Electronically-coupled up-conversion: an alternative approach to impurity photovoltaics in crystalline silicon

The traditional approach to harnessing the impurity-photovoltaic effect to improve solar cell performance is plagued by additional recombination caused by the impurity centres. This extra recombination channel is usually deemed to outweigh the benefits of additional generation of electron–hole pairs via sub-band-gap absorption through the impurity levels. Here we consider an alternative approach that restricts the impurity levels to a film with a wider band gap at the rear of a solar cell, isolating the impurities from the minority carriers generated in the base, but, in principle, still allowing impurity-generated carriers to contribute to the cell current. Initial proof-of-concept experiments show that implantation of silicon ions into amorphous silicon films on the rear of crystalline silicon wafers results in the desired increase in sub-band-gap absorptance of infrared photons, without degrading the surface passivation properties of the amorphous layer. However, these defect states are not thermally stable, and in any case do not result in additional carriers being injected into the silicon wafer itself, either because the infrared-generated carriers relax back to the valence band before the second photon can be absorbed, or because the free carriers recombine before reaching the wafer interface. Subsequent attempts involving implantation of iron and erbium impurities to generate stable absorption centres in the amorphous silicon films also failed to inject additional carriers into the crystalline wafer. Possible modifications that may alleviate these problems are discussed.