Plastic solar cells: disorder means improvement

Scientists part-funded by the European Research Council have spent decades trying to build flexible plastic solar cells that are efficient enough to compete with conventional cells made of silicon. Now, scientists at Stanford University have discovered that disorder at the molecular level actually improves the polymers’ performance. This discovery  could speed up the development of low-cost, commercially available plastic solar cells.

Study co-author Alberto Salleo, an associate professor of materials science and engineering, said: “People used to think that if you made the polymers more like silicon they would perform better. However, we found that polymers don’t naturally form nice, well-ordered crystals. They form small, disordered ones, and that’s perfectly fine.”

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 that of a crooked fingerprint. While some of the polymer chains were found to be particularly long, others formed only tiny crystals just a few molecules long.

By analysing 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,” Salleo added. “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.

“Our conclusion is that you don’t need to make something so rigid that it forms large crystals. 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.”

The findings are published in the journal Nature Materials.