Discrete Optical Field Manipulation by Ag-Al Bilayer Gratings for Broadband Absorption Enhancement in Thin-Film Solar Cells

Plasmonic gratings have been widely used for light harvesting in thin-film solar cells (TFSCs). However, the detrimental parasitic metal absorption loss limits the actual light absorption in the active layer and reduces the power conversion efficiency. In this paper, it is found that the localized surface plasmon resonance (LSPR) used to increase long-wavelength light absorption has significant field concentration around the bottom corners of metal gratings, but the field distribution for the short-wavelength absorption band localizes around the top corners of gratings. Due to the differences between the spatial field distributions and the related mechanisms of metal loss, discrete optical field manipulation is proposed to suppress the ohmic loss mainly associated with LSPR and the interband transition loss associated with metal materials by using Ag-Al bilayer gratings, where Ag has a small absorption coefficient and Al has a high plasmon frequency. Fifteen to forty percent improvements of photocurrents in TFSCs with Ag-Al bilayer gratings are observed in simulation compared to the ones with single-layer metal gratings. This combined metal nanostructure scheme suppresses the loss issue of metal and extends the application potential of plasmonic light-harvesting techniques.