Effect of Alumina as an Anti-Soiling Nanomaterial for Enhancing Photovoltaic Performance ^†

Abstract

Anti-soiling coatings are an essential tool for repelling or protecting surfaces from all sorts of particles, sand, and dust. It is usually used on photovoltaic (PV) cells and solar cells to generate electricity in dry regions such as Qatar and the Gulf countries. However, due to soiling, the performance of solar and PV cells significantly degrades, and they are unable to achieve their potential for success. Thus, an anti-soiling coating is applied to prevent dust accumulation, which interferes with the function of solar panels by restricting the required power output. In this study, an alumina nanomaterial was used in the preparation of the ink, which was coated onto a normal glass substrate using the spin coating technique, and the coated samples were characterized using SEM and XRD. The results showed that the coating was able to significantly reduce the surface energy of the glass substrate while improving its hydrophobicity. The anti-soiling performance of the coating was evaluated using a gravimetric method, which showed that the coating had excellent anti-soiling properties. The reference and coated glass substrates were placed outdoors for a given period of time, and the results showed that the amount of dust that was deposited on the coated sample that was outdoors was greatly reduced.

1. Introduction

Solar photovoltaic (PV) deployment is scaling up worldwide to a great extent. Nevertheless, this deployment has failed to reach its potential for success regarding soiling, as soiling immensely affects its performance [1]. In particular, soiling sabotages the solar photovoltaic panels in arid areas such as Qatar and the rest of the Gulf region. Fortunately, gulf regions have high solar irradiation. Therefore, this region has an increased ability to broaden its energy capabilities for electric generation. However, the Gulf region is a dry and parched environment, which makes achieving this goal a challenge. As dust collection and moisture are natural phenomena, both scientific and technical avenues can be used to address these problems. Throughout the past 30 years, numerous scientific studies have been conducted to stabilize PV systems with appropriate and trustworthy mechanical and electrical components. However, dust buildup on the surfaces of these systems can readily interfere with their intended function by limiting the useful power output, which can terminate the entire system’s performance [2]. Solar energy is a green energy source that is steadily expanding, and both big businesses and private investors are prepared to enter this industry. It is not surprising that photovoltaic establishments attempt to eliminate any downsides associated with solar farms, as the necessity to reduce the carbon footprint in our daily lives is important. One such challenge concerns the buildup of pollutants on the PV module’s surface, which directly correlates with the installation’s ability to produce less electricity. The transparency of the glass of the front module cover decreases as soil buildup progresses. As a result, the semiconductor cells receive less solar irradiation, resulting in a drop in the current output. If precipitation happens and some of the deposited pollution is washed off, these effects may be somewhat alleviated [3]. The two primary characteristics of the coating film are anti-reflectiveness and self-cleaning ability, both of which can improve the performance of solar modules [4]. Coatings can be applied to the covered glass surface to reduce adhesion and make the surfaces easier to clean. These coatings should be resilient and resistant to environmental damage. A hydrophobic anti-soiling coating was exposed to various environmental and abrasion stress tests. The hydrophobic performance of the coating was measured by monitoring the water contact angle and the water roll-off angle after exposure to a range of environmental and mechanical stress tests. The coating was highly resistant to damp heat and thermal cycling [5]. These characteristics are required in anti-soiling coatings, as their purpose is to reduce dust accumulation and allow rays of light from the sun to pass through. Anti-soiling coatings are especially needed in areas like Qatar, as mentioned before, as they can help enhance the energy in Qatar, which can be used for the generation of electricity. Anti-soiling coatings can also make the deployment of solar photovoltaic (PV) systems exceed their potential. Transparent superhydrophobic surfaces were produced on glass, polycarbonate, and poly (methyl methacrylate) (PMMA) substrates using surface-functionalized SiO₂, ZnO, and indium tin oxide (ITO) nanoparticles. Contact angle, contact angle hysteresis, and optical transmittance were measured. Multiscale wear studies were conducted using water jet equipment and an atomic force microscope [6]. The study developed a simple and controlled method for producing superhydrophobic TiO₂ coatings with increased transparency on glass substrates. The TiO₂ coating had a hierarchical topology formed by radial nanowires, efficiently producing rough surface topography and collecting enough air pockets. The as-prepared coating demonstrated outstanding super hydrophobicity after thermal annealing and modification with stearic acid, with a water contact angle (WCA) as high as 157, a low roll-off angle of 2, and strong bounce performance. The dual roughness and roughness control were investigated [7]. Another study compared the energy generated by solar modules with and without an anti-soiling coating. In the photovoltaic laboratory at Málaga University in southern Spain, six solar modules from the same manufacturer and with the same technology (three with coated surfaces and three with uncoated surfaces) were placed outside and evaluated for a year. The results revealed that dust deposition on the module’s surface affects performance in terms of energy and power due to a decrease in transmittance and non-homogeneity of dust dispersion. During dry times (without rainfall), energy losses for both types of solar modules reach considerable levels. The average daily energy soiling loss for coated modules is 2.5%, while uncoated modules have a loss of 3.3% [5]. Because of the great potential of solar irradiance, global energy demands have steadily shifted toward photovoltaic solar energy. Nevertheless, one of the key obstacles to such a technology in the area is soiling, which has been found to severely reduce the power output of solar modules. Anti-soiling coatings are a promising technology for reducing the impact of dust on photovoltaic solar panels. As a result, the aim of this research was to synthesize aluminum, zinc, titanium, and tin oxides using mixed-based and nanoparticle-based precursors via inkjet printing techniques and to examine their potential use in anti-soiling applications in PV panels [8]. Aluminum oxide (Al₂O₃) thin films are an amazing ceramic material. The sol-gel dip coating process deposits Al₂O₃ thin films on Si (100) and quartz substrates. The films are deposited in 15 dips and annealed at various temperatures. The structural and optical characteristics of the films are studied using an X-ray diffractometer (XRD) and a twin-beam UV–visible spectrophotometer. The absorbance of the films on quartz substrates was 265–205 nm, showing a bandgap of 4.68 eV, 4.82 eV, 5.37 eV, and 6.1 eV at annealing temperatures of 100 °C, 200 °C, 400 °C, and 600 °C. Depending on the annealing temperature, the films transmit 90–95% of the light [9]. In summary, anti-soiling coatings (regarding the surfaces) can either be hydrophobic or hydrophilic. Hydrophobic coatings repel water and hydrophilic coatings do not. Some examples of metal oxides that are considered hydrophilic are titanium oxide (the most popular metal oxide used in research on this topic) and zinc oxide. Also, hydrophilic coatings are preferred over hydrophobic ones, especially in dry areas like the Gulf region. In this study, Al₂O₃ was chosen for testing the applications of anti-soiling coatings on PV panels. Al₂O₃ is known for creating a strong hydrophilic surface, which is highly absorbent and can help prevent dust and dirt from sticking to the surface of photovoltaic (PV) panels. This prevents the soiling of the panels, which is responsible for reducing their efficiency. Additionally, Al₂O₃ is a naturally occurring material and is not reliant on other hazardous chemicals for its effectiveness. Because it is an inert material, it does not rust or react with other PV system components, resulting in a long lifespan of anti-soiling protection.

This study presents a novel approach to enhancing the performance of photovoltaic (PV) cells in arid regions using an anti-soiling coating made of alumina (Al₂O₃) nanomaterial. The innovation lies in the preparation of the ink and the application of the coating via the spin coating technique, which significantly reduces surface energy and improves hydrophobicity, thereby effectively preventing dust accumulation. A comprehensive characterization of the coated samples using SEM and XRD, along with an evaluation of the anti-soiling performance of the coating via a gravimetric method, highlights the coating’s effectiveness. This research addresses a critical issue for solar energy generation in dry regions, showcasing the coating’s potential to maintain the efficiency of PV cells by mitigating the adverse effects of soiling.

2. Materials and Methods

2.1. Materials

All chemicals in this study were used without further purification. Ethylene glycol, poly vinyl acetate (PVA), acetone, aluminum oxide (Al₂O₃) nanopowder (<100 nm) were purchased from Sigma-Aldrich and were 99% pure. All the experiments were performed with distilled water.

2.2. Substrate Preparation

In this study, a glass slide with dimensions of 5 cm  ×  5 cm was used as the substrate. The substrates were cleaned using ethanol to remove the contaminants on the glass surface. Then, the glass slides were rinsed with distilled water to remove the hydroxyl contaminants, and these slides were then dried at 80 °C for a period of one hour.

2.3. Deposition by Spin Coating

A saturated solution of Al₂O₃ nanopowder in deionized water was prepared. This was carried out by heating 25 mg of Al₂O₃ in 5 mL of deionized water at 80 °C with constant stirring until no more of the solid dissolved. The solution was cooled to room temperature. A poly vinyl acetate (PVA) solution dissolved in acetone and ethylene glycol was added. The ratio of Al₂O₃, PVA, and ethylene glycol was 5:2:5. The solution was then subjected to sonication in an ultrasonic bath at 40 °C for another 30 min in order to efficiently disperse the nanoparticles (Figure 1). The solution was kept overnight in a sealed glass vial on a magnetic stirrer with constant stirring. The prepared solution was coated onto the glass plates using the spin coating technique. A few drops of Al₂O₃ ink were placed on the glass substrate, and the experiment was carried out at 2000 rpm for 180 s. The substrate was placed in the furnace at 70 °C for 5 min, and the spin coating steps were repeated 5 times. After 12 h, the glass substrate was ready for further experimentation and characterization.

3. Results and Discussion

The XRD pattern of the coated film was obtained to determine the phase and crystallographic structure, as shown in Figure 2. The XRD data showed that the main mineral phase was Al₂O₃ (corundum) and revealed that the deposited film was polycrystalline in nature, with peaks at 26.5°, 33.7°, 37.7°, 43.4°, 51.5° 54.5°, 61.6°, and 65.6° belonging to the (012), (104), (110), (113), (024), (116), (122) and (024) planes, respectively, which indicated preferential growth at higher intensities over these planes. The hydrophilicity of the Al₂O₃ coating was analyzed using water contact angle measurements. The contact angles of the droplets of distilled water were measured using an optical contact angle instrument, and a 5 μL water droplet was dispensed at 2 μL/s on the coated glass and bare glass surfaces at three different places using an automated dispensing system. The images of the dispensed water droplets were recorded in order to measure the static contact angle. The contact angle (CA) measurement in Figure 3 revealed that the CA was higher in the coated sample than in the bare glass. The average CA of the coated sample was 76.65° and 69.9° [10].

It can be concluded that both the uncoated and coated samples are hydrophilic, and the coated sample showed a decrease in hydrophilicity, which is expected to result in improvements in its anti-soiling properties. This was confirmed by the outdoor testing results, in which the samples were exposed to nature. The outdoor experiment was carried out by exposing the bare and coated samples to real-world environmental conditions to accurately assess their anti-soiling properties. Two coated glass substrates were located in one of the outdoor sites of the Center of Advanced Materials at Qatar University and Al Muftah village, Al Wakrah.

Table 1. Glass substrate samples before and after exposure to the outdoor environment.

Sample Al2O3	Type	Coordinates of Glass Sample Location	Before (g)	After (g)	Diff. (g)
Glass substrate 1	Bare glass	25°22′45.1″ N	13.5727	13.5792	0.0065
	Coated	51°29′21.0″ E	13.6375	13.6412	0.0037
Glass substrate 2	Bare glass	25°10′23.2″ N	14.0297	14.0374	0.0077
	Coated	51°35′35.9″ E	14.1035	14.1077	0.0042

shows the glass substrate samples before and after exposure to the outdoor environment.

The samples were placed at a tilt angle of 0° (facing the sky) to test the worst-case scenario regarding the dust deposition rate. The masses of all the samples were measured and recorded using a balance with an accuracy of 0.0001 g before the outdoor experiment began. Two samples were exposed to nature for ten days (from 2 March 2023 to 12 March 2023). Then, the mass of the samples after exposure was recorded. Similar to the work in [8], the dust deposition rate of each sample was calculated by dividing the mass difference by the glass’s surface area using Equation (1). Then, the dust deposition rate per day was calculated for the three experiments.

$$\mathrm{D}\mathrm{e}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}={\displaystyle \frac{\mathrm{S}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{a}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{e}\mathrm{x}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}-\mathrm{S}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{b}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{e} \mathrm{e}\mathrm{x}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}}{\mathrm{S}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e} \mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}}}$$

(1)

The coated sample significantly reduced the amount of the deposited dust by around 18%. Accordingly, it is clear from Table 1 that the coated sample had enhanced anti-soiling properties due to a reduction in the amount of deposited dust. Cleaning the glass layer could be considered a more sustainable technique (in PV applications). Reducing the amount of deposited dust will help reduce the number of cleaning cycles and, thus, the water consumption used for cleaning the panels. The study heavily focused on anti-soiling coatings for solar panels, the aim of which is to prevent dust accumulation and maintain optimal performance using a simple and sustainable technique. The metal-doped Al₂O₃ was prepared and coated onto the PV glass samples to evaluate its anti-soiling properties. The spin coating technique was adopted for the Al₂O₃ (aluminum oxide) nanomaterial coating on the glass substrate, which showed a significant reduction in the surface energy of the glass, making it more hydrophobic. The subsequent outdoor exposure demonstrated a substantial reduction in the deposition of the dust on the coated sample in comparison with the uncoated sample. Therefore, Al₂O₃ anti-soiling coatings hold promise for enhancing solar panel performance, especially in arid regions like Qatar. Our current research focused on the exposure of glass substrates to an outdoor environment for 10 days in two areas of Qatar. In future studies, we will focus on exposing the glass substrate to an outdoor environment for the lifetime of the substrate, which is typically 8 years, and performing the experiments in other areas of Qatar that have greater water retention. Also, additional studies will focus on the morphology of the coated samples in a broader context to further understand the relationship between their surface textures and anti-soiling properties.

4. Conclusions

The performance of the solar and PV cells was significantly degraded due to soiling, and they were unable to achieve their potential for success. Thus, an anti-soiling coating was applied to prevent dust accumulation, which interferes with the function of solar panels by restricting the required power output. The application of an alumina nanomaterial-based anti-soiling coating on the glass substrates using the spin coating technique significantly reduced the surface energy and enhanced hydrophobicity, effectively preventing dust accumulation. The reference and coated glass substrates were placed outdoors over a period of time, and the results showed that the coated sample had a large reduction in the amount of deposited dust. The gravimetric method demonstrated the coating’s excellent anti-soiling properties, with outdoor tests in Qatar confirming a significant reduction in deposited dust on coated samples compared to uncoated ones. The XRD analysis revealed the polycrystalline nature of the Al₂O₃ coating, while contact angle measurements showed improved hydrophobicity. These findings suggest that Al₂O₃-based anti-soiling coatings are highly recommended for use on PV panels, especially in arid regions, to maintain the optimal performance of these panels by reducing dust-related efficiency losses.