Interchain Charge Transport Research Articles

Increasing the Content of Edge-On Orientation to Improve the Hole Mobility of IDTBT Film by the van der Waals Interaction between the Side Chain and Alkane Additives.

The molecular orientation in the conjugated polymer film critically impacts the performance of organic electronic devices. As for organic field-effect transistors (OFETs), the edge-on orientation is beneficial for efficient interchain charge transport. However, for the near amorphous indacenodithiophene-co-benzothiadiazole (IDTBT) film, the face-on orientation was dominated in the drop cast thin film. Herein, we proposed a strategy to increase the edge-on orientation in IDTBT films by inserting n-hexadecane (16C) additives with a high boiling point between the side chains. Because the 16C additives had a similar structure to that of the side chain, the 16C additives were distributed around the side chain in the nucleation stage. Then, the backbone aggregated quickly with the nuclei formation speed (k) nearly twice as high as that without the 16C additives. In the growth stage, some face-on nuclei turned to edge-on nuclei due to the van der Waals interactions between the side chain and 16C additives. This was verified by the results of in situ GIWAXS, where the intensity of (010) decreased and the intensity of (100) increased in the out-of-plane direction. The edge-on orientation content of the film with 16C additives reached 57%, higher than that of the neat film (33%). Moreover, compared to the neat IDTBT film, the lamellar packing distance of the film with 16C additives increased the lamellar packing distance from 16.97 to 23.26 Å and the π-π stacking distance decreased from 4.36 to 4.19 Å. The hole mobility of the film with 16C additives (3.24 ± 0.19 cm2 V-1 s-1) was higher than that of the neat film (0.94 ± 0.07 cm2 V-1 s-1).

Urea/Thiourea Imine Linkages Provide Accessible Holes in Flexible Covalent Organic Frameworks and Dominates Self‐Adaptivity and Exciton Dissociation

Unraveling the robust self‐adaptivity and minimal energy‐dissipation of soft reticular materials for environmental catalysis presents a compelling yet unexplored avenue. Herein, a top‐down strategy, tailoring from the unique linkage basis, flexibility degree, skeleton electronics to trace‐guest adaptability, is proposed to fill the understanding gap between micro‐soft covalent organic frameworks (COFs) and photocatalytic performance. The thio(urea)‐basis‐dominated linkage within benzotrithiophene‐based COFs induce the framework contraction/swelling (intralayer micro‐flexibility) in response to tetrahydrofuran or water. Adaptability of micro‐flexible thiourea‐COF with pore hydrophilicity not only contributes to the favorable mass transfer, but also enhances the accessible redox active sites, culminating in nearly 100% removal of micropollutant with low‐energy dissipation in wastewater. The incorporating urea/thiourea into imine linkage facilitates polarization reduction and exciton dissociation within skeleton wall, inducing strong localization for holes. This transformation facilitates interchain charge transport and unbalanced distribution conducive to oxidative holes‐mediated micropollutant decomposition.

Urea/Thiourea Imine Linkages Provide Accessible Holes in Flexible Covalent Organic Frameworks and Dominates Self-Adaptivity and Exciton Dissociation.

Unraveling the robust self-adaptivity and minimal energy-dissipation of soft reticular materials for environmental catalysis presents a compelling yet unexplored avenue. Herein, a top-down strategy, tailoring from the unique linkage basis, flexibility degree, skeleton electronics to trace-guest adaptability, is proposed to fill the understanding gap between micro-soft covalent organic frameworks (COFs) and photocatalytic performance. The thio(urea)-basis-dominated linkage within benzotrithiophene-based COFs induce the framework contraction/swelling (intralayer micro-flexibility) in response to tetrahydrofuran or water. Adaptability of micro-flexible thiourea-COF with pore hydrophilicity not only contributes to the favorable mass transfer, but also enhances the accessible redox active sites, culminating in nearly 100% removal of micropollutant with low-energy dissipation in wastewater. The incorporating urea/thiourea into imine linkage facilitates polarization reduction and exciton dissociation within skeleton wall, inducing strong localization for holes. This transformation facilitates interchain charge transport and unbalanced distribution conducive to oxidative holes-mediated micropollutant decomposition.

In situ continuous hydrogen-bonded engineering for intrinsically stretchable and healable high-mobility polymer semiconductors.

As a key component for wearable electronics, intrinsically stretchable and healable semiconducting polymers are scarce because carrier mobility is often reduced with increasing stretchability and self-healability. Here, we combine stepwise polymerization and thermal conversion to introduce in situ continuous hydrogen bonding sites in a polymer backbone without breaking the conjugation or introducing bulky softer side chains, benefiting the intrachain and interchain charge transport. We demonstrate that a regular sequence structure facilitated the formation of big nanofibers with a high degree of aggregation, providing the loose and porous thin film with simultaneously improved charge transport, stretchability, and self-healability. The mobility of damaged devices can be recovered to 81% after a healing treatment. Fully stretchable transistor based on the designed polymer exhibited a greatly enhanced mobility up to 1.08 square centimeters per volt per second under 100% strain, which is an unprecedented value and constitutes a major step for the development of intrinsically stretchable and healable semiconducting polymers.

Using Molecular Structure to Tune Intrachain and Interchain Charge Transport in Indacenodithiophene-Based Copolymers.

In this work, we compare two structurally near-amorphous rigid-rod polymers─poly(indacenodithiophene-co-benzothiadiazole), p(IDT-BT), and poly(indacenodithiophene-co-benzopyrollodione), p(IDT-BPD)─with orders of magnitude different mobilities to understand the effect charge carrier intrachain delocalization has on electronic transport. Quantum chemical calculations show that p(IDT-BPD) has a barrier to torsion that is significantly lower than that of p(IDT-BT) and is thus more likely to have reduced conjugation lengths. We utilize absorption and photoluminescence spectroscopy to characterize energetic disorder and show that p(IDT-BPD) has higher energetic disorder. Charge modulation spectroscopy (CMS) and model calculations are used to show that charge carriers are substantially delocalized in p(IDT-BT) and occupy near-uniform energetic environments. We find that mobility activated hopping barriers are similar in these two materials. Electronic structure calculations show that both intrachain and interchain couplings of monomer units are poor enough in p(IDT-BPD) that charge carriers collapse to single IDT units and transport via a through-space tunneling mechanism. This work highlights the remarkable charge transport properties of p(IDT-BT) by showing that high mobilities are achievable on device-relevant length scales with only 1D carrier delocalization.

Microstructural Model of Indacenodithiophene-co-benzothiadiazole Polymer: π-Crossing Interactions and Their Potential Impact on Charge Transport.

Morphological and electronic properties of indacenodithiophene-co-benzothiadiazole (IDTBT) copolymer with varying molecular weights are calculated through combined molecular dynamics (MD) and quantum chemical (QC) methods. Our study focuses on the polymer chain arrangements, interchain connectivity pathways, and interplay between morphological and electronic structure properties of IDTBT. Our models, which are verified against GIWAXS measurements, show a considerable number of BT-BT π-π interactions with a (preferential) perpendicular local orientation of polymer chains due to the steric hindrance of bulky side chains around IDT. Although our models predict a noncrystalline structure for IDTBT, the BT-BT (interchain) crossing points show a considerable degree of short-range order in spatial arrangement which most likely result in a mesh-like structure for the polymer and provide efficient pathways for interchain charge transport.

High n-type and p-type conductivities and power factors achieved in a single conjugated polymer.

The charge transport properties of conjugated polymers are commonly limited by the energetic disorder. Recently, several amorphous conjugated polymers with planar backbone conformations and low energetic disorder have been investigated for applications in field-effect transistors and thermoelectrics. However, there is a lack of strategy to finely tune the interchain π-π contacts of these polymers that severely restricts the energetic disorder of interchain charge transport. Here, we demonstrate that it is feasible to achieve excellent conductivity and thermoelectric performance in polymers based on thiophene-fused benzodifurandione oligo(p-phenylenevinylene) through reducing the crystallization rate of side chains and, in this way, carefully controlling the degree of interchain π-π contacts. N-type (p-type) conductivities of more than 100 S cm-1 (400 S cm-1) and power factors of more than 200μW m-1 K-2 (100 μW m-1 K-2) were achieved within a single polymer doped by different dopants. It further demonstrated the state-of-the-art power output of the first flexible single-polymer thermoelectric generator.

Tuning the Charge Transport Property and Photocatalytic Activity of Anthracene-Based 1D π-d Conjugated Coordination Polymers by Interlayer Stacking.

One-dimensional (1D) π-d-conjugated coordination polymers (CCPs) with charge delocalization have attracted significant attention due to their potential application in energy conversion and storage. However, the fundamental understanding of the correlation of their structural parameters with photophysical and photocatalytic properties remains underexplored. Herein, we report three novel Cu-node anthracene-based 1D π-d CCPs with systematic variation of steric groups (Ph > Me > H) at the 9 and 10 position of anthracene (denoted as AnPh, AnMe, and AnH), which is aimed at altering the stacking of the polymer chains and its impact on the inter-chain charge transport property. Using the combination of steady-state X-ray absorption spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and electrochemical impedance spectroscopy, we show that the linear ligands (AnPh, AnMe, and AnH) with different degrees of steric groups (Ph > Me > H) introduced at the 9 and 10 position of anthracene can alter the stacking of the polymer chains and thus impact their crystallinity, charge separation, and charge transport property, which in turn impacts their photocatalytic performance for hydrogen evolution reaction.

Systematic Investigation of Solution-State Aggregation Effect on Electrical Conductivity in Doped Conjugated Polymers

Systematic Investigation of Solution-State Aggregation Effect on Electrical Conductivity in Doped Conjugated Polymers

Highly conductive and thermally stable nanoparticle-conjugated polymer compounds through environmentally friendly in situ synthesis

In this paper, an environmentally-friendly in situ synthesis of highly conductive and thermally stable nanoparticle-conjugated polymer nano-compounds is introduced. The water-based in situ chemical synthesis enables the successful polymerization of conductive, conjugated polypyrrole: polystyrene sulfonate (PPY:PSS) polymer and the incorporation of silver (Ag) nanoparticles without morphological deformation. The Ag nanoparticles dispersed within the PPY:PSS acted as a mass transport barrier, thus enhancing the thermal stability of the obtained Ag-PPY:PSS. In addition, the enhanced electrical conductivity of Ag-PPY:PSS was attributed to the embedded Ag nanoparticles generating a low-contact resistance conduction pathway, thereby facilitating inter-chain charge transport. Moreover, the conductivity of Ag-PPY:PSS was well tuned by controlling the weight percentage of Ag nanoparticles in the reaction mixture. The environmentally-friendly synthesis of highly conductive and thermally stable nano-compounds will contribute to the realization of next-generation green electronic applications.

Control of Crystallite Orientation in Diketopyrrolopyrrole-Based Semiconducting Polymers via Tuning of Intermolecular Interactions.

Numerous previous studies have focused on the notion that semiconducting polymers with an edge-on dominant orientation are advantageous for horizontal charge transport, whereas polymers with a face-on dominant orientation are advantageous for vertical charge transport, since the crystallite orientation determines the π-π stacking direction, which in turn affects the interchain charge transport direction. Here, we report that the crystallite orientation is dependent on the intermolecular interactions in the semiconducting polymer. In this study, we control the intermolecular interactions in a donor-acceptor (D-A) semiconducting polymer via side chain engineering. To perform side chain engineering, we use two different polymers: one with side chains on only A units (PDPP-B) and the other with side chains on both D and A units (PDPP-C8). We observe that PDPP-C8 is characterized by weaker intermolecular interactions due to the additional side chains on D units. A morphological analysis reveals that PDPP-B and PDPP-C8 films have microstructures that are characterized by edge-on and face-on dominant orientations, respectively. Therefore, we demonstrate that our strategies effectively control intermolecular interactions and, consequently, the crystallite orientation. Finally, we compare the vertical and horizontal mobilities of both polymer films. These results show that the crystallite orientation has significant influence on charge transport behaviors.

Tunable Three-Dimensional Nanostructured Conductive Polymer Hydrogels for Energy-Storage Applications.

Three-dimensional (3D) nanostructured conducting polymer hydrogels represent a group of high-performance electrochemical energy-storage materials. Here, we demonstrate a molecular self-assembly approach toward controlled synthesis of nanostructured polypyrrole (PPy) conducting hydrogels, which was "cross-linked" by a conjugated dopant molecule trypan blue (TB) to form a 3D network with controlled morphology. The protonated TB by ion bonding aligns the free sulfonic acid groups into a certain spatial structure. The sulfonic acid group and the PPy chain are arranged by a self-sorting mechanism to form a PPy nanofiber structure by electrostatic interaction and hydrogen bonding. It is found that PPy hydrogels doped with varying dopant concentrations and changing dopant molecules exhibited controllable morphology and tunable electrochemical properties. In addition, the conjugated TB dopants promoted interchain charge transport, resulting in higher electrical conductivity (3.3 S/cm) and pseudocapacitance for the TB-doped PPy, compared with PPy synthesized without TB. When used as supercapacitor electrodes, the TB-doped PPy hydrogel reaches maximal specific capacitance of 649 F/g at the current density 1 A/g. The result shows that PPy nanostructured hydrogels can be tuned for potential applications in next-generation energy-storage materials.

Carbon nanotubes assisting interchain charge transport in semiconducting polymer thin films towards much improved charge carrier mobility

Conjugated polymers attracted much attention in the past few decades due to their wide applications in various optoelectronic devices and circuits. The charge transport process in conjugated polymers mainly occurs in the intrachain and interchain parts, where the interchain charge transport is generally slower than intrachain transport and may slow down the whole charge transport properties. Aiming at this issue, herein we employ semiconducting single-walled carbon nanotubes (s-SWNTs) as efficient charge-transporting jointing channels between conjugated polymer chains for improving the charge transport performance. Taking thetypical conjugated polymer, ploy- N -alkyl-diketopyrrolo-pyrrole-dithienylthieno[3,2- b ]thiophene (PDPP-TT) as an example, polymer thin film transistors (PTFTs) based on the optimized blended films of PDPP-TT/s-SWNTs exhibit an obviously increasing device performance compared with the devices based on pure PDPP-TT films, with the hole and electron mobility increased from 2.32 to 12.32 cm2 V−1 s−1 and from 2.02 to 5.77 cm2 V−1 s−1, respectively. This result suggests the importance of forming continuous conducting channelsin conjugated polymer thin films, which can also be extended to other polymeric electronic and optoelectronic devices to promote their potential applications in large-area, low-cost and high performance polymeric electronic devices andcircuits.

Exploiting π–π Stacking for Stretchable Semiconducting Polymers

Although intermolecular charge transport is known to occur via π–π stacking, the influence of π–π stacking on the mechanical properties of polymers has received little attention compared with other dynamic noncovalent interactions. Herein, we demonstrate a method to enhance stretchability via lowering crystallinity and increasing π–π stacking of thiophene-based random copolymer chains, which causes π–π stacking-induced polymer networks to form within the fully conjugated semiconducting polymer matrix. The polymer networks contain coiled amorphous chains that aid energy dissipation when the polymer film is subjected to strain; furthermore, the π–π stacking prevents the chains from irreversible sliding out of place due to the applied strain and provides interchain charge transport. Consequently, we are able to improve the polymer’s mechanical properties such as elongation at break, tensile strength, and toughness along with charge mobility. Additionally, our polymer shows great tolerance to a 40% strain wit...

"Double-Cable" Conjugated Polymers with Linear Backbone toward High Quantum Efficiencies in Single-Component Polymer Solar Cells.

A series of "double-cable" conjugated polymers were developed for application in efficient single-component polymer solar cells, in which high quantum efficiencies could be achieved due to the optimized nanophase separation between donor and acceptor parts. The new double-cable polymers contain electron-donating poly(benzodithiophene) (BDT) as linear conjugated backbone for hole transport and pendant electron-deficient perylene bisimide (PBI) units for electron transport, connected via a dodecyl linker. Sulfur and fluorine substituents were introduced to tune the energy levels and crystallinity of the conjugated polymers. The double-cable polymers adopt a "face-on" orientation in which the conjugated BDT backbone and the pendant PBI units have a preferential π-π stacking direction perpendicular to the substrate, favorable for interchain charge transport normal to the plane. The linear conjugated backbone acts as a scaffold for the crystallization of the PBI groups, to provide a double-cable nanophase separation of donor and acceptor phases. The optimized nanophase separation enables efficient exciton dissociation as well as charge transport as evidenced from the high-up to 80%-internal quantum efficiency for photon-to-electron conversion. In single-component organic solar cells, the double-cable polymers provide power conversion efficiency up to 4.18%. This is one of the highest performances in single-component organic solar cells. The nanophase-separated design can likely be used to achieve high-performance single-component organic solar cells.

Approaching Intra- and Interchain Charge Transport of Conjugated Polymers Facilely by Topochemical Polymerized Single Crystals.

Charge transport of small molecules is measured well with scanning tunneling microscopy, conducting atomic force microscopy, break junction, nanopore, and covalently bridging gaps. However, the manipulation and measurement of polymer chains remain a long-standing fundamental issue in conjugated polymers and full of challenge since conjugated polymers are naturally disordered materials. Here, a fundamental breakthrough in generating high-quality conjugated-polymer nanocrystals with extended conjugation and exceptionally high degrees of order using a surface-supported topochemical polymerization method is demonstrated. In the crystal the conjugated-polymer chains are extended along the long axis of the crystal with the side chains perpendicular to the long axis. Devices with conducting channels along the polymer chains show efficient charge transport, nearly two orders of magnitude greater than the interchain charge transport along the π-π stacking direction. This is the first example to clarify intra- and interchain charge transport based on an individual single crystal of conjugated polymers, and demonstrate the importance of intrachain charge transport in plastic electronics.

Conducting Properties of Polyaniline in the THz Region: Investigations of Various Processing Conditions

The conduction properties of polyaniline (PAni) with various processing conditions were demonstrated using terahertz time-domain spectroscopy. The effects of protonation were observed by varying the doping time and annealing time. The measurements agreed with the Mott-Davis hopping model with the exponent varying inversely with the protonation conditions. Also, the density of the samples increased with compression pressure thereby increasing the inter-chain and inter-particle charge transport. Lastly, the THz response on PAni-polyethylene pellets showed dominant contribution of PAni network.

Supramolecular control over the morphology of bio-inspired poly(3-hexylthiophene) for organic thin film transistors

We have developed a strategy for modifying the channel layer of organic thin film transistors (OTFTs) through side-chain induced self-organization into a well-ordered film. To obtain selectively self-patterned layers, we treated an adenine-functionalized poly(3-hexylthiophene) (PAT) with adenosine triphosphate (ATP). Using this strategy, interchain charge transport resulting from π-conjugation was selected to control the polymer morphology, without the need of additional chemical synthetic processing. The side chain–induced self-organization can be understood in terms of supramolecular interactions. The π-electrons were delocalized among the thiophene rings, thereby improving the interchain charge transport ability; the resulting planar π-electron system in PAT:ATP resulted in closer intermolecular π–π distances, facilitating enhanced charge carrier mobility within a fibrillar structure. The PAT:ATP-based OTFT device exhibited moderate to improved electronic characteristics, with an average field mobility of 3.2 × 10−4 cm2 V−1 s−1 at −30 V and a threshold voltage (Vth) of 5 V, and an on/off current ratio of 106. This method has great potential for inducing selective intermolecular interactions in fully solution processed electronic devices.

Dopant-Enabled Supramolecular Approach for Controlled Synthesis of Nanostructured Conductive Polymer Hydrogels.

Conducting polymer hydrogels emerge as a novel class of polymeric materials that show great potential in many energy, environmental, and biomedical devices. We describe here for the first time a general supramolecular approach toward controlled in situ synthesis of one-dimensional nanostructured conductive hydrogels (polypyrrole (PPy) as a model system) using a rational dopant counterion, which is a disc-shaped liquid crystal molecular copper phthalocyanine-3,4',4″,4‴-tetrasulfonic acid tetrasodium salt (CuPcTs). The dopant molecule CuPcTs cross-linked the PPy chains to form a three-dimensional network that gelated into a hydrogel. The PPy hydrogel could be synthesized in bulk quantities with uniform morphology of self-assembled interconnected nanofibers. The tetra-functional dopant favors a supramolecular self-assembly mechanism to form one-dimensional PPy nanostructures. Furthermore, the enhanced interchain charge transport of CuPcTs doped PPy resulted in greatly enhanced conductivity and pseudocapacitance compared with pristine PPy.

A comparative MD study of the local structure of polymer semiconductors P3HT and PBTTT

Atomistic molecular dynamics simulations of P3HT and PBTTT-C12 at finite temperatures are carried out to investigate the nanoscale structural properties that lead to higher measured hole mobility in PBTTT versus P3HT field-effect transistors. Simulations of the polymer melts show that the structural properties in PBTTT facilitate both intra- and inter-chain charge transport compared with P3HT due to a greater degree of planarity, closer and more parallel stacking of the thiophene and thienothiophene rings, and possible interdigitation of the dodecyl side chains. The crucial role played by the bulky dodecyl side chain and thienothiophene ring, respectively, in determining intra-chain and inter-chain structural order is clarified.