Photoconductivity

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Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.<ref name="radio conductivity">Template:Cite journal</ref>

When light is absorbed by a material such as a semiconductor, the number of free electrons and holes increases, resulting in increased electrical conductivity.<ref>Template:Cite journal</ref> To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. When a bias voltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current through the circuit.

Classic examples of photoconductive materials include:

photographic film: Kodachrome, Fujifilm, Agfachrome, Ilford, etc., based on silver sulfide and silver bromide.<ref name="pears1">Template:Cite book</ref>
the conductive polymer polyvinylcarbazole,<ref name="OrganicPhotoconductors">Template:Cite journal</ref> used extensively in photocopying (xerography);
lead sulfide, used in infrared detection applications, such as the U.S. Sidewinder and Soviet (now Russian) Atoll heat-seeking missiles;
selenium,<ref>Template:Cite journal</ref> employed in early television and xerography.

Molecular photoconductors include organic,<ref>Template:Cite journal</ref> inorganic,<ref>Template:Cite journal</ref> and – more rarely – coordination compounds.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

ApplicationsEdit

Template:Further When a photoconductive material is connected as part of a circuit, it functions as a resistor whose resistance depends on the light intensity. In this context, the material is called a photoresistor (also called light-dependent resistor or photoconductor). The most common application of photoresistors is as photodetectors, i.e. devices that measure light intensity. Photoresistors are not the only type of photodetector—other types include charge-coupled devices (CCDs), photodiodes and phototransistors—but they are among the most common. Some photodetector applications in which photoresistors are often used include camera light meters, street lights, clock radios, infrared detectors, nanophotonic systems and low-dimensional photo-sensors devices.<ref>Template:Cite journal</ref>

SensitizationEdit

Sensitization is an important engineering procedure to amplify the response of photoconductive materials.<ref name="pears1"/> The photoconductive gain is proportional to the lifetime of photo-excited carriers (either electrons or holes). Sensitization involves intentional impurity doping that saturates native recombination centers with a short characteristic lifetime, and replacing these centers with new recombination centers having a longer lifetime. This procedure, when done correctly, results in an increase in the photoconductive gain of several orders of magnitude and is used in the production of commercial photoconductive devices. The text by Albert Rose is the work of reference for sensitization.<ref name="rose1">Template:Cite book</ref>

Negative photoconductivityEdit

Some materials exhibit deterioration in photoconductivity upon exposure to illumination.<ref name="Joshi1990">Template:Cite book</ref> One prominent example is hydrogenated amorphous silicon (a-Si:H) in which a metastable reduction in photoconductivity is observable<ref name="StaeblerWronski1977">Template:Cite journal</ref> (see Staebler–Wronski effect). Other materials that were reported to exhibit negative photoconductivity include ZnO nanowires,<ref name=":0">Template:Cite journal</ref> molybdenum disulfide,<ref name="Serpi1992">Template:Cite journal</ref> graphene,<ref name="HeymanStein2015">Template:Cite journal</ref> indium arsenide nanowires,<ref>Template:Cite journal</ref> decorated carbon nanotubes,<ref>Template:Cite journal</ref> and metal nanoparticles.<ref name="NakanishiBishop2009">Template:Cite journal</ref>

Under an applied AC voltage and upon UV illumination, ZnO nanowires exhibit a continuous transition from positive to negative photoconductivity as a function of the AC frequency.<ref name=":0" /> ZnO nanowires also display a frequency-driven metal-insulator transition at room temperature. The responsible mechanism for both transitions has been attributed to a competition between bulk conduction and surface conduction.<ref name=":0" /> The frequency-driven bulk-to-surface transition of conductivity is expected to be a generic character of semiconductor nanostructures with the large surface-to-volume ratio.

Magnetic photoconductivityEdit

In 2016 it was demonstrated that in some photoconductive material a magnetic order can exist.<ref name="Náfrádi 2016">Template:Cite journal</ref> One prominent example is CH₃NH₃(Mn:Pb)I₃. In this material a light induced magnetization melting was also demonstrated<ref name="Náfrádi 2016"/> thus could be used in magneto optical devices and data storage.