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Ishii-Tanaka Laboratory started full-scale activities in April 2007. By using complimentary approaches of bpth spectroscopic methods and electric anlaysis, we are making efforts to investigate the basic understanding of organic electronics to further improve the device performance. The spectroscoic research focuses on the electronic structure of organic materials and their interfaces. Especially, photoemission spectroscopic methods give us useful information on electrons, for example, what level of electrons are occupied and how they are lined up at the interfaces. That is, we have to know the seat where electrons sit on. In addtion, the information on dynamic behavior of electrons stored in organic molecule is also important to understand device performance. That is, we have to know how electrons move in device, too. To that, we have developped the various measurement methods with motto of By developping original methods available only in our lab, we performe unique researches. Besides tools, basic understanding and interrest to interdisciplinary fileds such as physics, chemistry, electronics engineering, organic and inorganic materials science are indispensable. We'd like to get such a wide range of understanding and knowledge.
In order to elucidate the operating principle of organic electronics, it is necessary to elucidate how electrons are packed in molecules. In our laboratory, we are measuring electrons in organic semiconductors by photoelectron spectroscopy while developing our own experimental equipment. When a substance is exposed to high-energy light such as ultraviolet light, electrons can be extracted by the photoelectric effect. By measuring the velocity distribution of the ejected photoelectrons, the distribution of electrons in the original molecule can be known. So far, 1) search for basic issues such as "how do electron levels line up at the interface between the organic molecule layer and the electrode layer?", And 2) "do organic molecules also cause band dispersion like silicon?" ? ”, 3) By observing a few negative ions existing on the surface of the thin film of organic EL material, and examining the electrons that have entered LUMO, 4) The arrangement of electron levels of the device structure such as solar electrons We are proceeding with highly original research, such as determining and 5) developing high-sensitivity photoelectron spectroscopy to make it possible to observe weak trap levels that could not be measured by conventional photoelectron spectroscopy.
As mentioned above, photoelectron spectroscopy is a powerful measurement method, but it also has some problems. One is that the measurement environment is limited to high vacuum. Since actual organic electronic devices move under the influence of atmospheric components, it is a mistake to measure only the clean state in a vacuum. There is also the problem that it is not possible to measure samples with high insulation. Photoelectron yield spectroscopy (PYS) does not measure the velocity distribution of photoelectrons, but measures the total amount of photoelectrons as a function of the energy of incident light, and determines the ionization energy from the energy at which photoelectron emission begins. Since no velocity distribution is required, it can be measured even in the atmosphere, and even an insulating sample can be measured if a potential is applied so as to draw out electrons appropriately. We use this method to 1) study the difference in ionization energy of organic semiconductors in vacuum and air, 2) measure the ionization energy of liquid semiconductors and biomolecules in liquids, and 3). You can measure the ionization energy of negative ions in the organic transistor in the operating state, 4) measure the ionization energy of negative ions present on the surface of the organic EL material thin film, and observe the electrons that have entered LUMO. ) We are conducting research such as exploring the mystery of frictional charge reduction by measuring the weak level in the band gap of insulating polymers.
Furthermore, as a method to supplement the research on photoelectron spectroscopy, we are also proceeding with research using the Kelvin probe. This method has long been used as a method for measuring the work function of a sample. In this method, a constant voltage is applied by facing the sample and the reference electrode. At that time, it utilizes the fact that the AC signal that flows when the distance between the plates is vibrated is proportional to the work function difference. We use this method to measure the huge surface potential generated by the spontaneous polarization that occurs in organic EL devices. We have also found that this method can be applied to liquids to measure work functions in liquids and to measure potential profiles in liquids. We have also developed a rotary KP device that measures by rotating the electrodes, and are conducting unique research such as simultaneously performing sample film formation and work function measurement.
In our laboratory, we have been developing a displacement current evaluation method, which is a method for exploring the behavior of carriers in devices. It has been found that this method can be used to obtain important information for understanding the device, such as carrier injection characteristics, carrier spatial distribution, trap characteristics, and polarization characteristics in the device. Specifically, we are conducting research focusing on the operating mechanism of organic transistors, doping characteristics, interface characteristics of organic electroluminescent (EL) devices, and the relationship between carrier behavior and device deterioration.
In order to supply power to sensors and low power consumption devices, an electric vibration power generator (VEG) that can obtain electrical energy from the vibrations that exist around us is attracting attention. However, it was difficult to make an electret, which was one of the causes of increasing the manufacturing cost of VEG. Therefore, we are working on the realization of inexpensive and high-performance VEG using spontaneously oriented polar molecules as electrets. Using Alq3, which is a material for typical organic electroluminescence (EL) devices, as an electret, we have realized a VEG that does not require any charge processing. This study also demonstrates that "polar organic EL materials function as self-assembled electrets (SAEs)." Therefore, in the future, we will develop organic EL materials into sensors, microphones, filters, etc., which will lead to the creation of innovative electret devices.
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