Topology and Ground State Control In Open-Shell Donor-Acceptor Conjugated Polymers

Donor-acceptor (DA) conjugated polymers (CPs) with narrow bandgaps and open-shell (diradical) character represent an emerging class of materials whose rich behavior emanates from their collective electronic properties and diminished electron pairing. However, the structural and electronic heterogeneities that define these materials complicate bandgap control at low energies and connections linking topology, exchange interactions, and (opto)electronic functionality remain nascent. To address these challenges, we demonstrate structurally rigid and strongly π-conjugated copolymers comprised of a solubilizing thiadiazoloquinoxaline acceptor and cyclopenta[2,1-b:3,4-b′]dithiophene or dithieno[3,2-b:2′,3′-d]thiophene donors. Atom-specific substitution modulates local aromatic character within the donor resulting in dramatic differences in structural, physicochemical, electronic, and magnetic properties of the polymers. These long-range π-mediated interactions facilitate control between low-spin aromatic and high-spin quinoidal forms. This work provides a strategy to understand the evolution of the electronic structure within DA CPs, control the ground state spin multiplicity, tune spin-spin interactions, and articulate the emergence of their novel properties

Backbone-Driven Host-Dopant Miscibility Modulates Molecular Doping In NDI Conjugated Polymers

Molecular doping is the key to enabling organic electronic devices, however, the design strategies to maximize doping efficiency demands further clarity and comprehension. Previous reports focus on the effect of the side chains, but the role of the backbone is still not well understood. In this study, we synthesize a series of NDI-based copolymers with bithiophene, vinylene, and acetylenic moieties (P1G, P2G, and P3G, respectively), all containing branched triethylene glycol side chains. Using computational and experimental methods, we explore the impact of the conjugated backbone using three key parameters for doping in organic semiconductors: energy levels, microstructure, and miscibility. Our experimental results show that P1G undergoes the most efficient n-type doping owed primarily to its higher dipole moment, and better host–dopant miscibility with N-DMBI. In contrast, P2G and P3G possess more planar backbones than P1G, but the lack of long-range order, and poor host–dopant miscibility limit their doping efficiency. Our data suggest that backbone planarity alone is not enough to maximize the electrical conductivity (σ) of n-type doped organic semiconductors, and that backbone polarity also plays an important role in enhancing σ via host–dopant miscibility. Finally, the thermoelectric properties of doped P1G exhibit a power factor of 0.077 μW m−1 K−2, and ultra-low in-plane thermal conductivity of 0.13 W m−1K−1 at 5 mol% of N-DMBI, which is among the lowest thermal conductivity values reported for n-type doped conjugated polymers

Backbone-driven host-dopant miscibility modulates molecular doping in NDI conjugated polymers

Molecular doping is the key to enabling organic electronic devices, however, the design strategies to maximize doping efficiency demands further clarity and comprehension. Previous reports focus on the effect of the side chains, but the role of the backbone is still not well understood. In this study, we synthesize a series of NDI-based copolymers with bithiophene, vinylene, and acetylenic moieties (P1G, P2G, and P3G, respectively), all containing branched triethylene glycol side chains. Using computational and experimental methods, we explore the impact of the conjugated backbone using three key parameters for doping in organic semiconductors: energy levels, microstructure, and miscibility. Our experimental results show that P1G undergoes the most efficient n-type doping owed primarily to its higher dipole moment, and better host–dopant miscibility with N-DMBI. In contrast, P2G and P3G possess more planar backbones than P1G, but the lack of long-range order, and poor host–dopant miscibility limit their doping efficiency. Our data suggest that backbone planarity alone is not enough to maximize the electrical conductivity (σ) of n-type doped organic semiconductors, and that backbone polarity also plays an important role in enhancing σ via host–dopant miscibility. Finally, the thermoelectric properties of doped P1G exhibit a power factor of 0.077 μW m(−1) K(−2), and ultra-low in-plane thermal conductivity of 0.13 W m(−1)K(−1) at 5 mol% of N-DMBI, which is among the lowest thermal conductivity values reported for n-type doped conjugated polymers

Water-Assisted Mechanical Testing of Polymeric Thin-Films

Thin films with a nanometer-scale thickness are of great interest to both scientific and industrial communities due to their numerous applications and unique behaviors different from the bulk. However, the understanding of thin-film mechanics is still greatly hampered due to their intrinsic fragility and the lack of commercially available experimental instruments. In this review, we first discuss the progression of thin-film mechanical testing methods based on the supporting substrate: film-on-solid substrate method, film-on-water tensile tests, and water-assisted free-standing tensile tests. By comparing past studies on a model polymer, polystyrene, the effect of different substrates and confinement effect on the thin-film mechanics is evaluated. These techniques have generated fruitful scientific knowledge in the field of organic semiconductors for the understanding of structure–mechanical property relationships. We end this review by providing our perspective for their bright prospects in much broader applications and materials of interest

Elucidating the Role of Hydrogen Bonds for Improved Mechanical Properties in a High-Performance Semiconducting Polymer

Incorporation of hydrogen bond moieties into the backbone or side chain of conjugated polymers is an effective strategy to enhance mechanical performance, facilitate morphological organization, and promote self-healing ability. However, the understanding of hydrogen bonds, particularly the effect of bond strength and directionality, on thermomechanical and optoelectronic performance is still in its infancy due to the competing influence of morphology, glass transition phenomena, and the measurement process itself. Here, we compare the influence of statistically incorporated amide and urea moieties on the mechanical properties of DPP-TVT parent polymers. We observed a profound difference in ductility; amide functionalization increases the strain at failure by over 100% relative to the pure DPP-TVT polymer, while urea functionalization results in a loss of strain at failure by 50%. This is attributed to the crystalline behavior of functionalized conjugated polymers that is promoted by intermolecular interactions of urea groups, which we elucidated via an in-depth investigation of the swelling, crystalline packing, thermal behavior, and strain-dependent charge transport. Furthermore, we employed a novel free-standing tensile test to validate our mechanical measurements supported on a water surface. Our results demonstrated that hydrogen bond moieties must be carefully chosen to achieve a delicate balance of morphological control and mechanical performance, as simply increasing the hydrogen bond strength can result in detrimental mechanical and electrical performance

N-Type Complementary Semiconducting Polymer Blends

Complementary semiconducting polymer blends (c-SPBs) have been demonstrated as an effective approach to balance performance and processing of semiconducting polymers for organic field-effect transistors. All previously reported c-SPBs have been exclusively based on p-type polymers. In this report, we designed and synthesized naphthalene diimide (NDI) based matrix polymers and systematically studied n-type charge transport behaviors of their corresponding polymer blends. NDI-Cm (m = 3–7) polymers displayed low melting points (55–105 °C) allowing for the lowest temperature melt-processing of organic transistors to date with mobilities up to 1.01 × 10–3 cm2 V–1 s–1. NDI-Cm polymers were revealed to be nearly amorphous by GIXRD and thin film UV–vis which explain the lowered thermal transitions and observed poor charge transport. Utilizing a c-SPB with 5% fully conjugated P(NDI2OD-T2), the transistor performance improved up to 100-fold of the pure matrix polymer despite the low crystallinity of NDI-Cm thin films

Backbone Flexibility On Conjugated Polymer\u27s Crystallization Behavior and Thin Film Mechanical Stability

Extensive efforts have been made to develop flexible electronics with conjugated polymers that are intrinsically stretchable and soft. We recently systematically investigated the influence of conjugation break spacers (CBS) on the thermomechanical properties of a series n-type naphthalene diimide-based conjugated polymer and found that CBS can significantly reduce chain rigidity, melting point, as well as glass transition temperature. In the current work, we further examined the influence of CBS on the crystallization behaviors of PNDI-C3 to C6, including isothermal crystallization kinetics, crystal polymorphism and subsequently time-dependent modulus, in a holistic approach using differential scanning calorimetry, X-ray scattering, polarized optical microscopy, atomic force microscopy, and pseudo-free-standing tensile test. Results demonstrate that increasing the length of CBS increases the crystallization half-time by 1 order of magnitude from PNDI-C3 to PNDI-C6 from approximately 103 to 104 s. The crystallization rate shows a bimodal dependence on the temperature due to the presence of different polymorphs. In addition, crystallization significantly affects the mechanical response, a stiffening in the modulus of nearly three times is observed for PNDI-C5 when annealed at room temperature for 12 h. Crystallization kinetic is also influenced by molecular weight (MW). Higher MW PNDI-C3 crystallizes slower. In addition, an odd–even effect was observed below 50°C, odd-number PNDI-Cxs (C3 and C5) crystallize slower than the adjacent even-numbered PNDI-Cxs (C4 and C6). Our work provides an insight to design flexible electronics by systematically tuning the mechanical properties through control of polymer crystallization by tuning backbone rigidity

Directly Probing the Fracture Behavior of Ultrathin Polymeric Films

Understanding fracture mechanics of ultrathin polymeric films is crucial for modern technologies, including semiconductor and coating industries. However, up to now, the fracture behavior of sub-100 nm polymeric thin films is rarely explored due to challenges in handling samples and limited testing methods available. In this work, we report a new testing methodology that can not only visualize the evolution of the local stress distribution through wrinkling patterns and crack propagation during the deformation of ultrathin films but also directly measure their fracture energies. Using ultrathin polystyrene films as a model system, we both experimentally and computationally investigate the effect of the film thickness and molecular weight on their fracture behavior, both of which show a ductile-to-brittle transition. Furthermore, we demonstrate the broad applicability of this testing method in semicrystalline semiconducting polymers. We anticipate our methodology described here could provide new ways of studying the fracture behavior of ultrathin films under confinement

Strain-Induced Nanocavitation in Block Copolymer Thin Films for High Performance Filtration Membranes

A new pore formation process was investigated for the manufacture of composite ultrafiltration membranes. Phase-separated block copolymer (BCP) thin films supported on a compliant macroporous poly(ether sulfone) (PES) support craze under tensile strain, leaving behind pores of predictable size based on the self-assembled nanoscopic domains. The high aspect ratio pores formed in this process were used to create membranes that were highly permeable (959 L/(m2 h bar) with near complete rejection of 40 nm diameter gold nanoparticles (AuNP). By use of BCP’s inherent ability to cavitate under strain, tedious block removal steps are avoided. Membranes can thus be prepared in a simple, roll-to-roll ready, one-step process. In this initial study, BCP craze formation and filtration performance were characterized for various polymer types, molecular weights, and thicknesses. All these factors influenced the BCP’s thin film morphology, mechanical performance, deformation mechanism, and ultimately filtration performance. This work demonstrates a possible new path toward achieving scalable, BCP-based ultrafiltration membranes

Influence of Side-Chain Isomerization On the Isotheram Crystallization Kinetics of Poly(3-Alkylthiophenes)

Flexible alkyl side chain in conjugate polymers (CPs) improves the solubility and promotes solution processability, in addition, it affects interchain packing and charge mobilities. Despite the well-known charge mobility and morphology correlation for these semi-crystalline polymers, there is a lack of fundamental understanding of the impact of side chain on their crystallization kinetics. In the present work, isothermal crystallization of five poly(3-alkylthiophene-2,5-diyl) (P3ATs) with different side-chain structures were systematically investigated. To suppress the extremely fast crystallization and trap the sample into amorphous glass, an advanced fast scanning chip calorimetry technique, which is able to quench the sample with few to tens thousands of K/s, was applied. Results show that the crystallization of P3ATs was greatly inhibited after incorporation of branched side chains, as indicated by a dramatic up to six orders of magnitude decrease in the crystallization rate. The suppressed crystallization of P3ATs were correlated with an increased π–π stacking distance due to unfavorable side-chain steric interaction. This work provides a pathway to use side-chain engineering to control the crystallization behavior for CPs, thus to control device performance