Marsha A. Loth

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.

Guiding Crystallization around Bends and Sharp Corners

Control over the molecular orientation in organic thin films is demonstrated with precise in-plane spatial resolution over large areas. By exploiting the differential crystallization rates on substrates with different surface energies, the radial symmetry of spherulitic growth can be disrupted by preferentially selecting the molecular orientations that promote growth along the paths of the underlying patterns.

Orientation-independent charge transport in single spherulites from solution-processed organic semiconductors.

Due to the rapidity of morphological development during deposition, solution-processed organic semiconductor thin films exist in semicrystalline or polycrystalline states, incorporating a high degree of local variations in molecular orientation compared to their single-crystal counterparts. Spherulites, a common crystalline superstructure found in these systems, for example, incorporate a large distribution of molecular orientations about the radial axis to maintain their space-filling growth habit. Here, we aim to determine how this distribution of molecular orientations influences charge transport by fabricating arrays of devices on single spherulites. Given that the orientation distribution that is present about the radial axis mandates the presence of low-angle grain boundaries within single spherulites, we find intraspherulitic charge transport to be independent of the general direction of π-stacking; organic field-effect transistors exhibit comparable mobilities regardless of how their channels are oriented with respect to the general π-stacking direction.

Tailored interfaces for self-patterning organic thin-film transistors

Patterning organic thin-film transistors (OTFTs) is critical in achieving high electronic performance and low power consumption. We report on a high-yield, low-complexity patterning method based on exploiting the strong tendency of halogen-substituted organic semiconductors to crystallize along chemically tailored interfaces. We demonstrate that the organic semiconductor molecules self-align on the contacts, when the halogen–halogen interaction is allowed by the chemical structures and conformations of the self-assembled monolayer and organic semiconductor. The ordered films exhibit high mobilities and constrain the current paths. The regions surrounding the devices, where the interaction is inhibited, consist of randomly oriented molecules, exhibiting high-resistivity and electrically insulating neighboring devices. To identify the role of F–F interactions in the development of crystalline order, we investigate OTFTs fabricated on mono-fluorinated benzene thiol treated contacts, which allows us to isolate the interactions between the F originating from the organic semiconductor and the F in each position on the benzene ring of the thiol, and to selectively study the role of each interaction. Combining the results obtained from quantitative grazing incidence X-ray diffraction and Kelvin probe measurements, we show that the surface treatments induce structural changes in the films, but also alter the injection picture as a result of work function shifts that they introduce. We show that both effects yield variations in the field-effect transistor characteristics, and we are able to tune the field-effect mobility more than two orders of magnitude in the same material.

Quantifying resistances across nanoscale low- and high-angle interspherulite boundaries in solution-processed organic semiconductor thin films.

The nanoscale boundaries formed when neighboring spherulites impinge in polycrystalline, solution-processed organic semiconductor thin films act as bottlenecks to charge transport, significantly reducing organic thin-film transistor mobility in devices comprising spherulitic thin films as the active layers. These interspherulite boundaries (ISBs) are structurally complex, with varying angles of molecular orientation mismatch along their lengths. We have successfully engineered exclusively low- and exclusively high-angle ISBs to elucidate how the angle of molecular orientation mismatch at ISBs affects their resistivities in triethylsilylethynyl anthradithiophene thin films. Conductive AFM and four-probe measurements reveal that current flow is unaffected by the presence of low-angle ISBs, whereas current flow is significantly disrupted across high-angle ISBs. In the latter case, we estimate the resistivity to be 22 MΩμm(2)/width of the ISB, only less than a quarter of the resistivity measured across low-angle grain boundaries in thermally evaporated sexithiophene thin films. This discrepancy in resistivities across ISBs in solution-processed organic semiconductor thin films and grain boundaries in thermally evaporated organic semiconductor thin films likely arises from inherent differences in the nature of film formation in the respective systems.

Aggregate formation and its effect on (opto)electronic properties of guest-host organic semiconductors

We quantify guest molecule aggregation and its effect on the photoconductive properties of guest-host thin films, depending on the guest concentration and host material. A high-performance anthradithiophene (ADT) derivative served as a guest, while functionalized benzothiophene (BTBTB) and polymethylmethacrylate (PMMA) were chosen as hosts. Aggregates exhibited redshifted optical absorption and photoluminescence (PL) spectra, as well as reduced PL quantum yields. Propensity toward guest aggregation differed for PMMA and BTBTB hosts. Photocurrents dramatically increased as the percentage of aggregated guest molecules increased due to considerably higher charge carrier mobility in the aggregates. At low guest concentrations, BTBTB films outperformed PMMA films.

Single-step solution processing of small-molecule organic semiconductor field-effect transistors at high yield

Here, we report a simple, alternative route towards high-mobility structures of the small-molecular semiconductor 5,11-bis(triethyl silylethynyl) anthradithiophene that requires one single processing step without the need for any post-deposition processing. The method relies on careful control of the casting temperature of the semiconductor and allows rapid production of transistors with uniform and reproducible device performance over large areas.

Characterization of a soluble anthradithiophene derivative

The structural and electrical properties of a solution processable material, 2,8-difluoro-5,11-tert-butyldimethylsilylethynyl anthradithiophene (TBDMS), were measured for single crystal transistors. TBDMS is observed to readily form single crystals from physical vapor zone sublimation. A columnar packing crystal structure, with an approximate π/4 radian rotational offset between neighboring molecules, is observed. Single crystal TBDMS transistors display a maximum observed saturation mobility μS of 0.07 cm2/V s, current on-off ratio >107, and subthreshold swing S≈1 dec/V. The spectral current noises of single crystal devices display a 1/f flicker noise, while the metal-semiconductor charge injection barrier is estimated by ultraviolet photoemission spectroscopy.

Charge transport in solution processable polycrystalline dual-gate organic field effect transistors

Dual gate organic thin film transistors based on solution processable fluorinated 5,11 bis(triethylsilylethynyl) anthradithiophene semiconductor were fabricated. Top (Teflon, er =2.1) and bottom (SiO2, er =3.9) gate dielectrics with different dielectric constants were chosen. Top gate mobilities >1 cm2 /Vs and bottom gate mobilities >0.1 cm2/V s were achieved. Temperature dependent mobility measurements show thermally activated charge transport and a comparative analysis is performed in the framework of two models representing polaron hopping as well as hopping in Gaussian density of states (DOS), respectively.

Effect of Processing Parameters on Performance of Spray-Deposited Organic Thin-Film Transistors

The performance of organic thin-film transistors (OTFTs) is often strongly dependent on the fabrication procedure. In this study, we fabricate OTFTs of soluble small-molecule organic semiconductors by spray-deposition and explore the effect of processing parameters on film morphology and device mobility. In particular, we report on the effect of the nature of solvent, the pressure of the carrier gas used in deposition, and the spraying distance. We investigate the surface morphology using scanning force microscopy and show that the molecules pack along the π-stacking direction, which is the preferred charge transport direction. Our results demonstrate that we can tune the field-effect mobility of spray-deposited devices two orders of magnitude, from 10−3 cm2/Vs to 10−1 cm2/Vs, by controlling fabrication parameters.