William T. Gruenbaum

Hole transport in tri-p-tolylamine doped polymers: the role of the polymer dipole moment

Abstract Hole mobilities have been measured in tri-p-tolylamine (TTA) doped polymers with polymer dipole moments that range from near-zero to 1.7 debye. The results are described within the framework of a formalism based on disorder, due to Bassler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameter of the formalism is σ, the variance of the hopping site energies. The principal observations of this work are: (1) σ increases with increasing intersite distance for all polymers studied, and (2) σ increases with increasing dipole moment of the polymer. The interpretation of the results leads to the conclusion that for weakly polar dopant molecules, a major contribution to the width of the distribution of hopping site energies is the component due to van der Waals forces. For TTA doped poly(styrene)s, the van der Waals component is estimated as between 0.074 and 0.116 eV, increasing with incresing intersite distance or decreasing TTA concentration.

HOLE TRAPPING IN TRI-P-TOLYLAMINE-DOPED POLY(STYRENE)

Abstract Hole mobilities have been measured in tri-p-tolylamine-doped poly(styrene) containing different concentrations of di-p-tolyl-p-anisylamine (DTA), di-p-anisyl-p-tolylamine (DAT), and tri-p-anisylamine (TAA). DTA, DAT and TAA are traps with depths of 0.08, 0.15 and 0.22 eV. The mobilities decrease with increasing trap concentration and trap depth while the field and temperature dependencies remain unchanged. The prefactor mobilities are independent of trap concentrations and depths. The results are discussed within the framework of the recent simulations of Wolf et al. The results are in good agreement and confirm the argument that for shallow trapping the basic phenomenology of transport, as revealed by the field and temperature dependencies of the mobility, remain unchanged. Quantitatively, the effects of traps can be accounted for by the replacement of the energy width of the hopping site manifold by an effective width whose square increases linearly with trap depth and the logarithm of the trap concentration.

Hole transport in vapor-deposited triphenylmethane glasses

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 Bassler 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.

Hole transport in bis(ditolylaminostyryl)benzene doped poly(styrene)

Hole mobilities have been measured in poly(styrene) (PS) doped with bis(ditolylaminostyryl)benzene (TASB). TASB is a weakly polar molecule that contains two aniline donor functionalities. The dipole moment is 0.54 D. The results are described by a formalism based on disorder, due to Bassler and coworkers. According to the formalism, charge transport occurs by hopping within a manifold of localized states that are subject to a distribution of energies. A key parameter of the formalism is the energy width of the hopping site manifold. The width is described by a model based on dipolar disorder. The model is based on the assumption that the total width is comprised of a dipolar component and a van der Waals component. For TASB, with a near-zero dipole moment, the dipolar component vanishes and the total width becomes equal to the van der Waals component. For TASB, the van der Waals component is 0.102 eV and is independent of concentration. The absence of a concentration dependence is attributed to an intramolecular interaction between the two aniline functionalities associated with each TASB molecule.

Hole trapping in molecularly doped polymers

Hole mobilities have been measured in di-p-tolylphenylamine-doped poly(styrene) containing 1-phenyl-3-p-diethylaminostyryl-5-p-diethylamino-phenylpyrazoline (DEASP), an 0.38 eV trap. For molar concentrations of less than a few multiples of 10−7, DEASP has no effect on the mobility. For concentrations in excess of 10−6, the mobility decreases with concentration as c−1.5. A concentration of 10−3 suppresses the trap-free mobility by approximately 5 orders of magnitude. The results are described within the framework of a formalism due to Hoesterey–Letson and the recent simulations of Wolf et al. and Borsenberger et al.

HOLE TRANSPORT IN TRIPHENYLMETHANE DOPED POLYMERS

Hole mobilities have been measured in poly(styrene) (PS) doped with a series of triphenylmethane (TPM) derivatives with different dipole moments. The results are described within the framework of a formalism based on disorder, due to Bassler 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. The key parameter of the formalism is the energy width of the hopping site manifold. For TPM doped PS, the widths increase with increasing TPM concentration and increasing dipole moment. The 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, while 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 (T AA) molecules. The difference in the van der Waals components is the principal reason for the difference in mobility of TPM and T AA doped polymers.

Hole transport in vapor deposited bis(ditolylaminostyryl)benzene

Hole mobilities have been measured in vapor deposited bis(ditolylaminostyryl)benzene (TASB). TASB is a weakly polar molecule that contains two aniline donor functionalities. The results are described by a formalism based on disorder, due to Bassler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states that are distributed in energy. The key parameter of the formalism is the energy width of the hopping site manifold. The width is described by a model based on dipolar disorder. According to the model, the total width is comprised of a dipolar component and a van der Waals component. For TASB, the dipolar component is 0.012 eV and the van der Waals component 0.091 eV. The van der Waals component is considerably larger than values reported for other triarylamine donor glasses. This is attributed to differences in stereochemistry of the aniline donor functionalities.

High-Mobility Doped Polymers

Hole mobilities have been measured in 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) doped in a series of segmented thermoplastic polymers. For TAPC concentrations of 25 wt%, the mobilities are as high as 3×10-3 cm2/Vs. To our knowledge, these values are a factor of 100 higher than any hole mobilities described in the literature for this dopant concentration.

The Effect of Dopant Concentration on the Mobility of a Triphenylmethane Doped Polymer

Hole mobilities have been measured in poly(styrene) (PS) doped with bis(4-N,N-diethylamino-2-methylphenyl) (4-4-methoxyphenyl)methane (TPM). TPM is a weakly polar donor molecule with a dipole moment of 2.1 D. The results are described by a formalism based on disorder. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The parameters of the formalism are σ, the energy width of the hopping site manifold, Σ the degree of positional disorder, µ0 a prefactor mobility, and ρ0 a wavefunction decay constant. The energy widths are between 0.105 and 0.113 eV, increasing with increasing TPM concentration. The concentration dependence of the widths is described by an argument based on dipolar disorder. Values of Σ are between 2.4 and 3.8, increasing with increasing dilution. The prefactor mobilities are between 1.6×10-5 and 7.4×10-2 cm2/ Vs and can be described by a wavefunction decay constant of 1.2 A.

Effects of Dopant Concentration on the Mobilities of Molecularly Doped Polymers

Hole mobilities have been measured for poly(styrene) (PS) doped with triphenylmethane (TPM) and triarylamine (TAA) derivatives with the same dipole moment. The results are described by a formalism based on disorder. According to the formalism. charge transport occurs by hopping through a manifold of localized states that are distributed in energy. The key parameter of the formalism is the width of the hopping site energies. For TPM doped PS, the widths decrease with increasing dilution while for TAA doped PS, the widths increase with dilution. The widths are described by a model based on dipolar disorder. The model is based on the assumption that the total width is comprised of a dipolar component and a van der Waals component. The selection of dopant molecules with the same dipole moment provides a method by which the van der Waals component can be determined from an analysis of the total widths of both. For TPM doped PS, the van der Waals component is constant while for TAA doped PS the van der Waals component increases with increasing dilution. The difference is described by a charge delocalization argument.