## 3 Results
### Photon Harvesting in Thin Film Solar Cells
Normal incidence IBS thus appears to be an effective and fast process for tailoring the glass morphology, competitive with respect to other plasma treatments implemented on the transparent conductive oxide [2; 39]. The effect of the treatment on the light scattering properties of the glasses is severe after a 4 min process, since the haze parameter drops to values in the range 40% \(\div\) 20% across the spectral interval (Figure 6b). After a 2 min cycle, however, the haze maintains relevant values around 60% \(\div\) 50%, which qualifies the latter sample as a promising substrate for thin film solar cells. Table 1 reports the photovoltaic parameters and relative variation of integrated reflectance of the co-deposited a-Si:H solar cells grown on the reference flat glass, on the pristine texture (0 min equivalent to Sample 2) and on treated nanostructured glass (2 min and 4 min). A relevant optical gain versus the flat reference is confirmed for the cells on pristine substrate and on the sample treated for 2 min (\(J_{\text{SC}}\) above 13 mA/cm\({}^{2}\) against 11.1 mA/cm\({}^{2}\) for the flat case), while as expected the gain is negligible for the cell on the glass treated for 4 min (\(\Delta J_{\text{SC}}\) = 0.7 mA/cm\({}^{2}\)). The lowered light harvesting capability, as determined by the total reflectance measurements on the cells, confirms the effect of the smoothing treatment. The relative variation of integrated reflectance \(\mid\Delta R\mid/R_{\text{flat}}\) of the cells decreases dramatically from 50% for the cell on the pristine texture down to only 2% for the sample treated for 4 min. Remarkably, for the sample treated with a 2 min cycle, the \(\mid\Delta R\mid/R_{\text{flat}}\) value remains almost unchanged at around 40%, while the electrical performance is positively impacted. The electrical parameters of the flat reference cell are modest since the p-layer/TCO contact has not been adjusted to the AZO layer here in use as front electrode; nevertheless, the improving trend after the smoothening treatment is clear. For the cell on the pristine texture, a severe drop is measured both for \(V_{\text{OC}}\) and FF (the latter caused by low shunt resistance), which can be ascribed to the V-shaped rough morphology of the substrate which prevents a conformal growth of the semiconducting layers. Conversely, the cells grown on treated substrates show a promising partial recovery of the electrical parameters. In particular \(\Delta V_{\text{OC}}\), evaluated versus the flat cell, is reduced to \(\sim\)60 mV (for the 2 min substrate) and \(\sim\)10 mV (for the 4 min substrate), against \(\sim\)150 mV for the cell on the pristine texture. The relevant mechanism is the adjustment of the local slopes. It has been shown, at least for microcrystalline Si, that V-shaped valleys with unfavorable opening angles--corresponding to slopes higher than 35\({}^{\circ}\) in our reference system--are prone to developing voids and fissures during the silicon layer growth, with a detrimental impact on \(V_{\text{OC}}\)[39]. The almost complete \(V_{\text{OC}}\) recovery achieved with the substrate treated for 4 min can thus be explained with the slope distribution fully constrained between \(\pm\)35\({}^{\circ}\) (green line in Figure 6b). Further work is in progress to optimize the proposed treatment.