What is a Photovoltaic Cell?

A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect. The photovoltaic cell is the electrical building block.

Solar Panels

Multiple solar cells in an integrated group, all oriented in one plane, constitute a solar photovoltaic panel or module. So one solar “panel” comprises dozens of solar cells.

Solar Array

Solar panels are then connected together in electrical “series”, and this comprises an “array”. An array will typically consist of all the solar panels on a single roof.

System

The whole solar “system” comprises your solar array(s), your inverter, all the wiring and ancillary elements of such as the isolation switches and the generation meter.

The science behind photovoltaics

Note: We have tried really hard to make this information comprehensible to the non-scientist. If we have failed to communicate this clearly, please do get in touch to help us improve this page.

Silicon

A solar cell is made of semiconducting materials, usually silicon (periodic table element – Si).

The atomic structure of silicon has a deficit of electrons in its outer shell. This means that the atom is always seeking additional electrons to fill its outer shell, and it does this by a process known as covalent bonding. In covalent bonding, atoms share a pair of electrons, and this sharing process – covalent bonding – completes the outer shell of the atom.

Crystalline Silicon

Atoms may form multiple covalent bonds. In the case of silicone, since it has a deficit of 4 electrons, it forms a bond with 4 nearby atoms (one per atom). In so doing, it crystalises.

Here we see a crystalline regular structure, with the nuclei of each atom sharing electrons, creating a bond with 8 electrons around each nucleus.

Now interestingly, electricity cannot conduct in this pure monocrystalline silicon state. This is because none of the electrons in pure crystalline silicone are free to move about.

Using a process called doping (literally the process of adding impurities) the properties of the silicone can be changed, so that it does conduct electricity.

Doping

Crystalline silicon has impurities added to make it conduct electricity. The silicon is categorised as p-type or n-type, depending on which impurities are added (doped).

The type of impurity that is added will either be phosphorus (P) or boron (B). Where phosphorus is added, this is referred to as n-type; where boron is added, this is referred to as p-type.

This diagram shows that when an atom of phosphorus (which has 5 electrons in its outer shell) is substituted for an atom of silicon, it will bond with the neighbouring silicon atoms, but there will be one electron left over, that doesn’t form part of the bond. This is indicated by the blue electron in the diagram.

When making solar cells, only a tiny amount of phosphorus is used – roughly every thousandth silicon atom is replaced by a phosphorus atom.

This is unbonded electron is referred to as a “free electron”

Light energy (photons) can move this spare electron, so when voltage is applied, it changes to a state where the electron is drawn to the positive (+) and can move freely (in other words, current flows).

With a p-type, we have the same doping process, however this time we are using boron (B). Boron only has three electrons in its outer shell. So instead of having “free electrons” it has “free openings or vacancies” known as “holes” (indicated by the green circle above), where there is no electron.

Again, only tiny amounts of boron is used – roughly every millionth silicon atom is replaced by a boron atom.

How current flows in a p-type structure

1. Electrons are drawn to the positive (+) pole and thy actually move to into vacant holes, so as to fill them up.
2. Then, as the electrons move to vacant holes, new holes are created and so the next adjacent electrons move to the newly vacant holes.
3. This process is repeated. Electrons move toward the positive (+) pole, and at the same time, the holes appear to move toward the negative (-) pole.

Only electrons are actually moving, but the holes can be considered as having positively charged particles.

The movement of electrons in semiconductors

N-type: Electrons move toward the plus pole. Current flows in the opposite direction of the electrons’ movement.

P-Type: Electrons are what is actually moving, but the holes appear to be moving toward the direction of the minus pole.

As we see in these diagrams, both the p-type and n-type semiconductors can have current flow.

However, they are not as conductive as metal. Therefore, there is no need to use semiconductors if the only purpose is for current flow or conductivity.

The advantages or characteristics of a semiconductor are its ability to allow or stop current flow based on certain conditions. The basic principle behind a semiconductor is its rectification behaviour using a p-n junction.

p-n Junction

When newly doped N-type and P-type semiconductor materials are fused together, they create what is known as a “p-n junction.” The “p–n junction” is the boundary or interface between the p-type and n-type, inside a single crystal of semiconductor. 

The “p” (positive) side contains an excess of “holes”.

The “n” (negative) side contains an excess of electrons in the outer shells of the atoms.

We can consider the “p–n junction” to be the elementary “building block” of a solar cell. They are the active sites where the electronic action of the device takes place.

So what occurs at the p-n junction?

Initially, some of the free electrons from the N side (phosphorous impurity atoms) begin to migrate across this newly formed junction to fill up the “holes” on the P side.

This produces negative ions (NA) on the P side, and leaving behind positively charged donor ions on the N side (ND).

Separately, the holes from the P side migrate across the junction in the opposite direction into the region where there are large numbers of free electrons.

As a result, the charge density of the P-type material along the junction becomes negative as it is left with negatively charged acceptor ions (NA). Similarly, the charge density of the N-type material along the junction becomes positive. This charge transfer of electrons and holes across the PN junction is known as diffusion.

This process continues back and forth until the number of electrons which have crossed the junction have a large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction.

Eventually a state of equilibrium occurs producing a ‘potential barrier zone’ around the area of the junction as the donor atoms (ND) repel the holes and the acceptor atoms (NA) repel the electrons.

The regions on either side of the “p-n junction” become completely depleted of free carriers when compared to the N and P type materials further away from the junction; free charge carriers cannot rest in a position where there is a potential barrier.

This area around the PN Junction is now called the Depletion Layer.

There is a neutral charge condition around the “p-n junction” – it is in a state of equilibrium.

It is evident therefore that the N-type material has lost electrons and the P-type has lost holes. The N-type material has become positive with respect to the P-type and the N-side is at a positive voltage relative to the P-side.

A free charge can only cross the depletion region if it is given extra energy by an external source.

What happens when the sun hits the solar cell?

Energy from the sunlight (photons) hits the solar cell and is absorbed by the semiconductor material.

These photons contain energy, and this energy breaks apart electron-hole pairs. Each photon will usually free exactly one electron, resulting in a free hole as well.

The electric field at the p-n junction will send the electron to the N side and the hole to the P side. This disrupts the electrical equilibrium. There is now an excess of electrons on one side and an excess of holes on the other.

If an external current path is provided, electrons will flow through the path to the P side to unite with holes that the electric field sent there. In other words when the solar cell is connected to an external circuit, equilibrium is restored by the excess electrons going around the external circuit.

This flow of electrons is the electric current. It is Direct Current (DC), since the flow is moving in only one direction.

The solar cell’s electric field causes a voltage.

The composition of a solar cell

The cell has an anti-reflective coating adhered to it, and then is covered by glass to protect it.

The electrical contacts are provided by a screen printed silver paste, one to the top and one to the bottom.

Most cells have a polymer backing sheet.

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Hopefully you found this explanation of the science of Solar PV comprehensible!

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