Cz-silicon wafers and solar cells from compensated solar-grade silicon feedstock: Potential and challenges

Light-induced degradation (LID) due to boron-oxygen complex formation seriously diminishes the minority carrier lifetime of p-type Czochralski-grown (Cz) wafers. Depending linearly on the boron concentration NA in uncompensated silicon, the boron-oxygen defect density was suggested to depend on the net doping concentration p0 = NA-ND in compensated p-type samples, containing similar amounts of boron and phosphorus. However, this dependence contradicts observations of LID in compensated n-type silicon wafers, which are confirmed in this study by investigating the boron-oxygen complex formation on a large variety of compensated p- and n-type samples. In spite of their high boron content, compensated n-type samples show a less pronounced LID than p-type samples containing less boron. Our experiments indicate that in compensated Cz silicon, the defect concentration is only a function of the compensation ratio RC = (NA+ND)/(NA-ND) and may be described consistently in compensated p- and n-type Cz-Si by an exponential decay. Beyond these fundamental effects, the lifetime study reveals on a variety of compensated p-type Cz crystals from industrial solar-grade feedstock that there are no feedstock-related losses in material quality in addition to the intrinsic losses due to LID. By manufacturing solar cells in an industrial solar cell process this is confirmed by stable degraded cell efficiencies of 17.4% which are achieved on average over the whole crystal length on crystals made from feedstock with and without blend of 33% SoG feedstock.