Editing Phosphor (section)

==Phosphor degradation==
Many phosphors tend to lose efficiency gradually by several mechanisms. The activators can undergo change of [[valence (chemistry)|valence]] (usually [[oxidation]]), the [[crystal lattice]] degrades, atoms – often the activators – diffuse through the material, the surface undergoes chemical reactions with the environment with consequent loss of efficiency or buildup of a layer absorbing the exciting and/or radiated energy, etc.

The degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature; moisture impairs phosphor lifetime very noticeably as well.

Harder, high-melting, water-insoluble materials display lower tendency to lose luminescence under operation.<ref name="advelectr"/>

Examples:
* BaMgAl<sub>10</sub>O<sub>17</sub>:Eu<sup>2+</sup> (BAM), a [[plasma display|plasma-display]] phosphor, undergoes oxidation of the dopant during baking. Three mechanisms are involved; absorption of oxygen atoms into oxygen vacancies on the crystal surface, [[diffusion]] of Eu(II) along the conductive layer, and [[electron transfer]] from Eu(II) to absorbed oxygen atoms, leading to formation of Eu(III) with corresponding loss of emissivity.<ref>{{cite journal|doi=10.1016/j.jlumin.2004.09.119|title=On phosphor degradation mechanism: thermal treatment effects|year=2005|last1=Bizarri|first1=G|first2=B|journal=[[Journal of Luminescence]]|volume=113|page=199|last2=Moine|bibcode = 2005JLum..113..199B|issue=3–4 }}</ref> Thin coating of [[aluminium phosphate]] or [[lanthanum(III) phosphate]] is effective in creating a [[barrier layer]] blocking access of oxygen to the BAM phosphor, for the cost of reduction of phosphor efficiency.<ref>Lakshmanan, p. 171.</ref> Addition of [[hydrogen]], acting as a [[reducing agent]], to [[argon]] in the plasma displays significantly extends the lifetime of BAM:Eu<sup>2+</sup> phosphor, by reducing the Eu(III) atoms back to Eu(II).<ref>{{cite journal|doi=10.1143/JJAP.48.092303|title=Lifetime Improvement of BaMgAl<sub>10</sub>O<sub>17</sub>:Eu<sup>2+</sup> Phosphor by Hydrogen Plasma Treatment|year=2009|last1=Tanno|first1=Hiroaki|first2=Takayuki|first3=Shuxiu|first4=Tsutae|first5=Hiroshi|journal=Japanese Journal of Applied Physics|volume=48|page=092303|last2=Fukasawa|last3=Zhang|last4=Shinoda|last5=Kajiyama|bibcode = 2009JaJAP..48i2303T|issue=9 |s2cid=94464554 }}</ref>
* Y<sub>2</sub>O<sub>3</sub>:Eu phosphors under electron bombardment in presence of oxygen form a non-phosphorescent layer on the surface, where [[electron–hole pair]]s [[Carrier generation and recombination|recombine]] nonradiatively via surface states.<ref>{{cite journal|doi=10.1002/pssc.200404813|title=Degradation of Y<sub>2</sub>O<sub>3</sub>:Eu phosphor powders|year=2004|last1=Ntwaeaborwa|first1=O. M.|first2=K. T.|first3=H. C.|journal=Physica Status Solidi C|volume=1|page=2366|last2=Hillie|last3=Swart|bibcode = 2004PSSCR...1.2366N|issue=9 }}</ref>
* ZnS:Mn, used in AC thin-film electroluminescent (ACTFEL) devices degrades mainly due to formation of [[deep-level trap]]s, by reaction of water molecules with the dopant; the traps act as centers for nonradiative recombination. The traps also damage the [[crystal lattice]]. Phosphor aging leads to decreased brightness and elevated threshold voltage.<ref>{{cite journal|doi=10.1143/JJAP.36.2728|title=Deep Traps and Mechanism of Brightness Degradation in Mn-doped ZnS Thin-Film Electroluminescent Devices Grown by Metal-Organic Chemical Vapor Deposition|year=1997|last1=Wang|first1=Ching-Wu|first2=Tong-Ji|first3=Yan-Kuin|first4=Meiso|journal=Japanese Journal of Applied Physics|volume=36|issue=5A|page=2728|last2=Sheu|last3=Su|last4=Yokoyama|bibcode = 1997JaJAP..36.2728W |s2cid=98131548 }}</ref>
* ZnS-based phosphors in [[cathode-ray tube|CRT]]s and [[field emission display|FED]]s degrade by surface excitation, coulombic damage, build-up of electric charge, and thermal quenching. Electron-stimulated reactions of the surface are directly correlated to loss of brightness. The electrons dissociate impurities in the environment, the [[reactive oxygen species]] then attack the surface and form [[carbon monoxide]] and [[carbon dioxide]] with traces of [[carbon]], and nonradiative [[zinc oxide]] and [[zinc sulfate]] on the surface; the reactive [[hydrogen]] removes [[sulfur]] from the surface as [[hydrogen sulfide]], forming nonradiative layer of metallic [[zinc]]. Sulfur can be also removed as [[sulfur oxide]]s.<ref>Lakshmanan, pp. 51, 76</ref>
* ZnS and CdS phosphors degrade by reduction of the metal ions by captured electrons. The M<sup>2+</sup> ions are reduced to M<sup>+</sup>; two M<sup>+</sup> then exchange an electron and become one M<sup>2+</sup> and one neutral M atom. The reduced metal can be observed as a visible darkening of the phosphor layer. The darkening (and the brightness loss) is proportional to the phosphor's exposure to electrons and can be observed on some CRT screens that displayed the same image (e.g. a terminal login screen) for prolonged periods.<ref>{{cite web|url=http://tubedevices.com/alek/pwl/luminofory/luminofory.ppt |title=PPT presentation in Polish (Link to achieved version; Original site isn't available) |publisher=Tubedevices.com |access-date=2016-12-15 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20131228072818/http://tubedevices.com/alek/pwl/luminofory/luminofory.ppt |archive-date=2013-12-28 }}</ref>
* Europium(II)-doped alkaline earth aluminates degrade by formation of [[F-center|color centers]].<ref name="advelectr"/>
* {{chem|Y|2|SiO|5}}:Ce<sup>3+</sup> degrades by loss of luminescent Ce<sup>3+</sup> ions.<ref name="advelectr"/>
* {{chem|Zn|2|SiO|4}}:Mn (P1) degrades by desorption of oxygen under electron bombardment.<ref name="advelectr"/>
* Oxide phosphors can degrade rapidly in presence of [[fluoride]] ions, remaining from incomplete removal of flux from phosphor synthesis.<ref name="advelectr"/>
* Loosely packed phosphors, e.g. when an excess of silica gel (formed from the potassium silicate binder) is present, have tendency to locally overheat due to poor thermal conductivity. E.g. {{chem|InBO|3}}:Tb<sup>3+</sup> is subject to accelerated degradation at higher temperatures.<ref name="advelectr"/>