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Copper(I) oxide
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===Semiconductor and related uses=== [[Rectifier|Rectifier diode]]s based on this material have been used industrially as early as 1924, long before [[silicon]] became the standard. Copper(I) oxide is also responsible for the pink color in a positive [[Benedict's reagent|Benedict's test]]. In the history of [[semiconductor]] physics, Cu<sub>2</sub>O is one of the most studied materials. Many [[Semiconductor]] applications have been demonstrated first in this material: *Semiconductor [[diode]]s<ref>L. O. Grondahl, Unidirectional current carrying device, Patent, 1927</ref> *Phonoritons ("a coherent superposition of [[exciton]], [[photon]], and [[phonon]]")<ref>{{Cite journal|last1=Hanke|first1=L.|last2=Fröhlich|first2=D.|last3=Ivanov|first3=A. L.|last4=Littlewood|first4=P. B.|last5=Stolz|first5=H.|date=1999-11-22|title=LA Phonoritons in Cu<sub>2</sub>O|journal=Physical Review Letters|volume=83|issue=21|pages=4365–4368|doi=10.1103/PhysRevLett.83.4365|bibcode=1999PhRvL..83.4365H}}</ref><ref>L. Brillouin: ''Wave Propagation and Group Velocity'', [[Academic Press]], [[New York City]], 1960 {{ISBN|9781483276014}}.</ref> The lowest excitons in Cu<sub>2</sub>O are extremely long lived; absorption lineshapes have been demonstrated with [[electronvolt|neV]] linewidths, which is the narrowest bulk exciton resonance ever observed.<ref>{{cite journal | last1=Brandt | first1=Jan | last2=Fröhlich | first2=Dietmar | last3=Sandfort | first3=Christian | last4=Bayer | first4=Manfred | last5=Stolz | first5=Heinrich | last6=Naka | first6=Nobuko | title=Ultranarrow Optical Absorption and Two-Phonon Excitation Spectroscopy of Cu<sub>2</sub>O Paraexcitons in a High Magnetic Field | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=99 | issue=21 | date=2007-11-19 | issn=0031-9007 | doi=10.1103/physrevlett.99.217403 | page=217403| pmid=18233254 | bibcode=2007PhRvL..99u7403B }}</ref> The associated quadrupole [[polariton]]s have low [[group velocity]] approaching the speed of sound. Thus, light moves almost as slowly as sound in this medium, which results in high polariton densities. Another unusual feature of the [[ground state]] excitons is that all primary scattering mechanisms are known quantitatively.<ref>J. P. Wolfe and A. Mysyrowicz: Excitonic Matter, ''[[Scientific American]]'' '''250''' (1984), No. 3, 98.</ref> Cu<sub>2</sub>O was the first substance where an entirely parameter-free model of [[absorption (electromagnetic radiation)|absorption]] [[linewidth]] broadening by [[temperature]] could be established, allowing the corresponding [[absorption coefficient]] to be deduced. It can be shown using Cu<sub>2</sub>O that the [[Kramers–Kronig relation]]s do not apply to polaritons.<ref name="Hopfield1958">{{cite journal|last1=Hopfield|first1=J. J.|title=Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals|journal=Physical Review|volume=112|issue=5|year=1958|pages=1555–1567|issn=0031-899X|doi=10.1103/PhysRev.112.1555|bibcode=1958PhRv..112.1555H}}</ref> In December 2021, [[Toshiba]] disclosed a transparent cuprous oxide (Cu<sub>2</sub>O) thin-film [[solar cell]]. The cell achieved an 8.4% [[energy conversion efficiency]], the highest efficiency ever reported for any cell of this type as of 2021. The cells could be used for [[Atmospheric satellite#High-altitude platform station|high-altitude platform station]] applications and [[Electric vehicle|electric vehicles]].<ref>{{cite news |last=Bellini |first=Emiliano |url=https://pv-magazine-usa.com/2021/12/22/toshiba-claims-8-4-efficiency-for-transparent-cuprous-oxide-solar-cell/ |title=Toshiba claims 8.4% efficiency for transparent cuprous oxide solar cell |work=pv magazine |date=2021-12-22 |accessdate=2021-12-22 }}</ref>
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