The black wings of a butterfly have provided a simple way to improve the light-gathering abilities of solar cells. It is the latest example of how science turns to nature to find elegant solutions to technical problems.
Solar cells are a wonderfully clean way to turn sunlight directly into electricity with no moving parts and no pollution. But since their current invention more than 60 years ago, they have suffered from inefficiency. Early versions only converted about 1 to 2 per cent of the sunlight falling upon them. That has been improved to roughly 20 per cent today, but that still means 80 per cent of the sunlight is not being used.
Part of the problem is that solar cells tend to have smooth surfaces, which act as partial mirrors, reflecting sunlight back into the sky rather than absorbing it. It was in an attempt to make the surface of solar cells less reflective, and therefore absorb more light, where the scientists turned to the butterfly.
The wings of the butterfly Pachliopta aristolochiae, or Indian Common Rose, are a very dark black. This means they absorb just about all of the light that falls on them. (Total reflection of light produces white.) Scientists from Karisruhe Institute of Technology report in the journal Science that under a powerful microscope, they saw tiny holes in the butterfly wings known as nano structures. These little structures, invisible to the naked eye, come in different sizes and shapes, something like the surface of a sponge.
The holes allow light to penetrate deeper into the wing, increasing the surface area so more light is absorbed without making the wings larger. And the different sizes of holes enable the butterflies to absorb all the colours of light that fall on them, resulting in their deep black colour.
For the butterfly, their light-absorbing wings are used for temperature control. For the scientists, they wanted to apply the same technique to solar cells, so more light could be turned into electricity.
After examining these natural structures under an electron microscope, the scientists reproduced a similar pattern onto the surface of a thin film solar cell. It is made of a modern flexible material that is usually used in smaller applications. When they shone light straight down on the surface, absorption increased by 97 per cent. That number shot up to 207 per cent when the light beamed down at an angle, important for solar panels that can't always directly face the sun.
It seems that the little holes act as caves, letting the light penetrate deeper into the surface and be absorbed along the cave walls.
This could become a tremendous step forward to improving the efficiency of this type of solar panel and making it more competitive in the alternative energy market.
Seeds inspire Velco invention
This is not the first time nature has inspired science, often leading to useful inventions. In the 1940s, Swiss inventor George de Mestral was walking his dog and noticed seeds that had firmly attached themselves to the dog's fur and his own pant legs. When he examined the seeds closely, he saw a miniature hook system that grabbed onto loops in fabric and fur. The result was one of our most useful sticky materials: Velcro.
NASA also looked to nature when it developed robots that resemble insects with legs instead of wheels to explore other planets. That's because walking is better for covering uneven terrain.
And then there is the gecko, a small lizard with the amazing ability to climb walls, cling to the ceiling and even stick to smooth surfaces such as a mirror. The secret, discovered under a microscope, is a large number of tiny hairs that increase the footpads contact area with the surface. They use what are known as van der Waals forces, which are very weak and only act at microscopic distances, but add up when surface contact is very large.
Materials scientists are very interested in gecko feet not just because they stick to almost any surface, but they also instantly let go on demand, leaving no residue, allowing the gecko to run. A tape with that ability would be very valuable, and gloves made of gecko-like material would allow people to climb walls.
It makes sense that nature can show us better ways to do things. After all, it has had millions of years to experiment and evolve the best solutions to problems such as sticking to a surface, absorbing light, flying, swimming, seeing in the dark and countless other adaptations that have allowed animals and plants to survive in different environments.
In the case of solar cells, nature has shown us how to manage energy more efficiently, which is a lesson we we need to learn in so many other systems as well.