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| Fig. 1: Molecular structure of monocrystalline (left) and polycrystalline (right) materials (Source: Wikimedia Commons) |
As demand for clean energy resources has grown, solar energy has emerged as a cornerstone innovation in renewable electricity generation. Indeed, solar arrays represent a reliable source of electricity that does not emit greenhouse gases or pollutants, while being inexpensive to manufacture. Solar panels are composed of multiple solar cells, typically made from silicon or other semiconductors, which convert energy from sunlight into electric current. This conversion is driven by the photovoltaic effect, in which photons from sunlight excite electrons on the active semiconducting layer allowing them to flow freely through the material. The two dominant semiconductor materials used in photovoltaics are monocrystalline silicon—a uniform crystal structure—and large-grained polycrystalline silicon—a heterogeneous composition of crystal grains (Fig. 1). [1] Owing to differences in material properties, expense of manufacturing, and energy efficiency, both materials have distinct advantages and disadvantages that guide decision-making in solar energy adoption.
The physical differences between monocrystalline and large-grained polycrystalline silicon originate from differences in their production methods. Monocrystalline silicon is produced via the Czochralski process in which a seed crystal is dipped and rotated into a melt of highly purified silicon, forming a cylindrical crystal, typically with a diameter on the order of 10 cm (Fig. 2). [2] Polycrystalline silicon, on the other hand, is produced by pouring melted silicon into a rectangular cast followed by controlled cooling, resulting in a silicon block with visible crystal grains on the order of mm to cm. [3]
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| Fig. 2: The Czochralski process for monocrystalline silicon production (Source: Wikimedia Commons) |
Because the power output of a solar module depends on the incident solar irradiance, which is the power per unit area of sunlight meeting the modules surface, packing geometry plays an important role in efficiency. The Czochralski process produces cylindrical silicon rods. Cutting circular panels as simple cross-sections of these solids would yield a maximum packing efficiency of 0.91 unit area of solar panel surface per unit area of array, while rectangular panels pack with 100 percent efficiency. [4] Thus, to avoid this 9 percent drop in efficiency, monocrystalline Czochralski rods must be cut into rectangles, a costly process that wastes between 30 and 50 percent of the material. [3] Because large-grained polycrystalline silicon is cast directly into a rectangular shape, this manufacturing disadvantage is avoided, significantly reducing production costs.
In the production of monocrystalline silicon, great care is taken to ensure a uniform crystal structure is grown with minimal impurities and defects. Because silicon is highly reactive with every material, only ultra-high purity quartz crucibles can be used to hold the melt, since the reaction between silicon and quartz produces SiO, which evaporates readily from the melt. [3] Also, as mentioned above, the production process stipulates significant material waste. These realities beg the question: why use monocrystalline solar panels at all?
An explanation lies in the different electrical properties of the materials. To examine this, we use the following electrical properties of typical monocrystalline and block-cast large-grained polycrystalline solar panels at 25°C under an irradiance of 1000 W/m2 taken from modules produced at the Silesian University of Technology with identical metallic contacts, anti-reflective coatings, and dimensions:
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| Table 1: Electrical characteristics of typical 50 mm x 50 mm monocrystalline and polycrystalline solar cells at 25°C and 1000 W/m2 [5] |
where VOC is the cell's open circuit voltage, ISC is the cell's short circuit current, and FF is the cell's fill factor, defined as
| FF | = | Pmax VOC ISC |
To calculate the cells' power transfer efficiencies, we divide the maximum power output Pmax by the solar power incident on the surface of the cell. Let J be the irradiance and A be the area of the cell:
| Efficiency | = | Pmax J A |
= | VOC ISC FF J A |
| Efficiencymono | = | 0.66V × 0.85A × 0.655 1000 W/m2 × 0.05m × 0.05m |
= | 14.7% |
| Efficiencypoly | = | 0.63V × 0.72A ×
0.694 1000 W/m2 × 0.05m × 0.05m |
= | 12.6% |
We see from these calculations that monocrystalline cells transfer solar power into electricity at an efficiency 2% higher than block-cast large-grained polycrystalline cells, amounting to a significant energy saving over the lifetime of the cell. Indeed, the superior efficiency of monocrystalline silicon has been experimentally verified across the literature. [6][7][8] This efficiency difference arises from structural differences at the molecular level. As previously mentioned, this form of polycrystalline silicon has many more point defects and grain boundaries than monocrystalline silicon. These impurities act as sources of internal resistance because they shorten the mobile charge carrier lifetime, thus impeding current flow through the material. [3]
At this point, we have identified a key tradeoff between large-grained polycrystalline and monocrystalline solar cells. While the efficient manufacturing process for polycrystalline silicon is attractive, the drop in power transfer compared to monocrystalline cells might be an unjustifiable sacrifice depending on the application. For example, monocrystalline modules are ideal for residential or rooftop applications where space is strictly limited. On the other hand, the cost efficiency of polycrystalline systems may be justified in cases where the solar installation is in a location with ample sunlight and space to install a larger number of panels. The choice between these materials ultimately hinges on the specific budget, space, and energy yield priorities of the application. By carefully evaluating these factors, decision makers can select the most suitable option for the given scenario.
© Jack Glad. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] L. Jiang et al., "Comparison of Monocrystalline and Polycrystalline Solar Modules," IEEE 9141722, 5th Information Technology and Mechatronics Engineering Conference (ITOEC), 12 Jun 20.
[2] W. Zulehner, "Czochralski Growth of Silicon," J. Cryst. Growth 65, 198 (1983).
[3] A. Goetzberger and V. U. Hoffmann, Photovoltaic Solar Energy Generation (Springer, 2005).
[4] J. H. Conway and N. J. A. Sloane, Sphere Packings, Lattices and Groups, (Springer, 1999).
[5] L. A. Dobrzański et al., "Electrical Properties Mono- and Polycrystalline Silicon Solar Cells, J. Achiev. Mater. Manuf. Eng. 59 67 (2013).
[6] O. Ayadi et al., "Experimental Comparison Between Monocrystalline, Polycrystalline, and Thin-film Solar Systems Under Sunny Climatic Conditions," Energy Rep. 8, 218 (2022).
[7] M. Benghanem et al., "Evaluation of the Performance of Polycrystalline and Monocrystalline PV Technologies in a Hot and Arid Region: An Experimental Analysis," Sustainability 15, 14831 (2023).
[8] M. B. Chatta et al., "Experimental Investigation of Monocrystalline and Polycrystalline Solar Modules at Different Inclination Angles," J. Therm. Eng. 4, 2137 (2018).
