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1、Passage18 Theneedforsolarelectricityisclear.Itissafe,ecologicallysound,efficient,continuouslyavailable,andithasnomovingparts.Thebasicproblemwiththeuseofsolarphotovoltaicdevicesiseconomics,butuntilrecentlyverylittleprogresshadbeenmadetowardthedevelopmentoflow-costphotovoltaicde
2、vices.Thelargerpartofresearchfundshasbeendevotedtothestudyofsingle-crystalsiliconsolarcells,despitetheevidencethatthistechniqueholdslittlepromise.Thereasonforthispatternisunderstandableandhistorical.Crystallinesilicon,however,isparticularlyunsuitabletoterrestrialsolarcells. Cr
3、ystallinesiliconsolarcellsworkwellandaresuccessfullyusedinthespaceprogram,wherecostisnotanissue.Whilesinglecrystalsiliconhasbeenproveninextraterrestrialusewithefficienciesashighas18percent,andothermoreexpensiveandscarcematerialscanhaveeverhigherefficiencies,costsmustbereducedby
4、afactorofmorethat100tomakethempracticalforcommercialuses.Besidethefactthatthestartingcrystallinesiliconisexpensive,95percentofitiswastedanddoesnotappearinthefinaldevice.Recently,therehavebeensomeimaginativeattemptstomakepolycrystallineandribbonsiliconwhicharelowerincostthanhigh
5、-qualitysinglecrystals；buttodatetheefficienciesoftheseapparentlylower-costmaterialshavebeenunacceptablysmall.Moreover,thesematerialsarecheaperonlybecauseoftheintroductionofdisorderingincrystallinesemiconductors,anddisorderdegradestheefficiencyofcrystallinesolarcells. Thisdiffi
6、cultycanbeavoidedbypreparingcompletelydisorderedoramorphousmaterials.Amorphousmaterialshavedisorderedatomicstructureascomparedtocrystallinematerials:thatis,theyhaveonlyshort-rangeorderratherthatthelong-rangeperiodicityofcrystals.Theadvantagesofamorphoussolarcellsareimpressive.W
7、hereascrystallinesiliconmustbemade200micronsthicktoabsorbasufficientamountofsunlightforefficientenergyconversion,only1micronoftheproperamorphousmaterialsisnecessary.Crystallinesiliconsolarcellscostinexcessof100persquarefoot,butamorphousfilmscanbecreatedatacostofabout50centspers
8、quarefoot. Althoughmanyscientistswereawareoftheverylo
