Abstract
Micro-Raman (μRS) and micro-photoluminescence spectroscopy (μPLS) are demonstrated as valuable characterization techniques for fundamental research on silicon as well as for technological issues in the photovoltaic production. We measure the quantitative carrier recombination lifetime and the doping density with submicron resolution by μPLS and μRS. μPLS utilizes the carrier diffusion from a point excitation source and μRS the hole density-dependent Fano resonances of the first order Raman peak. This is demonstrated on micro defects in multicrystalline silicon. In comparison with the stress measurement by μRS, these measurements reveal the influence of stress on the recombination activity of metal precipitates. This can be attributed to the strong stress dependence of the carrier mobility (piezoresistance) of silicon. With the aim of evaluating technological process steps, Fano resonances in μRS measurements are analyzed for the determination of the doping density and the carrier lifetime in selective emitters, laser fired doping structures, and back surface fields, while μPLS can show the micron-sized damage induced by the respective processes.
Highlights
Silicon solar cells contribute by far the largest share to the world’s photovoltaic facilities [1]
We demonstrate the latest advances on these research fields, which are based on micro-Raman spectroscopy and micro-photoluminescence spectroscopy
We demonstrated the high resolution (< 1 μm) measurement of (1) the Shockley-Read-Hall lifetime by μRS and μPLS, (2) of the doping density by μRS and μPLS, and (3) of stress with both methods
Summary
Silicon solar cells contribute by far the largest share to the world’s photovoltaic facilities [1]. While this may be caused by the high surface recombination at the aluminum contact, the nature at the second interface is less clear We investigated this area with μPLS and showed an increased defect luminescence at 1,250 nm in this area (Figure 6), which is an indication for a higher defect density in this area, which could cause the drop in lifetime. This is due to the fact that high and low injection lifetimes are both proportional to the inverse defect density [20] This highlights the usefulness of μPLS for the characterization of solar cells, which are typically working under low injection conditions. The measurement shows the strongly different recombination activities of the three grain boundaries and reveals micron-sized denuded zones around the left grain boundary The linescan across this grain boundary highlights the spatial resolution of micro-photoluminescence lifetime mapping. Details on the impact of stress on the recombination activity and a quantitative analysis can be found in [25,26]
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