Iain Mcculloch

Indolo-naphthyridine-6,13-dione thiophene building block for conjugated polymer electronics: Molecular origin of ultrahigh n-type mobility (Conference Presentation)

We present the synthesis and characterization of four conjugated polymers containing a novel chromophore for organic electronics based on an indigoid structure. These polymers exhibit extremely small band gaps of ∼1.2 eV, impressive crystallinity, and extremely high n-type mobility exceeding 3 cm2 V s–1. The n-type charge carrier mobility can be correlated with the remarkably high crystallinity along the polymer backbone having a correlation length in excess of 20 nm. Theoretical analysis reveals that the novel polymers have highly rigid nonplanar geometries demonstrating that backbone planarity is not a prerequisite for either narrow band gap materials or ultrahigh mobilities. Furthermore, the variation in backbone crystallinity is dependent on the choice of comonomer. We find that electron mobility can be correlated to the degree of order along the conjugated polymer backbone. Finally, we use this novel system to begin to understand the complicated effect of alkyl chain variation on the solid state packing in all 3 dimensions.

All-printed organic transistors: integrating devices for flexible circuits (Conference Presentation)

Over the past several decades, conventional electronic circuits have been used for both analytical and digital logic circuits. Printed electronics has the potential to reduce fabrication complexity of electronic circuits and using lower-cost and large area manufacturing techniques. The performance of film transistors (OTFTS) has also improved and these devices could be applied to circuit applications where the high performance, high speed, and high energy consumption offered by conventional electronics is not needed. Amongst many factors that govern circuit design, the scale factor (W/L) serves as a crucial variable for tuning a circuit performance. Here we present printing techniques developed in order to adjust aspect ratios of printed transistors using solution processed electronic materials on to flexible substrates. By combining high-speed doctor blade and surface energy patterning we can demonstrate arrays of OTFTs that are later integrated to form circuits. In the surface energy patterning process, a hydrophobic self-assembled monolayer is deposited on a plastic substrate, and plasma etching is used to create hydrophilic regions. The desirable ink is deposited on the hydrophilic regions using doctor blading and only hydrophilic regions are patterned with the ink. Device aspect ratios are increased and controlled by patterning intermitted SD electrodes and controlling the size of the semiconductor island. We utilize screen printing method to interconnect devices to demonstrate several circuit designs such as enhancement-load Inverter, NAND and NOR on the same printing batch. We will discuss how machine learning is used to train this circuits and applied to sensing applications.

Organic thin-film transistor fabrication using a laser printer (Conference Presentation)

Organic electronic materials are desirable due to facile and low-cost manufacturing through solution deposition. However, the inherit difficulties of reproducibility and solvent compatibility, as well as the hazards associated with the solvents, have stifled the progress of realizing practical solution-deposition methods. As a result, organic thin-films used in industry are typically produced by thermal evaporation techniques, which largely negate the benefits due to the higher cost and complexity of vacuum and evaporation equipment. Here we report the use of a conventional office laser printer to electrophotographically deposit the organic semiconductor layer in thin-film transistors. We have successfully used this solvent-free, low-cost method to produce the first laser-printed organic semiconductor layer in thin-film transistors. We printed on flexible and transparent polyethylene terephthalate (PET) substrates. We used the highly hydrophobic fluoropolymer Cytop as the dielectric in a bottom-gate, bottom-contact configuration, a feat that is not possible with traditional solution-deposition. The organic semiconductor layer consisted of a toner powder based on triisopropylsilylethynyl pentacene (TIPS Pn). Grazing incidence wide-angle X-ray scattering (GIWAXS) images indicated both edge- and face-on orientations of the semiconductor for these devices while electrical measurements revealed field-effect mobilities up to 10-3 cm2V¬-1s-1 and on/off current ratio of 103. Our method has the combined advantages of low temperature and ambient pressure deposition while eliminating the drawbacks of solvents or the high cost of evaporation equipment. Further, as a digital printing method, the laser-printed layer is easily patternable as designed by any convenient graphics software. Since the powder is transferred in a dry state, surface dewetting is no longer an issue, which opens the door to even more substrate/dielectric materials that would otherwise reject solutions from adhering.

Probing the origins of temperature dependence of charge transport in organic single crystal transistors (Conference Presentation)

Low temperature transport measurements of classical semiconductors are a well-defined method to determine the physics of transport behavior. These measurements are also used to evaluate organic semiconductors, though physical interpretation is not yet fully developed. The similar energy ranges of the various processes involved in charge transport in organic semiconductors, including excitonic coupling, charge-phonon coupling, and trap distributions, result in ambiguity in the interpretation of temperature dependent electrical measurements. The wide variety of organic semiconductors, ranging from well-ordered small molecule crystals to disordered polymers, manifest varying degrees of “ideal” device behavior and require intensive studies in order to capture the full range of physical mechanisms involved in electronic transport in this class of materials. In addition, the physics at electrical contacts and dielectric material interfaces strongly affect device characteristics and results in temperature dependent behavior that is unrelated to the semiconductor itself. In light of these complications, our group is working toward understanding the origins of temperature dependent transport in single crystal, small molecule organic semiconductors with ordered packing. In order to disentangle competing physical effects on device characterization at low temperature, we use TEM and Raman spectroscopy to track changes in the structure and thermal molecular motion, correlated with density functional theory calculations. We perform electrical characterization, including DC current-voltage, AC impedance, and displacement current measurements, on transistors built with a variety of contact and dielectric materials in order to fully understand the origin of the transport behavior. Results of tetracene on silicon dioxide and Cytop dielectrics will be discussed.