Browsing by Author "Boulfrad, Yacine"

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  • Enhanced performance in the deteriorated area of multicrystalline silicon wafers by internal gettering
    (2013) Boulfrad, Yacine; Haarahiltunen, Antti; Savin, Hele; Øvrelid, Eivind J.; Arnberg, Lars
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The deteriorated area of the multicrystalline silicon (mc-Si) ingots grown by directional solidification, commonly known as the Red Zone, is usually removed before wafering. This area, characterized by poor minority carrier lifetime, is located on the sides, at the top, and the bottom of the mc-Si ingots. In this study, the effect of internal gettering by oxygen precipitates and structural defects has been investigated on the bottom zone of a mc-Si ingot. Nucleation and growth of oxygen precipitates as well as low temperature annealing were studied. Photoluminescence imaging, lifetime mapping, and interstitial iron measurements performed by μ-PCD reveal a considerable reduction of the bottom Red Zone. An improvement of lifetime from below 1 µs to about 20 µs and a reduction of interstitial iron concentration from 1.32 × 1013 at/cm3 to 8.4 × 1010 at/cm3 are demonstrated in this paper.
  • Experimental evidence on removing copper and light-induced degradation from silicon by negative charge
    (2014) Boulfrad, Yacine; Lindroos, Jeanette; Wagner, Matthias; Wolny, Franziska; Yli-Koski, Marko; Savin, Hele
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In addition to boron and oxygen, copper is also known to cause light-induced degradation (LID) in silicon. We have demonstrated previously that LID can be prevented by depositing negative corona charge onto the wafer surfaces. Positively charged interstitial copper ions are proposed to diffuse to the negatively charged surface and consequently empty the bulk of copper. In this study, copper out-diffusion was confirmed by chemical analysis of the near surface region of negatively/positively charged silicon wafer. Furthermore, LID was permanently removed by etching the copper-rich surface layer after negative charge deposition. These results demonstrate that (i) copper can be effectively removed from the bulk by negative charge, (ii) under illumination copper forms a recombination active defect in the bulk of the wafer causing severe light induced degradation.
  • Preventing light-induced degradation in multicrystalline silicon
    (2014) Lindroos, Jeanette; Boulfrad, Yacine; Yli-Koski, Marko; Savin, Hele
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Multicrystalline silicon (mc-Si) is currently dominating the silicon solar cell market due to low ingot costs, but its efficiency is limited by transition metals, extended defects, and light-induced degradation (LID). LID is traditionally associated with a boron-oxygen complex, but the origin of the degradation in the top of the commercial mc-Si brick is revealed to be interstitial copper. We demonstrate that both a large negative corona charge and an aluminum oxide thin film with a built-in negative charge decrease the interstitial copper concentration in the bulk, preventing LID in mc-Si.
  • Reduction of Light-induced Degradation of Boron-doped Solar-grade Czochralski Silicon by Corona Charging
    (2013) Boulfrad, Yacine; Lindroos, Jeanette; Inglese, Alessandro; Yli-Koski, Marko; Savin, Hele
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This study aims at the reduction of light-induced degradation of boron-doped solar-grade Czochralski silicon wafers by corona charging. The method consists of deposition of negative charges on both surface sides of wafer and keeping the wafer in dark for 24 hours to allow the diffusion of positively-charged interstitial copper towards the surfaces. This method proves to be useful to reduce or eliminate light-induced degradation caused by copper. The degradation was significantly reduced in both intentionally (copper-contaminated of the negative charge was found to be proportional to the reduction strength
  • Valon aiheuttama rappeutuminen monikiteisessä piissä
    (2012-12-30) Hyvärinen, Sampo
     Sähkötekniikan korkeakoulu | Bachelor's thesis