Evolution of c-Si PV Cell Technologies: Page 4 of 4
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What Lies Ahead?
It is very difficult to improve silicon cell efficiency while reducing costs. Consider that it has taken manufacturers more than 20 years to commercialize some high-efficiency cell architectures. This speaks largely to the difficulty of designing affordable, reliable and repeatable manufacturing techniques that utilize existing equipment and supply chains as much as possible. While IBC and PERL are good technologies for improving cell efficiency, they may not be the most promising options for commercial mass manufacturing because of the higher costs associated with the more-complex production processes. As a result, most manufacturers are looking to improve module output based on existing cell technologies, since these efforts require much lower investments.
Half-cell modules. A general manufacturing trend in the industry, going back several decades, is to slice as many silicon wafers as possible from a single ingot, resulting in increasingly thin wafers. Since cells are susceptible to breakage in the production process, there is an inherent tradeoff between saving material cost during production and incurring material cost associated with broken and scrapped cells. One clever method discovered for reusing broken silicon cells is to trim them in half for use in half-cell PV modules. Half-cut cells generate half the current of a standard cell, which reduces resistive losses in the interconnecting busbars within the module. Reducing internal resistance between the cells increases module power output. As a result, manufacturers today can increase module power output by 5 W–10 W, potentially at a lower cost per watt, by intentionally cutting cells in half. Manufacturers commercializing half-cell module designs include JinkoSolar, LONGi Solar, Mitsubishi, REC Solar and Trina Solar.
Bifacial modules. The global uptick in PERC module production is likely to lead to a future increase in the production of bifacial modules. As shown in Figure 6 (p. 28), cell manufacturers can produce bifacial solar cells by adding just one processing step to the standard PERC cell production line. Bifacial modules convert light captured on the backside of the module into electrical power, which could increase energy captured in the field by 10%–15% with only a modest increase in manufacturing and installation costs. (See “Bifacial PV Systems,” SolarPro, March/April 2017.)
Module manufacturers often use a glass-on-glass package for bifacial PV cells rather than the usual glass-on-film package. Bifacial PV systems also require specialty racking systems and unique mounting considerations to capture the maximum bifacial benefit. This bifacial ecosystem is emerging now. Over the last few years, PV module glass suppliers have begun offering ultra-thin PV glass (<2 mm thick), which reduces the weight of the resulting glass-on-glass module. Mounting system manufacturers are now offering specialty bifacial mounting systems, including single-axis trackers for large-scale PV power plants. Module manufacturers commercializing bifacial PV modules include LG, LONGi, SolarWorld, Sunpreme and Yingli Solar.
Multi-junction cells. Another way to improve cell performance is to stack multiple p-n junctions to selectively filter out light passing through the cell based on its energy level. For example, the manufacturer can tune a p-n junction near the top surface of the solar cell to absorb more light in the blue spectrum and a p-n junction toward the rear of the cell to absorb more red-spectrum light. Multi-junction cell designs have been around for decades and have carved out a niche in space applications and in concentrating PV. However, manufacturers have struggled to find commercial applications for this cell technology in conventional terrestrial applications due to prohibitively high manufacturing costs.
This is beginning to change, as it is increasingly common to see PV cell designs with additional p-n junctions built by depositing thin-film materials on a c-Si base layer. These so-called hybrid or heterojunction solar cells can take advantage of many of the benefits of thin film’s light-absorbing properties at only a fraction of the cost of building a pure c-Si multi-junction solar cell. The multi-junction cell trend will likely evolve as researchers and manufacturers learn more about perovskite materials, which some have dubbed a “wonder material.” Greentech Media reports (see Resources) that Oxford PV claims to have achieved 27.3% efficiency using a perovskite-silicon tandem junction cell technology and believes the technology is capable of breaking the 29% silicon cell efficiency limit. Since perovskites are affordable and can be tuned to low-energy wavelengths, these materials could begin to replace the top thin-film layer in heterojunction cells as they make their way into commercial production.
While multi-junction perovskite cells are admittedly complex, this is just one example of the exciting work under way to develop higher-performing silicon cells. As new cell technologies come to market, researchers can optimize PV module encapsulation materials to better match the light-absorption capabilities of these new cell designs. Each marginal increase in module output and efficiency is important because it ultimately serves to reduce the per-watt costs associated with a whole range of project variables, from transportation to land acquisition to the entire BOS ecosystem.
Blair Reynolds / SMA America / Rocklin, CA / sma-america.com
PV Education Network / pveducation.org
Deign, Jason, “New Efficiency Record for Perovskite Solar—Can Oxford PV Hit 30% by 2020?” Greentech Media, June 28, 2018
Martin, Green, “Developments in Crystalline Silicon Cells,” Solar Cell Manufacturing: Developing Technologies, edited by Gavin Conibeer and Arthur Willoughby, John Wiley & Sons, 2014
Osborne, Mark, “ISFH Pushes P-Type Mono Cell to Record 26.1% Conversion Efficiency,” PVTech, February 7, 2018
Perlin, John, From Space to Earth: The Story of Solar Electricity, Harvard University Press, 2002
SunPower, White Paper: SunPower Panels Generate the Highest Financial Return for Your Solar Investment, Summer 2008