MSDE View Article Online REVIEW View Journal | View Issue Morphological effects on polymeric mixed ionic/ Cite this: Mol.Syst.Des.Eng.,2019, electronic conductors 4,310 Jonathan W. Onorato a and Christine K. Luscombe *abc Mixed ion/electron conducting polymers have recently received significant interest from a number of re- search communities, spanning from biological to mechanical. Their ability to conduct ions and electrons in $0 Received 16th November 2018, the same material enables their use in a wide range of electrochemical devices. This functionality can be Accepted 13th February 2019 used to improve performance of more traditional devices or enable completely novel ones. Herein the use of blended polymers, block copolymers, and homopolymers as mixed conducting polymer systems is DOI: 10.1039/c8me00093j discussed, with special emphasis on connecting polymer structure and morphology to mixed conduction rsc.li/molecular-engineering performance. Following this discussion, the outlook for the future of this field is presented. Design, System, Application Polymeric materials which can conduct electrons and ions have already seen wide usage in applications ranging from biological sensors that improve electrocardiogram sensitivity to improving the capacity of lithium ion batteries. Their usage has resulted in impressive improvements in performance of these devices; however, understanding is limited about the structure of these polymers as it relates to their performance. This review article seeks to address the connection between the structure that polymer chains adopt in the solid state and the ion and electron transporting properties of said polymers. Specifically, this review addresses three different motifs that are used for generating these polymers: blended polymers, block copolymers, and homopolymers. Each type presents different strengths and weaknesses, and responds to changes in morphologies in different ways. With an improved understanding of the influence of morphological factors on these transport properties, rational selection of processing methods to control the ionic and electronic transport in each material to target its usage in a specific application would be possible. It is the authors' hope that this review will encourage greater study of the relationship between morphology and performance, increasing our understanding and control over ionic and electronic transport. 13,14 3XEOLVKHGRQ)HEUXDU\'RZQORDGHGRQ 1. Introduction chitectures include block copolymers, and homopoly- mers.15 Due to the highly complex phase-behavior of these Mixed ionic/electronic conductors (MIECs) are a class of ma- materials and the difficulty of devising new synthetic terials with growing interest from the academic community methods, the links between morphology and MIEC perfor- due to their wide variety of potential uses. MIECs are mate- mance are still poorly understood. However, in order to en- rials that can conduct both electrons and ions, and as such able rational design, and long-term gains in MIEC perfor- serve a unique function for applications in electrochemical mance, the connection between ionic conduction, electronic 1,2 3,4 5,6 devices, such as sensors, actuators, batteries, fuel conduction, and the adopted morphology must be better un- 7 8,9 cells, and organic electrochemical transistors (OECTs). derstood. This rigorous study is still in its infancy, but some MIECs can be fabricated from both ceramics and polymers; early trends have started to be understood, and will be however, this work will be focused specifically on polymeric presented here. To understand MIEC performance, it is im- MIECs, and will use the term MIEC to refer only to poly- portant to understand the fundamentals of electronic con- 10,11 meric MIECs. duction independently of ionic conduction, and likewise for MIECs come in several distinct architectures. The most ionic conduction independent of electronic conduction. common type currently is a blended architecture, where a polymer that possesses electronic conduction is blended with 5,12 a polymer that possesses ionic conduction. Additional ar- 1.1 Electronic conduction in conjugated polymers Electronic conduction in polymeric materials occurs through a Department of Materials Science and Engineering, University of Washington, the overlap of π-orbitals. In conjugated polymers, there is a Seattle, WA, 98195-2120, USA. E-mail: [email protected] continuous pathway of overlapping π-orbitals, allowing for b Department of Molecular Engineering and Sciences, University of Washington, free electron migration along the whole polymer backbone, Seattle, WA, 98195-1652, USA c Department of Chemistry, University of Washington, Seattle, WA, 98195-1700, as shown in Fig. 1. Interchain charge transfer is possible USA through a hopping mechanism, allowing electrons to hop 310 | Mol. Syst. Des. Eng.,2019,4,310–324 This journal is © The Royal Society of Chemistry 2019 View Article Online MSDE Review from the π-orbitals of adjacent polymer chains. This delocali- zation results in the generation of a band-gap. By increasing the coplanarity of the conjugated backbone, the range over which atomic orbitals interact is extended, decreasing the size of the bandgap, as shown in Fig. 1.16 Planarity of a poly- mer backbone is highly important to charge conduction, as it extends the range over which a charge can be conducted be- fore a relatively slow interchain hopping process occurs, in- creasing the electron mobility.17 For charges to be transported at bulk, device-level scales, good charge transport must exist in at least two directions. This is because charges must have alternative pathways to travel when defects in the polymer chain or chain ends cause Fig. 1 Generation of a band gap with increasing number of the along-backbone conduction pathway to end. This inter- monomers incorporated into a conjugated polymer. Reproduced from ref. 16 with permission from the Royal Society of Chemistry. chain transport is enabled and improved by the quality of the $0 polymer's π–π stacking. Similar to transport along the back- bone, planarizing groups are beneficial as they ease interac- tions between π-orbitals on adjacent chains. As previously ethyl-branch into a hexyl side chain, polyIJ3-(2- mentioned, this stacking is the basis for crystallite formation ethylhexyl)thiophene), reduces the observed field effect mobil- in organic electronics, and typically the higher the percentage ity by an order of magnitude.25 Increasing the rigidity of the of crystallinity, the higher the mobility.18,19 This is due in backbone also increases the mobility, polyIJ2,5-bisIJ3- part to the lack of order in the amorphous regions, resulting alkylthiophen-2-yl)thienoij3,2-b]thiophene) has a charge mobil- in a high percentage of trap states and an increased likeli- ity approximately an order of magnitude higher than P3HT hood of charge recombination.20,21 Tie-chains are single poly- by substituting two thiophene repeat units for a fused thieno- mer chains that bridge multiple crystalline domains, provid- thiophene unit.26 Other strategies for improving mobility in- – ing along-backbone charge conduction between adjacent clude synthesizing donor/acceptor copolymers,27 29 adjusting crystallites. In order for charges to cross amorphous regions, surface energy levels,30,31 and increasing regioregularity.32,33 tie-chains are critical, providing improvements in mobility Most strategies for improving charge transport revolve − − − − from 10 5 to 10 2 cm2 V 1 s 1 for a commonly studied semi- around rigidifying the polymer backbone and increasing the conducting polymer, polyIJ3-hexylthiophene) [P3HT].22 π-stacking interactions, both features which would be The chemical structure of the polymer repeat unit has a expected to negatively contribute to ionic conduction.21 How- significant impact on the charge mobility. By increasing the ever, a recent discovery provides a unique opportunity, show- length of the solubilizing alkyl side chain from P3HT to ing that high charge mobilities are possible even in highly 3XEOLVKHGRQ)HEUXDU\'RZQORDGHGRQ − polyIJ3-octylthiophene), a reduction from 1.1 × 10 2 to 1.4 × amorphous materials, if a polymer has a high molecular − − − 10 4 cm2 V 1 s 1 in charge mobility is observed.23,24 A similar weight and is sufficiently planar.34 This is possible because phenomenon is seen with branching chains; introducing an of the connections between local aggregates by the tie chains Jonathan Onorato received his Christine Luscombe received her BSE in Polymer Science and En- B.A.,M.A.,MSci,andPhDatthe gineering from Case Western Re- University of Cambridge where serve University in Cleveland, she performed research under Ohio, where he worked for Prof. the direction of Prof. Steven Ley Stuart Rowan. He is a PhD Can- (for B.A. and MSci) and Prof. didate at the University of Wash- Andrew Holmes and Dr Wilhelm ington in the Materials Science Huck (for PhD). She subse- and Engineering department quently moved to UC Berkeley as where he received the Wagstaff a Lindeman Research Fellow fellowship and is currently a DI- and Trinity College Junior Re- RECT fellow with the Clean En- search Fellow to do her post- Jonathan Onorato ergy Institute at the University of Christine Luscombe doctoral work with Prof. Jean M. Washington. He works under the J. Fréchet. She has been at the direction of Prof. Christine Luscombe, and is broadly interested in University of Washington since 2006 and is currently the Camp- conjugated polymers, with specific focus on mixed conduction and bell Development Professor in the Materials Science and Engineer- structure–property relationships in conjugated polymers. ing Department. This journal is © The Royal Society of Chemistry 2019 Mol. Syst. Des. Eng.,2019,4,310–324 | 311 View Article Online Review MSDE formed from the long polymer chains, as described in Fig. 2. In addition to the size of the ion, the identity of the salt From this, it could be possible to develop a highly open, from which the ion is dissociated is highly important. If the aggregate-based conjugated polymer that could co-optimize ion pair does not dissociate, it will not respond to an applied conduction of electrons and ions.