The efficient utilization of solar energy is a cornerstone of the global transition towards sustainable and clean power. Among the various technologies, photovoltaic (PV) power generation stands out due to its zero-emission operation. However, the performance and economic viability of solar panels are significantly compromised by the natural accumulation of dust and particulate matter on their surfaces. This layer of dust reduces the amount of sunlight reaching the photovoltaic cells, leading to substantial losses in power output. Research indicates that within a week without rain, dust can reduce the output power of solar panels by approximately 7.4%. In arid, desert environments, the degradation over several weeks can be even more severe. Therefore, developing effective and efficient cleaning methods for solar panels is a critical operational challenge.
Traditional cleaning methods, such as water washing or mechanical brushing, are often labor-intensive, water-consuming, and can risk damaging the panel surface. A novel, non-contact approach gaining attention is electrostatic dust removal. This method involves applying a transparent conductive film onto the surface of the solar panels. A high-voltage electrode is then positioned above the panel. When activated, the electrode and the conductive film create an electric field. Dust particles on the panel acquire an electric charge, primarily from contact with the conductive film, and are subsequently lifted off the surface by electrostatic forces, thereby cleaning the panel without physical contact. The success of this technology hinges on the properties of the transparent conductive film, which must exhibit excellent optical transparency to avoid reducing light absorption by the solar cells, while also providing sufficient electrical conductivity to facilitate the charging process.
While materials like Indium Tin Oxide (ITO) are common, they are brittle and expensive. Our previous research identified carbon nanotube (CNT)-based films, particularly those using single-walled carbon nanotubes (SWCNTs), as a superior alternative for this application on solar panels. SWCNT films offer a compelling combination of good electrical conductivity, high optical transparency, mechanical flexibility, and potentially lower cost. Importantly, we found that within a broad sheet resistance range (500 Ω/□ to 100 kΩ/□), the dust removal efficiency remains high, allowing for a design trade-off that prioritizes optical transparency to maximize the energy yield of the solar panels. However, a major unanswered question for the practical deployment of this technology on outdoor solar panels is the long-term durability of these SWCNT films when exposed to harsh environmental conditions. Solar panels are subject to continuous ultraviolet (UV) radiation, temperature cycles, humidity, and, in coastal areas, salt spray. The aging performance of high-transparency SWCNT films under these stressors is largely unknown.
This study addresses this critical gap. We fabricated a high-transparency SWCNT conductive film using a rod-coating method suitable for scalable production. To simulate years of outdoor exposure in a condensed timeframe, we conducted accelerated aging tests under four key environmental stressors: continuous light (simulating UV and solar radiation), salt spray (simulating coastal/marine environments), high temperature, and high-low temperature cycling. The films were subjected to 2000 hours of each aging protocol. We systematically investigated the changes in the film’s structural morphology, chemical composition, and key functional properties—namely, optical transmittance, haze, sheet resistance, and surface energy. Furthermore, we directly evaluated the impact of aging on the core application: the electrostatic dust removal efficiency for solar panels. Finally, we employed a time-series forecasting model to predict the long-term degradation trend and estimate the functional service life of the SWCNT films under these different environmental conditions. The findings provide essential insights and predictive tools for assessing the viability and lifespan of SWCNT-based transparent electrodes in real-world solar panel cleaning applications.
Film Preparation and Experimental Methodology
Fabrication of SWCNT Transparent Conductive Films
We employed a rod-coating technique to prepare the SWCNT films, chosen for its potential for large-area, uniform coating. First, a stable SWCNT conductive ink was formulated. This involved blending an aqueous SWCNT dispersion with a dispersant (Sodium Dodecyl Sulfate, SDS), a polyurethane binder to enhance adhesion to the glass substrate, a leveling agent, and deionized water. The mixture was magnetically stirred at 50°C for 15 minutes to achieve a homogeneous dispersion.
The coating substrate was a standard 10 cm x 10 cm glass sheet, simulating a segment of a solar panel surface. A controlled volume of the SWCNT ink was deposited on the glass and spread uniformly using a precision coating rod. After allowing the film to level for approximately one minute, it was transferred to a hot plate and annealed at 120°C for 5 minutes. This process yielded a robust SWCNT transparent conductive film with a thickness of about 8 µm. The primary goal was to achieve very high optical transparency, accepting a higher sheet resistance to maximize light transmission for the solar panels.
Accelerated Aging Test Protocols
To evaluate environmental durability, we established four distinct accelerated aging platforms, each mimicking a key stress factor for outdoor solar panels.
| Aging Type | Simulated Condition | Equipment / Standard | Key Parameters | Duration |
|---|---|---|---|---|
| Light Aging | Continuous Solar UV/Radiation | Xenon Lamp Chamber with Daylight Filter | 50°C, 300 W/m² irradiance | 2000 h |
| Salt Spray Aging | Coastal/Marine Atmosphere | Salt Fog Chamber (IEC 60068-2-11) | 35°C, 5% NaCl, >85% RH, continuous spray | 2000 h |
| High Temperature Aging | Prolonged Thermal Exposure | Constant Temperature Oven | 80°C, dry air | 2000 h |
| High-Low Temp Cycling | Diurnal/Night Temperature Swings | Oven (-35°C) & Freezer (80°C) | 12h at 80°C / 12h at -35°C per cycle | 2000 h (83 cycles) |
Characterization and Performance Evaluation
The films were characterized before and after aging using the following techniques:
Structure & Morphology: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM).
Functional Properties: Optical transmittance and haze were measured using a hazemeter. Sheet resistance (Rs) was measured using a four-point probe. Surface energy was calculated from water and ethylene glycol contact angles.
Figure of Merit (FoM): The optoelectronic quality was quantified using the Haacke formula:
$$FoM = \frac{T^{10}}{R_s}$$
where T is the transmittance (at 550 nm) and R_s is the sheet resistance. A higher FoM indicates better overall performance for transparent conductive applications on solar panels.
Electrostatic Dust Removal Test for Solar Panels
The ultimate performance metric was the dust removal efficiency. We constructed a test platform using a small solar panel (10 cm x 10 cm) coated with the SWCNT film. Desert sand (140 mesh) was evenly distributed on the panel surface at a density of 5 mg/cm². A metal plate electrode was positioned 1 cm above the panel and connected to a +6 kV DC power supply. Upon voltage application, the dust removal process was observed. The cleaning efficiency (ω) was calculated by weighing the panel before and after the process:
$$ω = \frac{M^* – M}{M’} \times 100\%$$
where M* is the initial mass (panel + dust), M is the final mass after cleaning, and M’ is the mass of the deposited dust. The post-cleaning power output of the solar panel was also measured and normalized against a clean, uncoated reference panel to assess the impact of the film and any residual dust on the energy generation of the solar panels.
Results and Discussion: Impact of Aging on Film Properties
Structural and Morphological Changes
The analysis revealed distinct degradation mechanisms for different aging types. XRD spectra showed a slight right-shift in the characteristic SWCNT peak after all aging processes, indicating minor lattice distortions or structural changes within the carbon nanotubes due to prolonged stress. The shift was most pronounced for salt spray and light aging. XPS analysis of the carbon (C1s) peak indicated a decrease in C-C bonds and an increase in C-O and C=O bonds after aging, suggesting surface oxidation of the SWCNTs. FTIR spectra corroborated this, showing a reduction in hydroxyl groups and an increase in carbonyl groups, consistent with oxidation and possible degradation of the polymeric binder.
SEM imaging provided the most visually striking differences. Films subjected to light, high temperature, and temperature cycling showed no major cracking or delamination. In contrast, salt spray aging caused severe, multi-area delamination of the SWCNT conductive layer from the glass substrate. This physical detachment is a critical failure mode, as it destroys the local electrical conductivity essential for the electrostatic cleaning function on solar panels.
Degradation of Optoelectronic Properties
The functional properties critical for solar panel application deteriorated at different rates. The table below summarizes the key property changes after 2000 hours of aging.
| Aging Type | Transmittance (T) at 550 nm | Haze | Sheet Resistance (Rs) | Figure of Merit (FoM) |
|---|---|---|---|---|
| Initial (Unaged) | 91.34% | 2.15% | 32.5 kΩ/□ | 12.0 × 10-7 Ω-1 |
| Light Aging | 90.39% (-0.95 pp) | 3.41% | 475.3 kΩ/□ (x14.6) | 0.74 × 10-7 Ω-1 |
| Salt Spray Aging | 90.88% (-0.46 pp) | 4.98% | 933.7 kΩ/□ (x28.7) | 0.71 × 10-7 Ω-1 |
| High Temp Aging | 90.95% (-0.39 pp) | 2.89% | 84.18 kΩ/□ (x2.6) | 10.1 × 10-7 Ω-1 |
| Temp Cycling Aging | 91.01% (-0.33 pp) | 2.76% | 86.33 kΩ/□ (x2.7) | 10.3 × 10-7 Ω-1 |
Optical Properties: Light aging caused the largest drop in transmittance, primarily due to the photo-yellowing of the polyurethane binder. Salt spray aging led to the highest increase in haze, a result of salt particle deposition and surface roughening from delamination. The high transmittance was largely preserved, which is beneficial for the solar panels’ light absorption.
Electrical Properties: The sheet resistance increased dramatically under light and salt spray aging. The 14.6-fold increase from light aging is attributed to photo-induced defects in the SWCNTs and disruption of the conductive network. The catastrophic 28.7-fold increase from salt spray aging is directly linked to the physical delamination observed in SEM, which severs conductive pathways. In contrast, thermal aging (both constant and cycling) had a relatively mild effect on sheet resistance, demonstrating the excellent thermal stability of the SWCNT network itself.
Figure of Merit: The combined optoelectronic quality, as measured by the FoM, plummeted for light and salt spray aged films to about 6% of their initial value. The thermally aged films retained over 84% of their initial FoM, highlighting their robustness against temperature-related stress on solar panels.
Surface Energy and Dust Removal Efficacy
Surface energy, which influences dust adhesion, was calculated from contact angle measurements. After aging, the water and glycol contact angles increased, leading to a decrease in total surface energy. A lower surface energy typically implies weaker adhesion to dust particles, which could theoretically benefit the electrostatic cleaning process on solar panels by making dust easier to dislodge.
However, the practical dust removal test revealed a more complex outcome. The electrostatic cleaning efficiency for the solar panels remained high (97.5-97.7%) after light, high temperature, and temperature cycling aging, showing only a minor decrease from the unaged film’s 98.4%. This demonstrates that the increases in sheet resistance under these conditions were still within a range that supports effective particle charging. Despite the drop in FoM for light-aged films, their functional performance for solar panel cleaning was preserved.
The result for salt-spray-aged films was starkly different. The cleaning efficiency dropped significantly to 81.8%. This failure is directly attributable to the film delamination. Areas where the conductive coating detached from the solar panel substrate became electrically inert, preventing local dust particles from acquiring sufficient charge to be removed by the electric field. This physical failure mode outweighs any potential benefit from reduced surface energy.
The normalized power output of the cleaned solar panels correlated with the cleaning efficiency. Panels with films aged by light, heat, or temperature cycling recovered over 93% of the reference panel’s power. The panel with the salt-spray-aged film, due to its poorer cleaning, recovered only 89.7%.
Lifetime Prediction and Practical Implications for Solar Panels
To translate accelerated aging data into practical lifespan estimates for solar panel applications, we defined a failure threshold. Based on our functional tests, we consider the SWCNT film on solar panels to be effective if it maintains a dust removal efficiency ≥90% and allows the panel to recover ≥92% of its nominal power after cleaning. Analysis of our data correlates this performance threshold with a Figure of Merit (FoM) value of approximately $$FoM_{threshold} \geq 7.4 \times 10^{-7} \Omega^{-1}$$.
The degradation of FoM over time exhibited clear time-series characteristics. We employed an Autoregressive Integrated Moving Average (ARIMA) model to forecast the FoM decay beyond our 2000-hour test period. The model was trained on data from 0-1500 hours and validated against the 1600-2000 hour data, showing low prediction errors (e.g., MAE and RMSE on the order of 10-7), confirming its reliability for this purpose.
By extrapolating the ARIMA model forecasts until the predicted FoM falls below the threshold, we can estimate the functional service life of the SWCNT film under each accelerated aging condition. The results are summarized below:
| Aging Condition (Accelerated) | Predicted Functional Service Life | Primary Degradation Mechanism |
|---|---|---|
| Continuous Light Exposure | ~2,300 hours | SWCNT oxidation/defect formation, binder yellowing |
| Salt Spray (Coastal Environment) | ~1,000 hours | Physical delamination of film from substrate |
| High Temperature (80°C) | ~2,900 hours | Minimal; excellent thermal stability of SWCNTs |
| High-Low Temperature Cycling | ~3,000 hours | Minimal; excellent thermal fatigue resistance |
Interpretation for Real-World Solar Panels:
These accelerated life predictions indicate that SWCNT transparent conductive films possess excellent resistance to thermal stresses, which is promising for solar panels installed in hot or climates with large diurnal temperature swings. Their resistance to continuous UV/light exposure is good but represents a likely limiting factor over decades of outdoor operation. The most severe threat is posed by saline environments, where adhesion failure drastically shortens the functional lifespan. For practical deployment on solar panels, this underscores the critical need to improve the adhesion between the SWCNT coating and the glass substrate, possibly through optimized binders or surface treatments, especially for marine applications.
Conclusion
This investigation provides a comprehensive evaluation of the aging performance of high-transparency SWCNT conductive films for the emerging application of electrostatic cleaning on solar panels. We successfully fabricated films using a scalable rod-coating method and subjected them to four key accelerated aging environments.
Our findings reveal that the SWCNT films exhibit remarkable stability against thermal aging (both constant and cyclic), with minimal degradation in optoelectronic properties and preserved electrostatic cleaning functionality for solar panels. This makes them highly suitable for solar panel installations in regions with high temperatures or significant thermal cycling.
Resistance to continuous light exposure is good, with the films maintaining effective dust removal performance for solar panels despite a measurable increase in sheet resistance and decrease in optical FoM. The primary concern under light aging is the gradual oxidation of the SWCNTs and binder yellowing.
The most critical vulnerability is to salt spray corrosion, which induces severe physical delamination of the conductive layer from the glass substrate of the solar panels. This mechanical failure, rather than the chemical degradation of the SWCNTs themselves, leads to a catastrophic loss of local conductivity and a significant drop in cleaning efficiency. Enhancing the interfacial adhesion is therefore the paramount development need for deploying this technology on solar panels in coastal regions.
The application of the ARIMA model provides a valuable tool for predicting the long-term degradation trend and estimating service life under different climatic conditions relevant to solar panels. In summary, SWCNT transparent conductive films are a highly promising material for integrated electrostatic cleaning systems on solar panels, offering a balance of transparency, functionality, and durability, particularly in non-corrosive, thermally challenging environments. Addressing the adhesion issue in humid, saline atmospheres will be key to unlocking their full potential for global solar panel maintenance.