A researcher in Germany has analyzed how top-cell transparency levels may affect bottom-cell performance in stacked tandem solar cells and has found that imperfect transmission not only impacts the bottom cell but also the top cell itself in a current-matched device.

“The central insight of my work is that the transparency of the top cell in a tandem – often underestimated – is critical for the overall device performance,” Martina Schmid, a scientist at Germany’s University of Duisburg-Essen, told pv magazine. “In particular, in a current-matched 2-terminal configuration, poor transmission through the top cell directly undermines its own performance! This is a striking—and surprisingly underemphasized—fact, even though it’s fundamentally obvious.”

Schmid explained that the top cell must achieve higher single-cell efficiency to compensate for reduced transparency. “As a general rule of thumb: if the top cell fully transmits photons below its bandgap and the solar spectrum is evenly split between the top and bottom cells, then the top cell needs to reach at least 50% of the bottom cell’s efficiency for the tandem to match the performance of a bottom cell alone,” she added.

In case of reduced transparency – for instance, to 35% – the top cell must be even more efficient, according to her analysis. If, additionally, the tandem is to outperform the single bottom cell by 30%, an 85% top-to-bottom single-cell efficiency ratio is required.

“Given these challenges, spectrum splitting may be a more practical route than trying to optimize transparency alone,” Schmid stressed. “Moreover, tandems have a material efficiency disadvantage,  which can be mitigated by combining them with light concentration techniques. Furthermore, to make the most of the available solar irradiation, bifacial operation is recommended – an essential step anyway, especially since most silicon modules today are already bifacial.”

In the study “Heuristic Rule of Thumb for Tandem Solar Cells and Perspectives for the Future,” published in Solar RRL, Schmid explained that crucial for tandem devices is that the pn-junctions of the two cells are aligned so that the current can flow directly between them, often facilitated by a tunnel junction.

This design is ideal for lowering the number and thickness of intermediate layers, which minimizes parasitic optical absorption and maximizes light, but is also problematic for the tandem cell’s overall efficiency without current matching between the top 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 top-cell transparency issues analyzed in the paper.

“While optimizing the absorber material to maximize above-bandgap absorption and minimize defect-related sub-gap losses is essential, other layers must also be considered,” she added. “Contact layers, for instance, are a major source of transparency losses due to effects such as free charge carrier absorption.”

Schmid also suggested improving the balance between the cell’s optical transparency and electrical conductivity, as well as identifying new strategic designs for thin-film stacks exhibiting complex light propagation. Furthermore, she recommended using spectrum-splitting to replace transmission losses with the anticipated lower losses of spectrum-splitting optics.

“To relax particularly current-matching constraints, concepts like luminescent coupling, wavelength-selective intermediate reflectors, or bifacial illumination may 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 adaptability to variable illumination, offer an innovative pathway.”