| 27 January 2013
Quantum scale photosynthesis in biological systems that inhabit extreme low-light environments could hold the key to more efficient solar cell designs. Recent research work at the University of Cambridge, United Kingdom, aimed at providing a better understanding of the surprisingly long quantum coherence times between the exciton states that carry energy in pigment-protein complexes (PPCs), the?biomolecular?structures that perform light absorption, energy transport and charge?separation?in natural photosynthetic organisms.
While scientists knew the quantum wave-like dynamics are present during energy?transport?over a much longer time than previously assumed, it was not well understood why these quantum dynamics are robust. ?Finding mechanisms that support quantum effects at room temperature in?what is effectively a functional device is an interesting problem from the viewpoint of future technologies that harness non-classical properties of light and matter,? says Dr Alex Chin, advanced research fellow in the Winton Programme for the Physics of Sustainability at the University of Cambridge.
By building and simulating a sophisticated model of how excitons move in the presence of the vibrations of the pigments and protein, Chin?s team was able to show that strong coupling to particular vibrations in the molecular environment can stabilise quantum dynamics against "decoherence", the normally unavoidable and continuous degradation of quantum effects due to noise of the fluctuating surroundings. ?Our results showed that particular vibrations can be excited by the?passage?of the excitons, driving them out of?equilibrium,?which can?transfer?their excitation energy back to the excitons at later times,? Chin reports. ?The way they do this actually creates new coherence in the excitons, effectively extending?the lifetime of their coherences.?
The Cambridge researchers learned that these interactions with specific vibrations lead to a partially reversible exchange of energy between excitons and the environment as they move ? the?environmental interactions are not completely?dissipative. ?This means that when the excitons arrive at their final positions, not all of the energy expended in reaching that point is lost to the exciton, some is stored in vibrational states and may be used to drive further processes, such as charge?separation,? Chin says. These results?could suggest?that the PPC complexes,?perhaps as a result of evolution, have a mechanical (i.e. vibrational) structure that is matched to the properties of the excitons to produce?efficient, "hot"?energy transport of optical energy.
This newfound knowledge could now deliver clues about efficient, long-range transport of excitons in solar cells. ?In many artificial devices, this can be impeded by poor exciton diffusion, trapping, etc.? Chin says.??Moreover, the idea of exciton delivery in vibrationally excited ? or ?hot? ? states has been proposed as a way to increasing the harvesting of?energy?from sunlight, as this prevents energy losses that arise from exciton cooling if charge extraction can take place from these non-equilibrium states.? The mechanisms in Chin?s work suggest that strong coupling and a tuning of excitonic energy levels to sharp?vibrational modes can drive these phenomena. ?This design principle might be?possible?to explore in organic?solar?cell materials,?whose?molecular and optical properties are often modelled in a similar way to PPCs,? suggests Chin, who also is the author of the paper ?The role of non-equilibrium vibrational structures in electronic coherence and recoherence in pigment?protein complexes? published in Nature Physics.
Chin says the most unexpected finding was the rich, dynamical interplay between excitons?and?specific vibrations that leads?to?the support and spontaneous generation of coherence amongst the excitons. ?From a quantum physics perspective, this was a new type of?mechanism leading to a complicated energy?dynamics?that falls outside of our usual notions of?dissipative?transport.?That these, relatively unexplored, dynamics might be advantageous for?energy?efficiency or some other?processes?is very exciting.?
Going forward, Chin intends to explore how these dynamics might appear in other, yet more complex, processes in photosynthetic complexes, particularly in the act of charge separation, central to solar cell operation.??With the exciting new, non-equilibrium physics that seem to operate in these systems, we hope to isolate and establish the key conceptual features of excitonic and charge dynamics at the boundary of quantum and classical physics, which might be profitably exploited in future technologies,? the scientist concludes.
Written by?Sandra Henderson, Research Editor,?Solar Novus Today
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