When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. If a piece of p-type silicon is placed in close contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). In practice, p-n junctions of silicon solar cells are not made in this way, but rather by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. The most commonly known solar cell is configured as a large-area p-n junction made from silicon. Main articles: semiconductor and p-n junction These higher energy photons will be absorbed by a silicon solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy. However, the solar frequency spectrum approximates a black body spectrum at about 5,800 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon (1.12eV), which is near to the ideal value for a terrestrial solar cell (1.4eV). It can be said that photons absorbed in the semiconductor create electron-hole pairs.Ī photon only needs to have energy greater than that of the band gap in order to excite an electron from the valence band into the conduction band. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole", leaving another hole behind, thus propagating holes throughout the lattice. The network of covalent bonds that the electron was previously a part of now has one fewer electron. #Diagrams of solar energy freeThe energy given to the electron by the photon "excites" it into the conduction band where it is free to move around within the semiconductor. Usually this electron is in the valence band. When a photon is absorbed, its energy is given to an electron in the crystal lattice. This generates an electron-hole pair and sometimes heat depending on the band structure.īand diagram of a silicon solar cell, corresponding to very low current (horizontal Fermi level), very low voltage (metal valence bands at same height), and therefore very low illumination The photon can be absorbed by the silicon if the photon energy is higher than the silicon band gap value.The photon can reflect off the surface.The photon can pass straight through the silicon - this (generally) happens for lower energy photons. When a photon hits a piece of semiconductor, one of three things can happen:
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