High-performance Organic Field-Effect Transistors Based on Monodisperse Glassy-Nematic Oligofluorenes

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This study was motivated to identify factors affecting hole mobility in uniaxially oriented п-conjugated systems. In addition to the monodisperse oligofluorenes that have been reported recently, thermotropic liquid crystalline fluorene-thiophene oligomers were synthesized and characterized for hole mobility using field-effect transistors. Both series of monodisperse conjugated oligomers and their polymer analogues were prepared into glassy-amorphous, and polydomain and monodomain glassy-nematic films for a systematic study of the effects on hole mobility of chain length, molecular aggregation, orientational order parameter, and the contact resistance in a field-effect transistor.Field-effect transistors were fabricated using hepta- and dodecafluorenes, F(Pr)5F(MB)2 and F(MB)10F(EH)2, and a polyfluorene. Hole mobilities were measured for devices comprising monodomain and polydomain glassy-nematic films as well as glassy-amorphous films. Oligofluorenes were more readily aligned into monodomain films via thermal annealing at a lower temperature, resulting in a higher R_<abs> than polyfluorene. Glassy-amorphous films possess the lowest mobility of all because of the geometric and energetic disorder. The mobility of a monodomain glassy-nematic F(MB)10F(EH)2 film is enhanced over a glassy-amorphous film by a factor of 800. In monodomain films, the field-effect mobility value is determined by the oligofluorene's chain length and the polyfluorene's persistence length. For F(MB)10F(EH)2, the field-effect mobility was evaluated at 0.012 cm^2/Vs with an on/off ratio of 1.0×10^4. This mobility value is twice that of polyfluorene in the same device structure. It is concluded that monodisperse glassy-nematic conjugated oligomers hold great potential for use in field-effect transistors because of superior chemical purity, structural uniformity and regularity, and ease of processing into uniaxially aligned defect-free films.