External energy source
Solar energy is the only external energy source we have on earth. Everything that grows, blossoms, and lives come indirectly from the sun, even fossil fuels are in fact nothing but stored plant remains that have only been able to grow with heat and light from the sun. In short, solar energy is the most direct form of energy we have on earth! Did you know that every hour more sunlight falls on the earth than we need for energy in a year? The trick is to use solar panels to convert this energy from sunlight into a more useful form for us, such as electricity. There are different ways to convert energy from the sun that we distinguish between photothermal and photovoltaic.
With photothermal techniques, sunlight is used to heat up a fluid. This can be done, for example, by using heat panels to create hot water for the home. But there are also large solar parks where mirrors are aimed at a central tower filled with water. Steam is generated in this tower to drive a turbine, just like in a fossil fuel power plant.
Standard solar panels are based on the photovoltaic effect. We, therefore, call them PV (photovoltaic) panels. Light photons falling on the panel create a voltage difference with which electricity is generated.
With Solar Monkey we focus only on PV panels. The first panel was built in 1883 by the American Charles Fritts. And although technology has changed a lot, the operating principle of a PV panel remains unchanged. After reading this article you will know all about the basic principles of PV panels!
Different ways to convert solar energy
To produce PV panels, we need materials with specific properties for conducting electricity. Electricity and the conduction of electricity are nothing more than the movement of charged particles, electrons. Metals are good conductors of electricity because electrons can move freely through the material. In most non-metallic materials, the electrons are bound to the atom and cannot move freely through the material. These materials do not conduct electricity and we call them insulators.
Semiconductors and the photovoltaic effect
For solar panels, we need so-called semiconductors. These materials normally conduct little or no electricity, but by certain addition of energy, free electrons can be created, and these then ‘jump’ loose from the atomic lattice. We then say that an electron jumps from the valence band to the conduction band. An electron in the conduction band can therefore move freely through the material. The energy required for an electron to jump from the valence band to the conduction band is called the bandgap. The reason for the existence of this bandgap lies in quantum mechanics and will not be considered further here. When a negatively charged electron jumps free from the atom, it leaves behind a positively charged ‘electron hole’. The energy to release an electron can be supplied by heat, for example, but also by light.
Schematic representation of a metal and a semiconductor.
In a solar panel, this energy is supplied by a particle of light, also known as a photon. This photon is absorbed by the material, the negatively charged electron is released from the material grid and can now move freely through the material. What remains is a positively charged particle, also called a hole. So now there is an electron-hole pair that can both move freely through the material.
Illustration of the generation of an electron-hole pair by the absorption of a light particle (photon). An electron becomes detached and ‘jumps’ from the valence band to the conduction band.
The shedding of free electrons and free-electron holes does not in itself create a voltage difference. First, these charged particles must be separated. This is done by artificially creating two different types of semiconductors and placing them on top of each other. We speak of a p-type and an n-type semiconductor.
In an n-type semiconductor, atoms are added during the production process that has a free electron available and donates to the material. Adding phosphorus atoms to silicon releases an electron, leaving behind a positively charged phosphorus atom. This phosphorus atom is fixed in the lattice and, unlike the electron that has been released, cannot move. We call this N-doping. So now there are more freely moving electrons than holes. This can also be done the other way round, by adding a p-type doping atom that takes up an electron. There are then more free-moving electron holes or positively charged particles. In practice, Boron is often used for this.
On the left, a phosphorus-doped material (n-doped) with free negatively charged electrons and positively charged P atoms in the grid. On the right, a boron-doped material (p-doped) with free positively charged holes and negatively charged B-atoms in the lattice.
On the left, a material doped with phosphorus (n-doped) with free negatively charged electrons and positively charged P-atoms in the grid. On the right, a material doped with boron (p-doped) with free positively charged holes and negatively charged B-atoms in the lattice.
Because the density of electrons (or holes) on one side is many times lower than on the other, there is a driving force to flow from one side to the other. You can compare this to two connected barrels of liquid or gas, where this always flows out until the pressure on both sides is equal.
This is the force of an electric voltage caused by the charged particles in the lattice. The positively charged solid atoms in the lattice repel the negatively charged electrons and vice versa.
Diffusion and drift are opposite, and when a solar panel is in the dark, these drift and diffusion are in balance. So no electricity is generated yet. At the moment of illumination, new electrons and holes are formed throughout the material, causing the electrical voltage (drift) to exceed diffusion. This causes the electrons to be drawn to the n-type side, while the holes are drawn to the p-type side. This results in a voltage difference and the solar panel has become a voltage source. We connect the plus pole to the p-type and the minus pole to the n-type. We now have an electrical circuit in which electrons flow from minus to plus. And there we have our solar panel!