Semiconductors and P-N junction

1. The silicon atom and crystalline structure

Silicon is one of the most abundant semiconductors present on Earth. A semiconductor is made up of individual atoms covalently bonded together in an orderly arrangement, sharing a pair of electrons each. This is useful in solar cells. The neutral silicon atom has 14 protons, 14-16 neutrons, and 14 electrons. It is these outer shell electrons, also known as the valence electrons that are involved in bonding with four other Si atoms, forming the silicon crystal, c-Si.

Fig.1: Covalent bonds in a silicon crystal lattice [1]

There are two types of semiconductors namely intrinsic and extrinsic. An intrinsic semiconductor is essentially a pure semiconductor material with no impurity atoms. In its intrinsic state, silicon is a poor conductor of electricity. An extrinsic semiconductor can be formed from an intrinsic semiconductor by adding impurity atoms to the crystal in a process known as doping. If a pentavalent atom is added as impurity, only four of its five valence electrons participate in covalent bonding with adjacent silicon atoms. One electron is left and is known as the free electron. Examples of pentavalent atoms are Phosphorus, Antimony, Arsenic and Bismuth. For a trivalent atom, all three valence electrons participate in covalent bonding and there is an electron vacancy or a hole with one of the adjacent silicon atoms. Examples of trivalent atoms are Boron, Indium and Gallium [2]

2. Doping process and p-n junction formation

The process of doping is used to create n and p-materials that have either an excess of electrons in the crystalline structure or a deficiency of electrons. Doping involves the addition of very small concentration of impurity atoms to control the conductivity in a precise manner by creating an excess of free electrons (n-type impurity). Dopants that create holes are known as acceptors and those that donate electrons in the crystal are known as donors. When a p-n junction is formed from a single crystal of Si, an n-region is on one side and a p-region is on the other side, i.e. some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes. A depletion region is formed at the boundary consisting of positive charges at the top side (pentavalent ions) and negative charges at the bottom side (trivalent ions) of the junction. The p-n junction is the key property of ordinary semiconductor diode and allows the diode to pass current in one direction only. Creation of the depletion region continues until an equilibrium point is reached whereby the number of negative charges, i.e. electrons is saturated within the p-n junction such that no further electrons can diffuse into the p-region as they are repelled by the large number of electrons that have been moved at the bottom side of the junction. An electric field is then created within the depletion zone which consists of many positive charges and negative charges on opposite sides. This electric field acts as a barrier to the free electrons blocked in the n-region. An external source should be applied to move them across the p-n junction which in the case of photovoltaic is sunlight that generates a dc voltage from the solar panel [2].

Fig.2: : P-N junction [2]

3. Penetration depth, δ The ideal efficiency of a solar cell depends on the construction properties of the solar cell and the optical properties of the material of which the solar cell is made. The optical property is characterized by the absorption coefficient, α which describes the number of photons that can be absorbed per unit depth (cm-1). This depends mainly on the internal crystalline structure and the presence of boundary defects that act as electrons trap in the solar panel. Hence, the absorption coefficient indicates the penetration depth of photons. A low absorption coefficient represents a high penetration depth. The contrary is also true. All photons which have energy greater than the energy band-gap of the material or have a wavelength less than the critical wavelength of the material will be absorbed and will be generating one electron lone pair. However, only photons landing in the absorber / active region will be generating EHPs that may contribute to the conduction process. This is why the ideal efficiencies of solar cells are limited to approximately 25% for Silicon-type and up to 40% for Galium-Arsenide based [2].


References:

[1] “PVEducation.” [Online]. Available: https://www.pveducation.org/pvcdrom.

[2] D. Buchla, Renewable Energy Systems, 1st ed. Pearson, 2014.