Three-dimensional carbazole-based dendrimers: model structures for studying charge transport in organic semiconductor films

We report the synthesis and charge transport properties of a series of three-dimensional dendrimers up to the third generation that have a 9,9'-spirobifluorene core, carbazole-based dendrons and di-n-propylfluorene surface groups. The dendrimers can all be spin-coated to form good quality amorphous films. The charge carrier mobility of the dendrimers was measured by two different methods; in an organic field-effect transistor (OFET) architecture, and by Charge Extraction by Linearly Increasing Voltage (CELIV). In the OFET configuration the first generation dendrimer had a maximum mobility of 4.1 x 10(-4) cm(2) V-1 s(-1) and an ON/OFF ratio of 1.1 x 10(5). Unexpectedly, in spite of the third generation dendrimer having a volume approximately six times that of the first generation, the mobility was found to decrease by only an order of magnitude. A similar trend in mobility was seen in the CELIV results. Photoluminescence (PL) measurements in solution showed that the first generation dendrimer was comprised of non-interacting chromophores, while the second and third generation dendrimers had substantial intra-dendrimer interchromophore interactions. In the solid-state, PL measurements showed that for the first generation dendrimer there were clear inter-dendrimer interchromophore interactions with little change for the second and third generations. Comparison of the dendrimer molecular volumes in solution and the solid-state showed that in the latter, the dendrimers took up a smaller volume suggesting that there was interdigitation of the dendrons. For the first generation dendrimer the interdigitation leads to trap sites for charge transport, with the small decrease in mobility in moving from the first to the second and third generation being due to the extra intra-dendrimer interchromophore interactions. Model dendritic systems such as these can be used to gain significant insight into the subtly of charge transport phenomena in solution processable macromolecular organic semiconductors, since they offer a level of molecular control that is difficult to achieve with polymers.