# From Self-Cleaning to Self-Aware Solar Mirror Skin
## 1 Introduction
All dipoles add up over the entire material and produce a net polarization that results in electric field generation, which can be piezoelectrically transduced to electric voltage and current and hence detected. In other terms, if we coat a solar mirror with a piezoelectric material, we can turn it into a dust sensor. In desert environments, where dust conditions are extreme, the amount of dust that accumulates on a solar mirror can be significant. In regions like the Sahara or the Middle East, dust deposition rates range from 10 to 100 g/m\({}^{2}\) per day, with an average of about 1.5 kg/m\({}^{2}\) over the course of a month [16]. This dust accumulation exerts pressure on the surface of the mirror, which can be detected by the piezoelectric material as an electrical signal. The main challenge in this context is that dust does not accumulate uniformly. On a relatively small area, such as 1 cm\({}^{2}\), the piezoelectric signal generated by dust pressure would be very small, on the order of picovols (pV), which is too low to be easily detectable by standard sensors. However, by increasing the sensitive area to 1 m\({}^{2}\), the generated signal could increase significantly, reaching microvols (mV), which is much more detectable than the signal from a smaller area. The importance of distributing sensing in this case is crucial. A larger sensitive area not only amplifies the piezoelectric signal enough to surpass the noise threshold (typically in the nanovolt range) but also allows for more precise monitoring of the entire surface of the mirror, averaging out localized pressure variations caused by dust. A larger surface area better distributes pressure fluctuations, improving the stability and reliability of the detection system. In summary, by using a larger sensitive area and optimizing the piezoelectric material properties, measurable signals can be obtained even under harsh desert conditions, where dust accumulates rapidly and unevenly. This dual strategy of enhancing piezoelectric material properties and defining an effective device architecture is crucial for achieving robust, measurable signals in distributed dust sensing. In this work, we focus on maximizing the piezoelectric effect of the material in the form of a thin film. The goal is to optimize the piezoelectric properties to generate a strong and measurable signal in response to dust accumulation. This study aims to enhance the material's performance by fine-tuning its composition and structure to achieve the highest possible sensitivity. In a future study, we will investigate the optimal device area to further improve the sensor's efficiency and performance, ensuring that the sensitive skin can reliably detect dust accumulation across the entire surface of the solar mirror. In particular, the first step of fabrication of thin AlN films by means of reactive sputtering processes was directed to maximize piezoelectric coefficients of the material, in such a way that dust deposition (and hence the soiling of the surface) can be treated as mechanical strain and thus detected, preserving at the same time all requirements of self-cleaning and optical clarity in the full solar range. Moreover, considering the use of piezoelectricity to give sensing properties for detecting soiling on a solar mirror's surface, and recognizing that zinc oxide (ZnO) is another transparent piezoelectric material [17], sputtered ZnO-based inorganic composites were investigated. The decision to incorporate zinc oxide into a transparent, insulating matrix was made to preserve the optical properties of solar mirrors, especially their refractive index. In this case, our goal was to evaluate both the piezoelectric and electrical conductivity properties of these composites, aiming to create a versatile sensing skin capable of providing different outputs for potential neural sensor architectures [18].