## 3 Results
### Photon Harvesting in Thin Film Solar Cells  
The optical performance of the patterned glass when applied as substrate for thin film solar cells has been investigated by fabricating p-i-n amorphous silicon solar cells and evaluating EQE and cell reflectance. The EQE measured for identical devices grown on flat and nanostructured substrates is shown in Figure 5a for Sample 2 and 2-T, which exhibited both antireflective and scattering effects. The response measured for both the rough cells is significantly enhanced when compared to the flat reference cell (black line). The EQE enhancement in the long wavelength range is indicative of efficient light trapping by means of multiple passes of scattered light within the absorber layer. In addition, a reduction of the interference fringes is observed, due to scattering from the uncorrelated roughness of the internal interfaces conformed to the pattern of the substrate. Quantitatively, \(J_{\mathrm{SC}}\) reaches 11.8 mA/cm\({}^{2}\) and 13.5 mA/cm\({}^{2}\) for Sample 2-T and 2, respectively, against 10.8 mA/cm\({}^{2}\) for the flat cell. The total reflectance of the same solar cells is reported in Figure 5b. A significant reduction with respect to the flat case (black line) is measured for both the cells on textured glass and for Sample 2 in particular. This is in agreement with the results on the bare glass (Figure 4a), where Sample 2 is characterized by higher haze with respect to 2-T, which translates into a more effective light trapping in the solar cell. Similarly to what has been done for the bare glass substrates, we evaluated the relative variation \(\mid\Delta R/R\mid_{\mathrm{cell}}=\mid R_{\mathrm{nano}}-R_{\mathrm{flat}} \mid/R_{\mathrm{flat}}\) of the integrated reflectance over the full spectral range for the cells on different nanostructured substrates versus the flat cell. The latter parameter is shown as blue bars in Figure 5c,d, in order to quantify the efficiency of light handling within the cell. In general, a stronger reduction of reflectivity, i.e., improved light harvesting, is observed as the ion fluence increases, although for the samples without Ti adhesion layer (Figure 5d), only two cells are available. A further observation is that, for fixed ion dose, the cell without Ti has a better light harvesting efficiency. These results compare well with the characterization of the substrates shown in the previous section; as expected the light trapping efficiency scales progressively with samples endowed with higher haze. Figure 5c,d also reports the corresponding relative enhancement of \(J_{\mathrm{SC}}\) measured for the same solar cells \(\Delta J_{\mathrm{SC}}/J_{\mathrm{SC-flat}}\). From Figure 5c, for the glasses obtained with Ti layer, it appears that this parameter increases with ion fluence, reaching a maximum for the intermediate value (Sample 2-T). For the largest fluence, on the other hand, the relative enhancement of \(J_{\mathrm{SC}}\) drops while the optical enhancement still increases. We attribute this divergent behavior to the monotonic increase of the RMS height of the nanopattern--as shown in Figure 3b--which likely does not guarantee the full conformal growth of the semiconductor layers of the cell due to shadowing effects, with formation of defective regions. In fact, this issue is also found for the cell on Sample 3, without Ti layer, and is confirmed by the low shunt resistance and reduced open circuit voltage measured in both cases (data not shown). The severe deterioration of the electrical performance likely disturbs the charge collection with impact on the EQE and \(J_{\mathrm{SC}}\), thus masking the optical enhancement. It is known that high aspect ratio features, while valuable for an optical improvement, are usually detrimental to the electrical performance of the solar cells, as they can easily induce the formation of low-density faulty regions [2; 38; 39]; a trade-off between optical and electrical properties must thus be found.