This study investigates the role of polysilicon in solar cell production, focusing on material properties and their environmental impact, using the manufacturing process in Lu'an, Shanxi Province, as a case study. The research establishes an experimental framework to evaluate cell performance across varying resistivity levels and proposes optimization strategies for green production practices. The findings reveal that the optimal resistivity range for polycrystalline solar cells is 1.4-2.0 Ω·cm. Additionally, solar cells manufactured from silicon wafers with a minority carrier lifetime exceeding 6.0 μs achieve a 0.07% higher conversion efficiency compared to those with a lifetime below 5.5 μs. Further analysis of the production process highlights opportunities for improving efficiency and sustainability. Polycrystalline solar cells dominate the photovoltaic industry, serving as a cornerstone for advancing solar technology. However, key material properties—such as resistivity, minority carrier lifetime, and oxygen-carbon content—directly influence the performance and lifespan of solar cells. This study emphasizes how optimizing these properties, along with adopting sustainable production practices, can enhance the efficiency and environmental compatibility of polycrystalline solar cells.
This study utilized P-type multicrystalline silicon wafers, measuring 155.75 mm × 155.75 mm with a thickness of 190 ± 20 μm, supplied by Shanxi Lu'an. A total of over 60,000 wafers were tested for resistivity (ρ) and categorized into seven distinct ranges:
Each range included 400 silicon wafers, which were processed through the standard solar cell production sequence: acid texturing, diffusion, etching, phosphorus washing, PECVD (plasma-enhanced chemical vapor deposition) coating, screen printing, and performance testing using a Berger tester. Two sample groups were selected based on minority carrier lifetimes: one group with lifetimes exceeding 6.0 μs (1000 wafers) and another with lifetimes below 5.5 μs (700 wafers). Both groups underwent identical manufacturing processes to produce solar cells, allowing for comparative performance analysis.
To further evaluate performance variations within ingots, a G6 furnace ingot was divided into three distinct regions: Area A (corners), Area B (sides), and Area C (center). A total of 400 samples from each region were processed using the same production and testing procedures. This ensured a consistent basis for analyzing the influence of ingot regions on solar cell performance.
The open-circuit voltage (Uoc) of polycrystalline solar cells is inversely related to their resistivity. This relationship exists because Uoc is influenced by the reverse saturation current, which depends on carrier concentration in the base region. Higher resistivity reduces carrier concentration, increasing the reverse saturation current and lowering Uoc. Polysilicon characteristics affect cell resistivity through carrier mobility and density, with density primarily determined by temperature and material composition, and mobility influenced by crystal structure and defects.
Although the chemical composition of a single silicon ingot is generally uniform, variations in carrier mobility caused by differences in grain boundary conditions affect resistivity. For example, smaller metal impurity grains in polysilicon create larger monocrystalline regions, reducing resistivity. Consequently, the short-circuit current (Isc) is impacted by reduced metal impurities (increasing current gain) and increased reverse saturation current (causing current loss). Analysis indicates that when resistivity is between 1.0 and 1.4 Ω·cm, Isc increases due to reduced reverse saturation current. Beyond 1.4 Ω·cm, further increases in resistivity (up to 3.0 Ω·cm) yield minimal changes in Isc.
The light-to-electricity conversion efficiency (η) of polycrystalline solar cells depends directly on resistivity, as shown in the formula below:
Here, FF represents the fill factor, and Pin denotes incident power (W). This formula demonstrates that variations in resistivity, influenced by silicon wafer properties, significantly affect photoelectric conversion efficiency. Statistical analysis reveals that the optimal resistivity range for polycrystalline solar cells is 1.4 to 2.0 Ω·cm. Figure 1 illustrates data from improved polysilicon materials supplied by the manufacturer. Following process optimizations, the average resistivity of silicon wafers decreased by approximately 0.0552 Ω·cm, and wafers with resistivity above 2.0 Ω·cm were nearly eliminated. Moreover, the resistivity distribution became more concentrated, displaying a normal distribution pattern.
Figure 1: Distribution Histogram of Resistivity (ρ) of Silicon Wafers After Improvement
Minority carrier lifetime is a crucial parameter for evaluating the performance and durability of semiconductor materials. It provides insights into material quality, manufacturing processes, and device efficiency. Measuring the minority carrier lifetime of silicon wafers from small square ingots and assessing its correlation with resistivity are essential for improving solar cell performance.
Figure 2 illustrates that silicon wafers from small ingots with a minority carrier lifetime exceeding 6.0 μs exhibit higher resistivity compared to those with a lifetime below 5.5 μs. Performance tests of solar cells produced from these ingots reveal that when the minority carrier lifetime exceeds 6.0 μs, the photoelectric conversion efficiency increases by 0.07%. Specifically, cells from ingots with a lifetime above 6.0 μs achieve a conversion efficiency of 18.47%, compared to 18.40% for those below 5.5 μs.
Key performance indicators for ingots with a minority carrier lifetime greater than 6.0 μs include Uoc of 0.6314 V, Isc of 8.9295 A, a fill factor of 79.88%, and resistivity of 1.562 Ω·cm. These values are consistently higher than those for ingots with a lifetime below 5.5 μs, highlighting the positive impact of extended carrier lifetimes on solar cell efficiency.
Given these findings, minority carrier lifetime serves as a reliable standard for assessing the quality of initial polycrystalline wafer materials. Enhancing the carrier lifetime and improving detection accuracy can significantly boost the photoelectric conversion efficiency of polycrystalline solar cells, offering considerable benefits for the photovoltaic industry.
To evaluate how different regions within a crystalline silicon ingot affect solar cell performance, silicon wafers were sampled from regions A (corner areas), B (side regions excluding corners), and C (central area) of the segmented ingot matrix. Solar cells were manufactured under identical production conditions, and their performance was tested for photoelectric conversion efficiency.
The results, summarized in Table 1, reveal that solar cells produced from wafers in region C exhibit a conversion efficiency 0.18% higher than those from region A and 0.08% higher than those from region B. This performance gradient is primarily linked to differences in material purity and the thermal field distribution during the ingot heating process. The central region (C) typically benefits from more uniform thermal conditions, leading to higher material purity and fewer crystal defects, which enhance photoelectric conversion efficiency.
These findings highlight the importance of optimizing the thermal field distribution during ingot growth to improve material quality across all regions. Enhanced uniformity in the ingot production process could significantly reduce performance discrepancies between regions, thereby increasing overall solar cell efficiency and production yield.
a) Minority carrier lifetime for small square ingot type A b) Minority carrier lifetime for small ingot type B
Figure 2: Statistics of Resistivity and Minority Carrier Lifetime for Different Small Ingots
Compared to single-crystal silicon production, the casting process for polycrystalline inherently results in higher impurity levels. This is particularly evident in the edge regions of the ingot, which are more susceptible to impurity contamination due to proximity to the crucible walls. During the casting process, a temperature gradient develops between the ingot core and its surroundings, disrupting the uniform distribution of solid and liquid phases. This gradient leads to higher crystallographic quality and superior electrical properties—such as increased minority carrier lifetime and reduced resistivity—at the center of the ingot compared to the peripheral regions. These findings underscore the importance of optimizing the casting process to minimize temperature gradients and impurity contamination, thereby enhancing the overall quality of polycrystalline and its suitability for high-efficiency solar cell production.
Table 1 Test results of solar cell performance made of silicon wafers in regions A, B, and C
Item |
Sample Quantity |
Uoc (V) |
Isc (A) |
FF (%) |
η (%) |
Ingot Region A |
400 |
0.6323 |
8.9272 |
79.37 |
18.36 |
Ingot Region B |
400 |
0.6332 |
8.9444 |
79.50 |
18.46 |
Ingot Region C |
396 |
0.6345 |
8.9525 |
79.61 |
18.54 |
Field test data reveals that polysilicon production, cell manufacturing, and cell assembly are the primary contributors to pollution in solar cell production. Polysilicon production alone accounts for 50% of total pollutants, primarily due to its high energy consumption. The process relies heavily on electricity and heat from coal combustion, leading to substantial CO2 emissions. During the solar cell assembly phase, aluminum and plastic usage contribute to 76% of the total CO2 emissions. In terms of human toxicity emissions, the polysilicon production stage generates significant quantities of arsenic (As), chromium (Cr), carbon monoxide (CO), and hydrogen fluoride (HF). These findings highlight the urgent need for process optimization to reduce environmental and human health impacts.
Cold hydrogenation technology offers a more sustainable alternative for the raw material supply process. This system integrates energy-efficient measures, including material heating, cyclone separation, and internal component enhancements. The advanced material control design reduces the use of additives during polysilicon preparation and improves the yield of high-quality silicon wafers from ingot zones A and B.
By replacing the outdated trichlorosilane (SiHCl3) synthesis and thermal hydrogenation systems with cold hydrogenation technology, energy consumption during the polysilicon casting process can be significantly reduced. This transition also increases silicon wafer purity and streamlines production workflows, further lowering environmental impacts.
The traditional thermal hydrogenation process, while efficient, is a significant source of environmental pollution. Transitioning to a polysilicon deposition system provides a greener alternative that improves raw material utilization and produces higher-purity polysilicon. This upgrade requires minimal changes to existing production facilities, as the deposition system can be integrated with the original thermal hydrogenation equipment. This approach not only reduces raw material consumption but also enhances overall process efficiency, aligning with the industry's sustainability goals.
As previously discussed, the impurity content in polysilicon significantly impacts the photoelectric conversion efficiency of solar cells. To enhance polysilicon quality, the production process can be refined by optimizing the purification and distillation stages. This approach involves the use of advanced technologies such as high-efficiency packed towers and thermal-coupling distillation to remove complex impurities from SiH3Cl.
Additionally, this process enables the purification of SiCl4 and the adsorption of boron compounds, which are common contaminants in polysilicon production. The adoption of this technology can also be extended to other applications, such as optical fiber manufacturing, offering substantial economic benefits. Furthermore, distillation systems can recover and convert Cl2H2Si, a valuable byproduct, improving material utilization and reducing waste.
Non-condensable gas deep cold recovery technology addresses tail gas treatment in polysilicon production. By incorporating deoxygenation purification, temperature-swing adsorption, and pressure-swing adsorption, this system recovers 82% of SiH3Cl from the exhaust gas. The recovered chlorosilane can be reintegrated into the production cycle, reducing raw material consumption and environmental emissions. This system primarily processes non-condensable gases such as N2 and H2, which contain trace amounts of SiH3Cl. The deep cold recovery technology not only minimizes waste but also enhances the sustainability of the polysilicon production process.
Cooling water flash evaporation technology harnesses waste heat from the polysilicon deposition system to generate steam by lowering the pressure of cooling water. This method improves heat recovery efficiency, reduces energy consumption, and lowers operational costs. From an environmental perspective, this technology mitigates thermal pollution by repurposing waste heat, aligning with sustainable production practices. By integrating flash evaporation systems into the manufacturing process, companies can enhance both economic and environmental performance.
This study evaluates the photoelectric conversion performance of solar cells manufactured from polysilicon, with a specific focus on silicon wafer resistivity, minority carrier lifetime in small ingots, and regional variations within ingot casting. The findings reveal that optimizing material properties such as resistivity and impurity control can significantly enhance solar cell efficiency. To support sustainable manufacturing, this study also outlines green production strategies, including the adoption of cold hydrogenation, advanced purification, and energy recovery technologies. These recommendations provide actionable insights for improving the efficiency and environmental performance of polysilicon-based solar cell production. By integrating these findings, this research offers a practical framework for enhancing polysilicon quality and lays a foundation for future innovations in solar cell manufacturing.