Optical metastructures for trapping light in thin Si solar cells

We propose nano-scale split ring structures and fishnet structures on thin film solar cells to increase absorption near the band edge of Si by 9× and 27× respectively. These structures originate in the metamaterials literature and in the context of Si solar cells; we refer to them as `metastructures´. This nomenclature is adopted since we are only using a single layer distribution of such structures or geometries. The structures are highly tunable and can be designed to obtain enhanced absorption at multiple wavelengths. The results are obtained using full wave simulation for both 500 nm and 2 μm thick solar cells with and without back electrodes. In addition the total power absorbed by the cell is computed using Poynting´s theorem and the intensity of the electric field is studied of various wavelengths as a function of depth. These computations are obtained from numerically simulated field values on a surface just below the metastructures. The maximum photocurrent that can be generated at various wavelengths in the visible spectrum will be calculated and the photocurrent enhancement produced by the nanoplasmonic split ring and fishnet structures will be evaluated. The results for the split rings and fishnet structures are compared with light trapping produced by nano sized spheres, hemispheres and flakes that have been reported in the literature. We conclude that fishnet and split ring structures and other designs based on metamaterial concepts are better suited for the development of high efficiency thin film solar cells.