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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT4.6 Photonics and deviceCHARACTERISATION4.6.1 Photoluminescence basedcharacterisation of siliconUniversity StaffA/Prof. Thorsten TrupkeProject Scientists andTechniciansAllen YeeUndergraduate StudentsMatthias JuhlPostgraduate Students• Yael Augarten• Henner Kampwerth• Bernhard Mitchell• Li HuaExternal Collaborators• Ron Sinton, Sinton Consulting• BT Imaging Pty Ltd• Otwin Breitenstein, Jan Bauer, MaxPlanck Institute for MicrostructurePhysics, Halle, Germany4.6.1.1 BackgroundIn photoluminescence (PL) imaging an intenselight source is used to illuminate large areasamples such as silicon bricks, wafers orcomplete solar cells homogeneously withtypically one-sun equivalent illuminationintensity. A CCD camera captures a highresolution picture of the luminescent lightthat the sample emits. Figure 4.6.1.1 shows aschematic diagram of a luminescence imagingsystem. The power supply that is shown onthe bottom left can be used as an excitationsource in electroluminescence (EL) imagingexperiments on finished cells [4.6.1.3] and alsofor applications in which a solar cell is heldat specific operating points under externalillumination, such as the series resistanceimaging methods that were introduced by ourgroup [4.6.1.4-4.6.1.6].High resolution (typical one megapixel)luminescence images can be capturedwithin seconds or even fractions of a second,allowing very fast and detailed studies ofspecific material and device properties.Over the last five years PL imaging has seenthe development of a range of specificapplications and also rapid adoption by thePV community. Specific applications thatare applicable across the PV value chainare now available and the number of theseapplications is growing rapidly. Examplesinclude series resistance imaging anddiffusion length imaging on fully processedcells. The research of the PL group at UNSWcontinues to contribute significantly to thesedevelopments. This year the research by ourgroup was focussed on the application of thephotoluminescence intensity ratio methodfor measurements of the bulk lifetime onsilicon bricks (Section 4.6.1.2), proof of conceptstudies on an application of luminescenceimaging for measuring the emitter sheetresistance on diffused wafers (4.6.1.4) and onanalysing local shunt or defect currents fromPL images on metallised and non-metallisedsamples in a quantitative fashion (4.6.1.5).The commercialisation of PL imaging byUNSW spin-off company BT Imaging PtyLtd has also made significant progress, witha growing number of leading wafer andcell manufacturers and research institutesworldwide now using the BTi-R1 R&D toolthat was introduced into the market in 2008.In addition, as-cut wafer inspection with PLimaging (see Section 4.6.1.3) is increasinglyrecognised as an ideal tool for incomingwafer inspection in solar cell productionor for outgoing quality control in wafermanufacturing [4.6.1.2, 4.6.1.7-4.6.1.8].New in-line PL imaging tools that are fastenough to keep up with typical productionline throughput at full one megapixel image100
Schematic diagram of aluminescence imaging system. Inphotoluminescence (PL) imaging anintense laser is used to illuminatethe entire sample and a sensitiveIR camera then captures theluminescent emission from thesample. In PL imaging the samplecan be a brick prior to wafering,an as-cut or partially processedwafer or a fully processed solarcell (as shown). A power supplyis used in electroluminescenceimaging on finished solar cellsand in combination with thelight source in series resistanceimaging techniques.ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTFigure 4.6.1.1resolution were introduced by BT Imaging in 2010and are currently entering the market.4.6.1.2 PL imaging on Si bricksPhotoluminescence imaging is an ideal tool forfast characterisation of silicon bricks, giving instantinformation about the position of low lifetimeregions that are commonly observed near thebottom and top of a brick and also exposing areasof high structural defect density. This informationcan be used as valuable feedback during theproduction of silicon wafers. Initial work onluminescence imaging on bricks showed excellentqualitative correlation between the as measured PLintensity and for example μ-PCD effective minoritycarrier lifetime maps [4.6.1.9]. More recently wealso demonstrated a quantitative interpretationof the PL signal in terms of the bulk minoritycarrier lifetime m b.The bulk lifetime is a more usefulparameter than the effective lifetime, especially inunpassivated samples, where the measurementresult is strongly influenced by surfacerecombination. PL imaging thus has advantagescompared to other measurement techniques suchas μ-PCD, which have commonly been used to datefor the characterisation of bricks in production, andwhich report only effective lifetime values.In our previous approach to measure bulk lifetimefrom PL images we converted the measured PLintensity form a single PL image into bulk lifetimeusing a pre-determined tool specific transferfunction. While that approach proved to be areliable approach for obtaining bulk lifetime ona number of samples and without the need foradditional calibration, one disadvantage is thefact that it requires a separate doping densitymeasurement. The PL intensity in low injectionconditions is given as PL~Dn∙N D, i.e. it is proportionalto both the effective lifetime and the backgrounddoping density N D. Normalising the measurementfor doping variations is required in order to obtaininformation on Dn and thereby the lifetime.Recent work in the Photoluminescence Groupfocussed on a modified approach for bulk lifetimemeasurements on silicon bricks, which avoidsthe need for a separate resistivity measurement.The method is based on analysing thephotoluminescence intensity ratio (PLIR) betweentwo PL images taken with different spectral filtersmounted in front of the camera [4.6.1.10-4.6.1.11].It is equivalent to the diffusion length imagingmethod proposed and first demonstrated atUNSW by Peter Würfel on finished solar cells usingelectroluminescence imaging [4.6.1.12]. In fact, theelimination of voltage variations from a diffusionlength image that is achieved in the PLIR methodon cells is in perfect analogy to the eliminationof doping density variations when applying thismethod to PL images on bricks. Silicon bricksrepresent an ideal test case for the PLIR method,since uncertainties associated with variations insurface texture and with the optical and electronicproperties of the rear surface are almost completelyavoided. In cells and wafers these uncertainties canlead to large experimental errors.Figure 4.6.1.2 shows a comparison of twobulk lifetime images obtained from a dopingnormalised PL image and from the PL intensityratio, respectively. The latter image was obtainedusing a transfer function that was calculatedtaking into account sample temperatureand the exact spectral properties of themeasurement system.Deviations are observed near the lowlifetime edges at the bottom and top,where the PLIR overestimates the bulklifetime (see also Fig.4.6.1.3, showing acomparison of cross sections throughthe two bulk lifetime images). Defectluminescence contributions, which becomemore noticeable in the low lifetime regionswith reduced band to band luminescencesignal is currently believed to cause thosedeviations. Spectral PL measurementscurrently under way will shed more light on thisissue. Several approaches to mitigate the impactof the above measurement errors in the PLIRnear the bottom and top regions are currentlyunder investigation.Bulk lifetime images obtained froma single doping density normalisedPL image (upper) and from the PLIR(lower), see text for details.Figure 4.6.1.2Cross sections through the bulklifetime images from Fig. 4.6.1.2,showing good agreement in thecentre and artefacts near thebottom and top regions.Figure 4.6.1.3101
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