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High-performance Liquid Crystalline Semiconductor Material: Ph-BTBT-10

No.178(July 2018)
D5491
Ph-BTBT-10  (1)
A liquid crystalline semiconductor material Ph-BTBT-10 (1) has been recently reported by Hanna et al.1) as a p-type semiconductor material which possesses excellent transport properties. 1 exhibits an ultra-high mobility (μmax = 14.7 cm2/Vs) comparable to oxide-based semiconductors (IGZO), and remarkable air stability.1) TCI has examined the fabrication and evaluation of 1-based Organic Field-Effect Transistor (OFET) devices by vacuum deposition methods. Furthermore, TCI has investigated the vacuum-deposited thin film structure of 1 and the correlation between device performances and thin-film crystallinities through 2D grazing-incidence X-ray diffraction (2D-GIXD) analysis.2,3)

Figure 1. OFET device configuration

Figure 2. Transfer characteristics of Ph-BTBT-10 devices
(a) w/o annealing (ODTS) (b) annealing 120 °C, 5 min (ODTS)


Table 1. OFET characteristics of vacuum deposition Ph-BTBT-10 devices

The field-effect mobility of 1 was measured using top-contact thin-film field-effect transistor (FET) geometry (Figure 1). The performances of the OFET devices are summarized in Table 1 and Figure 2. All the devices exhibited pure typical p-channel FET characteristics. FET mobilities were quite dependent on the thermal annealing treatment regardless of the self-assemble-monolayer (SAM) (Figure 2). The device fabricated on bare substrate demonstrated good performance with a hole carrier mobility of 4.86 cm2/Vs and threshold voltage (Vth) of −8 V. Moreover, although Vth increased (Vth = −22 V), the ODTS-treated devise exhibited a large increase in drain current (IDS) and the highest transport performance with a hole carrier mobility of 14.0 cm2/Vs.

Figure 3. 2D-GIXD analysis (Thermal in-situ measurement)

Fogure 4. 2D-GIXD analysis (Out-of-plane, bare substrate)

Figure 5. Phase transition Image of Ph-BTBT-10


Figure 3 and 4 show the thermal in situ 2D-GIXD data of 1-thin-film crystal structure.2,3) The data at RT and 60 °C indicated a similar series of peaks assignable to a monolayer structure with a d-spacing of 27 Å. When the substrate temperature was increased to 120 °C, the diffraction peaks clearly changed, which suggests a transformation from a monolayer structure (d = 27 Å) to a bilayer structure with a d-spacing of 54 Å as shown in Figure 5. Since the phase transition temperature to the smectic E (SmE) mesophase is 144 °C,1) a series of peaks assignable to SmE were observed when heated to 180 °C. In addition, a mixed layer of monolayer and bilayer structures appeared when the thin film of 1 was rapidly cooled from 180 °C to RT. This result indicates that the cooling speed might be a significant factor in forming a well-uniformed crystalline thin film structure.
Based on these results, 1 can be handled through vacuum deposition method, and the phase transition from the monolayer to the bilayer structure can occur in the same way in which it occurs for solution-processed thin films of 1.1) Finally, we demonstrated top-ranked FET performances via vacuum deposition process using our in-house equipment.

References

  • 1)Liquid crystals for organic thin-film transistors
  • 2)2D-GIXD experiments were performed at the BL46XU and BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2017B1817 and 2017B1629).
    We acknowledge Prof. Noriyuki Yoshimoto, Assist. Prof. Daiki Kuzuhara and Mr. Shimpei Miura (Iwate Univ.), Assist. Prof. Mitsuharu Suzuki (NAIST) and Dr. Tomoyuki Koganezawa (JASRI) for technical support in GIXD.
  • 3)Temperature-Dependent Thin-Film Structures of the Liquid Crystalline Organic Semiconducting Material
    • S. Miura et al. The 65th JSAP Spring Meeting, 2018, 20a-P7-1.
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