Research method

Accurate Analysis of Molecular Orientation ofOrganic Semiconductor Material

X-ray diffraction/scattering

Clarification of excellent electric properties of transistor devices fabricated by vacuum deposition

Role of SPring-8

Achievements

Successful fabrication of transistor device with high mobility by vacuum
deposition* using 2-decyl-7-phenyl [1] benzothieno [3, 2-b] [1] benzothiophene(Ph-BTBT-10), an organic semiconductor with excellent charge transport properties
Demonstration of excellent charge transport properties (hole mobility, >10 cm2 / Vs)
by transistor device evaluation
Confirmation of thin-film crystal structure (molecular arrangement) that gives
Ph-BTBT-10 its high mobility by analysis of deposited thin film by
two-dimensional grazing incidence X-ray diffraction (2D-GIXD)**
R&D Institution: Tokyo Chemical Industry Co., Ltd.
*Spin coating and vacuum deposition: These are both typical methods for fabricating semiconductor thin films. In spin coating, a semiconductor
solution is dropped onto a substrate, which is rotated (spun) at a high speed to coat its surface with the semiconductor. In vacuum deposition, athin film of a material is formed by vaporizing (evaporating) the material in vacuum, and the vaporized material is deposited on the surface of asubstrate.
**2D-GIXD: When a sample surface is irradiated with an X-ray beam nearly parallel to the surface, faint diffraction signals can be obtained from athin film layer. This is called grazing incidence X-ray diffraction (GIXD). When the signals are detected two-dimensionally, the preferred orientationof the sample can be evaluated. This is called 2D-GIXD.

Ph-BTBT-10
Ph-BTBT-10 (upper) is a commercially available (lower)p-type organic semiconductor material with excellent
transistor properties.

BackgroundResults

Organic semiconductors are lightweight andflexible. Focusing on these advantages, the researchand development of organic field-effect transistors(OFETs) have been intensively carried out towards
their application in next-generation devices, such as
stretchable and bendable electronic circuits andmedical sensors that can be attached to the skin.
Ph-BTBT-10 is a liquid crystalline organic
semiconductor that is soluble in common organic
solvents and has high thermal durability, film
formability, and air stability. In a previous study, it
was found that an OFET device fabricated using
Ph-BTBT-10 by spin coating showed a high holemobility (>10 cm2/ Vs).1) In this study, it was also
found that a comparable mobility was obtained fora device fabricated by vacuum deposition. To
determine the reasons for the excellent electric
properties, it was necessary to reveal the thin-film
crystal structure (molecular arrangement) in thevacuum-deposited thin film of the organic
semiconductor using crystallography techniques.

An OFET device using Ph-BTBT-10 was fabricatedby vacuum deposition and its transistor
characteristics were evaluated. The results revealed
that Ph-BTBT-10 was not thermally decomposed
and its electrical properties were not deteriorated
during the vacuum deposition process. The device
fabricated by vacuum deposition showed excellent
electric charge transport characteristics (holemobility, >10 cm2/Vs) comparable to those of adevice fabricated by spin coating.
The molecular arrangement of the deposited
thin film was analyzed by 2D-GIXD at SPring-8. The
X-ray obtained from SPring-8 beamlines has anintensity much higher than that of general X-ray
diffraction systems and enables the fast andaccurate analysis of the molecular arrangement oforganic thin films. The analysis results suggest thatthe crystal structure (molecular arrangement) of thevacuum deposited thin film was the same as that of
the spin-coated thin film, verifying the high mobilityof the device fabricated by vacuum deposition.

Results of 2D-GIXD analysis
OFET device fabricated using Ph-BTBT-10
A Ph-BTBT-10 thin film was formed on a silicon wafer (n+−Si/SiO2). A gold (Au)layer was deposited on the thin film. Silicon (n+−Si), silicon dioxide (SiO2),
Ph-BTBT-10, and Au were used as the gate electrode, gate insulating layer,
semiconductor layer, and source/drain electrodes, respectively.

The interlayer distance in the out-of-plane direction was calculated from the diffraction peaks along the qz -axis (indicated by white boxes). The
molecular arrangement of Ph-BTBT-10 in the deposited thin film was predicted at each substrate temperature. The obtained diffraction peaks
suggest that the crystal structure (molecular arrangement) of the post-annealed deposited thin film was the same as that of the spin-coated thinfilm.1)

2D-GIXD analysis

Images of molecular arrangements

semiconductor layer

gate insulating layer
source electrodes
drain electrodes

Out-of-plane diffraction
profiles along the qz axis

gate electrode

OFET device performance
Stable p-type semiconductor characteristics were observed
for the transistor device fabricated by depositing aPh-BTBT-10 thin film (right table). By annealing the deposited
thin film, the transistor performance was greatly improved,that is, a maximum hole mobility of μmax = 14.0 cm2/Vs wasachieved for the octadecyltrichlorosilane (ODTS)-treatedsubstrate.

180 °C → RT
180 °C
120 °C
60 °C
RTReference H.Ⅰino, T. Usui, J. Hanna, Nat. Commun. 2015, 6, 6828.

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