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

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 (μ_max = 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 (V_th) of −8 V. Moreover, although V_th increased (V_th = −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.