RESEARCH NEWS STORY
January 15, 2024
Chiba University

The Power of Pause: Controlled Deposition for
Effective and Long-Lasting Organic Devices
Researchers move a step closer to making conventional optoelectronic devices more
lightweight and flexible.
In organic optoelectronic devices, the control of molecular deposition on thin films isimportant for optimal surface arrangement and device performance. In a recent study,
researchers developed a new method for achieving stable deposition on thin films
effectively. They also developed a tool to track real-time potential changes on the surface.
These findings are expected to aid the improvement of organic devices, such as organic
light-emitting diodes, in terms of efficacy and durability.

Image title: A schematic of the apparatus developed for the vacuum deposition method.
Image caption: The primary component of the rotary Kelvin probe features a rotary electrode and its
surroundings. Additionally, it shows the vacuum deposition system, with a crucible and a computer-controlledshutter.
Image credit: Prof. Hisao Ishii of Chiba University, Japan
License type: Original content
Usage restrictions: Cannot be reused without permission.

Organic optoelectronic devices, such as organic light-emitting diodes (OLEDs), use moleculeswith specific structures arranged on thin films. Additionally, the arrangement of thesemolecules on any surface is crucial for various processes that occur within these devices. This
arrangement is guided by two primary factors: the deposition rate (how fast the moleculesare placed) and the surface temperature. Slower deposition rates and higher temperatures
facilitate the proper arrangement, resulting in more stable structures. Finding the right timescale for this process is also critical, and researchers are now looking for ways to control thesefactors for optimal molecular arrangement on surfaces.
In a new study, a team from Japan led by Prof. Hisao Ishii from the Graduate School of Scienceand Engineering and the Center for Frontier Science at Chiba University, along with Masahiro
Ohara from Chiba University and Dr. Yuya Tanaka from the Graduate School of Science andTechnology at Gunma University, has introduced a new method of deposition that achieves
suitable molecular arrangement. The article was made available online on December 4, 2023,
and published in Volume 15, Issue 49 of the journal ACS Applied Materials and Interfaces on
December 13, 2023. "When depositing organic molecules by vacuum deposition, theorientation of the molecules is changed over time by pausing the deposition. Moreover, bychanging the deposition conditions, it is possible to invert the orientation of both the headand tail-end of the molecules," explains Prof. Ishii.
In their study, the team found a simple yet ingenious way to control the orientation ofmolecules deposited on aluminum and benzene-containing thin films, denoted as Alq3 andTPBi, respectively. They used a method called "intermittent deposition," which introduces
breaks during the deposition process, and developed an updated version of a tool called"rotary Kelvin probe" (RKP). This was used to measure the surface potential (voltage on thematerial's surface) during and after the deposition in real-time. By repeatedly opening andclosing the deposition shutter at specific intervals, the researchers could change thepolarization (the distribution of charges), influencing how the molecules were oriented on thefilms.
The new approach of intermittent deposition created a relaxed and stable surface layer with
controllable polarization. The study also revealed how surface relaxation affected molecularorientation and the formation of a potential valley (shaped like a "V"). In fact, this deposition
method enables the creation of an arbitrary potential profile for desired molecularorientations on the thin film of interest.
In terms of applications, this intermittent deposition technique can enhance the effectivity
and lifetime of OLED materials. In addition, it can also be used for non-polar organic molecules,
making it useful for devices like organic photovoltaic cells and transistors. Prof. Ishiiemphasizes, "This method is expected to further improve the efficiency and lifetime of OLEDs.
Beyond OLEDs, it also promotes the development of other organic devices, such as organic
memory devices. Therefore, replacing conventional inorganic devices with organic devices will
make lightweight and flexible devices readily available."
In summary, this study explores the relaxation processes that impact the orientation ofmolecules on the surface of organic thin films and utilizes intermittent deposition to create astable surface layer effectively. Additionally, an RKP tool is developed to analyze changes insurface potential over time. The proposed deposition method is expected to work withvarious organic molecules (not just polar ones) and could pave the way for the improvementof existing organic devices and the development of new ones.
About Professor Hisao Ishii
Dr. Hisao Ishii is a Professor at the Graduate School of Science and Engineering and the Center
for Frontier Science and Molecular Chirality Research Center at Chiba University, Japan. He
obtained his Ph.D. from the University of Tokyo in 1991 and specializes in organic electronics
with over 13,000 citations and 250 publications. He has won several awards, including a Best
Paper Award from the Surface Science Society of Japan (2010), a research award from theJapan OLED forum (2009), and the 13th fellow award from the Japan Society of Applied Physics(2019). He also holds memberships in organizations like The Surface Science Society of Japanand The Chemical Society of Japan.
Funding:
This research was supported by the Japan Society for the Promotion of Science (JSPS)
KAKENHI Grant-in-Aid for Scientific Research (grant nos. 20H02810 and 21K05208) and Grantin-Aid for JSPS Fellows (grant no. 22J21883).
Reference:
Title of original paper:Impact of Intermittent Deposition on Spontaneous Orientation
Polarization of Organic Amorphous Films Revealed by Rotary
Kelvin Probe

Authors:

Masahiro Ohara1, Hokuto Hamada1, Noritaka Matsuura1, Yuya
Tanaka2, and Hisao Ishii1,3

Affiliations:

1

Journal:DOI:ACS Applied Materials and Interfaces
10.1021/acsami.3c12914

Graduate School of Science and Engineering, Chiba University
Graduate School of Science and Technology, GunmaUniversity
3
Center for Frontier Science and Molecular Chirality ResearchCenter, Chiba University
2

Contact: Hisao Ishii
Professor, Graduate School of Science and Engineering, Chiba University
Email: ishii130@faculty.chiba-u.jp
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