The photoconductive response of a sensitized molecularly doped photoconductor is reported. The photoconductor consists of triphenylamine dispersed in a bisphenol-A-polycarbonate polymer, sensitized by a thin layer of vacuum-deposited amorphous selenium. The wavelength and field dependence of the photoinjection efficiency has been measured using photoinduced discharge techniques. An efficiency of 0.7 was measured at a field of 6.4×105 V/cm.
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.
Electron mobilities have been measured in N,N′-bis(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide doped poly(styrene) containing a series of acceptor traps: 4-(cyanocarboethoxymethylidene)-2-methyl-1,4-naphthoquinone (MNQ), 3,5-dimethyl-3′,5′-diisopropyl-4,4′-diphenoquinone (DPQ), 4H-1,1-dioxo-2,6-di-tert-butyl-4-(dicyanomethylidene)thiopyran (TBS), N,N′-dicyano-2-tert-butyl-9,10-anthraquinonediimine (DCAQ), and 4H-1,1-dioxo-4-dicyanomethylidene-2-p-tolyl-6-phenylthiopyran (PTS). From reduction potential measurements, the trap depths of MNQ, DPQ, TBS, DCAQ, and PTS are 0.19, 0.19, 0.20, 0.35, and 0.40 eV, respectively. The mobilities decrease with increasing trap depth and trap concentration. The results are discussed within the framework of the Hoesterey-Letson formalism and the recent simulations of Wolf and co-workers and Borsenberger and co-workers.
Hole mobilities of vapor-deposited p-diethylaminobenzaldehyde diphenylhydrazone glasses have been measured over a range of temperatures that includes the glass transition temperature Tg. Discontinuities in the temperature dependence were observed at Tg. For T > Tg, the activation energy is approximately 1/2 its value for T < Tg. The results are described within the framework of a formalism based on disorder, due to Bässler 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 and distance. The key parameters of the formalism are σ, the energy width of the hopping site manifold, Σ the degree of positional disorder, and μ0 a prefactor mobility. For T < Tg, the results yield σ = 0.106 eV, μ0 = 4.6×10—3 cm2/Vs, and Σ = 1.0. Analyzing the data for T > Tg leads to the conclusion that σ increases with increasing temperature while Σ remains constant.
The field and temperature dependencies of the hole mobility of 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) doped polystyrene have been measured and compared to results obtained for the TAPC doped polycarbonate and pure TAPC. The results are described by the disorder formalism, due to Bässler and co-workers. The mobility of TAPC doped polystyrene is approximately 100-fold greater than that observed for TAPC doped polycarbonate. This effect is interpreted in terms of (1) the elimination of random dipolar fields due to static dipole moments of the polycarbonate that affect the energetic disorder, and (2) improved electronic intermolecular coupling with a concomitant reduction of positional disorder.
The field and temperature dependencies of free carrier photogeneration efficiencies of vapor deposited molecular glasses have been studied by time-of-flight photocurrent techniques. The measured photogeneration efficiencies were analyzed by a theory of geminate recombination due to Onsager. In spite of the wide distribution of the charge mobilities and molecular dipole moments of the materials studied, thermalization distances and primary quantum yields were 27–36 Å and 10−3 to 10−2, respectively. The results suggest that the thermalization process in molecular glasses is not driven by processes that determine bulk transport properties.
