
Where I(x,y
y) is the irrad
diance at the
e measurem ent point, n is a numberr of LEDs, Ils is the irradiance of LED,,
θ and r are the angle an
nd the distan
nce from the
e light source
e to the mea
asurement p
point, shown in Fig.3.(a).
The last step is to warp the plane LED lattice in
nto a global as
a if process
s graphene in
nto a Bucky Ball. At the
same time, a high-perfo
ormance batttery and a ccontrol syste
em of circuit is hidden in this global to
t drive the
whole artific
cial sun.
Fig.4
4. The progre
ess of fabric
cated an artifficial sun
4.
RESU
ULT AND DISCUSSIION (a merre fiction)
Finally, I tes
sted this optical system of
o multi-LED
Ds so that ens
sure its spec
ctral power d
distribution simulates
s
the
e
sunlight acc
curately. Using spectrogrraph, the em
mission specttrum line of my artificiall sun is measured. From
m
the figure below we can
n see that the curve of L
LED spectrum
m is close to the curve off real sunligh
ht spectrum.
As a result, artificial solar irradiation is confirme
ed.
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:10.1038/nphoton.2006.78. Retrieved 2009-03-04.
• en.wikiversity.org/wiki/PN_Junction
• "The life and times of the LED: a 100-year history". Nature Photonics 1 (4): 189–192.
doi:10.1038/nphoton.2007.34