## 3 Results
### Photon Harvesting in Thin Film Solar Cells
A post-growth refinement of the nanostructured glass morphology for improved electrical performance has been attempted via ion beam sputtering at normal incidence (normal-IBS), prior to solar cell deposition. The treatment exploits ballistic smoothening of the nanostructured surface [40]. Figure 5: (**a**) External quantum efficiency (EQE) and (**b**) total integrated reflectance of a-Si:H solar cells grown on flat and nanostructured 2 and 2-T glasses; (**c,d**) Relative variation versus the flat cell of integrated reflectance over the full spectral range (blue bars) and \(J_{\mathrm{SC}}\) evaluated from the EQE spectra (red bars) for cells grown on nanostructured substrates prepared at different ion fluence (**c**) with and (**d**) without Ti adhesion layer. We selected Sample 2 as a reference, since this substrate produced the largest \(J_{\mathrm{SC}}\) enhancement, while other relevant photovoltaic parameters where below the values obtained with the flat reference cell (e.g., open circuit voltage (V\({}_{\mathrm{OC}}\)) reads 776 mV versus 840 mV for the flat cell). We synthesized three nanostructured glasses with morphology equivalent to Sample 2 and followed the evolution of the morphological properties after 2 min and 4 min of normal-IBS process (see sketch in Figure 6e). The pristine and the two treated glasses were finally used as substrates for co-deposited a-Si:H solar cells together, with a reference flat glass. Figure 6 shows a morphological and optical comparison of the three samples. The high aspect ratio nanostructures typical of Sample 2 (see Figure 2a) are heavily smoothed by 2 min and 4 min cycles of normal-IBS treatment (the AFM image in Figure 6a refers to a 4 min cycle), and the V-shaped valleys typical of the pristine pattern are tailored to shallower U-shaped ones, as demonstrated by the superimposed AFM profiles in Figure 6d. The RMS roughness \(\sigma\) decreases from the initial value of -150 nm, to 120 nm after 2 min of normal-IBS, and down to only 56 nm when doubling the treatment time. In order to gain insight into the smoothing mechanisms active with normal incidence ion irradiation, an analysis of the slope distribution of the AFM topography has been carried out. The first derivatives of the AFM images were computed orthogonally to the ripple axis and converted into normalized histograms of the local slope distribution (Figure 6b). For the pristine sample (blue line), the distribution is broad in the range of \(-60^{\circ}\)/\(+60^{\circ}\). After 2 min of normal-IBS process (red line), a partial slope selection of the structures is observed, as evidenced by the two peaks in the distribution located approximately at \(-50^{\circ}\) and \(+50^{\circ}\). A flattening effect in correspondence to the bottom of the U-shape valleys is evidenced by the growing peak around \(0^{\circ}\). If the ion irradiation at normal incidence is protracted for 4 min (green line), the slope distribution is heavily impacted: the peak around \(0^{\circ}\) becomes dominant (strong smoothing effect) and two small peaks centered around \(\pm 15^{\circ}\) are observed. The morphological reorganization is likely driven by the reduction of the surface free energy. It involves a smoothening of the corrugations caused mainly by ion induced relaxation  
Figure 6: (**a**) AFM topography (6 μm \(\times\) 6 μm) of nanostructured glass (Sample 2 type) after IBS at normal incidence for 4 min on the rippled glass (sketched in (**e**)); (**b**) histograms of local slopes extracted from AFM images of pristine texture (0 min) and treated glass after 2 min and 4 min of normal incidence ion beam sputtering (IBS); (**c**) comparison of haze spectra on pristine (sample 2 type) and treated glasses; (**d**) AFM topographic line profiles superimposed. mechanisms such as ion induced viscous flow, thermally activated surface self-diffusion, and ion induced surface diffusion [40].