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.
Hole mobilities of 4-diethylaminobenzaldehyde diphenylhydrazone (HDZ-F) doped poly(styrene) have been measured over a wide concentration range. The results have been described by a formalism based on disorder. The formalism is premised on the argument that charge transport occurs by hopping through a manifold of localized states that are distributed in energy and distance. The key parameter of the formalism is σ, the energy with of the hopping site manifold. For HDZ-F doped PS, σ is concentration-dependent. The maximum value is 0.121 eV and occurs at approximately 15% HDZ-F. The width decreases sharply for concentrations above and below. The concentration dependence is described by a model of dipolr disorder. The model is based on the assumption that the total widths are comprised of a dipolar component and a van der Waals component. The interpretation of the experimental results leads to the conclusion that the concentration dependence of the total width is largely determined by the van der Waals component.
Abstract Hole mobilities have been measured in bis (4‐ N , N ‐diethylamino‐2‐methylphenyl)‐4‐methylphenylmethane (MPMP) doped polystyrene (PS) and polycarbonate (PC) over a range of MPMP concentrations. The results are described by the disorder formalism. The formalism is based on the assumption that charge transport occurrs by hopping through a manifold of hopping sites subject to energetic and positional disorder. The results show that the width of the density of hoppiny states increases with increasing PC concentration and decreases with increasing PS concentration. The concentration dependencies are attributed to dipolar fields associated with dipole moments of MPMP or the host polymer. The positional disorder increases with increasing polymer concentration in both materials.
Hole transporting properties of tris-(8-hydroxyquinolinato) aluminum (Alq) were investigated using time-of-flight (TOF) photocurrent transients and photoinduced discharge techniques. A thin layer of Alq was inserted between two hole transporting layers. Within the time domain of the photocurrent transients,the fraction of holes escaping from one side of the Alq layer to the other side is strongly field, temperature, and Alq thickness dependent. The results of photoinduced discharge experiments indicate that eventually the holes escaped the trilayer samples. The injection barrier is estimated to be approximately 0.12 eV. The hole penetration range is estimated to be 10-30 angstrom in the range of field strengths studied, therefore suggesting the recombination zone of electrons and holes in Alq-containing electroluminescent (EL) devices is confined close to the interface of Alq and the hole transporting layer. The results demonstrate the importance of the hole injection barriers and hole trapping to the performance of organic EL devices using Alq as the electron transporting and emissive layer.
Hole and electron mobilities have been measured in vapor deposited layers of N-(p-(di-p-tolylamino)phenyl)- N ′ -(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (TAND). The TAND molecule contains a naphthalene diimide acceptor functionality and a triarylamine donor functionality. The mobilities are of comparable magnitude and show similar field and temperature dependencies. The results are described within the framework of a formalism based on disorder. The formalism is premised on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic disorder. The key parameter of the formalism is the energy width of the hopping site manifold. For TAND, the widths are 0.110 and 0.091 eV for hole and electron transport, respectively. The result leads to the conclusion that the hole and electron transport manifolds are independent and not influenced by the transport states of the oppositely charged carriers.
Abstract Photocurrent transients are determined in polycarbonate doped with 17 wt% of p‐p‐EFTP (see structure below) as a function of the electric field strength and the temperature. The hole mobilities are obtained both by a graphical method and by computational analysis based on an analytical expression for the transient photocurrent which has been recently proposed by Scott et al. The comparison of the results obtained with the two methods shows that, although the absolute values of the mobilities differ by a factor of 1.5, the field and temperature dependencies of the hole mobility are similar. The system studied shows an anomalous field dependence of the mobility which at low fields increases with increasing field to a maximum above which it decreases with increasing field. Such a behaviour is analysed in the framework of two different theoretical models: one based on the Marcus theory of electron‐transfer processes and Bässler's model which takes into account the fluctuations in hopping sites due to the simultancous presence of diagonal and off‐diagonal disorder. While the first model does not provide a satisfactory description of the experimental data, the latter cannot explain qualitatively the observed field dependence over a wide range of applied fields. The predicted saturation of the drift velocity of the holes at high fields does not agree with the observed temperature dependence of the mobility high fields. This disagreement with the conventional models is probably due to the large dimensions of the molecules allowing a change in the free energy for nearest‐neighbour hops which is large compared to the polaron binding energy or the width of the distribution of the density of states.
Abstract Site-selective fluorescence spectra have been measured to investigate the origin of the polaronic contribution to charge transport in 4-[bis(4-ethylphenyl)amino]phenyl-N,N,N′,N′-tetrakis(4-ethylphenyl)-[1,1′:3′1″-terphenyl]-4,4″-diamine doped poly(styrene). The spectra feature an unusually large Stokes shift of σ = 750 cm−1 and remain broad over the entire range of excitation energies. This is a clear indication of a torsional mode of relaxation that can account for the polaronic contribution observed in charge transport experiments.
Xerographic photoreceptors charge acceptance and potential discharge models experimental methods photogeneration theories photogeneration charge transport theories hole transport electron and bipolar transport photoreceptors summary and concluding remarks. Appendices: polymers generation materials transport materials.
