Solar cell working principle - Explain working of solar cells

Solar cell working principle is developed in 1883 by Charles Fritts. Covering layers of the material selenium with a thin layer of gold. This cell had a yield of only 1% and was, therefore, more of a 'proof of concept'. The first 'modern' silicon cell was developed in 1941 by Russell Ohl.

The first crystalline silicon panel is existing from 1954 with a yield of 4%. Especially for space travel, these were interesting because for terrestrial applications the power was too low. In the following years, the efficiency improved to around 6%.

Solar cell working principle - Explain working of solar cells

Principle of a silicon singlet fission solar cell with incorporated organic crystals. Credit: M. Künsting/HZB

A photovoltaic panel is, in fact, a collection of a large number of cells. A cell is also called a photovoltaic cell (PV system, from the English photovoltaic cell) with an ambiguous word. Most cells made of a layer of the material silicon.

Making from this silicon a semiconductor, a layer of phosphorus is added at the top. A layer of boron at the bottom (see the page semiconductors for more explanation). They place it between two glass plates for protection.

As soon as sunlight shines on a solar energy panel, electrons are 'detached' from the top of the panel under the influence of this radiation. There is a free electron with a corresponding hole. Because an electric field arises at the interface due to the uneven charge distribution, the electrons can only go one way.

As a result, a voltage difference between the top and bottom of the panel occurs. If you connect the top and bottom, a current will flow across the wire. Since the voltage across the cell is very low (only half a volt), several cells are often together in a panel.

Different types:

Then an inverter (transformer) will have to be placed. The inverter ensures that the voltage of a series of PV panels (usually around 24V DC) is translating into an alternating current of 230V.

Another type of pv cells are the so-called thin-film cells. These cells are, as the name suggests, significant (100 to 200 times) thinner than their crystalline counterparts. The cells are flexible and have a little weight, making them easily applicable on various surfaces.

The efficiency of thin film solar cells is much lower (about 6%), and the production process is very complex, but the required quantity of raw material (crystalline silicon) is deficient. It is making the cells cheaper per unit of area than crystalline cells. This lower price, however, does not outweigh the loss of efficiency, so you're not or hardly better off with a thin film cell.

Solar cell working principle - Explain working of solar cells

This video explains it perfectly,  made by

There are three types of silicon available for photovoltaic cells: monocrystalline, polycrystalline and amorphous silicon. The so-called Czochralsi process obtains monocrystalline silicon: inserting a rod into a vessel of molten silicon. The result is round slices that consist of one silicon crystal.

These cells achieve the highest yield but are relatively expensive. Polycrystalline silicon is by pouring liquid silicone into a square shape. A variety of silicon crystals will grow with the cooling, which does not all connect seamlessly. This form is cheaper, but therefore also lower return.

The last type, amorphous silicon, contains no crystals at all. This material is the cheapest to obtain but also gives the least cost-effective PV panels.

Solar cell working principle - Explain working of solar cells

Other materials are also available, but these are too expensive to use on a large scale. These techniques often achieve high returns, but because of their price, they usually find their application in space travel. The highest yield produced so far (around 40%) belongs to a cell developed by Spectrolab (part of Boeing).

The cell uses mirrors and lenses to increase the intensity of the incident light. Besides, they making different layers of material (multi-junction principle) to utilise more wavelengths. The materials for the subcells are Gallium indium phosphorite, Gallium indium arsenide and Germanium.

A new and very promising technique is the so-called organic cells. These cells work according to a less cost-effective principle but are cheap to manufacture and flexible in an application that science has completely collapsed.

Solar cell working principle - Explain working of solar cells

The highest efficiency is with cells that operate as a sort of sieve. These cells consist of various layers of different material, each of which converts part of the sunlight into electricity. After all, different materials have different energy states and thus absorb different wavelengths.

A more significant portion of the sunlight is effective, instead of only reflecting. A variant of this technique uses a prism to split the light before it reaches the panel. Each type of light falls on another kind of photovoltaic cell so that a more substantial part of the sunlight is effective.

With these techniques, it is possible to realise photovoltaic energy cells with a yield of no less than 40% in the future. Panels based on this are so expensive, however, that they are not yet interesting for daily applications. Even more when they can control the heat in a solar cell, to use it for generating more electricity.

Another, cheaper, method to increase the efficiency of the panels is the bundling of sunlight through optical instruments (lenses, mirrors). After all, bundled light is stronger and can therefore in principle release more electrons from the panel.

By placing the solar cells in the focal point of the optical system, the yield is increasing substantially. In Australia, people are working on a plant that 'follows' the sun as it were and kept the light constantly focused on the panels.

Solar cell working principle - Explain working of solar cells

Every type of material has its characteristics. One of these characteristics is the so-called band gap, the energy difference that an electron has to bridge to jump from the valence band to the conduction band. It is this property that determines which part of the sun radiation is absorbed by the cell.

The other radiation has either insufficient energy to 'wake up' the electrons and passes through the material or has so much power that the atoms absorb it and converted into vibrations (heat). Because the sun does not shine equally well in all wavelengths, this means that some materials have a better maximum efficiency than others. Below is a diagram in which for some materials you see the bandgap plotted against the maximum achievable yield.

Not only does the bandgap limit the efficiency of a solar cell. Reflective properties of the material and defects in the crystal matrix also play an essential role. To overcome all these limitations and to realise higher efficiency, a combination of several elements is necessary. Here, the light that is not absorbing by the first layer is reflecting on the second layer.