多孔硅在太阳电池中应用有着很大的 潜力。
484R.Bilyalovetal./SolarEnergyMaterials&SolarCells65(2001)477}485
di!usioncoe$cientmightdi!usefromthePSintotheepi-layer(Fig.3)andcauseadecreaseofthecarrierlifetimeinthespace-chargeregion,leadingtoalargerrecombinationcurrent.ApossiblewaytoavoidthisproblemisaddinganappropriateamountofHCltothechemicaletchingsolution,becauseHClisknowntoa!ecttheplatingbehaviorofCu[13].Theratherlow"llfactorof&60%isduetothefactthattheunderlyingp>-Silayerhasathicknessofonly1 m,whichresultsinanincreasedseriesresistance.
Theshort-circuitcurrent(J)ofthecellhavingadualPSintermediatelayeris muchhigherthanthatofthepreviouscellwithsingle-layerPS[3](18.7versus11.2mA/cm ).Thisfactwhichisexplainedbytheimprovedcrystallographicqualityoftheepi-layerclearlyindicatestheadvantageoftheuseofdualPSlayer.TheJofdual PSisquitecomparablewiththereferencevalueforathin"lmonaSisubstratewithoutPSlayer(Table2).Internalquantume$ciency(IQE)analysisshowsthatthebulkdi!usionlengthincaseofadualPSintermediatelayerisonlyslightlylowerwhichindicatesthequalityofthematerialsissimilar.Intheinfraredregion,IQEofthecellwithPSintermediatelayerdoesnotexhibitahumpwhichistypicalforthecellswithlightcon"nement.Thisseemstocon"rmtheexpectationsfromtheopticalanalysisthattheopticalgainislimitedbecauseofthemainlyspecularre#ectancebehavioroftheburiedporouslayerandbecauseofthepore"llinge!ectthatstronglyreducestheinitialporosity.
4.Conclusions
Theconceptofathin-"lmsolarcellonlow-costSi-basedsubstrateusingadualporousSiintermediatelayerhasbeeninvestigated.MorphologicalanalysisclearlyshowsthataSilayerwithepitaxialqualitycanbedepositedbyCVDondual(PSwhichpreservesitsporousstructure,althoughaloweringofporosityduetopore"llingisobserved.SIMSanalysisshowsthatPSactsasane!ectivegetteringlayerforCrandCuandasadi!usionbarrierforFe.AlthoughthePSformationprocessincombinationwiththesubsequent(medium-temperature)CVDandsolarcellprocessmightgiverisetoaloweringofcarrierlifetimeandVduetometalcontamination.It hasbeenshownthatasolarcellcanberealizedwithintheSi"lmontopofaporouslayerwithdualporositystructure.Despitetheimprovedcrystallographicqualityoftheepitaxiallayerontheporoussurface,norealimprovementofJduetooptical e!ectsisobserved.Thismatcheswiththeexpectationsfromtheopticalanalysisthattheopticalgainislimitedbecauseofthemainlyspecularre#ectancebehavioroftheburiedporouslayerandbecauseofthepore"llinge!ect,whichstronglyreducestheporosity.
AthinSi"lmwithlightdi!usoronthetopandPSmultilayerstructureonthebottomformedalow-costSi-basedsubstratemightbeagoodsolutionifthepore"llinge!ectcouldbeavoided.Principally,twopossibilitiesexisttoovercometheproblemofthelowgrowthrateandrelatedpore"lling.Ontheonehand,onecouldmakeuseofassistedCVD-techniquesthatallowtoobtainahighergrowthrateatevenlowertemperatures,whichcouldreservetheporouslayer.Ontheotherhand,