p-channel organic semiconductors based on hybrid acene-thiophene molecules for thin-film transistor applications

We report the structural and electrical characterization of two new p-channel organic semiconductors, 5,5′-bis(2-tetracenyl)-2,2′- bithiophene (1) and 5,5′-bis(2-anthracenyl)-2,2′-bithiophene (2). Both compounds exhibited a high degree of thermal stability with decomposition temperatures of 530 °C and 425 °C for 1 and 2, respectively. The thin-film structures of 1 and 2 were examined using wide-angle X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXD), and atomic force microscopy (AFM). Films of 1 and 2 pack in similar triclinic unit cells with the long axes of the molecules nearly perpendicular to the substrate. Thin-film transistors (TFTs) based on 1 and 2 exhibit contact-corrected linear regime hole mobility as high as 0.5 cm²/Vs and 0.1 cm²/Vs, respectively. The specific contact resistance at high gate voltages for gold top contacts was 2 × 10⁴ Ω cm and 3 × 10⁴ Ω cm for 35 nm thick films of 1 and 2, respectively. Long-term air stability tests revealed less degradation of the electrical properties of 1 and 2 in comparison to pentacene. Variable temperature measurements revealed activation energies as low as 22 and 27 meV for 1 and 2, respectively. The temperature and gate voltage dependence of the mobility are discussed in terms of a double exponential distribution of trap states and a model accounting for the layered structure of the organic films. The enhanced air and thermal stability over pentacene, combined with good electrical performance characteristics, make 2 a promising candidate for future organic TFT applications.