Near-IR improvement of Si photovoltaic conversion by a nanoscale modification

We have studied near-IR internal quantum efficiency (IQE) transformations produced by a highly doped amorphized and nanostructured layer inserted in the emitter of multi-interface solar cells. This active substructure is created by P ion implantation followed by an adequate thermal treatment necessary to form two sharp a-Si/c-Si heterointerfaces and the corresponding transition zones. We report a large near-IR IQE improvement (View the MathML source) for a series of (multi-interface novel device (MIND)) model solar cells in comparison with an excellent classical solar cell (η=20%). This effect is reproducible and can be explained by the presence in the band gap of an extrinsic band energy allowing direct optical transitions and originating from post-implantation defects. The IQE improvement appears even in non-optimized devices. The improved IR generation is possible due to phonons aiding transitions from extrinsic centers to the Si indirect conduction band. The effect depends on the device temperature: the higher the temperature, the better is the IR conversion efficiency, which is in contradiction with classical cells. The deconvolution of the IQE difference between a MIND and a classical cell gives four characteristic energies related to post-implantation defects in the recrystallized c-Si zones.