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) × 10-5 cm2/V s and an ON/OFF ratio of 3.0 × 104. 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) × 10-5 cm2/V s and an ON/OFF ratio of 2.7 × 103.
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.
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