Matthew J. Panzer

Low-voltage operation of a pentacene field-effect transistor with a polymer electrolyte gate dielectric

Large operating voltages are often required to switch organic field-effect transistors (OFETs) on and off because commonly used gate dielectric layers provide low capacitive coupling between the gate electrode and the semiconductor. We present here a pentacene OFET gated by a solution-deposited polymer electrolyte film in which the current was modulated over four orders of magnitude using gate voltages less than 2V. A subthreshold slope of 180mV per decade of current was observed during transistor turn on at a source-drain bias of −1V; the estimated dielectric layer specific capacitance was 5μF∕cm2. Sweep rate-dependent hysteresis may be attributed to a combination of ion migration and charge carrier trapping effects. Strategies to improve switching speeds for polymer electrolyte-gated OFETs are also discussed.

Alternating Current Driven Electroluminescence from ZnSe/ZnS:Mn/ZnS Nanocrystals

We present a novel technique for room temperature, solution-based fabrication of alternating current thin-film electroluminescent (AC-TFEL) devices using phosphor-doped nanocrystals. Synthesis for stable ZnSe/ZnS:Mn/ZnS nanocrystals that exhibit a quantum yield of 65 +/- 5% is outlined, and their electroluminescence is demonstrated in structures consisting of only wide band gap ceramic layers. Both the nanocrystal and the ceramic films have minimal absorption across the visible light spectrum, enabling us to demonstrate transparent AC-TFEL devices.

Selection of Metal Oxide Charge Transport Layers for Colloidal Quantum Dot LEDs

We investigate the effect of the electronic energy level positioning, conductivity, and morphology of metal oxide charge transport layers on the performance of light emitting devices (LEDs) that consist of a colloidally synthesized quantum dot (QD) luminescent film embedded between electron and hole injecting ceramic layers. We demonstrate that understanding of these material properties and their effect on charging processes in QDs enables the systematic design of higher efficiency QD-LEDs and excitation of QDs with different emission colors using the same device structure.

Air-Stable Operation of Transparent, Colloidal Quantum Dot Based LEDs with a Unipolar Device Architecture

We report a novel unipolar light-emitting device architecture that operates using direct-current, field-driven electroluminescence of colloidally synthesized quantum dots (QDs). This device architecture, which is based only on transparent ceramics and QDs, enables emission from different color QDs and, for the first time, constant QD electroluminescence during extended operation in air, unpackaged.

High charge carrier densities and conductance maxima in single-crystal organic field-effect transistors with a polymer electrolyte gate dielectric

High charge carrier densities have been realized in organic field-effect transistors based on single crystals of the organic semiconductors rubrene and tetracene using a high capacitance polymer electrolyte gate dielectric. The source-drain current was modulated by five orders of magnitude in a rubrene single-crystal organic field-effect transistor (SC-OFET) with gate voltages ranging from 0to−3V. A peak in the field-effect conductance was also observed in SC-OFETs at induced carrier densities of ∼1015charges∕cm2. Key to successful device fabrication was the introduction of a thin, insulating spacer layer between the organic single crystal (OSC) and the polymer electrolyte gate dielectric. Further improvement of the device fabrication procedure may eliminate suspected solvent-related degradation effects and raise mobility values in these SC-OFETs, opening the door to a wide spectrum of experiments on OSCs at high charge carrier densities.

Mass transfer properties of monoliths

The mass transfer properties of polyglycidylmethacrylate–ethylenedimethacrylate monolithic ion-exchangers (convective interaction media disks) were evaluated. As a reference material, the particulate ion-exchanger Source 30 was selected. The model proteins lysozyme, bovine serum albumin, and IgG were loaded at different concentrations and velocities. The mass transfer zones obtained with the monoliths were affected by neither the linear flow velocity nor the protein concentration in the mobile phase. The reduced height equivalent to one theoretical plate (HETP) of monoliths were independent of the reduced velocity. This was not the case for the particulate material.

High-performance, mechanically compliant silica-based ionogels for electrical energy storage applications

The development of an ionic liquid-rich (∼94% by mass), mechanically compliant, silica-supported ionogel (ionic liquid-based gel electrolyte) is described. This new form of ionogel was created using a straightforward sol–gel process with a novel formulation of reactants, resulting in a versatile, stable, and high-performance solid electrolyte. The mechanically compliant ionogels described herein show both capacitance and ionic conductivity values at room temperature nearly equivalent to those of the neat ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EMI TFSI), in addition to exhibiting a wide window of electrochemical stability. Compliant ionogels (elastic modulus of ∼5 kPa) show markedly different mechanical character than previously reported silica ionogels, which are mechanically brittle and exhibit lower ionic conductivity and double-layer capacitance. The silica microstructure is shown to be tunable by simple alterations of the reagent formulation. Compliant ionogels may be suitable for in situ fabrication of flexible, thin-film energy storage devices such as supercapacitors.

High mobility top-gated pentacene thin-film transistors

A common device geometry for measuring the electrical characteristics of organic semiconductors is the thin-film organic field-effect transistor (OTFT). Mostly for reasons of cost, convenience, and availability, this usually involves depositing the organic material on a prefabricated gate structure such as Si∕SiO2, the surface chemistry of which is often modified with self-assembled monolayers. The interactions between these surfaces and the deposited organic can have a profound effect on thin-film growth and the resulting electrical characteristics since most of the charge transport in these structures occurs near the organic-insulator interface. An alternative to this traditional technique is to assemble the transistor on top of the organic semiconductor. We have used chemical-vapor deposition of the polymeric dielectric material parylene to create pentacene OTFTs with gate electrodes both on top of and below the semiconductor film, with field-effect mobilities as high as 0.1cm2∕Vs and Ion∕Ioff ratios g...

Poly(Ethylene Glycol) Diacrylate-Supported Ionogels with Consistent Capacitive Behavior and Tunable Elastic Response

Harnessing the many favorable properties of ionic liquids in a solid electrolyte thin film form is desirable for a host of electrical energy storage applications, including electrochemical double layer capacitors. Using a cross-linked polymer matrix to provide structural support, freestanding ionogel materials can be achieved with a wide range of polymer weight fractions. Compression testing and impedance spectroscopy have been used to characterize the mechanical and electrical responses of ionogels containing between 4.9 and 44.7 wt % poly(ethylene glycol) diacrylate. Although the elastic modulus of these solid electrolyte materials is observed to vary by more than 4 orders of magnitude within the composition range studied, concomitant changes in gel ionic conductivity and double layer capacitance were much less dramatic.

Poly(dimethylsiloxane)‐Supported Ionogels with a High Ionic Liquid Loading

The immiscibility of poly(dimethylsiloxane) (PDMS) and ionic liquids (ILs) was overcome to create PDMS-supported IL gels (ionogels) with IL loadings of up to 80% by mass through a simple sol-gel reaction at room temperature. By stirring a mixture of a functionalized PDMS oligomer, formic acid, and an IL (or lithium-in-IL solution), a resin was formed that could be cast to create a freestanding, flexible ionogel. PDMS-supported ionogels exhibited favorable ionic conductivity (ca. 3 mS cm(-1)) and excellent mechanical behavior (elastic modulus: ca. 60 kPa; fatigue life: >5000 cycles; mechanically stable at temperatures up to 200 °C). The activation energy of ionic conductivity was shown to be nearly identical for the ionogel and the neat IL, in contrast to ionogel systems wherein the scaffold material is miscible with the IL. This similarity indicates that IL/scaffold chemical interactions are key to the understanding of ionogel electrical performance, especially at elevated temperatures.