Back in black: record efficiency for black-silicon solar cells

Crystalline silicon solar cells have been around for a long time, and their blue sheen now shines anywhere from residential homes to airport rooftops. Their design and manufacturing methods have matured greatly since their first appearance in 1954¹ such that most experts do not envision more dramatic improvements to their current efficiency levels around 20-25%².

This puts the pressure on new photovoltaic designs to reach similar efficiency levels with lower manufacturing costs in order to push the price of solar energy closer to parity with fossil fuels. A collaboration between scientists in Finland and Spain has taken a great step in this direction by developing a method to construct a black silicon solar cell with record 22.1% efficiency³. This darker, strongly light-absorbing cousin of the crystalline solar cell should be cheaper to manufacture and is now efficient enough to possibly compete commercially.

Black silicon has long been known to have absorption advantages over traditional solar cells. An orderly lattice of silicon atoms make up crystalline silicon, giving the material a smooth and reflective surface that reduces the amount of absorbed light. Antireflective coatings such as silicon dioxide or titanium oxide must be deposited on top to augment the absorption to adequate levels. This increases manufacturing costs and pushes the all-important cost per kilowatt-hour upward.

In contrast, scientists create black silicon by bombarding the surface of a smooth silicon crystal with high-energy laser pulses that essentially transform the mirror-like surface into a jumble of nanostructures - rods, cones, and spikes on the nanometer scales - that almost look like a jumble of trees or pillars, as seen below.

This disordered, unruly shape is both the material's boon and bane. Since materials absorb light best when photons arrive perpendicular to the surface, crystalline silicon only absorbs light well when the Sun is at its peak in the sky. In contrast, black silicon has an undulating surface pointing in many directions, allowing it to absorb more light at larger angles. In addition, any reflected light has a good chance of running into another nanostructure and being absorbed before leaving the material. This is a huge benefit - a static black-silicon solar cell on a rooftop can absorb light more efficiently throughout the day as the Sun scans across the sky. In addition, manufacturing of black silicon cells is cheaper because the antireflective coating is no longer needed to minimize reflectance.

The improved absorption from the nanostructured shape comes at a price. Such a disordered surface has many dangling bonds that trap electrons, which consequently recombine with holes (the positively charged particle left behind by a photoexcited electron). This type of recombination increases with surface area, which is very large in black silicon with its many pillars and crevasses. Each electron trapped at the surface that recombines with a hole cannot escape to the external circuit and produce energy, greatly limiting the efficiency of black silicon solar cells.

Savin and colleagues have finally found a solution. Knowing that dangling bonds are the source of the trapping problem, they deposited a thin film of aluminum oxide on top of the black silicon. This oxide is an insulator that does not affect the absorption properties of the material but bonds to the silicon at their interface to saturate the dangling bonds. This process, known as passivation, removes the majority of potential electron trapping sites and increases electron lifetimes to the millisecond range typical in crystalline solar cells. This timescale gives them ample opportunity to diffuse from the silicon into the connected circuit and provide electrical power with record 22.1% efficiency.

The new design has multiple benefits: improved efficiency by reducing recombination, but also increased daily energy production by 3% due to better absorption of low-angle solar rays. This should be especially beneficial for energy production at high latitudes, as well as in the morning or evening,s when the Sun is close to the horizon. Such a small increment in production may not seem dramatic, but these small gains can cause substantial economic shifts since solar cells are so close to being competitive with fossil fuels.

While many other types of solar cells are also being researched, this is a rare example of research from the lab that could quickly have an impact on commercial development. Findings such as these always give a burst of hope that innovation will continue to lower prices and solar cells will soon provide a dominant source of power for our energy-hungry civilization.

References

Chapin DM et al. "A new silicon p-n junction photocell for converting solar radiation into electrical power." Journal of Applied Physics, 25(5), 676-677 (1954).
Saga T. "Advances in crystalline silicon solar cell technology for industrial mass production." NPG Asia Materials, 2, 96-102 (2010).
Savin H. et al. "Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency". Nature Nanotechnology, published online May 18, 2015.