Applications of Photovoltaic Technologies Why Solar Cells?

Applications of Photovoltaic Technologies

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.

Greenhouse Effect

• Human activities have now reached a scale where they are impacting on the planet's environment and its attractiveness to humans.

Spectrum of light

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.

Solar Radiation

• Power emitted from Sun =3.8×1023 (kw)

• Power direct to Earth=1.8×1014 (kW)

• Solar constant=1353 W/m2

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.

Standard Solar Spectra

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.

Part of periodic table

Compound semiconductors

• Elemental semiconductors: Si, Ge

• Compound semiconductors: GaAs, InP

• Ternary semiconductors: AlGaAs, HgCdTe

• Quaternary semiconductors: InGaAsP, InGaAlP

Direct and indirect semiconductor

Crystal Structures

Commercial Si solar cells

Metal-insulator-conductor

Photoelectric effect

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

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)

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.

Solar Cell-structure