Percolation paths for charge transports in N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD)

We investigated charge transports in N,N′-diphenyl-N,N′-di(m  -tolyl)benzidine (TPD), a well-known hole-transport material for organic light-emitting diodes (OLEDs), in the orthorhombic and monoclinic polymorphs. Based on Marcus theory, charge-transfer rate constants in both the polymorphs were investigated by calculating the reorganization energies and charge transfer integrals. The rate constants for hole transfers, kCT+, were calculated to be two to three orders of magnitude larger than those for electron transfers, kCT-, for both the polymorphs. The small kCT- values result not only from the large reorganization energies for electron transfers but also from the small electron transfer integrals. To investigate the charge transfer integrals in more detail, the contributions of respective moieties of TPD molecules, nitrogen, central biphenylene, outer phenyl, and outer tolyl moieties, to charge transfer integrals were analyzed separately. From the analysis, the small electron transfer integrals compared to hole transfer integrals were found to originate from little contribution of LUMO to the outer phenyl and outer tolyl moieties, which has close intermolecular contacts. In contrast, HOMO spreads over these moieties, which results in relatively large hole transfer integrals. Holes can transport through these moieties with close contacts. Percolated hole-transport paths, consisting of consecutive molecular pairs with large hole transfer integrals, and therefore, large kCT+, exist in both the polymorphs, showing the good hole-transport property.