While the chemical structure of organic semiconductors has an obvious effect on their proclivity for charge transport, the ways with which they are processed have a dramatic effect on the performance of plastic electronics devices incorporating them. In some cases, morphological defects and misalignment of crystalline grains can completely obscure the materials' intrinsic charge transport properties. Although some deposition methods, especially vapor-phase ones, can produce single crystals and thus avoid some of these problems, it is desirable to gain a fundamental understanding of how to improve charge transport when using solution-phase deposition techniques. In this review, we present both a survey of solution-based processing techniques for plastic electronics relevant on both the commercial and research scale and a set of strategies to control thin film morphology towards enhancing their electronic transport properties.

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Morphology control strategies for solution-processed organic semiconductor thin films

Organic electronics is a promising field spanning a wide range of applications, with unrivaled advantages in low production cost, versatility in material synthesis, and compatibility with a wide range of substrates including flexible polymeric materials. Organic molecules are characterized by weak van der Waals interactions, which grant access to multiple crystalline packing states (crystal polymorphism) at near ambient conditions. Different polymorphs, even with the slightest changes in their molecular packing can have electronic properties that differ by orders of magnitude. Therefore, accessing metastable polymorphs can serve as a novel design strategy for attaining high device performance. Recently, this strategy has been successfully applied to small organic molecules such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and high hole mobilities have been attained in organic field-effect transistors fabricated using their metastable structures. In addition, polymorphism serves as an excellent platform for advancing the fundamental understanding of charge transport in π-conjugated systems. The relationship between molecular packing and charge transport can be unequivocally established since the chemical structures are identical amongst polymorphs, leaving molecular packing as the only variable in the case of packing polymorphism.

Polymorphism as an emerging design strategy for high performance organic electronics

Solution-processed hybrid perovskite semiconductors attract a great deal of attention, but little is known about their formation process. The one-step spin-coating process of perovskites is investigated in situ, revealing that thin-film formation is mediated by solid-state precursor solvates and their nature. The stability of these intermediate phases directly impacts the quality and reproducibility of thermally converted perovskite films and their photovoltaic performance.

Hybrid Perovskite Thin‐Film Photovoltaics: In Situ Diagnostics and Importance of the Precursor Solvate Phases

The infl uence of polymer entanglement on the self-assembly, molecular packing structure, and microstructure of low- M w (lightly entangled) and high- M w (highly entangled) poly (3-hexylthiophene) (P3HT), and the carrier transport in thin-fi lm transistors, are investigated. The polymer chains are gradually disentangled in a marginal solvent via ultrasonication of the polymer solution, and demonstrate improved diffusivity of precursor species (coils, aggregates, and microcrystallites), enhanced nucleation and crystallization of P3HT in solution, and self-assembly of well-ordered and highly textured fi brils at the solid‐liquid interface. In low- M w P3HT, reducing chain entanglement enhances interchain and intrachain ordering, but reduces the interconnectivity of ordered domains (tie molecules) due to the presence of short chains, thus deteriorating carrier transport even in the face of improving crystallinity. Reducing chain entanglement in high- M w P3HT solutions increases carrier mobility up to ≈ 20-fold, by enhancing interchain and intrachain ordering while maintaining a suffi ciently large number of tie molecules between ordered domains. These results indicate that charge carrier mobility is strongly governed by the balancing of intrachain and interchain ordering, on the one hand, and interconnectivity of ordered domains, on the other hand. In high- M w P3HT, intrachain and interchain ordering appear to be the key bottlenecks to charge transport, whereas in low- M w P3HT, the limited interconnectivity of the ordered domains acts as the primary bottleneck to charge transport.

Entanglement of Conjugated Polymer Chains Influences Molecular Self-Assembly and Carrier Transport

Blending of small-molecule organic semiconductors (OSCs) with amorphous polymers is known to yield high performance organic thin film transistors (OTFTs). Vertical stratification of the OSC and polymer binder into well-defined layers is crucial in such systems and their vertical order determines whether the coating is compatible with a top and/or a bottom gate OTFT configuration. Here, we investigate the formation of blends prepared via spin-coating in conditions which yield bilayer and trilayer stratifications. We use a combination of in situ experimental and computational tools to study the competing effects of formulation thermodynamics and process kinetics in mediating the final vertical stratification. It is shown that trilayer stratification (OSC/polymer/OSC) is the thermodynamically favored configuration and that formation of the buried OSC layer can be kinetically inhibited in certain conditions of spin-coating, resulting in a bilayer stack instead. The analysis reveals here that preferential loss of the OSC, combined with early aggregation of the polymer phase due to rapid drying, inhibit the formation of the buried OSC layer. The fluid dynamics and drying kinetics are then moderated during spin-coating to promote trilayer stratification with a high quality buried OSC layer which yields unusually high mobility >2 cm2 V−1 s−1 in the bottom-gate top-contact configuration.

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Vertical Phase Separation in Small Molecule:Polymer Blend Organic Thin Film Transistors Can Be Dynamically Controlled

We demonstrate a new way to investigate and control the solvent vapor annealing of solution-cast organic semiconductor thin films. Solvent vapor annealing of spin-cast films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) is investigated in situ using quartz crystal microbalance with dissipation (QCM-D) capability, allowing us to monitor both solvent mass uptake and changes in the mechanical rigidity of the film. Using time-resolved grazing incidence wide angle X-ray scattering (GIWAXS) and complementary static atomic force microscopy (AFM), we demonstrate that solvent vapor annealing in the molecular regime can cause significant performance improvements in organic thin film transistors (OTFTs), whereas allowing the solvent to percolate and form a liquid phase results in catastrophic reorganization and dewetting of the film, making the process counterproductive. Using these lessons we devise processing conditions which prevent percolation of the adsorbed solvent vapor molecules for extended peri...

Yi Ren

Papers

Solvent vapor annealing in the molecular regime drastically improves carrier transport in small-molecule thin-film transistors.