Charge-transport properties of the 1,4-diiodobenzene crystal:: A quantum-mechanical study

The 1,4-diiodobenzene (DIB) crystal stands out among molecular organic semiconducting crystals because of its remarkable room-temperature hole mobility (>10 cm(2)/(V s)). Here, on the basis of a density functional theory study, we demonstrate that the high mobility in DIB is primarily associated with the heavy iodine atoms. We find that along specific crystal directions, both electrons and holes are characterized by a very small effective mass of about 0.5 m(0). Interestingly, iodine substitution also leads to a significant decrease in the local hole-vibration coupling compared to benzene; as a result, the electronic coupling for holes is calculated to be much larger than the hole-vibration coupling, which is consistent with the observation of large hole mobility. In marked contrast, the polaron binding energy in the case of electrons is found to be significantly higher than the electronic coupling; this implies that electrons in DIB are strongly localized even at room temperature.