Editing Photoconductivity

{{short description|Material property in which absorbing EM radiation increases electrical conductivity}}

'''Photoconductivity''' is an [[optical phenomenon|optical]] and [[electrical phenomenon]] in which a material becomes more [[electric conductance|electrically conductive]] due to the absorption of [[electromagnetic radiation]] such as [[Visible spectrum|visible light]], [[ultraviolet]] light, [[infrared]] light, or [[gamma rays|gamma radiation]].<ref name="radio conductivity">{{cite journal |first=L. A. |last=DeWerd |author2=P. R. Moran |pmid=634229 |title=Solid-state electrophotography with Al<sub>2</sub>O<sub>3</sub> |journal=Medical Physics |volume=5 |issue=1 |pages=23–26 |year=1978|bibcode = 1978MedPh...5...23D | doi=10.1118/1.594505 }}</ref>

When light is absorbed by a material such as a [[semiconductor]], the number of [[Free electron model|free electron]]s and [[Electron hole|holes]] increases, resulting in increased [[Electrical resistivity and conductivity|electrical conductivity]].<ref>{{cite journal|last1=Saghaei|first1=Jaber|last2=Fallahzadeh|first2=Ali|last3=Saghaei|first3=Tayebeh|title=Vapor treatment as a new method for photocurrent enhancement of UV photodetectors based on ZnO nanorods|journal=Sensors and Actuators A: Physical|date=June 2016|volume=247|pages=150–155|doi=10.1016/j.sna.2016.05.050|bibcode=2016SeAcA.247..150S }}</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 [[Biasing|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 Photo|Ilford]], ''etc.'', based on [[silver sulfide]] and [[silver bromide]].<ref name="pears1">{{cite book|last1=Pearsall|first1=Thomas|title=Photonics Essentials, 2nd edition|publisher=McGraw-Hill|date=2010|url=https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html|isbn=978-0-07-162935-5}}</ref>
* the [[conductive polymer]] poly[[N-Vinylcarbazole|vinylcarbazole]],<ref name="OrganicPhotoconductors">{{cite journal |doi=10.1021/cr00017a020 |first=Kock Yee |last=Law |title=Organic photoconductive materials: recent trends and developments |journal=Chemical Reviews|volume=93 |pages=449–486 |year=1993}}</ref> used extensively in [[photocopy]]ing ([[xerography]]); 
* [[Lead(II) sulfide|lead sulfide]], used in infrared detection applications, such as the U.S. [[Sidewinder missile|Sidewinder]] and [[Soviet Union|Soviet]] (now [[Russia|Russian]]) [[Vympel K-13|Atoll]] heat-seeking missiles; 
* [[selenium]],<ref>{{Cite journal |last1=Belev |first1=G. |last2=Kasap |first2=S. O. |date=2004-10-15 |title=Amorphous selenium as an X-ray photoconductor |url=https://www.sciencedirect.com/science/article/pii/S0022309304006386 |journal=Journal of Non-Crystalline Solids |series=Physics of Non-Crystalline Solids 10 |language=en |volume=345-346 |pages=484–488 |doi=10.1016/j.jnoncrysol.2004.08.070 |bibcode=2004JNCS..345..484B |issn=0022-3093|url-access=subscription }}</ref> employed in early television and xerography.
Molecular photoconductors include organic,<ref>{{Cite journal |last1=Weiss |first1=David S. |last2=Abkowitz |first2=Martin |date=2010-01-13 |title=Advances in Organic Photoconductor Technology |url=https://doi.org/10.1021/cr900173r |journal=Chemical Reviews |volume=110 |issue=1 |pages=479–526 |doi=10.1021/cr900173r |pmid=19848380 |issn=0009-2665|url-access=subscription }}</ref> inorganic,<ref>{{Cite journal |last1=Cai |first1=Wensi |last2=Li |first2=Haiyun |last3=Li |first3=Mengchao |last4=Wang |first4=Meng |last5=Wang |first5=Huaxin |last6=Chen |first6=Jiangzhao |last7=Zang |first7=Zhigang |date=2021-05-13 |title=Opportunities and challenges of inorganic perovskites in high-performance photodetectors |url=https://iopscience.iop.org/article/10.1088/1361-6463/abf709 |journal=Journal of Physics D: Applied Physics |language=en |volume=54 |issue=29 |pages=293002 |doi=10.1088/1361-6463/abf709 |bibcode=2021JPhD...54C3002C |s2cid=234883317 |issn=0022-3727|url-access=subscription }}</ref> and – more rarely – coordination compounds.<ref>{{Cite journal |last1=Aragoni |first1=M. Carla |last2=Arca |first2=Massimiliano |last3=Devillanova |first3=Francesco A. |last4=Isaia |first4=Francesco |last5=Lippolis |first5=Vito |last6=Mancini |first6=Annalisa |last7=Pala |first7=Luca |last8=Verani |first8=Gaetano |last9=Agostinelli |first9=Tiziano |last10=Caironi |first10=Mario |last11=Natali |first11=Dario |date=2007-02-01 |title=First example of a near-IR photodetector based on neutral [M(R-dmet)2] bis(1,2-dithiolene) metal complexes |url=https://www.sciencedirect.com/science/article/pii/S1387700306004011 |journal=Inorganic Chemistry Communications |language=en |volume=10 |issue=2 |pages=191–194 |doi=10.1016/j.inoche.2006.10.019 |issn=1387-7003|url-access=subscription }}</ref><ref>{{Cite journal |last1=Pintus |first1=Anna |last2=Ambrosio |first2=Lucia |last3=Aragoni |first3=M. Carla |last4=Binda |first4=Maddalena |last5=Coles |first5=Simon J. |last6=Hursthouse |first6=Michael B. |last7=Isaia |first7=Francesco |last8=Lippolis |first8=Vito |last9=Meloni |first9=Giammarco |last10=Natali |first10=Dario |last11=Orton |first11=James B. |date=2020-05-04 |title=Photoconducting Devices with Response in the Visible–Near-Infrared Region Based on Neutral Ni Complexes of Aryl-1,2-dithiolene Ligands |url=https://doi.org/10.1021/acs.inorgchem.0c00491 |journal=Inorganic Chemistry |volume=59 |issue=9 |pages=6410–6421 |doi=10.1021/acs.inorgchem.0c00491 |pmid=32302124 |s2cid=215809603 |issn=0020-1669|hdl=11311/1146329 |hdl-access=free }}</ref>

==Applications==
{{further|Photoresistor}}
When a photoconductive material is connected as part of a circuit, it functions as a [[resistor]] whose [[Electrical resistance|resistance]] depends on the [[intensity (physics)|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 [[photodetector]]s, i.e. devices that measure light intensity. Photoresistors are not the ''only'' type of photodetector—other types include [[charge-coupled device]]s (CCDs), [[photodiode]]s and [[phototransistor]]s—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]]s, nanophotonic systems and low-dimensional photo-sensors devices.<ref>{{cite journal |last1=Hernández-Acosta |first1=M A |last2=Trejo-Valdez |first2=M |last3=Castro-Chacón |first3=J H |last4=Torres-San Miguel |first4=C R |last5=Martínez-Gutiérrez |first5=H |last6=Torres-Torres |first6=C |title=Chaotic signatures of photoconductive Cu ZnSnS nanostructures explored by Lorenz attractors |journal=New Journal of Physics |date=23 February 2018 |volume=20 |issue=2 |pages=023048 |doi=10.1088/1367-2630/aaad41|bibcode=2018NJPh...20b3048H |doi-access=free }}</ref>

==Sensitization==
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 (physicist)|Albert Rose]] is the work of reference for sensitization.<ref name="rose1">{{cite book|last1=Rose|first1=Albert|title=Photoconductivity and Allied Problems|publisher=Wiley Interscience|date=1963
|url=https://www.amazon.com/Concepts-photoconductivity-problems-Interscience-astronomy/dp/B0006AYVDG|isbn=0-88275-568-4|series=Interscience tracts on physics and astronomy }}</ref>

==Negative photoconductivity==
Some materials exhibit deterioration in photoconductivity upon exposure to illumination.<ref name="Joshi1990">{{cite book|author=N V Joshi|title=Photoconductivity: Art: Science & Technology|url=https://books.google.com/books?id=lv-Nb5-H3pQC&pg=PA272|date=25 May 1990|publisher=CRC Press|isbn=978-0-8247-8321-1}}</ref> One prominent example is [[hydrogenated amorphous silicon]] (a-Si:H) in which a metastable reduction in photoconductivity is observable<ref name="StaeblerWronski1977">{{cite journal|last1=Staebler|first1=D. L.|last2=Wronski|first2=C. R.|title=Reversible conductivity changes in discharge-produced amorphous Si|journal=Applied Physics Letters|volume=31|issue=4|year=1977|pages=292|issn=0003-6951|doi=10.1063/1.89674|bibcode = 1977ApPhL..31..292S }}</ref> (see [[Staebler–Wronski effect]]). Other materials that were reported to exhibit negative photoconductivity include [[Zinc oxide|ZnO nanowires]],<ref name=":0">{{Cite journal |last1=Javadi |first1=Mohammad |last2=Abdi |first2=Yaser |date=2018-07-30 |title=Frequency-driven bulk-to-surface transition of conductivity in ZnO nanowires |url=https://aip.scitation.org/doi/abs/10.1063/1.5039474 |journal=Applied Physics Letters |volume=113 |issue=5 |pages=051603 |doi=10.1063/1.5039474 |bibcode=2018ApPhL.113e1603J |issn=0003-6951|url-access=subscription }}</ref> [[molybdenum disulfide]],<ref name="Serpi1992">{{cite journal|last1=Serpi|first1=A.|title=Negative Photoconductivity in MoS2|journal=Physica Status Solidi A|volume=133|issue=2|year=1992|pages=K73–K77|issn=0031-8965|doi=10.1002/pssa.2211330248|bibcode = 1992PSSAR.133...73S }}</ref> [[graphene]],<ref name="HeymanStein2015">{{cite journal|last1=Heyman|first1=J. N.|last2=Stein|first2=J. D.|last3=Kaminski|first3=Z. S.|last4=Banman|first4=A. R.|last5=Massari|first5=A. M.|last6=Robinson|first6=J. T.|title=Carrier heating and negative photoconductivity in graphene|journal=Journal of Applied Physics|volume=117|issue=1|year=2015|pages=015101|issn=0021-8979|doi=10.1063/1.4905192|arxiv = 1410.7495 |bibcode = 2015JAP...117a5101H |s2cid=118531249 }}</ref> [[indium arsenide]] [[nanowire]]s,<ref>{{Cite journal|last1=Alexander-Webber|first1=Jack A.|last2=Groschner|first2=Catherine K.|last3=Sagade|first3=Abhay A.|last4=Tainter|first4=Gregory|last5=Gonzalez-Zalba|first5=M. Fernando|last6=Di Pietro|first6=Riccardo|last7=Wong-Leung|first7=Jennifer|last8=Tan|first8=H. Hoe|last9=Jagadish|first9=Chennupati|date=2017-12-11|title=Engineering the Photoresponse of InAs Nanowires|journal=ACS Applied Materials & Interfaces|language=EN|volume=9|issue=50|pages=43993–44000|doi=10.1021/acsami.7b14415|pmid=29171260|issn=1944-8244|doi-access=free|hdl=1885/237356|hdl-access=free}}</ref> decorated carbon nanotubes,<ref>{{Cite journal|last1=Jiménez-Marín|first1=E.|last2=Villalpando|first2=I.|last3=Trejo-Valdez|first3=M.|last4=Cervantes-Sodi|first4=F.|last5=Vargas-García|first5=J. R.|last6=Torres-Torres|first6=C.|date=2017-06-01|title=Coexistence of positive and negative photoconductivity in nickel oxide decorated multiwall carbon nanotubes|url=https://www.sciencedirect.com/science/article/pii/S0921510717300569|journal=Materials Science and Engineering: B|language=en|volume=220|pages=22–29|doi=10.1016/j.mseb.2017.03.004|issn=0921-5107|url-access=subscription}}</ref> and metal [[nanoparticle]]s.<ref name="NakanishiBishop2009">{{cite journal|last1=Nakanishi|first1=Hideyuki|last2=Bishop|first2=Kyle J. M.|last3=Kowalczyk|first3=Bartlomiej|last4=Nitzan|first4=Abraham|last5=Weiss|first5=Emily A.|last6=Tretiakov|first6=Konstantin V.|last7=Apodaca|first7=Mario M.|last8=Klajn|first8=Rafal|last9=Stoddart|first9=J. Fraser|last10=Grzybowski|first10=Bartosz A.|title=Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles|journal=Nature|volume=460|issue=7253|year=2009|pages=371–375|issn=0028-0836|doi=10.1038/nature08131|bibcode = 2009Natur.460..371N|pmid=19606145|s2cid=4425298 }}</ref>

Under an applied AC voltage and upon UV illumination, [[Zinc oxide|ZnO]] [[Nanowire|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-area-to-volume ratio|surface-to-volume ratio]].

==Magnetic photoconductivity==
In 2016 it was demonstrated that in some photoconductive material a magnetic order can exist.<ref name="Náfrádi 2016">{{cite journal|last1=Náfrádi|first1=Bálint|title=Optically switched magnetism in photovoltaic perovskite CH3NH3(Mn:Pb)I3|journal=Nature Communications|date=24 November 2016|volume=7|issue=13406|page=13406|doi=10.1038/ncomms13406|pmid=27882917|pmc=5123013|arxiv=1611.08205|bibcode=2016NatCo...713406N}}</ref> One prominent example is CH<sub>3</sub>NH<sub>3</sub>(Mn:Pb)I<sub>3</sub>. 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.

==Photoconductivity spectroscopy==
{{Main|Photocurrent#Photocurrent spectroscopy}}
The characterization technique called '''photoconductivity spectroscopy''' (also known as '''photocurrent spectroscopy''') is widely used in studying optoelectronic properties of semiconductors.<ref>{{cite web |url=https://www.rsc.org/publishing/journals/prospect/ontology.asp?id=CMO:0002602&MSID=C2JM15027A |title=RSC Definition - Photocurrent spectroscopy |website=RSC |access-date=2020-07-19 }}</ref><ref>
{{cite book |last1=Lamberti |first1=Carlo |last2=Agostini |first2=Giovanni |date=2013 |title=Characterization of Semiconductor Heterostructures and Nanostructures |chapter=15.3 - Photocurrent spectroscopy |edition=2 |location=Italy |publisher=Elsevier |pages=652–655 |isbn=978-0-444-59551-5 |doi=10.1016/B978-0-444-59551-5.00001-7 }}</ref>

==See also==
* [[Photodiode]]
* [[Photoresistor]] (LDR)
* [[Photocurrent]]
* [[Photoconductive polymer]]
* [[Infrared detector]]
** [[Lead selenide]] (PbSe)
** [[Indium antimonide]] (InSb)

==References==
{{reflist}}

[[Category:Condensed matter physics]]
[[Category:Electrical phenomena]]
[[Category:Optical phenomena]]