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SOLAR CELL Two films are better for solar cell efficiency, say researchers

| Author / Editor: Luke James / Jochen Schwab

Researchers at Penn State University in the United States theorize that two layers are better than one when it comes to solar cell efficiency, and that they are much better and more efficient than tandem cells.

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Researchers at Penn State University in the United States theorize that two layers are better than one when it comes to solar cell efficiency.
Researchers at Penn State University in the United States theorize that two layers are better than one when it comes to solar cell efficiency.
(Source: gemeinfrei / Unsplash)

Tandem cells have long since been hailed as the future of solar cell technology, and they’ve been the focus of research for several years now.

Essentially, a tandem solar cell works by stacking two solar cells on top of one another. In this setup, the top cell is semi-transparent and efficiently converts large energy photons into electricity while the bottom cell converts the transmitted low energy photons. This enables a larger portion of light energy to be converted into electricity, dramatically improving efficiency and making them a promising alternative.

To date, however, no commercial devices make use of this technology. They are inherently complex and there are still many questions that have gone unanswered in their research and development, which is holding back progress.

A different approach

Now researchers at Penn State University in the United States have suggested a different approach which they claim will eliminate this inherent complexity. In a paper published in the journal Applied Physics Letters, the research team theorizes that stacking two absorber layers into one cell—rather than stacking two cells on top of one another—could lead to better efficiency and lower production costs.

A schematic of a double thin-film layered solar cell. The sun enters at the top and reaches the CIGS and CZTSSe layers which then absorb the light and create positive and negative particles that travel to the top and bottom contact layers, producing electricity.
A schematic of a double thin-film layered solar cell. The sun enters at the top and reaches the CIGS and CZTSSe layers which then absorb the light and create positive and negative particles that travel to the top and bottom contact layers, producing electricity.
(Source: Penn State University/Akhlesh Lakhtakia)

In this study, the Penn State researchers report how they used a model that treated both electrical and optical “sides” equally. In solar cell research, the electrical “side” looks at how collected sunlight can be converted into electricity and the optical “side” looks at how sunlight can be collected. The typical approach is for researchers to look at and attempt to optimize electrical and optical aspects independently.

In doing this, the research team modeled a dual absorber solar cell incorporating both copper-indium-gallium selenide (CIGS) and copper-zinc-tin-sulfide/selenide (CZTS) layers into a single cell. They chose these materials because they have a similar lattice structure and can both be deposited using the same vapor deposition technique.

The team’s optoelectronic models showed that where the bandgaps in both layers are optimized, such a device could achieve an efficiency of 34.45 %.

Promising, but still only a theory

Although this sounds promising, it’s important to remember it’s all still on paper as mathematics. The lead researcher behind the project, Professor Akhlesh Lakhtakia, is a theoretician—his mathematical model is designed to test the possibility of a dual absorber solar cell. It will be down to others to apply the study’s theory in practice.

“Fabrication of this double-absorber thin-film solar cell, or an approximative variant thereof, will require the attention of experimentalists for careful compositional grading and may not perform as well as predicted by a detailed optoelectronic model,” the study says. “Nevertheless, this solar cell is promising for ubiquitous in-device microwatt-scale generation of electricity.”

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