2 Fundamentals of silicon photovoltaics

• Radiation quantum must have at least the energy of the band gap

• Only the energy of the radiation quantum is necessary for charge separation

• in addition to the energy gap, there

  is also an impulse gap.

• To generate the electron-hole pair,

  a three-body impact must occur:

  1. Photon

  2. Phonon

  3. Charge carrier

• Phonons are the energy quanta of

  the lattice vibration.

• Probability much smaller

Direct semiconductors have a significantly higher absorption rate than indirect semiconductors.

Therefore, much less material can be used.

The peak of the valence band and the minimum of the conduction band are at the same lattice momentum.

Thin film solar cells are always direct semiconductors.

In indirect semiconductors, bringing the electron from the peak of the valence band to the minimum of the conduction band requires energy and lattice momentum.

The probability of the electron-hole pair generation is much lower.

• The path length of the light through the atmosphere weakens it and changes the spectral composition (blue sky at noon, morning or evening red)

• AM0 is outside of the atmosphere

• AM1 with vertical passage through the atmosphere

• AM1.5 is standard spectrum for solar cell measurement

Future solar cells will be characterized by low manufacturing costs, not by a large efficiency improvement.

1. Manufacturing of raw silicon from quartz sand

2. Purification of the silicon

3. Creating silicon blocks with Siemens or Czochralski process

4. Production of silicon wafers by wire saw method

5. Doping

6. Placement of electrical contacts

7. Deposition of antireflection coating

8. Encapsulation of the cell

Monocrystalline silicon (mono-Si)

• Homogeneous crystal structure

• Efficiency ~ 20-23%

• Energy-intensive and expensive production

Polycrystalline silicon (poly-Si)

• No uniform crystal direction, but different alignment

• Efficiency ~ 15-20%

• Lower energy demand for production

30 years lifetime, maybe longer

cell test

glass cleaning

foil cut

cell soldering

cross soldering

inspection

lamination

Final inspection of the laminates

final assembly

high-voltage test

performance test

quality control

encampments

• Metal: Fermi level in the middle

  between valence and conduction band;

  overlapping of valence and conduction

  band

• Insulators: Fully occupied valence band

  and a large energy gap to the

  conduction band; thermally excited

  electrons cannot overcome it at room

  temperature

• Semiconductors: Fully occupied

  valence band and a smaller band gap

p-doped

• The Fermi level is closer to the valence band

• Having an excess of free holes

n-doped

• The Fermi level is closer to the conduction band

• Having an excess of free electrons