Transcript Chetan - I-Shou University

Applications of Photovoltaic
Technologies
Referenced website:
http://www.udel.edu/igert/pvcdrom/
http://solarpv.itri.org.tw/memb/main.aspx
Why Solar Cells?
•
•
•
•
•
•
Finite fossil fuel supply
Less environmental damage
No radiation risk (meltdown)
Nearly infinite supply of FREE energy
Sun gives us 32 x1024 joules a year,
Cover 0.1% of the Earth’s surface with 10%
efficient solar cells with an efficiency of would
satisfy our present needs.
2
Greenhouse Effect
• Human activities have now reached a scale where they are
impacting on the planet's environment and its attractiveness to
humans.
3
Spectrum of light
E  h  h
c

h: Planck’s constant 6.626×10-34 (J-s)
ν: frequency (s-1)
λ: wavelength (m)
c : light speed 3.0× 108 (m/s)
4
Atmospheric Effects
Atmospheric effects have several impacts on the
solar radiation at the Earth's surface. The major
effects for photovoltaic applications are:
• a reduction in the power of the solar radiation
due to absorption, scattering and reflection in
the atmosphere;
• a change in the spectral content of the solar
radiation due to greater absorption or scattering
of some wavelengths;
• the introduction of a diffuse or indirect
component into the solar radiation; and
• local variations in the atmosphere (such as
water vapor, clouds and pollution) which have
additional effects on the incident power,
spectrum and directionality.
Hu, C. and White, R.M., "Solar Cells: From Basic
to Advanced Systems", McGraw-Hill, New York,
1983.
5
Solar Radiation
 Power emitted from Sun =3.8×1023 (kw)
 Power direct to Earth=1.8×1014 (kW)
 Solar constant=1353 W/m2
T=5762 K
6
Air Mass (AM)
• AM0 : The standard
spectrum outside the
Earth's atmosphere.
• AM 1: Light incident with
the angle of 0 degree.
• AM 1.5: Light incident
with the angle of 48
degree.
1
AM 
cos 
Intensity
I D  1353  0.7
AM 0.687
I G  1.1I D
Meinel A.B. and Meinel M.P., "Applied Solar
Energy", Addison Wesley Publishing Co., 1976
•ID : Direct beam intensity (W/m2)
•I : Global irradiance (W/m2)
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Standard Solar Spectra
8
Standard Solar Spectra-cont.
• The AM1.5 Global spectrum is designed for flat plate modules
and has an integrated power of 1000 W/m2 (100 mW/cm2).
• The AM1.5 Direct (+circumsolar) spectrum is defined for solar
concentrator work. It includes the direct beam from the sun
plus the circumsolar component in a disk 2.5 degrees around
the sun. The direct plus circumsolar spectrum has an integrated
power density of 900 W/m2.
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Part of periodic table
II
III
IV
V
VI
B
C(6)
Al
Si(14)
P
S
Zn
Ga
Ge(32)
As
Se
Cd
In
Sb
Te
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Compound semiconductors
•
•
•
•
Elemental semiconductors: Si, Ge
Compound semiconductors: GaAs, InP
Ternary semiconductors: AlGaAs, HgCdTe
Quaternary semiconductors: InGaAsP, InGaAlP
Elemental
IV
Compounds
Binary III-V
Binary II-VI
Si
Ge
As
SiGe
SiC
AlP
GaAs
InP
GaP
CdTe
CdS
ZnS
CdSe
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Direct and indirect semiconductor
E
E
Ec
Ec
Ev
photon
phonon
photon
P
Direct Semiconductor
P
Ev
Indirect Semiconductor
High absorption probability
Low absorption probability
GaAs; InP etc.
c-Si
12
Crystal Structures
In a crystalline
solid atoms
making up the
crystal are
arranged in a
periodic fashion
Crystalline
•Commercial Si solar cells
Amorphous
In some solids there is no
periodic structure of atoms
at all and called amorphous
solids
Some solids are
composed of
small regions of
single crystal
material, known
as polycrystalline.
Polycrystalline
13
Commercial Si solar cells
SINGLECRYSTAL
POLYCRYSTAL
AMORPHOUS
14
Metal-insulator-conductor
Empty States (CB)
Eg
Filled States (VB)
metal semiconductor insulator
• In metal conduction band (CB) and valence band (VB) overlap, in
insulator and semiconductor CB and VB are separated by a energy
band (Eg).
• Eg for Si is 1.1242eV (semiconductor) as compared to 5eV for
diamond (Insulator)
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Photoelectric effect
Photon
Electron
•Semiconductor
Metal
Photon is a particle with
energy E = hv
Photon
Eg
Eph( hv)>Eg
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Absorption of Light
•
Eph < EG Photons with energy Eph less than the band gap
energy EG interact only weakly with the semiconductor,
passing through it as if it were transparent.
•
Eph = EG have just enough energy to create an electron hole
pair and are efficiently absorbed.
•
Eph > EG Photons with energy much greater than the band
gap are strongly absorbed
17
N- and P-type
•Addition of impurities with
five valence electrons results
an extra electron available
current conduction
• P, As, Sb (donor impurities
• Addition of impurities with
three valence electrons
results in available empty
energy state, a hole
• B, Al, In, Ga (Acceptor
impurities)
18
Physics of Photovoltaic Generation
If energy of inclined light (Ehp)
> Energy band of material
(EG).
Then, emit electron-hole pair
(EHP) to produce the
electric current.
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Physics of Photovoltaic Generation
n-type
semiconductor
+ + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - -
Depletion Zone
p-type
semiconductor
Solar Cell-structure
• A solar cell is a P-N junction device
• Light shining on the solar cell produces both a current and a
voltage to generate electric power.
Busbar
Antireflection
coating
Fingers
Emitter
Antireflection
texturing
Base
(grid pattern)
Rear contact
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