Paul M. Borsenberger

Hole photogeneration in poly(N‐vinylcarbazole)

The photogeneration of holes in poly(N‐vinylcarbazole) has been investigated as a function of applied electric field, wavelength, and temperature. For fields above 3.0×104 V/cm, the field and temperature dependences of the photogeneration efficiency are in good agreement with predictions based on the Onsager theory of geminate recombination. The quantum efficiency can be described by a thermalization distance which varies from 26 to 30 A as a function of wavelength and a primary quantum yield which varies between 0.20 and 0.03 as a function of temperature. The photogeneration efficiency increases in steps with decreasing wavelength. Transitions occur between the ground state and the first, second, and third excited singlet states. For a given transition, the efficiency is constant and depends only upon the electric field and temperature. The increase in efficiency which occurs with decreasing wavelength can be explained by an increase in the thermalization distance. The primary quantum yield shows an exponential dependence on reciprocal temperature with an activation energy of 0.05 eV. The thermalization distance is independent of temperature and determined only by the excitation wavelength.

Hole transport in tri‐p‐tolylamine‐doped bisphenol‐A‐polycarbonate

Hole mobilities of binary solid solutions of tri‐p‐tolylamine and bisphenol‐A‐polycarbonate have been measured as functions of electric field and temperature for concentrations between 6% and 50% tri‐p‐tolylamine. The mobilities can be described by an exponential dependence on the square root of the electric field and a temperature‐dependent activation energy. The results are discussed within the framework of a model involving hopping within a Gaussian distribution‐of‐states broadened by disorder.

A comparative study of hole transport in vapor-deposited molecular glasses of N,N′,N″,N‴-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine

Abstract Hole mobilities have been measured in vapor-deposited layers of N,N′,N″,N‴-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TTB) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD). The results are described within the framework of a formalism based on disorder, due to Bassler and co-workers. 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 parameters of the formalism are σ, the energy width of the distribution of hopping site energies, and Σ, the degree of positional disorder. For TTB, σ = 0.078 eV and Σ = 1.7. The values for TPD are 0.077 eV and 1.6. The width of the hopping site energies can be described by a model based on dipolar disorder. The degree of positional disorder is in agreement with results observed for a wide range of vapor-deposited molecular glasses and is attributed to packing constraints. The results show that transport in both compounds can be described by simple disorder-controlled hopping without invoking polaronic contributions.

Hole transport in binary solid solutions of triphenylamine and bisphenol‐A‐polycarbonate

By means of potential discharge techniques, hole transport has been investigated in binary solid solutions of triphenylamine and bisphenol‐A‐polycarbonate. Charge was injected from a photoemitting electrode of amorphous selenium. Under space‐charge‐limited conditions, hole transport can be described by a drift mobility that is linearly dependent upon the electric field. For fields of 105 V/cm, values of the mobility were between 10−7 and 10−5 cm2/V s, depending upon the temperature and concentration. The variation with concentration suggests that hole transport occurs by phonon‐assisted hopping between triphenylamine molecules with a charge localization radius of approximately 1.8 A.

An aggregate organic photoconductor. II. Photoconduction properties

The photoconductive properties of an organic aggregate photoconductor are reported. This material shows both electron and hole conduction. The photogeneration efficiency is field dependent and about 0.5 at 106 V/cm. The field and temperature dependences for hole generation are qualitatively consistent with the Onsager theory of geminate recombination. The corresponding dependences of the electron‐generation process are obscured by a field‐dependent trapping. Drift mobilities, while not directly measured, appear to be in the range of 10−8 cm2/V s. At fields of 6×104 V/cm, the electron and hole ranges are at least 10−8 cm2/V.

Hole transport in tri‐p‐tolylamine‐doped polymers

Hole mobilities have been measured in tri‐p‐tolylamine‐doped polymers as functions of the polymer composition, temperature, and electric field. Depending on the composition of the polymer, the mobilities vary by over two orders of magnitude. In all cases, the electric field and temperature dependencies can be described as exp(βE1/2) and exp−(T0/T)2, respectively. The effect of the host polymer on the mobility is largely related to the prefactor μ0. It is suggested that the dependence of μ0 on the polymer composition is related to the wave‐function overlap factor, 2αR, and due to differences of the ionization potential of the host polymer.

Transient photocurrents across organic–organic interfaces

Hole photocurrent transients in organic–organic bilayers are described. Transitions in the photocurrents for holes moving across the organic–organic interfaces are observed. The magnitude of the photocurrent increases (or decreases) when holes transfer from a lower (or higher) mobility material into a higher (or lower) one. These results demonstrate a novel technique for studying energetic barriers and hole injection dynamics at organic–organic interfaces.

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.

Tail Broadening of photocurrent transients in molecularly doped polymers

Tail broadening of nondispersive photocurrent transients has been measured in 1,1‐bis(di‐4‐tolylaminophenyl)cyclohexane‐doped polystyrene over a range of concentrations, fields, and temperatures. The results are described by the parameter W, defined as W=(t1/2−t0)/t1/2, where t1/2 is the time for the transient to decay to one‐half of its plateau value and t0 the time defined by the intersection of asymptotes of the plateau and trailing edge of the transient. The interpretation of the experimental results leads to the conclusion that energetic disorder is the principal source of broadening of the transients.

Electron transport in N,N'-bis(2-phenethyl)-perylene-3,4:9,10-bis(dicarboximide)

By time‐of‐flight techniques, electron mobilities have been measured in vapor deposited films of the title compound. The results were compared to predictions of the disorder formalism, due to Bassler and co‐workers, and models based on polaron formation. The results lead to the conclusion that fluctuations in hopping site energies are the major contribution to the activation energy. For consistency between experiment and predictions of the formalism, however, an additional source of activation is required. The source of this activation is believed due to either polaron formation or trapping. The width of the hopping site manifold, σ, is determined as 0.080 eV and the positional disorder parameter, Σ, as 1.0. The interpretation of these results by a model in which transport occurs only by polaron displacement leads to inconsistencies with both the temperature and field dependencies of the mobility.