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.

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)