Chapter 3: PV Technologies
3.1 Introduction to Solar Cells
3.1.1 Materials and Energy Levels
Solar cells are semiconductor devices made of semiconductor materials. In order to better understand the working mechanism of solar cells we need to know what is a semiconductor. In the valence band, electrons are bound to the atomic structure. In the conduction band, electrons are free to move through the material. Between the two, there is an energy gap that the electron in the valence band must overcome to become a free carrier. This is called the bandgap energy, Eg.
In an insulator, the bandgap energy, Eg is more than 5 eV and electrons cannot jump into the conduction band. In a conductor, the conduction and valence bands overlap. In a semi-conductor, the bandgap energy, Eg is less than 5 eV. With a little thermal energy, electrons can jump into the conduction band.
There are two types of semiconductor materials:
- Some materials (Ge and Si for example) are intrinsic semiconductors, i.e. they have semiconductor properties in their pure state. the electron and hole concentrations in intrinsic semiconductors are small.
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Most semiconductors used in electronic components are of the extrinsic type, i.e. they are made more conducting by the doping with microscopic amounts of another element.
3.1.2 The P-N Junction
Construction of a PN Junction:
- P-type Semiconductor:
- In a P-type semiconductor, such as silicon or germanium, atoms from Group III of the periodic table (trivalent, with three valence electrons) are intentionally introduced as impurities into the crystal lattice of the semiconductor material.
- The most common dopants for P-type semiconductors include boron. These impurity atoms create “holes” or locations where an electron is missing in the semiconductor crystal structure.
- N-type Semiconductor:
- In an N-type semiconductor, atoms from Group V of the periodic table (pentavalent, with five valence electrons) are introduced as impurities.
- Common dopants for N-type semiconductors include phosphorus or arsenic. These impurity atoms introduce extra electrons into the crystal lattice.
- Junction Formation:
- When a P-type semiconductor is brought into physical contact with an N-type semiconductor, a PN junction is formed at the interface between the two regions.
- At the junction, electrons from the N-type region diffuse across the junction into the P-type region, and holes from the P-type region diffuse into the N-type region. This movement of charge carriers establishes a region near the junction known as the depletion zone.
Depletion Zone:
The interaction between the free electrons from the N-type region and the holes from the P-type region creates a depletion zone at the PN junction. In this region:
- Electron Diffusion: Electrons diffuse from the N-type side to the P-type side, leaving behind positively charged donor ions in the N-type region.
- Hole Diffusion: Holes diffuse from the P-type side to the N-type side, leaving behind negativel