Discoveries Featured

Nature’s Approach to Solar Power

By: Carol A. Rouzer, VICB Communications
Published: March 3, 2010

Novel photovoltaic cells exploit light-harvesting complexes from photosynthetic plants.

The development of efficient, affordable, and non-polluting sources of renewable energy is one of the most important challenges facing mankind. The solution to this problem would be simple if we could find a way to harness the massive amounts of solar energy that reach the earth every day. While most efforts to achieve this goal have utilized crystalline silica-based photovoltaic cells or water-based passive solar panels, Vanderbilt Institute of Chemical Biology members David Cliffel and Sandra Rosenthal, along with their collaborators in the Kane Jennings lab (Vanderbilt Interdisciplinary Materials Science Program, Department of Chemical and Biomolecular Engineering) have chosen to exploit the most efficient photochemical system available, namely photosynthesis.

Figure 1. Photosystem I complexes isolated from spinach leaves form the foundation for the development of a new form of photovoltaic cell. (Image courtesy of Wikimedia Commons under the GNU Free Documentation License.)

In plants, photosynthesis harnesses energy from sunlight and converts it into chemical energy. The first step of this process is carried out by Photosystem I (PSI), an assembly of chlorophyll and phylloquinone pigments along with iron-sulfur clusters embedded in a complex of twelve proteins. Light energy absorbed by the PSI pigments is funneled to a special chlorophyll molecule called P₇₀₀. The energy excites a P₇₀₀ electron, allowing it to escape and travel through the PSI system to a terminal iron sulfur cluster, F_B. As a result, a charge separation is achieved between P₇₀₀+ (now positively charged after losing the electron) and F_B- (now negatively charged after gaining the electron). This charge separation is then harnessed for further chemical reactions required by the cell. PSI is highly efficient in its ability to convert light to chemical energy, and it does so nearly 100 times faster than a typical silicon diode used in most commercial photovoltaic cells.

Figure 2. Schematic diagram of PSI. Green, chlorophyll; orange, philloquinone, yellow and red, iron sulfur centers, white/gray, protein.
(Images courtesy of Wikimedia Commons under the GNU Free Documentation License.)

To exploit PSI’s ability to convert light to chemical energy requires the construction of a device, or electrochemical cell, that can harness the charge separation generated between P₇₀₀+ and F_B-. The Vanderbilt investigators made an important advance toward this goal when they discovered that a monolayer of PSI complexes, isolated from spinach leaves, could be coated onto the surface of a nanoporous gold leaf electrode. When used as a component of an electrochemical cell, the PSI-coated electrode effectively converted light energy into an electric current [Ciesielski et al. (2008) ACS Nano, 2, 2465]. This promising achievement formed the foundation for continued work to perfect a PSI-based cell. The most recent advance came withthe finding that replacing the monolayer with a dense multilayer of PSI complexes markedly increased the efficiency and output of the cell [Ciesielski et al. (2010) Bioresource Technol., published online January 9, DOI: 10.1016/j.biortech.2009.12.045]. The resulting devices produced a current density of ~2 μA/cm² at moderate light intensities, and their performance was limited only by the speed at which chemical species could move within the liquid component of the cell. The PSI-based cells were remarkably stable, retaining their ability to generate electricity from light for periods of at least 280 days with simple room temperature storage, and the investigators point out that the cost to produce the cells in their laboratories was only ~10¢/cm². Although still far from ready for commercial applications, these promising devices provide a critical framework for ongoing efforts to capitalize on nature’s most efficient solar energy system.