Abstract
This paper gives a review of some recent developments in the field of contactless silicon wafer characterization techniques based on lifetime spectroscopy and infrared imaging. In the first part of the contribution, we outline the status of different lifetime spectroscopy approaches suitable for the identification of impurities in silicon and discuss—in more detail—the technique of temperature- and injection-dependent lifetime spectroscopy. The second part of the paper focuses on the application of infrared cameras to analyze spatial inhomogeneities in silicon wafers. By measuring the infrared signal absorbed or emitted from light-generated free excess carriers, high-resolution recombination lifetime mappings can be generated within seconds to minutes. In addition, mappings of non-recombination-active trapping centers can be deduced from injection-dependent infrared lifetime images. The trap density has been demonstrated to be an important additional parameter in the characterization and assessment of solar-grade multicrystalline silicon wafers, as areas of increased trap density tend to deteriorate during solar cell processing.
Highlights
The performance of today’s commercially produced silicon solar cells is to a great extent limited by recombination via defects and impurities in the bulk
The main advantages of lifetime spectroscopy (LS) over deep-level transient spectroscopy (DLTS) are that (i) it is a contactless technique, facilitating the sample preparation, and (ii) it is most sensitive to those centers that contribute to the total recombination rate
In order to combine the advantages of Temperature-dependent lifetime spectroscopy (TDLS) and injection-dependent lifetime spectroscopy (IDLS), we have recently introduced temperature- and injectiondependent lifetime spectroscopy (TIDLS) [5]
Summary
The performance of today’s commercially produced silicon solar cells is to a great extent limited by recombination via defects and impurities in the bulk. In addition to the spatial analysis of recombination centers, the infrared camera allows to analyze the spatial distribution of non-recombination-active minority-carrier trapping centers. This infrared trap mapping (ITM) method [11] is of particular relevance to mc-Si wafers, as the trapdensity mapping predicts how a multicrystalline silicon wafer behaves during high-temperature processing. The second part of this review presents the current state of these infraredcamera-based measurement techniques, which—due to their short data acquisition times—evolve into in-line characterization tools for solar cell production lines in the near future
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