Complete Report - University of New South Wales

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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTFilter WavelengthCut-offs (nm)Jsc III-V Top Cell(Am-2)Jsc III-V MiddleCell (Am-2)Jsc III-V BottomCell (Am-2)Without Filter 152.41 147.27 199.24 38.8Design 1 932-1070 152.41 146.50 146.46 4.09Design 2 930-1068 152.31 146.89 144.96 6.41Design 3 930-1066 151.99 146.89 146.12 4.54CurrentMismatch (%)Effect of the use of dielectricreflectors on reducing currentmismatch within the III-V device.Table 4.3.1.3Seasonsin LaParguera24AnglebetweenIII-V & Sicells (°)Effect of varying angle betweenSi and III-V arrays to maximise celloutput from the various seasonalspectral contents shown inFig. 4.3.1.9(b).Table 4.3.1.2Seasonal averaged direct spectralcontents for (a) Canberra and (b)La Parguera.Figure 4.3.1.9Fixed Demi-Cube StructureFiltercut-offs(nm)III-V cellefficiency(%)Si cellefficiency(%)Total cellefficiency(%)Filtercut-offs(nm)Variable Demi-Cube StructureAnglebetweenIII-V & Sicells (°)III-V cellefficiency(%)at 500 suns (or higher) sunlight concentration is nolonger essential. Also, the requirements on reflectordesign are considerably relaxed due to the greaterease of designing for high reflection rather than forhigh transmission (Fig. 4.3.1.8), with design nowsimplifying to that of a noncritical band pass filter.The extra cell area required is not a factor since thelarger array is formed from lowcost silicon cells.Another advantage of the“demi-cube” concept is that theangle between the Si and III-Varrays can be varied to takeadvantage of the non-idealreflection and transmissionproperties of the dielectricreflector. As the incidenceangle on the dielectric reflectorchanges, the reflector cut-offwavelengths vary and so dothe slopes in the transitionregion between reflectionand transmission. This allowsthe reflector to capture thespectrum with the mostsuitable wavelength range forthe Si and III-V arrays as thespectral content of sunlightvaries daily and seasonally, asshown in Fig. 4.3.1.9.Table 4.3.1.2 shows the effect of varying anglebetween Si and III-V arrays to maximise output fromvarious spectral contents using the spectra of Fig.4.3.1.9 as an example. Note the spectra shown inFigure 4.3.1.9 and used for Table 4.3.1.2 are seasonalaverages only. Hence the advantage of variableSi cellefficiency(%)Summer 33.9 874-1060 41.0 4.1 45.1 874-1060 33.9 41.0 4.1 45.1Autumn 33.9 874-1060 40.2 4.1 44.3 894-1080 35.3 40.8 4.2 45.0Winter 33.9 874-1060 41.3 4.1 45.4 874-1060 33.9 41.3 4.1 45.4Spring 33.9 874-1060 40.3 4.1 44.4 894-1080 35.3 40.9 4.2 45.1Total cellefficiency(%)demi-cube structure will beeven greater if used within aday as the changes in dailyspectrum content will begreater than those observedin Figure 4.3.1.9. Table 4.3.1.3shows the effect of the useof dielectric reflectors onreducing current mismatchwithin the III-V device.References:4.3.1.1 M.A. Green, “Lambertian light trapping in texturedsolar cells and light-emitting diodes: analyticalsolutions”, Progress in Photovoltaics: Research andApplications 10(4), pp. 235-241, 2002.4.3.1.2. Richard King, 51st Electronic Materials Conference,June, 2009.4.3.1.3 J. Lasich et al., “World’s First Demonstration of a140kWp Heliostat Concentrator PV (HCPV) system”,34 th IEEE Photovoltaic Specialists Conference,Philadelphia, 7-12 June 2009.4.3.2 Industry Collaborative Research andCommercialisation4.3.2.1 IntroductionThe Centre’s Technology Transfer team (TTT) wasestablished in 2008 to accelerate the developmentof the Centre’s commercially significanttechnologies and to carry out technology transfersto industry. This includes assisting companies toestablish and optimise the large scale productionof UNSW photovoltaic technology. With the highdemand for UNSW technology and new licences,the TTT grew significantly during 2010 with theinclusion of Phil Hamer, Xue Bai and Emily Mitchell.The already strong academic record of the TTT wasalso further enhanced with an additional two teammembers having been awarded University Medalsfor being placed first in their respective universityprograms. This included Emily Mitchell who receivedthe University Medal for first place in PhotovoltaicEngineering at UNSW in 2007 prior to completingher PhD at the Fraunhofer Institute in Germany andXue Bai who received the University Medal for beingplaced 1 st in Photovoltaic Engineering at UNSW in
ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTTTT photo June 2010 with Matt Edwards overseas atthe time and new comers Emily Mitchell, Phil Hamerand Xue Bai about to join the team.Figure 4.3.2.12010. This takes the number of University Medallistswithin the TTT to six at the end of 2010, but withanother university medallist Nicole Kuepper beingtemporarily on leave from the TTT while completingher PhD. Figure 4.3.1 shows the TTT in June 2010with members from left to right being D. Jordan, A.Sugianto, S. Wenham, A. Lennon, B. Hallam, L. Mai, B.Tjahjono and N. Kuepper.4.3.2.2 Industry Collaborators andLicensees of TechnologyThe Centre has a large number of collaboratorsand licensees including many cell manufacturersworking with the Centre to improve or develop newsolar cell technology for commercialisation. Someof these fund the Centre quite generously and areseen as being partners to NewSouth Innovations(NSi), the commercial arm of the University thatmanages the entire intellectual property portfoliofor the Centre. Interest from companies wantingto become commercial partners with the Centre orlicense technology grew rapidly during the last twoyears despite the international financial downturnin late 2008. This appears to be due in part to theperceived world leadership of UNSW in this area,but also due to the growing long-term importanceplaced on photovoltaics by a world struggling todeal with climate change, diminishing resourcesand increasing energy requirements. Collaboratorswith the Centre in the first generation photovoltaicsarea come predominantly from China, Germany,the United States, Australia, South Korea andTaiwan and include Suntech-Power, Roth and Rau,Guodian Solar, Centrotherm, Tianwei, Optomec,Global Sunrise Energy, BP Solar, Sunergy, CSG Solar,E-ton Solar, Spectraphysics, Schott Solar, HyundiHeavy Industries, China Light, Corum Solar, Advent,Shinsung, JA Solar and Yunnan Tianda. Several ofthese companies are amongst the world’s largestsolar cell and equipment manufacturers.4.3.2.3 Commercially RelevantTechnologies4.3.2.3.1 IntroductionWith screen-printed solar cells continuingto dominate commercial manufacturingwith well over 50% market share, thebroad aim of this work has been todevelop, in conjunction with severalindustry partners, the next generationof screen-printed solar cell. In particular,the fundamental limitations of theconventional screen-printed solar cellthat have limited its performance for thelast 30 years have been identified, and innovativeapproaches to redesigning the emitter and frontmetal contact have been devised, developed andanalysed in this work. In addition to overcomingthe current and voltage limitations imposed bythe design shown below, a further aim of this workhas been to retain compatibility with existingequipment and infrastructure currently used by ourindustry partners for the manufacture of screenprintedsolar cells.Despite the dominance of this technology,this solar cell design shown in Fig. 4.3.2.2 hassignificant performance limitations that limit thecell efficiencies to well below those achievablein research laboratories around the world. Inparticular, the front surface screen-printedmetallisation necessitates a heavily diffused emitterto achieve low contact resistance and also toachieve adequate lateral conductivity in the emittersince the metal lines need to be widely spacedcompared to laboratory cells to avoid excessiveshading losses. Such cells therefore typically haveemitters with sheet resistivities in the range of 50-60ohms per square, which inevitably give significantlydegraded response to short wavelength light.Good work in recent years by partners Dupontand Ferro has improved paste formulations,allowing ohmic contacts to be formed to morelightly doped emitters in the vicinity of 60 ohms/square while simultaneously lowering the bulkresistivity of the fired paste. To further raise this150-200 mp +p-typemetaln ++3 m m patterned metal contactStandard screen-printed solar cell.Figure 4.3.2.2phosphorusbulk of waferrear metal contact25
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