Photoluminescence imaging for quality control in silicon solar cell manufacturing
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Photoluminescence imaging for quality control in silicon solar cell manufacturing Daniel Chung1, Bernhard Mitchell1, Jürgen W. Weber2, Neil Yager2, Thorsten Trupke1,2 1 Australian Centre for Advanced Photovoltaics, University of New South Wales, Sydney, Australia 2 BT Imaging Pty Ltd, Sydney, Australia ABSTRACT We report on progress with PL imaging applications in silicon solar cell production, specifically focusing on the characterization of silicon bricks prior to wafer cutting. Silicon bricks represent an ideal opportunity to characterize and quantify the electronic material quality at an early stage of the PV value chain. Quantitative data on bulk lifetime can be obtained on bricks without any specific sample preparation, unlike unprocessed wafers. Spatially resolved bulk lifetime, interstitial iron concentration, and defect density measurements are demonstrated on bricks from different manufacturers including both high performance multicrystalline and older generation multicrystalline bricks. We find significant variability in bulk lifetime and iron concentration across the samples which is not related to its date of manufacture. However we do see a qualitative reduction in crystallographic defects in the newer high performance multicrystalline bricks. Data is parameterized in different ways to suggest possible paths to better predict solar cell efficiencies from an early stage of inspection. Brick level PL measurements were previously performed using a conventional area scanning PL imaging system, which is associated with light spreading artefacts of weakly absorbed light. To overcome these artefacts, a new line scanning photoluminescence imaging system is used. We show a reduction in contrast smearing between high- and low lifetime regions in the new setup leading to image quality suitable for defect detection and quantitative measurements without deconvolution correction. INTRODUCTION Roughly half of the photovoltaics market is based on multicrystalline silicon (mc-Si) in recent times [1], which is lower cost , but also lower quality than monocrystalline silicon. Advances in the production of mc-Si and techniques to control defects in solar cell processing have kept mc-Si competitive, in particular the recent advent and rapid adoption of so-called high performance multicrystalline (HPM) wafers. One of the ingredients needed to improve mc-Si production and its use downstream is to have characterization tools, which can quickly and reliably measure relevant properties of the material. In this work, we advance a photoluminescence (PL) based method for inspecting mc-Si bricks prior to wafering, which is the earliest practical stage for characterization of the crystalized silicon. Since the quality of mc-Si has a large bearing on final solar cell performance [2], PL brick measurements are expected to provide valuable feedback for crystal growth and provide guidance for downstream solar cell processing. One of the advantages of measuring at the brick stage, is that bulk lifetime can be measured directly without th
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