© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION Controlling Molecular Ordering in Aqueous Conducting Polymers Using Ionic Liquids Seyoung Kee, Nara Kim, Bong Seong Kim, Seongjin Park, Yun Hee Jang, Seoung Ho Lee, Jehan Kim, Junghwan Kim, Sooncheol Kwon, and Kwanghee Lee* S. Kee, N. Kim, B. S. Kim, S. Park, Prof. Y. H. Jang, Dr. S. H. Lee, Dr. J. Kim, Dr. S. Kwon, Prof. K. Lee School of Materials Science and Engineering and Department of Nanobio Materials and Electronics Heeger Center for Advanced Materials and Research Institute for Solar and Sustainable Energies Gwangju Institute of Science and Technology Gwangju 500-712, Republic of Korea E-mail: klee@gist.ac.kr J. Kim Pohang Accelerator Laboratory Pohang University of Science and Technology Pohang 790-784, Republic of Korea DOI: 10.1002/adma.201505473 complex (Figure 1a). [5] However, films coated from PEDOT:PSS solutions exhibit poor electrical properties [i.e., conductivity (σ dc ) ≈ 10 0 S cm –1 and mobility (μ) ≈ 10 –3 cm 2 V –1 s –1 ]. This is mainly because of the inevitably excessive PSS content, which induces a stable and reproducible dispersion but also inhibits dense packing and alignment of the PEDOT chains. However, it was recently found that the electrical conduction of PEDOT:PSS can be dramatically improved by pre- and/or post-deposition treat- ment with various polar solvents, [6] surfactants, [7] ionic com- pounds, [8] and acids. [9,10] Regarding the origin of conductivity enhancement, numerous studies have suggested that dipoles or ions in these solvents/solutions reduce the electrostatic interaction between PEDOT and PSS, resulting in their phase separation and the restructuring of the PEDOT:PSS complex toward more ordered structures. Nonetheless, the controlling factors for such structural realignments have not been clearly established, and therefore, deeper understanding is required to derive highly ordered structures from this type of CP colloid system. Herein, we demonstrate that the molecular ordering of the solution-processed PEDOT:PSS complex can be controlled by adding different ionic liquids (ILs) into the PEDOT:PSS solutions. In the mixture of PEDOT:PSS and ILs, the counter- ion exchange occurred to different extents and could be con- trolled by modulating the electrostatic interactions in ILs. By investigating a correlation between the degree of counter-ion exchange and the structural transformation, we found that robust counter-ion exchange led to the formation of highly ordered nanofibrillar structures of PEDOT chains based on a more expanded conformation and a reduced π–π stacking dis- tance, thereby resulting in a 5000-fold enhancement of the σ dc with the maximum σ dc of ≈2100 S cm –1 . ILs are molten salts at low temperature and exhibit many unique features, including thermal and chemical stability, negligible volatility, high ionic conductivity, and a broad elec- trochemical window. [11] The physical and chemical properties of ILs substantially depend on the electrostatic interactions between ILs’ cations and anions, and the binding energy (ΔE) can be tuned via rational design of the molecular structures because most ILs are composed of organic ions. By introducing various ILs with tailored ΔE values to the PEDOT:PSS complex, we intend to manipulate the counter-ion exchange between PEDOT:PSS and ILs. As counter-ion exchange reagents, we employed four different types of ILs with various anions (X), chloride (Cl), ethyl sulfate (ES), tricyanomethanide (TCM), and tetracyanoborate (TCB), coordinated with 1-ethyl-3-methylimid- azolium (EMIM) cation (Figure 1b). The cation part was fixed to EMIM + , which is the archetypal cation in ILs, to maintain the Organic electronics originate from the combination of the semi- conducting and metallic properties of π-conjugated molecular systems and the organic materials’ excellent mechanical prop- erties (i.e., flexibility and lightweight), thereby being expected to be main components of future ubiquitous applications, such as wearable devices, foldable displays, and individual power supplies. [1] The other intriguing feature of organic electronic materials is their processing advantage, which enables their solution-based fabrication using various printing techniques in a cost-effective way, namely, so-called “printed electronics.” [2] The solution processability of conjugated molecules can be achieved using two strategies: i) the functionalization of mole- cular backbones with alkyl side chains, which is principally used for semiconducting molecules, and ii) the utilization of bifunctional counter ions acting as soluble templates, which is mainly used for doped conducting polymers (CPs). However, introducing bulky and insulating side chains/counter ions, which induce solubility by decreasing interchain interactions in the solution state, impedes interchain coupling in the solid state and thus limits charge transport through π-orbital overlap- ping. [3] Therefore, it has been a significant challenge to obtain organic materials that simultaneously provide both the highly ordered π–π packing structure required for efficient charge transport and the excellent solution processability needed for the facile fabrication of electronic devices. A complex consisting of conducting poly(3,4-ethylenedioxy- thiophene) (PEDOT) and insulating polyanion, poly(4-styrene- sulfonate) (PSS), has been the most successful organic electronic material because of its excellent aqueous processability. [4] The negative charges on the sulfonate groups (SO 3 – ) in PSS neutralize the positively doped PEDOT, while extra sulfonic acid groups (SO 3 – H + ) induce aqueous colloidal dispersion in the PEDOT:PSS Adv. Mater. 2016, DOI: 10.1002/adma.201505473 www.advmat.de www.MaterialsViews.com