The effects of Ag nano-strips with triangle, rectangular and trapezoid cross sections on the optical absorption, generation rate, and short-circuit current density of ultra-thin solar cells were investigated. By putting the nano-strips as a grating structure on the top of the solar cells, the waveguide, surface plasmon polariton (SPP), and localized surface plasmon (LSP) modes, which are excited with the assistance of nano-strips, were evaluated in TE and TM polarizations. The results show, firstly, the TM modes are more influential than TE modes in optical and electrical properties enhancement of solar cell, because of plasmonic excitations in TM mode. Secondly, the trapezoid nano-strips reveal noticeable impact on the optical absorption, generation rate, and short-circuit current density enhancement than triangle and rectangular ones. In particular, the absorption of long wavelengths which is a challenge in ultra-thin solar cells is significantly improved by using Ag trapezoid nano-strips.

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.