Photovoltaics, which convert sunlight into electricity, have long been touted as one of the most promising solutions to our energy needs. Unfortunately, today's devices reflect a lot of solar energy as heat, which means that solar power is currently not as cheap as other forms of energy. Now, however, researchers in the Netherlands have developed an anti-reflective coating based on the nanostructure of a moth’s eyes, which could reduce the reflection from photovoltaic cells and thereby make them more efficient.

Jaime Gomez Rivas and colleagues at the AMOLF institute in Eindhoven say that their “moth-eye” technology is superior to other known anti-reflection measures. Additionally, they have developed a new eco-friendly production technique that can apply the coating with high precision.

Attracted to the light

Everyone knows that moths seek light in the dark, but it seems that they really can’t get enough of those rays. To maximize the amount of light entering their eyes, to help them see at night, the insects’ eyes are covered in tapered nanostructures. This creates an “effective medium” where the refractive index gradually increases as light travels from air through to the insects’ optical nerve. The resulting effective index is graded from close to one at the top to close to 3.4 at the bottom, which means that very little light is reflected out of the eye.

Inspired by these biostructures, Gomez Rivas and colleagues have mimicked the effect by growing nanowires of different lengths, creating a metamaterial with optical properties that change gradually as a function of distance. The group has been reporting, over the past few years, a significant reduction in reflection over a broad range of colours and angles of incidence. Until now, however, it has not been clear whether this effect is the result of increased light transmission, or just scattering and absorption in the nanowires.

This time, the team used gallium phosphide (GaP) nanorods on top of a GaP substrate, then measured reflection and transmission simultaneously (Adv. Mater.:2009.21.1 ). “We showed for the first time that light transmission was dominant, with only a minor part of the [reduced] reflection linked with scattering losses and absorption” said Rivas.

Green coat

Early versions of a “graded index” nanostructure have been created from etching a silicon substrate with different kinds of nanostructure. A major drawback of this approach, however, is that the antireflection layer is also absorbing. Two years ago, the first “bottom up” graded index nanostructure was produced by combining silicon dioxide with titanium dioxide. Now, this new research offers a significantly more energy efficient approach, by creating a graded index in a single material.

“The idea of slowly changing the impedance of air to that of the material is well known, but the method by which they do it is rather elegant,” said Pete Vukusic, who studies the optical properties of natural materials at the University of Exeter, UK.

Rivas told that his team’s long-term goal is to turn this into a product but, for the next few years, they will continue to seek even lower reflection. “In theory we could get 99% transmission; the difficulty is that our nanowires will need to get longer and thicker and then scattering come into play,” he said.

Francisco J Garcia-Vidal, an optics researcher at the Autonomous University of Madrid, said, “This bottom-up technique will very flexible because, allowing us to choose almost independently the material for the nanowires and the substrate.”