Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1790252 | Journal of Crystal Growth | 2014 | 6 Pages |
Abstract
We evaluate minority-carrier lifetime and defect content of n-type photovoltaic silicon grown by the noncontact crucible method (NOC-Si). Although bulk impurity concentrations are measured by inductively coupled plasma mass spectroscopy to be less than one part per million, homogeneously throughout the as-grown material we observe lifetimes in the ~150 µs range, well below the theoretical entitlement of single-crystalline silicon. These observations suggest the presence of homogeneously distributed recombination-active point defects. We compare an industry-standard gettering profile to an extended gettering profile tailored for chromium extraction, to elucidate potential gains and limitations of impurity gettering. Near the ingot top, gettering improves lifetimes to 750 and >1800 µs for standard and extended profiles, respectively. Relatively lower gettered lifetimes are observed in wafers extracted from the ingot middle and bottom. In these regions, concentric-swirl patterns of low lifetime are revealed after gettering. We hypothesize that gettering removes a large fraction of fast-diffusing recombination-active impurities, while swirl microdefect regions reminiscent of Czochralski silicon can locally limit gettering efficiency and lifetime. Apart from these swirl microdefects, a low dislocation density of <103 cmâ2 is observed. The millisecond lifetimes and low dislocation density suggest that, by applying appropriate bulk microdefect and impurity control during growth and/or gettering, n-type NOC-Si can readily support solar cells with efficiencies >23%.
Keywords
Related Topics
Physical Sciences and Engineering
Physics and Astronomy
Condensed Matter Physics
Authors
Maulid Kivambe, Douglas M. Powell, Sergio Castellanos, Mallory Ann Jensen, Ashley E. Morishige, Kazuo Nakajima, Kohei Morishita, Ryota Murai, Tonio Buonassisi,