Complete Report - University of New South Wales

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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT0.5 cm(a)(b)PL images taken on a test structure. An externalresistor is connected in parallel to the structure using acontact probe (visible in the bottom right). The imagesshow the impact of a large resistance (mild shunt,left) and a small resistance (strong shunt, right) on thePL image.Figure 4.6.1.9Extracted current as calculated from PL imagesusing a new analysis methodology as a functionof the measured current extracted over a resistor,demonstrating that PL imaging allows a quantitativeanalysis of the extracted current from localised shuntsor defects.Figure 4.6.1.104.6.1.5 Luminescence based shuntimagingQualitative shunt detection from luminescenceimages has been demonstrated in the past by theUNSW PL group (amongst others) [4.6.1.15-4.6.1.17].The vicinity of shunted areas typically appears as ablurred region of reduced luminescence intensityin both PL and EL images. This blurring is caused byvoltage drops associated with lateral current flowthrough the emitter and the front surface grid.A luminescence imaging based method fordetermining quantitative shunt values in siliconsolar cells from open circuit photoluminescenceimages has been developed and demonstratedin collaboration with the Max Planck Institute forMicrostructure Physics, Halle, Germany. The methodis based on interpretation of the luminescenceintensity around a local shunt in terms of theextracted current density [4.6.1.18]. Under theassumption of a unity ideality factor the localreduction in PL count rate in the vicinity of a shuntor local defect is proportional to the current beingextracted. The theoretical framework for quantifyingthe total current extracted by a local defect byintegrating the reduction of the PL signal in thesurrounding of the shunt or defect and by applyingsuitable calibration procedures was developed.Experimental verification of the method wasachieved using a specifically prepared test structure,i.e. a solar cell structure with single point contacton the front surface. That structure allowedgenerating local shunts with variable and welldefined external shunt resistance. Figure 4.6.9shows two PL images of that structure with a high(left) and low (right) Ohmic resistor connected inparallel with that structure. Figure 4.6.1.10 showsthe extracted current, as obtained from the abovemethodology as a function of the current flowingacross the resistor, as measured with a multimeter.Excellent agreement is observed. Combining theabove current measurement technique with aninterpretation of the local luminescence signal interms of the diode voltage near a shunt site allowsquantification of individual local shunts.Applications of this method to finished cellsare affected by current extraction via the metalgrid, which causes experimental errors. Theimpact of this effect was assessed experimentallyusing specific test structures, where the relativeposition with respect to the nearest grid finger ofartificially introduced shunts could be varied. Theseexperiments indicate that the above experimentalerrors in fully processed cells can at least partiallybe corrected using empirically determined lookup tables.When applied to partially processed cells prior tometallisation the method can be used to quantifylocal shunts, to determine the total recombinationcurrent in local and distributed defects and alsoto quantify recombination channels that areintroduced by specific processing steps such aslaser processing.4.6.1.6 SummaryAn exceptional variety of material and solar cellparameters can be measured on silicon bricks,silicon wafers and silicon solar cells with highlateral spatial resolution and short measurementtime using luminescence imaging techniques.The range of applications continues to grow, withnew applications for PL imaging being developedat UNSW, and increasingly also in other researchinstitutes and by R&D groups in PV companies.PL imaging, introduced at UNSW only five yearsago, has now been broadly adopted as a standardcharacterisation method, with PL tools now inuse at virtually all leading research institutesworldwide and also by leading wafer and solar cellmanufacturers. A range of opportunities for in-linequality control, process monitoring and processcontrol are enabled by the high resolution andspeed of PL imaging. Demonstration by UNSWspin-off company BT Imaging of those applicationsin beta trials at several company sites is currentlyin progress.104
References4.6.1.1 McMillan, W. et al., In-line monitoring of electricalwafer quality using photoluminescence imaging.Proceedings of 25th EPVSC, Valencia, Spain,September, 2010.4.6.1.2 Trupke, T., Nyhus, J. and Haunschild, J.,Luminescence imaging for inline characterisation insilicon photovoltaics. Phys.Stat.Solidi RRL, 2011: p.published online February 2011.4.6.1.3 Fuyuki, T. et al., One shot mapping of minoritycarrier diffusion lenth in polycrystalline Si solar cellsusing electroluminescence. 31st IEEE PhotovoltaicSpecialists Conference, Orlando, USA, 2005.4.6.1.4 Trupke, T. et al., Fast photoluminescence imaging ofsilicon wafers. WCPEC-4, Waikoloa, USA, 2006.4.6.1.5 Trupke, T., et al., Spatially resolved series resistanceof silicon solar cells obtained from luminescenceimaging. Appl. Phys. Lett., 2007. 90: p. 093506.4.6.1.6 Kampwerth, H. et al., Advanced luminescence basedeffective series resistance imaging of silicon solarcells. Appl. Phys. Lett., 2008. 93: p. 202102.4.6.1.7 Haunschild, J. et al., Quality control of ascutmulticrystalline silicon wafers usingphotoluminescene imaging for solar cell production.Solar Energy Materials and Solar Cells, 2010.94(12): p. 2007-2012.4.6.1.8 You, D. et al., The dislocation distributioncharacterisitics of multi-crystalline silicon ingotand its impact on cell efficiency. 35th IEEE PVSC,Honululu, USA, June, 2010.4.6.1.9 Trupke, T., Bardos, R.A. and Nyhus, J.,Photoluminescence characterisation of silicon wafersand solar cells. 18th workshop on Crystalline SiliconSolar cells & Modules, 2008.4.6.1.10 Trupke, T. et al., Bulk minority carrier lifeitmefrom luminescence intensity ratios measuredon silicon bricks. 25th EPVSC, Valencia, Spain,September 2010.4.6.1.11 Mitchell, B. et al., Bulk minority carrier lifetimes anddoping of silicon bricks from photoluminescenceintensity ratios. J. Appl.Phys., 2011: p. acceptedfor publication.4.6.1.12 Würfel, P. et al., Diffusion lengths of silicon solarcells obtained from luminescence images. Journal ofApplied Physics, 2007. 101: p. 123110.4.6.1.13 Demant, M. et al., Analysis of luminescenceimages applying pattern recoginition techniques.Proceedings of 25th EPVSC, Valencia, Spain,September, 2010.4.6.1.14 Sinton, R.A., Contactless electroluminescence forshunt-value measurement in solar cells. Proceedingsof the 23rd EPVSC, Valencia, September, 2008:p. 1157.4.6.1.15 Trupke, T. et al., Luminescence imaging for fastshunt localisation in silicon solar cells and siliconwafers. International Workshop on Science andTechnology of Crystalline Silicon Solar Cells,Sendai, Japan, 2-3 October, 2006.4.6.1.16 Kasemann, M. et al., Shunt detection capabilities ofluminescence imaging on silicon solar cells. 22ndEuropean Photovoltaic Solar Energy Conference,Milan, Italy, 2007.4.6.1.17 Breitenstein, O. et al., On the detection of shunts insilicon solar cells by photo- and electroluminescenceimaging progress in Photovoltaics, 2008. 16: p. 325.4.6.1.18 Augarten, Y. et al., Luminescence shunt imaging:qualitative and quantitative shunt images usingphotoluminesccence imaging. 24th EuropeanPhotovoltaic Solar Energy Conference, Hamburg,September, 2009.4.6.2 General CharacterisationUniversity StaffDr Henner KampwerthProject Scientists and TechniciansAlan YeeThe range of research undertaken by the Centreinvolves a variety of measurement techniquesfor the characterisation of photovoltaic materialsand devices. Understanding and utilising thesetechniques appropriately, and improving themwhere possible, is crucial in order to maintain theworld-class reputation of our research output.Improvements to measurement techniques with thegoal of producing more accurate and meaningfulresults will have a direct impact both on researchactivities and on the optimization of proceduresused in manufacturing. The time and numberof experiments needed to understand a certainphenomena can be greatly reduced. The need tomeasure parameters of interest more precisely,without the possibility of misinterpretation, istherefore high. The General Characterisation groupwas formed in July 2010 to address this need.The group primarily focuses on opticalcharacterization methods such asphotoluminescence spectroscopy and absorptionspectroscopy. In recent years, it has been shownthat these optical measurements can be used toextract a large number of material and deviceproperties. In addition the ability to performcontactless non-destructive probing of small areasdown to a few ten micrometers in diameter makesoptical measurement techniques very attractive forsamples that cannot be measured otherwise.In addition, the group will review somecommonly used measurement methods withthe aim of automating parameter extractionfrom measurement data. It is common that lessexperienced researchers misinterpret data and drawincorrect conclusions. Computer programs thatassist in various levels of interpretation thereforecan prove their value. Some computer programswritten in the course of this work will be madeavailable for public use.ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT105
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