Applications of photoluminescence imaging of silicon bricks

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Copyright: Chung, Daniel
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Abstract
The manufacturing of silicon solar cells starts with the crystallisation of silicon into ingots. This process has a large impact on the final efficiency of solar cells. Therefore, measurements of electronic quality at the ingot or brick stage are advantageous for fast process feedback and guidance for downstream manufacturing. In this thesis, advancements were made to a previously developed photoluminescence (PL) imaging approach to measure the bulk lifetime of silicon bricks. The improved approach was found to be accurate in measuring the bulk lifetime trends across silicon bricks, with good agreement to established wafer-level lifetime measurements. Two main applications were then explored for the improved PL imaging technique. In the first, applying the lifetime measurement technique to wafers was explored. Modelling revealed a minimum sample thickness of several millimetres is needed for measuring the bulk lifetimes of high quality silicon from industry without surface passivation. Although this is thicker than typically produced, specially made samples like these are ideal for bulk lifetime analysis without the uncertainties of surface passivation. Using n-type Czochralski silicon cross-sections as an example, lifetimes up to 20 milliseconds were measured and the impact of thermal donors could be quantified. In the next application, multicrystalline silicon brick lifetime measurements using PL were compared to the resulting solar cells produced from the bricks. A large trial on standard and high efficiency multicrystalline silicon cells was undertaken to assess this correlation. Statistical techniques were introduced to separate random variations due to solar cell processing from material quality and it was found that dislocation density measured using wafer PL imaging was the most important material property. For the higher efficiency cells, brick bulk lifetime was a significant predictor, but it explained less cell performance variations than the wafer dislocation metric. Finally, an alternative method was presented for the bulk lifetime measurement on silicon bricks using two excitation wavelengths. Although some limitations were found, this technique is more practical to implement and is robust against uncertainties in the measurement system properties. It is expected that bulk lifetime measurements for material qualification will become invaluable as more advanced solar cells enter production.
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Author(s)
Chung, Daniel
Supervisor(s)
Trupke, Thorsten
Abbott, Malcolm
Mitchell, Bernhard
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Publication Year
2018
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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