# From Self-Cleaning to Self-Aware Solar Mirror Skin
## 3 Results and Discussion
However, it is important to note that further increasing the thickness could compromise the coating's optical clarity across the full solar spectrum. Increasing the thickness of the coating beyond a certain point may lead to light scattering and absorption, which would reduce the transparency of the film. This would be particularly problematic in the context of maintaining optical clarity across the full solar spectrum, as any reduction in transmittance could negatively impact the overall efficiency of solar mirror systems. Therefore, while thicker films can improve piezoelectric properties, the trade-off between thickness and optical clarity must be carefully balanced to ensure optimal performance for both the self-cleaning and reflective functions of the mirror. To support this, it is useful to observe the UV-VIS-NIR transmittance trend of sputtered AlN thin coating films shown in Figure 4. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Sample**} & **Power Target** & **Pressure** & **Ar Flux** & **N2 Flux** & **Thickness** & **WCA** \\  & **(W)** & **(Pa)** & **(sccm)** & **(sccm)** & **(nm)** & **(\({}^{\circ}\))** \\ \hline \multirow{3}{*}{AlNpiezo} & P = 3600 W & & & & & \\  & I = 5.90 A & 0.4 & 200 & 70 & 341 \(\pm\) 1.5 & 95.3 \(\pm\) 1.0 \\  & V = 618 V & & & & & \\ \hline \multirow{3}{*}{AlNaux} & P = 1800 W & & & & & \\  & I = 6.8 A & 4 & 200 & 90 & 96.1 \(\pm\) 0.3 & 100.8 \(\pm\) 2 \\  & V = 269 V & & & & & \\ \hline \multirow{3}{*}{AlNam} & P = 1800 W & & & & & \\  & I = 6.80 A & 2.4 & 200 & 70 & 219.5 \(\pm\) 0.5 & 90.3 \(\pm\) 1.0 \\ \cline{1-1}  & V = 270 V & & & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental fabrication conditions and WCA of auxetic and piezoelectric AlN. Starting with the XG glass substrate, one can observe the difference in transmittance of the films at varying thicknesses where it should be taken into account that the optimal optical behavior is the one closest to that of the uncoated substrate. The piezoelectric coefficient \(d_{33}\) of the fabricated samples was measured, and for the selected sample AlNpiezo, it was found to be 1.8 pC/N under an applied force of 5 N. This value enables the use of this material as a piezoelectric soiling transducer, advancing the development of a soiling-detecting solar mirror skin. Another useful property for detecting solar mirror failures in different configurations is a change in the coating's electrical conductivity. The key idea is that a sensitive skin should be multifunctional, meaning that it should provide different outputs, similar to the smart materials used in the building sector [27]. In this way, the future step of fabricating a neural network of devices starting from selected materials can be simplified, choosing properties that can be easily integrated. Figure 4: UV-VIS-NIR transmittance of AlN films of different thickness in comparison to the glass substrate Eagle XG. Figure 3: XRD patterns for amorphous (AlN_am) and piezoelectric (AlN_piezo) films. Aluminum nitrides are well known for their insulating properties, but ongoing studies are exploring doping with metallic particles to develop resistive composites capable of altering their electrical resistivity in response to various failure-related inputs. Following this approach, we also tested another material as a potential sensitive solar mirror skin: a family of inorganic composites containing different amounts of zinc oxide (ZnO). These composites exhibit both piezoelectric properties--due to the combined effect of the thin film's overall anisotropy and ZnO's intrinsic piezoelectric behavior--and resistive properties, resulting from the presence of conductive crystalline ZnO islands embedded within an insulating silicate matrix. In other words, each composite can be optimized for a particular task (detecting failure, sensing soiling, monitoring relative humidity, and so on) depending on its specific electric behavior.