The cost of silicon contributes a significant portion of the overall price of solar cells, and manufacturers have a strong incentive to make devices that use ever thinner layers of the material. Unfortunately, silicon is not a strong absorber of light – especially at long wavelengths – which means that photons can pass through silicon films that are too thin before their energy can be deposited in the cell.

Patterning the surface of the silicon can improve how the cell absorbs and traps light, but etching unavoidably wastes some valuable material. Furthermore, the increased surface area results in greater energy loss through surface recombination of charge carriers – already significant for such thin films. Even the texturing process itself affects the electronic performance through the damage that it causes to the top of the cell.

Writing in Nano Futures, a research team representing the international PhotoNVoltaics project, and led by Valerie Depauw of imec in Leuven, Belgium, reports an approach that seeks to balance these competing factors. Depauw explains the motivation for the work: "We strived to enable the development of a new and disruptive solar-cell generation, resulting from the marriage of crystalline-silicon photovoltaics with advanced light-trapping schemes from the field of nanophotonics. The core of this project was to bring together researchers from the different fields that were needed (photovoltaics, photonics, nanostructuring) to interact directly."

The group started with a micron-thick silicon film that they separated from a monocrystalline wafer. Conventional methods of slicing thin sections from bulk silicon result in a loss of material from the cut – termed "kerf loss". Depauw and colleagues instead used the kerfless "empty-space-in-silicon technique", which makes more efficient use of the raw material. This process involves the etching of an array of narrow pits on the silicon which, upon annealing, form a single planar void beneath the surface. The thin layer remaining above the void can undergo passivation and metallization before being bonded to a substrate and removed.

Nanopatterns in close-up

The researchers knew that surface nanopatterns in which the periodicity is disrupted can improve silicon’s light-absorption properties as compared to perfectly periodic structures. They therefore imprinted a nanotexture by reactive ion etching (RIE) after shaping the mask on a self-assembled layer of charged polystyrene beads. The result was an imperfectly periodic pattern of gently sloping, rounded nanocups. Compared to the microscale structures more commonly used on solar cells, the new technique represents a more efficient use of material, with far less silicon discarded during the patterning process.

Although surfaces of nanowires or nanocolumns have been shown previously to be even more effective absorbers of incident light, the hugely increased surface area leads to more surface recombination losses when used in solar cells. High-quality passivation layers and antireflective coatings are also difficult to apply to such surfaces, lowering the overall efficiency. The method and pattern used by Depauw and colleagues represent the best compromise between these conflicting needs, and yielded an overall conversion efficiency higher than any yet seen for a silicon film of such thinness.

A complete solar cell based on this nanophotonic structure, and using kerfless fabrication methods, could be as thin as desired, finding uses not available to conventional devices. "The main applications of our thin c-Si solar cells," says Depauw, "could be in buildings as windows and skylights, where they will bring more freedom for integration, and possibly lighter and thinner module designs".

Having optimized the pattern of the nanotexture, the team plans to focus next on squeezing further gains from the cell design and the texturing process. Given discrepancies between the electronic characteristics of patterned and unpatterned silicon films, the researchers propose that the etching procedure itself can affect the performance of the cell. "For dry nanotexturing processes such as RIE, there are four main causes of damage," explains Depauw. "These are: surface roughening; ion bombardment with a penetration of a few nanometres to tens of nanometers; ultraviolet radiation; and residual etching precursor on the surface. Wet etching by an alkaline solution avoids such effects and allows higher-quality surface passivation."