日本千叶大学 Hisao Ishii（石井 久夫）教授 11月28日下午学术报告

报告人：Hisao Ishii（石井 久夫）教授，日本千叶大学

报告题目：Spontaneous Orientation Polarization in Vacuum-Evaporated Films of OLED-Related Molecules 

     Investigated by Rotary Kelvin Probe

报告时间：2023年11月28日下午15:00

报告地点：909-B厅

报告摘要：

Most organic materials in organic light emitting diodes (OLEDs) have molecular structure with lower symmetry for realizing uniform amorphous film. So their molecular structure often has permanent electric dipole moment (PDM). When these polar molecules are deposited by vacuum vapour deposition, spontaneous orientation polarization (SOP) of their PDM often occurs [1]. SOP is common in the evaporated films of many organic semiconducting molecules with a permanent dipole moment, but the mechanism of SOP formation is still open question. A significant electric field is formed in the film due to SOP. Consequently, the properties of OLEDs incorporating such films are influenced. The polarization charge appearing at hetero-interfaces dominates the charge injection and accumulation properties. Moreover, SOP correlates with device degradation. Recently, these polarized films can be used as electret for vibrational energy generators [2]. In this presentation, we will report on our recent research on SOP. Topics are as follows.

1)How we can control the orientation order and polarity by changing evaporation conditions and additional perturbation like light illumination: The higher deposition rate was found to give higher SOP for Alq3 and TPBi [3]. Simultaneous UV light illumination during the deposition was found to increase SOP for TPBi [4].

2) Effect of intermittent evaporation: Recently we found the different surface potential due to SOP of Alq3 between a film evaporated to some thickness at once and that made by repeated cycles of small amount of evaporation to the same thickness. The latter process consists of a set of 1nm thick evaporation and time interval, giving smaller polarization than usual successive deposition. This phenomenon should be related to a surface relaxation of the film during the deposition. In the case of successive deposition, the molecules which lands on the surface do not have enough time to look for energetically stable site/orientation, but in the case of intermittent deposition, the molecules have a enough time to find energetically relaxed state. The possible mechanism will be discussed on the results by rotary Kelvin probe measurement.

3) Whether wet process deposition induce SOP or not: By using automatic measurement system of Rotary Kelvin probe method during the film deposition, we have tried to investigate whether SOP can occur even for wet process deposition in which complete random orientation is expected.[5]

The above results on SOP are useful to control and improve organic electronic devices and vibration generator etc. They are also hints to solve the mysterious mechanism of SOP formation.

个人简介 ：

Hisao Ishii received his MS and PhD degrees in chemistry from the University of Tokyo, in 1988 and1991, respectively.   His   thesis research focused on the metastable atom electron spectroscopy of clean and adsorbate-covered silicon surfaces. In 1991, he started to investigate the film and interface electronic structure of organic semiconductors at Nagoya University as an assistant professor. In 2002, he was promoted to associate professor in Research Institute of Electrical Communication, Tohoku University, to extend his research to organic device physics. Now, he is working for Chiba University since 2006 as a professor. His current research interests include photoelectron spectroscopy of functional organic materials, electronic structure  of organic/ inorganic interfaces, and molecular orientation of thin organic films. His efforts are also focusing on developing new techniques such as high-sensitivity photoelectron spectroscopy and rotary Kelvin probe etc.

联系人：唐建新 教授