New record efficiency for black silicon solar cells

Solar cells built with black silicon are much more light-absorbent, and can capture incident photons from very low angles(Credit: Aalto University)

Researchers at the University of Aalto, Finland have broken the efficiency record for black silicon solar cells a type of cell that can gather sunlight even from tight angles by almost four percent.

Black silicon can be manufactured simply by adding a dense network of nanoscale needles on top of a standard piece of silicon. Modifying the material in this way makes it a lot less reflective, allowing solar cells that use it to trap light even when it's coming from very low angles. This could be a good way to increase the yield of solar cells throughout the day, particularly in countries at higher latitudes. On top of this, black silicon cells could also be cheaper, as they don't need the antireflection coatings used by many other types of solar cells.

The main issue that has stifled the progress of black silicon cells is something known as carrier recombination. When a photon hits a silicon atom inside a solar cell, the excess energy frees up an electron that is later used to generate electricity. Occasionally, though, the electron simply recombines with a silicon atom, effectively wasting the energy provided by the photon. Recombination is proportional to the surface area of the silicon, and the needles on the surface of dark silicon raise surface area so much that about half of the freed electrons are "lost" in this way.

Now, a team of researchers led by assistant professor Hele Savin has managed to get around the issue, and in so doing, it has increased the record efficiency of black silicon cells by almost four percentage points, up to 22.1 percent. Their real-life performance is however better than that as the researchers were also able to show that, thanks to their ability to accept sunlight from lower angles, black silicon cells can gather three percent more energy than a cell with the same nominal efficiency over the course of the entire day.

Savin and colleagues put recombination in check by applying a thin aluminum film, acting like a chemical and electronic shield, on top of the nanostructures. They also integrated all the metal contacts on the back side of the cell, for added absorption.

These two changes meant that only four percent of the freed up electrons recombined, as opposed to the previous 50 percent. The new cell design is however likely not pushing this technology to its limit just yet, since it made use of p-type silicon rather than the more durable n-type silicon. According to the scientists, a better choice of materials or a better cell structure would push efficiencies even further.

The near-term goal for the researchers is to apply their technology to other cell structures, thin and multi-crystalline cells in particular – but also, Savin tells us, other devices like screens and photodetectors.

The results appear in this week's edition of the journal Nature Technology.

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