Intrachain charge transport is unique to conjugated polymers distinct from inorganic and small molecular semiconductors and is key to achieving high-performance organic electronics. Polymer backbone planarity and thin film morphology sensitively modulate intrachain charge transport. However, simple, generic nonsynthetic approaches for tuning backbone planarity and the ensuing multiscale assembly process do not exist. We first demonstrate that printing flow is capable of planarizing the originally twisted polymer backbone to substantially increase the conjugation length. This conformation change leads to a marked morphological transition from chiral, twinned domains to achiral, highly aligned morphology, hence a fourfold increase in charge carrier mobilities. We found a surprising mechanism that flow extinguishes a lyotropic twist-bend mesophase upon backbone planarization, leading to the observed morphology and electronic structure transitions.

Tuning structures of solution‐state aggregation and aggregation‐mediated assembly pathways of conjugated polymers is crucial for optimizing their solid‐state morphology and charge‐transport property. However, it remains challenging to unravel and control the exact structures of solution aggregates, let alone to modulate assembly pathways in a controlled fashion. Herein, aggregate structures of an isoindigo–bithiophene‐based polymer (PII‐2T) are modulated by tuning selectivity of the solvent toward the side chain versus the backbone, which leads to three distinct assembly pathways: direct crystallization from side‐chain‐associated amorphous aggregates, chiral liquid crystal (LC)‐mediated assembly from semicrystalline aggregates with side‐chain and backbone stacking, and random agglomeration from backbone‐stacked semicrystalline aggregates. Importantly, it is demonstrated that the amorphous solution aggregates, compared with semicrystalline ones, lead to significantly improved alignment and reduced paracrystalline disorder in the solid state due to direct crystallization during the meniscus‐guided coating process. Alignment quantified by the dichroic ratio is enhanced by up to 14‐fold, and the charge‐carrier mobility increases by a maximum of 20‐fold in films printed from amorphous aggregates compared to those from semicrystalline aggregates. This work shows that by tuning the precise structure of solution aggregates, the assembly pathways and the resulting thin‐film morphology and device properties can be drastically tuned.

Conjugated polymers are rapidly emerging as an attractive class of semiconductors for next-generation electronics thanks to their low-cost, high-throughput solution processability, mechanical flexibility, stretchability, self-healing properties, and ability to interface and communicate with biological systems. Accordingly, the last four decades has seen a surge of studies that have provided seminal contributions to the thorough understanding of conjugated polymers. One of the key factors that dictates the electronic performance of conjugated polymers is their assembly and crystallization behavior, which has remained intriguing and challenging to study. The complex solution processing environment and rapid kinetics strongly couple with the conjugated polymer assembly process, further complicating a full mechanistic picture. In this perspective, we summarize the charge transport mechanism, fundamentals of conjugated polymer assembly, and solution printing. We further discuss central strategies that have been developed to control and enhance their multiscale assembly during solution printing. Finally, we hope that our perspective will stimulate more studies on how processing can control morphology and charge transport of conjugated polymers and applications of these concepts to other advanced functional materials.

Intimately connected to the rule of life, chirality remains a long-time fascination in biology, chemistry, physics and materials science. Chiral structures, e.g., nucleic acid and cholesteric phase developed from chiral molecules are common in nature and synthetic soft materials. While it was recently discovered that achiral but bent-core mesogens can also form chiral helices, the assembly of chiral microstructures from achiral polymers has rarely been explored. Here, we reveal chiral emergence from achiral conjugated polymers, in which hierarchical helical structures are developed through a multistep assembly pathway. Upon increasing concentration beyond a threshold volume fraction, dispersed polymer nanofibers form lyotropic liquid crystalline (LC) mesophases with complex, chiral morphologies. Combining imaging, X-ray and spectroscopy techniques with molecular simulations, we demonstrate that this structural evolution arises from torsional polymer molecules which induce multiscale helical assembly, progressing from nano- to micron scale helical structures as the solution concentration increases. This study unveils a previously unknown complex state of matter for conjugated polymers that can pave way to a field of chiral (opto)electronics. We anticipate that hierarchical chiral helical structures can profoundly impact how conjugated polymers interact with light, transport charges, and transduce signals from biomolecular interactions and even give rise to properties unimagined before.

Polymorphism has emerged as an important design consideration in organic semiconductors (OSCs). Previously, in many OSCs, even small changes in molecular stacking can cause drastic changes to the optical and electronic properties. However, investigation into n-type semiconductors has significantly lagged behind their p-type counterparts. In this work, we present the prolific polymorphism of 2-dimensional quinoidal terthiophene (2DQTT-o-B) and systematically investigate each of 5 polymorphs, 3 of which have been previously unreported. Grazing incidence X-ray diffraction provided a key method to understanding the structure of each polymorph. Via the polymorphic transitions mapped, we tuned the electron mobility by 5 orders of magnitude, from 5.63 × 10 −5 to 0.22 cm 2 V −1 s −1 . These were accompanied by modifications to the optical properties, namely we observed substantial differences in the refractive index noted by intensity differences under polarized optical microscopy and a large shift in optical band gap from 1.18 eV up to 1.40 eV. Finally, we suggest that changes to these properties may be related to the unique quinoidal to aromatic transition observed in quinoidal molecules.

Organic semiconductors are usually polycyclic aromatic hydrocarbons and their analogs containing heteroatom substitution. Bioinspired materials chemistry of organic electronics promises new charge transport mechanism and specific molecular recognition with biomolecules. We discover organic semiconductors from deoxyribonucleic acid topoisomerase inhibitors, featuring conjugated backbone decorated with hydrogen-bonding moieties distinct from common organic semiconductors. Using ellipticine as a model compound, we find that hydrogen bonds not only guide polymorph assembly, but are also critical to forming efficient charge transport pathways along π−conjugated planes when at a low dihedral angle by shortening the end-to-end distance of adjacent π planes. In the π−π stacking and hydrogen-bonding directions, the intrinsic, short-range hole mobilities reach as high as 6.5 cm2V−1s−1 and 4.2 cm2V−1s−1 measured by microwave conductivity, and the long-range apparent hole mobilities are up to 1.3 × 10–3 cm2V−1s−1 and 0.4 × 10–3 cm2V−1s−1 measured in field-effect transistors. We further demonstrate printed transistor devices and chemical sensors as potential applications.

Nanosizing is rapidly emerging as an alternative approach to enhance solubility and thus the bioavailability of poorly aqueous soluble active pharmaceutical ingredients (APIs). Although numerous techniques have been developed to perform nanosizing of API crystals, precise control and modulation of their size in an energy and material efficient manner remains challenging. In this study, we present meniscus-guided solution coating as a new technique to produce pharmaceutical thin films of nanoscale thickness with controlled morphology. We demonstrate control of aspirin film thickness over more than 2 orders of magnitude, from 30 nm to 1.5 μm. By varying simple process parameters such as the coating speed and the solution concentration, the aspirin film morphology can also be modulated by accessing different coating regimes, namely the evaporation regime and the Landau-Levich regime. Using ellipticine-a poorly water-soluble anticancer drug-as another model compound, we discovered a new polymorph kinetically trapped during solution coating. Furthermore, the polymorphic outcome can be controlled by varying coating conditions. We further performed layer-by-layer coating of multilayer nanocomposites, with alternating thin films of ellipticine and a biocompatible polymer, which demonstrate the potential of additive manufacturing of multidrug-personalized dosage forms using this approach.

Interfacial out-of-plane molecular orientation critically influences the electronic performance of organic semiconductor thin films. The appearance of a lyotropic liquid crystalline (LC) mesophase during solution coating of conjugated polymers may directly determine the interfacial out-of-plane molecular orientation. However, a lack of studies on the packing structure of the liquid crystalline mesophase and its evolution to the solid state impedes the general understanding of the molecular orientation transformation from the liquid crystalline mesophase to solid-state thin films. This work addresses this unanswered question using poly [[2,5-(DPP-BTz) as a model compound. From near-edge X-ray adsorption fine structure spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements, we observe distinct edge-on orientation at the top interface compared to a primarily face-on orientation in the bulk in solution-coated DPP-BTz thin films. Interestingly, the interfacial orientations in thin films are strongly correlated with those of the lyotropic liquid crystalline mesophase of DPP-BTz appearing during solution coating. Specifically, the LC mesophase adopts an edge-on orientation near the air−liquid interface and a face-on orientation in the bulk liquid layer. The multiscale structure and interfacial orientation of the mesophase are characterized by solution-state small-angle Xray scattering and in situ GIWAXS measurements. We attribute the edge-on LC orientation at the top interface to surface energy minimization of alkyl side chains with air, while the anisotropic broad LC orientation distribution in the bulk arises from twisted molecular packing in the LC mesophase. The out-of-plane molecular orientation is preserved in the LC mesophase and is carried over to the solid-state thin film, creating the distinct edge-on interfacial alignment at the thin-film top surface.