Revealing bipolar charge-transport property of 4,4′-N,N′- dicarbazolylbiphenyl (CBP) by quantum chemical calculations

4,4′-N,N′-Dicarbazolylbiphenyl (CBP) has both the hole- and electron-transport ability, that is, bipolar charge-transport property, and has been widely used for organic light-emitting diodes (OLEDs). In this study, we explain the bipolar charge-transport property of CBP by quantum chemical calculations for the crystal. Both the reorganization energies and the charge transfer integrals were investigated, and charge-transfer rate constants were calculated based on Marcus theory. The hole- and electron-transfer rate constants thus calculated were found to be similar in magnitude. This is in sharp contrast with the case of a structurally similar but a poor electron-transport material, N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD), for which the hole-transfer rate constants were calculated to be much larger than the electron-transfer rate constants. From the detailed analysis of the charge transfer integrals for CBP, it was found that holes transfer through all the segments of the molecules, using the delocalized HOMO over the whole molecule as in the case of TPD. On the other hand, electrons transfer advantageously through the delocalized LUMO on the carbazole moieties of CBP, which have close intermolecular contacts. The LUMO of TPD localizes on the central biphenylene moiety, resulting in small electron transfer integrals. The results clearly show the difference of the electron-transport property between CBP and TPD.