关于多孔硅(6)

多孔硅在太阳电池中应用有着很大的 潜力。

482R.Bilyalovetal./SolarEnergyMaterials&SolarCells65(2001)477}485

Fig.5.Simulatedre#ectanceattheSi/porousSi/Sisurfaceforadualporouslayerconsistingof(a)a20%and60%porouslayerand(b)a20%and80%porouslayer.Inbothcases,there#ectanceisshownforonedualsequenceaswellasatwofoldandthreefoldrepititionthereof.

lowerthanthatintheinitialMG-Sisubstrate.Asaresult,theconcentrationofFeinanactivelayerissigni"cantlylowerthaninaSi"lmgrowndirectlyonMG-Si.Itisobviousthatadditionalresearchisrequiredtoexplainthebehaviorofthedi!erentimpuritiesintheburiedporousSilayer.

3.3.Opticalanalysis

Inordertobeoptimalfromanopticalpointofview,eachlayerofthisdualporousstructureshouldhavethethicknessasfollowsfromthequarter-wavelengthcriterion.Theexpectedre#ectancebehaviorissimulatedfortwoconditions ,beingalayerstructureconsistingof20%and60%porosityontheonehandandastructureconsistingof20%and80%ontheotherhand.Furthermore,besidesthedualPSstructure,thetwofold(2sequences)andthreefold(3sequences)repetitioniscon-sidered(Fig.5a,b).Generaltendenciesarethatforanequalnumberofsequences,there#ectanceincreasesiftheporositydi!erencebecomeslargerandthattheregionoverwhichthehighre#ectanceisobtainedextendswhenthenumberofsequencesin-creases.Bothopticalmeasurements[8]aswellasrandommediummodelingrevealthatparticularlyinthecaseofalowremainingporosityoftheburiedporouslayer,thisbackre#ectormainlyexhibitsaspecularre#ectance.

IndependentresearchonusingamultilayerporousSisystemhasbeenreportedintheliteratureaswell[4].There,itisreportedthatforverythinepitaxialSi"lms(i.e.below5 m)theadvantageofhavingalargenumberofrepetitionsreducesbecausemorelightisabsorbedinsuchalargenumberofporouslayersthaninjustoneporouslayerattheexpenseoflightabsorptioninthethinSi"lm.Itisconcludedthatahighspecularre#ectancecanbeobtainedinthecaseofadualporouslayerwithtwoextremeporosities.Athreefoldrepetitionofa20}80%duallayerstructureresultsina'90%re#ectancewithinthe600}900nmwavelengthrange.Practicalconsider-ationshoweverarethat(1)suchane$cientspecularbackre#ectorshouldgoalong Thespecularre#ectancecalculationsarebasedonthegeneralFresnel-equationsandareperformedusingtheIMEC-developed`Multilayer-Proasoftware.