The story of electricity, its importance and some of the electricity generation methods were discussed thoroughly in the last set of articles. Solar energy is a renewable source of energy for the death of sun is almost synonymous with the extinction of mankind. Sun’s energy can be tapped in different ways and photovoltaics is one of the most efficient of them. In photovoltaics, sun’s energy is directly converted into electrical current unlike the case of concentrated solar power where there is intervention of turbines. Due to the absence of large-moving parts in the process, maintenance costs come down greatly. Also for the same reason, power generation is decentralised and does not require a lot of time and capital to kick start. Decentralisation also reduces the losses in the transmission of power. The science of a photovoltaic solar cell is discussed in this article.
The physics described here is for a generic solar cell model and there might be improvised variants which may not look the same. Nevertheless the physics remains almost the same. A typical solar cell consists of N-type silicon semiconductor wafer in contact with a P-type silicon semiconductor wafer. The N-type semiconductor wafer is made of silicon, which is doped with a material rich in electrons and the name N-type suggests the presence of large amounts of negative charge. In case of a P-type semiconductor, the wafer is a positively doped silicon material. N-type wafer is covered by an anti-reflecting material and is the negative terminal of the cell. P-type wafer is the positive terminal of the cell. Photoelectric effect says that when radiation of a particular frequency falls onto material rich in electrons, like a metal or a doped semiconductor, the electrons are relinquished from the forces binding it to the atom. These free electrons move along a direction where they lose their electrostatic potential energy.
When light falls on the solar cell described above, most of it gets absorbed due the anti-reflecting surface and reaches the next layer. N-type semiconductor present in the next layer is rich in electrons and these electrons are freed in the process. There is a wire connecting the N-type region to the P-type region. The electrostatic potential of an electron in the N-type region is higher than that in the P-type region. Hence these electrons move along the wire to the P-type region. As learnt during our A Level Physics tuition class, since the direction of current is opposite to that of flow of electrons, the current flow is pointed away from the P-type semiconductor. The wire is connected to a load, like a bulb, and the electricity generated can be used for running various appliances.