Solar light harvesting

According to University of Toronto Professor Greg Scholes, over 100 billion tons of biomass are produced annually by photosynthetic activity. To put this in perspective, Scholes said that the equivalent biomass of two (Egyptian) great pyramids is produced every day. Half of this production occurs in the ocean.

The ability for such enormous biomass production comes from high levels of coherence between chlorophyll bearing proteins in a given photosynthesizing organism. A typical light absorption reaction occurs in 350 seconds at near unity in low light conditions. To understand how awesome this level of coherence is, consider a trimer. A trimer contains 300 identical photosynthetic elements. Each of the 300 elements absorbs its own “packet” of light at the beginning of a reaction and all 300 elements synthesize their individual “packets” simultaneously in near perfect coherence.

The first question Scholes posed to the audience in his lecture was, “In 350 seconds, how is nearly perfect coherence maintained between all photosynthetic elements of an organism, especially as energy is transferred?” In other words, how has nature developed the ability to transfer solar energy so efficiently?

Scholes followed the first question with another, “Is it possible to correlate the unique signatures produced by proteins (in a given algae or bacteria) absorbing light from a specific range of the spectrum to the subsequent movement of the energy throughout that organism?”

Scholes’ explanation for the near unity observed in energy absorption and transfer is based in a quantum mechanical understanding of absorption resonance as well as an understanding of the Forster Mechanism. Operating over a greater separation distance, the dipole-dipole moments between donor and acceptor of the light are coupled electro-dynamically during the transition of solar-based energy ownership. This model is based in the Forster Mechanism where a dipole-dipole approximation is valid. Unfortunately, the model breaks down the closer the acceptor and donor are to each other.

Hence the formation of a theory based in the physics quantum energy transfer and eigenstates. On the quantum level, there is a quantum mechanism that essentially enables the solar energy absorbed to “jump” between molecular sites. This quantum electrodynamic model of photon transfer only works for the transfer of a “virtual photon.” Photon creation and annihilation incidents occur at the donor and acceptor locations. For more information on this model of solar energy transfer, refer to the paper Scholes co-authored in 2011 called “Resonance energy transfer: Beyond the limits.”

What does this mean for engineers in field of solar cell development? Being able to utilize Resonance Energy Transfer (RET), the term given to the highly coherent procedure by which solar energy is processed (on the non-quantum level), would enable highly efficient energy capture in solar cells. Increased understanding of the quantum theory of energy transfer will enable a further refinement of solar cells for an even greater increase in solar cell efficiency. Although manipulation of the quantum theory for technological development requires expensive and difficult experimentation, the work to develop a functional understanding of the theory is beneficial. The use of the quantum theory in cell refinement would enable the production of solar panels that would outperform the RET theory-based cells because of the “filters” or “screens” that are induced by the separation of donors and acceptors on the RET scale. These filters and screens make the RET process only 95% efficient, but this loss of efficiency is not present on the quantum level where the separation is unimaginably minute. The quantum energy transfer theory explains that the quantum scale processing of energy uses orbital overlap to create either simultaneous electron and hole transfer or simultaneous donor and acceptor excitation.