Recombination via point defects and their complexes in solar silicon

Electronic grade Czochralski and float zone silicon in the as grown state have a very low concentration of recombination generation centers (typically <10 ¹⁰ cm ^-3). Consequently, in integrated circuit technologies using such material, electrically active inadvertent impurities and structural defects are rarely detectable. The quest for cheap photovoltaic cells has led to the use of less pure silicon, multi-crystalline material, and low cost processing for solar applications. Cells made in this way have significant extrinsic recombination mechanisms. In this paper we review recombination involving defects and impurities in single crystal and in multi-crystalline solar silicon. Our main techniques for this work are recombination lifetime mapping measurements using microwave detected photoconductivity decay and variants of deep level transient spectroscopy (DLTS). In particular, we use Laplace DLTS to distinguish between isolated point defects, small precipitate complexes and decorated extended defects. We compare the behavior of some common metallic contaminants in solar silicon in relation to their effect on carrier lifetime and cell efficiency. Finally, we consider the role of hydrogen passivation in relation to transition metal contaminants, grain boundaries and dislocations. We conclude that recombination via point defects can be significant but in most multi-crystalline material the dominant recombination path is via decorated dislocation clusters within grains with little contribution to the overall recombination from grain boundaries. Achieving high efficiency in low cost silicon solar cells is a key goal in the quest for effective renewable energy sources. In this Feature Article the authors have studied the recombination process in solar silicon involving defects and impurities which degrade the cell efficiency. Lifetime mapping measurement using microwave detected photoconductivity decay shows that the parasitic recombination is concentrated in specific regions of multi-crystalline ingots. Localised Laplace Deep Level Transient Spectroscopy has been used to distinguish isolated point defects, small precipitate complexes and decorated extended defects. It is concluded that in most multi-crystalline materials the dominant recombination path is via decorated dislocation clusters within grains with little contribution to the overall recombination from grain boundaries.