Photovoltaic effect

Definition Photovoltaic effect, is the process where two dissimilar materials that are in close contact generate an electric voltage after being hit by incident light or any other radiant energy. When light strikes crystals similar to silicon or germanium - where electrons are typically not free to move from atom to atom inside the crystal- it provides the energy required to free some electrons from their bound state. Free electrons then cross the junction between the two dissimilar crystals with more ease in a single direction than in the other, as a result giving a negative charge to one side of the junction and, as a result, a negative voltage in relation to the other side, similar to one electrode of a battery having a negative voltage in relation to the other. The photovoltaic effect can keep providing voltage and current as long as there is light falling on the two materials. The produced current can be used in measuring the incident light’s brightness or as a power source in an electrical circuit, like in a solar power systems   Light consists of photons, that are just small packets of electromagnetic energy. A photovoltaic cell - the type of cell that make up a solar panel- can absorb these photons. When light that is of a suitable wavelength strikes these cells, photon energy is transferred to an atom of the PN junction’s semiconducting material. Precisely, the energy is transported to the electrons in the semiconducting material. This makes the electrons jump to a state of higher energy which is known as the conduction band. Thus a hole is left behind in the valence band which the electron leaped up from. The movement of the electrons resulting from the extra energy generates two charge carriers, which is the electron hole pair.                                          

Photovoltaic effect and electron energy

Initially in the photovoltaic effect the electron  is in a respective “ground states.” After that, the electron is elevated to its excited state by consuming the energy received from the incident light. Now the available energy in the excited state can be expended. In the absence of any junction forming materials, no incentive exists for free, excited electrons to move along in a certain direction; so they fall back eventually to the ground state. On the other side, when two different materials are positioned in contact, an electric field is therefore generated along this contact. This is the defined as a built-in field, and it applies a force on free electrons, leading to tilting the electron states and forcing the free excited electrons into the external electrical load so the excess energy of electrons can be dissipated.   This external load can simply be a resistor, or any number of complex of electrical devices ranging from lights to heaters. The motion of the electron is in a single direction. In short, the photovoltaic effect produces a DC current ( direct current), and this current constantly flows in only one direction.  
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