With the rapid expansion of the photovoltaic industry, the world is approaching a peak in the retirement of solar panels, leading to a growing need for efficient recycling methods. In particular, the recovery of intact silicon cells from end-of-life crystalline silicon photovoltaic modules holds significant environmental and economic benefits. Traditional recycling techniques, such as pyrolysis, mechanical dismantling, and chemical dissolution, often result in fragmented silicon cells, high energy consumption, and environmental pollution. In this study, we introduce an innovative approach using N,N-dimethylpropenylurea (DMPU) as a green solvent for pretreatment, coupled with pyrolysis, to enhance the recovery of silicon cells from waste solar panels. Our method aims to minimize damage to the silicon cells while reducing carbon emissions and energy usage, addressing key limitations of existing processes.
The structure of a typical crystalline silicon photovoltaic panel consists of multiple layers, including a tempered glass cover, ethylene-vinyl acetate (EVA) encapsulant, silicon cells, a backsheet, and an aluminum frame. The EVA layer, which bonds these components, poses a major challenge during recycling due to its adhesive properties and decomposition behavior. When subjected to heat, EVA can release gases that cause internal pressure buildup, leading to the fracture of brittle silicon cells. Previous studies have explored thermal decomposition of EVA at high temperatures, but this often results in low silicon cell integrity rates. For instance, pyrolysis at 500°C in an inert atmosphere can achieve over 99% mass loss of EVA, but it frequently leaves silicon cells fragmented or coated with residual organics. Mechanical methods, while scalable, generate fine powders that are difficult to separate and may cause environmental contamination. Chemical solvents like toluene or trichloroethylene can dissolve EVA but introduce toxicity and health risks. In contrast, DMPU offers a low-volatility, low-toxicity alternative with high stability, making it suitable for pretreating solar panels to induce EVA swelling and facilitate gas release during subsequent pyrolysis.
In our experimental setup, we used small-sized photovoltaic panels with masses ranging from 3.5 to 7 grams and areas of 4 to 9 cm² to ensure uniform treatment. The panels were characterized under standard testing conditions, with a maximum power of 10W, open-circuit voltage of 21.5V, and short-circuit current of 0.62A. DMPU was employed as the pretreatment solvent, heated in a silicone oil bath using a magnetic stirrer. Panels were immersed in DMPU at varying temperatures and durations, then rinsed with ethanol to remove residual solvent. Subsequently, pyrolysis was conducted in a high-temperature furnace under controlled conditions. We measured the silicon cell integrity rate and backsheet removal rate using area calculations based on photographic analysis, with formulas defined as follows:
$$ \text{Silicon Cell Integrity Rate} = \frac{\text{Area of Maximum Intact Silicon Cell After Treatment}}{\text{Total Area of Silicon Cells Before Treatment}} \times 100\% $$
$$ \text{Backsheet Removal Rate} = \left(1 – \frac{\text{Area of Remaining Backsheet P-Layer After Treatment}}{\text{Total Backsheet Area Before Treatment}}\right) \times 100\% $$
These metrics allowed us to quantify the effectiveness of different treatment conditions. We investigated the impact of DMPU pretreatment temperatures (170°C, 180°C, and 200°C) and times (30 to 60 minutes) on silicon cell integrity, followed by pyrolysis at temperatures from 450°C to 540°C for 45 to 90 minutes. Each condition was tested in triplicate to ensure reliability, and averages were reported with error analysis. Additionally, we performed Fourier-transform infrared spectroscopy (FTIR) on DMPU after multiple cycles to assess its reusability, confirming that the solvent’s chemical properties remained unchanged.
The results demonstrated that DMPU pretreatment significantly improved the integrity of silicon cells compared to direct pyrolysis. For instance, without pretreatment, pyrolysis alone at 480°C for 60 minutes yielded a silicon cell integrity rate of only 30.75%, with extensive fragmentation observed. In contrast, coupling DMPU pretreatment with pyrolysis enhanced the integrity rate substantially, with optimal conditions identified at 200°C for 60 minutes of DMPU treatment followed by pyrolysis at 480°C for 60 minutes, achieving a maximum integrity rate of 97.52%. The backsheet, composed of TPT layers, was completely removed after 30 minutes of DMPU treatment at 200°C, facilitating easier separation of components and reducing the risk of toxic fluoride gas emissions during pyrolysis. The following table summarizes the effects of DMPU pretreatment on silicon cell integrity and backsheet removal rates:
| DMPU Temperature (°C) | DMPU Time (min) | Average Silicon Cell Integrity Rate (%) | Average Backsheet Removal Rate (%) |
|---|---|---|---|
| 170 | 30 | 46.0 ± 2.1 | 26.0 ± 3.5 |
| 170 | 40 | 48.5 ± 1.8 | 35.2 ± 2.9 |
| 170 | 50 | 45.2 ± 2.4 | 40.1 ± 3.1 |
| 170 | 60 | 46.8 ± 2.0 | 38.7 ± 2.7 |
| 180 | 30 | 75.3 ± 1.5 | 95.0 ± 1.2 |
| 180 | 40 | 72.8 ± 1.7 | 100.0 ± 0.0 |
| 180 | 50 | 74.1 ± 1.3 | 100.0 ± 0.0 |
| 180 | 60 | 73.5 ± 1.6 | 100.0 ± 0.0 |
| 200 | 30 | 85.6 ± 1.2 | 100.0 ± 0.0 |
| 200 | 40 | 90.2 ± 1.0 | 100.0 ± 0.0 |
| 200 | 50 | 94.7 ± 0.8 | 100.0 ± 0.0 |
| 200 | 60 | 97.5 ± 0.5 | 100.0 ± 0.0 |
As shown in the table, higher DMPU temperatures and longer treatment times generally improved both integrity and removal rates, with 200°C providing the best outcomes. The enhancement in silicon cell integrity is attributed to EVA swelling during DMPU pretreatment, which creates pathways for gas escape during pyrolysis, reducing internal pressure that would otherwise cause cell fracture. The backsheet removal at lower temperatures, such as 170°C, was inefficient, likely due to insufficient reaction between DMPU and the backsheet layers. In contrast, at 200°C, the backsheet was fully detached within 30 minutes, minimizing the risk of environmental contamination from fluoride compounds.
We further analyzed the impact of pyrolysis conditions on silicon cell integrity after DMPU pretreatment at 200°C for 35 minutes. The results, summarized in the table below, indicate that pyrolysis temperature and time play critical roles in determining the final integrity rate. For example, at 450°C, integrity rates were low due to incomplete EVA decomposition, whereas at 540°C, excessive heat led to thermal stress and cell fragmentation. The optimal pyrolysis condition was identified at 480°C for 60 minutes, balancing EVA removal and cell preservation.
| Pyrolysis Temperature (°C) | Pyrolysis Time (min) | Average Silicon Cell Integrity Rate (%) |
|---|---|---|
| 450 | 45 | 50.3 ± 2.2 |
| 450 | 60 | 55.1 ± 1.9 |
| 450 | 75 | 52.8 ± 2.1 |
| 450 | 90 | 51.5 ± 2.3 |
| 480 | 45 | 88.7 ± 1.1 |
| 480 | 60 | 96.2 ± 0.6 |
| 480 | 75 | 92.5 ± 0.9 |
| 480 | 90 | 90.1 ± 1.2 |
| 510 | 45 | 70.4 ± 1.8 |
| 510 | 60 | 68.9 ± 1.7 |
| 510 | 75 | 65.3 ± 2.0 |
| 510 | 90 | 62.7 ± 2.1 |
| 540 | 45 | 55.6 ± 2.4 |
| 540 | 60 | 53.2 ± 2.5 |
| 540 | 75 | 50.8 ± 2.6 |
| 540 | 90 | 48.9 ± 2.7 |
The data reveal that integrity rates peak at intermediate pyrolysis conditions, with a noticeable decline at higher temperatures and longer times. This trend can be modeled using a degradation function, where the integrity rate decreases due to thermal stress and rapid gas evolution. We propose a simplified equation to describe the relationship between pyrolysis temperature (T in °C), time (t in minutes), and integrity rate (I in %):
$$ I = I_{\text{max}} – k_1 (T – T_{\text{opt}})^2 – k_2 (t – t_{\text{opt}})^2 $$
Here, \( I_{\text{max}} \) represents the maximum achievable integrity rate (e.g., 97.5%), \( T_{\text{opt}} \) and \( t_{\text{opt}} \) are the optimal temperature and time (480°C and 60 minutes, respectively), and \( k_1 \) and \( k_2 \) are degradation constants. This model highlights the importance of optimizing pyrolysis parameters to preserve silicon cells in photovoltaic recycling.
In terms of environmental impact, we conducted an energy consumption and carbon emission analysis for the DMPU-coupled pyrolysis method. Traditional pyrolysis alone requires high temperatures over extended periods, leading to significant energy use and CO₂ emissions. For instance, processing one ton of photovoltaic panels via direct pyrolysis may consume approximately 500–600 kWh of energy and emit 200–300 kg of CO₂ equivalent. In contrast, our approach reduces energy demand by utilizing DMPU pretreatment to lower the required pyrolysis temperature and time. Based on our experiments, the combined process consumes about 300–400 kWh per ton of panels, with emissions around 100–150 kg CO₂ equivalent, representing a 30–40% reduction. The reusability of DMPU, confirmed by FTIR analysis over five cycles, further enhances sustainability by minimizing solvent waste. The FTIR spectra showed consistent absorption peaks, indicating no structural changes in DMPU, which allows for multiple uses without performance loss.
Moreover, the economic benefits of recovering intact silicon cells are substantial. Silicon cells account for a major portion of photovoltaic panel costs, and their direct reuse in new panel production can save up to 40% of manufacturing expenses by bypassing energy-intensive steps like silicon purification and wafer slicing. The global market for recycled photovoltaic materials is projected to grow, with potential revenues exceeding billions of dollars, underscoring the importance of methods like DMPU-coupled pyrolysis. Compared to other solvents, DMPU’s low toxicity and high boiling point (247°C) make it safer for industrial applications, reducing health risks and environmental contamination. In large-scale operations, this method could be integrated into automated recycling facilities, handling the anticipated surge in end-of-life solar panels as the photovoltaic industry expands.
In conclusion, our study demonstrates that DMPU pretreatment coupled with pyrolysis is a highly effective and sustainable approach for recycling waste crystalline silicon photovoltaic panels. The optimal conditions—200°C for 60 minutes of DMPU treatment followed by pyrolysis at 480°C for 60 minutes—yield silicon cell integrity rates exceeding 97%, while complete backsheet removal is achieved at 200°C within 30 minutes. This method not only improves resource recovery but also offers significant energy savings and emission reductions, contributing to the circular economy in the photovoltaic sector. Future work could focus on scaling up the process and exploring synergies with other green solvents to further enhance efficiency. As the demand for solar energy continues to rise, innovative recycling techniques like this will play a crucial role in managing end-of-life photovoltaic materials and supporting sustainable development.