Near-field study of light scattering at rough interfaces of a-Si:H/µc-Si:H tandem solar cells

Light scattering at randomly textured interfaces in tandem solar cells consisting of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) is studied in the optical near-field experimentally by near-field scanning optical microscopy (NSOM) and theoretically by a rigorous solution of Maxwell's equations using a Fourier ansatz. The work concentrates on the influence of evanescent light modes on the absorption in the a-Si:H top cell. It is found that the improvement of solar cell efficiency due to the surface texturing is borne by evanescent light modes. For both, the theoretical and the experimental study, techniques are developed to extract the evanescent part of the light from the obtained data. From the theoretical results, the principal mechanism of light scattering in the optical near-field regime is studied and, in particular, light which is trapped in the a-Si:H top cell by total internal reflection is visualized. From the experimental results, a map of the local light trapping efficiency is generated and the correlation to morphological properties is studied.