Charge-carrier mobilities in binary mixtures of discotic triphenylene derivatives as a function of temperature

The pulse-radiolysis time-resolved microwave conductivity technique was used to study charge transport in three binary mixtures of triphenylene derivatives and in three of these components separately. In the liquid-crystalline mesophase the hole mobilities in the mixtures are found to be an order of magnitude higher than those in the separate components. This is partly due to the more stable columnar structure in the mixtures and partly due to efficient ``alloy band formation.'' In the crystalline phase the mobility in the mixtures is only a few times lower than that of the separate components. The ionization energies of the components as measured by cyclic voltammetry are up to similar to0.5 eV apart. It turns out that this energy difference is easily compensated by the relatively large charge-transfer integrals for hole transport in the binary mixtures, which was obtained from ab initio Hartree Fock calculations. The mobilities estimated theoretically for ordered systems largely exceed the experimental values. This is most likely due to structural disorder along the columns in the material. Theoretical estimations of the hole mobility suggest that mobilities in excess of 1 cm(2)/V s could be attainable in well-ordered crystalline triphenylene samples.