Impact of Molecular Weight on the Mechanical and Electrical Properties of a High-Mobility Diketopyrrolopyrrole-Based Conjugated Polymer

Improved Mobility-Stretchability Properties of Diketopyrrolopyrrole-Based Conjugated Polymers with Diastereomeric Conjugation Break Spacers

Stretchable conjugated polymers with conjugation break spacers (CBSs) synthesized via random terpolymerization have gained considerable attention because of their efficacy in modulating mobility and stretchability. This study incorporates a series of dianhydrohexitol diastereomers of isosorbide (ISB) and isomannide (IMN) units into the diketopyrrolopyrrole-based backbone as CBSs. It is found that the distorted CBS (IMN) improves the mobility-stretchability properties of the polymer with a highly coplanar backbone, whereas the extended CBS (ISB) enhances those of the polymer with a noncoplanar backbone. Additionally, the different configurations of ISB and IMN sufficiently affect the solid-state packing, aggregation capabilities, crystallographic parameters, and mobility-stretchability properties of the polymer. The IMN-based polymers exhibit the highest mobility of 1.69 cm2 V-1 s-1 and crystallinity retentions of (85.7, 78.6)% under 20% and 60% strains, outperforming their ISB-based or unmodified counterparts. The improvement is correlated with a robust aggregation capability. Furthermore, the CBS content affects aggregation behavior, notably affecting mobility. This result indicates that incorporating CBSs into the polymer can enhance backbone flexibility via movement and rotation of the CBS without affecting the crystalline regions.

Trade-off between processability and device performance in donor-acceptor semiconductors revealed using discrete siloxane side chains

Donor-acceptor polymeric semiconductors are crucial for state-of-the-art applications, such as electronic skin mimics. The processability, and thus solubility, of these polymers in benign solvents is critical and can be improved through side chain engineering. Nevertheless, the impact of novel side chains on backbone orientation and emerging device properties often remains to be elucidated. Here, we investigate the influence of elongated linear and branched discrete oligodimethylsiloxane (oDMS) side chains on solubility and device performance. Thereto, diketopyrrolopyrrole-thienothiophene polymers are equipped with various oDMS pendants (PDPPTT-Sin) and subsequently phase separated into lamellar domains. The introduction of a branching point in the siloxane significantly enhanced the solubility of the polymer, as a result of increased backbone distortion. Simultaneously, the charge carrier mobility of the polymers decreased by an order of magnitude upon functionalization with long and/or branched siloxanes. This work unveils the intricate balance between processability and device performance in organic semiconductors, which is key for the development of next-generation electronic devices.

Unraveling the Molecular Weight Dependence of High Magnetic Field to Manipulate the Semiconducting Polymer Molecular Orientation

The magnetic alignment of molecules, which exploits the anisotropy of diamagnetic susceptibility, provides a clean and versatile approach to the structural manipulation of semiconducting polymers. Here, the magnetic-alignment dynamics of two molecular-weight (MW) batches of a diketopyrrolopyrrole (DPP)-based copolymer (PDVT-8) were investigated. Microstructural characterizations revealed that the magnetically aligned, high-MW (Mn = 53.7 kDa) PDVT-8 film exhibited a higher degree of backbone chain alignment and film crystallinity compared with the low-MW (Mn = 17.6 kDa) PDVT-8 film grown via the same magnetic alignment method. We found that as the MW increases, the degree of preaggregation of the polymer molecules in solution significantly increases and the aggregation mode changes from H-aggregation to J-aggregation through a cooperative assembly mechanism. These events improved the responsiveness of high-MW polymer molecules to magnetic fields. Field-effect transistors based on the magnetic aligned high-MW PDVT-8 films exhibited a 6.8-fold increase in hole mobility compared to the spin-coated films, along with a mobility anisotropy ratio of 12.6. This work establishes a significant correlation among chain aggregation behavior in solution, polymer film microstructures, magnetic responsiveness, and carrier transport performance in donor-acceptor polymer systems.

Microstructural Evolution of P(NDI2OD-T2) Films with Different Molecular Weight during Stretching Deformation

Conjugated polymers exhibit excellent electrical and mechanical properties when their molecular weight (Mw) is above the critical molecular weight (Mc). The microstructural changes of polymers under strain are crucial to establish a structure-performance relationship. Herein, the tensile deformation of P(NDI2OD-T2) is visualized, and cracks are revealed either along the (100) crystal plane of side chain packing or along the main chain direction which depends on the Mw is below or above the Mc. When Mw < Mc, the film cracks along the (100) plane under small strains. When Mw > Mc, the polymer chains first undergo stretch-induced orientation and then fracture along the main chain direction at large strains. This is attributed to the fact that the low Mw film exhibits large crystalline domains and the absence of interdomain connectivity, which are vulnerable to mechanical stress. In contrast, the high Mw film displays a nearly amorphous morphology with adequate entanglements, the molecular chains can endure stresses in the stretching direction to release substantial strain energy under greater mechanical deformation. Therefore, the film with Mw > Mc exhibits the optimal electrical and mechanical performances simultaneously, i.e., the electron mobility is retained under 100% strain and after 100 stretching-releasing cycles.

Enhanced N-type Semiconducting Performance of Asymmetric Monochlorinated Isoindigo-based Semiregioregular Polymers under Dynamic Forces

The asymmetric monochlorination strategy not only effectively addresses the steric issues in conventional dichlorination but also enables the development of promising acceptor units and semiregioregular polymers. Herein, monochlorinated isoindigo (1CIID) is successfully designed and synthesized by selectively introducing single chlorine (Cl) atoms. Furthermore, the 1CIID copolymerizes with two donor counterparts, centrosymmetric 2,2'-bithiophene (2T) and axisymmetric 4,7-di(thiophen-2-yl)benzo[1,2,5]thiadiazole (DTBT), forming two polymers, P1CIID-2T and P1CIID-DTBT. These polymers exhibit notable differences in backbone linearity and dipole moments, influenced by the symmetry of their donor counterparts. In particular, P1CIID-2T, which contains a centrosymmetric 2T unit, demonstrates a linear backbone and a significant dipole moment of 10.20 D. These properties contribute to the favorable film morphology of P1CIID-2T, characterized by highly ordered crystallinity in the presence of fifth-order (500) X-ray diffraction peaks. Notably, P1CIID-2T exhibits a significant improvement in molecular alignment under dynamic force, resulting in over 8-fold improvement in the performance of organic field-effect transistor (OFET) devices, with superior electron mobility up to 1.22 cm2 V-1 s-1. This study represents the first synthesis of asymmetric monochlorinated isoindigo-based conjugated polymers, highlighting the potential of asymmetric monochlorination for developing n-type semiconducting polymers. Moreover, our findings provide valuable insights into the relationship between the molecular structure and properties.

Impact of doping on the mechanical properties of conjugated polymers

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

Real-time monitoring polymerization degree of organic photovoltaic materials toward no batch-to-batch variations in device performance

Polymerization degree plays a vital role in material properties. Previous methodologies of molecular weight control generally cannot suppress or alleviate batch-to-batch variations in device performance, especially in polymer solar cells. Herein, we develop an in-situ photoluminescence system in tandem with a set of analysis and processing procedures to track and estimate the polymerization degree of organic photovoltaic materials. To support the development of this protocol, we introduce polymer acceptor PYT constructed by near-infrared Y-series small molecule acceptors via Stille polymerization, and shed light on the correlations between molecular weight, spectral parameters, and device efficiencies that enable the design of the optical setup and confirm its feasibility. The universality is verified in PYT derivatives with stereoregularity and fluoro-substitution as well as benzo[1,2-b:4,5-b']dithiophene-based polymers. Overall, our result provides a tool to tailor suitable conjugated oligomers applied to polymer solar cells and other organic electronics for industrial scalability and desired cost reduction.

Chain Length Dependence of Electron Transport in an n-Type Conjugated Polymer with a Rigid-Rod Chain Topology

Most currently known n-type conjugated polymers have a semiflexible chain topology, and their charge carrier mobilities are known to peak at modest chain lengths of below 40-60 repeat units. Herein, we show that the field-effect electron mobility of a model n-type conjugated polymer that has a rigid-rod chain topology grows continuously without saturation, even at a chain length exceeding 250 repeat units. We found the mechanism underlying the novel chain length-dependent electron transport to originate from the reduced structural disorder and energetic disorder with the increasing degree of polymerization inherent to the rigid-rod chain topology. Furthermore, we demonstrate a unique chain length-dependent decay of threshold voltage, which is rationalized by decreased trap densities and trap depths with respect to the degree of polymerization. Our findings provide new insights into the role of polymer chain topology in electron transport and demonstrate the promise of rigid-rod chain architectures for the design of future high-mobility conjugated polymers.

Highly stretchable polymer semiconductor thin films with multi-modal energy dissipation and high relative stretchability

Stretchable polymer semiconductors (PSCs) have seen great advancements alongside the development of soft electronics. But it remains a challenge to simultaneously achieve high charge carrier mobility and stretchability. Herein, we report the finding that stretchable PSC thin films (<100-nm-thick) with high stretchability tend to exhibit multi-modal energy dissipation mechanisms and have a large relative stretchability (rS) defined by the ratio of the entropic energy dissipation to the enthalpic energy dissipation under strain. They effectively recovered the original molecular ordering, as well as electrical performance, after strain was released. The highest rS value with a model polymer (P4) exhibited an average charge carrier mobility of 0.2 cm2V-1s-1 under 100% biaxial strain, while PSCs with low rS values showed irreversible morphology changes and rapid degradation of electrical performance under strain. These results suggest rS can be used as a parameter to compare the reliability and reversibility of stretchable PSC thin films.

Chain-Kinked Design: Improving Stretchability of Polymer Semiconductors through Nonlinear Conjugated Linkers

The manipulation of the polymer backbone structure has a profound influence on the crystalline behavior and charge transport characteristics of polymers. These strategies are commonly employed to optimize the performance of stretchable polymer semiconductors. However, a universal method that can be applied to conjugated polymers with different donor-acceptor combinations is still lacking. In this study, we propose a universal strategy to boost the stretchability of polymers by incorporating the nonlinear conjugated linker (NCL) into the main chain. Specifically, we incorporate meta-dibromobenzene (MB), characterized by its asymmetric linkage sites, as the NCL into the backbone of diketopyrrolopyrrole-thiophene-based (DPP-based) polymers. Our research demonstrates that the introduction of MB prompts chain-kinking, thereby disrupting the linearity and central symmetry of the DPP conjugated backbone. This modification reshapes the polymer conformation, decreasing the radius of gyration and broadening the free volume, which consequently adjusts the level of crystallinity, leading to a considerable increase in the stretchability of the polymer. Importantly, this method increases stretchability without compromising mobility and exhibits broad applicability across a wide range of donor-acceptor pair polymers. Leveraging this strategy, fully stretchable transistors were fabricated using a DPP polymer that incorporates 10 mol % of MB. These transistors display a mobility of approximately 0.5 cm2 V-1 s-1 and prove remarkably durable, maintaining 90% of this mobility even after enduring 1000 cycles at 25% strain. Overall, we propose a method to systematically control the main-chain conformation, thereby enhancing the stretchability of conjugated polymers in a widely applicable manner.

Achieving High Performance Stretchable Conjugated Polymers via Donor Structure Engineering

A backbone engineering strategy is developed to tune the mechanical and electrical properties of conjugated polymer semiconductors. Four Donor-Acceptor (D-A) polymers, named PTDPPSe, PTDPPTT, PTDPPBT, and PTDPPTVT, are synthesized using selenophene (Se), thienothiophene (TT), bithiophene (BT), and thienylenevinylenethiophene (TVT) as the donors and siloxane side chain modified diketopyrrolopyrrole (DPP) as acceptor. The influences of the donor structure on the polymer energy level, film morphology, molecular stacking, carrier transport properties, and tensile properties are all examined. The films of PTDPPSe show the best stretchability with crack-onset-strain greater than 100%, but the worst electrical properties with a mobility of only 0.54 cm2  V-1  s-1 . The replacement of the Se donor with larger conjugated donors, that is, TT, BT, and TVT, significantly improves the mobility of conjugated polymers but also leads to reduced stretchability. Remarkably, PTDPPBT exhibits moderate stretchability with crack-onset-strain ≈50% and excellent electrical properties. At 50% strain, it has a mobility of 2.37 cm2 V-1  s-1 parallel to the stretched direction, which is higher than the mobility of most stretchable conjugated polymers in this stretching state.

Revealing the Molecular Weight Effect on Highly Efficient Polythiophene Solar Cells

Polythiophenes (PTs) are promising electron donors in organic solar cells (OSCs) due to their simple structures and excellent synthetic scalability. Benefiting from the rational molecular design, the power conversion efficiency (PCE) of PT solar cells has been greatly improved. Herein, five batches of the champion PT (P5TCN-F25) with molecular weights ranging from 30 to 87 kg mol^-1 were prepared, and the effect of the molecular weight on the blend film morphology and photovoltaic performance of PT solar cells was systematically investigated. The results showed that the PCEs of the devices improved first and then maintained a high value with the increase of molecular weight, and the highest PCE of 16.7% in binary PT solar cells was obtained. Further characterizations revealed that the promotion in photovoltaic performance mainly comes from finer phase separation structures and more compact molecular packing in the blend film. The best device stabilities were also achieved by polymers with high molecular weights. Overall, this study highlights the importance of optimizing the molecular weight for PTs and offers directions to further improve the PCE of PT solar cells.

A Semi-Crystalline Polymer Semiconductor with Thin Film Stretchability Exceeding 200

Despite the emerging scientific interest in polymer-based stretchable electronics, the trade-off between the crystallinity and stretchability of intrinsically stretchable polymer semiconductors-charge-carrier mobility increases as crystallinity increases while stretchability decreases-hinders the development of high-performance stretchable electronics. Herein, a highly stretchable polymer semiconductor is reported that shows concurrently improved thin film crystallinity and stretchability upon thermal annealing. The polymer thin films annealed at temperatures higher than their crystallization temperatures exhibit substantially improved thin film stretchability (> 200%) and hole mobility (≥ 0.2 cm²  V^-1  s^-1 ). The simultaneous enhancement of the crystallinity and stretchability is attributed to the thermally-assisted structural phase transition that allows the formation of edge-on crystallites and reinforces interchain noncovalent interactions. These results provide new insights into how the current crystallinity-stretchability limitation can be overcome. Furthermore, the results will facilitate the design of high-mobility stretchable polymer semiconductors for high-performance stretchable electronics.

An Amorphous n-Type Conjugated Polymer with an Ultra-Rigid Planar Backbone

High charge carrier mobility polymer semiconductors are always semi-crystalline. Amorphous conjugated polymers represent another kind of polymer semiconductors with different charge transporting mechanism. Here we report the first near-amorphous n-type conjugated polymer with decent electron mobility, which features a remarkably rigid, straight and planar polymer backbone. The molecular design strategy is to copolymerize two fused-ring building blocks which are both electron-accepting, centrosymmetrical and planar. The polymer is the alternating copolymer of double B←N bridged bipyridine (BNBP) unit and benzobisthiazole (BBTz) unit. It shows a decent electron mobility of 0.34 cm²  V^-1  s^-1 in organic field-effect transistors. The excellent electron transporting property of the polymer is possibly due to the ultrahigh backbone stiffness, small π-π stacking distance, and high molecular weight.

Unraveling the Stretch-Induced Microstructural Evolution and Morphology-Stretchability Relationships of High-Performance Ternary Organic Photovoltaic Blends

The stretchability and stretch-induced structural evolution of organic solar cells (OSCs) are pivotal for their collapsible, portable, and wearable applications, and they are mainly affected by the complex morphology of active layers. Herein, a highly ductile conjugated polymer P(NDI2OD-T2) is incorporated into the active layers of high-efficiency OSCs based on nonfullerene small molecule acceptors to simultaneously investigate the morphological, mechanical, and photovoltaic properties and structural evolution under stretching of ternary blend films with various acceptor contents. The structural robustness of the blend films is indicated by their stretch-induced structural evolution, which is monitored in real-time by a combination of in situ wide/small angle X-ray scattering. It is found that adding the soft P(NDI2OD-T2) can enhance the stretchability and structural robustness of ternary blend films by more entangled chains and tie chains to dissipate strain. Furthermore, the stretchability of the ternary blends can be superbly predicted by a 3D equivalent box model. This work provides instructive insight and guidance for designing stretchable electronics and predicting the stretchability of multicomponent blends.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Polyurethane-Based Stretchable Semiconductor Nanofilms with High Intrinsic Recovery Similar to Conventional Elastomers

Polymer semiconductors with large elastic recovery (ER) under high strain in thin film state are highly desirable for stretchable electronics. Here we report a type of stretchable semiconductor PU(DPP)x, by copolymerization of oligodiketopyrrolopyrrole-based conjugated block and hydrogenated polybutadiene flexible block via urethane linkage for intermolecular hydrogen bonding. By regulating block ratio, PU(DPP)35 with 35 wt % conjugated block exhibits high intrinsic ER > 80% under 175% strain (ε) in pseudo free-standing thin film state, comparable with commercial elastomers, and crack onset strain (COS) > 300% along with maximum hole mobility of 0.19 cm2 V-1 s-1 in organic thin film transistors to bring it to the best performing block copolymer-type stretchable semiconductors. Enhanced mobility is achieved using PU(DPP)35 as the binder for conjugated polymer PDPPT3. The 25 wt %-PDPPT3 blend displays mobility up to 1.28 cm2 V-1 s-1 along with COS ∼120%, and 10 wt %-PDPPT3 blend exhibits ER of 78% at ε = 150%, COS of ∼230%, modulus of 36.5 MPa, maximum mobility of 0.62 cm2 V-1 s-1 and no obvious degradation of mobility at ε = 150% after 100 cycles of strain. Moreover, the structural similarity enables the blend film uniform and stable microstructure against mechanical and thermal deformation. Notably, PU(DPP)35 and the blend are characterized by high mechanical performance similar to that of commercial elastomers in thin film state, and demonstrate their potential for high performance stretchable electronics.

Gaining control over conjugated polymer morphology to improve the performance of organic electronics

Conjugated polymers (CPs) are widely used in various domains of organic electronics. However, the performance of organic electronic devices can be variable due to the lack of precise predictive control over the polymer microstructure. While the chemical structure of CPs is important, CP microstructure also plays an important role in determining the charge-transport, optical and mechanical properties suitable for a target device. Understanding the interplay between CP microstructure and the resulting properties, as well as predicting and targeting specific polymer morphologies, would allow current comprehension of organic electronic device performance to be improved and potentially enable more facile device optimization and fabrication. In this Feature Article, we highlight the importance of investigating CP microstructure, discuss previous developments in the field, and provide an overview of the key aspects of the CP microstructure-property relationship, carried out in our group over recent years.

Role of Molecular Weight in the Mechanical Properties and Charge Transport of Conjugated Polymers Containing Siloxane Side Chains

The molecular weight is a key factor affecting the properties of conjugated polymers. To determine the critical molecular weights of conjugated polymers modified with siloxane side chains, poly-diketo-pyrrolopyrrole-selenophene (PTDPPSe-5Si) samples with molecular weights ranging from 20 to 350 kDa are synthesized. The critical molecular weight of the polymer is determined in the range of 60-100 kDa by testing the viscosity of the solution. When the molecular weight of the 27-60 kDa polymers is below the critical molecular weight, they exhibit a high crystallinity and low ductility. When the molecular weight of the 100 kDa polymer reaches the critical molecular weight, the crystallinity decreases, and the ductility increases. As the molecular weight increases, the polymer film also gradually changes from brittle to ductile. Furthermore, when the molecular weight of the 315 kDa polymer is much higher than the critical molecular weight, the film exhibits a significant ductility, which results in the polymer films showing no pronounced cracks after high-percentage stretching. Additionally, due to the oriented alignment of the molecular chains caused by stretching, the carrier mobility in the parallel direction becomes 2.14-fold of the initial film.

Unraveling the Photovoltaic, Mechanical, and Microstructural Properties and Their Correlations in Simple Poly(3-pentylthiophene) Solar Cells

The power conversion efficiency of polythiophene organic solar cells is constantly refreshed. Despite the renewed device efficiency, very few efforts have been devoted to understanding how the type of electron acceptor alters the photovoltaic and mechanical properties of these low-cost solar cells. Herein, we conduct a thorough investigation of photovoltaic and mechanical characteristics of a simple yet less explored polythiophene, namely poly(3-pentylthiophene) (P3PT), in three different types of organic solar cells, where ZY-4Cl, PC₇₁ BM, and N2200 are employed as three representative acceptors, respectively. Compared with the reference P3HT-based solar cells, P3PT-based devices all perform more efficiently. Particularly, the P3PT:ZY-4Cl blend exhibits the highest efficiency (nearly 10%) among the six combinations and outperforms the prior top-performance system P3HT:ZY-4Cl. Furthermore, the blend films based on N2200 exhibit a high crack-onset strain of ∼38% on average, which is approximately 15 and 17 times higher than those of ZY-4Cl and PC₇₁ BM, respectively. The microstructural origins for the above difference are well elucidated by detailed grazing incidence X-ray scattering and microscopy analysis. This work not only underlines the potential of P3PT in prolific solar cell research but also demonstrates the superior tensile properties of polythiophene-based all-polymer blends for the preparation of stretchable solar cells. This article is protected by copyright. All rights reserved.

Alignment of organic conjugated molecules for high-performance device applications

High-performance organic semiconductor materials as the electroactive components of optoelectronic devices have attracted much attention and made them ideal candidates for solution-processable, large-area, and low-cost flexible electronics. Especially, organic field-effect transistors (OFETs) based on conjugated semiconductor materials have experienced stunning progress in device performance. To make these materials economically viable, comprehensive knowledge of charge transport mechanisms is required. The alignment of organic conjugated molecules in the active layer is vital to charge transport properties of devices. The present review highlights the recent progress of processing-structure-transport correlations that allow the precise and uniform alignment of organic conjugated molecules over large areas for multiple electronic applications, including OFETs, organic thermoelectric devices (OTEs), and organic phototransistors (OPTs). Different strategies for regulating crystallinity and macroscopic orientation of conjugated molecules are introduced to correlate the molecular packing, the device performance and charge transport anisotropy in multiple organic electronic devices. This article is protected by copyright. All rights reserved.

Optimizing the Intercrystallite Connection of Donor-Acceptor Conjugated Semiconductor Polymer by Controlling the Crystallization Rate via Temperature

The charge carrier transport of conjugated polymer thin film is mainly decided by the crystalline domain and intercrystallite connection. High density tie-chain can provide an effective bridge between crystalline domains. Herein, the tie-chain connection behavior is optimized by decreasing the crystal region length (l_c ) and increasing the crystallization rate. Poly[4-(4,4-bis(2-octyldodecyl)-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT-ODD) is dissolved in nonpolar solvent isooctane and high ordered rod-like aggregations are formed. As the temperature increases, the changes of solution state and crystallization behavior lead to three different chain arrangement morphologies in the films: (1) at 25°C, large and separated crystal regions are formed; (2) at 55°C, small and well-connected crystal regions are formed due to faster crystallization rate and smaller nucleus size; (3) at 90°C, the amorphous film is formed. Further results show that the film prepared at 55°C has a smaller crystal region length (l_c , 7.6 nm) and higher tie-chains content. Thus, the film exhibits the best device mobility of 2.3 × 10^-3 cm² V^-1 s^-1 . This result shows the great influence of crystallization kinetics on the microstructure of conjugated polymer films and provides an effective way for the optimization of the intercrystallite tie-chain. This article is protected by copyright. All rights reserved.

Molecular Design of Stretchable Polymer Semiconductors: Current Progress and Future Directions

Stretchable polymer semiconductors have advanced rapidly in the past decade as materials required to realize conformable and soft skin-like electronics become available. Through rational molecular-level design, stretchable polymer semiconductor films are now able to retain their electrical functionalities even when subjected to repeated mechanical deformations. Furthermore, their charge-carrier mobilities are on par with the best flexible polymer semiconductors, with some even exceeding that of amorphous silicon. The key advancements are molecular-design concepts that allow multiple strain energy-dissipation mechanisms, while maintaining efficient charge-transport pathways over multiple length scales. In this perspective article, we review recent approaches to confer stretchability to polymer semiconductors while maintaining high charge carrier mobilities, with emphasis on the control of both polymer-chain dynamics and thin-film morphology. Additionally, we present molecular design considerations toward intrinsically elastic semiconductors that are needed for reliable device operation under reversible and repeated deformation. A general approach involving inducing polymer semiconductor nanoconfinement allows for incorporation of several other desired functionalities, such as biodegradability, self-healing, and photopatternability, while enhancing the charge transport. Lastly, we point out future directions, including advancing the fundamental understanding of morphology evolution and its correlation with the change of charge transport under strain, and needs for strain-resilient polymer semiconductors with high mobility retention.

Side-Chain Engineering of Conjugated Polymers for High-Performance Organic Field-Effect Transistors

Past decades have witnessed the rapid development of conjugated polymers because of their promising semiconducting properties and applications in organic field-effect transistors (OFETs). Recent studies have shown that side-chain engineering of conjugated polymers is an efficient strategy to increase semiconducting performance. This Perspective focuses on the side-chain modulation of conjugated polymers and evaluating their effects on the performance of OFETs. The challenges and potential applications of functional high-performance OFETs through side-chain engineering are also discussed.

A simplified approach to thermally activated delayed fluorescence (TADF) bipolar host polymers

Herein, we compare a series of solution-processible TADF polymers with different host pendant groups to achieve balanced charge transport properties through the combination of unipolar co-hosts.

Contrasting the Charge Carrier Mobility of Isotactic, Syndiotactic, and Atactic Poly((N-carbazolylethylthio)propyl methacrylate)

Isotactic nonconjugated pendant electroactive polymers (NCPEPs) have recently shown potential to achieve comparable charge carrier mobilities with conjugated polymers. Here we report the broader influence of tacticity in NCPEPs, using poly((N-carbazolylethylthio)propyl methacrylate) (PCzETPMA) as a model polymer. We utilized the thiol-ene reaction as an efficient postpolymerization functionalization method to achieve pendant polymers with high isotacticity and syndiotacticity. We found that a stereoregular isotactic polymer showed ∼100 times increased hole mobility (μh) as compared to both atactic and low molecular weight syndiotactic PCzETPMA, achieving μh of 2.19 × 10-4 cm2 V-1 s-1 after annealing at 120 °C. High molecular weight syndiotactic PCzETPMA gave ∼10 times higher μh than its atactic counterpart, comparable to isotactic PCzETPMA after annealing at 150 °C. Importantly, high molecular weight syndiotactic PCzETPMA showed a dramatic increase in μh to 1.82 × 10-3 cm2 V-1 s-1 when measured after annealing at 210 °C, which surpassed the well-known conjugated polymer poly(3-hexylthiophene) (P3HT) (μh = 4.51 × 10-4 cm2 V-1 s-1). MD simulations indicated short-range π-π stacked ordering in the case of stereoregular isotactic and syndiotactic polymers. This work is the first report of charge carrier mobilities in syndiotactic NCPEPs and demonstrates that the tacticity, annealing conditions, and molecular weight of NCPEPs can strongly affect μh.

Thermoplastic Elastomer Tunes Phase Structure and Promotes Stretchability of High-Efficiency Organic Solar Cells

Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here, a simple and effective strategy is reported to improve the stretchability and reduce the stiffness of high-efficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents and the molecular weight dependence of SEBS on microstructure and mechanical properties are thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl and PBDB-T:ITIC. More crucially, the elastic modulus of such complex ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.

Simultaneous Incorporation of Two Types of Azo-Groups in the Side Chains of a Conjugated D-A Polymer for Logic Control of the Semiconducting Performance by Light Irradiation

A new design strategy for photoresponsive semiconducting polymer with tri-stable semiconducting states is reported by simultaneous incorporation of tetra-ortho-methoxy-substituted azobenzene (mAzo) and arylazopyrazole (pAzo) in the side chains. The trans-to-cis transformations for mAzo and pAzo groups can sequentially occur within the polymer thin film after sequential 560 and 365 nm light irradiation. Remarkably, the trans-cis isomerization of mAzo and pAzo groups can modulate the thin film crystallinity. Accordingly, the performances of the resulting field-effect transistors (FETs) can be reversibly modulated, leading to tri-stable semiconducting states after sequential 560, 365, and 470 nm light irradiation. Therefore, the device performance can be logically controlled by light irradiation at three different wavelengths. In addition, with light irradiation and device current as the input and output signals, the three-value logic gate by using single FET device can be successfully mimicked.

Pairing Suzuki–Miyaura cross-coupling and catalyst transfer polymerization

Borylation strategies to make AB Suzuki–Miyaura monomers for use in catalyst-transfer polymerization with nickel or palladium catalysts.

Intrinsically stretchable naphthalenediimide–bithiophene conjugated statistical terpolymers using branched conjugation break spacers for field–effect transistors

A series of naphthalene−diimide based conjugated polymers was synthesized through statistical terpolymerization with branched conjugation break spacers to enhance their mobility−stretchability properties in field-effect transistors.

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.