According to a report by the Physicists Organization Network on August 4, American scientists published a study in "Nature Materials" online on the same day, revealing that molecular-level chaos can actually enhance the performance of polymers. This breakthrough could significantly advance the development of low-cost, commercial plastic solar cells.
For many years, researchers have aimed to develop flexible plastic solar cells that match the efficiency of traditional silicon-based solar panels. To achieve this, they needed to create plastic materials that allow electric charges to move more efficiently. Some teams tried to design flexible polymers into ordered, silicon-like crystals, but this approach did not improve charge mobility as expected.
Alberto Cerro, a materials engineering associate professor at Stanford University and one of the research collaborators, explained, “People used to believe that making polymers more like crystalline silicon would improve their performance. However, in reality, polymers don’t naturally form large, ordered crystals. Instead, they tend to form small, disordered ones—something that may actually be beneficial. Scientists should learn to embrace the inherent disorder in these materials.â€
The research team focused on a type of organic material known as semiconductor polymers. These materials have the flexibility of plastics but also possess the ability to absorb sunlight and conduct electricity. Since their discovery 40 years ago, they have been considered ideal for creating ultra-thin solar cells, light-emitting diodes, and transistors.
Unlike silicon crystals used in today’s solar panels, semiconductor polymers are lightweight and can be processed at room temperature using cost-effective methods such as inkjet printing. However, Cerro pointed out, “In solar cells, electrons need to move quickly through the material. Unfortunately, the electron mobility in these polymers is poor, which has limited their commercial use.â€
To address this issue, some scientists attempted to make the polymers more rigid to encourage the formation of ordered crystals, but this didn’t help. Others found that certain disordered polymers exhibited high charge mobility instead.
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 appeared disordered, resembling tangled spaghetti, while others formed tiny crystals just a few molecules long. Cerro said, “These small, disordered crystals were hard to detect with X-rays. In fact, scientists previously thought they didn’t exist.â€
By analyzing the light emitted as charges moved through the samples, the researchers discovered that numerous small crystals were scattered throughout the material, connected by long polymer chains. Cerro explained, “The small size of the crystals is key to improving performance. Their compact size allows electrons to pass through them quickly and move to the next crystal. Meanwhile, the long polymer chains help carry the electrons across the material, resulting in higher electron mobility compared to larger, disconnected crystals.â€
Additionally, he noted that larger polymers are often insoluble in water, making them unsuitable for low-cost manufacturing techniques like inkjet printing. This discovery opens new possibilities for developing efficient, affordable, and flexible solar cell technologies. (Liu Xia)
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