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

Abstract

IntroductionOver the past decade, several conjugated organic oligomers and polymers have been developed for potential use in low-cost semiconductor applications to replace their higher-cost inorganic counterparts. The most attractive aspect of these organic materials is that their structures can be diversifi ed and their physical or chemical properties tuned through strategic molecular design. [1][2][3][4][5][6][7][8][9] Among the variety of organic semiconductors developed, thiophene-based materials have emerged as an important class because of their high chemical and electrochemical stability, the accessibility of their preparation for thiophene synthesis, and the availability of welldeveloped/regioselective ring-ring coupling methodologies. [ 10 , 11 ] Furthermore, the properties of oligo/polythiophene cores can be effi ciently tuned by introducing appropriate substitutions. To date, the fi eld-effect transistor (FET) mobilities in devices composed of oligomeric thiophene semiconductors are generally higher than those obtained from conjugated polymers. Oligomeric thiophene semiconductor polymers engage in longrange packing, and effi cient charge transport is directly related to the long-range packing of molecules in semiconductor fi lms. Many synthetic methods have been developed to functionalize the α -and ω -ends of conjugated thiophene ring systems to increase the solubility and stability toward oxidation or to infl uence the solid-state ordering of oligothiophenes without affecting the planarity of the conjugated backbone. [ 12 , 13 ] The present study was devised based on the following three considerations: i) previously reported organic semiconductors have generally shown J-aggregation [ 14 ] with head-to-tail molecular stacking; [ 15 ] ii) large area π -stacking between adjacent molecules can be realized by H-aggregation, which occurs when molecules stack side by side; [ 16 ] and iii) H-aggregation induces stable morphologies in thin fi lms and reproducible transistor performances. We describe a novel synthetic strategy to induce H-aggregation between adjacent molecules in the thin fi lm state. We designed four types of quaterthiophene derivative with end-groups composed of cyclohexyl ethyl (CE4T), cyclohexyl butyl (CB4T), dicyclohexyl ethyl (DCE4T), and dicyclohexyl butyl (DCB4T). UV-vis absorption and grazing-incidence wideangle X-ray scattering (GIWAXS) analyses indicated that the asymmetric derivatives, CE4T and CB4T, tended to undergo H-Aggregation Strategy in the Design of Molecular Semiconductors for Highly Reliable Organic Thin Film TransistorsFour 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 symmetrica...

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