Although a very high electroluminescence (EL) efficiency has been demonstrated with organic light emitting diodes (OLEDs) using electrophosphorescence, enhancement of energy conversion efficiency, that is decreasing driving voltage, has been required from the standpoint of practical device applications. In this paper, we report on a decrease in operation voltage by doping carrier transport layers and discuss the possible mechanisms. In particular, we demonstrate an OLED with very low driving voltage (2.9 V at J = 100 mA/cm2) with a p-i-n structure.
Insulator-semiconductor and electrode-semiconductor interfaces are crucial to the performance of organic thin-film transistors (TFTs). This paper reviews how to control the interfaces for improving the performance of p-type and n-type organic TFTs and p-type polymer TFTs. In particular, modifying the gate insulator surface by using self-assembled monolayers (SAMs) is our main focus. Changes in the water contact angle and surface energy by forming SAMs on the gate insulator surface are shown, and the TFT performance in the organic semiconductor layers grown on the modified surfaces is presented. Crystal growth of the organic semiconductors and charge trap sites on the gate insulator surface are discussed. They explain the improved TFT performance such as field effect mobility and current on/off ratio.
Although changes in the layer structure of an OLED as well as the diffusion phenomena in cathode materials and in EIM (electron-injection material) have been discussed actively, fundamental questions have remained unanswered. Using backside SIMS technique after optimised preprocessing of OLED samples, it has been possible to measure more accurate profiles of diffused elements, which was quite difficult under conventional SIMS analysis from the front surface due to so called knock-on effect. We have utilized the backside SIMS in the deterioration evaluation of OLED devices under the high temperature storage condition. Diffusion of EIM has been found with the change in layer structure depending on Tg (glass transition temperature) of HTM (hole transport material). On the basis of the analysis, it was found that the diffusion of EIM and the change in the layer structure depended on the Tg (glass transition temperature) of the HTM (hole-transport material) at the deterioration evaluation during high-temperature storage.
Kelvin probe force microscopy (KFM), which has been a common method for studying electrical properties of nanometer-scale structures, is a dynamic-mode atomic force microscopy (AFM) technique combined with a traditional Kelvin probe method used for measuring macroscopic contact potential differences. It allows us to map electron affinity or work function on a nanometer scale. In this article KFM using a frequency modulation detection method (FM-KFM) is first explained briefly. The frequency modulation method plays an essential role in high-sensitive interaction force detection. We also describe applications of FM-KFM to the nanometer-scale surface potential investigations of organic materials including phase-separated self-assembled monolayer films (PS-SAM films) and single polymer crystal surfaces. Furthermore, recent results on the local surface potential study of carbon nanotube field effect transistors using FM-KFM is presented.
Recently electronic devices using organic materials have been extensively studied. The elucidation of the electronic structures of the materials and interfaces of such devices is indispensable for understanding and improving devices. Since the properties of organic devices are affected by atmospheric gas effect, the electronic structures not only in vacuum but also in atmospheric condition should be examined. Recently, we have developed an apparatus for measuring photoelectron yield spectroscopy (PYS) for that purpose. Photoelectrons emitted into atmosphere are extracted by an external field and probed as a current with an ammeter. Such current-mode detection is operative in any condition including vacuum and air. In this paper, we report on the details of the apparatus, the application to various organic films and powders. The trial to probe the electronic structures of organic/metal interfaces by PYS is also reported.
Infrared spectroscopic techniques for surface analysis are introduced. Infrared spectra provide qualitative and quantitative molecular information for most of the chemical groups. In particular, the quantitative polarized infrared spectra of surface species reflects molecular orientation, which is recognized to be a key to develop new functionalized materials. In this manuscript, conventionally useful techniques are described first, and a recent trend is also introduced, so that benefits of infrared spectroscopy would be enhanced.
We found a simple method to prepare polymer nanoparticles by evaporation of solvent from a polymer solution. This method is applicable to prepare of nanoparticles of many kinds of functional materials (eg. engineering plastics, fluorescence polymers, conductive polymers, biodegradable polymers and so on.) with controlling their size ranging from several tens nm to several µm. By using this method, we fabricated particles with phase-separation structures from polymer blends or block-copolymers. Furthermore, it is shown that novel nanostructures including hemispherical particles and nano-disks by selective cross-linking and selective elution of polymer moieties that forms phase-separation structures are prepared.
Development of highly sensitive sensing techniques is needed for the analysis of biological functions of biomaterials such as DNAs and proteins. We have been developing biosensors by using infrared absorption spectroscopy in the multiple internal reflection geometry (MIR-IRAS). MIR-IRAS provides us with valuable information about chemical states of biomolecules, biomaterials and cells. In this report, we describe some experimental techniques that would be helpful in performing MIR-IRAS measurements.