This cloudy-day black silicon solar cell can hit a record 22.1% efficiency

When most people think of solar power, they’re usually thinking of warmer climates with lots of sun. Despite all the huge advances solar has made in recent years, the fact remains that the cells perform best when there’s plenty of sunlight. That may no longer be necessary. Researchers at Finland’s Aalto University have achieved a record-breaking 22.1% efficiency for a nanostructured silicon, or black, solar cell. They accomplished this by
overlaying a thin, passivating film on the nanostructures by a process known as atomic layer deposition, and by integrating all of the metal contacts on the cell’s back side.

Perhaps the best part: Black solar cells work really well on cloudy days. “This is an advantage particularly in the north, where the sun shines from a low angle for a large part of the year,” said professor Hele Savin from Aalto University, who coordinated the study, in a statement. “We have demonstrated that in winter Helsinki, black cells generate considerably more electricity than traditional cells, even though both cells have identical efficiency values.”

Using the aforementioned process, the team managed to beat their previous record by almost 4%, which is a stunning achievement. The new cells have a certified external quantumefficiency of 96% at 300nm wavelengths, which the team said shows that charged carrier surface recombination is no longer a problem — and that for the first time, the black silicon isn’t limiting energy conversion efficiency. And thanks to the inherent properties of black solar cells, they can capture solar radiation at low angles, generating more electricity over the full duration of a day as compared with traditional cells.

“Our record cells were fabricated using p-type silicon, which is known to suffer from impurity-related degradation. There is no reason why even higher efficiencies could not be reached using n-type silicon or more advanced cell structures,” Savin said. Next, the team will take the same process and apply it to other kinds of cell structures, such as thin and multi-crystalline cells. They also want to try fabricating cells using n-type silicon instead of p-type silicon. The “best” cells used in this example were 3×3 centimeters in size; later examples could be upscaled to full wafers on to an industrial scale, the team said.

The data shows the use of black silicon can result in a 3% increase in daily energy production when compared with a reference cell with the same efficiency, due to its better angular acceptance, the researchers said. The Aalto University’s team’s results were published in Nature Nanotechnology.

Back in August 2014, researchers at Michigan State University developed a fully transparent solar cell that improved on existing translucent designs, although that cell is capped at roughly 1% efficiency for now.