Mass production of non-reflective polymer surfaces (nanofabrication) yields more efficiency in solar energy

A key hurdle in realizing high-efficiency, cost-effective solar energy technology is the low efficiency of current power cells. In order to achieve maximum efficiency when converting solar power into electricity, ideally there is a need for solar panel that can absorb nearly every single photon of light across the entire spectrum of sunlight and irrespective of the sun's position in the sky.

One way to achieve suppression of sunlight's reflection over a broad spectral range is by using nanotextured surfaces that form a graded transition of the refractive index from air to the substrate. Researchers in Finland have now demonstrated a scalable, high-throughput fabrication method for such non-reflecting nanostructured surfaces.

The main advances of this work are in the field of nanofabrication. It was published in a recent paper in Advanced Materials ("Non-Reflecting Silicon and Polymer Surfaces by Plasma Etching and Replication").

The process involves a maskless deep reactive ion etching process that produces nanospikes on a silicon wafer. The process is known as black silicon process. The geometry of the nanospikes i.e. height, width and also the density can be controlled by changing the etching parameters. The main strength of the maskless method is its high throughput.

Different applications require different types of surfaces, and in this study the Finnish team shows that the densest arrays of nanospikes with slightly positively tapered sidewalls had the lowest optical reflectance, while pyramid-shaped nanospikes were ideal for use as templates for polymer replication. Polymer replication techniques are typically high-throughput and low-cost methods which make them very attractive.

In this research it has been shown that both hot-embossing and UV-embossing of polymer is possible with the PDMS stamp. The use of polymers instead of silicon would be useful in high-volume applications due to lower costs. Nanospike-structured polymeric and silicon surfaces are non-reflective and additionally they can be made ultrahydrophobic and self-cleaning, by coating them with a low-surface energy coating. These kinds of inexpensive, non-reflective and self-cleaning surfaces have many applications, for instance in sensors and solar cells.

Another important issue is the mechanical durability of the nanostructured surfaces. At the moment the nanostructured surfaces damage quite easily but the team is studying ways to make the surfaces more robust.

To do this, first, an elastomeric stamp is produced by casting a PDMS layer on top of the nanospike-structured silicon surface (the original nanospikes were fabricated on full silicon wafers using the black silicon process). The PDMS is thermally cured and peeled off. Then, the PDMS stamp can be used to replicate the original nanospike pattern into other polymers, such as PMMA.