Synthesis and characterization of organic semiconducting materials for organic thin-film transistor applications

Abstract

The thin-film morphologies and thin-film transistor (TFT) characteristics of fluorene-and thieno[3,2-b]thiophene-based alternating copolymer, poly(9,9´-n-dioctylfluorene-alt-thieno[3,2-b]thiophene) (F8TT) are reported. The crystalline properties of the copolymer were confirmed through X-ray diffraction studies. Solution-processed thin-film transistor was fabricated using the bottom contact geometry. F8TT has a field-effect mobility of $1.1 × 10^{-3}cm^2V^{-1}s^{-1}$ without any alignment technique; this mobility was three times higher than that of poly(9,9´-n-dioctylfluorene-alt-bithiophene) (F8T2) under the same conditions. The present results indicate that the greater planarity and rigidity of thieno[3,2-b]thiophene compared to bithiophene results in higher crystallinity of the polymer backbone, which leads to improved TFT performance. The thin-film transistor (TFT) performances of two thiophene-based semiconducting materials (T2TT and T2FL) are examined. Through studies of the thin-film morphology using X-ray diffraction (XRD) and atomic force microscopy (AFM), I could access the behavior and structure of the thin film devices at the nanoscale level in single molecular layers and determine the correlation between molecular alignment and device performance. Due to the extended π-conjugation, efficient hole injection, and good molecular alignment, the T2TT and T2FL TFT devices exhibited hole mobilities of up to $0.03 cm^2 V^{-1}s^{-1}$, when the films were deposited at $T_{sub}$ = RT and $80^\circ C$, respectively. In spite of the large number of small grains in the film deposited at room temperature, the T2TT molecules were aligned perpendicular to the substrate when deposited at this low substrate temperature, which favored π-π overlap between adjacent molecules, resulting in high OTFT performance. The thin-film morphologies and TFT characteristics of a series of binary blends of poly(9,9´-dioctylfluorene-alt-bithiophene) (F8T2) and α,ω-dihexylquarterthi...