Design of plasmonic backcontact nanogratings for broadband and polarization-insensitive absorption enhancement in thin-film solar cell

We discuss the rules for designing nanostructured plasmonic backcontact of thin-film crystalline silicon solar cells using two-dimensional finite-difference time-domain (2D-FDTD) method. A novel efficient quasi-periodic plasmonic nanograting is designed. Numerical calculations demonstrate that broadband and polarization-insensitive absorption enhancement is achieved by the proposed structure which is based on a supercell geometry containing N subcells in each of which there is one Ag nanowire deposited on the backcontact of the solar cell. The proposed structure offers the possibility of controlling the number and location of photonic and plasmonic modes and outperforms the periodic plasmonic nanogratings which only utilize plasmonic resonances. We start by tuning the plasmonic mode of one subcell and then construct the supercell based on the final design of the subcell. Our findings show that with a proper choice of key parameters of the nanograting, several photonic and plasmonic modes can be excited across the entire spectral region where crystalline silicon (c-Si) is absorbing. The absorption enhancement is significant, particularly in the long wavelength region where c-Si is weakly absorbing.