how the solar cell work.docx

how the solar cell work.docx

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howthesolarcellwork

出处：

Toothman,Jessika,andScottAldous. "HowSolarCellsWork" 01April2000. HowStuffW.< 28May2011.

IntroductiontoHowSolarCellsWork

RenewingtheGridImageGallery

©iS

Thisisoneplaceyou'reprobablyusedtoseeingsolarcells,butthey'llbepoppingupmoreastheyearsgoby.Seemore renewingthegridpictures.

You'veprobablyseencalculatorswithsolarcells--devicesthatneverneed batteries andinsomecases,don'tevenhaveanoffbutton.Aslongasthere'senough light,theyseemtoworkforever.Youmayalsohaveseenlargersolarpanels,perhapsonemergencyroadsigns,callboxes,buoysandeveninparkinglotstopowerthelights.

Althoughtheselargerpanelsaren'tascommonassolar-poweredcalculators,they'reoutthereandnotthathardtospotifyouknowwheretolook.Infact, photovoltaics --whichwereonceusedalmostexclusivelyinspace,poweringsatellites'electricalsystemsasfarbackas1958--arebeingusedmoreandmoreinlessexoticways.Thetechnologycontinuestopopupinnewdevicesallthetime,fromsunglassestoelectricvehiclechargingstations.

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10AmazingIntellectualAchievements

Thehopefora"solarrevolution"hasbeenfloatingaroundfordecades--theideathatonedaywe'llallusefreeelectricityfromthe sun.Thisisaseductivepromise,becauseonabright,sunnyday,thesun'sraysgiveoffapproximately1,000wattsofenergypersquaremeteroftheplanet'ssurface.Ifwecouldcollectallofthatenergy,wecouldeasilypowerourhomesandofficesforfree.

Inthisarticle,wewillexaminesolarcellstolearnhowtheyconvertthesun'senergydirectlyintoelectricity.Intheprocess,youwilllearnwhywe'regettingclosertousingthesun'senergyonadailybasis,andwhywestillhavemoreresearchtodobeforetheprocessbecomescost-effective.

PhotovoltaicCells:

ConvertingPhotonstoElectrons

Thesolarcellsthatyouseeoncalculatorsand satellites arealsocalledphotovoltaic（PV）cells,whichasthenameimplies（photomeaning"light"andvoltaicmeaning"electricity"）,convertsunlightdirectlyintoelectricity.Amoduleisagroupofcellsconnectedelectricallyandpackagedintoaframe（morecommonlyknownasasolarpanel）,whichcanthenbegroupedintolargersolararrays,liketheoneoperatingatNellisAirForceBaseinNevada.

GoingSolar,GoingGreen

Addingsolarpanelstoanexistinghomecanbeexpensive--buttherearelotsofotherwaystomakeyourhomegreener.LearnmoreaboutwhatyoucandotoprotecttheenvironmentatDiscoveryChannel's PlanetGreen.

Photovoltaiccellsaremadeofspecialmaterialscalledsemiconductorssuchassilicon,whichiscurrentlyusedmostcommonly.Ba¬sically,whenlightstrikesthecell,acertainportionofitisabsorbedwithinthesemiconductormaterial.Thismeansthattheenergyoftheabsorbedlightistransferredtothesemiconductor.Theenergyknockselectronsloose,allowingthemtoflowfreely.

PVcellsalsoallhaveoneormoreelectricfieldthatactstoforceelectronsfreedbylightabsorptiontoflowinacertaindirection.Thisflowofelectronsisacurrent,andbyplacingmetalcontactsonthetopandbottomofthePVcell,wecandrawthatcurrentoffforexternaluse,say,topoweracalculator.Thiscurrent,togetherwiththecell'svoltage（whichisaresultofitsbuilt-inelectricfieldorfields）,definesthepower（orwattage）thatthesolarcellcanproduce.

That'sthebasicprocess,butthere'sreallymuchmoretoit.Onthenextpage,let'stakeadeeperlookintooneexampleofaPVcell:

thesingle-crystalsiliconcell.

HowSiliconMakesaSolarCell

APPhoto/CharlesDharapak

PresidentBarackObama,SenateMajorityLeaderHarryReidofNevada,andCol.HowardBelote,checkedoutthesolarpanelsatNellisAirForceBaseinNevadainMayof2009.

Siliconhassomespecialchemicalproperties,especiallyinitscrystallineform.An atom ofsiliconhas14electrons,arrangedinthreedifferentshells.Thefirsttwoshells--whichholdtwoandeightelectronsrespectively--arecompletelyfull.Theoutershell,however,isonlyhalffullwithjustfourelectrons.Asiliconatomwillalwayslookforwaystofillupitslastshell,andtodothis,itwillshareelectronswithfournearbyatoms.It'slikeeachatomholdshandswithitsneighbors,exceptthatinthiscase,eachatomhasfourhandsjoinedtofourneighbors.That'swhatformsthecrystallinestructure,andthatstructureturnsouttobeimportanttothistypeofPVcell.

Theonlyproblemisthatpurecrystallinesiliconisapoorconductorofelectricitybecausenoneofitselectronsarefreetomoveabout,unliketheelectronsinmoreoptimumconductorslikecopper.Toaddressthisissue,thesiliconinasolarcellhas impurities --otheratomspurposefullymixedinwiththesiliconatoms--whichchangesthewaythingsworkabit.Weusuallythinkofimpuritiesassomethingundesirable,butinthiscase,ourcellwouldn'tworkwithoutthem.Considersiliconwithanatomofphosphoroushereandthere,maybeoneforeverymillionsiliconatoms.Phosphoroushasfiveelectronsinitsoutershell,notfour.Itstillbondswithitssiliconneighboratoms,butinasense,thephosphoroushasoneelectronthatdoesn'thaveanyonetoholdhandswith.Itdoesn'tformpartofabond,butthereisapositiveprotoninthephosphorousnucleusholdingitinplace.

When energy isaddedtopuresilicon,intheformofheatforexample,itcancauseafewelectronstobreakfreeoftheirbondsandleavetheiratoms.Aholeisleftbehindineachcase.Theseelectrons,called freecarriers,thenwanderrandomlyaroundthecrystallinelatticelookingforanotherholetofallintoandcarryinganelectricalcurrent.However,therearesofewoftheminpuresilicon,thattheyaren'tveryuseful.

Butourimpuresiliconwithphosphorousatomsmixedinisadifferentstory.Ittakesalotlessenergytoknocklooseoneofour"extra"phosphorouselectronsbecausetheyaren'ttiedupinabondwithanyneighboringatoms.Asaresult,mostoftheseelectronsdobreakfree,andwehavealotmorefreecarriersthanwewouldhaveinpuresilicon.Theprocessofaddingimpuritiesonpurposeiscalled doping,andwhendopedwithphosphorous,theresultingsiliconiscalled N-type （"n"fornegative）becauseoftheprevalenceoffreeelectrons.N-typedopedsiliconisamuchbetterconductorthanpuresilicon.

Theotherpartofatypicalsolarcellisdopedwiththeelementboron,whichhasonlythreeelectronsinitsoutershellinsteadoffour,tobecomeP-typesilicon.Insteadofhavingfreeelectrons, P-type （"p"forpositive）hasfreeopeningsandcarriestheopposite（positive）charge.

Onthenextpage,we'lltakeacloserlookatwhathappenswhenthesetwosubstancesstarttointeract.

AnatomyofaSolarCell

Beforenow,ourtwoseparatepiecesofsiliconwereelectricallyneutral;theinterestingpartbeginswhenyouputthemtogether.That'sbecausewithoutan electricfield,thecellwouldn'twork;thefieldformswhentheN-typeandP-typesiliconcomeintocontact.Suddenly,thefreeelectronsontheNsideseealltheopeningsonthePside,andthere'samadrushtofillthem.Doallthefreeelectronsfillallthefreeholes?

No.Iftheydid,thenthewholearrangementwouldn'tbeveryuseful.However,rightatthe junction,theydomixandformsomethingofabarrier,makingitharderandharderforelectronsontheNsidetocrossovertothePside.Eventually,equilibriumisreached,andwehaveanelectricfieldseparatingthetwosides.

Thiselectricfieldactsasa diode,allowing（andevenpushing）electronstoflowfromthePsidetotheNside,butnottheotherwayaround.It'slikeahill--electronscaneasilygodownthehill（totheNside）,butcan'tclimbit（tothePside）.

Whenlight,intheformof photons,hitsoursolarcell,itsenergybreaksapartelectron-holepairs.Eachphotonwithenoughenergywillnormallyfreeexactlyoneelectron,resultinginafreeholeaswell.Ifthishappenscloseenoughtotheelectricfield,oriffreeelectronandfreeholehappentowanderintoitsrangeofinfluence,thefieldwillsendtheelectrontotheNsideandtheholetothePside.Thiscausesfurtherdisruptionofelectricalneutrality,andifweprovideanexternalcurrentpath,electronswillflowthroughthepathtothePsidetounitewithholesthattheelectricfieldsentthere,doingworkforusalongtheway.Theelectronflowprovidesthe current,andthecell'selectricfieldcausesa voltage.Withbothcurrentandvoltage,wehave power,whichistheproductofthetwo.

Thereareafewmorecomponentsleftbeforewecanreallyuseourcell.Siliconhappenstobeaveryshinymaterial,whichcansendphotonsbouncingawaybeforethey'vedonetheirjob,so

an antireflectivecoating isappliedtoreducethoselosses.Thefinalstepistoinstallsomethingthatwillprotectthecellfromtheelements--oftena glasscoverplate.PVmodulesaregenerallymadebyconnectingseveralindividualcellstogethertoachieveusefullevelsofvoltageandcurrent,andputtingtheminasturdyframecompletewithpositiveandnegativeterminals.

HowmuchsunlightenergydoesourPVcellabsorb?

Unfortunately,probablynotanawfullot.In2006,forexample,mostsolarpanelsonlyreachedefficiencylevelsofabout12to18percent.Themostcutting-edgesolarpanelsystemthatyearfinallymuscleditswayovertheindustry'slong-standing40percentbarrierin