Vanderbilt University researches have developed a way to combine Photosystem 1 (PS1), the photosynthetic protein that converts light into electrochemical energy in spinach with silicon (the material used in solar cells), in a fashion that produces substantially more electrical current than has been reported by previous biohybrid solar cells.
“This combination produces current levels almost 1,000 times higher than we were able to achieve by depositing the protein on various types of metals. It also produces a modest increase in voltage,” said David Cliffel, associate professor of chemistry, who collaborated on the project with Kane Jennings, professor of chemical and biomolecular engineering.
The researchers’ next step is to build a functioning PS1-silicon solar cell using this new design. Jennings has an EPA award that will allow a group of undergraduate engineering students to build the prototype. The students won the award at the National Sustainable Design Expo in April based on a solar panel that they had created using a two-year old design. With the new design, Jennings estimates that a two-foot panel could put out at least 100 milliamps at one volt - enough to power a number of different types of small electrical devices.
The PS1/silicon combination produces nearly a milliamp (850 microamps) of current per square centimeter at 0.3 volts. The reason this combo works so well is because the electrical properties of the silicon substrate have been tailored to fit those of the PS1 molecule. This is done by implanting electrically charge atoms in the silicon to alter its electrical properties - a process called doping. In this case, the protein worked extremely well with silicon doped with positive charges and worked poorly with negatively doped silicon.
To make the device, the researchers extracted PS1 from spinach into an aqueous solution and poured the mixture on the surface of a p-doped silicon wafer. Then they put the wafer in a vacuum chamber in order to evaporate the water away leaving a film of protein. They found that the optimum thickness was about one micron, about 100 PS1 molecules thick.