Light-to-Electricity: What Happens in a Solar Cell

The photovoltaic effect (production of electricity due to light) was discovered by Edmond Becquerel as long ago as 1839. In 1921, Albert Einstein was awarded the Nobel Prize in Physics for his work on the photoelectric effect. The first ever solar cell was developed at Bell Labs (also the birthplace of the transistor which completely revolutionized electronics) in 1954. Many developments have taken place in this field since then, and there has been increased focus in recent times due to environmental considerations and the aim to increase non-polluting, ‘cleaner’ energy production.

The functioning of a solar cell is entirely based on the properties of its constituent materials. A solar cell essentially consists of an interface between two materials – one material which has a tendency to acquire electrons with relative ease (called the ‘acceptor’ or ‘p-type’ material) and the other having a tendency to lose electrons with relative ease (called the ‘donor’ or ‘n-type’ material). The area over which these two materials come into contact with each other is called a ‘junction’. At the onset, when these two materials come into contact with each other, the difference between concentration of electrons causes some electrons to cross the junction and formation of an electric field at the interface.

In any material, the electrons can occupy only specific, discrete energy levels. In semiconductor materials, there is a certain energy level above which the electrons can move around freely and the material acts as a conductor. Some portion of the spectrum of the sunlight can provide this required energy to the electrons in the material. When such electrons are released in the electric field region, they are pushed to one side by the electric field in the region. Suitable contacts are connected to the materials to channel the flow of these electrons into the external circuit so that other electrical equipment can be connected to the solar cell.

(Source: https://www.visualcapitalist.com/animation-how-solar-panels-work/)

(Note: The direction of current is the reverse of the direction of flow of electrons)

The most commonly used material is silicon. Metallurgical processes are used for converting ordinary silicon into n-type and p-type silicon, which involves ‘doping’ the silicon with tiny amounts of other elements. Commonly used elements are boron for p-type and phosphorus for n-type. A large variety of other materials, including organic materials and organometallic complexes, has been experimented with and yielded varying results. The performance of the materials depends on numerous factors such as which part of the spectrum is suitable for electron excitation, the microstructure of the material which affects electron movement, and other chemical properties which determine factors like lifetime, degradation etc. Cost and environmental impact also play a role in selection of materials. For example, there have been concerns over the use of some cadmium-based materials as cadmium is considered to be toxic.

The power output of a solar cell is defined under ‘standard test conditions’ – the power in the incident sunlight is 1000W/m², the surface temperature of the cell is 25^oC and and an ‘air mass 1.5’ factor is considered to account for the effect of the atmosphere as the sunlight has to encounter the entire stretch of the atmosphere before it is incident on the solar panel surface. The efficiency of light-to-electricity conversion is quite low for all solar cells, with commercially available panels scoring around the 16%-20% range. As mentioned earlier, this is highly dependent on the microstructure and properties of the materials used for preparing the cell. A cell consisting of six layers of different materials had demonstrated an efficiency of 47.1% in 2019, however this was under laboratory conditions with highly concentrated incident light. Environmental factors cause variations in performance at actual site conditions, and thus actual operation is not expected to be as efficient as laboratory conditions. The intensity of incident light also affects the output. Greater intensity means that more photons strike the solar cell, and this in turn enables formation of more free electrons, thus increasing the output current.

A single solar cell is quite small in size and produces a very low power output. A number of such cells  are connected in series to form a solar panel. 60-cell and 72-cell panels are the most common among those used in rooftop installations.

(Source: https://www.wholesalesolar.com/blog/solar-panel-size-guide)

Multiple panels can be connected in different arrangements to achieve the required voltage and power output. There are a number of other intermediate components which make up the entire photovoltaic system to which electrical equipment can be connected.

A major difference between solar cells and electromagnetism-based sources is that the variation in magnetic field necessary for electricity generation creates an alternating current (AC) waveform, whereas the output of a solar cell is direct current (DC).