Marcia M. Payne

Ultra-flexible solution-processed organic field-effect transistors

Organic semiconductors might enable new applications in low-cost, light-weight, flexible electronics. To build a solid foundation for these technologies, more fundamental studies of electro-mechanical properties of various types of organic semiconductors are necessary. Here we perform basic studies of charge transport in highly crystalline solution-processed organic semiconductors as a function of applied mechanical strain. As a test bed, we use small molecules crystallized on thin plastic sheets, resulting in high-performance flexible field-effect transistors. These devices can be bent multiple times without degradation to a radius as small as ~200 μm, demonstrating that crystalline solution-processed organic semiconductors are intrinsically highly flexible. This study of electro-mechanical properties suggests that solution-processable organic semiconductors are suitable for applications in flexible electronics, provided that integration with other important technological advances, such as device scalability and low-voltage operation, is achieved in the future.

All-Printed Flexible Organic Transistors Enabled by Surface Tension-Guided Blade Coating

A combination of surface energy-guided blade coating and inkjet printing is used to fabricate an all-printed high performance, high yield, and low variability organic thin film transistor (OTFT) array on a plastic substrate. Functional inks and printing processes were optimized to yield self-assembled homogenous thin films in every layer of the OTFT stack. Specifically, we investigated the effect of capillary number, semiconductor ink composition (small molecule-polymer ratio), and additive high boiling point solvent concentrations on film fidelity, pattern design, device performance and yields.

High Mobility Field‐Effect Transistors with Versatile Processing from a Small‐Molecule Organic Semiconductor

Trialkylgermyl functionalization allows the development of high-performance soluble small-molecule organic semiconductors with mobilities greater than 5 cm(2) V(-1) s(-1) . Spray-deposited organic thin-film transistors show a record mobility of 2.2 cm(2) V(-1) s(-1) and demonstrate the potential for incorporation in large-area, low-cost electronic applications.

Solution-Printed Organic Semiconductor Blends Exhibiting Transport Properties on Par with Single Crystals

Solution-printed organic semiconductors have emerged in recent years as promising contenders for roll-to-roll manufacturing of electronic and optoelectronic circuits. The stringent performance requirements for organic thin-film transistors (OTFTs) in terms of carrier mobility, switching speed, turn-on voltage and uniformity over large areas require performance currently achieved by organic single-crystal devices, but these suffer from scale-up challenges. Here we present a new method based on blade coating of a blend of conjugated small molecules and amorphous insulating polymers to produce OTFTs with consistently excellent performance characteristics (carrier mobility as high as 6.7 cm2 V−1 s−1, low threshold voltages of<1 V and low subthreshold swings <0.5 V dec−1). Our findings demonstrate that careful control over phase separation and crystallization can yield solution-printed polycrystalline organic semiconductor films with transport properties and other figures of merit on par with their single-crystal counterparts.

Exciton delocalization drives rapid singlet fission in nanoparticles of acene derivatives.

We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.

Isomerically Pure syn-Anthradithiophenes: Synthesis, Properties, and FET Performance

The synthesis of isomerically pure syn-anthradithiophene derivatives (syn-ADTs) is described. X-ray crystallography is used to compare the solid-state arrangement of syn-ADT derivatives 2a,b to the analogous mixture of syn- and anti-ADTs. Single-crystal OFETs based on isomerically pure syn-ADTs 2a,b display device performance comparable to those based on a mixture of ADT isomers syn/anti-2a,b with mobilities as high as 1 cm(2)/(V s).

Copper-phthalocyanine-based organic solar cells with high open-circuit voltage

We report on a copper phthalocyanine (CuPc)/3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) organic solar cell with an aluminum cathode exhibiting an open-circuit voltage (VOC) of 1150 mV and a short-circuit current density (JSC) of 0.125mA∕cm2. For comparison, aluminum and silver were used as electrode materials and their effects on device characteristics were studied; silver yielded lower VOC but higher JSC and higher efficiency. Results could be understood through a model that hypothesized surface modification of CuPc by PTCBI and the formation of a thin insulating layer between aluminum and PTCBI when aluminum was used as electrode.

Vibration‐Assisted Crystallization Improves Organic/Dielectric Interface in Organic Thin‐Film Transistors

Solution processability of organic semiconductors allows high-throughput fabrication on arbitrary substrates at low-cost, but the films often exhibit low performance. Here, we report on a new method for device fabrication, vibration assisted crystallization (VAC) that produces superior films, which approach the fundamental performance limits shown in corresponding single-crystal measurements.

Direct Structural Mapping of Organic Field‐Effect Transistors Reveals Bottlenecks to Carrier Transport

O N Organic fi eld-effect transistors (OFETs) continue to attract considerable attention due to the steady improvement of their performance over the past decade and their increasingly competitive position with respect to inorganic technologies, such as a-Si. The availability of solution-processable high-performance organic semiconductors makes this a particularly attractive technology for use in low-cost, fl exible and lightweight electronic devices. [ 1–4 ] However, it is often the case that the microstructure of organic semiconductors in devices is far from ideal and very challenging to understand and control when fi lms are deposited on top of a substrate patterned with electrodes. The organic fi lm can present heterogeneities across multiple length scales, this being a serious limitation for the performance and reproducibility of corresponding devices. In particular, it has been shown that the microstructure and morphology of the organic semiconductor, including its crystallinity, [ 5 , 6 ] polymorphism and texture, [ 7 , 8 ] grain and domain sizes, [ 9 , 10 ] grain boundary density and misorientation, [ 11 , 12 ] , as well as surface coverage and wettability of the substrate play pivotal roles in mediating device performance. [ 13 , 14 ] For instance, in the case of fl uorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT), treatment of the bottom Au contacts with pentafl uorobenzene thiol (PFBT) has been shown to dramatically improve carrier transport in bottom-contact OFETs. [ 15 , 16 ] The surface treatment prevents the < 111 > -textured crystallites exhibiting poor in-plane π -stacking from nucleating. It was shown that < 001 > grains grow on the treated electrodes and extend laterally several microns from their edges, thus preventing growth of the undesirable crystallite orientations in

Solvent Vapor Annealing in the Molecular Regime Drastically Improves Carrier Transport in Small-Molecule Thin-Film Transistors

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.