Nondispersive hole transport in carbazole- and anthracene-containing polyspirobifluorene copolymers studied by the charge-generation layer time-of-flight technique

Nondispersive hole transport in two polyspirobifluorene copolymers containing either 10% anthracene or 10% carbazole was studied in detail by the charge-generation layer time-of-flight (TOF) technique over a wide range of electric fields and temperatures. The TOF transients of both polymers showed a clear plateau indicating nondispersive transport of charge carriers. Zero-field mobilities were found to be in the order of ${10}^{-6}{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ at room temperature. Results were analyzed within the framework of the Gaussian disorder model to extract the parameters of the charge-carrier transport. The width of the transport density of states was determined to be $83\mathrm{meV}$ for the polyspirobifluorene-anthracene copolymer and $89\mathrm{meV}$ for the polyspirobifluorene-carbazole copolymer. At lower temperatures a change of slope in the temperature dependence of the zero-field mobility was observed. At higher temperatures the TOF transients were modified by a cusp. Both phenomena can be explained within the framework of the Gaussian disorder model.