Ryo Fujimoto1 Yu Yamashita1 Shohei Kumagai1 Junto Tsurumi1 Alexander Hinderhofer2 Katharina Broch2 Frank Schreiber2 Shun Watanabe1 3 Jun Takeya1

1, Material Innovation Research Center (MIRC) and Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba, Japan
2, Institut für Angewandte Physik, Universität Tübingen, Tübingen, , Germany
3, JST, PRESTO, Kawaguchi, Saitama, Japan

π-conjugated polymeric conductors are produced via simple solution processes on many different substrates and offer a wide variety of functionalities. Their potential is highlighted not only for a substantial conductor in solar cells and supercapacitors, but also for an energy conversion medium in thermoelectronics and spintronics devices. In the applications given above, doping is one of the most indispensable techniques to increase conductivity. Because a π-conjugated core is distorted by the addition or extraction of electrons, structural and energetic disorder is unavoidable. Most recently, a novel doping method, referred to as molecular implantation, which enables the introduction of a dopant into the semiconducting polymer network[1] and realizes a relatively high charge concentration of up to 5×1020 cm-3 with excellent controllability of the doping level[2]. Molecular implantation enhances the original lamellar structure and band-like transport, which is supported by the observation of Hall effect and Anderson localization. However, molecular implantation is a vacuum process, and dopants are limited to only those that are evaporative small molecules.
Here, we extend the concept of molecular implantation, and present a simpler, versatile, and efficient doping method[3]. A standard thiophene-based polymeric semiconductor, poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3.2-b]thiophenes) (PBTTT-C14), can be doped by various dopants in solution, where dopants are not necessarily fully dissolved in the solvent. A solid-state film of PBTTT-C14 was immersed into a dopant-dispersed fluorinated solvent CT-solv 180TM (commercially available from Asahi Glass Co.), which does not cause the damage to the polymer film. Efficient charge transfer and reasonably high electrical conductivity were demonstrated with two different molecular dopants, 2,3,5,6- tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and molybdenum tris(1-(trifluoroacetyl)-2-(trifluoromethyl)ethane- 1,2-dithiolene) (Mo(tfd-COCF3)3). It should be noted that even with non-evaporative, insoluble dopants such as the Mo(tfdCOCF3)3, a metal–organic complex, a significant increase in conductivity together with a metallic nature, i.e. observation of the Hall effect and Anderson localization, is confirmed. Comprehensive studies based on UV-vis-NIR absorption, electron spin resonance (ESR) spectroscopy, and X-ray reflectivity (XRR) measurements confirm that dopant molecules are likely stored within the lamellae in PBTTT-C14, which enables two-dimensional charge transport to be established.
[1] K. Kang, S. Watanabe, et al. Nature Materials, 2016, 15, 896
[2] R. Fujimoto, S. Watanabe, et al. Organic Electronics, 2017, 47, 139
[3] R. Fujimoto, Y. Yamashita, et al. Journal of Materials Chemistry C accepted.