Nanostructured metallic rear reflectors for thin solar cells: balancing parasitic absorption in metal and large-angle scattering

Abstract

Rear reflectors for solar cells comprised of metal films with periodic arrays of nanoscale features on their surface can provide significantly enhanced light trapping in the absorber layer. However these structures can also result in significantly increased parasitic absorption into the metal layer at various wavelengths of light. Conversely these highly absorbing resonances can also coincide with the wavelengths which display the largest enhancement to the cell’s photocurrent. As such it is important to understand the underlying causes for such photocurrent enhancements and losses in the metal in order to design the optimum structure for use. 3D Finite-difference-time-domain simulations have been used to model a variety of structures and analyze the spatial distribution of absorption within different materials which make up the structure, the angles at which light will be scattered from the rear surface, as well as the idealized short circuit current from each structure integrated across the AM1.5 spectrum. These reveal the properties of these modes at resonant wavelengths at which absorption into both materials is enhanced. Despite the enhanced coupling of light into the metal at these wavelengths, the amount of light scattered back into the absorber at large angles is also significantly boosted. For a large variety of geometries, the impact of this large angle scattering dominates leading to significant increases to a cell’s photocurrent. Our simulations allow us to understand the contributions of multiple plasmonic effects occurring in such structures, allowing selection of the most suitable geometries to achieve large-angle scattering in a desired wavelength range.