Photoconductivity is an optical and

electrical phenomenon This is a list of electrical phenomena. Electrical phenomena are a somewhat arbitrary division of electromagnetic phenomena. Some examples are: *Biefeld–Brown effect — Thought by the person who coined the name, Thomas Townsend Brown, to ...

in which a material becomes more

electrically conductive Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation. When light is absorbed by a material such as a semiconductor, the number of free electrons and holes increases, resulting in increased electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (i ...

, or to excite the impurities within the band gap. When a

bias Bias is a disproportionate weight ''in favor of'' or ''against'' an idea or thing, usually in a way that is closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group, ...

voltage and a load

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active ...

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. * the

conductive polymer Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers ...

poly vinylcarbazole, used extensively in

photocopy A photocopier (also called copier or copy machine, and formerly Xerox machine, the generic trademark) is a machine that makes copies of documents and other visual images onto paper or plastic film quickly and cheaply. Most modern photocopier ...

ing (

xerography Xerography is a dry photocopying technique. Originally called electrophotography, it was renamed xerography—from the roots el, ξηρός, label=none ''xeros'', meaning "dry" and -γραφία ''-graphia'', meaning "writing"—to emphasiz ...

); *

lead sulfide Lead sulfide refers to two compounds containing lead and sulfur: *Lead(II) sulfide Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is s ...

, used in infrared detection applications, such as the U.S. Sidewinder and Soviet (now Russian)

Atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gro ...

heat-seeking Infrared homing is a passive weapon guidance system which uses the infrared (IR) light emission from a target to track and follow it seamlessly. Missiles which use infrared seeking are often referred to as "heat-seekers" since infrared is radi ...

missiles; * selenium, employed in early television and

. Molecular photoconductors include organic, inorganic, and – more rarely – coordination compounds.

Applications

When a photoconductive material is connected as part of a circuit, it functions as a

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 device A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...

s (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 detector An infrared detector is a detector that reacts to infrared (IR) radiation. The two main types of detectors are thermal and photonic (photodetectors). The thermal effects of the incident IR radiation can be followed through many temperature depend ...

s, nanophotonic systems and low-dimensional photo-sensors devices.

Sensitization

Sensitization is an important engineering procedure to amplify the response of photoconductive materials. 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.

Negative photoconductivity

Some materials exhibit deterioration in photoconductivity upon exposure to illumination. One prominent example is

hydrogenated amorphous silicon Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs. Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposited in thin films onto ...

(a-Si:H) in which a metastable reduction in photoconductivity is observable (see Staebler–Wronski effect). Other materials that were reported to exhibit negative photoconductivity include molybdenum disulfide,

graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.

, indium arsenide nanowires, decorated carbon nanotubes, and metal nanoparticles.

Magnetic photoconductivity

In 2016 it was demonstrated that in some photoconductive material a magnetic order can exist. One prominent example is CH₃NH₃(Mn:Pb)I₃. In this material a light induced magnetization melting was also demonstrated thus could be used in magneto optical devices and data storage.

Photoconductivity spectroscopy

The characterization technique called photoconductivity spectroscopy (also known as photocurrent spectroscopy) is widely used in studying optoelectronic properties of semiconductors.

References