Rate Limiting Mechanism of Transition Metal Gettering in Multicrystalline Silicon

Abstract

We have performed studies on multicrystalline silicon used for solar cells in the as-grown state and after a series of processing and gettering steps. The principal goal of this work is to determine the rate limiting step for metal impurity gettering from multicrystalline silicon with an emphasis on the release of impurities from structural defects. Synchrotron-based x-ray fluorescence mapping was used to monitor the release process. Copper and nickel impurities were found to reside primarily at dislocations in the as-grown state of the material. Short annealing treatments rapidly dissolved the impurity agglomerates. Based on these results and modeling of the dissolution process, copper and nickel is in the form of small agglomerates (<lOnm) clustered together over micron-scale regions in the as-grown material. Aluminum gettering further disintegrated the agglomerates to below the sensitivity of the system, 2-5nm in radii. No significant barrier to release of copper or nickel from dislocations was observed. IntroductionMulticrystalline silicon (mc-silicon) is a very interesting material for terrestrial solar cells. Its low cost and respectable energy conversion efficiency (12-15%) makes it arguably the most cost competitive material for large-volume solar power generation. However, the efficiency of mcsilicon solar cells is severely degraded by regions of high minority carrier recombination which have been shown to largely possess dislocations and microdefects [ 1-31. These structural defects are known to increase in recombination activity with transition metal decoration [4-93. Therefore, frontside and/or backside segregation-type gettering of metal impurities from the mc-silicon bulk

Scanning infrared microscopy investigation of copper precipitation in cast multicrystalline silicon

“…But up to now, few papers have been published about copper precipitation in cast multicrystalline silicon (mc-Si) [19,20]. Moeller et al [19] observed with TEM the micro-feature of copper precipitates in mc-Si and found that copper nucleated preferentially at dislocations and at steps of grain boundaries.…”

“…Moeller et al [19] observed with TEM the micro-feature of copper precipitates in mc-Si and found that copper nucleated preferentially at dislocations and at steps of grain boundaries. McHugo et al [20] mainly studied the gettering effect of copper in mc-Si. However, the influence of annealing temperature and cooling rate on copper precipitation in mc-Si is still unclear.…”