Solar and The PV Effect
Solar panels are devices that convert light into electricity. So called because the most powerful source of free light available comes from the star called Sol by astronomers; our Sun. Solar panels are also referred to as photovoltaics which translates into ‘light-electricity’.
A solar panel is a collection of small solar cells, which when spread over a large enough area, together work to provide sufficient power to be used as a viable alternative to fossil fuels. The more light that hits a cell and the more efficiency of the cell design determines the amount of electricity produced by the panel.
The photovoltaic effect
This is a process in which two dissimilar materials in close contact produce an electrical voltage when struck by light or other radiant energy.
Light striking crystals such as silicon or germanium, in which electrons are usually not free to move from atom to atom within the crystal, provides the energy needed to free some electrons from their bound condition. Free electrons cross the junction between two dissimilar crystals more easily in one direction than in the other, giving one side of the junction a negative charge and therefore, a negative voltage with respect to the other side; just as one electrode of a battery has a negative voltage with respect to the other.
The photovoltaic effect can continue to provide voltage and current as long as light continues to fall on the two materials. This current can be used to measure the brightness of the incident light or as a source of power in an electrical circuit, as in a solar power system.
The photovoltaic effect in a solar cell can be illustrated with an analogy to a child at a slide. Initially, both the electron and the child are in their respective ‘ground states.’ Next, the electron is lifted up to its excited state by consuming energy received from the incoming light, just as the child is lifted up to an ‘excited state’ at the top of the slide by consuming chemical energy stored in his body. In both cases there is now energy available in the excited state that can be expended. In the absence of junction-forming materials, there is no incentive for excited, free electrons to move along a specific direction; they eventually fall back to the ground state. On the other hand, whenever two different materials are placed in contact, an electric field is generated along the contact.
This is the so-called ‘built-in field’, and it exerts a force on free electrons, effectively ’tilting’ the electron states and forcing the excited free electrons into an external electrical load where their excess energy can be dissipated. The external load can be a simple resistor, or it can be any of a myriad of electrical or electronic devices ranging from motors to radios.
Correspondingly, the child moves to the slide because of his desire for excitement. It is on the slide that the child dissipates his excess energy. Finally, when the excess energy is expended, both the electron and the child are back in the ground state, where they can begin the whole process over again. The motion of the electron, like that of the child, is in one direction. In short, the photovoltaic effect produces a ‘direct current'(DC); one that flows constantly in only a single direction.
The heart of a PV cell is the interface between two different types of semiconductor (called p-type and n-type). When a light photon with sufficient energy hits an atom in this region, it throws out an electron.
The electron, now free to move, travels through the n-type semiconductor to metal contacts on the surface. The hole left by the absence of the electron travels in the opposite direction, through the p-type semiconductor.
Once at the metal contact, the electron flows around an electrical circuit, doing work in the process, to meet up with a hole at the rear contact.
