Complete Report - University of New South Wales

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