Disorder can improve the performance of plastic solar cells

Scientists have spent decades trying to build flexible plastic solar cells efficient enough to compete with conventional cells made of silicon. Recently, scientists discovered that disorder at the molecular level actually improves the polymers' performance. Now Stanford University researchers have an explanation for this surprising result. Their findings, published in Nature Materials, could speed up the development of low-cost, commercially available plastic solar cells. Instead of trying to mimic the rigid structure of silicon, Study co-author Alberto Salleo, an associate professor of materials science and engineering at Stanford and his colleagues recommend that scientists learn to cope with the inherently disordered nature of plastics. In the study, the Stanford team focused on a class of organic materials known as conjugated or semiconducting polymers – chains of carbon atoms that have the properties of plastic, and the ability to absorb sunlight and conduct electricity. To observe the disordered materials at the microscopic level, the Stanford team took samples to the SLAC National Accelerator Laboratory for X-ray analysis. The X-rays revealed a molecular structure resembling a fingerprint gone awry. Some polymers looked like amorphous strands of spaghetti, while others formed tiny crystals just a few molecules long. "The crystals were so small and disordered you could barely infer their presence from X-rays," Salleo said. "In fact, scientists had assumed they weren't there." By analyzing light emissions from electricity flowing through the samples, the Stanford team determined that numerous small crystals were scattered throughout the material and connected by long polymer chains, like beads in a necklace. The small size of the crystals was a crucial factor in improving overall performance, Salleo said. "Being small enables a charged electron to go through one crystal and rapidly move on to the next one," he said. "The long polymer chain then carries the electron quickly through the material. That explains why they have a much higher charge mobility than larger, unconnected crystals." Another disadvantage of large crystalline polymers is that they tend to be insoluble and therefore cannot be produced by ink-jet printing or other cheap processing technologies, he added. "Our conclusion is that you don't need to make something so rigid that it forms large crystals," Salleo said. "You need to design something with small, disordered crystals packed close together and connected by polymer chains. Electrons will move through the crystals like on a superhighway, ignoring the rest of the plastic material, which is amorphous and poorly conducting.