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RenewableandSustainableEnergyReviewsHigh-brightnessLEDs—EnergyefficientlightingsourcesandtheirpotentialinindoorplantcultivationABSTRACTTherapiddevelopmentofoptoelectronictechnologysincemid-1980hassignificantlyenhancedthebrightnessandefficiencyoflight-emittingdiodes(LEDs.LEDshavelongbeenproposedasaprimarylightsourceforspace-basedplantresearchchamberorbioregenerativelifesupportsystems.TheraisingcostofenergyalsomakestheuseofLEDsincommercialcropcultureimminent.Withtheirenergyefficiency,LEDshaveopenednewperspectivesforoptimizingtheenergyconversionandthenutrientsupplybothonandoffEarth.ThepotentialsofLEDasaneffectivelightsourceforindooragriculturalproductionhavebeenexploredtoagreatextent.TherearemanyresearchesthatuseLEDstosupportplantgrowthincontrolledenvironmentssuchasplanttissuecultureroomandgrowthchamber.ThispaperprovidesabriefdevelopmenthistoryofLEDsandabroadbasereviewonLEDapplicationsinindoorplantcultivationsince1990.Contents1.Introduction2.LEDdevelopment.3.Colorratiosandphotosynthesis4.LEDsandindoorplantcultivation.4.1.Planttissuecultureandgrowth4.2.Spaceagriculture84.3.Algaculture4.4.Plantdiseasereduction5.Intermittentandphotoperiodlightingandenergysaving6.Conclusion1.IntroductionWithimpactsofclimatechange,issuessuchasmorefrequentandseriousdroughts,floods,andstormsaswellaspestanddiseasesarebecomingmoreseriousthreatstoagriculture.Thesethreatsalongwithshortageoffoodsupplymakepeopleturntoindoorandurbanfarming(suchasverticalfarmingforhelp.Withproperlighting,indooragricultureeliminatesweather-relatedcropfailuresduetodroughtsandfloodstoprovideyear-roundcropproduction,whichassistinsupplyingfoodincitieswithsurgingpopulationsandinareasofsevereenvironmentalconditions.Theuseoflight-emittingdiodesmarksgreatadvancementsoverexistingindooragriculturallighting.LEDsallowthecontrolofspectralcompositionandtheadjustmentoflightintensitytosimulatethechangesofsunlightintensityduringtheday.Theyhavetheabilitytoproducehighlightlevelswithlowradiantheatoutputandmaintainusefullightoutputforyears.LEDsdonotcontainelectrodesandthusdonotburnoutlikeincandescentorfluorescentbulbsthatmustbeperiodicallyreplaced.Nottomentionthatincandescentandfluorescentlampsconsumealotofelectricalpowerwhilegeneratingheat,whichmustbedispelledfromclosedenvironmentssuchasspaceshipsandspacestations.2.LEDdevelopmentLEDisauniquetypeofsemiconductordiode.Itconsistsofachipofsemiconductormaterialdopedwithimpuritiestocreateap–njunction.Currentflowseasilyfromthep-side(anode,tothen-side(cathode,butnotinthereversedirection.Electronsandholesflowintothejunctionfromelectrodeswithdifferentvoltages.Whenanelectronmeetsahole,itfallsintoalowerenergylevel,andreleasesenergyintheformofaphoton.Thecolor(wavelengthofthelightemitteddependsonthebandgapenergyofthematerialsformingthep–njunction.ThematerialsusedforanLEDhaveadirectbandgapwithenergiescorrespondingtonear-infrared,visibleornear-ultravioletlight.ThekeystructureofanLEDconsistsofthedie(orlight-emittingsemiconductormaterial,aleadframewherethedieisplaced,andtheencapsulationwhichprotectsthedie(Fig.1.Fig.1LEDdevelopmentbeganwithinfraredandreddevicesmadewithgalliumarsenide.Advancesinmaterialssciencehavemadepossibletheproductionofdeviceswithever-shorterwavelengths,producinglightinavarietyofcolors.J.Margolinreportedthatthefirstknownlight-emittingsolidstatediodewasmadein1907byH.J.Round.NopracticaluseofRound’sdiodewasmadeforseveraldecadesuntiltheinventionofthefirstpracticalLEDbyNickHolonyak,Jrin1962.HisLEDsbecamecommerciallyavailableinlate1960s.TheseGaAsPLEDscombinethreeprimaryelements:gallium,arsenicandphosphorustoprovidea655nmredlightwithbrightnesslevelsofapproximately1–10mcdat20mA.Astheluminousintensitywaslow,theseLEDswereonlyusedinafewapplications,primarilyasindicators.FollowingGaAsP,GaP(galliumphosphideredLEDsweredeveloped.Thesedevicesexhibitveryhighquantumefficienciesatlowcurrents.AsLEDtechnologyprogressedthroughthe1970s,additionalcolorsandwavelengthsbecameavailable.ThemostcommonmaterialswereGaPgreenandred,GaAsPorange,andhighefficiencyredandGaAsPyellow.Thetrendtowardsmorepracticalapplications(suchasincalculators,digitalwatches,andtestequipmentalsobegantodevelop.AstheLEDmaterialstechnologybecamemoreadvanced,thelightoutputwasincreased,andLEDsbecamebrightenoughtobeusedforillumination.In1980sanewmaterial,GaAlAs(galliumaluminumarsenidewasdevelopedfollowedbyarapidgrowthintheuseofLEDs.GaAlAstechnologyprovidessuperiorperformanceoverpreviouslyavailableLEDs.Thevoltagerequirementislower,whichresultsinatotalpowersavings.LEDscouldbeeasilypulsedormultiplexedandthusaresuitableforvariablemessageandoutdoorsigns.Alongthisdevelopmentperiod,LEDswerealsodesignedintobarcodescanners,fiberopticdatatransmissionsystems,andmedicalequipment.Duringthistime,theimprovementsincrystalgrowthandopticsdesignallowyellow,greenandorangeLEDsonlyaminorimprovementinbrightnessandefficiency.Thebasicstructureofthematerialremainedrelativelyunchanged.Aslaserdiodeswithoutputinthevisiblespectrumstartedtocommercializeinlate1980s,LEDdesignersusedsimilartechniquestoproducehigh-brightnes