New research from Germany has shown how to improve the transparency and performance of TOPCEL The key remains the commercialization of Tandem Perovskiet-Silicon solar cells, as well as for all other types of tandem devices. The analysis has shown in particular that the top cell must achieve a higher efficiency of one cells to compensate for reduced transparency.
A researcher in Germany has analyzed how the transparency levels of top cells can influence the soil cell performance in stacked tandem solar cells and has discovered that imperfect transmission not only influences the lower cell, but also the upper cell itself in a current matched device.
“The central insight into my work is that the transparency of the top cell in a tandem – often underestimated – is crucial for the general device performance,” Martina Schmid, a scientist at Germany University of Duisburg-Ess, told PV -Magazine. “In particular, a poor transmission through the upper cell in a current coordinated 2-terminal configuration is immediately undermining its own performance! This is a striking and surprisingly undertook-of-act, although it is fundamentally clear.”
Schmid explained that the top cell should achieve a higher efficiency of one cells to compensate for reduced transparency. “As a general rule of thumb: If the upper cell transmits the photons completely under the tire gap and the solar strum, the upper and soil cells are evenly distributed, then the upper cell must reach at least 50% of the efficiency of the lower cell for the tandem to match the performance of a soil cell alone,” she added.
In the case of reduced transparency – for example up to 35% – the top cell must be even more efficient, according to her analysis. Moreover, if the tandem exceeds the single soil cell by 30%, an 85% of above-to-bottom single-cell efficiency ratio is required.
“Given these challenges, Spectrum splitting can be a more practical route than trying to only optimize transparency,” Schmid emphasized. “In addition, tandems have a material efficiency – after which can be limited by combining them with light concentration techniques. Moreover, most of the available solar radiation, bifacial effect is recommended – an essential step anyway, especially because most silicon modules are already bifacial today.”
In the study “Heurist rule of thumb for tandem solar cells and perspectives for the future“Published in Solar RRL, Schmid explained that it is crucial for tandem devices that the PN directions of the two cells are aligned so that the current can flow directly between them, often facilitated by a tunnel connection.
This design is ideal for reducing the number and thickness of intermediate layers, which minimizes and slightly maximizes parasitic optical absorption, but is also problematic for the total efficiency of the tandem cell without electricity adjustment between the upper and bottom devices, due to the cell with the lower current output.
The researcher also described various approaches that can help solve some of the transparency problems of the top cell that are analyzed in the paper.
“Although the absorption material is optimized to maximize the absorption of the upper tires and defective Sub-Gap losing is essential, other layers must also be considered,” she added. “For example, contact layers are an important source of transparency losses due to effects such as the absorption of free charge.”
Schmid also suggested improving the balance between the optical transparency of the cell and the electrical conductivity, as well as the identification of new strategic designs for thin film stacks that show complex light pre-planting. She also recommends using spectrum splitting to replace transmission losses with the expected lower losses of spectrum-splitting optics.
“In order to relax the current limitations in particular, concepts such as LuminesCente coupling, wavelength-selective intermediary reflectors or bifacial lighting can be considered,” she concluded. “Looking ahead, bifacial tandem concentrator cells, in a three-terminal configuration and combined with spectrum-splitting optics to reduce optical losses and improve the adaptability of variable lighting, they offer an innovative route.”
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