While the exact future of energy production is unclear, it's obvious that the role of solar power will continue to grow. Achieving higher efficiencies for each panel would help drop costs further, but requires capturing more photons from across the electromagnetic spectrum. To this end, researchers have been developing silicon-based nanostructures—layered materials consisting of networks of microscopic wafers, pores, and wires forming an artificial lattice.

These nanostructured solar cells can also be incorporated into flexible devices, which conventional photovoltaics cannot. Additionally, nanostructured photocells don't require an antireflective coating, unlike their conventional cousins. The only downside is that their overall efficiencies have lagged expectations.

Researchers at the National Renewable Energy Laboratory (NREL) determined that the pores in nanostructured photocells have kept them from matching their more conventional brethren. By addressing the problem, Jihun Oh, Hao-Chih Yuan, and Howard M. Branz fabricated a solar cell with 18.2 percent efficiency, comparable to the 21 percent efficiency achievable by the best wafer silicon solar panels. Even more important than the particular design, this research points toward a general method of improvement, which potentially could lead to much higher efficiencies.

The internal structure of most photocells involves photodiodes: semiconductor devices in which incoming photons induce the flow of electric current. Diodes in general are made from two materials: p-type, in which the effective charge carriers are positively charged "holes" where an electron would normally reside; and n-type, where the charge carriers are negatively charged. The junction between p and n materials is structured to ensure current flows in one direction.

Efficiencies in solar cells depend on both the amount of light absorbed and the efficiency at which it's converted into electricity. This latter factor is strongly influenced by recombination—the process wherein holes and negative charges join together within the cell, ending the current flow.

Nanostructures make things both better and worse. Compared to conventional photocells, they have an increased light collection area. This is accomplished by a set of pores with diameters less than the wavelength of visible light. Light coming in impinges on the sides of the pores, exposing photodiodes embedded within the material. The size of the pores also reduces reflection, meaning antireflective coatings may not be necessary.

But nanostructured photovoltaic cells also provide more opportunities for recombination, both by increasing the surface area and because the pores reach into regions where charge carriers in the interior can interact.

By separating the contribution by surface and interior recombination effects, the NREL study found that Auger recombination—recombination by these interior charges—was actually more damaging to photocell efficiency. In other words, the very pores that offered advantages also led to problems, which was why nanostructured photovoltaics haven't lived up to their promises thus far.

The researchers found that etching the silicon material with tetramethylammonium hydroxide (TMAH) greatly increased the efficiency of the photocell. The result was shallower, slightly wider, and more irregularly shaped pores. Additionally, this process suppressed Auger recombination by controlling the number of excess charge carriers away from the surface. Together, these changes reduced recombination while keeping the effective surface area large. As a result, they achieved 18.2 percent efficiency, comparable to conventional solar cells.

The researchers also noted that they were getting more light reflected than expected compared to non-TMAH-etched silicon. They speculated that correcting this problem could immediately result in greater than 20 percent efficiency. However, even with the excess reflection, they already had corrected a major difficulty with nanostructured photovoltaic cells, a necessary step toward efficient, thin, and flexible solar panels.

Nature Nanotechnology, 2012. DOI: 10.1038/nnano.2012.166 (About DOIs).