Organic field-effect transistors (OFETs) have received significant attention because of the potential use in flexible and/or disposable portable electronic devices. The charge carrier mobility is strongly influenced by the organic semiconductor layer. But, it has been still lower than the charge carrier mobility for inorganic MOS FETs. Therefore, great efforts have been made toward the development of various semiconducting small molecules and polymers. Among these, fused π-conjugated semiconductors such as acene, thienoacene, and thienothiophene based compounds are promising materials for high-performance OFETs.

In this study we investigated the characterization of thin-film OFETs based on newly developed compound, asymmetric chrysene derivatives which regard phenyl chrysene as the main structure (Fig. 1). It is expected that the large and planar chrysene core would provide intermolecular interactions to achieve high charge carrier mobilities, and asymmetric molecular structure are promising for improvement of solubility.

Thin-films are fabricated by spin-coat and vapor deposition method on a n-doped Si wafer with a 220 nm thermally grown SiO₂. The spin-coated films (50 nm thick) were fabricated by 0.4 wt% toluene solution at 2000 rpm for 30 sec. The vapor deposited films were deposited on insulating SiO₂ layer was treated with a thin-film (ca. 30 nm) of a polymer insulator such as polystyrene (PS) or CYTOP (Asahi Glass Co.). Molecular packing and orientation was characterized by atomic force microscope (AFM), and X-ray diffractions　(XRDs) analysis. The electrical properties of the OFET devices were measured by using an Agilent 4155C semiconductor parameter analyzer under vacuum at room temperature.

The solution processed asymmetric chrysene based transistors showed typical p-channel FET characteristics. The field-effect carrier mobility of the device was determined to be 5.9×10^-4cm²V^-1s^-1 and on/off ratio of 10⁵ for P-28CR-6. The vaper deposited P-28CR-n based OFET shows also typical p-channel FET characteristics and their mobilities  are strongly correlated with the applied polymer insulator, and the best field-effect mobility (3.1 cm²V^-1s^-1 and on/off ratio of 10⁴) was obtained for the CYTOP-covered device. As shown in Fig. 2(b) carrier mobility is correlated by the length of alkyl chain for P-28CR-n. According to XRD analysis, orientation angle of chrysene core of P-28CR-n to the substrate is also correlated to the alkyl chain length of P-28CR-n. Interestingly, the field-effect mobility shows a similar trend to the orientation angle of the chrysene core for the alkyl chain length.