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Photoconductivity
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<p>Photoconductivity is an <a href="/facts/Optical_phenomenon/d0nGfmhT">optical</a> and <a href="/facts/Electrical_phenomenon/NoqGhn6z">electrical phenomenon</a> in which a material becomes more <a href="/facts/Electric_conductance/hxSt0UAj">electrically conductive</a> due to the absorption of <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a> such as <a href="/facts/Visible_spectrum/frPBh8o5">visible light</a>, <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet</a> light, <a href="/facts/Infrared/44XdJNcc">infrared</a> light, or <a href="/facts/Gamma_rays/5Drf913J">gamma radiation</a>. </p><p>When light is absorbed by a material such as a <a href="/facts/Semiconductor/zFf3mGTE">semiconductor</a>, the number of <a href="/facts/Free_electron_model/vIyhQHfK">free electrons</a> and <a href="/facts/Electron_hole/YymdTv9d">holes</a> increases, resulting in increased <a href="/facts/Electrical_resistivity_and_conductivity/akyCKvMy">electrical conductivity</a>. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the <a href="/facts/Band_gap/IjIeh95l">band gap</a>, or to excite the <a href="/facts/Impurities/FfPvIRoB">impurities</a> within the band gap. When a <a href="/facts/Biasing/lK0e6hbK">bias</a> <a href="/facts/Voltage/EN2po6N2">voltage</a> and a load <a href="/facts/Resistor/JrsGReM6">resistor</a> are used in series with the semiconductor, a <a href="/facts/Voltage_drop/zR5RmLKI">voltage drop</a> across the load resistors can be measured when the change in electrical conductivity of the material varies the current through the circuit. </p><p>Classic examples of photoconductive materials include: </p> <ul><li><a href="/facts/Photographic_film/37aFq6ls">photographic film</a>: <a href="/facts/Kodachrome/DTw4Xjow">Kodachrome</a>, <a href="/facts/Fujifilm/jkNJkzet">Fujifilm</a>, <a href="/facts/Agfachrome/GARgpz1p">Agfachrome</a>, <a href="/facts/Ilford_Photo/DVHSSFV8">Ilford</a>, <i>etc.</i>, based on <a href="/facts/Silver_sulfide/muzsPR6v">silver sulfide</a> and <a href="/facts/Silver_bromide/5YsSDplb">silver bromide</a>.</li> <li>the <a href="/facts/Conductive_polymer/JbwFx9IL">conductive polymer</a> poly<a href="/facts/N-Vinylcarbazole/dvGEM5zL">vinylcarbazole</a>, used extensively in <a href="/facts/Photocopy/aF7URdyU">photocopying</a> (<a href="/facts/Xerography/5S5CJdBe">xerography</a>);</li> <li><a href="/facts/Lead(II)_sulfide/IXauvWYq">lead sulfide</a>, used in infrared detection applications, such as the U.S. <a href="/facts/Sidewinder_missile/LPm7nQTT">Sidewinder</a> and <a href="/facts/Soviet_Union/m7wWsPto">Soviet</a> (now <a href="/facts/Russia/jfkCwyxn">Russian</a>) <a href="/facts/Vympel_K-13/mrUxSgGH">Atoll</a> heat-seeking missiles;</li> <li><a href="/facts/Selenium/k975NApC">selenium</a>, employed in early television and xerography.</li></ul> <p>Molecular photoconductors include organic, inorganic, and – more rarely – coordination compounds. </p>