In a recent report published by the Physicists Organization Network on August 4, American scientists revealed in an online article featured in "Nature Materials" that molecular-level chaos might actually enhance the performance of polymers. This discovery could significantly advance the development of affordable, commercial plastic solar cells.
For many years, researchers have aimed to create flexible plastic solar cells that match the efficiency of traditional silicon-based ones. To achieve this, they needed to develop plastic materials that allow electric charges to move more efficiently through the cells. Some teams tried to design flexible polymer structures that resemble crystalline silicon, but this approach didn’t improve charge mobility as expected.
Alberto Cerro, a materials engineering associate professor at Stanford University and a collaborator on the study, noted, “People used to believe that making polymers more like crystalline silicon would improve their performance. However, polymers don't naturally form large, ordered crystals—they tend to form small, disordered ones instead. It turns out that this disorder can be beneficial, and scientists should embrace the chaotic nature of plastics.â€
The research team focused on a group of organic materials known as semiconductor polymers. These materials share the properties of plastics but also have the ability to absorb sunlight and conduct electricity.
Since their introduction 40 years ago, semiconductor polymers have been considered ideal for creating ultra-thin solar cells, light-emitting diodes, and transistors. Unlike silicon, which is used in most solar panels today, these polymers are lightweight and can be processed at room temperature using low-cost methods such as inkjet printing. However, Cerro pointed out, “In solar cells, electrons need to move quickly through the material, but the electron mobility in semiconductor polymers has been too low for commercial use.â€
To address this issue, some scientists attempted to make the polymers more rigid to promote better crystal formation, but this didn’t help. Others found that certain seemingly disordered polymers had high charge mobility.
Cerro’s team sent these disordered materials to the SLAC National Accelerator Laboratory for X-ray analysis. The results showed that some of the molecular structures were not as ordered as previously thought. Some polymers looked like tangled spaghetti, while others formed tiny, short crystals. Cerro explained, “These small, disordered crystals are hard to detect with X-rays. Scientists even doubted their existence.â€
By studying the light emitted as charges moved through the samples, the team discovered that numerous small crystals were spread throughout the material, connected by long polymer chains. Cerro said, “The small size of the crystals is key to improving performance. Their small size allows electrons to pass through one crystal and quickly reach the next. Meanwhile, the long polymer chains help carry the electrons across the material, resulting in higher electron mobility compared to larger, disconnected crystals. Additionally, larger polymers are often insoluble, making them unsuitable for cost-effective processing methods like inkjet printing.†(Liu Xia)
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