Nano-scale materials are thought to provide new building blocks that may be key to more efficient solar-to-power and solar-to-fuel energy conversion. Biological photosynthetic systems can provide some important “bioinspiration” with regard to their design. With respect to the former photovoltaic applications, this concerns both unidirectional electric charge separation and subsequent unidirectional charge transport over large nano-scale distances, the latter being key to minimizing competitive charge recombination.
Nature accomplishes this via a complex macromolecular structure that precisely controls the orientational and positional ordering and local environments of multiple, spatially separated electron donor and acceptor cofactors. Unfortunately, the photosynthetic protein scaffolds responsible for this detailed 3-D organization of separated cofactors are not particularly robust from a materials perspective, although there is promise that their stability may be suitably enhanced by their incorporation into non-biological matrices.
An alternative approach involves the design of non-biological electron “donor-bridge-acceptor” (D-br-A) cofactors accomplishing the light-induced electron transfer process key to electric charge separation “through bonds” instead of “through space.” There are several challenges associated with this approach, namely gaining control over the conformation of the spatially extended cofactor to ensure efficient charge separation, stabilization of the charge separated state, organizing these in 2-D or 3-D ensembles to produce unidirectional electric charge separation over nano-scale distances, and the effective extraction of the generated electron-hole pair.