High-performance semiconductors based on oligocarbazole-thiophene derivatives for solution-fabricated organic field-effect transistors

A series of oligocarbazole-thiophenes based on a constant conjugate backbone (carbazole-bithiophene-carbazole) with various n-alkyl chain lengths was prepared for application to organic field-effect transistors (OFETs). The lengths of the n-alkyl substitutions attached on 9-position of carbazole moieties were methyl (CCzT2), hexyl (C6CzT2), dodecyl (C12CzT2), and octadecyl (C18CzT2), called CxCzT2. Variations of n-alkyl chain lengths are proposed to figure out the optimization of OFET performance via solution fabrication of the active layer. Before fabricating OFET devices, the thermal, optical, and electrochemical properties of CxCzT2 were fully characterized with thermogravimetric analysis, differential scanning calorimetry, ultraviolet-visible spectroscopy, and cyclic voltammetry to realize the relationships of the structure to the properties. After fabricating CxCzT2 on Si/SiO2 substrates via solution casting, the thin film morphologies were also studied with polarizing optical microscopy, atomic force microscopy, and X-ray diffraction to investigate the structural relationship to OFET performance. A higher hole mobility was observed with C12CzT2 (3.6x10(-2)cm(2) V-1 s(-1)) due to its liquid crystal properties, and the hole mobility could be further improved to 1.2x10(-1)cm(2) V-1 s(-1) by the introduction of a phenyl-self-assembled monolayer on the Si/SiO2 substrates. The excellent OFET performances of C12CzT2 by solution-fabrication could be considered as a promising candidate for high-end OFET application. (C) 2012 Elsevier B. V. All rights reserved.