The 83rd JSAP Autumn Meeting 2022 Highlighted Presentations
October 24, 2022

Charge transport properties and band-dispersion modulation induced by low-frequency intramolecular vibration
modes of organic semiconductors

A New Step Toward Improving Performance of n-Type
Organic Semiconductors
Development of molecules that suppress low-frequency
intramolecular vibration modes is the key
Hiroyuki Ishii (1), Hiroyuki Yoshida (2),(3), Nobuhiko Kobayashi (1)Faculty of Pure and Applied Sciences, University of Tsukuba (1), Graduate School of Engineering, Chiba University (2), MCRC, ChibaUniversity (3)Summary• The mechanism by which n-type organic semiconductors have extremely low electron
mobility compared to that of p-type organic semiconductors is revealed.
• Low-frequency intramolecular vibration modes could be ignored when calculating holemobility but are an important factor in determining the magnitude of electron mobility.
• This research may accelerate the performance improvement of n-type organic
semiconductors.

A research group led by Hiroyuki Ishii and Nobuhiko Kobayashi of the University of Tsukuba and Hiroyuki
Yoshida of the Chiba University, has elucidated the mechanism by which n-type organic semiconductors
have extremely low electron mobility compared to that of p-type organic semiconductors, which has long
been unexplained. Employing the time-dependent wave packet diffusion method, they increased theprecision of calculations for electron mobility by including low-frequency intramolecular vibration effects thatare neglected in calculations for hole mobility. This research will greatly improve the accuracy of simulations
for the molecular design of n-type organic semiconductors and contribute to the improvement of theirperformance in the future.

Detail

Elucidating the Mechanism Behind the Low Electron Mobility of n-Type Organic
Semiconductors, a Longstanding Mystery
Organic semiconductors, a type of plastic, are expected to find use in a wide variety of flexible
devices owing to their low thickness and weight. Currently, they are being used in flat panel
displays for TVs and smartphones, and applications in solar cells and various sensors are being
explored. In addition, organic semiconductors are expected to find a wide range of social
applications because they are inexpensive, printable materials with low environmental impact.
Organic semiconductors are either hole-transporting (p-type) or electron-transporting (n-type). In
organic semiconductor devices, semiconductors with different transport properties are joined.
However, n-type organic semiconductors have extremely low electron mobility (*1) compared tothose of p-type organic semiconductors. This is one of the factors that has reduced the overall
performance of organic semiconductor devices.
A research group led by Hiroyuki Ishii of the University of Tsukuba’s Faculty of Pure and Applied
Sciences succeeded for the first time in observing the energy band structure (*2/Fig. 1) of the
conduction band (*3) of pentacene, a typical organic semiconductor, by using angle-resolved lowenergy inverse photoelectron spectroscopy (*4). The obtained results helped clarify themechanism by which n-type organic semiconductors have extremely low electron mobility
compared to those of p-type organic semiconductors. By using the time-dependent wave packet
diffusion method (*5), the low-frequency intramolecular vibrational mode, which could be ignored in
the calculation of hole mobility, was shown to be a factor that significantly reduces electronmobility. This research will greatly improve the accuracy of simulations of the molecular design ofn-type organic semiconductors and will contribute to the improvement of their performance.
Ishii and his research team were the first to observe that electrons flowing in pentacene, anorganic semiconductor, deform the molecule and generate quasiparticles called polarons. They
found that the existence of polarons is closely related to the mystery of why the mobility ofelectrons is only one-tenth that of holes. Electrons induce intramolecular vibrations with a low
frequency of 254 cm-1, thereby deforming molecules and reducing their mobility. By applying thismechanism to the time-dependent wave packet diffusion method, they were able to accurately
calculate the electron mobility in other n-type organic semiconductors.
Ishii said that “Until now, calculations of electron mobility in n-type organic semiconductors havebeen based on the assumption that high-frequency intramolecular vibrational modes generate thepolaron, as is the case with holes. This caused an overestimation of the magnitude of the mobility.
When we recalculated the mobility considering low-frequency intramolecular vibrational modes, wefound that there was a large difference in the mobility of electrons and holes. The low-frequency
intramolecular vibrational modes were inhibiting the movement of electrons in n-type organic
semiconductors.”
Ishii further said that “By simulating electrical conduction, taking into account the low-frequency
intramolecular vibrational modes, we were able to calculate the difference in mobility between ntype and p-type organic semiconductors without any discrepancies. This has also allowed us tolook at ways to improve n-type organic semiconductors. If we can develop molecules that cansuppress low-frequency intramolecular vibrational modes, we will be able to increase the mobility

of electrons in n-type organic semiconductors. We hope that our findings will be utilized in the fieldof molecular design in the future.”
Electron mobility is the key to improving the performance of organic semiconductor
devices
The higher the electron mobility of organic semiconductors, the wider the range of theirapplications. For example, e-Paper, which can project a still image, has a mobility of approximately0.5 cm2/Vs; with a mobility above 1 cm2/Vs, it can be used to create a display, and with a mobility
above 10 cm2/Vs, it can be used to create an RF-ID tag that can perform simple calculations. In thefuture, it is expected to be used in wearable devices for which a mobility of 50 cm2/Vs is required.
The key to achieving the same is improving the performance of n-type organic semiconductors.
Ishii said that “Currently, there are several p-type organic semiconductors that exceed 20 cm2/Vs.
However, the mobility of n-type organic semiconductors is still mostly below 3–5 cm2/Vs. In thefuture, we will use simulations to gain knowledge on how to design molecules that will enable ntype organic semiconductors to have a mobility of 10 cm2/Vs or higher.”
Unlike inorganic semiconductors, organic semiconductors can be developed at a very low cost. For
example, the future Internet of things (IoT) will require a large number of sensors. In this regard,
the use of organic semiconductors will afford a great cost advantage. Further, widely applying
inexpensive solar cells made from organic semiconductors can reduce environmental impacts.
Realizing all of these possibilities requires a better understanding of electron mobility.
Annotations
1 Electron mobility: A quantity that indicates the ease of electron transfer in a material.
2 Energy band structure: The relationship between the energy and momentum of electrons and holes in a crystal
based on the laws of quantum mechanics.
3 Conduction band: An empty energy band in charge neutrality state. The conduction band is the path through which
injected electrons flow and is the element that determines electron mobility.
4 Angle-resolved low-energy inverse photoelectron spectroscopy: An experimental technique developed by Prof.
Yoshida that can measure the energy and momentum of electrons flowing in the conduction band.
5 Time-dependent wave packet diffusion method: A method developed by Dr. Ishii to accurately calculate the mobilityof electrons and holes in organic semiconductors based on quantum theory.
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figureFig. 1 Energy band structure of valence band and conduction band.
Holes flow in the valence band and electrons flow in the conduction band. Each band structure determines the holetransport property (hole conduction/p-type organic semiconductor) and electron transport property (electronconduction/n-type organic semiconductor) in organic semiconductors.