Biomimetic photovoltaics & artificial photosynthesis

The fundamental roadblock in molecular photovoltaics lies at the nano-scale: How to build nano-structures that reconcile the short diffusion lengths of charge carriers with the long optical absorption lengths for visible light? The Graetzel cell does this with a serpentine (and random) matrix. There are, in principle, better ways to do it. But the key enabling technology is a route to cheap and reliable self-assembly of complex nanostructures. Self-assembly based on DNA scaffolds will almost certainly allow us to assemble and test complex nanoscale photonic components that have been hitherto impossible to make. We will address the use of DNA nanotechnology as a robust and economic basis for manufacturing.

DNA nanotechnology is based on using DNA ‘sticky-ends’ and 3-way junctions to make complex self-assembled nanostructures simply by annealing the components together. We have demonstrated the self-assembly of a ‘molecular pegboard’, a nanoscale array onto which molecular components can be placed at any desired location, and we have used the technology to make nano-structured arrays of metal nanoparticles. A new approach, called DNA Origami, uses a single genomic-length strand that is folded by small helper strands. Folding is so cooperative that yields of even very complex structures are essentially 100%. The arrays were folded to have DNA protrusions that spell the letters ‘ASU’. We have also made arrays in which each site is occupied by a unique single stranded ‘probe’ that acts as addressable tether point for attaching molecules and nanoparticles in any desired pattern.

A key component of these assembled systems will be artificial reaction centers which perform the initial charge separation and injection. Artificial reaction centers have been developed in our labs that reproduce many of the important properties of the much more sophisticated natural reaction centers. In particular, they approach the performance of the natural ones in terms of the quantum yield of charge separation, the fraction of photon energy conserved, and the lifetime of photoinduced charge separation.
The molecular pentad shown above is an example of an artificial photosynthetic reaction center. It consists of a porphyrin dyad (P_A-P_B) covalently linked to a carotenoid polyene (C) and a diquinone moiety (Q_A-Q_B). Excitation of the pentad with visible light is followed by photoinduced electron transfer from the C-P-¹-P-Q_A-Q_B singlet state to yield a charge-separated state C-P-P·⁺-Q_A·^--Q_B and finally C·⁺-P_A-P_B-Q_A-Q_B·^-_. This final state preserves over one half of the excitation energy as chemical energy, is formed with a quantum yield of 0.83, and has a lifetime of hundreds of microseconds. This pentad is a molecular-scale photovoltaic that mimics the process by which photosynthetic organisms harvest sunlight and convert it to electrochemical potential.