Plasmonic nanogratings on MIM and SOI thin-film solar cells: comparison and optimization of optical and electric enhancements.

In this work, Ag nanogratings comprised of arrays of nanostrips with three different cross sections of triangular, rectangular, and trapezoidal shape were considered and put at the top of the thin-film metal-insulator-metal (MIM) and semiconductor-on-insulator (SOI) solar cells. Then, the optical absorption and the short-circuit current density (J_SC) enhancement (relative to a bare cell) were calculated and compared. In addition, the best strip cross section among three types of cross sections and the optimum grating period were found. The results showed that for the transverse electric (TE) mode, only the waveguide modes were excited inside the Si active layer with the assistance of Ag nanogratings. For the transverse magnetic (TM) mode, the waveguide as well as the localized surface plasmonic (LSP) modes were excited. The LSP modes, which were excited at the longer wavelengths centered on ∼600  nm, led to an additional and consequently a larger J_SC enhancement. Finally, among the various types of plasmonic SOI and MIM solar cells, a SOI cell with a 300 nm grating period, comprised of rectangular nanostrips, showed a 40% enhancement in J_SC, which is the highest possible value achieved in this work.