Fig. 1 SEM image of substrate solar cell consisting of ~500 nm ZnO NWs grown on an Al-ZnO seed layer with a coating of ~100 nm of a-Si:H. The sketch to the right shows the structure of the coated layers.

Fig. 2 Film formation (a) in the direction of substrate normal, (b) by 3D surface growth algorithm and (c) by simple geometrical model. For the film formation the metal back contact is used as substrate for the nominal film thickness of d and ZnO nanowire (NW) grows on transparent conductive oxide.

Fig. 3 Influence of periods (p, 200 nm (a), 360 nm (b), 600 nm (c) and 840 nm (d)) on the interface morphologies, optical thickness and electrical thickness (t_elec) at nominal p-i-n diode thickness (t_o, 120 nm), nanowire height (h, 200 nm), diameter of nanowire (d, 120 nm).

Fig. 4 (a) Schematic cross section of the unit cell of an amorphous silicon thin film solar cell based on ZnO nanowire at 400 nm nanowire height, 600 nm unit cell period, 100 nm i-layer thickness and 120 nm nanowire diameter. Simulated power loss profiles under monochromatic illumination at wavelength of (b) 360 nm, (c) 560 nm and (d) 620 nm for the structure (a).

Fig. 5 (a) Comparison of quantum efficiencies for solar cells (with and without nanowires (NW)) at different nanowire diameters (d) as a function of wavelength. (b) Comparison of quantum efficiencies for solar cells (with and without nanowires (NW)) at different unit cell periods (p) as a function of wavelength. The parameters of nanowires in (a) are 200 nm nanowire height, 600 nm period and 100 nm i-layer thickness, while those for the nanowires in (b) are 120 nm nanowire diameter, 200 nm nanowire height and 100 nm i-layer thickness. Without nanowire in the solar cells correspond to flat solar cell.

Fig. 6 (a) Comparison of quantum efficiencies for solar cells with nanowires at different i-layer thicknesses (t_i) as a function of wavelength. (b) Comparison of quantum efficiencies for solar cells (with and without nanowires (NW)) at different nanowires height (h) as a function of wavelength. Other parameters of nanowires in (a) are 600 nm unit cell period, 200 nm nanowire height and 120 nm diameter of nanowire, while those for nanowires in (b) are 600 nm unit cell period, 120 nm diameter of nanowire and 100 nm i-layer thickness. Without nanowire solar cell correspond to flat solar cell.

Fig. 7 (a) Change in optical thickness of i-layer of nanowire solar cell as a function of nanowire height for both realistic interfaces and geometric model and (b) the improved short circuit currents in nanowire solar cell (parameters: 600 nm unit cell period, 120 nm nanowire diameter, 100 nm i-layer thickness and 10 nm n-layer thickness) as a function of nanowire heights. The parameters for flat solar cell are t_i = 100 nm (flat) and t_i = t_opt (flat for optical thickness).