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SCIENCE ADVANCES | RESEARCH ARTICLE CONDENSED MATTER PHYSICS Copyright © 2021 The Authors, some rights reserved; Charge transport physics of a unique class of rigid-rod exclusive licensee American Association conjugated polymers with fused-ring conjugated units for the Advancement of Science. No claim to linked by double carbon-carbon bonds original U.S. Government Mingfei Xiao1†, Remington L. Carey1†, Hu Chen2†, Xuechen Jiao3,4, Vincent Lemaur5, Sam Schott1, Works. Distributed 1 6 1,7 8 1,9 under a Creative Mark Nikolka , Cameron Jellett , Aditya Sadhanala , Sarah Rogers , Satyaprasad P. Senanayak , Commons Attribution 6 1 1 1,10 1 Ada Onwubiko , Sanyang Han , Zhilong Zhang , Mojtaba Abdi-Jalebi , Youcheng Zhang , NonCommercial 1 8 1,11 1 1 Tudor H. Thomas , Najet Mahmoudi , Lianglun Lai , Ekaterina Selezneva , Xinglong Ren , License 4.0 (CC BY-NC). Malgorzata Nguyen1, Qijing Wang1, Ian Jacobs1, Wan Yue12, Christopher R. McNeill3, Guoming Liu1,13,14, David Beljonne5, Iain McCulloch2,6, Henning Sirringhaus1* We investigate the charge transport physics of a previously unidentified class of electron-deficient conjugated polymers that do not contain any single bonds linking monomer units along the backbone but only double-bond linkages. Such polymers would be expected to behave as rigid rods, but little is known about their actual chain Downloaded from conformations and electronic structure. Here, we present a detailed study of the structural and charge transport properties of a family of four such polymers. By adopting a copolymer design, we achieve high electron mobilities 2 −1 −1 up to 0.5 cm V s . Field-induced electron spin resonance measurements of charge dynamics provide evidence for relatively slow hopping over, however, long distances. Our work provides important insights into the factors that limit charge transport in this unique class of polymers and allows us to identify molecular design strategies for achieving even higher levels of performance. http://advances.sciencemag.org/ INTRODUCTION chains in the solid state is also responsible for energetic disorder and Conjugated polymers have attracted substantial research attention in broadening of the electronic density of states. Key factors that deter- recent decades and are being developed for a range of applications mine the chain conformation, backbone planarity, and persistence in displays, large-area electronics, and bioelectronics (1–7), which length include the potentials for torsion and for backbone deflection make use of their compatibility with large-area, low-temperature (10). Characterization of molecular-level chain conformations (11–13) solution-based manufacturing deposition imparted by functional- is arguably the starting point for understanding structure-property ization with solubilizing side chains (8, 9). Unlike inorganic semi- relationships (14–16) in specific conjugated polymer systems. conductors for which delocalized electronic states and band In recent years, the persistence length, which is defined as the structures stem from long-range crystallinity within atomic lattices, average projection of the end-to-end vector on the tangent to the on May 11, 2021 the charge transport and optoelectronic properties of conjugated chain contour at a chain end in the limit of infinite chain length (17), polymers are largely determined at the single molecular level by the has been uncovered to intimately correlate with the optoelectronic conformation of the polymer backbone, which governs the ability of characteristics of the polymer. A long persistence length has been the p-conjugated electronic states in the frontier molecular orbitals demonstrated to be desirable for applications such as high-efficiency (MOs) to delocalize along the polymer backbone. A distribution of organic photovoltaics and high-mobility organic field-effect transistors different conformations adopted across the network of polymer (OFETs) (12). Early examples of the importance of persistence length include studies of the photoluminescence quantum yield of poly- phenylene vinylene derivatives, which was found to be positively 1Optoelectronics Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK. 2KSC, King Abdullah University of Science and Technology (KAUST), correlated with the persistence length and stiffness of the backbone Thuwal 23955-6900, Saudi Arabia. 3Department of Materials Science and Engineering, (18). Unusually high optical absorption in specific donor-acceptor 4 Monash University, Clayton, Victoria 3800, Australia. Australian Synchrotron, 800 (D-A) conjugated polymers has been ascribed to long persistence Blackburn Road, Clayton, VIC 3168, Australia. 5Laboratory for Chemistry of Novel Materials, University of Mons, BE-7000 Mons, Belgium. 6Department of Chemistry, length as well (19). Already from early studies, it has been known Imperial College London, South Kensington SW7 2AZ, UK. 7Centre for Nano Science that persistence length is related to the degree of conformational and and Engineering, Indian Institute of Science, Bangalore 560012, India. 8ISIS Pulsed 9 energetic disorder (20). Stiffer chains not only reduce the number of Neutron Source, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK. School of possible backbone configurations and, hence, energetic disorder Physical Sciences, National Institute of Science Education and Research, HBNI, Jatni 752050, India. 10Institute for Materials Discovery, University College London, (21, 22) but also provide straight tie chains bridging neighboring Torrington Place, London WC1E 7JE, UK. 11Cambridge Graphene Centre, University crystallites, forming efficient percolation pathways required for macro- 12 of Cambridge, Cambridge CB3 0FA, UK. Key Laboratory for Polymeric Composite scopic charge transport within thin-film devices such as OFETs (23). and Functional Materials of Ministry of Education, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China. 13Beijing National One of the factors that limit the persistence length of traditional Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, conjugated polymers is the existence of single-bond linkages that CAS Research/Education Center for Excellence in Molecular Sciences, Institute of 14 connect rigid conjugated units along the polymer backbone. At these Chemistry, Chinese Academy of Sciences, Beijing 100190, China. University of single-bond linkages, cis-trans isomerism or variations of the tor- Chinese Academy of Sciences, Beijing 100049, China. *Corresponding author. Email: [email protected] sional angle between neighboring conjugated units can occur that †These authors contributed equally to this work. limit p-electron delocalization and may change the orientation of Xiao et al., Sci. Adv. 2021; 7 : eabe5280 28 April 2021 1 of 13 SCIENCE ADVANCES | RESEARCH ARTICLE the backbone. As a result, most conjugated polymers exhibit limited In this work, we systematically investigate the chain conforma- chain rigidity with random coil or worm-like conformations on tions, persistence lengths, microstructure, energetic disorder, and length scales above 1 to 10 nm (12) and more flexibility compared charge transport properties in four such polymers, herein named with other p-conjugated systems such as carbon nanotubes (24). To NN1, NN2, AN1, and AN2. In addition, we compare the air stability date, the conjugated polymers with the longest persistence lengths of these four polymers to the widely investigated electron-deficient have been found to be donor-acceptor copolymers with extended, polymer P(NDI2OD-T2). The chemical structures are shown in fused-ring conjugated units linked by single bonds designed to Fig. 1A. Compared to the systems first reported in (28), including exhibit steep torsion potentials (10, 19). In such rigid and low- NN1, we would like to further improve the mobility by modifying disorder polymers, the highest carrier mobilities on the order of 1 to the structure using two different strategies: extending the aromatic 5 cm2 V−1 s−1 have been observed in polymer-based OFETs to date. core and incorporation of branched side chains with branching A wide range of such donor-acceptor polymers have been investi- points further removed from the polymer backbone. By extension gated, but at present, there are no clear molecular design guidelines of the planar aromatic core, the backbone rigidity and coplanarity that would allow going beyond the level of performance that is could be increased. This strategy has proved useful to improve charge achievable with such systems, and for the last 5 years, there has been transport properties in other rigid-rod polymers (29). More specif- only gradual and slow progress in the performance of conjugated ically, we adopted a copolymer design with some units extended from polymers for OFETs (7, 25–27). There is an urgent need for new a naphthalene to an anthracene core. By designing derivatives in design motifs for conjugated polymers that might allow further reduc- which the branch point is moved further away from the backbone, tions in main-chain conformational flexibility and energetic disorder. we aim to combine the advantages of improved solubility from Downloaded from Recently, an aldol condensation reaction between bis-isatin and branched side chains with stronger p-p intermolecular interactions, bis-oxindole monomers has been used to synthesize an unusual series which are normally associated with linear side chains. This strategy of electron-deficient