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Finnish researchers achieve black silicon solar cell efficiency of 22.1%

19 May 2015

Aalto University researchers have beaten their previous record for the energy conversion of black silicon solar cells by over three absolute percentage points.

The surface area of the best cells in the study is nine square centimetres. The researchers believe this is a good starting point for up-scaling the results to full wafers, and then all the way to industrial scale panels (image: Aalto University)

The researchers obtained the record-breaking efficiency of 22.1 percent on nanostructured silicon solar cells (a result certified by Fraunhofer ISE CalLab). This is an almost 4 percent absolute increase on their previous record, and was achieved by applying a thin passivating film on the nanostructures via a process of atomic layer deposition, and by integrating all metal contacts on the back side of the cell.

Due to its structure, black silicon is a very good absorber of visible and infra-red light, but the surface recombination has long been a bottleneck in the production of black silicon solar cells and has so far limited their efficiencies to only modest values. The new record-breaking 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 percent at 300nm wavelength demonstrates that the increased surface recombination problem no longer exists and for the first time, black silicon is not limiting the final energy conversion efficiency.

Energy conversion efficiency, however, is not the only parameter that should be examined, according to Professor Hele Savin from Aalto University, who coordinated the study. Due to the ability of black cells to capture solar radiation from low angles, they generate more electricity in a single day when compared with traditional cells.

The team has demonstrated that in winter in Helsinki, black cells generate considerably more electricity than traditional cells, even if both have similar conversion efficiencies. This is an advantage, particularly in northern climes, where sunlight is at low incident angles for a large part of the year.

The team is hoping to apply the technology to other cell structures – in particular, thin and multi-crystalline cells. The record-breaking cells were fabricated using p-type silicon, which is known to suffer from impurity-related degradation. Hele Savin believes there is no reason why even higher efficiencies could not be reached using n-type silicon or more advanced cell structures.

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