Martyn A. McLachlan

High‐Mobility Low‐Voltage ZnO and Li‐Doped ZnO Transistors Based on ZrO2 High‐k Dielectric Grown by Spray Pyrolysis in Ambient Air

Sequential layers of the high-k dielectric ZrO2 and the electron transporting semiconductors ZnO and Li-doped ZnO are deposited onto conductive indium tin oxide electrodes using spray pyrolysis. With these structures, thin-film transistors are fabricated with operating voltages below 6 V and maximum electron mobilities on the order of 85 cm(2) V-1 s(-1).

High‐Performance ZnO Transistors Processed Via an Aqueous Carbon‐Free Metal Oxide Precursor Route at Temperatures Between 80–180 °C

An aqueous and carbon-free metal-oxide precursor route is used in combination with a UV irradiation-assisted low-temperature conversion method to fabricate low-voltage ZnO transistors with electron mobilities exceeding 10 cm(2) /Vs at temperatures <180 °C. Because of its low temperature requirements the method allows processing of high-performance transistors onto temperature sensitive substrates such as plastic.

Inverted organic photovoltaic devices with high efficiency and stability based on metal oxide charge extraction layers

A substantial increase in device performance and operational stability in solution processed inverted bulk heterojunction (BHJ) organic photovoltaic devices (OPV) is demonstrated by introducing a zinc oxide (ZnO) interlayer between the electron collecting bottom electrode and the photoactive blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). The structure and morphology of the dense, planar ZnO layers were controlled either by electro-deposition or spray pyrolysis techniques. Metal oxide sandwich OPV devices based on the photoactive blend on an electro-deposited ZnO interlayer with a (100) preferential crystal orientation, and using a tungsten oxide (WOx) interlayer on the opposite electrode, resulted in a remarkable increase in power conversion efficiency with a value of 4.91% under AM1.5 illumination and an external quantum efficiency of 74%. Electro-deposition of the ZnO at low temperature proved to be the most promising method for forming the ZnO interlayers, allowing the highest control of film structure and morphology, as well as leading to significantly improved device efficiency and stability.

Low-voltage ZnO thin-film transistors based on Y2O3 and Al2O3 high-k dielectrics deposited by spray pyrolysis in air

We report the application of ambient spray pyrolysis for the deposition of high-k polycrystalline Y2O3 and amorphous Al2O3 dielectrics and their use in low-voltage ZnO thin-film transistors. The films are studied by means of atomic force microscopy, UV-visible absorption spectroscopy, impedance spectroscopy, and field-effect measurements. ZnO transistors based on spray pyrolysed Y2O3 and Al2O3 dielectrics show low leakage currents, and hysteresis-free operation with a maximum electron mobility of 34 cm2/V s and current on/off ratio on the order of 105. This work is a significant step toward high-performance oxide electronics manufactured using simple and scalable processing methods.

Thin film photonic crystals: synthesis and characterisation

The results of an investigation of the major factors that influence colloidal self-assembly of thin film photonic crystals are reported. The effect of temperature, relative humidity, sphere diameter, colloidal concentration and substrate angle were investigated: the results establish clearly that temperature is the most critical factor. Quantitative analysis of the results using Design of Experiments methodology has identified the optimum conditions for the growth of large area, low defect density thin film photonic crystals.

High‐Efficiency, Solution‐Processed, Multilayer Phosphorescent Organic Light‐Emitting Diodes with a Copper Thiocyanate Hole‐Injection/Hole‐Transport Layer

Copper thiocyanate (CuSCN) is introduced as a hole-injection/hole-transport layer (HIL/HTL) for solution-processed organic light-emitting diodes (OLEDs). The OLED devices reported here with CuSCN as HIL/HTL perform significantly better than equivalent devices fabricated with a PEDOT:PSS HIL/HTL, and solution-processed, phosphorescent, small-molecule, green OLEDs with maximum luminance ≥10 000 cd m(-2) , maximum luminous efficiency ≤50 cd A(-1) , and maximum luminous power efficiency ≤55 lm W(-1) are demonstrated.

p-channel thin-film transistors based on spray-coated Cu2O films

Thin films of cuprous oxide (Cu2O) were grown using solution-based spray pyrolysis in ambient air and incorporated into hole-transporting thin-film transistors. The phase of the oxide was confirmed by X-ray diffraction measurements while the optical band gap of the films was determined to be ∼2.57 eV from optical transmission measurements. Electrical characterization of Cu2O films was performed using bottom-gate, bottom-contact transistors based on SiO2 gate dielectric and gold source-drain electrodes. As-prepared devices show clear p-channel operation with field-effect hole mobilities in the range of 10−4–10−3 cm2 V−1 s−1 with some devices exhibiting values close to 1 × 10−2 cm2 V−1 s−1.

Surface Structure Modification of ZnO and the Impact on Electronic Properties.

Zinc oxide (ZnO) is a widely utilized, versatile material implemented in a diverse range of technological applications, particularly in optoelectronic devices, where its inherent transparency, tunable electronic properties, and accessible nanostructures can be combined to confer superior device properties. ZnO is a complex material with a rich and intricate defect chemistry, and its properties can be extremely sensitive to processing methods and conditions; consequently, surface modification of ZnO using both inorganic and organic species has been explored to control and regulate its surface properties, particularly at heterointerfaces in electronic devices. Here, the properties of ZnO are described in detail, particularly its surface chemistry, along with the role of defects in governing its electronic properties, and methods employed to modulate the behavior of as-grown ZnO. An outline is also given on how the native and modified oxide interact with molecular materials. To illustrate the diverse range of surface modification methods and their subsequent influence on electronic properties, a comprehensive review of the modification of ZnO surfaces at molecular interfaces in hybrid photovoltaic (hPV) and organic photovoltaic (OPV) devices is presented. This is a case study rather than a progress report, aiming to highlight the progress made toward controlling and altering the surface properties of ZnO, and to bring attention to the ways in which this may be achieved by using various interfacial modifiers (IMs).

Optimised pulsed laser deposition of ZnO thin films on transparent conducting substrates

The growth of polycrystalline zinc oxide (ZnO) thin films by pulsed laser deposition (PLD) on indium tin oxide (ITO) is reported. For the first time the influence of deposition temperature over an extended range (50–650 °C) is investigated on ITO. We describe the role of temperature on the optical and crystalline properties of the deposited films, of 120–250 nm thickness. Additionally, the effect of the background oxygen pressure is reported. Under all of the deposition conditions highly textured c-axis oriented, transparent (>85%) and low roughness (RMS < 10 nm) ZnO films are formed. Growth temperatures ≥450 °C lead to the highest degree of crystallinity and film quality with measured full width half maximum (FWHM) of X-ray diffraction (XRD) peaks as small as 0.14°2θ. XRD measurements of films grown at <350 °C show a shift in the (002) diffraction peak to lower 2θ values, indicating that the deposited films are oxygen deficient. Increasing the oxygen pressure results in the preparation of stoichiometric films at temperatures as low as 50 °C. We demonstrate that in addition to forming high quality ZnO, the optical and electronic properties of ITO can be preserved—even at high temperature—presenting a methodology for preparing highly crystalline ZnO on ITO over a temperature window significantly larger than that of previous literature reports. Furthermore, the low temperature processing opens up the possibility of deposition on a wide range of substrates, especially those unsuitable for exposure to high temperatures.

High Electron Mobility Thin-Film Transistors Based on Solution-Processed Semiconducting Metal Oxide Heterojunctions and Quasi-Superlattices

High mobility thin‐film transistor technologies that can be implemented using simple and inexpensive fabrication methods are in great demand because of their applicability in a wide range of emerging optoelectronics. Here, a novel concept of thin‐film transistors is reported that exploits the enhanced electron transport properties of low‐dimensional polycrystalline heterojunctions and quasi‐superlattices (QSLs) consisting of alternating layers of In2O3, Ga2O3, and ZnO grown by sequential spin casting of different precursors in air at low temperatures (180–200 °C). Optimized prototype QSL transistors exhibit band‐like transport with electron mobilities approximately a tenfold greater (25–45 cm2 V−1 s−1) than single oxide devices (typically 2–5 cm2 V−1 s−1). Based on temperature‐dependent electron transport and capacitance‐voltage measurements, it is argued that the enhanced performance arises from the presence of quasi 2D electron gas‐like systems formed at the carefully engineered oxide heterointerfaces. The QSL transistor concept proposed here can in principle extend to a range of other oxide material systems and deposition methods (sputtering, atomic layer deposition, spray pyrolysis, roll‐to‐roll, etc.) and can be seen as an extremely promising technology for application in next‐generation large area optoelectronics such as ultrahigh definition optical displays and large‐area microelectronics where high performance is a key requirement.