A solar panel is made up of solar cells that convert solar energy (sunlight) to electricity. This conversion is known as photovoltaic effect. Solar energy is the best alternate energy source as it is renewable, easy to produce and saves the environment. The government of India too promotes it by providing remarkable subsidies.
Basic structure of a solar cell and its working principle
A solar cell, also known as a photovoltaic cell, works on the principle of photoelectric effect—ability of an object to emit electrons when light falls on it. To generate electricity from a solar cell, a material is required that emits electrons and is raised to a higher energy state by the absorption of light. These electrons have enough energy to drive the connected device or load. The cells are made of a semiconductor material that is used in the form of p-n junctions.
A solar cell consists of n-type (having electrons as a majority charge carrier) and p-type (having holes as a majority charge carrier) semiconductor materials. The depletion region is formed between n-type and p-type materials. The depletion layer is made up of positive and negative ions. Electrons and holes are not present inside the depletion layer. Light travels in the form of packets of energy called photons. These photons (sunlight) are applied on the depletion region.
Photons are absorbed in depletion regions and generate electron-hole pairs. These free electrons and holes have enough energy to jump out of the depletion zone. If a wire is connected from the cathode (n-type material) to the anode (p-type material), electrons flow through the wire. The electrons have negative charge and, hence, get attracted by p-type material (positive terminal), travel through the external load (device) and create a flow of electrons (current), as shown in Fig. 1.
As shown in Fig. 2, the sunlight concentrator focuses sunlight on a small part and tracks the Sun throughout the day for higher efficiency. The solar panel generates unstable or fluctuating electricity and, thus, has to be regulated. The power-conditioning block gives regulated stable electricity that is stored in the battery or a battery bank. The battery gets damaged if a proper power-conditioning circuit is not used. The device gets energy from the battery.
Sometimes, a solid copper foundation is laid over the solar panel that gives it massive strength. Solar panels may have anti-reflective (AR) coating over the surface to produce more energy and increase efficiency. AR layer usually has NaOH surface texturation with several tiny pyramids. The layer’s thickness should be designed for better efficiency.
Multi-junction structure of a solar cell
A few years ago, late Dr A.P.J. Abdul Kalam said that, we are not utilising solar energy efficiently. Researchers are working hard and have come up with different solar structures and technologies to increase the efficiency of solar cells. Till now, we have depended on single-junction solar cells that give an efficiency of about 20 per cent. To improve this efficiency, the concept of multi-junction solar cells has been adopted. These are solar cells with multiple p-n junctions made up of different semiconductor materials.
The gap between valance band and conduction band is called forbidden gap or energy gap (Eg). In an energy band diagram, electrons jump from valence to conduction band and become free when electrons absorb enough energy. These electrons are considered free electrons and take part in current flow. If an applied photon has wavelength greater than or equal to hc/Eg (h is Plank’s constant and c is velocity of light), electrons gain enough energy and jump from valence to conduction band.
Electrons that belong to conduction band take part in current flow and are thus responsible for generating electricity. In the above-mentioned relationship, h and c are constant, which means the number of free electrons depends on Eg. This Physics fundamental has been used in the structure of solar cells. A multi-junction structure (material with different values of Eg) absorbs maximum sunlight (broad range of wavelengths), generates more free electrons or current and, ultimately, increases the efficiency of solar cells.
The top-most junction has the widest band gap and, as we go down, it decreases. It is necessary to find the best semiconductor materials in terms of band gap to increase efficiency. Fig. 3 shows the different semiconductor materials used in a multi-junction solar cell. As photons (sunlight) strike the solar cell, highest energy-level photons are absorbed by the top-most layer, and photons with low energy are absorbed in the lower layers.
A window layer is used to reduce surface recombination velocity, and the back surface field (BSF) layer reduces the scattering of carriers towards the tunnel junction. It simply functions as an electrical contact and, thus, covers the entire back surface of the cell structure. It is always made of metal.