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

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 cm2 V−1 s−1, due to H‐aggregation, whereas the field‐effect mobilities of devices that incorporated symmetrical molecules, DCE4T and DCB4T, were poor, below 10−4 cm2 V−1 s−1, due to J‐aggregation. More importantly, H‐aggregation within the thin film provided stable crystalline morphologies in the spin‐coated films, and, thus, thin film transistors (TFTs) using cyclohexylated quaterthiophenes yielded highly reproducible transistor performances. The distributions of measured field‐effect mobilities in transistors based on cyclohexylated quaterthiophenes with H‐aggregation were remarkably narrow. A novel synthetic strategy to induce H‐aggregation between adjacent molecules in the thin film state is described. H‐aggregation occurs when molecules stack side‐by‐side and results in a large area π‐stacking between adjacent molecules. The large π‐overlap area in H‐type aggregation materials can produce high‐performance transistors. In addition, H‐aggregation induces uniform morphologies in thin films and reproducible transistor performances.