Fig. 1 Conceptual schematics showing light trapping and optical losses in Ag BR for (a) a Si thin-film solar cell and (b) a Si nanowire solar cell. (c) A three-layer structure consisting of a NW array, a thin nanohole-contact-grating layer, and a thick flat BR. (d, e) Top-view and cross-section of a unit cell. (f) Quarter-view of a unit cell and the location of the boundary conditions. The probe line ‘AB’ (denoted in red) is set along the center of a nanowire, and a probe area ‘C’ (blue) is set close to the bottom of the nanowire.

Fig. 2 (a) Absorptance spectra of SiNW arrays without Ag BR. (b) Absorptance spectra of SiNW arrays integrated with a flat Ag BR. (c) Optical properties of SiNW arrays that have a lattice constant of 100 nm and a wire diameter of 60 nm. The blue and black lines correspond to light absorption by Si with and without Ag BR, respectively. The solid red line corresponds to the absorption loss by the Ag BR with the same geometry, and the green line shows the reflectance. The dashed red line shows the absorption loss at the Ag/air interface calculated by Fresnel equations. (d) The line plots show the light intensities along the probe line ‘AB’ at four typical wavelengths labeled with (i-iv), as denoted in panel c. The insets show the two-dimensional electric field distribution as a function of different wavelengths for the probe rectangle ‘C’ depicted in Fig. 1(f).

Fig. 3 (a) Light absorptances of SiNW arrays (with a lattice constant of 500 nm and diameter of 300 nm) are compared to the absorptance (loss) by a Ag BR, where the blue and black lines correspond to the light absorption by Si with and without the Ag BR, respectively. The red line shows the absorption loss by the Ag BR. (b) The electric field distributions along the probe line ‘AB’ inside the SiNW are compared for the three wavelengths, (i) 833 nm, (ii) 983 nm, and (iii) 1000 nm. The insets show the corresponding two-dimensional plots of electric field for the probe rectangle ‘C’.

Fig. 4 (a) The ultimate efficiencies calculated using three different geometrical parameters are shown as a function of the embedded depth of Ag nanoholes. For reference, each dashed line (color-matched) denotes the ultimate efficiency value of bare SiNW arrays without a Ag BR. (b) Optical losses by the Ag BR are also shown for comparison.

Fig. 5 (a-c) Optical loss spectra for three different geometrical conditions of SiNW arrays as a function of embedded depth of Ag nanoholes. (d-f) Absorption enhancement (AE) in SiNWs with different embedded depths of nanoholes where the AE in the SiNWs integrated with a flat BR (p = 0, dark blue lines) was set to 1 for comparison.