Pentacene-Based Thin-Film Transistors With a Solution-Process Hafnium Oxide Insulator

Abstract—Pentacene-based organic thin-film transistors with solution-process hafnium oxide (HfOx) as gate insulating layer have been demonstrated. The solution-process HfOx could not only exhibit a high-permittivity (κ = 11) dielectric constant but also has good dielectric strength. Moreover, the root-mean-square surface roughness and surface energy (γs) on the surface of the HfOx layer were 1.304 nm and 34.24 mJ/cm2, respectively. The smooth, as well as hydrophobic, surface of HfOx could facilitate the direct deposition of the pentacene film without an additional polymer treatment layer, leading to a high field-effect mobility of 3.8 cm2/(V · s). Index Terms—Hafnium oxide, high permittivity, organic thinfilm transistor (OTFT), solution process, surface energy

Disorder engineering and conductivity dome in ReS2 with electrolyte gating

Atomically thin rhenium disulphide (ReS2) is a member of the transition metal dichalcogenide (TMDC) family of materials characterized by weak interlayer coupling and a distorted 1T structure. Here, we report on the electrical transport study of mono- and multilayer ReS2 with polymer electrolyte gating. We find that the conductivity of monolayer ReS2 is completely suppressed at high carrier densities, an unusual feature unique to monolayers, making ReS2 the first example of such a material. While thicker flakes of ReS2 also exhibit a conductivity dome and an insulator-metal-insulator sequence, they do not show a complete conductivity suppression at high doping densities. Using dual-gated devices, we can distinguish the gate-induced doping from the electrostatic disorder induced by the polymer electrolyte itself. Theoretical calculations and a transport model indicate that the observed conductivity suppression can be explained by a combination of a narrow conduction band and Anderson localization due to electrolyte-induced disorder.Comment: Submitted versio

Organic Single-Crystal Field-Effect Transistors

We present an overview of recent studies of the charge transport in the field effect transistors on the surface of single crystals of organic low-molecular-weight materials. We first discuss in detail the technological progress that has made these investigations possible. Particular attention is devoted to the growth and characterization of single crystals of organic materials and to different techniques that have been developed for device fabrication. We then concentrate on the measurements of the electrical characteristics. In most cases, these characteristics are highly reproducible and demonstrate the quality of the single crystal transistors. Particularly noticeable are the small sub-threshold slope, the non-monotonic temperature dependence of the mobility, and its weak dependence on the gate voltage. In the best rubrene transistors, room-temperature values of $\mu$ as high as 15 cm$^2$/Vs have been observed. This represents an order-of-magnitude increase with respect to the highest mobility previously reported for organic thin film transistors. In addition, the highest-quality single-crystal devices exhibit a significant anisotropy of the conduction properties with respect to the crystallographic direction. These observations indicate that the field effect transistors fabricated on single crystals are suitable for the study of the \textit{intrinsic} electronic properties of organic molecular semiconductors. We conclude by indicating some directions in which near-future work should focus to progress further in this rapidly evolving area of research.Comment: Review article, to appear in special issue of Phys. Stat. Sol. on organic semiconductor

Microstructure, morphology and device physics of gravure printed and solution processed organic field-effect transistors

This thesis explores the relationship between microstructure, morphology and device physics in gravure printed and solution processed organic field-effect transistors (OFETs). Chapter 1 introduces the key concepts encountered in this work: the properties of organic semiconductors and OFETs; the use of printing techniques in organic electronics; and the relationship between microstructure and OFET performance in poly(3-hexylthiophene) (P3HT). Chapter 2 details the materials and experimental techniques used in this thesis. In Chapter 3, gravure printing is demonstrated for high throughput fabrication of OFETs. Printed devices are achieved with typical saturated mobility of 0.03cm2/Vs and on/off ratio in the range 103:9-4:6, which exceeds that achieved with spin coated devices with the same material system and geometry. Chapter 4 presents a systematic comparison of the microstructure and OFET characteristics of gravure printed and spin coated P3HT thin films. First light scattering is used to understand the conformation of P3HT chains in various solvents, then grazing incidence wide angle X-ray scattering (GIWAXS), absorption characteristics and atom force microscopy (AFM) are used to characterise the microstructure of the P3HT lms. In turn, this is compared to OFET performance. In Chapter 5 two solvent based techniques are investigated as alternatives to thermal annealing as methods to enhance microstructure. A blend of a high and low boiling point solvent is first examined as the casting solvent for P3HT and is found to moderately improve P3HT field-effect mobility. Secondly, solvent vapour treatment (SVT) - exposing a P3HT film to a solvent vapour after spin coating - is studied by in-situ GIWAXS. The time resolved measurement of interchain and interlamella distances allowed the dynamics of SVT to be investigated. SVT was found to decrease P3HT crystallinity, although AFM showed it lead to smoother films. In Chapter 6 two emerging materials are investigated for use in OFETs. Preliminary work on fabricating OFETs with single crystal copper phthalocyanine is presented. Finally, work towards a metal-free OFET is described in which the source and drain electrodes are formed of high conductivity PEDOT deposited by vapour phase polymerisation

12CaO.7Al2O3 ceramic: A review of the electronic and optoelectronic applications in display devices

The alumina-based compound, 12CaO.7Al2O3, is a ceramic material with a unique cage-like lattice. Such a structure has enabled scientists to extract various new characteristics from this compound, most of which were unknown until quite recently. This compound has the ability to incorporate different anionic species and even electrons to the empty space inside its cages, thereby changing from an insulator into a conductive oxide. The cage walls can also incorporate different rare earth phosphor elements producing an oxide-based phosphor. All these characteristics are obtained without a significant change in the structure of the lattice. It is, therefore, reasonable to expect that this compound will receive attention as a potential material for display applications. This review article presents recent investigations into the application of 12CaO.7Al2O3 ceramic in various display devices, the challenges, opportunities and possible areas of future investigation into the development of this naturally abundant and environmental friendly material in the field of display.LP Displays Ltd, Blackburn, UK for partial funding of the studentship at Queen Mary, University of London. Dr Lesley Hanna of Wolfson Centre for Materials Processing, Brunel University Londo