Combining 1D and 2D waveguiding in an ultrathin GaAs NW/Si tandem solar cell.

As an effective means to surpass the Shockley-Queisser efficiency limit, tandem solar cells have been successfully designed and used for years. However, there are still economical and design set-backs hampering the terrestrial implementation of tandem solar cells. Introducing high efficiency, thin Si-based tandem cells that are flexible in design (shape and curvature) will be the next major step towards integrating highly efficient solar cells into fashionable designs of today's buildings and technologies. In this work we present an optically coupled tandem cell that consists of a GaAs nanowire array on a 2μm-thick Si film as the top and bottom cells, respectively. By performing FDTD simulations, we show that coupling the incident light to guided modes of the 1D wires not only boosts the absorption in the wires, but also efficiently transfers the below bandgap photons to the Si bottom cell. Due to diffraction by the nanowire array the momentum of the transmitted light is matched to that of guided modes of the 2D Si thin film. Consequently, infrared light is up to four times more efficiently trapped in the Si bottom cell compared to when the film is not covered by the nanowires.