Donor/Acceptor Type Polymer Semiconductor Applicable for Organic Thin-Film Transistors without Surface Modification on Printed Silver Electrodes
Authors: Murase Tomohide , Takeda Yasunori , Shiwaku Rei , Hayasaka Kazuma , Matsui Hiroyuki , Sekine Tomohito , Kumaki Daisuke , Tokito Shizuo 
Published: February 2020

<Introduction>
Organic thin-film transistor (OTFT) devices have attracted much attention in viewpoint of printed-electronics that realizes inexpensive, large-area and flexible devices in the near future. Mitsubishi Chemical Corporation (MCC) has investigated organic semiconductor (OSC) for more than a decade, and focuses recently on development of donor/acceptor (DA) type polymer semiconductors[1-4]. Improvement of OTFT device mobility by novel OSC materials is still one of the biggest issues in the development phase. However, their additional features that can make device fabrication process simpler are also an important industrialization standpoint in material selection. In this paper, we present DA type polymer semiconductor that enables uncomplicated device fabrication process without self-assembled monolayer (SAM) modification on printed silver electrode surface.
<Experiments>
Top-gate, bottom-contact devices were fabricated as follows. Silver source and drain (S/D) electrodes were patterned by ink-jet printing on Parylene underlayer on a glass substrate. After preparation of bank structure by a dispenser system using solution of amorphous fluoropolymer, semiconducting layer (MCC’s OSC; MOP-01) was printed into an area defined by the bank by a dispenser equipment using its dilute mesitylene solution. Parylene polymer dielectric (thickness: 300 nm) was then deposited on the surface under vaccum. Finally silver gate electrode was patterned by ink-jet printing on the polymer dielectric. Channel length and channel width of the devices are 30-40 μm and 1000 μm, respectively.
During the fabrication process described above, three kinds of the silver S/D electrode surface were prepared with or without SAM modification. The SAM molecule introduced at the electrode surface is either pentafluorobenzenethiol (PFBT) commonly used for p-type OSC or 4-methylbenzenethiol (4MBT) normally used for n-type OSC. Silver S/D electrodes without SAM modification were also prepared for comparison.
<Results and Discussion>
Transfer characteristics of the fabricated OTFT were measured by semiconductor parameter analyzer. The OTFTs exhibited a field-effect mobility of 0.4 cm2/Vs with 4MBT SAM, 0.5 cm2/Vs without SAM, and 0.6 cm2/Vs with PFBT SAM in a saturation region (VDS = -10V) as an average with an on/off current ratio of about 106. The mobility difference is quite small regardless of the presence or absence of the SAM and its species. In addition, mobility among several devices was adequately uniform in each case. According to these findings, we were able to fabricate complementary organic inverter circuits successfully with fewer process steps by combining MOP-1 and n-type OSC (TU-3, Future Ink corporation) where only 4MBT was utilized as an electrode surface modifier[5]. The inverter circuit realized low power operation with excellent performance.
<Acknowledgments>
T.M. would like to thank all the collaborators in Mitsubishi Chemical Center for Advanced Materials (MC-CAM) in University of California, Santa Barbara (UCSB). This work was partially supported by the Japan Science and Technology Agency (JST, the center of Innovation Program).
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