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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTIncreasing order of wavelengthDecreasing order of bandgap energyof the semiconductorIncreasing order of particle sizeSchematic representation showingan example of surface plasmonresonance tunability by varying theof size of the metal nanoparticlesfor wavelength dependent lighttrapping in a tandem solar cell.Figure 4.5.70can be optimised independently of each other. Ourrecent work has also made use of an additionalreflecting layer (paint) along with the nanoparticlelayer on the rear side of the cell to maximise therandomisation of light and to ensure no light islost [4.5.80].In this study we focus our interest on the use ofplasmonics for low-dimensional structures likequantum dot structures for possible applicationsin tandem solar cells. PL measurements wereused to characterise the samples. The analysisis based on the reciprocity of light that a goodabsorber can be a good emitter and the proofofconcept established with EL measurementsin an earlier study [4.5.82]. Hence the PLenhancement would be synonymous to anincrease in photocurrent for an optimised QD cell.The tunability of plasmon resonance allows thepossibility of increasing the photocurrent of eachcell in a tandem cell configuration. Optimisingthe nanoparticle parameters can help achievewavelength dependent light trapping which willbe an encouraging step towards increasing theperformance of a tandem solar cell. Figure 4.5.70shows one way of achieving the SPR tunability byvarying the size of the nanoparticles in a tandemcell configuration.The sample under study is a 4nm single layer siliconQD structure in an oxide matrix with a 6nm cappingoxide layer with an emission wavelength of 930nm(~1.3eV). Mass thicknesses of 10, 14, 18 and 22 nmsilver were deposited and the particles annealedat 200°C in nitrogen for an hour. A 532nm, 10mWNd:YAG laser was used to illuminate the samplesfor both the front (incident on QD layer) and rearside (incident on the quartz slide) locations of thesilver nanoparticles and the PL plots studied. PLmeasurements were carried out on these samplesand the effect of different size particles wereinvestigated both before and after the deposition ofthe nanoparticles.The self assembly techniques of nanoparticlefabrication give a varied size and shape distributionvery suitable for a broadband response. This isparticularly noted for particles corresponding tothicker Ag layers as can be seen from the PL plots inFig. 4.5.71. The red-shift of the enhancement peakswith larger particles are also clearly evident. Moredetails of this work has been reported in [4.5.83].Rear located nanoparticles perform better thanfront located nanoparticles, consistent with theresults from thin-film cells. We believe this isbecause of the change in scattering cross-section ofthe nanoparticles due to the changes in the drivingfield and also due to the change in the mode ofexcitation (from air for front located nanoparticlesand from Si for rear located nanoparticles) forthe two different locations on the samples. Morework need to be done to better understand themechanism. Further work will also concentrateon using alternate approaches for tuning theresonance position close to the band-edge(emission wavelength for PL) to increase thescattering properties and hence absorption in theweakly absorbing range.4.5.6 Concluding remarks for the ThirdGeneration sectionIn 2010 work has proceeded significantly in all theareas of Third Generation research, with improvedfabrication and characterisation of materials andcomplexity of modelling which together give anoverall better understanding and optimisationof devices.Group IV based nanostructure materials have seensignificant improvement in device design. Withinterlayers, in both SiN xand SiC matrices, used toeffectively control both the uniformity of size of SiQDs and carrier transport in the growth direction.Modelling now allows the confined energy levelsof complex QD shapes to be calculated and theiroverlap to form mini-bands estimated in moredetail. At the same tiome improved characterisationis giving a much clearer idea on the shape anddistributions of Si QDs and is now allowing choiceof growth parameters to give uniform size sphericalQDs to be grown. Ge nanostructure materials haveseen improved control of p-type conductivity in96
PL enhancementPL Enhancement76no MNP (a) Front10nm Ag14nm Ag18nm Ag522nm Ag43210600 700 800 900 1000Wavelength (nm)20no MNP (b) Rear18Ag reflector10nm Ag1614nmAg18nm Ag1422nm Ag121086420600 700 800 900Wavelength (nm)10001.00.90.80.70.60.50.40.30.20.10.03.22.82.42.01.61.2PL (a.u)PL (a.u)0.80.40.0PL plots from a 4nm singlelayer QD structure in an oxidematrix with a 6nm cappingoxide layer (right axis) alongwith the correspondingenhancement plots (left axis)with silver nanoparticlescorresponding to 10, 14, 18and 22 nm mass thickness ofsilver for (a) silver nanoparticleson the front and (b) silvernanoparticles on the rear.Figure 4.5.71ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTGe QDs and very highly crystalline growth of GeQWs. Homojunction Si nanostructure deviceshave demonstrated improved conduction, therehas been continued demonstration of rectifyingproperties for heterojunctions with c-Si andheterojunctions between two nanostructuredmaterials are now starting to exploit the advantagesof both.Hot carrier cells have seen further developmentof efficiency modelling which now not onlyincludes material parameters such as Augercoefficients for real promising absorber materialssuch as InN, but also includes the effects of realnon-ideal energy selective contacts. There hasbeen significant improvement in measurementof negative differential resistance in Si QD basedenergy selective contacts is now allowing and thisoverlaps well with continuing improvement inmodelling the transport through such structuresin various matrices. Carrier cooling measurementson the candidate absorber material InN haveshown evidence for phonon bandgaps building onthe slowed carrier cooling seen in InP previously.Modelling of analogues of these materials innanostructures now predicts the phonon propertiesof rea lIII-V QD arrays grown by collaborators. Inaddition colloidally dispersed Si nanoparticleshave now been fabricated as coherent single layernanoparticle arrays and are progressing towardsmulti-layer structures. Theoretical work on thepotential materials for absorbers with controlledphon dispersion and hence slowed carrier coolinghas progressed on approapratie analogues forInN and devices involving various nanostructureconfigurations are being analysed conceptually.Up-conversion has seen a further significantdevelopment of the use of porous-Si as a host for Erup converting species. The porosity of porous-Si canbe controlled to give layers of alternating refractiveindex which can be used as distributed Braggreflectors to modifiy the photonic density of statessuch that there is strong enhancement of photonicmodes at the critical Er absorption at 1500nm.This has been demonstrated and can lead directlyto large increase in the up-conversion efficiency,thus tackling one of the key problems of rare earthup-conversion that it is inherently non-linear anddependent on concentration.Plasmonics, whic has previously been shown toenhance emission and absorption from both 1 stand 2 nd generation cells, has now been appliedto the Si annostructured layers of 3 rd generatrionmaterials. This has shown significant enhancementin luminescence demonstrating the strong potentialto achieve local concentration with a global 1 sunillumination. This is particularly useful for several ofthe non-linear 3 rd generation approaches.The development of all the 3 rd generation projectsin 2010 now allows much greater understandingof the materials and devices. Work in 2011 will seeconsolidation of this into improved devices. Severalnew areas of funding will contribute to this and alsoallow development of new project areas.References:4.5.1 M. A. Green, “Third Generation Photovoltaics:Ultra-High Efficiency at Low Cost” (Springer-Verlag, 2003).4.5.2 J. Nelson, “The Physics of Solar Cells”, ImperialCollege press, 2003.4.5.3 G. Conibeer, “Third Generation Photovoltaics”,Materials Today, 10 (11) (2007) 42-50.4.5.4 E-C. Cho, S. Park, X. Hao, D. Song, G. Conibeer,S-C. Park, M.A. Green, “Silicon quantum dot/crystalline silicon solar cells”, Nanotechnology, 19(2008) 2452014.5.5 Hao X J, Podhorodecki A, Shen Y S, Zatryb G,Misiewicz J, Green M A, “Effects of Si-rich oxidelayer stoichiometry on the structural andoptical properties of Si QDs/SiO 2multilayer film”,Nanotechnology 20, 485703 (2009).4.5.6 M.A. Green, G. Conibeer, D. König, I. Perez-Wurfl,A. Gentle, S. Huang, D. Song, X.J. Hao, F. Gao, S.Park, C. Flynn, Y. So, B. Zhang, D. Di, P. Campbell,Y. Huang and T. Puzzer, “Progress with All-SiliconTandem Cells On Glass Using Silicon QuantumDots in Silicon-Based Dielectric Matrices”, 24thEuropean Photovoltaic Solar Energy Conference,Hamburg, Germany, 21-25 September 2009.4.5.7 M. Zacharias, J. Heitmann, R. Scholz, U. Kahler, M.Schmidt, Appl. Phys. Lett. 80(2002) 661.4.5.8 X.J. Hao, I. Perez-Wurfl, G. Conibeer, M.A. Green,19th PVSEC, Korea, Nov 2009.97
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