Aalto University researchers have obtained the record-breaking efficiency of 22.1 percent efficiency on nanostructured black silicon solar cells. This critical almost 4 percent absolute increase to their previous record was achieved by applying a thin passivating film on the nanostructures and by integrating all metal contacts on the back side of the cell.

Black Silicon Solar Cells Developed at Aalto University.  Image Credit: Hele Savin, Aalto University.  Click image for the largest view.

Black Silicon Solar Cells Developed at Aalto University. Image Credit: Hele Savin, Aalto University. Click image for the largest view.

Most folks simply think solar cells are a uniform item over all latitudes. Not so, as you go further north or south the sun light is lower in the sky and other matters come into play.

Professor Hele Savin from Aalto University, who coordinated the study explains, “This is an advantage particularly in the north, where the sun shines from a low angle for a large part of the year. We have demonstrated that in winter Helsinki, black cells generate considerably more electricity than traditional cells even though both cells have identical efficiency values.”

Due to the ability of black cells to capture solar radiation from low angles, they already generate more electricity over the duration of one day as compared to the traditional cells.

Savin added, “The energy conversion efficiency is not the only parameter that we should look at.”

The researchers improved their previous record by over three absolute % in cooperation with Universitat Polit├Ęcnica de Catalunya. They obtained the record-breaking efficiency of 22.1% on nanostructured silicon solar cells and the result has been certified by Fraunhofer ISE CalLab.

The thin passivating film on the nanostructures is made by the film on the nanostructures by atomic layer deposition, and by integrating all metal contacts on the back side of the cell.

The surface recombination has long been the bottleneck of black silicon solar cells and has so far limited the cell efficiencies to only modest values. The new record setting cells consist of a thick back-contacted structure that is known to be highly sensitive to the front surface recombination. The certified external quantum efficiency of 96% at the 300nm wavelength demonstrates that the increased surface recombination problem no longer exists and for the first time the black silicon is not limiting the final energy conversion efficiency.

The team’s results have been published Nature Nanotechnology.

For the near future, the goal of the team is to apply the technology to other cell structures, particularly, thin and multi-crystalline cells.

Professor Savin predicts, “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.”

The development of the cells fabricated last year will continue in the upcoming “BLACK” project, supported by the European Union, in which Professor Savin together with her team will develop the technology further in cooperation with industry.

“The surface area of the best cells in the study was already 9 cm2. This is a good starting point for upscaling the results to full wafers and all the way to the industrial scale, she added.

Snow bird solar cells are a very large market niche that is poorly addressed. As well as the low solar angle there are other concerns such as hail damage, snow loads, and the demands are quite high for power over longer nights. Imagine trying to design a solar cell that can survive a hail storm and be affordable and insurable, snow loads measured in feet and a means to clear the snow. These are challenging problems. Add to that the power demand for long nights and again, how do they get cleared after a snow, which might happen every day?

One step at a time, and allow for a lot of time. Meanwhile congratulations are in order for closing the gap with conventional cells.


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