Light harvesting by planar photonic crystals in solar cells: the case of amorphous silicon

In this paper we discuss on light management in silicon thin film solar cells, using photonic crystal (PhC) structures. We particularly focus on photovoltaic devices including amorphous silicon absorbers patterned as 2D PhCs. Physical principles and design rules leading to the optimized configuration of the patterned cell are discussed by means of optical simulations performed on realistic thin film solar cell stacks. Theoretically, a maximum increase of 40% rel in integrated absorption in the a-Si:H layer of the patterned cell is expected compared to the unpatterned case. Moreover, both simulation and optical characterization of the fabricated cells demonstrate the robustness of their optical properties with regard to the angle of incidence of the light and to the fabrication induced defects in the PhCs. Finally, the impact of surface recombination due to the generation of new free surfaces with higher defect densities is addressed. We demonstrate that patterning still induces a substantial increase in the conversion efficiency, with a reasonable surface recombination velocity.