New world record for solar cell efficiency achieved by CIGS Thin Films

The record efficiency of CIGS solar cells has been independently verified, showing that there is still life in the once popular technology that has been eclipsed in recent years.

Today, the vast majority of photovoltaic systems are based on silicon. However, silicon cells are approaching peak efficiency and require relatively high temperatures to produce, creating a floor in cost and embodied energy that is difficult to reach. The much newer perovskite technology is improving at an astonishing rate, but doubts remain about the longevity of perovskite cells.

Some researchers would therefore like to keep options open for an alternative approach. Moreover, stacking different types of solar cells on top of each other leads to efficiency improvements that no one could achieve on their own. Both keep CIGS relevant as a potential part of fossil fuel replacement, despite declines in market share since the early 2000s.

The letters in CIGS stand for copper, indium, gallium and selenide, a combination of which is applied to a plain glass plate, along with silver and sodium. A team from Uppsala University created such a cell that outperformed its predecessors, using several innovations, including adjusting the gallium concentrations in different parts of the cell.

This composition sounds expensive – some of those metals are quite pricey. The fact that they have to be treated with rubidium fluoride, a molybdenum layer behind and a transparent layer in front, does not improve the situation.

However, these types of cells are not called ‘thin film’ for nothing. The plates are so fine that the amounts of metals required should not be prohibitively expensive compared to silicon.

However, for CIGS to be worth using, it needs efficiency that is at least close to competing technologies. For decades, solar engineers were more concerned about reducing production costs than improving efficiency, but solar cells are now so cheap that efficiency gains have become a priority in recent years.

The standard efficiency of commercial panels has more than doubled in the past decade, allowing much more energy to be generated on rooftops with limited space, and easing concerns that utility solar will compete with agriculture for land.

Researchers at Uppsala University held the record for CIGS efficiency in the 1990s, but that performance was surpassed for decades. However, they recently produced a cell that their own tests suggested achieved an efficiency of 24 percent. Independent tests by the Fraunhofer Institute for Solar Energy Systems have now yielded a value of 23.54 percent.

“The measurements we made ourselves for this solar cell and other recently produced solar cells are within the margin of error for the independent measurement,” Professor Marika Edoff of Uppsala University said in a statement.

This claims the CIGS crown, beating Solar Frontier’s previous record in Japan by 0.29 percent, although still behind silicon’s 27.6 percent and perovskite’s 26.1.

However, CIGS doesn’t have to beat silicon’s efficiency to matter. Different cell technologies work best at capturing different parts of the spectrum of sunlight reaching Earth. Tandem cells are topped by a layer that is good at collecting blue to ultraviolet and relatively transparent at longer wavelengths, with another layer underneath that is better at collecting red light. These could be more efficient than any single-layer cell – with the record currently standing at 33.9 percent.

“Our research shows that CIGS thin-film technology is a competitive alternative as a standalone solar cell. The technology also has properties that can function in other contexts, such as the bottom cell of a tandem solar cell,” says Edoff. The high reliability of CIGS also gives it an advantage over some other candidates.

Inevitably, tandem cells are more expensive per unit area, and currently also per unit of energy. Therefore, they are only used in cases where efficiency is much more important than cost, such as powering satellites. However, as improvements continue across multiple technologies, tandem or even triple or quad cells could become much more widespread.

The research has been published in Nature Energy.

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