According to a report by the Physicists Organization Network on August 4, American scientists published an article in "Nature Materials" online, highlighting that molecular-level disorder can actually enhance the performance of polymers. This new research could significantly advance the development of affordable, commercial plastic solar cells.
For many years, researchers have aimed to develop flexible plastic solar cells that match the efficiency of silicon-based ones. To achieve this, they needed to create materials that allow electric charges to move more efficiently through the device. Some teams tried to design flexible polymer structures that resemble crystalline silicon, but these efforts didn't result in improved charge mobility.
"People used to believe that making polymers more like crystalline silicon would improve their performance," said Alberto Cerro, a materials engineering associate professor at Stanford University and a collaborator on the study. "But instead, the polymers don’t naturally form large, ordered crystals. Instead, they create small, disordered ones — which might actually be better. Scientists should learn to embrace the chaos in these materials."
The research team focused on a class of organic materials known as semiconductor polymers. These materials have the flexibility of plastics but also possess the ability to absorb sunlight and conduct electricity.
Since their introduction 40 years ago, semiconductor polymers have been seen as a promising candidate for ultra-thin solar cells, light-emitting diodes, and transistors. Unlike silicon, which is rigid and requires high-temperature processing, these polymers are lightweight and can be manufactured using low-cost methods like inkjet printing. However, their electron mobility has historically been too low for commercial use, according to Cerro.
To address this issue, some scientists attempted to make the polymers more rigid to promote better crystallization, but that approach didn’t work. Others found that certain disordered polymers showed unexpectedly high charge mobility.
Cerro’s team sent these disordered materials to the SLAC National Accelerator Laboratory for X-ray analysis. The results revealed that some of the molecular structures were not well-ordered. Some polymers looked like tangled spaghetti, while others formed tiny, short crystals. Cerro noted, "These small, disordered crystals are hard to detect with X-rays. Scientists even thought they didn’t exist."
By studying the light emitted as charges moved through the samples, the researchers discovered that numerous small crystals were spread throughout the material, connected by long polymer chains. Cerro explained, "The small size of the crystals is crucial. It allows electrons to move quickly from one crystal to another, and then the long polymer chains carry them across the material. This leads to higher electron mobility compared to larger, disconnected crystals. Plus, larger polymers are usually insoluble, making them unsuitable for cost-effective manufacturing techniques like inkjet printing."
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