## 2 Materials and Methods  
The A-IPL nanostructuring technique was carried out in a UHV chamber equipped with a thermal evaporator and a defocused ion beam extracted from a gridded multi-aperture Ar\({}^{+}\) source (Tectra instruments, Frankfurt, Germany). We evaporated 150 nm thick polycrystalline Au films [21; 22] (average Au grains diameter around 100 nm and root mean square (RMS) surface roughness around 2 nm) on bare borosilicate glass or, in the experiments aimed to investigate the Ti role, on glass coated with 2 nm of evaporated Ti. The samples were subsequently exposed to defocused ion beam irradiation under grazing incidence conditions (\(\vartheta\) = 82\({}^{\circ}\)), constant flux of 4.96 \(\times\) 10\({}^{15}\) ions cm\({}^{-2}\cdot\)s\({}^{-1}\) (measured in a plane orthogonal to the beam direction) and energy of 800 eV. The duration of the irradiation process was used as a control variable to determine the final ion fluence value. A biased tungsten filament (V = \(-\)13 V), providing electrons via thermionic emission, was placed close to the extraction grid in order to compensate surface charging effects due to ion irradiation.  
The morphology of the samples was investigated ex situ by means of atomic force microscopy (AFM) operating in intermittent-contact mode (Nanosurf Mobile S and NanoMagnetic ezAFM) equipped with a high aspect ratio Si tip (ACLA by AppNano, Mountain View, CA, USA). The topography images were numerically analyzed by using WsXM software (Nanotec Electronica SL, Madrid, Spain) [23] in order to quantitatively evaluate the surface RMS roughness, the one-dimensional power spectral density functions (_1D-PSD_) and the height-height correlation functions, _Hcf_(_r_) [24].  
In order to evidence the anti-reflective and scattering properties induced by the surface nanostructures, we performed far field optical reflectivity measurements by means of an integrating sphere (ISP-50-8-R from Ocean Optics, Dunedin, FL, USA) fiber coupled to a compensated deuterium-halogen lamp (DH-2000-BAL, Mikropak, from Ocean Optics, Dunedin, FL, USA) and to a high resolution solid state spectrometer (HR4000 from Ocean Optics, Dunedin, FL, USA). All spectra were normalized to the integrated reflectance signal of the standard reference sample (WS-1-SS-Spectralon from Ocean Optics, Dunedin, FL, USA).  
The glass templates, prepared with different textured morphologies, were tested as novel substrates for thin film silicon solar cells. Identical amorphous silicon p-i-n single junctions were codeposited on nanostructured and flat glass for a meaningful comparison. A 750 nm thick RF-sputtered Al-doped ZnO (AZO) layer, with optical transmission \(\sim\)80% and sheet resistance \(\sim\)10 \(\Omega\)/sq, was applied as a front transparent electrode. The p-i-n structure was made of a 7 nm thick p-type amorphous silicon-carbide layer, a 260 nm thick intrinsic amorphous silicon absorber layer, and a 30 nm thick n-type microcrystalline silicon layer. The silicon layers were deposited by plasma enhanced chemical vapor deposition (PECVD) in a commercial cluster tool; fabrication details are reported elsewhere [25]. On all the substrates, we finally applied 0.27 cm\({}^{2}\) back reflecting contact pads made of 80 nm thick sputtered AZO, followed by a thermally evaporated Ag film. The devices were characterized by measuring the illuminated current-voltage (I-V) characteristic under the global air mass 1.5 (AM 1.5 G) spectrum with a WACOM class AAA dual-lamp solar simulator and the external quantum efficiency (EQE) with a Bentham PV300 (Bentham Instruments Ltd, Reading, UK) setup. The short-circuit current density (\(J_{\text{SC}}\)) of the solar cells has been calculated from the EQE curves by convolution with the photon flux of the AM1.5 G solar spectrum, while the other parameters have been directly derived from the I-V curves. Total reflectance measurements were additionally carried out with the same setup described above.