Tin breakthrough may help solar panels beat their power limit

Solar panels waste most of the sunlight that hits them. Now a team of physicists from the Netherlands and Austria may have found a clue that could one day help them waste much less.

Researchers from the University of Groningen and the Vienna University of Technology (TU Wien) have discovered why a material made with tin holds on to the energy of sunlight far longer than expected. Their findings, published in the journal ACS Energy Letters, point toward a new type of solar cell that could, in theory, beat a limit scientists have accepted for decades.

The 33% efficiency limit

A standard solar cell built from a single layer of material can turn at most about a third of incoming sunlight into electricity. This cap, known as the Shockley-Queisser limit, sits at roughly 33 percent, and everyday panels fall well short of it.

Much of the loss happens in the blink of an eye. When sunlight hits a solar cell, it sets electrons in motion so they can flow as electric current. Light with a lot of energy, such as blue light, creates "hot" electrons that carry a bonus of extra energy. But these electrons cool down almost instantly, within a few trillionths of a second, and their bonus turns into useless heat before it can be collected.

Scientists have long dreamed of a "hot-carrier" solar cell that catches these electrons while they are still hot. The problem is time: there usually isn't any.

A surprising slowdown

Tin-based perovskites, a family of crystals that can be used in solar cells, seemed to break that rule. Measurements, including work by Groningen professor Maria Antonietta Loi, showed their electrons staying hot around a thousand times longer than normal, into the range of billionths of a second.

The result was so unusual that the team questioned it. "We even started to doubt the measurements ourselves," said Jan Anton Koster, a physics professor at Groningen and the study's corresponding author. Koster and PhD student Tim Faber then built detailed computer simulations to find out what was going on.

Two effects working together

The answer turned out to be a double act. The first effect works like a heat trap. As hot electrons cool, they pass their energy into tiny vibrations in the crystal. In these materials, the vibrations can hand some of that energy straight back to the electrons before it escapes.

The second effect is a kind of traffic jam. To cool down, electrons have to drop into lower energy levels. But when many electrons are excited at once, those lower levels fill up, leaving no room to fall into. Tin-based crystals are especially prone to this because their electrons behave as if they are unusually light.

Neither effect alone could explain the lab results. Only when the researchers included both did their simulations slow the cooling more than a hundredfold, close to what had been measured in experiments.

Not there yet

The team is clear that nobody has built a working hot-carrier solar cell yet. Engineers still need ways to pull the hot electrons out quickly and turn their extra energy into electricity. The researchers have, however, drawn up a recipe for future materials: electrons that behave as if they are light, a soft and flexible crystal structure, and very few impurities, since defects speed up energy loss.

Tin has another appeal. Many of today's best-performing perovskites contain lead, which is toxic, so tin offers a less harmful alternative.

"The measurements were clear, but we didn't understand the physics behind this," Koster said. Now they do, and that understanding could be a first step toward solar panels that squeeze more power out of every ray of sunlight.