Photovoltaic paints
Introduction
The photoelectric effect consists of the emission of electrons by a material when electromagnetic radiation (visible or ultraviolet light, in general) hits it.[1] Other types of interaction between light and matter are sometimes included in the term:
The photoelectric effect was discovered and described by Heinrich Hertz, in 1887, when he observed that the arc that jumps between two electrodes connected to high voltage reaches greater distances when illuminated with ultraviolet light than when left in the dark.
The theoretical explanation was made by Albert Einstein, who published the revolutionary article Heuristics of the generation and conversion of light in 1905, basing his formulation of photoelectricity on an extension of Max Planck's work on quanta. Robert Andrews Millikan later spent ten years experimenting, trying to prove that Einstein's theory was not correct, finally concluding that it was. This allowed Einstein and Millikan to be awarded Nobel Prizes in 1921 and 1923, respectively.
You could say that the photoelectric effect is the opposite of X-rays, since the photoelectric effect indicates that photons can transfer energy to electrons. X-rays (the nature of their radiation was not known, hence the unknown "X") are the transformation into a photon of all or part of the kinetic energy of a moving electron. This was discovered by chance before the work of Planck and Einstein became known (although it was not understood then).
Photons have a characteristic energy determined by the wave frequency of light. If an atom absorbs energy from a photon that has more energy than necessary to eject an electron from the material and also has a path directed toward the surface, then the electron can be ejected from the material. If the photon energy is too small, the electron is unable to escape from the surface of the material. Changes in the intensity of light do not modify the energy of its photons, only the number of electrons that can escape from the surface on which it falls and therefore the energy of the electrons emitted does not depend on the intensity of the radiation that reaches it, but on its frequency. If the photon is absorbed, part of the energy is used to release it from the atom and the rest contributes to providing kinetic energy to the free particle.
All electrons are susceptible to being emitted by the photoelectric effect, but only those that need less energy to be ejected from their orbit do so.
In an insulator (dielectric), the most energetic electrons are found in the valence band. In a metal, the most energetic electrons are in the conduction band. In an N-type semiconductor, it is the conduction band electrons that are most energetic. In a P-type semiconductor too, but there are very few of them in the conduction band. So in this type of semiconductor you have to take into account the electrons in the valence band.