Multilevel Investigation of Charge Transport in Conjugated Polymers

Article pubs.acs.org/accounts

Multilevel Investigation of Charge Transport in Conjugated Polymers Huanli Dong*,† and Wenping Hu†,‡ †

Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

‡

CONSPECTUS: Conjugated polymers have attracted the world’s attentions since their discovery due to their great promise for optoelectronic devices. However, the fundamental understanding of charge transport in conjugated polymers remains far from clear. The origin of this challenge is the natural disorder of polymers with complex molecular structures in the solid state. Moreover, an effective way to examine the intrinsic properties of conjugated polymers is absent. Optoelectronic devices are always based on spin-coated films. In films, polymers tend to form highly disordered structures at nanometer to micrometer length scales due to the high degree of conformational freedom of macromolecular chains and the irregular interchain entanglement, thus typically resulting in much lower charge transport properties than their intrinsic performance. Furthermore, a subtle change of processing conditions may dramatically affect the film formationinducing large variations in the morphology, crystallinity, microstructure, molecular packing, and alignment, and finally varying the effective charge transport significantly and leading to great inconsistency over an order of magnitude even for devices based on the same polymer semiconductor. Meanwhile, the charge transport mechanism in conjugated polymers is still unclear and its investigation is challenging based on such complex microstructures of polymers in films. Therefore, how to objectively evaluate the charge transport and probe the charge transport mechanism of conjugated polymers has confronted the world for decades. In this Account, we present our recent progress on multilevel charge transport in conjugated polymers, from disordered films, uniaxially aligned thin films, and single crystalline micro- or nanowires to molecular scale, where a derivative of poly(paraphenylene ethynylene) with thioacetyl end groups (TA-PPE) is selected as the candidate for investigation, which could also be extended to other conjugated polymer systems. Our systematic investigations demonstrated that 3−4 orders higher charge transport properties could be achieved with the improvement of polymer chain order and confirmed efficient charge transport along the conjugated polymer backbones. Moreover, with downscaling to molecular scale, many novel phenomena were observed such as the largely quantized electronic structure for an 18 nm-long TA-PPE and the modulation of the redox center of tetrathiafulvalene (TTF) units on tunneling charge transport, which opens the door for conjugated polymers used in nanometer quantum devices. We hope the understanding of charge transport in PPE and its related conjugated polymer at multilevel scale in this Account will provide a new method to sketch the charge transport properties of conjugated polymers, and new insights into the combination of more conjugated polymer materials in the multilevel optoelectronic and other related functional devices, which will offer great promise for the next generation of electronic devices.

1. INTRODUCTION The discovery of conducting polymers in the 1970s1,2 had completely changed people’s traditional concept from insulating polymer materials to the new research world of plastic electronics.3−6 Compared to inorganic materials, conjugated polymers possess attractive advantages of abundant material systems, lightweight, easy fabrication, and good flexibility, suggesting their promising applications in large-area optoelectronic devices, such as polymer light-emitting diodes (PLEDs), polymer solar cells (PSCs), and polymer field-effect transistors (PFETs).7,8 Remarkable advances in polymer device performance have been achieved recently, together with development of new polymer semiconductors and optimization of device fabrication techniques. Until now, the power conversion efficiency (PCE) of PSCs is over 10%,9,10 the charge carrier mobility of PFETs is over 10 cm2 V−1 s−1,11,12 and the © 2016 American Chemical Society

performance and stability of PLEDs have already been satisfactory for real applications.5,6 However, regardless of decades of focus of research, charge transport in conjugated polymers remains challenging. To date, various charge transport models such as band-like model, hopping transport mechanism, Peierls model, and Soliton model have been developed.13−16 Each of them definitely explained some experimental phenomena and greatly contributed to the development of theoretical and experimental research of conjugated polymers. However, the fundamental charge transport in conjugated polymers remains far from clear because of their naturally disordered features with complex molecular structures. Moreover, most recently investigated Received: July 18, 2016 Published: October 25, 2016 2435

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research polymer optoelectronic devices are based on spin-coated films; in this case, polymers tend to form highly disordered structures at nanometer to micrometer length scales due to the high degree of conformational freedom of macromolecular chains and the irregular interchain entanglement, thus typically resulting in much lower charge transport properties than their intrinsic performance. In addition, a subtle change of processing conditions may dramatically affect the film formation, inducing large variations in the morphology, crystallinity, and microstructures, and finally varying the effective charge transport and leading to great inconsistency over an order of magnitude even for the same polymer.17,18 Obviously, to fairly evaluate and investigate the nature of charge transport of conjugated polymers, it is crucial that the variation in both overall molecular order and device quality should be minimized and or eliminated, and the alignment of polymer chains in solid state should be improved to an extreme degree,19−21 such as using conjugated polymer crystals.19,20 In addition, to reveal the charge transport mechanism from the molecular level, it is better to construct devices based on a few or even a single polymer chain without consideration for the effect of localized electron states induced by the interchain entanglement and the chain-end tails.22 That is to say, except for polymer thin films, comprehensive investigation of charge transport in conjugated polymers from different aspects is highly essential to approach the intrinsic properties of a conjugated polymer. This Account highlights our recent progress on multilevel investigations of charge transport in conjugated polymers, from disordered films, uniaxially aligned thin films, and single crystals to molecular devices (Figure 1). Herein, a rigid-rod conjugated

efficient charge transport along the conjugated polymer backbones under different scales. Moreover, our comprehensive investigations of charge transport in TA-PPE and its related polymers in this Account provide new insights into the combination of conjugated polymer materials and multiscale devices and open a door for the development of polymer micro- and nanoscale and molecular devices.

2. DISORDERED FILMS OF CONJUGATED POLYMER Poly(para-phenylene ethynylene)s (PPEs), an important class of conjugated polymers, have been widely investigated in PLEDs and fluorescent chemical sensors due to their strong luminescence properties.24,28,29 Moreover, the ideal rigid-rod conjugated backbones indicate their potential good electronic properties; however, the optoelectronic properties for this class material of PPEs have rarely been investigated previously. Two kind of devices, PFETs and photoswitchers, were adopted in our experiment to examine their optoelectronic properties. Top-contact PFETs based on the spin-coated TA-PPE films were first constructed on bare Si/SiO2 substrates with gold as the source and drain electrodes (Figure 2a). Our initial research results demonstrated the typical p-type field-effect characteristics of TA-PPE but with very low charge carrier mobility of 10−6 to 10−5 cm2 V−1 s−1, and low photoconductivity of ∼2.49 × 10−5 S m−1 and on/off ratio of 8−12 under 5.76 mW cm−2 light illumination (Figure 2c−f),30 largely due to the disordered polymer chains and amounts of defects in the spin-coated polymer films (Figure 2b). 3. ORDERED FILMS OF CONJUGATED POLYMER It is well-known that polymer molecules are expected to exhibit intrinsically anisotropic properties because of the electronic delocalization along the conjugated backbones of the polymer chains, so that the intrachain mobility of carriers (electrons or holes) on an isolated polymer chain was orders higher than that of the films.31,32 Therefore, an important research direction to improve the performance of polymer devices is to establish optimal structure and order of polymers in films.33 Various techniques could be used to order materials, for example, the “friction transfer technique”,34 an effective approach for aligning a wide variety of materials. However, the detailed structural information and polymer chain alignment upon PPEs had been rarely addressed. To realize the enough self-assembly for good alignment of TA-PPE chains, here we used a combination of “drop-casting self-assembly” and the friction transfer technique to approach this target.30 For example, aligned Teflon rod (PTFE) substrate was preprepared by sliding a PTFE bar at a constant pressure against the substrate (Figure 3a), where the PTFE chains were aligned along the sliding direction. Then, oriented TA-PPE films were obtained by drop casting TA-PPE tetrahydrofuran (THF) solution (about 5 mg mL−1) onto the oriented PTFE layer (Figure 3b) with slow solvent evaporation in a closed jar, where the TA-PPE chains just aligned along the sliding direction of PTFE, as manifested by the atomic force microscopy (AFM) images and polarized UV−vis absorption and optical micrograph (Figure 3c−f). PFETs were further fabricated based on the ordered TA-PPE films on trichloro(octadecyl)silane (OTS) self-assembled monolayer modified the substrate (Figure 4a). In the active layer, polymer chains were oriented with side chains perpendicular to the substrate, which was beneficial for efficient

Figure 1. Multilevel investigation of charge transport in conjugated polymers.

polymer, poly(para-phenylene ethynylene) with thioacetyl end groups (TA-PPE) has been selected as our initial research target with the considerations of its ideal rigid-rod molecular structure,23 good conductivity,24 and good self-assembly ability (Figure 1). Moreover, the functional end groups of TA-PPE also provide its ability to form Au−S bonds with gold electrodes in molecular devices.25−27 Our systematic investigations demonstrated that much higher charge transport properties could be achieved with the improvement of TA-PPE polymer chain order in the solid state and confirmed the 2436

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research

Figure 2. (a,b) Disordered films of TA-PPE in (c,d) transistors and (e,f) photoswichers. Reproduced with permission from ref 30. Copyright 2008 American Chemical Society.

4. SINGLE CRYSTALS AT MICRO- AND NANOMETER SCALE Although the ordered films of conjugated polymers has been proven to be an effective approach for the enhancement of their charge transport at macroscopic state,34−37 obviously there are still some amount of inevitable defects and grain boundaries in thin films, which to some extent severely affects the further improvement of charge transport and device performance of conjugated polymers. Compared to macroscopic thin films, single crystals possess long-ranged molecular order, no grain boundaries, and a low density of defects, and have been proven to be the best candidates for the intrinsic charge transport investigation in organic small semiconductors.38,39 Recently, well-defined charge transport physics and affecting factors have been established based on organic small single crystal transistors, such as structure−property relationship, anisotropic transport property, and transport-temperature dependence.40,41 Inspired by the research of organic small single crystals and their transistors, we carried out the pioneering exploration of preparing TA-PPE crystals.19 While the generation of single crystals of flexible polymers from dilute solution was demonstrated more than a half century ago, producing single crystals from rigid, conjugated polymers remains elusive due to the inherent electron-rich backbone structures.42,43 Here, we developed a solvent-assisted self-assembly process, where a certain volume of low-boiling point solvent, such as THF was added at the bottom of the closed jar to provide a solvent vapor atmosphere.19,44 In this condition, the growth process of

carrier transport in the two-dimensional direction in the polymer films (i.e., via both the intrachain and interchain) (Figure 4b). The characteristics of their PFETs (Figure 4c,d) gave charge carrier mobility, on/off ratio, and threshold voltage of ∼4.3 × 10−3 cm2 V−1 s−1, 3.8 × 104, −28 V, respectively,35 definitely confirming the significant improvement of charge transport for TA-PPE due to the optimized molecular orientation in films. The ordered films of conjugated polymer will be beneficial not only for charge transport but also for exciton dissociation. In order to prove this, photoresponse characteristics of TA-PPE aligned film based devices with polymer chain alignment just along the charge transport direction in the channel were measured, and the results were shown in Figure 4e (the irradiated light was tuned at 0−5.76 mW cm−2). It was obvious that the photodevices worked well, and the current of the devices with molecular alignment was as high as 2.65 nA at 20 V under 5.76 mW cm−2. It was largely higher than that of the devices without molecular orientation (Figure 2e, only 0.12 nA at 20 V under 5.76 mW cm−2) under the same operational conditions. On the other hand, with light on or off the devices exhibited capability of switching between high and low current states (Figure 4f) with on/off ratio up to 330−400, which was several decades higher than that of devices without aligned polymer films (Figure 2f). 2437

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research

Figure 3. (a) Sliding a PTFE rod at a constant pressure against the substrates to obtain aligned PTFE layer. (b) TA-PPE films were obtained by drop casting TA-PPE on the oriented PTFE layer. (c, d) AFM images of aligned PTFE substrate and bilayer TA-PPE/PTFE films. (e) Polarized UV−vis absorption and (f) optical micrograph of aligned TA-PPE films on oriented PTFE film. Reproduced with permission from ref 30. Copyright 2008 American Chemical Society.

Figure 4. (a) Schematic bottom-gate, top-contact PFET geometry on ordered polymer films. (b) Representative “edge-on” molecular orientation in TA-PPE films suggested above. (c) Output and (d) transfer characteristics of an exemplary TA-PPE transistor. (e, f) Photocharacteristics of devices based on TA-PPE films with alignment (in panel f, constant voltage, 10 V; light intensity, 5.76 mW cm−2). Reproduced with permission from refs 30 and 35. Copyright 2008 and 2009 American Chemical Society.

polymer crystals can be depicted by following steps: the full extension of conjugated polymer chains in the dilute solution, the initial homogeneous nucleation−growth process with the evaporation of solvent and increase of solution concentration, the dissolution of disordered aggregates under the solvent interactions and the second stage of heterogeneous crystal growth, and finally the formation of high-quality polymer crystals by continuously optimizing this process. Through this process, well-defined TA-PPE nanowires were obtained with the average diameter of 5−10 nm (Figure 5a−d). Surprisingly, these obtained TA-PPE nanowires possessed very high crystallinity, demonstrating a typical single crystal diffraction pattern for an individual TA-PPE nanowire (Figure 5e,f). From Figure 5f, together with X-ray diffraction (XRD) results, we could conclude that the nanowire crystals have an orthorhombic crystal unit cell with lattice parameters of a ≈ 13.6 Å, b ≈ 3.74 Å, and c ≈ 5.0 Å. Moreover attractively, it was found that the π−π stacking direction in TA-PPE nanowires was predominantly perpendicular to the long axis of nanowires with their main chains arranged along the long axis of nanowires, which was different from previously proposed polymer chain packing models, for example, different from that of poly(3-hexylthiophene) (P3HT) molecular packing where polymer chains pack perpendicular to the long-axis of microcrystals,42 and π−π stacking had been seen as the main driving force for the growth of micro- and nanocrystals of organic π-conjugated molecules.38 Such conjugated polymer packing in crystal nanowires is beneficial for us to investigate charge transport along the polymer chains to get intrinsic property, that is, the fast charge transport method proposed for conjugated polymers. TA-PPE nanowires based PFETs (Figure

Figure 5. (a−c) Scanning electron microscopy (SEM) and (d) AFM images of TA-PPE nanowires. (e, f) Transmission electron microscopy (TEM) image and selected-area electron diffraction (SAED) of individual TA-PPE nanowire. (g) Polymer chain packing in the nanowire crystals. (h) AFM image of a representative transistor based on TA-PPE nanowire crystals. Reproduced with permission from ref 19. Copyright 2009 American Chemical Society.

2438

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research

Figure 6. (a) Molecular structure of TTF−PPE. (b) Schematic view of rGO top-contact test bed for molecular tunneling junctions. (c) Schematic view of self-assembled long conjugated polymers inside the junctions. The blue arrow indicates charge transfer by direct tunneling across the molecular barriers. (d) Representative I−V curves of rGO-only junctions and junctions based on PPE and TTF−PPE ultrathin films. (e) Modulation of TTF units on charge transport via chemical oxidation (□) before/as-prepared TTF−PPE junctions; (○) TTF−PPE2+ junctions after TTF−PPE was treated with excess iron perchlorate hexahydrate. Reproduced with permission ref 51. Copyright 2015 Macmillan Publisher Ltd.

5h) were further fabricated through an “organic ribbon mask” technique45 in a bottom-gate top-contact configuration to probe their charge transport. The obtained average charge transport mobility for TA-PPE crystal nanowires was around 10−2 cm2 V−1 s−1 with the highest mobility value approaching 0.1 cm2 V−1 s−1, which was 1 to 2 orders of magnitude higher than that of corresponding TA-PPE ordered thin film transistors and even 3−4 orders of magnitude higher than that of spin-coated films.35 Here with TA-PPE as example, we for the first time definitely confirmed the possibility of preparing conjugated polymer micro- and nanocrystals and their use for help investigating charge transport of polymers. Such a concept has been successfully extended to more conjugated polymer systems,20,33,46,47 and more interestingly, it is found that in these polymer nanowires, their polymer conjugated backbones are also arranged along the long axis of nanostructures. And much higher field-effect mobility has been achieved based on these crystalline polymer nanostructures than that of their corresponding thin film devices.

monolayer of conjugated polymers is highly challenging to self-assemble due to the long length and flexibility of their backbone chains, and a monolayer of conjugated polymers is seldom addressed due to the challenges. Here we overcome the challenges by using TA-PPE with anchors to graft on micropores to form a monolayer with their backbone parallel to substrate and reduced graphene oxide (rGO) as soft top electrodes on the micropore to form a molecular junction of conjugated polymers. Herein, besides TA-PPE, another PPE derivative incorporating redox-active tetrathiafulvalene (TTF) units into the conjugated backbones (TTF−PPE) was also synthesized for comparison (Figure 6a).48,49 By controlling the self-assembly process and with the solution-processed rGO films (5−7 nm) as top soft contact,50 we constructed a new type of molecular junction composed of conducting polymers, where the conjugated polymers of TA-PPE and TTF-PPE were self-assembled in a distinguishable “planar” manner with the anchoring groups on both ends bonded to the bottom Au electrode from the traditional vertically oriented small-molecule monolayers (Figure 6b,c).51 Electrical measurements on the junctions revealed molecular-specific characteristics of the polymeric molecules, with more efficient tunneling charge transport properties than that of less conjugated small

5. MONOLAYER OF CONJUGATED POLYMERS A monolayer of small molecules is easily self-assembled perpendicularly on substrates such as gold. However, a 2439

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research molecules, due to the higher conjugation and lower electron injection of the conjugated polymers. Moreover, the incorporation of TTF units in the conjugated backbones played an important role on modulating charge transport behavior of polymers via altering conjugation path and energy level alignment (Figure 6d), which could be further modulated under external stimuli, such as chemical oxide (Figure 6e), providing us a new strategy for the potential of employing conjugated polymers in multifunctional molecular devices, such as molecular memory via synthetic methods for desirable functionalities.

6. MOLECULAR DEVICES OF CONJUGATED POLYMERS Monolayers of conjugated polymers provide the chance to study the charge transport perpendicular to their backbone. However, charge transport along the polymer chains still remains unclear. Hoofman et al., studied the charge transport on isolated chains of semiconducting polymer poly(phenylene vinylene) by the pulse-radiolysis time-resolved microwave conductivity (TRMC) technique in dilute solution and found that the intrachain charge transport along one conjugated polymer chain was 4 orders of magnitude higher than that of interchain charge transport in films.52,53 Terao et al. reported the emergence of a highly conducting pathway for charges facilitated by the molecular structures, consequently suppressing thermal motion of the chain scattering from the molecular design concept, that is, insulating the π-conjugated chains (such as poly(phenylene−ethynylene)) with macrocycles, leading to band conduction at low temperature and a very high carrier mobility of 8.5 cm2 V−1 s−1 measured through the TRMC technique.54,55 In order to further realize the measurement of charge transport along polymer chains in actual nanoscale devices, a prerequisite is to fix polymer chains between two electrodes with an especially small gap, that is, nanogap electrodes. There are two challenges: one is how to prepare such nanogap electrodes down to the scale of polymer chains; the other is how to fix the polymer chains between the nanogap electrodes.56,57 To achieve the goals, several methods were developed to fabricate such nanogap electrodes: for example, by using electroplating to get nanogap electrodes of gold with gap width from 20 to 100 nm based on submicrometer gap electrodes of electron beam lithography,58,59 by using molecular nanowire crystal as a mask to get nanogap electrodes of gold down to 9 nm,60 or by using grain boundary (GB) break junction to get nanogap electrodes down to 1−2 nm.61 These nanogap electrodes provide us an ideal platform for further nanoscale and molecular device investigations.62−64 For example, based on the planar nanogap electrodes, we fabricated TA-PPE nanometer devices by in situ writing the Poly70-TAPPE molecules (n ≈ 70, with a length of around 45 nm) into the ∼40 nm electrodes, which were connected together through Au−S bonds (Figure 7a,b).59 I−V measurements of TA-PPE nanometer devices demonstrated clear nonlinear stepwise characteristics when changing the bias, confirming the hole injection determined by tunneling in the nanometer devices. Moreover, these nanometer devices also demonstrated obvious light-controlled properties, working as photoswitchers, with very high and fast optical response characteristics, which was because under light illumination more electrons had sufficiently high energy to jump over or tunnel through the Au−S barrier, resulting in high current (“on” state) (Figure 7c). With the back Si substrate as gate electrode, the whole device

Figure 7. (a) Schematic of self-assembled TA-PPE nanodevices. (b) SEM and fluorescent images of the nanodevices with TA-PPE nanowires in ∼20 nm. (c) Photoresponse characteristics of the tunneling junction, which can be switched on/off quickly by photoirradiation (white light, 52 mW). (d) Conductance oscillations of the TA-PPE transistor with gate bias variation at 147 K (VDS, 1.05 V). Reproduced with permission from ref 63. Copyright 2005 American Chemical Society.

shifted into a nanometer-scale transistor with typical p-type transporting characteristics (Figure 7d) and demonstrated highly periodic oscillation with gate voltage when measured at 147 K, like a quantum dot junction with the active conjugated TA-PPE molecules as a dot and the terminal sulfur atoms as two tunnel barriers.63 To better probe the properties of TA-PPE at molecular scale, we shortened our TA-PPE molecules to Poly24-TA-PPE oligomer (with a length of around 18.3 nm) assembled into the matched ∼18 nm nanogap electrodes (Figure 8a).64 Based on these molecular devices, we observed very interesting I−V characteristics (Figure 8b,c) at room temperature: (i) highly periodic, identical, and repeatable stepwise behaviors; (ii) a certain separation of ∼0.23−0.26 V between the neighboring steps; and (iii) the unsymmetrical I−V curves with respect to the voltage inversion. The nice steps in the I−V curve are not due to conductance quantization because the step height is 6 orders of magnitude smaller than the conductance quantum, 2e2/h. It cannot be caused by the molecular deformation and defects that often tend to swear out the structures. Although the polymer molecule is chemically bonded to the gold electrodes through the end sulfur atoms, the conjugated πorbitals of the polymer molecule might not be able to extend over to the electrodes. The self-assembled nanojunction could be similar to a quantum dot junction, with the conjugated polymer molecules as a quantum dot and the terminal sulfur atoms acting as two tunnel barriers. One possible mechanism for the stepwise behavior could be related to the resonant electron tunneling: each step is a sign of opening of each conducting channel of the polymer molecule. The calculated I− V characteristics of the poly24 are shown in Figure 8b,c together with the corresponding experimental results. The theoretical and experimental results agreed very well with each other. It is clear that the well-spaced molecular orbital in the poly24 is responsible for the stepwise feature in the I−V curve. Different steps correspond to the opening of different conducting channels. The calculated voltage spacing between different steps of poly24 is around 0.26 V, in perfect agreement 2440

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research

state due to the reduced grain boundaries and charge transport defects as well as the remarkably enhanced contribution of intrachain charge transport, for example, the charge carrier mobility of TA-PPE shifted from 10−6 to 10−5 cm2 V−1 s−1 of disordered films to 4.3 × 10−3 cm2 V−1 s−1 of ordered films and 0.01−0.1 cm2 V−1 s−1 for nanocrystals. (2) In conjugated polymers with certain rigidity, for example, TA-PPE, the π−π stacking direction in the nanowire crystal was perpendicular to the long axis of nanowires with their main chains arranged along the long axis of nanowires, which was different from previously proposed polymer chain packing models, for example, in P3HT microcrystals where polymer chains were perpendicular to the long-axis of microcrystals, and such novel packing in polymer crystals is beneficial for us to investigate charge transport along the polymer chains. (3) With downscaling to molecular scale, many novel phenomena were observed such as the largely quantized electronic structure for an 18 nm-long TA-PPE and the modulation of redox center of TTF units on tunneling charge transport. It opens a door for conjugated polymers used in nanometer quantum devices. (4) The understanding of conjugated polymers at multilevel scale provides a new method to sketch the charge transport properties of conjugated polymers and to approach a long-term fundamental challenge encountered for charge transport in conjugated polymers and provides new insights into the combination of more conjugated polymer materials and offers their great promise for the next generation of electronic devices.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 8. (a) Schematic of TA-PPE molecular junction. Comparison between the experimental (I, solid line) and the theoretical (II, dashed lines) I−V (b) and dI/dV (c) characteristics. The dephasing factor is assumed to be three under the negative bias and ten under the positive bias. Reproduced with permission from ref 64. Copyright 2006 American Physical Society.

The authors declare no competing financial interest. Biographies Huanli Dong received her Ph.D. degree (2009) in Physical Chemistry from Institute of Chemistry, Chinese Academy of Sciences (ICCAS). She is now a Professor of Key Laboratory of Organic Solids, ICCAS. She was awarded as an Outstanding Young Scientist of NSFC in 2012, and won the Prize for Young Chemists of Chinese Chemical Society in 2014. Her research interests include molecular materials, crystals and devices of organic/polymeric semiconductors.

with the experimental value of 0.23−0.26 V. It is interesting to see that the electronic structure of an 18 nm long TA-PPE polymer is still largely quantized. It is noted that the experimental features below 2.5 V are not well reproduced by the calculations, which could be caused by either the uncertainty of the calculated Fermi energy or the one electron charging effect that is not included in the modeling.

Wenping Hu received his Ph.D. degree of Physical Chemistry from ICCAS in 1999. Then he joined Osaka University and Stuttgart University as a research fellow of Japan Society for the Promotion of Sciences and Alexander von Humboldt, respectively. In 2003, he worked in Nippon Telephone and Telegraph (NTT) and then returned to ICCAS and was promoted to full professor. He was appointed as Cheung Kong Professor of Minstry of Education of China in 2014. He was elected as an assistant president of Tianjin University in 2013 and vice president of the university in 2016. His research focuses on molecular electronics.

7. SUMMARY AND OUTLOOK Exploring the fundamental charge transport in conjugated polymers remains greatly challenging in polymeric electronics due to the natural disorders of polymer systems in solid state. In this Account, with TA-PPE as an example of conjugated polymers, we introduced a model method of “multilevel investigation” of conjugated polymers to approach their charge transport properties, from disordered films, ordered films, and micro- and nanometer crystals to molecular devices. The following findings are presented: (1) Significant performance improvement was demonstrated with the increasing polymer molecular orders in the solid

■

ACKNOWLEDGMENTS This work was supported financially by the Ministry of Science and Technology of China (Grants 2016YFB04001100, 2013CB933403, and 2013CB933504), the National Natural 2441

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research

Molecules, Macromolecules, and Supramolecular Architectures. Acc. Chem. Res. 2012, 45, 1193−1202. (22) Hu, W. P.; Nakashima, H.; Wang, E. J.; Furukawa, K.; Li, H. X.; Luo, Y.; Shuai, Z. G.; Kashimura, Y.; Liu, Y. Q.; Torimitsu, K. Advancing Conjugated Polymers Into Nanometer-Scale Devices. Pure Appl. Chem. 2006, 78, 1803−1822. (23) Moroni, M.; Lemoigne, J.; Luzzati, S. Rigid-Rod Conjugated Polymers for Nonlinear Optics. 1. Characterization and Linear Optical-Properties of Poly(aryleneethynylene) Derivatives. Macromolecules 1994, 27, 562−571. (24) Bunz, U. H. F. Poly(aryleneethynylene)s: Syntheses, Properties, Structures, and Applications. Chem. Rev. 2000, 100, 1605−1644. (25) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Molecular SelfAssembly and Nanochemistry-A Chemistry Strategy for The Synthesis of Nanostructures. Science 1991, 254, 1312−1319. (26) Tour, J. M. Molecular Electronics. Synthesis and Testing of Components. Acc. Chem. Res. 2000, 33, 791−804. (27) Samori, P.; Severin, N.; Müllen, K.; Rabe, J. P. Macromolecular Fractionation of Rod-Like Polymers at Atomically Flat Solid-Liquid Interfaces. Adv. Mater. 2000, 12, 579−582. (28) Pschirer, N. G.; Miteva, T.; Evans, U.; Roberts, R. S.; Marshall, A. R.; Neher, D.; Myrick, M. L.; Bunz, U. H. F. Blue Solid-State Photoluminescence and Electroluminescence from Novel Poly(paraphenyleneethynylene) Copolymers. Chem. Mater. 2001, 13, 2691− 2696. (29) Wilson, J. N.; Windscheif, P. M.; Evans, U.; Myrick, M. L.; Bunz, U. H. F. Band Gap Engineering of Poly(p-phenyleneethynylene)s: Cross-Conjugated PPE-PPV Hybrids. Macromolecules 2002, 35, 8681− 8683. (30) Dong, H. L.; Li, H. X.; Wang, E. J.; Wei, Z. M.; Xu, W.; Hu, W. P.; Yan, S. K. Ordering Rigid Rod Conjugated Polymer Molecules for High Performance Photoswitchers. Langmuir 2008, 24, 13241−13244. (31) Montali, A.; Bastiaansen, G.; Smith, P.; Weder, C. Polarizing Energy Transfer in Photoluminescent Materials for Display Applications. Nature 1998, 392, 261−264. (32) Grell, M.; Bradley, D. D. C. Polarized Luminescence from Oriented Molecular Materials. Adv. Mater. 1999, 11, 895−905. (33) Yao, Y. F.; Dong, H. L.; Hu, W. P. Ordering of Conjugated Polymer Molecules: Recent Advances and Perspectives. Polym. Chem. 2013, 4, 5197−5205. (34) Wittmann, J. C.; Smith, P. Highly Oriented Thin-Films of Poly(tetrafluoroethylene) as A Substrate for Oriented Growth of Materials. Nature 1991, 352, 414−417. (35) Dong, H. L.; Li, H. X.; Wang, E. J.; Yan, S. K.; Zhang, J. M.; Yang, C. M.; Takahashi, I.; Nakashima, H.; Torimitsu, K.; Hu, W. P. Molecular Orientation and Field-Effect Transistors of a Rigid Rod Conjugated Polymer Thin Films. J. Phys. Chem. B 2009, 113, 4176− 4180. (36) Shao, W.; Dong, H. L.; Jiang, L.; Hu, W. P. Morphology Control for High Performance Organic Thin Film Transistors. Chem. Sci. 2011, 2, 590−600. (37) Liu, S. H.; Wang, W. C. M.; Briseno, A. L.; Mannsfeld, S. C. E.; Bao, Z. N. Controlled Deposition of Crystalline Organic Semiconductors for Field-Effect-Transistor Applications. Adv. Mater. 2009, 21, 1217−1232. (38) Li, R. J.; Hu, W. P.; Liu, Y. Q.; Zhu, D. B. Micro- and Nanocrystals of Organic Semiconductors. Acc. Chem. Res. 2010, 43, 529−540. (39) Tang, Q. X.; Jiang, L.; Tong, Y. H.; Li, H. X.; Liu, Y. L.; Wang, Z. H.; Hu, W. P.; Liu, Y. Q.; Zhu, D. B. Micrometer- and NanometerSized Organic Single-Crystalline Transistors. Adv. Mater. 2008, 20, 2947−2951. (40) Podzorov, V. Organic Single Crystals: Addressing the Fundamentals of Organic Electronics. MRS Bull. 2013, 38, 15−27. (41) Yao, Y. F.; Dong, H. L.; Hu, W. P. Charge Transport in Organic and Polymeric Semiconductors for Flexible and Stretchable Devices. Adv. Mater. 2016, 28, 4513−4523.

Science Foundation of China (Grants 51633006, 51222306, 91222203, 91233205, and 91433115), Chinese Academy of Sciences (Grant XDB12030300), Beijing NOVA Programme (Grant Z131101000413038), Beijing Local College Innovation Team Improve Plan (Grant IDHT20140512), and Youth Innovation Promotion Association CAS.

■

REFERENCES

(1) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; et al. Electrical Conductivity in Doped Polyacetylene. Phys. Rev. Lett. 1977, 39, 1098−1101. (2) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetylene, (CH)X. J. Chem. Soc., Chem. Commun. 1977, 578−580. (3) Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. All-Polymer Field-Effect Transistors Realized by Printing Techniques. Science 1994, 265, 1684−1686. (4) Baeg, K. J.; Caironi, M.; Noh, Y. Y. Toward Printed Integrated Circuits based on Unipolar or Ambipolar Polymer Semiconductors. Adv. Mater. 2013, 25, 4210−4244. (5) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911−918. (6) Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D. Organic Transistors in Optical Displays and Microelectronic Applications. Adv. Mater. 2010, 22, 3778−3798. (7) Hu, W. P. Organic Optoelectronics; Wiley-VCH: Weinheim, Germany, 2013. (8) Dong, H. L.; Zhu, H. F.; Meng, Q.; Gong, X.; Hu, W. P. Organic Photoresponse Materials and Devices. Chem. Soc. Rev. 2012, 41, 1754−1808. (9) Li, Y. F. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723−733. (10) Lu, L. Y.; Zheng, T. Y.; Wu, Q. H.; Schneider, A. M.; Zhao, D. L.; Yu, L. P. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666−12731. (11) Dong, H. L.; Fu, X. L.; Liu, J.; Wang, Z. R.; Hu, W. P. Key Points for High-Mobility Organic Field-Effect Transistors. Adv. Mater. 2013, 25, 6158−6182. (12) Feng, L. L.; Gu, P. C.; Yao, Y. F.; Dong, H. L.; Hu, W. P. HighMobility Polymeric Semiconductors. Chi. Sci. Bull. 2015, 60, 2169− 2189. (13) Sze, S. M. Physics of Semiconductor Devices; Wiley: New York, 1981. (14) Chaikin, P. M.; Lubensky, T. C. Principle of Condensed Matter Physics; Cambridge University Press: Cambridge, U.K., 1997. (15) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999. (16) Su, W. P.; Schrieffer, J. R.; Heeger, A. J. Solitons in Polyacetylene. Phys. Rev. Lett. 1979, 42, 1698−1701. (17) Salleo, A.; Kline, R. J.; DeLongchamp, D. M.; Chabinyc, M. L. Microstructural Characterization and Charge Transport in Thin Films of Conjugated Polymers. Adv. Mater. 2010, 22, 3812−3838. (18) Sirringhaus, H.; Bird, M.; Richards, T.; Zhao, N. Charge Transport Physics of Conjugated Polymer Field-Effect Transistors. Adv. Mater. 2010, 22, 3893−3898. (19) Dong, H. L.; Jiang, S. D.; Jiang, L.; Liu, Y. L.; Li, H. X.; Hu, W. P.; Wang, E. J.; Yan, S. K.; Wei, Z. M.; Xu, W.; Gong, X. Nanowire Crystals of a Rigid Rod Conjugated Polymer. J. Am. Chem. Soc. 2009, 131, 17315−17320. (20) Gu, P. C.; Yao, Y. F.; Dong, H. L.; Hu, W. P. Preparation, Characterization and Field-Effect Transistor Applications of Conjugated Polymer Micro/Nano-crystal. Acta Polym. Sin. 2014, 1029− 1040. (21) Saeki, A.; Koizumi, Y.; Aida, T.; Seki, S. Comprehensive Approach to Intrinsic Charge Carrier Mobility in Conjugated Organic 2442

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443

Article

Accounts of Chemical Research (42) Kim, D. H.; Han, J. T.; Park, Y. D.; Jang, Y.; Cho, J. H.; Hwang, M.; Cho, K. Single-Crystal Polythiophene Microwires Grown by SelfAssembly. Adv. Mater. 2006, 18, 719−723. (43) Lim, J. A.; Liu, F.; Ferdous, S.; Muthukumar, M.; Briseno, A. L. Polymer Semiconductor Crystals. Mater. Today 2010, 13, 14−24. (44) Liu, Y.; Dong, H. L.; Jiang, S. D.; Zhao, G. Y.; Shi, Q. Q.; Tan, J. H.; Jiang, L.; Hu, W. P.; Zhan, X. W. High Performance Nanocrystals of a Donor-Acceptor Conjugated Polymer. Chem. Mater. 2013, 25, 2649−2655. (45) Jiang, L.; Gao, J. H.; Wang, E. J.; Li, H. X.; Wang, Z. H.; Hu, W. P.; Jiang, L. Organic Single-Crystalline Ribbons of A Rigid ″H″-type Anthracene Derivative and High-Performance, Short-Channel FieldEffect Transistors of Individual Micro/Nanometer-Sized Ribbons Fabricated by An ″Organic Ribbon Mask″ Technique. Adv. Mater. 2008, 20, 2735−2740. (46) Kim, J. H.; Lee, D. H.; Yang, D. S.; Heo, D. U.; Kim, K. H.; Shin, J.; Kim, H.-J.; Baek, K.-Y.; Lee, K.; Baik, H.; Cho, M. J.; Choi, D. H. Novel Polymer Nanowire Crystals of Diketopyrrolopyrrole-Based Copolymer with Excellent Charge Transport Properties. Adv. Mater. 2013, 25, 4102−4106. (47) Wang, S. H.; Kappl, M.; Liebewirth, I.; Mueller, M.; Kirchhoff, K.; Pisula, W.; Müllen, K. Organic Field-Effect Transistors based on Highly Ordered Single Polymer Fibers. Adv. Mater. 2012, 24, 417− 420. (48) Wang, E. J.; Wang, C. L.; Meng, Q.; Li, H. X.; Hu, W. P.; Zhu, D. B. Syntheses of Molecular Wires Containing Redox Center: Reversible Redox Property and Good Energy Level Matching with Au Electrode. Chin. Chem. Lett. 2008, 19, 1285−1289. (49) Wang, E. J.; Li, H. X.; Hu, W. P.; Zhu, D. B. Synthesis and Characterization of New Type Molecular Wires with Tetrathiafulvalene as Redox Center. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2707−2713. (50) Li, T.; Hauptmann, J. R.; Wei, Z. M.; Petersen, S.; Bovet, N.; Vosch, T.; Nygard, J.; Hu, W. P.; Liu, Y. Q.; Bjornholm, T.; Norgaard, K.; Laursen, B. W. Solution-Processed Ultrathin Chemically Derived Graphene Films as Soft Top Contacts for Solid-State Molecular Electronic Junctions. Adv. Mater. 2012, 24, 1333−1339. (51) Wang, Z. R.; Dong, H. L.; Li, T.; Hviid, R.; Zou, Y.; Wei, Z. M.; Fu, X. L.; Wang, E. J.; Zhen, Y. G.; Norgaard, K.; Laursen, B. W.; Hu, W. P. Role of Redox Centre in Charge Transport Investigated by Novel Self-Assembled Conjugated Polymer Molecular Junctions. Nat. Commun. 2015, 6, 7478. (52) Hoofman, R. J. O. M.; de Haas, M. P.; Siebbeles, L. D. A.; Warman, J. M. Highly Mobile Electrons and Holes on Isolated Chains of The Semiconducting Polymer Poly(phenylene vinylene). Nature 1998, 392, 54−56. (53) Prins, P.; Grozema, F. C.; Schins, J. M.; Patil, S.; Scherf, U.; Siebbeles, L. D. A. High Intrachain Hole Mobility on Molecular Wires of Ladder-Type Poly(p-Phenylenes). Phys. Rev. Lett. 2006, 96, 146601. (54) Terao, J.; Wadahama, A.; Matono, A.; Tada, T.; Watanabe, S.; Seki, S.; Fujihara, T.; Tsuji, Y. Design Principle for Increasing Charge Mobility of p-Conjugated Polymers using Regularly Localized Molecular Orbitals. Nat. Commun. 2013, 4, 1691. (55) Terao, J. Permethylated Cyclodextrin-based Insulated Molecular Wires. Polym. Chem. 2011, 2, 2444. (56) Cui, A. J.; Dong, H. L.; Hu, W. P. Nanogap Electrodes Towards Solid State Single-Molecule Transistors. Small 2015, 11, 6115−6141. (57) Li, T.; Hu, W. P.; Zhu, D. B. Nanogap Electrodes: Nanogap Electrodes. Adv. Mater. 2010, 22, 286−300. (58) Jiang, L.; Dong, H. L.; Meng, Q.; Tan, J. H.; Jiang, W.; Xu, C. Y.; Wang, Z. H.; Hu, W. P. Molecular Crystal Lithography: A Facile and Low-Cost Approach to Fabricate Nanogap Electrodes. Adv. Mater. 2012, 24, 694−698. (59) Cui, A. J.; Liu, Z.; Dong, H. L.; Wang, Y. J.; Zhen, Y. G.; Li, W. X.; Li, J. J.; Gu, C. Z.; Hu, W. P. Single Grain Boundary Break Junction for Suspended Nanogap Electrodes with Gapwidth Down to 1−2 nm by Focused Ion Beam Milling. Adv. Mater. 2015, 27, 3002−3006. (60) Zhu, H. F.; Li, T.; Zhang, Y. J.; Dong, H. L.; Song, J. S.; Zhao, H. P.; Wei, Z. M.; Xu, W.; Hu, W. P.; Bo, Z. S. High-Performance

Organic Nanoscale Photoswitches Based on Nanogap Electrodes Coated with a Blend of Poly(3-hexylthiophene) and 6,6 -Phenyl-C61butyric Acid Methyl Ester (P3HT:PCBM). Adv. Mater. 2010, 22, 1645−1648. (61) Hu, W. P.; Nakashima, H.; Furukawa, K.; Kashimura, Y.; Ajito, K.; Torimitsu, K. Self-Assembled Rigid Conjugated Polymer Nanojunction and Its Nonlinear Current-Voltage Characteristics at Room Temperature. Appl. Phys. Lett. 2004, 85, 115−117. (62) Hu, W. P.; Nakashima, H.; Furukawa, K.; Kashimura, Y.; Ajito, K.; Han, C.; Torimitsu, K. Carrier Injection from Gold Electrodes into Thioacetyl-end-functionalized Poly(para-phenyleneethynylene)s. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 165207. (63) Hu, W. P.; Nakashima, H.; Furukawa, K.; Kashimura, Y.; Ajito, K.; Liu, Y. Q.; Zhu, D. B.; Torimitsu, K. A Self-Assembled Nano Optical Switch and Transistor based on A Rigid Conjugated Polymer, Thioacetyl-end-functionalized Poly(para-phenylene ethynylene). J. Am. Chem. Soc. 2005, 127, 2804−2805. (64) Hu, W. P.; Jiang, J.; Nakashima, H.; Luo, Y.; Kashimura, Y.; Chen, K. Q.; Shuai, Z.; Furukawa, K.; Lu, W.; Liu, Y. Q.; Zhu, D. B.; Torimitsu, K. Electron Transport in Self-Assembled Polymer Molecular Junctions. Phys. Rev. Lett. 2006, 96, 027801.

2443

DOI: 10.1021/acs.accounts.6b00368 Acc. Chem. Res. 2016, 49, 2435−2443