THE ROLE OF DIPOLE-MOMENTS ON HOLE TRANSPORT IN TRIPHENYLAMINE-DOPED POLYMERS

Hole mobilities were measured in a series of triphenylamine (TPA) mole cules with different dipole moments doped into apolar and highly polar poly(styrene)s. The results are described by a formalism based on dis order, due to Bassler and coworkers. The formalism is premised on the assumption that charge propagation occurs by hopping through a manifol d of localized states with superimposed energetic and positional disor der. A key parameter of the formalism is the energy width of the hoppi ng site manifold, or DOS. For the apolar poly(styrene), the width of t he DOS increases with increasing dipole moment of the TPA molecule, wh ereas for the highly polar poly(styrene), the width is independent of the dipole moment. The results are explained by an argument based on d ipolar disorder. The argument is premised on the assumption that the t otal width is determined by dipolar components due to the dopant molec ule and the polymer repeat unit, and a van der Waals component. For th e apolar poly(styrene), the width is determined by a TPA dipolar compo nent that increases with increasing dipole moment of the TPA molecule and a van der Waals component of 0.077 eV. For the highly polar poly(s tyrene), the total dipolar component is 0.090 eV, independent of TPA d ipole moment, and the van der Waals component 0.090 eV. (C) 1996 John Wiley & Sons, Inc.