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Applications of Photovoltaic Technologies
Referenced website:
http://www.udel.edu/igert/pvcdrom/
http://solarpv.itri.org.tw/memb/main.aspx
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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.
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Greenhouse Effect
• Human activities have now reached a scale where they are impacting on the planet's environment and its attractiveness to humans.
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Spectrum of light
c
hhE
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)
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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.
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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
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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.
cos
1AM
687.0
7.01353AM
DI
DG II 1.1•ID : Direct beam intensity (W/m2)
•IG : Global irradiance (W/m2)
Meinel A.B. and Meinel M.P., "Applied Solar Energy", Addison Wesley Publishing Co., 1976
Intensity
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Standard Solar Spectra
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• 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.
Standard Solar Spectra-cont.
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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 SiGe AlP CdTe
Ge SiC GaAs CdS
As InP ZnS
GaP CdSe
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Direct and indirect semiconductor
High absorption probability Low absorption probability
Ev
E
P
Ec
Direct Semiconductor
photon
Ev
E
P
Ec
Indirect Semiconductor
phonon
photon
GaAs; InP etc. c-Si
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Crystal Structures
Polycrystalline
AmorphousCrystalline
In a crystalline solid atoms making up the crystal are arranged in a periodic fashion
Some solids are composed of small regions of single crystal material, known as polycrystalline.
In some solids there is no periodic structure of atoms at all and called amorphous solids
•Commercial Si solar cells
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Commercial Si solar cells
SINGLECRYSTAL POLYCRYSTAL AMORPHOUS
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Metal-insulator-conductor
• 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)
Filled States (VB)
Empty States (CB)Eg
metal semiconductor insulator
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Photoelectric effect
Metal
Photon Electron
Photon is a particle with energy E = hv
•Semiconductor
EgPhoton
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
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N- and P-type
• Addition of impurities with three valence electrons results in available empty energy state, a hole
• B, Al, In, Ga (Acceptor impurities)
•Addition of impurities with five valence electrons results an extra electron available current conduction
• P, As, Sb (donor impurities
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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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n-type semiconductor
p-type semiconductor
+ + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - -
Physics of Photovoltaic Generation
Depletion Zone
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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
Fingers
Emitter
Base
Rear contact
Antireflection coating
Antireflection texturing
(grid pattern)