A Moment in the Sun for Biomimicry

To improve solar yields, for instance, one group of researchers has used a genetically-engineered virus to precisely arrange carbon nanotubes, which increased the amount of electricity the panel extracted.¹ Another team has repurposed the treatment that gives CDs their silver coatings to print circuits onto paper, broadening the range of photovoltaic materials.²

By pushing against the boundaries of existing technology, these and other advances help create a space for more solar projects. However, a highly-regarded limit still exists for the amount of energy a solar cell can convert — one that continues to challenge the field's feasibility.

This theoretical cap, known as the Shockley-Queisser limit, maintains that an ideal semiconductor can convert into electrical energy about a third of the energy it gets from a photon of sunlight. Since this maximum efficiency of 33% was reported for solar cells in the 1960s, this benchmark has dared scientists to surpass it .³

Several labs have transcended the limit by stacking solar cells,⁴ but a group of researchers in Switzerland have recently created a single-layer design that jumps past it and reaches new heights in efficiency and economy.⁵ As described in this video by epflnews, to achieve this feat, the team left the laboratory and looked outside to "our teachers in the management of light: trees."

In trials of the model, the increased exposure and vertical funneling of the standing nanowires collected up to 12 times more sunlight, and the energy produced by the prototype surpassed the Shockley-Queisser limit by almost 10%.⁶ Additionally, rethinking the structure reduced the amount of the semiconducting material, gallium arsenide, by a factor of one thousandth. The device, hence, not only converted more sunlight into energy, but it also used significantly less material to do so.

These results will impact the work of solar researchers, but they can remind all of us to pay closer attention to living systems too. With outstretched tulips and daffodils now filling flowerboxes everywhere here in Brooklyn, it seems almost obvious that standing nanowires could lift solar cells to the next level. And it makes me wonder what other biomimetic solutions we're overlooking, if solar power can be progressed this far by seeing the forest for the trees.

Credits: Image of daffodils by Flickr's maccath. Video about Anna Fontcuberta i Morral and her team's research on solar cell nanotechnology by YouTube's epflnews.

1. Dang, X., Yi, H., Ham, M., Qi, J., Yun, D., Ladewski, R., Strano, M., Hammond, P., & Belcher, A. (2011). Virus-templated self-assembled single-walled carbon nanotubes for highly efficient electron collection in photovoltaic devices. Nature Nanotechnology, 6, 377-384 DOI: 10.1038/nnano.2011.50

2. Barr, M., Rowehl, J., Lunt, R., Xu, J., Wang, A., Boyce, C., Im, S., Bulović, V., & Gleason, K. (2011). Direct monolithic integration of organic photovoltaic circuits on unmodified paper. Advanced Materials, 23, 3500-3505 DOI: 10.1002/adma.201101263

3. Shockley, W., & Queisser, H. (1961). Detailed balance limit of efficiency of p-n junction solar cells. Journal of Applied Physics, 32 DOI: 10.1063/1.1736034

4. King, R., Law, D., Edmondson, K., Fetzer, C., Kinsey, G., Yoon, H., Sherif, R., & Karam, N. (2007). 40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells. Applied Physics Letters, 90 DOI: 10.1063/1.2734507

5. Krogstrup, P., Jørgensen, H., Heiss, M., Demichel, O., Holm, J., Aagesen, M., Nygard, J., & Fontcuberta i Morral, A. (2013). Single-nanowire solar cells beyond the Shockley–Queisser limit. Nature Photonics, 7, 306-310 DOI: 10.1038/nphoton.2013.32

6. Sanctuary, H. "Nanowires Have the Power to Revolutionize Solar Energy." PhysOrg. April 8, 2013.