PORTLAND, Ore. -- Carbon nanotubes studded with phospholipid disks enable solar cells to perform self-repairing operations similar to plants performing photosynthesis. The resulting photoelectrochemical solar cells are claimed by the Massachusetts Institute of Technology (MIT) researchers to be twice as efficient as the best solid-state solar panels.
The main difference between man-made and natural solar power conversion is that engineers aim to armor solar cells against gradual degradation with solid-state inorganic materials, whereas natural solar conversion uses photosynthesis to anticipate and repair inevitable damage to liquid-state organic materials.
In nature, examples abound of sustainable solar energy usage. For instance, the organic compounds that enable a leaf to perform photosynthesis are regularly damaged by sunlight, but the leaf has self-repair mechanisms. By studying the process by which biological photosynthesis constantly renews its solar-to-fuel transformation machinery, scientists now believe they can create solar cells that mimic that self-repair ability.
MIT researchers are not claiming to have unlocked the secret to photosynthesis, but to have mimicked the self-repairing mechanism used by plants to continually replenish their energy harvesting capabilities.
Photosynthesis includes built-in mechanisms that periodically breaks down a plant's chemical-based engine into basic building blocks, then reassembling a refurbished engine from the renewed components. According to MIT professor Michael Strano, plants perform this function every hour, renewing and recycling their protein-based photosynthesis capabilities to keep them continually working at optimal efficiency.
Strano's formulation mimics the reversibility of photosynthetic processes with synthetic disk molecules called phospholipids, each of which carries its own internal reaction center that coverts light into an electric current. When mixed into a solution with carbon nanotubes, the disks self-assemble around them. Since carbon conducts electricity even better than metals, the nanotubes enhance the transport of electrons freed inside the disks by their exposure to sunlight.
Inside the disks, Strano's team created a photosynthesis-like cyclical mechanism using seven different compounds that self-assemble into harvesters of light. By adding a surfactant, similar to those used to disperse oil in the Gulf this spring, the entire seven-compound assembly can be broken apart into its original components. Then, by forcing the whole solution through a filter that removes the surfactants, the original building blocks again self-assemble into a rebuilt solar cell.
Compared to under 20 percent for the best solid-state solar cells, the liquid-state photoelectrochemical cell achieved 40 percent efficiency; higher concentrations of nanotube-disks could increase efficiency further, according to researchers.
Funding for the research was provided by the MIT Energy Initiative and the U.S. Energy Department.