Charge transport properties of carbazole dendrimers in organic field-effect transistors

We report three generations of p-type dendrimer semiconductors comprised of spirobifluorene cores, carbazole branching units and fluorene surface groups for use in organic field-effect transistors (OFETs). The group of dendrimers are defined by their generation and noted as SBF-(Gx)(2), where x is the generation. Top contact-bottom gate OFETs were fabricated by spin-coating the dendrimers onto an n-octyltrichlorosilane (OTS) passivated silicon dioxide surface. The dendrimer films were found to be amorphous. The highest mobility was measured for the first generation dendrimer (SBF-(G1)(2)), which had an average mobility of (6.6 +/- 0.2) x 10(-5) cm(2)/V s and an ON/OFF ratio of 3.0 x 10(4). As the generation of the dendrimer was increased there was only a slight decrease in the measured mobility in spite of the significantly different molecular sizes of the dendrimers. The mobility of SBF-(G3)(2), which had a hydrodynamic radius almost twice of SBF-(G1)(2), still had an average mobility of (4.7 +/- 0.6) x 10(-5) cm(2)/V s and an ON/OFF ratio of 2.7 x 10(3). Density functional theory calculations showed that the highest occupied molecular orbital was distributed over the core and carbazole units meaning that both intra-and intermolecular charge transfer could occur enabling the hole mobility to remain essentially constant even though the dendrimers would pack differently in the solid-state.