A research team from the University of Birmingham used an advanced imaging technique to identify defects in a process that is widely used to fabricate the polymers found in many devices. Lightweight, low-cost, and electrically conductive, conjugated polymers are used in the fabrication of optoelectronic, computing, biosensing, and other applications. They can be printed in thin layers onto flexible substrates, making them a good fit for next-generation technologies. Conjugated polymers, which are broadly used in optoelectronics, biosensors, and OLEDs, can be synthesized using cross-coupling polycondensation reactions catalyzed by transition metals. But, these reactions often use precious metals as catalysts, making them impractical for large-scale industrial use. Aldol condensation has emerged as a versatile, inexpensive, environmentally friendly approach to synthesizing semiconducting polymers. However, limited information is available on the quality of the polymers resulting from this process. Computing is just one area where conjugated polymers, known for their flexibility and electrical conductivity, are used. Courtesy of the University of Birmingham. “The aldol condensation process can create defects in the polymer sequences, like missteps in a molecular dance, which can disrupt the flow of electrons through the material, reducing efficiency and reliability in devices,” professor Giovanni Costantini said. Using scanning tunneling microscopy (STM) with electrospray deposition (ESD), the researchers analyzed conjugated polymers at the molecular level, characterizing the chemical structure of four types of conjugated polymers that were synthesized using aldol condensation. The team uncovered defects that cannot be detected using traditional characterization techniques such as nuclear magnetic resonance and mass spectrometry. The high-resolution STM images provided the researchers with detailed insight into the microstructure of the conjugated polymers, enabling them to identify how the comonomers (building-block polymers) were connected, one molecule at a time. Further, the images revealed previously unrecognized defects in the backbones of all four polymer samples. These defects fell into two categories. Sequence defects resulted from deviations from the ideal comonomer sequence, resulting from the wrong ordering of the building-block polymers. Coupling defects, meanwhile, showed up as kinks in the backbone structure, resulting from the incorrect coupling position or orientation of building-block polymers. A quantitative analysis uncovered a substantial number of defects in the polymers synthesized by aldol condensation, indicating that both types of defects are intrinsic to the aldol condensation reaction itself. Based on the experimental results, the researchers defined the reaction pathways leading to defect formation. This information provides insight into the mechanism behind the aldol condensation reaction. Then, by adjusting the chemical design and purifying the building block polymers before polymerization, the researchers reduced the number of defects significantly. One approach they used to produce cleaner polymer chains involved the use of aldol condensation to create small, well-defined molecules, which were then linked using a different method. The insights gained through this study could provide a basis for the design of more effective polymerization methods to better control the polymer structure and its performance in optoelectronic applications. “This is a major step forward in understanding how to make better-performing, more sustainable materials for electronics,” Costantini said. “It shows that even green chemistry needs careful control to deliver the best results.” Conjugated polymers are also used in/for organic field-effect transistors, neuromorphic computing, and energy storage, in addition to optoelectronics and optoelectronic devices. “Our findings could have wide-reaching implications for the development of high-performance, flexible, low-cost electronics and help reduce reliance on rare or toxic metals in manufacturing,” Costantini said. The research was published in Nature Communications (www.doi.org/10.1038/s41467-025-62221-y).