Air-Stable n-Type Organic Field-Effect Transistors Based on Solution-Processable, Electronegative Oligomers Containing Dicyanomethylene-Substituted Cyclopenta[b]thiophene

Solution-processable, electronegative, p-conjugated systems containing dicyanomethylene-substituted cyclopenta[b]thiophene were synthesized as potential active materials for air-stable n-type organic field-effect transistors (OFETs). Electrochemical measurements revealed that these compounds exhibited electrochemical stability and that the lowest unoccupied molecular orbital (LUMO) had an energy level less than -4.0 eV. Flashphotolysis time-resolved microwave conductivity (FP-TRMC) measurements were performed, and the value of intradomain electron mobility was determined to be as high as 0.1 cm(2)V(-1)s(-1). The OFETs were fabricated by spin-coating thin films of the compounds as an active layer. The electron mobility of the OFETs was 3.5 x 10(-3) cm(2)V(-1)s(-1) in vacuum. Furthermore, electron mobility of the same order of magnitude and stable characteristics were obtained under air-exposed conditions. X-ray diffraction measurements of the spin-coated thin films revealed the difference of molecular arrangements depending on the inner conjugated units. Atomic force microscopy measurements of crystalline-structured films exhibited the formation of grains. The accomplishment of air-stability was attributed to the combined effect of the low-lying LUMO energy level and the molecular arrangements in the solid state, avoiding both the quenching of electron carriers and the intrusion of oxygen and/or moisture.