Stability of ternary interfaces and its effects on ideal switching characteristics in inverted coplanar organic transistors

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Stability of ternary interfaces and its effects on ideal switching characteristics in inverted coplanar organic transistors

Keito Murata, Gyo Kitahara, Satoru Inoue, Toshiki Higashino, Satoshi Matsuoka, Shunto Arai, Reiji Kumai, and Tatsuo Hasegawa

Phys. Rev. Applied 21, 024005 – Published 2 February 2024

Abstract

Inverted coplanar or bottom-gate bottom-contact (BGBC)-type thin-film transistors (TFTs) present several advantages for the manufacture and application of organic TFTs, although serious difficulties are encountered when trying to achieve sufficiently high performance. Recently, it was demonstrated that both high mobility and ideal on-off switching are attainable in BGBC-type printed organic TFTs with highly clean semiconductor-gate dielectric interfaces. However, an unknown channel material dependence in the device performance is found. Here, we show that the stability of semiconductor/metal/dielectric ternary interfaces is a crucial factor in the operation of BGBC-type organic TFTs. We fabricate single-crystal organic semiconductor (OSC) films with various numbers of layers using two different materials (phenyl/alkyl-substituted benzothieno[3,2-b]benzothiophene and phenyl/alkyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene) on highly lyophobic Cytop gate dielectric surfaces. The transfer characteristics exhibit notable time-dependent degradation, which clearly depends on the material, layer number, and encapsulation. Kelvin-probe force microscopy measurements reveal that the degradation is ascribed to contact resistance at the source electrodes, while it can be more suppressed in multilayer (two or more layers) OSCs. Atomic force microscopy and in-plane x-ray diffraction profiles present signs of the transformation in single molecular bilayer OSCs laid on the electrodes. The results suggest the importance of the quality of the OSC layer at ternary interfaces, providing a clue for improving the performance of BGBC-type organic TFTs.

Received 3 May 2023
Revised 18 July 2023
Accepted 22 December 2023

DOI:https://doi.org/10.1103/PhysRevApplied.21.024005

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Organic transistors Surface & interfacial phenomena

Field-effect transistors Organic semiconductors Solid-solid interfaces

Atomic force microscopy Resistivity measurements X-ray diffraction

Condensed Matter, Materials & Applied PhysicsPolymers & Soft Matter

Authors & Affiliations

Keito Murata ^1,*, Gyo Kitahara ¹, Satoru Inoue ¹, Toshiki Higashino ², Satoshi Matsuoka ¹, Shunto Arai ^1,†, Reiji Kumai ³, and Tatsuo Hasegawa ¹

¹Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
²National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
³Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan

^*murata@hsgw.t.u-tokyo.ac.jp
^†Present address: National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan.

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Vol. 21, Iss. 2 — February 2024

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Images

Figure 1
Single-crystal TFTs with various numbers of layers of OSC films. (a) Chemical structures of $Ph$ - $BTBT$ - $C_{n}$ and $Ph − BTNT − C_{n}$ . (b) Schematic cross section of TFTs. (c) Optical image of various numbers of layers of OSC films with a mixed solution of $Ph$ - $BTBT$ - $C_{n}$ . The red dashed line shows the boundary between regions with uniform thickness of OSC films. The black dashed line rectangle shows one of the TFTs with a uniform layer number. (d) Transfer characteristics of four layers of $Ph$ - $BTBT$ - $C_{n}$ TFTs. The mobility value is estimated as 9.1 cm² V^–1 s^–1. The SS value is extracted by the linear fit of current slope in the range from |I_d| = 10^−11.5 to 10⁻¹⁰ A. (e) Layer-number dependence of the device mobility in the linear regime several days after device fabrication.
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Figure 2
Transfer characteristics of TFTs with various OSC materials and layer numbers. All devices are stored under nitrogen atmosphere. (a) Transfer curves in the linear regime measured at various times after device fabrication. The schematic of each device structure above the gate dielectric layer is also shown. (b) Temporal variation in the device mobility extracted from the transfer curves. (c) Temporal variation in the SS value of the single-layer $Ph − BTNT − C_{n}$ TFTs.
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Figure 3
Transfer characteristics of single-layer $Ph$ - $BTBT$ - $C_{n}$ TFTs under various atmospheres. (a) Device structures above the gate dielectric layer. (b),(c) Temporal variation in the device mobility. TFTs stored in a laboratory, in a glove box filled with $N_{2}$ gas, and in a chamber (10⁻⁴ Pa) are shown as “Air”, “Nitrogen,” and “Vacuum”, respectively.
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Figure 4
Surface potential distribution in the linear regime (V_d= −0.5 V, V_g= −3 V) along the TFT channel. (a) Time dependence of surface potential of specific (upper) single-layer $Ph$ - $BTBT$ - $C_{n}$ and (lower) single-layer $Ph − BTNT − C_{n}$ TFTs. (b) Layer-number dependence of surface potential of (upper) $Ph$ - $BTBT$ - $C_{n}$ and (lower) $Ph − BTNT − C_{n}$ TFTs, respectively. $Ph$ - $BTBT$ - $C_{n}$ and single-layer $Ph − BTNT − C_{n}$ TFTs are measured within 20–40 h. The multilayer $Ph − BTNT − C_{n}$ TFTs are measured at approximately 250 h. “S” and “D” correspond to source and drain electrodes, respectively. Each dashed line represents the boundary between the electrode and the channel.
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Figure 5
Morphology of (a),(c),(e) single-layer and (b),(d),(f) multilayer (five layers) of $Ph$ - $BTBT$ - $C_{n}$ films (a),(b) around contact edge, (c),(d) on Cytop, and (e),(f) on electrodes. Images of the single-layer film show the temporal variation of a specific area. Those of the multilayer film are obtained more than one month after device fabrication. Color scale is common for each row. (g) Height profiles of the blue dashed lines on the right-hand image of (e). The black dashed line at 5.3 nm corresponds to the thickness of a single layer. (h) Statistical distribution extracted from (e). The average height of the base region corresponds to 0 nm. (i) Schematic illustration of structure of single-layer films. Single-layer film is partially defective, disordered, and stacked.
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Figure 6
In-plane x-ray diffraction (XRD) measurements of $Ph$ - $BTBT$ - $C_{n}$ films. (a) XRD profile of single layer on PFBT $/ Au$ collected by scanning the φ angle with 2θ fixed at 19.88° corresponding to the (020) diffraction. (b) XRD profile of single layer on PFBT/ $Au$ and on Cytop collected by scanning the 2θ angle with φ fixed at the optimal angle. q is the scattering vector. The inset is the layered-herringbone packing motif of $Ph − BTBT − C_{10}$ . (c) Crossed-Nicols micrograph of the single layer on PFBT $/ Au$ . The yellow dashed line shows a single-domain crystal film, as used for XRD measurements.
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