X‐ray microbeam scattering is used to map the microstructure of the organic semiconductor along the channel length of solution‐processed bottom‐contact OFET devices. Contact‐induced nucleation is ...known to influence the crystallization behavior within the channel. We find that microstructural inhomogeneities in the center of the channel act as a bottleneck to charge transport. This problem can be overcome by controlling crystallization of the preferable texture, thus favoring more efficient charge transport throughout the channel.
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 periods, thus extending the benefits of solvent vapor annealing and improving carrier mobility by nearly two orders of magnitude. Ultimately, it is demonstrated that QCM-D is a very powerful sensor of the state of the adsorbed solvent as well as the thin film, thus making it suitable for process development as well as in-line process monitoring both in laboratory and in future manufacturing settings.
Herein, we report on the fabrication of large-area printed low-voltage organic thin film transistor arrays via minimal-solution bar-coating. We established the bar-coating of the chemically ...cross-linked polymer dielectric based on poly(4-vinylphenol) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride by investigating the effects of composition, reaction and printing conditions on film thickness, cross-linking efficacy, and dielectric properties. Subsequently, we elucidated various aspects of large-area (up to 4-inch wafer) bar-coated cross-linked polymeric dielectric prepared from minimal solution (∼100 μL, ∼1.2 μL cm −2 ) by addressing film uniformity, thickness control, capacitance variation, underlying step coverage, patternability, etc. The resultant polymeric dielectric exhibited good insulating properties as exemplified by a low leakage current density of ∼10 −8 A cm −2 (at 1 MV cm −1 ) and a high areal capacitance of 42.6 nF cm −2 . Finally, a highly-crystallized organic semiconductor layer based on 2,8-difluorinated 5,11-bis(triethylsilylethynyl)anthradithiophene was deposited on the bar-coated cross-linked polymeric dielectric via bar-coating, leading to the realization of printed low-voltage organic transistor arrays with minimum ink solution wasted.
In bottom‐contact organic field‐effect transistors (OFETs), the functionalization of source/drain electrodes leads to a tailored surface chemistry for film growth and controlled interface energetics ...for charge injection. This report describes a comprehensive investigation into separating and correlating the energetic and morphological effects of a self‐assembled monolayers (SAMs) treatment on Au, Ag, and Cu electrodes. Fluorinated 5,11‐bis(triethylsilylethynyl) anthradithiophene (diF‐TES‐ADT) and pentafluorobenzenethiol (PFBT) are employed as a soluble small‐molecule semiconductor and a SAM material, respectively. Upon SAM modification, the Cu electrode devices benefit from a particularly dramatic performance improvement, closely approaching the performance of OFETs with PFBT‐Au and PFBT‐Ag. Ultraviolet photoemission spectroscopy, polarized optical microscopy, grazing‐incidence wide‐angle X‐ray scattering elucidate the metal work function change and templated crystal growth with high crystallinity resulting from SAMs. The transmission‐line method separates the channel and contact properties from the measured OFET current–voltage data, which conclusively describes the impact of the SAMs on charge injection and transport behavior.
Functionalizing gold, silver, and copper electrodes by PFBT self‐assembled monolayers leads to dramatic performance improvement in solution‐processed diF‐TES‐ADT transistors. In‐depth understanding of this transformation is elucidated by ultraviolet photoemission spectroscopy, polarized optical microscopy, and grazing‐incidence wide‐angle X‐ray scattering measurements, in correlation with charge carrier mobility and contact resistance.
We report on a systematic investigation on the performance and stability of p-channel and n-channel top-gate OFETs, with a CYTOP/Al2O3 bilayer gate dielectric, exposed to controlled dry oxygen and ...humid atmospheres. Despite the severe conditions of environmental exposure, p-channel and n-channel top-gate OFETs show only minor changes of their performance parameters without undergoing irreversible damage. When correlated with the conditions of environmental exposure, these changes provide new insight into the possible physical mechanisms in the presence of oxygen and water. Photoexcited charge collection spectroscopy experiments provided further evidence of oxygen and water effects on OFETs. Top-gate OFETs also display outstanding durability, even when exposed to oxygen plasma and subsequent immersion in water or operated under aqueous media. These remarkable properties arise as a consequence of the use of relatively air stable organic semiconductors and proper engineering of the OFET structure.
Functionalized acenes have proven to be promising compounds in the field of molecular electronics because of their unique features in terms of the stability, performance, and ease of processing. The ...emerging concept of large-area-compatible techniques for flexible electronics has brought about a wide variety of well-established techniques for the deposition of soluble acenes, with spray-coating representing an especially fruitful approach. In the present study, electrostatic spray deposition is proposed as an alternative to the conventional spray-coating processes, toward the realization of high-performance organic field-effect transistors (OFETs), on both rigid and flexible substrates. Particularly, a thorough study on the effect of the solvent and spraying regime on the resulting crystalline film’s morphology is performed. By optimization of the process conditions in terms of control over the size as well as the crystallization scheme of the droplets, desirable morphological features along with high-quality crystal domains are obtained. The fabricated OFETs exhibit excellent electrical characteristics, with high field-effect mobility up to 0.78 cm2/(V s), I on/I off >104, and near-zero threshold voltages. Additionally, the good performance of the OFETs realized on plastic substrates gives great potentiality to the proposed method for applications in the challenging field of large-area electronics.
Understanding the interactions at interfaces between the materials constituting consecutive layers within organic thin‐film transistors (OTFTs) is vital for optimizing charge injection and transport, ...tuning thin‐film microstructure, and designing new materials. Here, the influence of the interactions at the interface between a halogenated organic semiconductor (OSC) thin film and a halogenated self‐assembled monolayer on the formation of the crystalline texture directly affecting the performance of OTFTs is explored. By correlating the results from microbeam grazing incidence wide angle X‐ray scattering (μGIWAXS) measurements of structure and texture with OTFT characteristics, two or more interaction paths between the terminating atoms of the semiconductor and the halogenated surface are found to be vital to templating a highly ordered morphology in the first layer. These interactions are effective when the separating distance is lower than 2.5 dw, where dw represents the van der Waals distance. The ability to modulate charge carrier transport by several orders of magnitude by promoting “edge‐on” versus “face‐on” molecular orientation and crystallographic textures in OSCs is demonstrated. It is found that the “edge‐on” self‐assembly of molecules forms uniform, (001) lamellar‐textured crystallites which promote high charge carrier mobility, and that charge transport suffers as the fraction of the “face‐on” oriented crystallites increases.
The role of interfacial halogenation in mediating texture formation and the self‐patterning of organic semiconductor films, as well as the resulting effects on charge transport in organic thin‐film transistors, are explored. The presence of two or more anchoring sites between a halogenated semiconductor and a halogenated self‐assembled monolayer, closer than about twice the corresponding van der Waals distance, alter the microstructure and improve electrical properties.
Since the grain boundaries (GBs) within the semiconductor layer of organic field-effect transistors (OFETs) have a strong influence on device performance, a substantial number of studies have been ...devoted to controlling the crystallization characteristics of organic semiconductors. We studied the intrinsic effects of GBs within 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT) thin films on the electrical properties of OFETs. The GB density was easily changed by controlling nulceation event in TES-ADT thin films. When the mixing time was increased, the number of aggregates in as-spun TES-ADT thin films were increased and subsequent exposure of the films to 1,2-dichloroethane vapor led to a significant increase in the number of nuleation sites, thereby increasing the GB density of TES-ADT spherulites. The density of GBs strongly influences the angular spread and crystallographic orientation of TES-ADT spherulites. Accordingly, the FETs with higher GB densities showed much poorer electrical characteristics than devices with lower GB density. Especially, GBs provide charge trapping sites which are responsible for bias-stress driven electrical instability. Dielectric surface treatment with a polystyrene brush layer clarified the GB-induced charge trapping by reducing charge trapping at the semiconductor-dielectric interface. Our study provides an understanding on GB induced bias instability for the development of high performance OFETs.
We report the synthesis of a series of crystalline acenedithiophenes with up to seven linearly fused rings and silylethynyl substituents. These functional groups are designed to both improve ...solubility and enhance cofacial interactions in the solid. We discuss the crystal packing of these materials, as well as their physical properties such as oxidation potential, UV−vis absorption, fluorescence emission, and decomposition pathways.
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