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DPh-BTBT: High-mobility and Air-stability p-type OFET Material

High-mobility and air-stability p-type OFET material

Flexible, thin, and lightweight organic field-effect transistors (OFET) have attracted much attention for their potential uses in electronic devices, such as flexible displays, electronic paper, radio frequency identification tags, and electronic artificial skins. Since OFETs are operated by injecting carriers from electrodes to HOMO (p-type) or LUMO (n-type) of semiconductor materials, matching the work function of electrodes with the HOMO or the LUMO level of OFET materials is important. Organic materials (HOMO level < -5 eV), such as pentacene, are generally affected by the oxygen of ambient air, resulting in degradation of the OFET performance. Therefore, to keep sufficient air-stability of p-type materials, its HOMO level should be deeper than -5 eV.
DPh-BTBT [D3526], a thienothiophene-fused compound, has been reported by Takimiya et al1) as a valuable p-type OFET material having both high performance and air-stability. It should be noted that the DPh-BTBT has a deep HOMO level (-5.6 eV) resulting in remarkable air-stability of the OFET performance. Also, the HOMO is well-distributed on the sulfur atoms of the thienothiophene moiety, indicating that DPh-BTBT has an advantage of hole carrier transport in the thin-film form.

TCI have commercialized “the DPh-BTBT: high-mobility and air-stability p-type OFET material”, and also have started fabrication and evaluation of OFET devices in our laboratory.

  • K. Takimiya, H. Ebata, K. Sakamoto, T. Izawa, T. Otsubo, Y. Kunugi, J. Am. Chem. Soc. 2006, 128, 12604.

DPh-BTBT-based OFET Device Fabrication and Evaluation

The field-effect mobility of DPh-BTBT was measured using the top-contact thin-film field-effect transistors geometry (Figure 1). The thin film of DPh-BTBT as active layer (60 nm) was vacuum-deposited onto octadecyl trichlorosilane (ODTS)[O0079]-treated Si/SiO2 substrates at different substrate temperatures (Tsub = 80, 100 and 120 ℃). The drain and source electrodes (40 nm) then were prepared by gold evaporation through a shadow mask on top of the DPh-BTBT film; the drain-source channel length (L) and width (w) are 45 µm and 1.5 mm, respectively. The characteristics of the OFET devices were measured under nitrogen or ambient .conditions.

An example of OFET evaluation

The performances of the OFET devices are summarized in Table 1. All DPh-BTBT-based devices exhibited pure typical p-channel field-effect transistor (FET) characteristics. The FET mobilities were quite dependent on Tsub; the device fabricated at Tsub = 100 ℃ demonstrated the highest performance with a hole carrier mobility of 3.5 cm2/Vs, and an on/off ratio of 2.1 × 107 under the nitrogen conditions (Figure 2). Interestingly, in the ambient condition, the DPh-BTBT-based device also demonstrated excellent typical p-channel FET characteristics with a high hole mobility of 3.0 cm2/Vs. These results indicate that DPh-BTBT is a promising p-type OFET material possessing high hole mobility and good air-stability.

Typical OFET characteristics of top-contact devices fabricated using the DPh-BTBT (Tsub = 100°C).  (a) Transfer curves in the saturated region. (b) Output curves at different gate voltages

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"The purity of materials" is an important factor directly influencing their OFET device parameters. To check and enhance the quality and purity of the transistor materials, TCI has already started the fabrication and evaluation of the OFET devices using our products. We constantly seek to improve our technology and skill in order to provide high-purity and quality materials.

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