Jan 24, 2011
“Solar cell”. The first thing that comes to mind is a solid slab of over-priced silicon that we slap on our roofs to help offset our energy costs, write off some of our taxes, and help us sleep at night knowing we are doing our part to realize a greener world. To most of us, this represents the latest in cutting-edge technology; both in our evolved outlook on finding a more sustainable energy source and in our ability to process silicon precisely and efficiently on a high-throughput scale.
These heavily processed circuits are a far cry from what I expected a solar cell to be back in my middle school days when I first learned about plant biology and photosynthesis. In fact, I think most people today still half-expect solar cells to do some sort of variation of what plants do when they harvest energy from the sun. The truth is that these solar cells have little, if any, similarity, to plants when it comes to capturing solar energy. Not only do plants harness energy, but they use it to grow, reproduce, and repair themselves when they are damaged by the sun’s bombarding rays [1-4]. So why haven’t we developed a solar cell that can sustain itself; something that could automatically heal itself when it becomes damaged?
In a recent study  led by Professor Michael S. Strano, Charles and Hilda Roddey Professor of Chemical Engineering, researchers developed the first synthetic solar device capable of mimicking the self-repair process used by plants during photosynthesis. Unlike conventional, static solar cells, this dynamic solar cell is capable of spontaneously assembling and disassembling itself simply through the removal and addition of soap, or surfactant, molecules. Specifically, a mixture containing an assortment of molecules (Figure 1, left), including nanotubes, light-harnessing proteins contained in plants, and soap molecules, is treated to selectively remove the soap. Upon removal of the surfactant, the remaining components spontaneously self-assemble into the light-harnessing complex shown in Figure 1 (right). This self-assembly process is completely reversible in that the re-addition of the surfactant to the mixture once more disassembles the photoactive complex . Like the process used in plants, this reversible assembly process requires no additional energy, as the assembly and disassembly processes occur spontaneously. The system is only photoactive when the complex is assembled, exhibiting a per-complex efficiency of 40%.
The question remains as to how this dynamically assembled solar cell could be used to create a device capable of regenerating itself. To address this, researchers developed a prototype  in which the assembled complex is continuously illuminated to produce energy. Over time, the proteins in the solar cell become damaged, and the photo-efficiency begins to decrease. When the efficiency decreases to ~25% of its initial value, a surfactant-containing solution is introduced to disassemble the complex as the damaged proteins are replaced with new, functional proteins. Upon introduction of the new protein, surfactant is once more removed from the system as the remaining components re-assemble into a fully functional, photoactive complex. Using this regeneration process, the research group was able to increase solar cell efficiency by over 300% over 168 hours of illumination. The most recent efforts in the lab include increasing overall cell efficiencies by increasing the density of these complexes in solution and automating the assembly-disassembly process.
Though the development of a dynamic, regenerative solar cell may be far from the sci-fi, self-replicating machinery we’d like to see on our rooftops, it is nonetheless a step in the right direction. And how do we know if this is, in fact, the right direction? Is mimicking plants really the way to go? The truth is that no one can say for sure, but I figure that after billions of years of evolution, nature probably has a thing or two to teach us.
(1) Aro, E. M.; Virgin, I.; Andersson, B. Biochimica Et Biophysica Acta 1993, 1143, 113-134.
(2) Melis, A.; Nemson, J. A.; Harrison, M. A. Biochimica Et Biophysica Acta 1992, 1100, 312-320.
(3) Aro, E. M.; Kettunen, R.; Tyystjarvi, E. Febs Lett 1992, 297, 29-33.
(4) Melis, A. Trends Plant Sci. 1999, 4, 130-135.
(5) Ham, M. H.; Choi, J. H.; Boghossian, A. A.; Jeng, E. S.; Graff, R. A.; Heller, D. A.; Chang, A. C.; Mattis, A.; Bayburt, T. H.; Grinkova, Y. V.; Zeiger, A. S.; Van Vliet, K. J.; Hobbie, E. K.; Sligar, S. G.; Wraight, C. A.; Strano, M. S. Nat. Chem. 2010, 2, 929-936.
(6) Boghossian, A. A., Choi, J.H, Ham, M.H., Strano, M.S. Accepted in Langmuir 2010.