Fig. 1 (a) Schematic of the simplified structure made of a ZnO:Al/a-Si:H/Ag stack. A 1D silver grating (width w, pitch p, metal thickness h_m) is deposited on the a-Si:H layer (thickness h_s) and embedded in a ZnO:Al anti-reflection coating layer (thickness h₁). (b) Spectra of the numerically computed total absorption of the reference structure under TM (orange) and TE (green) polarized light at normal incidence. The parameters of the metallic grating are p=200 nm, w=80 nm and h_m=20 nm. The total absorption spectrum of a planar ZnO:Al (50 nm)/a-Si:H (90 nm)/Ag structure is shown with the black dashed curve for the sake of comparison.

Fig. 2 Study of the short wavelengths resonances (A_E) (TE) and (A_M) (TM). (a) Sketch of the asymmetric Fabry-Perot resonator model used to fit the numerical calculations. The influence of the metallic grating is neglected. ϕ₁ and ϕ₂ are the phase shifts induced by reflection at the ZnO:Al/Air and ZnO:Al/a-Si:H interfaces. (b, c) Total light absorption spectrum in the simplified structure depicted in Fig. 1 as a function of the wavelength and the ZnO:Al layer thickness h₁. Excitation at normal incidence light in (b) TE and (c) TM polarizations. The position of the absorption bands with low dependence on h₁ can be attributed to resonances B_E, C_E, B_M and C_M as shown with orange dashed lines. The resonance position h₁ = f(λ) given by the Fabry-Perot model (Eq.(1)) is shown with black dashed curves for q = 0, 1.

Fig. 3 Study of the resonances (B_E) (TE) and (B_M) (TM).(a) Sketch of the asymmetric Fabry-Perot resonator model used to fit the numerical calculations. The influence of the metallic grating is neglected. ϕ₁ and ϕ₂ are the phase shifts induced by reflection at the a-Si:H/ZnO:Al and a-Si:H/Ag interfaces. (b, c) Total light absorption spectrum in the simplified structure depicted in Fig. 1 as a function of the wavelength and the absorber layer thickness h_s. Excitation at normal incidence light in (b) TE and (c) TM polarizations. The resonance position h_s = f(λ) given by the Fabry-Perot model (Eq.(1)) with h₁ = h_s and n₁ = n_s, is shown in black dashed lines for q = 0, 1, 2, 3. (d,e) Electric field intensity maps for a 1D silver grating with w= 80 nm, p= 200 nm, h_m= 20 nm for an excitation at (d) λ_BE = 631 nm under TE polarized light and (e) λ_BM = 649 nm under TM polarized light.

Fig. 4 Study of the long wavelength (C_E) resonance for TE polarization. (a) Total light absorption spectrum as a function of the wavelength and the angle of incidence (plane of incidence perpendicular to the wires). (b) Total light absorption spectrum as a function of the wavelength and the grating period at normal incidence. The absorption bands attributed to resonances A_E and B_E are shown in orange dashed lines in Figs. 4 (a) and (b). (c) Electric field intensity map for an excitation at λ_CE = 687 nm for a TE polarization at normal incidence.

Fig. 5 Numerically computed optical absorption in each material of an ultra-thin a-Si:H solar cell for an excitation under TE (a, left) and TM (b, right) polarizations at normal incidence (red curve: absorption only in a-Si:H; blue curve: absorption in a-Si:H and the ITO and ZnO:Al spacing layers; grey curve: total absorption). The bandgap of amorphous silicon is shown in grey. Inset of Fig. (a): Sketch of the structure investigated: solar cell made of a Si₃N₄(Ag)/ITO/a-Si:H/ZnO:Al/Ag stack with a 90 nm-thick p-i-n a-Si:H absorber layer. A 1D silver grating (thickness=20 nm, width=80 nm and period=200 nm) is embedded in the front Si₃N₄ layer. Other geometrical parameters are hSi₃N₄ =60 nm, hITO=10 nm, h_ZnO:Al=15 nm.

Fig. 6 (a) Evolution of the absorption in the a-Si:H layer for the complete cell as a function of the ZnO:Al layer thickness h_AZO for a TM polarized light at normal incidence. (b, c) Electric field intensity maps for an excitation at λ_CM = 770 nm (simplified structure)(b) and λ_G= 709 nm (complete cell)(c) at normal incidence for a TM polarization.