# Band gap of silicon - Energy band in semiconductor

The band gap of silicon. To gain a fundamental understanding of the operation of a solar cell, it is essential to know what the concept of gap theory exactly entails. It is this material-specific property that explains the behaviour of the semiconductor in a solar cell to a large extent. The principle of Pauli exclusion teach us that no two electrons in a solid may have the same condition.

It is for this reason that electrons per atom in different energy-specific peels organise; the so-called electron shells. Because a substance contains billions and billions of particles. None of which may have the same energy state.

## Band gap of silicon - Energy band in semiconductor

These discrete shells in a solid become almost continuous bands, the so-called energy bands. The highly energetic group in which electrons are located is called the valence band. The lowest-energy gap that is free from atoms is known as the conduction band. The precise boundary between the validation and conduction band is called the Fermi level.

The electrons in the conduction band are such as all name, responsible for the conduction in a material. These electrons can move freely through the material. They are bound with a life metal and ensures that a possible voltage difference comes in a current. Because not all energy levels are allowed for the electrons, there is a 'prohibited' energy level in the case of isolators and semiconductors between the valence band and the conduction band. It is this forbidden band that is called the band gap.

In case of electrical conductors this gap is absent, in the fact of isolators it is 0.5 V, and with semiconductors, this is usually around 1V. By using Planck's constant (E = hc / λ, where h = Planck's constant, E = energy and c = light velocity), it is possible to calculate which wavelength belongs to which energy. This is useful because we can now use the gap to predict which wavelength is absorbed by a material to provide the forwarding of electrons from the valence to the conduction band. In this way, for example, we can calculate that the violet part of the visible light can bridge a band gap of up to 3,26 V. As soon as an incident photon is absorbed, the required energy is drained to promote the electron. The rest is converted into heat.

### Band gap of silicon - Energy band in semiconductor

Photons with insufficient energy are either transmitted or reflected.Generally speaking, the following applies to a solar cell: the higher the bridged band gap, the higher the resulting voltage. This implies a limit because the amount of solar radiation spectrum ( see article about solar radiation spectrum) in high energy spheres (UV and higher) is relatively limited.The higher voltage is therefore nice, but the resulting current is low. To maximise the yield (in watts), a material with a bandgap that is high enough, but not so high that there is little sunlight available, must be chosen.

The ideal band gap, i.e. the gap for which the product, is about 1.4 eV for sunlight. Because another gap implies a different absorption wavelength, it is possible to use larger parts of the solar radiation spectrum by placing solar cells of different gap above each other. These so-called 'tandem cells' have a very high theoretical maximum regarding efficiency, depending on which and how many gaps are used.