Scientists are striving to develop organic solar cells that can be produced as easily and inexpensively as thin films. A major obstacle is coaxing these carbon-based materials to reliably form the proper structure at the nanoscale. The goal is to develop cells made from low-cost plastics that will transform at least 10 percent of sunlight into electricity.
A team headed by David Ginger, associate professor of Chemistry at University of Washington, uses an atomic force microscope to quickly determine whether certain polymers are ever likely to reach the 10 percent efficiency threshold.
Most researchers make plastic solar cells by blending two materials together in a thin film, then baking them to improve their performance. In the process, bubbles and channels form much as they would in a cake batter. The bubbles and channels affect how well the cell converts light into electricity, and how much of the electric current actually gets to the wires leading out of the cell.
The exact structure of the bubbles and channels is critical to the solar cell's performance, but the relationship between baking time, bubble size, channel connectivity, and efficiency has been difficult to understand.
For the current research, the scientists worked with a blend of polythiophene and fullerene, model materials considered basic to organic solar cell research because their response to forces such as heating can be readily extrapolated to other materials. Ginger noted that the polymer tested is not likely to reach the 10 percent efficiency threshold, but that the results will be a useful guide.
The testing was accomplished using an atomic force microscope, which uses a needle to make a nanoscale image of the solar cell. The microscope, developed in Ginger's lab to record photocurrent, comes to a point just 10 to 20 nanometers across. The tip is coated with platinum or gold to conduct electrical current, and it traces back and forth across the solar cell to record the properties.
As the microscope traces back and forth over a solar cell, it records the channels and bubbles that were created as the material was formed. Using the microscope in conjunction with the knowledge gained from the current research, Ginger said, can help scientists determine quickly whether polymers will transform at least 10 percent of sunlight into electricity.
As researchers approach that threshold, nanostructured plastic solar cells could be incorporated into purses or backpacks to charge cellular phones or mp3 players, but eventually they could make in important contribution to the electrical power supply.