Diffraction-optimized surface structures for enhanced light harvesting in organic solar cells

Surface phase gratings for organic solar cells are designed, compared and optimized with respect to their ability to diffract incident light and thus increase its path length in the active layer and finally its absorption probability. The gratings are based on concentric rings, a Fermat's spiral or an Archimedean spiral. They exhibit circular symmetric diffraction patterns and therefore diffract light independent of its azimuthal angle. The grating pillars are arranged on these patterns according to a periodic sequence or the deterministic aperiodic Thue-Morse or Rudin-Shapiro sequences to further tailor the desired diffraction pattern. After additional parameter optimization, one of the most promising patterns, the Thue-Morse sequence on an Archimedean spiral, leads to a current density increase of 5%.