25th anniversary article: high-mobility hole and electron transport conjugated polymers: how structure defines function

Thermal Annealing for High Performance and Memory Behavior in n-Type Organic Electrochemical Transistors

N-type organic mixed ionic electronic conductors (n-OMIECs) struggle to match the performance of p-type counterparts, particularly in bioelectronics' flagship device, the organic electrochemical transistor. Enhancing n-type transistor performance typically necessitates the synthesis of new materials. More sustainable post-synthetic treatments, known to improve organic devices in dry and oxygen-free conditions, are not applied to n-OMIECs. This study introduces thermal annealing to enhance n-OMIECs' electron mobility without sacrificing their ability to take up ionic charges. Annealing increases the crystallinity of p(gNDI-gT2), the first designed n-OMIEC, enhancing its transistor performance to compete with new-generation NDI-based materials. Annealing reduces passive and in operando electrolyte uptake without compromising the device threshold voltage, keeping the device power demand low. The microstructure obtained by annealing, combined with the film's strong near-infrared (NIR) absorption and reduced water swelling, enables the creation of a device that retains photocurrent generated upon frequency-dependent light training. This leads to a microscale, water-compatible memory device that emulates the learning process of biological neurons triggered by light. This simple device can be implemented in artificial neural networks and face recognition platforms and achieve vector-matrix multiplication when fabricated in an array form, showcasing the potential for innovative applications in bioelectronics.

Molecular orientation of dielectric layers at indigo/dielectric interfaces impacts the ordering of indigo films in organic field-effect transistors

Organic multilayer systems, which are stacked layers of different organic materials, are used in various organic electronic devices such as organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). In particular, OFETs are promising as key components in flexible electronic devices. In this study, we investigated how the inclusion of an insulating tetratetracontane (TTC) interlayer in ambipolar indigo-based OFETs can be used to alter the crystallinity and electrical properties of the indigo charge transport layer. We find that the inclusion of a 20-nm-thick TTC film thermally annealed at a low temperature of 70 °C acts to significantly increase the ambipolar electrical transport of the indigo layer. X-ray diffraction, atomic force microscopy, and vibrational sum frequency generation measurements showed that annealing the TTC film significantly improved its ordering. The electronic sum-frequency generation spectra of TTC/indigo bilayers show that this improved ordering of TTC films promotes the growth of crystalline indigo films that exhibit charge mobilities in OFET that are nearly an order of magnitude larger than those measured for devices grown on unannealed TTC layers. Furthermore, using vibrational sum-frequency generation spectroscopy, we found that pre-annealing the TTC layer prior to indigo deposition can suppress the formation of defects within the TTC layer during indigo film growth, which also contributes to enhanced charge transport. Our results highlight the importance of controlling the molecular ordering within the interlayer contacts in OFET structures to achieve an enhanced performance.

Unveiling Nanoscale Ordering in Amorphous Semiconducting Polymers Using Four-Dimensional Scanning Transmission Electron Microscopy

We present four-dimensional (4D) scanning transmission electron microscopy (STEM) analysis to obtain a high level of detail regarding the nanoscale ordering within largely disordered organic semiconducting polymers. Understanding nanoscale molecular ordering in semiconducting polymers is crucial due to its connection to the materials' important properties. However, acquiring such information in a spatially localized manner has been limited by the lack of a nanoscale experimental probe, weak signal from ordering, and radiation damage to the sample. By collecting nanodiffraction patterns with a high dynamic range pixelated detector, we acquired statistically robust, high signal-to-noise ratio diffraction patterns from semiconducting organic materials, including poly(3-hexylthiophene-2,5-diyl) (P3HT), P3HT/[6,6]-phenyl C61 butyric acid methyl ester, and indacenodithiophene-co-benzothiadiazole (IDTBT), which largely have disordered structures. Real-space images of the ordered domains were reconstructed from the 4D-STEM data set for a variety of scattering vectors and in-plane angles to capture the different molecular stacking distances and their in-plane orientation. These were then analyzed to obtain the average size of the ordered domains within the sample. Such measurements were arranged in a two-dimensional (2D) histogram, which showed a direct relationship between the type and size of molecular ordering. Complementary analyses, such as intensity variance and angular correlation, were applied to obtain ordering and symmetry information. These analyses enabled us to directly characterize the alkyl and π-π stacking of P3HT, as well as the fullerene domains caused by donor segregation in the P3HT sample. Furthermore, the analysis also captured changes in the P3HT domains when the fullerenes are incorporated. Lastly, IDTBT showed a much lesser degree of ordering without much disinclination between the domains within the 2D histogram. The 4D-STEM analysis that we report here unveils new details of molecular ordering that can be used to optimize the properties of this important class of materials.

Polymers Containing Phenothiazine, Either as a Dopant or as Part of Their Structure, for Dye-Sensitized and Bulk Heterojunction Solar Cells

Potential photovoltaic technology includes the newly developed dye-sensitized solar cells (DSSCs) and bulk heterojunction (BHJ) solar cells. Owing to their diverse qualities, polymers can be employed in third-generation photovoltaic cells to specifically alter their device elements and frameworks. Polymers containing phenothiazine, either as a part of their structure or as a dopant, are easy and economical to synthesize, are soluble in common organic solvents, and have the potential to acquire desired electrochemical and photophysical properties by mere tuning of their chemical structures. Such polymers have therefore been used either as photosensitizers in dye-sensitized solar cells, where they have produced power conversion efficiency (PCE) values as high as 5.30%, or as donor or acceptor materials in bulk heterojunction solar cells. Furthermore, they have been employed to prepare liquid-free polymer electrolytes for dye-sensitized and bulk heterojunction solar cells, producing a PCE of 8.5% in the case of DSSCs. This paper reviews and analyzes almost all research works published to date on phenothiazine-based polymers and their uses in dye-sensitized and bulk heterojunction solar cells. The impacts of their structure and molecular weight and the amount when used as a dopant in other polymers on the absorption, photoluminescence, energy levels of frontier orbitals, and, finally, photovoltaic parameters are reviewed. The advantages of phenothiazine polymers for solar cells, the difficulties in their actual implementation and potential remedies are also evaluated.

Effect of Chemical Modification on Molecular Ordering in Polydiketopyrrolopyrrole Copolymers: From Liquid Crystalline to Crystalline

The chemical architecture of conjugated polymers is often designed by contemplating and understanding the consequences of structural changes on electronic properties at the molecular level. However, even minor changes to the chemical structure of a polymer can significantly influence the packing arrangement, which also influences the electronic properties of the bulk material. Here, we investigate the molecular arrangement in the ordered state at room temperature of a series of three different polydiketopyrrolopyrroles (PDPPs) in bulk and oriented thin films in detail by wide-angle X-ray scattering and by atomic force microscopy. The changes in the chemical structure of the investigated PDPPs, namely, an additional side chain or a different flanking unit, lead to an increase in long-range order and thereby to a change in the phase state from sanidic ordered via sanidic rectangular or oblique to crystalline.

Accessing the electronic structure of liquid crystalline semiconductors with bottom-up electronic coarse-graining

Understanding the relationship between multiscale morphology and electronic structure is a grand challenge for semiconducting soft materials. Computational studies aimed at characterizing these relationships require the complex integration of quantum-chemical (QC) calculations, all-atom and coarse-grained (CG) molecular dynamics simulations, and back-mapping approaches. However, these methods pose substantial computational challenges that limit their application to the requisite length scales of soft material morphologies. Here, we demonstrate the bottom-up electronic coarse-graining (ECG) of morphology-dependent electronic structure in the liquid-crystal-forming semiconductor, 2-(4-methoxyphenyl)-7-octyl-benzothienobenzothiophene (BTBT). ECG is applied to construct density functional theory (DFT)-accurate valence band Hamiltonians of the isotropic and smectic liquid crystal (LC) phases using only the CG representation of BTBT. By bypassing the atomistic resolution and its prohibitive computational costs, ECG enables the first calculations of the morphology dependence of the electronic structure of charge carriers across LC phases at the ∼20 nm length scale, with robust statistical sampling. Kinetic Monte Carlo (kMC) simulations reveal a strong morphology dependence on zero-field charge mobility among different LC phases as well as the presence of two-molecule charge carriers that act as traps and hinder charge transport. We leverage these results to further evaluate the feasibility of developing mesoscopic, field-based ECG models in future works. The fully CG approach to electronic property predictions in LC semiconductors opens a new computational direction for designing electronic processes in soft materials at their characteristic length scales.

Functionalization and Structural Evolution of Conducting Quasi-One-Dimensional Chevrel-Type Telluride Nanocrystals

Interfacing organic molecular groups with well-defined inorganic lattices, especially in low dimensions, enables synthetic routes for the rational manipulation of both their local or extended lattice structures and physical properties. While appreciably studied in two-dimensional systems, the influence of surface organic substituents on many known and emergent one-dimensional (1D) and quasi-1D (q-1D) crystals has remained underexplored. Herein, we demonstrate the surface functionalization of bulk and nanoscale Chevrel-like q-1D ionic crystals using In2Mo6Te6, a predicted q-1D Dirac semimetal, as the model phase. Using a series of alkyl ammonium (-NR4+; R = H, methyl, ethyl, butyl, and octyl) substituents with varying chain lengths, we demonstrate the systematic expansion of the intrachain c-axis direction and the contraction of the interchain a/b-axis direction with longer chain substituents. Additionally, we demonstrate the systematic expansion of the intrachain c-axis direction and the contraction of the interchain a/b-axis direction as the alkyl chain substituents become longer using a combination of powder X-ray diffraction and Raman experiments. Beyond the structural modulation that the substituted groups can impose on the lattice, we also found that the substitution of ammonium-based groups on the surface of the nanocrystals resulted in selective suspension in aqueous (NH4+-functionalized) or organic solvents (NOc4+-functionalized), imparted fluorescent character (Rhodamine B-functionalized), and modulated the electrical conductivity of the nanocrystal ensemble. Altogether, our results underscore the potential of organic-inorganic interfacing strategies to tune the structural and physical properties of rediscovered Chevrel-type q-1D ionic solids and open opportunities for the development of surface-addressable building blocks for hybrid electronic and optoelectronic devices at the nanoscale.

Recent advancements in implantable neural links based on organic synaptic transistors

The progress of brain synaptic devices has witnessed an era of rapid and explosive growth. Because of their integrated storage, excellent plasticity and parallel computing, and system information processing abilities, various field effect transistors have been used to replicate the synapses of a human brain. Organic semiconductors are characterized by simplicity of processing, mechanical flexibility, low cost, biocompatibility, and flexibility, making them the most promising materials for implanted brain synaptic bioelectronics. Despite being used in numerous intelligent integrated circuits and implantable neural linkages with multiple terminals, organic synaptic transistors still face many obstacles that must be overcome to advance their development. A comprehensive review would be an excellent tool in this respect. Therefore, the latest advancements in implantable neural links based on organic synaptic transistors are outlined. First, the distinction between conventional and synaptic transistors are highlighted. Next, the existing implanted organic synaptic transistors and their applicability to the brain as a neural link are summarized. Finally, the potential research directions are discussed.

Spatially Resolved Chiroptical Spectroscopies Emphasizing Recent Applications to Thin Films of Chiral Organic Dyes

Instrumental techniques able to identify and structurally characterize the aggregation states in thin films of chiral organic π-conjugated materials, from the first-order supramolecular arrangement up to the microscopic and mesoscopic scale, are very helpful for clarifying structure-property relationships. Chiroptical imaging is currently gaining a central role, for its ability of mapping local supramolecular structures in thin films. The present review gives an overview of electronic circular dichroism imaging (ECDi), circularly polarized luminescence imaging (CPLi), and vibrational circular dichroism imaging (VCDi), with a focus on their applications on thin films of chiral organic dyes as case studies.

Achieving ideal transistor characteristics in conjugated polymer semiconductors

Organic thin-film transistors (OTFTs) with ideal behavior are highly desired, because nonideal devices may overestimate the intrinsic property and yield inferior performance in applications. In reality, most polymer OTFTs reported in the literature do not exhibit ideal characteristics. Supported by a structure-property relationship study of several low-disorder conjugated polymers, here, we present an empirical selection rule for polymer candidates for textbook-like OTFTs with high reliability factors (100% for ideal transistors). The successful candidates should have low energetic disorder along their backbones and form thin films with spatially uniform energetic landscapes. We demonstrate that these requirements are satisfied in the semicrystalline polymer PffBT4T-2DT, which exhibits a reliability factor (~100%) that is exceptionally high for polymer devices, rendering it an ideal candidate for OTFT applications. Our findings broaden the selection of polymer semiconductors with textbook-like OTFT characteristics and would shed light upon the molecular design criteria for next-generation polymer semiconductors.

Computational Approaches for Organic Semiconductors: From Chemical and Physical Understanding to Predicting New Materials

While a complete understanding of organic semiconductor (OSC) design principles remains elusive, computational methods─ranging from techniques based in classical and quantum mechanics to more recent data-enabled models─can complement experimental observations and provide deep physicochemical insights into OSC structure-processing-property relationships, offering new capabilities for in silico OSC discovery and design. In this Review, we trace the evolution of these computational methods and their application to OSCs, beginning with early quantum-chemical methods to investigate resonance in benzene and building to recent machine-learning (ML) techniques and their application to ever more sophisticated OSC scientific and engineering challenges. Along the way, we highlight the limitations of the methods and how sophisticated physical and mathematical frameworks have been created to overcome those limitations. We illustrate applications of these methods to a range of specific challenges in OSCs derived from π-conjugated polymers and molecules, including predicting charge-carrier transport, modeling chain conformations and bulk morphology, estimating thermomechanical properties, and describing phonons and thermal transport, to name a few. Through these examples, we demonstrate how advances in computational methods accelerate the deployment of OSCsin wide-ranging technologies, such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), organic thermoelectrics, organic batteries, and organic (bio)sensors. We conclude by providing an outlook for the future development of computational techniques to discover and assess the properties of high-performing OSCs with greater accuracy.

On-surface synthesis of non-benzenoid conjugated polymers by selective atomic rearrangement of ethynylarenes

Here, we report a new on-surface synthetic strategy to precisely introduce five-membered units into conjugated polymers from specifically designed precursor molecules that give rise to low-bandgap fulvalene-bridged bisanthene polymers. The selective formation of non-benzenoid units is finely controlled by the annealing parameters, which govern the initiation of atomic rearrangements that efficiently transform previously formed diethynyl bridges into fulvalene moieties. The atomically precise structures and electronic properties have been unmistakably characterized by STM, nc-AFM, and STS and the results are supported by DFT theoretical calculations. Interestingly, the fulvalene-bridged bisanthene polymers exhibit experimental narrow frontier electronic gaps of 1.2 eV on Au(111) with fully conjugated units. This on-surface synthetic strategy can potentially be extended to other conjugated polymers to tune their optoelectronic properties by integrating five-membered rings at precise sites.

Enhancing the Backbone Coplanarity of n-Type Copolymers for Higher Electron Mobility and Stability in Organic Electrochemical Transistors

Electron-transporting (n-type) conjugated polymers have recently been applied in numerous electrochemical applications, where both ion and electron transport are required. Despite continuous efforts to improve their performance and stability, n-type conjugated polymers with mixed conduction still lag behind their hole-transporting (p-type) counterparts, limiting the functions of electrochemical devices. In this work, we investigate the effect of enhanced backbone coplanarity on the electrochemical activity and mixed ionic-electronic conduction properties of n-type polymers during operation in aqueous media. Through substitution of the widely employed electron-deficient naphthalene diimide (NDI) unit for the core-extended naphthodithiophene diimide (NDTI) units, the resulting polymer shows a more planar backbone with closer packing, leading to an increase in the electron mobility in organic electrochemical transistors (OECTs) by more than two orders of magnitude. The NDTI-based polymer shows a deep-lying lowest unoccupied molecular orbital level, enabling operation of the OECT closer to 0 V vs Ag/AgCl, where fewer parasitic reactions with molecular oxygen occur. Enhancing the backbone coplanarity also leads to a lower affinity toward water uptake during cycling, resulting in improved stability during continuous electrochemical charging and ON-OFF switching relative to the NDI derivative. Furthermore, the NDTI-based polymer also demonstrates near-perfect shelf-life stability over a month-long test, exhibiting a negligible decrease in both the maximum on-current and transconductance. Our results highlight the importance of polymer backbone design for developing stable, high-performing n-type materials with mixed ionic-electronic conduction in aqueous media.

Resolving the backbone tilt of crystalline poly(3-hexylthiophene) with resonant tender X-ray diffraction

The way in which conjugated polymers pack in the solid state strongly affects the performance of polymer-based optoelectronic devices. However, even for the most crystalline conjugated polymers the precise packing of chains within the unit cell is not well established. Here we show that by performing resonant X-ray diffraction experiments at the sulfur K-edge we are able to resolve the tilting of the planar backbones of crystalline poly(3-hexylthiophene) (P3HT) within the unit cell. This approach exploits the anisotropic nature of the X-ray optical properties of conjugated polymers, enabling us to discern between different proposed crystal structures. By comparing our data with simulations based on different orientations, a tilting of the planar conjugated backbone with respect to the side chain stacking direction of 30 ± 5° is determined.

Small-molecule ambipolar transistors

Ambipolar transistor properties have been observed in various small-molecule materials. Since a small energy gap is necessary, many types of molecular designs including extended π-skeletons as well as the incorporation of donor and acceptor units have been attempted. In addition to the energy levels, an inert passivation layer is important to observe ambipolar transistor properties. Ambipolar transport has been observed in extraordinary π-electron systems such as antiaromatic compounds, biradicals, radicals, metal complexes, and hydrogen-bonded materials. Several donor/acceptor cocrystals show ambipolar transport as well.

Physical and Electrochemical Properties of Soluble 3,4-Ethylenedioxythiophene (EDOT)-Based Copolymers Synthesized via Direct (Hetero)Arylation Polymerization

Over the past decades, π-conjugated polymers (CPs) have drawn more and more attention and been essential materials for applications in various organic electronic devices. Thereinto, conjugated polymers based on the 3,4-ethylenedioxythiophene (EDOT) backbone are among the high-performance materials. In order to investigate the structure-property relationships of EDOT-based polymers and further improve their electrochemical properties, a series of organic solvent-soluble EDOT-based alternative copolymers consisting of electron-rich fragments (fluorene P1, carbazole P2, and 3,4-alkoxythiophene P3) or electron-deficient moieties (benzotriazole P4 and thieno[3,4-c]pyrrole-4,6-dione P5) were synthesized via direct C-H (hetero)arylation polymerization (DHAP) in moderate to excellent yields (60-98%) with medium to high molecular weights (M n = 3,100-94,000 Da). Owing to their various electronic and structural properties, different absorption spectra (λ max = 476, 380, 558, 563, and 603 nm) as well as different specific capacitances of 70, 68, 75, 51, and 25 F/g with 19, 10, 21, 26, and 69% of capacity retention after 1,000 cycles were observed for P1-P5, respectively. After careful study through multiple experimental measurements and theoretical calculation, appropriate electronic characteristics, small molecular conformation differences between different oxidative states, and well-ordered molecular stacking could improve the electrochemical performance of CPs.

Zwitterionic side chain-modified conjugated polymers with greatly enhanced ambipolar charge-transport mobilities

A small amount of the 3-(hexyldimethylammonio)propane-1-sulfonate zwitterionic side chain was integrated into a diketopyrrolopyrrole ambipolar polymer to modulate its field-effect carrier-transport characteristics. It was found that such a modification can strengthen the interchain interaction, promote crystallization, and thus improve the hole and electron mobilities by 3.9- and 8.2-fold, respectively.

Charge injected proton transfer in indigo derivatives

In analogy with excited-state proton transfer, proton transfer is significantly facilitated in cationic and anionic molecules of indigo derivatives generated in field-effect transistors. We have prepared extended and truncated indigo derivatives and investigated their ambipolar transistor properties. Since the proton transfer reduces the energy gap from 2.2 to 0.4 eV, the proton transferred states are stabilized in the charge injected cationic and anionic states; the energy increase is as small as 0.5 eV, which is half of that in the neutral state. The intermolecular proton transfer enlarges the equilibrium N-H distance typically by 0.03 Å, and improves the donor and acceptor abilities by 0.2-0.4 eV, though the reorganization energy is practically unchanged. In addition, the transfer integrals along the hydrogen bonds are as large as one third of the columnar transfers, to facilitate the two-dimensional carrier conduction. The influence of proton transfer is most significant in indigo and truncated indigo derivatives, though isoindigo and quinacridone exhibit similar properties. Accordingly, indigo derivatives show much better donor and acceptor abilities than those expected from isolated molecules.

Spiers Memorial Lecture. Carbon nanostructures by macromolecular design - from branched polyphenylenes to nanographenes and graphene nanoribbons

Nanographenes (NGs) and graphene nanoribbons (GNRs) are unique connectors between the domains of 1D-conjugated polymers and 2D-graphenes. They can be synthesized with high precision by oxidative flattening processes from dendritic or branched 3D-polyphenylene precursors. Their size, shape and edge type enable not only accurate control of classical (opto)electronic properties, but also access to unprecedented high-spin structures and exotic quantum states. NGs and GNRs serve as active components of devices such as field-effect transistors and as ideal objects for nanoscience. This field of research includes their synthesis after the deposition of suitable monomers on surfaces. An additional advantage of this novel concept is in situ monitoring of the reactions by scanning tunnelling microscopy and electronic characterization of the products by scanning tunnelling spectroscopy.

Approaching Crystal Structure and High Electron Mobility in Conjugated Polymer Crystals

Conjugated polymers usually form crystallized and amorphous regions in the solid state simultaneously, making it difficult to accurately determine their precise microstructures. The lack of multiscale microstructures of conjugated polymers limits the fundamental understanding of the structure-property relationships in polymer-based optoelectronic devices. Here, crystals of two typical conjugated polymers based on four-fluorinated benzodifurandione-based oligo(p-phenylene vinylene) (F₄ BDOPV) and naphthalenediimide (NDI) motifs, respectively, are obtained by a controlled self-assembly process. The strong diffractivity of the polymer crystals brings an opportunity to determine the crystal structures by combining X-ray techniques and molecular simulations. The precise polymer packing structures are useful as initial models to evaluate the charge transport properties in the ordered and disordered phases. Compared to the spin-coated thin films, the highly oriented polymer chains in crystals endow higher mobilities with a lower hopping energy barrier. Microwire crystal transistors of F₄ BDOPV- and NDI-based polymers exhibit high electron mobilities of up to 5.58 and 2.56 cm²  V^-1  s^-1 , respectively, which are among the highest values in polymer crystals. This work presents a simple method to obtain polymer crystals and their precise microstructures, promoting a deep understanding of molecular packing and charge transport for conjugated polymers.

Bifuran-bridged bisboranes: highly luminescent B-doped oligohetarenes

Boron-doping of oligohetarenes – via classical metathesis or silicon/boron exchange routes – led to strongly luminescent and twofold reversibly reducible oligomers.

An insight into the role of side chains in the microstructure and carrier mobility of high-performance conjugated polymers

The effect of linear-chain interdigitation on device performance was studied in detail by both experimental and theoretical methods.

High-performance polymer field-effect transistors: from the perspective of multi-level microstructures

The multi-level microstructure of conjugated polymers is the most critical parameter determining the charge transport property in field-effect transistors (FETs). However, controlling the hierarchical microstructures and the structural evolution remains a significant challenge. In this perspective, we discuss the key aspects of multi-level microstructures of conjugated polymers towards high-performance FETs. We highlight the recent progress in the molecular structures, solution-state aggregation, and polymer crystal structures, representing the multi-level microstructures of conjugated polymers. By tuning polymer hierarchical microstructures, we attempt to provide several guidelines for developing high-performance polymer FETs and polymer electronics.

Design Rules to Maximize Charge-Carrier Mobility along Conjugated Polymer Chains

The emergence of polymeric materials displaying high charge-carrier mobility values despite poor interchain structural order has spawned a renewal of interest in the identification of structure-property relationships pertaining to the transport of charges along conjugated polymer chains and the subsequent design of optimized architectures. Here, we present the results of intrachain charge transport simulations obtained by applying a robust surface hopping algorithm to a phenomenological Hamiltonian parametrized against first-principles simulations. Conformational effects are shown to provide a clear signature in the temperature-dependent charge-carrier mobility that complies with recent experimental observations. We further contrast against molecular crystals the evolution with electronic bandwidth and electron-phonon interactions of the room-temperature mobility in polymers, showing that intrachain charge-carrier mobility values in excess of 100 cm²/(V s) can be achieved through a proper chemical engineering of the backbones.

Ordered Solid-State Microstructures of Conjugated Polymers Arising from Solution-State Aggregation

Controlling the solution-state aggregation of conjugated polymers for producing specific microstructures remains challenging. Herein, a practical approach is developed to finely tune the solid-state microstructures through temperature-controlled solution-state aggregation and polymer crystallization. High temperature generates significant conformation fluctuation of conjugated backbones in solution, which facilitates the polymer crystallization from solvated aggregates to orderly packed structures. The polymer films deposited at high temperatures exhibit less structural disorders and higher electron mobilities (up to two orders of magnitude) in field-effect transistors, compared to those deposited at low temperatures. This work provides an effective strategy to tune the solution-state aggregation to reveal the relationship between solution-state aggregation and solid-state microstructures of conjugated polymers.

Learning how planarization can affect dichroic patterns in polyfluorenes

The circular dichroism spectra of a single chain of polyfluorene was predicted for a p-twisted helix conformation and local planarized polymer sections in the presence and in the absence of thermal vibrations. Under thermal vibrations at 300 K, the planarized section of polyfluorene affords a red-shifted positive dichroic band between 446 and 456 nm, preserving a degree of chirality. The S1 ← S0 excitation energy decreases from 3.29 eV, for the p-twisted helix to 2.77 or 2.71 eV, for planarized sections with one or two coplanar twists, respectively. Thermal vibrations and intramolecular rotations eventually affect the circular dichroism spectrum patterns, where planarized bent conformers induce a positive band towards 450 nm.

Structural Transition and Interdigitation of Alkyl Side Chains in the Conjugated Polymer Poly(3-hexylthiophene) and Their Effects on the Device Performance of the Associated Organic Field-Effect Transistor

Direct grazing-angle X-ray scattering evidence of the order-disorder transition and interdigitation of side chains in a conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. The free methyl ends of the side chains exhibit closest packing, as in n-alkane crystallization, and cause a structural mismatch due to the difference between their packing density and the areal density of the attached ends. This mismatch is resolved by increases in the tilt angle of the side chains and local interdigitation. In situ X-ray scattering and electrical measurements show that the structural transition and interdigitation of these side chains strongly affect its surface morphology as well as the charge transport properties of the resulting P3HT-based organic field-effect transistor. Since most conjugated polymers have side chains, the results of this study provide a deeper understanding of the effects of side chains on the structural and electrical properties of conjugated backbones. These results also provide a new perspective on the formation of a metastable polymorph consisting of interdigitated P3HT.

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

Recent decades have witnessed the rapid development of semiconducting polymers in terms of high charge mobilities and applications in transistors. Significant efforts have been made to develop various conjugated frameworks and linkers. However, studies are increasingly demonstrating that the side chains of semiconducting polymers can significantly affect interchain packing, thin film crystallinity, and thus semiconducting performance. Ways to modify the side alkyl chains to improve the interchain packing order and charge mobilities for conjugated polymers are first discussed. It is shown that modifying the branching chains by moving the branching points away from the backbones can boost the charge mobilities, which can also be improved through partially replacing branching chains with linear ones. Second, the effects of side chains with heteroatoms and functional groups are discussed. The siloxane-terminated side chains are utilized to enhance the semiconducting properties. The fluorinated alkyl chains are beneficial for improving both charge mobility and air stability. Incorporating H bonding group side chains can improve thin film crystallinities and boost charge mobilities. Notably, incorporating functional groups (e.g., glycol, tetrathiafulvalene, and thymine) into side chains can improve the selectivity of field-effect transistor (FET)-based sensors, while photochromic group containing side chains in conjugated polymers result in photoresponsive semiconductors and optically tunable FETs.

Facile Incorporation of "Aggregation-Induced Emission"-Active Conjugated Polymer into Mesoporous Silica Hollow Nanospheres: Synthesis, Characterization, Photophysical Studies, and Application in Bioimaging

Typical aggregation-induced emission (AIE) luminogens tetraphenylethylene (TPE) and triphenylamine have been used to construct an AIE-active conjugated polymer, namely, poly(N,N-diphenyl-4-(4-(1,2,2-triphenylvinyl)styryl)aniline) (PTPA), which consist of D-π-A architecture by Wittig polymerization. We fabricated mesoporous silica hollow nanospheres (MSHNs) which were encapsulated with the AIE-active polymer for applications in cellular imaging. It exhibits a positive solvatochromism effect by increasing solvent polarity, supported by theoretical calculation using density functional theory. The structure of the monomers and polymer was confirmed by Fourier transform infrared, nuclear magnetic resonance, and high-resolution mass spectrometry techniques. Considering the advantage of high brightness in the fluorescence of PTPA, it was encapsulated into MSHNs by a noncovalent approach, and the surface was functionalized with an anti-EpCAM (antiepithelial cell adhesion molecule) aptamer through conjugation with γ-glycidoxypropyltrimethoxysilane for targeting cancer cells specifically. The aptamer-functionalized Apt-MSHNs exhibited excellent biocompatibility with the liver cancer-Huh-7 cells used for this study and was efficiently internalized by these cells. Because EpCAM are overexpressed in multiple carcinomas, including liver cancer, these aptamer-conjugated AIE MSHNs are therefore good candidates for targeted cellular imaging applications.

Structure-Function Relationship of Organic Semiconductors: Detailed Insights From Time-Resolved EPR Spectroscopy

Organic photovoltaics (OPV) is a promising technology to account for the increasing demand for energy in form of electricity. Whereas the last decades have seen tremendous progress in the field witnessed by the steady increase in efficiency of OPV devices, we still lack proper understanding of fundamental aspects of light-energy conversion, demanding for systematic investigation on a fundamental level. A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. Illuminating conjugated polymers not only leads to excited states, but sheds light on some of the most important aspects of device efficiency in organic electronics as well. The interplay between electronic structure, morphology, flexibility, and local ordering, while at the heart of structure-function relationship of organic electronic materials, is still barely understood. (Time-resolved) electron paramagnetic resonance (EPR) spectroscopy is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior spectral resolution compared to optical methods. This article aims at giving a non-specialist audience an overview of what EPR spectroscopy and particularly its time-resolved variant (TREPR) can contribute to unraveling aspects of structure-function relationship in organic semiconductors.

1 : 2 charge-transfer complexes of perylene and coronene with perylene diimide, and the ambipolar transistors

The title 1 : 2 charge-transfer complexes have DAA-type mixed stacks and show electron-dominant ambipolar transistor properties.

Diblock copolymers consisting of a redox polymer block based on a stable radical linked to an electrically conducting polymer block as cathode materials for organic radical batteries

Coupling a conjugated P3HT block to a radical polymer block leads to improved PTMA battery performances.

Charge-transfer complexes of sulfur-rich acceptors derived from birhodanines

The title acceptors form charge-transfer complexes with mixed stacks, whose transistors are affected by the S–S interaction between the acceptors.