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1、MultijunctionInorganic Solar CellsEfficiency limitsBelow band gap photons not absorbedThermalization of excess energySources of energy lossCBVBIncreasing VOC and decreasing JSCTriple-junction cellsAppl.Phys.Lett.90,183516(2007)Emcore Corporation,May(2006)World Record:40.7%under 240 suns!1.7-1.9 eV1.
2、3-1.4 eV0.67 eVThe cells are in series;current is passed through deviceThe current is limited by the layers that produces the least current.The voltages of the cells addThe higher band gap must see the light first.Current MatchingIndividually Optimized CellsCurrent-matched cells for MJSCSynopsys,Inc
3、.,February 2009Current MatchingCurrent limited by lowest-current subcellTotal VOC is equal to sum of subcell VOCsTunnel JunctionsCommon 3-cell structure 10-nm-thick layers of very highly doped semiconductor on each side allows carriers to tunnel from conduction band of n+material to valence band of
4、p+materialhttp:/web.phys.ksu.edu/papers/sds/image90.gifEfficiency for various top and middle cells when the substrate is germaniumBaur,C.et.al.,Journal of Solar Energy Engineering,2007Lattice matchingOne can tune the band gap of the III-V materials by varying the elemental compositionTo avoid defect
5、s(dislocations),one must match the lattice constants of each layer.For this reason,it is not yet possible to make an efficient cell with the ideal band gaps shown on the previous slide.MMLMC.Baur et.al,Trans.Of the ASME,2007Lattice MatchingLattice MatchedPrevent formation of dislocationsMetamorphic
6、Buffer LayerDislocations terminate in bufferSubstrateLM AbsorberTJAbsorberTJ,etc.BufferLayerAbsorberTJWith metamorphic materials,lattice matching does not have to be perfect.Structure of 40.7%world record cellLattice MismatchedBuffer layerTunnel JunctionGaInP top cell(1.8eV)GaInAs middle cell(1.3eV)
7、Ge bottom cell(0.67eV)R.R.King et.al,Appl.Phys.Lett.90,2007LMMMR.R.King et.al,Appl.Phys.Lett.90,2007Lattice matched vs.Mismatched These cells are highly optimized!Lattice matched vs.mismatchedMMMMLMLM40.7%for MM and 40.1%for LM at 240,135 suns(respectively)Lattice mismatched(metamorphic)cell structu
8、reAreal defect density is 8106cm-2Higher voltage upper sub-cell uses larger part of solar spectrumMore freedom to choose upper and middle cell materials R.R.King et.al,Appl.Phys.Lett.90,2007Future-more junctionsGaInNAs introduced for 6-junction cell Bandgap 1.0eV,lattice matched with Ge.G.Strobl,et
9、al,IEEE 4th world conf.2,2006A Very Different ApproachAlan Barnett et al.PROGRESS IN PHOTOVOLTAICS:RESEARCH AND APPLICATIONS,Prog.Photovolt:Res.Appl.2009;17:7583The two sets of sets do nothave to be lattice matched to each other or current matched.Summary Highest efficiencySpace&terrestrial concentr
10、ator Current and lattice matchingFuture research New semiconductors with ideal band gapsMore junctionsImproved tunnel junctionsKing,R.Nature Photonics 2 2,284-286(2008)Karem,N.et.al.,IEEE Transactions on Electron Devices,4646(10),2116-2125,1999Spectrolab Photovoltaic Products Data sheets,http:/ Specialists Conference,2005.Conference Record of the Thirty-first IEEE,17-22,Jan.3-7,2005Baur,C.et.al.,Journal of Solar Energy Engineering,129129,258-265,Aug.2007King,R.R.et.al.,Applied Physics Letters,9090,183516,2007.References
