Total photon absorption $A_{\mathrm{photon}}$ and its enhancement $(A_{\mathrm{photon}}/A_{\mathrm{photon}\text{-}\mathrm{ref}}-1)·100\%$ in the OPV active layers as a function of period $P$, (a) for different thicknesses $t_1$ of the 2-D Ag nanogrids with the linewidths $w$ fixed at 70 nm; and (b) for different linewidths $w$, with the thickness $t_1$ fixed at 30 nm [all other geometric parameters are the same as those in Fig. 3(a)].