Impurities in polycrystalline silicon solar cells

The effects of grain boundaries and impurities on silicon-solar-cell performance have been studied in single-crystal and polycrystalline ingots by intentionally incorporting 2 x 10 to the 12th/cu cm Mo, 10 to the 14th/cu cm Ti, and about 10 to the 15th/cu cm Cr during Czochralski growth. Mo, Ti, and Cr degrade solar-cell performance by inducing deep levels in silicon. Electrically active grain boundaries also reduce cell performance by inducing carrier recombination in the bulk as well as in the depletion region. At low impurity concentrations, polycrystalline cell performance is controlled by grain-boundary recombination, while at high concentrations, it is dominated by the impurity, resulting in equal efficiencies for single-crystal and polycrystalline cells. Impurity/grain-boundary interactions are species dependent and result in a decrese in impurity-induced deep-level concentration: Cr exhibits the largest interaction, Ti a weak interaction, and Mo no measurable effect.