Capturing Solar Light and Transferring Energy Efficiently

Whether an electron is powering a cell phone or a cellular organism makes little difference to the electron; it is the ultimate currency of modern society and biology, and electricity is the most versatile and relevant energy available to man.

The ability to capture light and then to transfer that energy to do work are the two main steps in a photovoltaic system. In Nature, such steps happen too fast for energy to be wasted as heat and in green plants the light energy is captured by highly effective photosynthetic complexes and then transferred with almost 100% efficiency to reaction centers, where long term energy storage is initiated.

Traditional silicon-based solar PVsystems, however, do not follow Nature’s model. In Nature, the energy transfer process involves electronic quantum coherence. Indeed, this wavelike characteristic of the energy transfer within the photosynthetic complex can explain its extreme efficiency, as it allows the complexes to sample vast areas of phase space to find the most efficient path. Two-dimensional electronic spectroscopy investigation of the bacteriochlorophyll complex has, in fact, shown direct evidence for remarkably long-lived electronic quantum coherence. The lowest-energy exciton (a bound electron–hole pair formed when an incoming photon boosts an electron out of the valence energy band into the conduction band) gives rise to a diagonal peak that clearly oscillates. Surprisingly, this quantum beating lasted the entire 660 femtoseconds, contrary to the older assumption that the electronic coherences responsible for such oscillations are rapidly destroyed.

Sunlight absorbed by bacteriochlorophyll (green)
within the FMO protein (gray) generates a wavelike motion of
excitation energy whose quantum mechanical properties can be
mapped through the use of two-dimensional electronic
spectroscopy.(Image courtesy of Greg Engel, Lawrence Berkeley
National Laboratory).

It may therefore come as no surprise that the first plastic solar cells are largely based on biomimetics, that is, on artificial photosynthesis based on human ability to gather and organize complex materials and organic molecules to replicate photosynthesis in a practical way.