Hole Transport in Triphenylmethane-Doped Poly(styrene)

Abstract

Hole mobilities have been measured for a series of triphenyl-methane (TPM) derivatives with different dipole moments doped into poly(styrene) (PS). The results are described within the framework of a formalism based on disorder, due to Bässler and coworkers. The formalism is premised on the assumption that transport occurs by hopping through a manifold of localized states that are subject to a distribution of energies and distances. The key parameters of the formalism are the energy width of the hopping site manifold, the degree of positional disorder, and a prefactor mobility. For TPM-doped PS, the widths are between 0.104 and 0.124 eV, increasing with increasing TPM concentration and increasing dipole moment. Values of the positional disorder parameter are between 2.0 and 4.5, increasing with increasing dilution. The prefactor mobilities decrease with increasing dilution and can be described by wavefunction decay constants of approximately 1.0 Å. The energy widths are described by a model based on dipolar disorder. According to the model, the widths are comprised of a dipolar component and a van der Waals component. The dipolar components are between 0.012 and 0.067 eV, and the van der Waals components are 0.104 eV. The van der Waals components are significantly larger than literature values for PS doped with a wide range of triarylamine (TAA) molecules. The difference in the van der Waals components is the principal reason for the very considerable difference in mobility of TPM- and TAA-doped polymers. For constant dopant concentrations, the degree of positional disorder and the prefactor mobilities are essentially the same for all TPM- and TAA-doped polymers.