Efficient molecular doping of polymeric semiconductors driven by anion exchange

• Published in: Nature (London) 2019-08, Vol.572 (7771), p.634-638
• Main Authors: Yamashita, Yu, Tsurumi, Junto, Ohno, Masahiro, Fujimoto, Ryo, Kumagai, Shohei, Kurosawa, Tadanori, Okamoto, Toshihiro, Takeya, Jun, Watanabe, Shun
• Format: Article
• Language: English
• Subjects: 639/301/1005/1007; 639/301/923/1028; 639/766/25; 639/925/357/341; Anion exchange; Anion exchanging; Carrier density; Charge density; Chemical properties; Current carriers; Dopants; Doping; Efficiency; Electric properties; Electrochemistry; Electrode potentials; Electromagnetism; Electron affinity; Entropy; Fourier transforms; Humanities and Social Sciences; Ionic liquids; Ionization; Ionization potentials; Ions; Letter; Methods; Molecular electronics; multidisciplinary; Organic chemistry; Polymers; Redox potential; Salts; Science; Science (multidisciplinary); Semiconductor doping; Semiconductors; Solvents; Transport properties
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1504-9

The efficiency with which polymeric semiconductors can be chemically doped—and the charge carrier densities that can thereby be achieved—is determined primarily by the electrochemical redox potential between the π-conjugated polymer and the dopant species 1 , 2 . Thus, matching the electron affinity of one with the ionization potential of the other can allow effective doping 3 , 4 . Here we describe a different process—which we term ‘anion exchange’—that might offer improved doping levels. This process is mediated by an ionic liquid solvent and can be pictured as the effective instantaneous exchange of a conventional small p-type dopant anion with a second anion provided by an ionic liquid. The introduction of optimized ionic salt (the ionic liquid solvent) into a conventional binary donor–acceptor system can overcome the redox potential limitations described by Marcus theory 5 , and allows an anion-exchange efficiency of nearly 100 per cent. As a result, doping levels of up to almost one charge per monomer unit can be achieved. This demonstration of increased doping levels, increased stability and excellent transport properties shows that anion-exchange doping, which can use an almost infinite selection of ionic salts, could be a powerful tool for the realization of advanced molecular electronics. The limitations of conventional chemical doping of polymeric semiconductors can be overcome by adding a second ionic species into the system, leading to enhanced doping, electrical conductivity and stability.