How do Solar PV modules work?

Theory

The photovoltaic effect is an electrical phenomenon in which light energy is converted to electrical energy. This works on the principle of how semiconductor materials behave when exposed to light. Photons from sun rays are absorbed by the semiconductor crystal which causes an array of free electrons to be produced in the crystal. These electrons act as transmitters of electricity.

A photovoltaic cell is the basic unit of the system where the photovoltaic effect is utilised to produce electricity from light energy.

Structure

Silicon used as silicon wafers are the most widely used semiconductor material for constructing the photovoltaic cell. In chemistry, the silicon atom has four valence electrons. Similar to any crystal structure, each silicon atom shares each of its four valence electrons with another nearest silicon atom hence creating covalent bonds between them. In this way, silicon crystal forms a tetrahedral lattice structure. As light ray strikes on any materials, some portion of the light is reflected and the other portion is transmitted through the materials while the rest is absorbed by the material.

Silicon is refined from sand or quartz, and then formed into a large crystal ingot, which is then sliced into thin wafers. Wafers are then prepared by adding different layers of phosphorous, anti-reflective coatings and screen-printed silver contacts for catching electrons. There are different variations on this theme, but this is your typical process.

Once the intensity of incident light is high enough to excite some of the electrons of covalent bonds, these excited electrons get sufficient energy to migrate from valence band to conduction band. As the energy level of these electrons is in the conduction band, they leave from the covalent bond leaving a hole in the bond behind each removed electron. These free electrons move randomly inside the crystal structure of the silicon. These free electrons and holes have a vital role in generating electricity in a photovoltaic cell. These electrons and holes are therefore referred to as light-generated electrons and holes. These light-generated electrons and holes cannot produce electricity in the silicon crystal alone.

To cater to that,  an impurity such as phosphorus is added to silicon. The four valence electrons of each pentavalent phosphorous atom are shared through covalent bonds with four neighbour silicon atoms, and the fifth valence electron is left mobile.

This fifth valence electron then loosely binds with the parent atom. These free electrons come from pentavalent impurity are always ready to conduct current in the semiconductor. Although there are numbers of free electrons, yet the substance is electrically neutral as the number of positive phosphorous ions locked inside the crystal is equal to the number of the free electrons come out from them. The process of inserting impurities in the semiconductor is known as doping, and the impurities are doped are known as dopants. The pentavalent dopants which donate their fifth free electron to the semiconductor crystal are known as donors. The semiconductors doped by donor impurities are known as n-type or negative type semiconductor as there are plenty of free electrons which are negatively charged by nature.

Working

In n-type semiconductor mainly the free electrons carry a negative charge and in p-type semiconductor mainly the holes, in turn, carry positive charge, therefore, free electrons in n-type semiconductor and free holes in p-type semiconductor are known majority carrier respectively.

There is a potential barrier between n-type and p-type material. This potential barrier is essential for working of the solar cell. While n-type semiconductor and p-type semiconductor contact each other, the free electrons near to the contact surface of n-type semiconductor get plenty of adjacent holes of p-type material. Hence free electrons in n-type semiconductor near to its contact surface jump to the adjacent holes of p-type material to recombine. As the covalent bonds are broken, there will be a number of holes created in the n-type material near the contact surface. Therefore, near the contact zone, the holes in the p-type materials disappear due to recombination on the other hand holes appear in the n-type material near the same contact zone. This is as such equivalent to the migration of holes from p-type to the n-type semiconductor. Following this, as soon as one n-type semiconductor and one p-type semiconductor come into contact, their respective electrons and holes transfer between one and another. This is a quick process but occurs continually. After some instant, there will be a layer of negative charge (excess electrons) in the p-type semiconductor adjacent to the contact along the contact surface. Similarly, there will be a layer of positive charge (positive ions) in the n-type semiconductor adjacent to contact along the contact surface. The thickness of these negative and positive charge layer increases up to a certain extent, after which no more migration occurs. This is due to the fact that while any electron of n-type semiconductor tries to migrate over p-type semiconductor it faces a sufficiently thick layer of positive ions in n-type semiconductor itself where it will drop without crossing it. Similarly, holes will no more migrate to n-type semiconductor from p-type. T

To simplify the above, negative charge layer in the p-type side and positive charge layer in n-type side together form a barrier which opposes the migration of charge carriers from its one side to other.  Due to the positive and negative charged layer, an electric field is generated across the region which is known as the depletion layer.

So what happens when light strikes?

As light strikes the n-type semiconductor, electrons from light-generated electron-hole pairs are unable to migrate to the p-region since they are not able to cross the potential barrier because to the repulsion of an electric field across depletion layer. At the same time, the light-generated holes cross the depletion region due to the attraction of the electric field of the depletion layer where they recombine with electrons.

As the negative charge (light generated electrons) are trapped on one side and positive charge (light generated holes) are trapped on the other,  potential difference is generated between these two sides of the cell. This difference typically varies between 0.3 – 0.5 V.

This is how a single solar cell produce a potential difference.

Solar cells are combined to create a solar module. An average solar module consists of 60 solar cells connected together in series

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