H-Aggregation Strategy in the Design of Molecular Semiconductors for Highly Reliable Organic Thin Film Transistors

Abstract

Four new quaterthiophene derivatives with end-groups composed of dicyclohexyl ethyl (DCE4T), dicyclohexyl butyl (DCB4T), cyclohexyl ethyl (CE4T), and cyclohexyl butyl (CB4T) were designed. All materials showed high solubility in common organic solvents. UV-vis absorption measurements showed that the quaterthiophene derivatives with asymmetrically substituted cyclohexyl end-groups (CE4T and CB4T) preferred H-type aggregation whereas those with symmetrically substituted cyclohexyl end-groups (DCE4T and DCB4T) preferred J-type aggregation. The molecular structure-dependent packing (H or J) of the new quaterthiophene derivatives was analyzed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The field-effect mobilities of devices that incorporated the asymmetrical molecules, CE4T and CB4T, were quite high, above 10(-2) cm (2) V-1 s(-1), due to H-aggregation, whereas the fi eld-effect mobilities of devices that incorporated symmetrical molecules, DCE4T and DCB4T, were poor, below 10(-4) cm (2) V-1 s(-1), due to J-aggregation. More importantly, H-aggregation within the thin fi lm provided stable crystalline morphologies in the spin-coated fi lms, and, thus, thin fi lm transistors (TFTs) using cyclohexylated quaterthiophenes yielded highly reproducible transistor performances. The distributions of measured fi eld-effect mobilities in transistors based on cyclohexylated quaterthiophenes with H-aggregation were remarkably narrow.