Hole Transport in Vapor-Deposited Triphenylmethane Glasses

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Hole mobilities have been measured in a series of vapor-deposited triphenylmethane (TPM) glasses with different dipole moments. The results are described by a formalism based on disorder, due to Bässler and coworkers. The formalism is premised on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. A key parameter of the formalism is the energy width of the hopping site manifold. For TPM glasses, the width is between 0.093 and 0.123 eV, increasing with increasing dipole moment. The width is described by a model based on dipolar disorder. The model assumes that the total width is comprised of a dipolar component and a van der Waals component. The dipolar components are between 0.037 and 0.089 eV, increasing with increasing dipole moment. The van der Waals components are approximately 0.085 eV, and independent of the dipole moment. The van der Waals components are significantly larger than literature values reported for a wide range of triarylamine (TAA) glasses. The difference between the van der Waals components is the principal reason for the differences in mobility between TPM and TAA glasses and is attributed to differences in charge delocalization of the TPM and TAA molecules.