Black Silicon Solar Cells Smash Efficiency Record

Scientists in Finland have achieved record-breaking 22.1% photovoltaic efficiency using a nanostructured material known as black silicon. The efficiency of a solar cell is the ratio between the electrical output and incident solar energy input. In a field known for progressing in minute steps, this result, which is a full 4% higher than any previous black silicon cell is particularly remarkable and brings this new photovoltaic material very suddenly into the mainstream. The work was published this week in Nature Nanotechnology.

Best solar cell efficiencies: chart created at the National Renewable Energy Laboratory (NREL), Golden, CO.Best solar cell efficiencies: chart created at the National Renewable Energy Laboratory (NREL), Golden, CO.

Black silicon is the term for a traditional silicon surface that has been functionalized by a variety of possible means with silicon nanostructures (cones, tubes, wires, pores, particles). The structures are created such that their length-scales are smaller than the wavelength of incoming light. The resultant surface absorbs light at nearly all wavelengths making it appear black and lending it its moniker.

Black silicon: the color of this solar cell is indicative of its ability to absorb 99% of incident light making it a highly efficient photovoltaic. Image credit: Aalto University.Black silicon: the color of this solar cell is indicative of its ability to absorb 99% of incident light making it a highly efficient photovoltaic. Image credit: Aalto University.

Aside from a broad light absorption spectrum, black silicon has two other major advantages. First, unlike traditional solar cells, it does not require an antireflective coating which is typically a significant contributor to manufacturing costs. Second, because it is not a flat surface, it absorbs more light from a variety of incident angles. This is particularly relevant when you consider that the research was conducted in Finland, regions of which see only low angle sun for large stretches of the year and have so far been considered unsuitable for solar power generation. This second benefit also demonstrates the importance of considering more than just efficiency measurements when analyzing the value of a technology. Even with power conversion equivalent to traditional cells in a lab, the black silicon cells will generate more electricity in a given day. So why isn’t this material already regularly used in solar cells?

Of course, with the good always comes the bad and black silicon is no exception. The greater surface area of this material means that charges generated by the light are more likely to hit a surface feature before they hit an electrode to yield useable electric current, a problem termed surface recombination. This downfall has always limited black silicon cell efficiencies to well below 20 % despite its obvious potential. The newly reported breakthrough was to passivate the surface with an extremely thin (20 nm) layer of alumina. Aside from minimizing the issue of surface recombination, the alumina coating was found to even further reduce the already very low reflectivity of the material to approximately 1% in the visible region of the spectrum producing the record-smashing 22% efficiency.

The standard bearer: traditional silicon cells like this one may no longer be the best option in the near future. Image from the US Dept. of Energy.The standard bearer: traditional silicon cells like this one may no longer be the best option in the near future. Image from the US Dept. of Energy.

 

The researchers note in their publication that 22% is only the beginning as this particular result was primarily a proof-of-concept experiment that did not seek to optimize all factors. Relatively easy adjustments like changing the type of silicon used or the structure of the electrodes could boost the efficiency above 23% shortly. This is approaching that of the very best traditional silicon solar cells (25%) without even taking into account the wider angle absorption of the black silicon. This remarkable research could be a true game changer in the solar cell market, finally unseating single-crystalline silicon from dominance and expanding the use of solar energy to higher latitudes.