An inverse relationship between mechanical ductility and mobility/molecular ordering in conjugated polymer systems was determined definitively through systematic interrogation of poly(3-hexylthiophene) (P3HT) films with varied degrees of molecular ordering and associated charge transport performance. The dilemma, whereby molecular ordering required for efficient charge transport conclusively undermines the applicability of these materials for stretchable, flexible device applications, was resolved using a polymer blend approach. Specifically, the molecular interactions between dissimilar polymer materials advantageously induced semiconducting polymer ordering into efficient π−π stacked fibrillar networks. Changes in the molecular environment surrounding the conjugated polymer during the elastomer curing process further facilitated development of high mobility networked semiconductor pathways. A processed P3HT: poly(dimethylsiloxane) (PDMS) composite afforded a semiconducting film that exhibits superior ductility and notable mobility versus the single-component polymer semiconductor counterpart.

Conjugated semiconducting polymers have been the subject of intense study for over two decades with promising advances toward a printable electronics manufacturing ecosystem. These materials will deliver functional electronic devices that are lightweight, flexible, large-area, and cost-effective, with applications ranging from biomedical sensors to solar cells. Synthesis of novel molecules has led to significant improvements in charge carrier mobility, a defining electrical performance metric for many applications. However, the solution processing and thin film deposition of conjugated polymers must also be properly controlled to obtain reproducible device performance. This has led to an abundance of research on the process-structure-property relationships governing the microstructural evolution of the model semicrystalline poly(3-hexylthiophene) (P3HT) as applied to organic field effect transistor (OFET) fabrication. What followed was the production of an expansive body of work on the crystallization, self-assembly, and charge transport behavior of this semiflexible polymer whose strong π-π stacking interactions allow for highly creative methods of structural control, including the modulation of solvent and solution properties, flow-induced crystallization and alignment techniques, structural templating, and solid-state thermal and mechanical processing. This Account relates recent progress in the microstructural control of P3HT thin films through the nucleation, growth, and alignment of P3HT nanofibers. Solution-based nanofiber formation allows one to develop structural order prior to thin film deposition, mitigating the need for intricate deposition processes and enabling the use of batch and continuous chemical processing steps. Fiber growth is framed as a traditional crystallization problem, with the balance between nucleation and growth rates determining the fiber size and ultimately the distribution of grain boundaries in the solid state. Control of nucleation can be accomplished through a sonication-based seeding procedure, while growth can be modulated through supersaturation control via the tuning of solvent quality, the use of UV irradiation or through aging. These principles carry over to the flow-induced growth of P3HT nanofibers in a continuous microfluidic processing system, leading to thin films with significantly enhanced mobility. Further gains can be made by promoting long-range polymer chain alignment, achieved by depositing nanofibers through shear-based coating methods that promote high fiber packing density and alignment. All of these developments in processing were carried out on a standard OFET platform, enabling us to generalize quantitative structure-property relationships from structural data sources such as UV-vis, AFM, and GIWAXS. It is shown that a linear correlation exists between mobility and the in-plane orientational order of nanofibers, as extracted from AFM images using advanced computer vision software developed by our group. Herein, we discuss data-driven approaches to the d...

Very few studies have reported oriented crystallization of conjugated polymers directly in solution. Here, solution crystallization of conjugated polymers in a microfluidic system is found to produce tightly π-stacked fibers with commensurate improved charge transport characteristics. For poly(3-hexylthiophene) (P3HT) films, processing under flow caused exciton bandwidth to decrease from 140 to 25 meV, π-π stacking distance to decrease from 3.93 to 3.72 Å and hole mobility to increase from an average of 0.013 to 0.16 cm(2) V(-1) s(-1), vs films spin-coated from pristine, untreated solutions. Variation of the flow rate affected thin-film structure and properties, with an intermediate flow rate of 0.25 m s(-1) yielding the optimal π-π stacking distance and mobility. The flow process included sequential cooling followed by low-dose ultraviolet irradiation that promoted growth of conjugated polymer fibers. Image analysis coupled with mechanistic interpretation supports the supposition that "tie chains" provide for charge transport pathways between nanoaggregated structures. The "microfluidic flow enhanced semiconducting polymer crystal engineering" was also successfully applied to a representative electron transport polymer and a nonhalogenated solvent. The process can be applied as a general strategy and is expected to facilitate the fabrication of high-performance electrically active polymer devices.

The pursuit of intelligent optoelectronics could have profound implications on our future daily life. Simultaneous enhancement of the electrical performance, mechanical stretchability, and optical transparency of semiconducting polymers may significantly broaden the spectrum of realizable applications for these materials in future intelligent optoelectronics, i.e., wearable devices, electronic skin, stretchable displays, and a vast array of biomedical sensors. Here, semiconducting films with significantly improved mechanical elasticity and optical transparency, without affecting the film’s electronic conductivity even under 100% strain, were prepared by blending only a small amount (below 1 wt %) of either p-type or n-type commercial semiconductor polymers. We demonstrate that a self-organized versatile conjugated polymer film displaying an interpenetrating polymer network is formed in the semiconducting films and is crucial for the observed enhancement of elasticity, optical transparency, and charge-carrier mobility. On the basis of this versatile semiconducting film, we explored a new practical approach to directly integrate all the stretchable components for a large area transistor array through solution processing and a final single, mechanical peel-off step. We demonstrate robust transistor arrays exhibiting charge carrier mobilities above 1.0 cm2/V s with excellent durability, even under 100% strain. We believe our achievements will have great impact on stretchable optoelectronic devices for practical applications and represent promising directions for industry-scale production of stretchable displays and wearable electronic devices.

The deposition of conjugated polymers is typically subject to chain-entanglement effects, which can severely hinder chain unfolding, alignment, and π−π stacking during rapid solution-coating processes. Here, long-range ordering and highly aligned poly(3-hexylthiophene) (P3HT) thin films were demonstrated by preprocessing the polymer solution with ultraviolet (UV) irradiation/solution aging and then depositing via the blade-coating method, which is compatible with roll-to-roll printing processes. The surface morphologies and optical anisotropy of deposited films revealed that the degree of chain alignment was greatly improved with increased levels of polymer assembly that can be precisely controlled by solution-aging time. The correlations between oriented nanofibrillar structures and their charge-transport anisotropy were further systematically investigated by blade coating pretreated solutions parallel and perpendicular to the direction of the source and drain electrodes. Interestingly, charge transport across the well-aligned P3HT nanofibers was more efficient than along the long-axis of nanofibrillar structures owing to enhanced intramolecular charge transport and tie-chains. The facile and scalable solution-coating method investigated here suggests an effective approach to induce anisotropic crystalline structures, which are readily obtained by directly controlling their intrinsic solution properties without the need for extrinsic techniques such as surface templating or shearing blade patterning.

The design of polymer semiconductors possessing effective π–π intermolecular interactions coupled with good solution processability remains a challenge. Structure‐property relationships associated with side chain structure, π–π intermolecular interactions, polymer solubility, and charge carrier transport are reported for a donor–acceptor(1)‐donor–acceptor(2) polymer: 5‐Decylheptadecyl (5‐DH), 2‐tetradecyl (2‐DT), and linear n‐octadecyl (OD) chains are substituted onto a polymer backbone consisting of terthiophene units (T) between two different electron acceptors, benzothiadiazole (B), and diketopyrrolopyrrole (D), pTBTD, to afford pTBTD‐5DH, pTBTD‐2DT, and pTBTD‐OD, respectively. In the 5‐DH side chain, the branching position is remote from the polymer backbone, whereas it is proximal in 2‐DT. This study demonstrates that incorporation of branched side chains where the branching position is remote from the polymer backbone merges the advantages of improved solubility from branched units with effective π–π intermolecular interactions normally associated with linear chains on conjugated polymers. pTBTD‐5DH exhibits superior qualities with respect to the degree of polymerization, solution processability, π–π interchain stacking, and charge carrier transport relative to the other analogs. pTBTD‐5DH exhibits a field‐effect hole mobility of up to 2.95 cm2 V–1 s–1, a factor of 3–7 times that achieved with pBDT6‐DT and pBDT6‐OD.

The electron deficiency and trans-planar conformation of bithiazole is potentially beneficial for the electrontransport performance of organic semiconductors. However, the incorporation of bithiazole into polymers through a facile synthetic strategy remains a challenge. Herein, 2,2′-bithiazole was synthesized in one step and copolymerized with dithienyldiketopyrrolopyrrole to afford poly(dithienyldiketopyrrolopyrrole-bithiazole), PDBTz. PDBTz exhibited electron mobility reaching 0.3 cm 2 V −1 s −1 in organic field-effect transistor (OFET) configuration; this contrasts with a recently discussed isoelectronic conjugated polymer comprising an electron-rich bithiophene and dithienyldiketopyrrolopyrrole, which displays merely hole-transport characteristics. This inversion of charge-carrier transport characteristics confirms the significant potential for bithiazole in the development of electron-transport semiconducting materials. Branched 5-decylheptacyl side chains were incorporated into PDBTz to enhance polymer solubility, particularly in nonhalogenated, more environmentally compatible solvents. PDBTz cast from a range of nonhalogenated solvents exhibited film morphologies and field-effect electron mobility similar to those cast from halogenated solvents.

Decreased conjugated polymer regioregularity can provide for mechanical flexibility, which is favorable for the fabrication of flexible electronics; however, concomitant decreased thin‐film crystallinity severely deteriorates charge transport performance. Here, the advantage of this inverse relationship has been taken to fabricate flexible organic field effect transistors using ultraviolet irradiated regioregular (RR) and regiorandom (RRa) poly(3‐hexylthiophene) (P3HT) blend thin‐films as the device active layer. The blend films exhibit prominent charge carrier mobility up to 0.22 cm2 V‐1 s‐1. Even when the RR‐P3HT component comprises only a small proportion of the blend, vertical phase separation of the components ensures formation of effective charge transport pathways. The underlying mechanism is correlated to molecular self‐assembly of RR‐P3HT and its unfavorable intermolecular interactions with RRa‐P3HT. Transistor performance is retained upon application of high external strain and bending 1000 times. The top‐gate architecture enables the dielectric to serve as a self‐encapsulation layer to protect the underlying semiconductor from atmospheric oxygen and moisture, resulting in over 700 h ambient device stability. This facile blend approach is expected to be an alternative strategy for the fabrication of high performance flexible electronics.