The accumulation of dust on the surface of solar panels significantly reduces their power generation efficiency, posing a major challenge for the widespread adoption of photovoltaic technology. In recent years, electrostatic dust removal methods utilizing transparent conductive films have emerged as a promising solution. This approach involves applying a transparent conductive film to the photovoltaic panel surface and placing high-voltage electrodes above it. When energized, these electrodes charge dust particles, causing them to levitate and detach from the panel under electrostatic forces. This method offers a non-contact, energy-efficient cleaning mechanism that can be automated and integrated into existing photovoltaic systems. Among various transparent conductive materials, single-walled carbon nanotubes (SWCNTs) have shown exceptional potential due to their unique combination of electrical conductivity, optical transparency, mechanical flexibility, and environmental stability. However, the long-term performance of SWCNT films in outdoor environments, where they are exposed to various aging factors, remains a critical concern for practical applications.
In this study, we focus on the aging behavior of SWCNT transparent conductive films specifically designed for electrostatic dust removal in photovoltaic panels. We prepared SWCNT films using a rod-coating method and subjected them to accelerated aging tests simulating various environmental conditions: photoaging, salt-fog aging, high-temperature aging, and high-low temperature cycling. The films were characterized for their structural, morphological, electrical, optical, and surface properties before and after aging. Additionally, we evaluated their performance in electrostatic dust removal applications and developed predictive models for their service life under different environmental conditions. Our findings provide valuable insights into the durability and reliability of SWCNT films for solar panel applications, contributing to the development of more efficient and sustainable photovoltaic systems.
Experimental Methods
The SWCNT transparent conductive films were fabricated using a rod-coating technique. We began by preparing a stable SWCNT conductive ink composed of aqueous SWCNT dispersion, sodium dodecyl sulfate (SDS) as a dispersant, polyurethane as a binder, and polyurethane leveling agent. The mixture was stirred at 50°C for 15 minutes to ensure homogeneous dispersion. The resulting ink was then applied to glass substrates (10 cm × 10 cm) using a coating rod, followed by leveling for approximately 1 minute and annealing at 120°C for 5 minutes. This process yielded uniform SWCNT films with a thickness of 8 μm, suitable for photovoltaic panel applications.
We conducted four types of accelerated aging tests to simulate different environmental conditions that solar panels might encounter during outdoor operation. The photoaging test was performed using a xenon lamp aging chamber with a daylight filter, maintaining a constant temperature of 50°C and an irradiance of 300 W/m² at the film surface for 2000 hours. The salt-fog aging test followed the IEC 60068-2-11 standard, using a 5% sodium chloride solution at 35°C with continuous spraying for 2000 hours. The high-temperature aging test involved exposing the films to a constant temperature of 80°C in a drying oven for 2000 hours. The high-low temperature aging test consisted of cyclic exposure between 80°C (12 hours) and -35°C (12 hours) for a total of 2000 hours.
We characterized the films using various techniques: X-ray diffraction (XRD) for structural analysis, X-ray photoelectron spectroscopy (XPS) for chemical composition, Fourier transform infrared spectroscopy (FTIR) for functional groups, scanning electron microscopy (SEM) for morphology, transmittance haze tester for optical properties, four-point probe resistivity tester for electrical properties, and contact angle measurements for surface energy. For electrostatic dust removal evaluation, we used a custom-built setup with a photovoltaic panel (10 cm × 10 cm) coated with the SWCNT film, a high-voltage DC power supply (6 kV), and 140-mesh desert sand as simulated dust. The dust removal efficiency and power generation efficiency were calculated using the following equations:
$$ \omega = \frac{M^* – M}{M’} \times 100\% $$
where $\omega$ is the dust removal efficiency, $M^*$ is the initial mass of the panel with dust, $M$ is the mass after dust removal, and $M’$ is the total mass of deposited dust.
$$ \eta = \frac{P_{\text{max1}}}{P_{\text{max2}}} \times 100\% $$
where $\eta$ is the normalized power generation efficiency, $P_{\text{max1}}$ is the maximum power of the panel with SWCNT film after dust removal, and $P_{\text{max2}}$ is the maximum power of a standard photovoltaic panel under the same conditions.
Results and Discussion
Structural and Morphological Analysis
The XRD analysis revealed that all aging types caused a slight rightward shift in the (100) diffraction peak, indicating changes in the crystal structure of the SWCNTs. The shifts were approximately 0.14° for salt-fog aging, 0.12° for photoaging, and 0.06° for both high-temperature and high-low temperature aging. This suggests that high salinity and high irradiance have more significant impacts on the atomic structure of SWCNTs. The XPS results showed a decrease in C-C bonds and an increase in C-O and C=O bonds after aging, indicating oxidation of the SWCNTs. FTIR spectra confirmed a reduction in hydroxyl groups and an increase in carbonyl groups, consistent with oxidative degradation. No new functional groups were detected, indicating that the aging processes did not introduce new chemical species.
SEM images showed that the SWCNT networks remained largely intact after photoaging, high-temperature aging, and high-low temperature aging, with only minor localized damage. However, salt-fog aging caused significant, non-uniform detachment of the conductive film from the glass substrate. This detachment is attributed to the combined effects of high humidity, salt corrosion, and poor adhesion between the SWCNT layer and the substrate. The morphological changes observed in salt-fog aged films are particularly concerning for long-term performance in coastal or high-salinity environments where solar panels are frequently deployed.
Electrical and Optical Properties
The electrical and optical properties of the SWCNT films changed significantly during aging. The sheet resistance ($R_s$) increased with aging time across all conditions, but to different extents. After 2000 hours of aging, the sheet resistance values were:
| Aging Type | Sheet Resistance ($\Omega/\square$) | Increase Factor |
|---|---|---|
| High-temperature | $8.418 \times 10^4$ | 2.6 |
| High-low temperature | $8.633 \times 10^4$ | 2.7 |
| Photoaging | $4.753 \times 10^5$ | 14.6 |
| Salt-fog | $9.337 \times 10^5$ | 28.7 |
The optical transmittance ($T$) at 550 nm decreased from 91.34% (unaged) to 90.39% after photoaging, 90.78% after salt-fog aging, 91.02% after high-temperature aging, and 91.08% after high-low temperature aging. The haze increased from 1.52% (unaged) to 4.98% after salt-fog aging, 2.67% after photoaging, 1.89% after high-temperature aging, and 1.76% after high-low temperature aging. The decrease in transmittance is primarily attributed to the yellowing of polyurethane binder and surface contamination, while the increase in haze results from light scattering due to surface roughness and particle deposition.
We evaluated the overall optoelectronic performance using the quality factor ($K_{FoM}$) proposed by Haacke:
$$ K_{FoM} = \frac{T^{10}}{R_s} $$
The quality factor decreased with aging time, with the most significant reduction observed for salt-fog aging (to 3.42% of initial value) and photoaging (to 6.16% of initial value). The high-temperature and high-low temperature aging showed relatively minor effects on the quality factor (maintaining 68.3% and 65.7% of initial value, respectively). This demonstrates that SWCNT films have excellent thermal stability but are more vulnerable to photoaging and salt-fog conditions in photovoltaic panel applications.
Surface Energy Analysis
The surface energy of the SWCNT films was calculated from water and glycol contact angles using the following equations:
$$ \gamma_s = \gamma_s^d + \gamma_s^p $$
$$ \gamma_L (1 + \cos \theta) = 4 \left( \frac{\gamma_s^d \gamma_L^d}{\gamma_s^d + \gamma_L^d} + \frac{\gamma_s^p \gamma_L^p}{\gamma_s^p + \gamma_L^p} \right) $$
where $\gamma_s$, $\gamma_s^d$, and $\gamma_s^p$ are the total surface energy, dispersive component, and polar component of the film, respectively; $\gamma_L$, $\gamma_L^d$, and $\gamma_L^p$ are the corresponding values for the test liquid; and $\theta$ is the contact angle.
The contact angles and calculated surface energies are summarized below:
| Aging Type | Water Contact Angle (°) | Glycol Contact Angle (°) | Surface Energy (mJ/m²) |
|---|---|---|---|
| Unaged | 52.36 | 38.45 | 52.17 |
| Photoaging | 69.24 | 55.98 | 43.28 |
| Salt-fog | 60.74 | 46.77 | 48.63 |
| High-temperature | 65.83 | 51.42 | 45.71 |
| High-low temperature | 64.19 | 49.86 | 46.54 |
The increase in contact angles and decrease in surface energy after aging indicate reduced surface wettability. This change is beneficial for electrostatic dust removal as it reduces the adhesion force between dust particles and the film surface, facilitating particle detachment under electrostatic forces. The most significant reduction in surface energy occurred after photoaging, consistent with the observed chemical changes in the SWCNTs and polymer matrix.
Electrostatic Dust Removal Performance
The performance of aged SWCNT films in electrostatic dust removal was evaluated using desert sand with a density of 5 mg/cm². The dust removal efficiency and normalized power generation efficiency after 2000 hours of aging are shown below:
| Aging Type | Dust Removal Efficiency (%) | Normalized Power Efficiency (%) |
|---|---|---|
| Unaged | 98.35 | 97.81 |
| Photoaging | 97.52 | 93.52 |
| Salt-fog | 81.76 | 89.71 |
| High-temperature | 97.71 | 96.33 |
| High-low temperature | 97.63 | 96.21 |
The significant decrease in dust removal efficiency for salt-fog aged films is primarily attributed to the detachment of conductive areas, which disrupts the uniform charging of dust particles. Although the increased sheet resistance after photoaging did not substantially affect dust removal efficiency, it contributed to the reduced power generation efficiency due to lower light transmittance. The high-temperature and high-low temperature aged films maintained excellent dust removal performance and power generation efficiency, demonstrating their suitability for thermal cycling environments commonly experienced by photovoltaic panels.
Lifetime Prediction
We used the Autoregressive Integrated Moving Average (ARIMA) model to predict the long-term behavior of the quality factor under different aging conditions. The model was trained on data from 0 to 1500 hours and tested on data from 1600 to 2000 hours. The prediction accuracy was evaluated using Mean Absolute Error (MAE) and Root Mean Squared Error (RMSE):
$$ \text{MAE} = \frac{1}{n} \sum_{t=1}^{n} |x_t – \hat{x}_t| $$
$$ \text{RMSE} = \sqrt{\frac{1}{n} \sum_{t=1}^{n} (x_t – \hat{x}_t)^2} $$
where $x_t$ is the actual quality factor value, $\hat{x}_t$ is the predicted value, and $n$ is the number of data points.
The error metrics for each aging type are summarized below:
| Aging Type | MAE ($\times 10^{-7} \Omega^{-1}$) | RMSE ($\times 10^{-7} \Omega^{-1}$) |
|---|---|---|
| Photoaging | 0.4574 | 0.7381 |
| Salt-fog | 0.0628 | 0.0685 |
| High-temperature | 1.3917 | 1.9472 |
| High-low temperature | 1.0641 | 1.4466 |
The low error values indicate that the ARIMA model provides accurate predictions of the quality factor degradation. Based on the minimum application requirements for photovoltaic panels (dust removal efficiency ≥90% and normalized power efficiency ≥92%), we defined the end-of-life criterion as a quality factor of $7.4 \times 10^{-7} \Omega^{-1}$. The predicted service lives under different aging conditions are:
| Aging Type | Predicted Service Life (hours) |
|---|---|
| Photoaging | 2300 |
| Salt-fog | 1000 |
| High-temperature | 2900 |
| High-low temperature | 3000 |
These results provide valuable guidance for the application of SWCNT films in different climatic conditions. For instance, in coastal areas with high salinity, more frequent maintenance or replacement may be necessary, while in regions with high temperature variations, the films can be expected to maintain performance for extended periods.
Conclusion
This comprehensive study on the aging performance of SWCNT transparent conductive films for electrostatic dust removal in photovoltaic panels reveals several important findings. The films exhibit excellent resistance to high-temperature and high-low temperature aging, with minimal changes in electrical and optical properties after 2000 hours of accelerated testing. Photoaging causes moderate degradation, primarily through oxidative mechanisms that increase sheet resistance and reduce transmittance. Salt-fog aging presents the most severe challenge, causing significant film detachment and substantial increases in sheet resistance that impair dust removal performance.
The electrostatic dust removal efficiency remains high (>97%) for all aging types except salt-fog aging, where it drops to 81.76% due to localized loss of conductivity. The power generation efficiency of photovoltaic panels using SWCNT films after dust removal closely correlates with both dust removal efficiency and film transmittance, highlighting the importance of maintaining optical properties during aging.
The ARIMA model successfully predicts the long-term behavior of the films’ quality factor, enabling service life estimation under different environmental conditions. The predicted service lives range from 1000 hours for salt-fog conditions to 3000 hours for high-low temperature cycling, providing practical guidance for the deployment of SWCNT films in various photovoltaic applications.
These findings demonstrate that SWCNT transparent conductive films are promising materials for electrostatic dust removal in solar panels, particularly in environments with thermal cycling but less so in high-salinity coastal regions. Future work should focus on improving the adhesion between SWCNT films and glass substrates to enhance salt-fog resistance, potentially through surface treatments or alternative binder systems. The methodology and results presented here contribute to the development of more durable and efficient self-cleaning photovoltaic systems, supporting the broader adoption of solar energy technology.