|Fig. 1: The compound eye of a moth has a highly antireflective structure that maximizes solar absorption.(Source: Wikimedia Commons)|
The sun provides nearly 105 TW of power which is significantly higher than what the entire world population needs in a year.  Sunlight has been the main source energy for bacteria, algae and plants for billions of years throughout evolution. Among organisms that have nearly perfect efficiency in absorbing light and converting it to energy is the Green Sulfur bacteria.  On the other hand, the most efficient man made solar panel, made of single crystal Silicon cells, has efficiency of only 44.7% with the conventional solar panels having efficiencies much lower than that.  The Green Sulfur bacteria proved to be capable of exploiting light intensities as low as 0.015 μmol quanta m-2 s-1 for photosynthetic and CO2 fixation purposes.  Therefore, understanding and translating the solar harvesting mechanisms such organisms use could enable us to make more efficient solar technologies.
Bioinspired solar technologies have been using inspirations from diverse sets of optical phenomenon manipulated by insects and plants. Compound eyes and wings of moth have been investigated for inspiration for antireflective structures that reduce loss from reflectance.The compound eyes of moths, shown in Fig. 1, are structured in a way that the reflection of light is significantly inhibited hence maximizing solar absorption. Synthetic forms of these structures have also been developed. Compared to bare solar cells, the synthetic moth eye coated panels reportedly showed 33% improvement in efficiency. 
|Fig. 2: The Papilio nephelus Boisduval butterfly has wing scale architecture that enables efficient light absorption. (Source: Wikimedia Commons)|
Dye sensitized solar cells have also been inspired by photosynthetic systems in nature. These systems include biophotonic crystals and molecular antennas. Biophotonic crystals are highly ordered crystalline structure resulting in bright colors. Where as, molecular antennas are organells responsible for the initial photoexcitation process in photosynthesis.  For this purpose, opal crystals have been found to be relatively easy to fabricate via self assembly and these photonics crystals reportedly increased efficiency by 50% between 500 and 600 nm wavelengths. 
Researchers have also used the wing structure of the Papilio nephelus Boisduval butterfly as a template to grow hierarchically structured photocatalysis with high solar absorption capability. This butterfly as shown in Fig. 2, gets its dark color from inverse V-type ridges and hole arrays having significant photo-absorbing capability. This work showed the capability of using such structures in the realm of artificial photosynthesis and solar fuel application. 
In summary, nature had billions of years experience in harvesting the solar energy for sustenance of life. Several of the highly efficient solar harvesting organisms have evolved structures that could serve as inspiration for making more efficient solar cells. Researchers have been able to use some of these and shown promising results however, more work is needed both in understanding the biological system such as the Green Sulfur bacteria and optimizing the biomimetic/bioinspired fabrication process.
© Loza Tadesse. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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