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...

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.

Long-range ordering emerges in poly­(3-hexylthiophene) (P3HT) solutions during time-dependent aggregation. Here, aggregation of P3HT in chloroform solution was induced by ultrasonication, aging, and combinations thereof. UV–vis spectroscopy and polarized optical microscopy demonstrated that long-range ordering in the solution and subsequently the solid state depends on assembled P3HT fiber length, as determined by film atomic force microscopy. Ultrasonication induced the formation of fibers that were relatively short compared to those obtained through aging. As a result, ultrasonication afforded isotropic solutions and films, whereas aging afforded anisotropic solutions and films. The impact of fiber length and anisotropy on macroscopic charge transport performance was evaluated using an organic field-effect transistor (OFET) architecture. Both aged and sonicated solutions exhibited charge carrier mobilities that were an order of magnitude higher than that obtained for pristine samples. Aging of sonicated solutions enabled semiconducting thin films with significantly higher mobilities (1.5 × 10–1 cm2 V–1 s–1) than those of either solution processing technique. Furthermore, the results indicate that grain boundary morphology has a significant impact on macroscopic charge carrier mobility. Grazing incidence wide-angle X-ray scattering demonstrated that the combined sonication/aging method affords a solidified film where the semiconductor exhibits a highly edge-on orientation. The results suggest that the nucleation and growth of aggregates can be controlled via solution processing methods and thus may allow the manipulation of active layer orientation, crystal packing density, and crystallite size. The investigation provides insight into the conjugated polymer solution process parameters that impact polymer ordering and aggregation in solution and resultant thin films for high-performance organic electronic devices.

Nanofibers are a ubiquitous structural motif in a variety of functional materials. In the field of organic electronics, π–π-stacking of conjugated polymers leads to fibrillar morphologies with a wide array of fiber packing behavior. Fiber orientation and alignment are known to influence the charge transport properties of devices such as organic field effect transistors. The solution processing methods used to create these devices give rise to large variations in these structural parametershowever, they are only observable with imaging techniques such as atomic force microscopy (AFM). To bring more rigorous quantification of orientation and alignment to these materials, a comprehensive image analysis tool is introduced to quantify the two-dimensional orientation and alignment of nanofibers from AFM images. It has been developed in MATLAB and packaged as a stand-alone application, so that researchers with no computational expertise can produce publication-ready figures directly from their images. AFM frequently yields images with low contrast and moderate noise, making quantitative feature extraction a significant challenge. In this protocol, each image is analyzed in the context of an Orientation Map, in which nanofibers are thinned to single-pixel width and an orientation is extracted for each of these pixels. The Orientation Map is obtained through a five-step process: fiber smoothing by anisotropic diffusion filtering, contrast enhancement by top hat filtering, binarization by adaptive thresholding, skeletonization, and recovery of orientations from the result of diffusion filtering. Each step involves parameters that can be set using physical heuristics, which are examined in detail. This Orientation Map yields an orientation distribution and a plot of S 2D, an orientational order parameter, as a function of frame size. The image analysis procedure is used to quantify differences in P3HT nanofiber morphology induced by various solution processing recipes, as well as the effect of spin-coating when used to deposit solutions of nanofibers. All examples presented in this protocol can be reproduced from beginning to end using the included software, with visualizations produced at each stage of processing.