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We have examined the effect of the post-processing of spin-coated tetracene and DAST (4′-dimethylamino- N -methyl-4-stilbazolium tosylate) films in their solvent vapor to examine the possibility of precisely controlling the vapor pressure and studying the effect of applying electric fields to their structural properties. It was demonstrated that the solvent concentration in the vapor phase can be monitored by infrared spectroscopy. The thickness uniformity of the tetracene films was improved by the formation of a mixture of the tetracene crystals with the saturated solution (“solution ultrathin film”) on the surface. Crystals with an ordered orientation were obtained by additionally applying an electric field during the vapor exposure of the DAST. The present technique for monitoring the solvent vapor pressure is also applicable for controlling the drying speed during the inkjet printing of functional organic materials.

By using stochastic mechanical simulations, we numerically investigate electron transfers in oligoacenes (anthracene, tetracene and pentacene). These π-conjugated oligomers are applied to organic devices and the performance depends on the electronic transfer properties. From quantum mechanical or quantum chemical calculations, estimation of the transport properties have been so far discussed via transfer integrals. Stochastic mechanics provide the quantum motion of electrons from the wave functions. From the analysis of the quantum motion of electrons, we calculate some dynamical properties, such as mean-square displacement relating with the mobility and the electron transfer rate between two anthracenes. Our dynamical approach is efficient and practical and especially important for analysis of molecular or nanoscale devices.

This paper examines the influence of charge carrier diffusion on photoenhanced current characteristics. The results of our analysis indicates that in the case of thin layers the diffusion of charge carriers leads to the following type of photoenhanced current–voltage relationship: j∼Un with 1≤n

Steady state and pulsed photoconduction ( i ) and photovoltaic ( U oc , i sc measurements were made on vacuum-evaporated tetracene films sandwiched between two aluminium or two gold semitransparent electrodes. The properties of the cells are shown to be strongly dependent on whether the substrate (S) or the non-substrate (NS) electrode is illuminated. A theoretical model is presented to explain the photoconduction and photovoltaic characteristics of the cells. The model is based on the assumption that charge carriers (holes) are injected into the band and to surface traps at the contacts by triplet excitons created by the exciting light in the tetracene layer. The charge introduced into the sample, in turn, affects the diffusion flux of triplet excitons reaching the electrodes. The energy and space distributions of traps are different at the S and the NS electrode. This causes triplet excitons to be more strongly quenched on their way to the S electrode than to the NS electrode. Predictions of the model are shown to agree with the experimental action spectra and the dependence of i , U oc and i sc on the light intensity I o . In particular, it allows the sublinear dependence of the emission-limited photocurrent on I o and the characteristic reversals of sign in the action spectra and in the dependence of U oc on I o to be explained. The cells studied entirely under vacuum (2 x 10 -3 Pa) showed no chane in the shape of the action spectra but the value of U oc decreased by a maximum of about one order of magnitude. The influence of the temperature could be rationalized by a temperature-induced variation in the trap parameter l and a change in the ratio of the dark to the photoinjection efficiency.

Tetracene, pentacene and nickel phthalocyanine films were prepared by vacuum deposition onto glass substrates. Their reflection spectra on illumination of the substrate (S) and the non-substrate (NS) surfaces were measured as a function of the sample thickness d. The reflectivity R and band shape of the spectra change with d and differ when measured from the S surface and the NS surface. These changes and asymmetries are interpreted as due to four factors: (i) the difference in surface roughnness, (ii) the difference in the refraction index at the film-air interface (NS) and the film-glass interface (S), (iii) the presence of stresses in the films and (iv) the Rayleigh-type scattering of light enhanced by the contact medium. A detailed analysis indicates the first and second factors to be dominant. Previous data for asymmetries in photoconductivity and photovoltaic effects can be consistently explained in terms of roughness-induced differentiation in the energy and spatial distributions of the structural defects in the near-surface layers of such films.

The charge carrier drift mobility in tetracene layers was measured using the time-of-flight method. Free carriers were generated by short electron pulses. At electric fields E > 2 × 10 4 V cm −1 the drift velocity of holes is constant, i.e. the drift mobility is proportional to the reciprocal electric field: μ h ∝ E −1 .

Metal-insulator-metal and semiconductor-insulator-metal cells containing tetracene (T) sandwiched between two electrodes (two different combinations of aluminium, gold or CuI) were prepared and their photoconduction properties were studied. It was found that the relationship between the first band of the absorption spectrum of polycrystalline tetracene and the action spectra of the photocurrent is determined by the dark injection efficiency of the illuminated contact and the conditions of illumination. An antibatic response in the Al/T/Al system at low voltages U and high active light intensities I o transforms into symbatic behaviour at high U and low I o .

Electronic transport phenomena in thin polycrystalline organic films were studied using transient currents and a.c. techniques. The results obtained for tetracene and p-terphenyl layers are very similar and may be explained by a hopping model. Additional effects are caused by inter-grain barriers. The heights of these barriers (0.07–0.08 eV) are reduced by an external electric field.

An investigation of electrical conductance switching phenomena in thin organic films has been carried out. For thin films of p-quaterphenyl and tetracene grown in high vacuum the electrical conductance, dynamic capacitance and temperature patterns corresponding to the distribution of electrical current density have been measured. The measurements were carried out in vacuum as well as in humid air. It is suggested that previously reported switching phenomena are caused by field-induced bipolar injection of charge carriers.

We measured the charge carrier mobility and the a.c. conductivity in polycrystalline tetracene layers. At low frequencies the a.c. conductivity is controlled by intergrain barriers. The mobility and a.c. measurements both give approximately 0.07 eV for the heights of these barriers. At high frequencies the a.c. conductivity refers to the inside of the grains and may be interpreted as hopping transport in localized states in a band situated in the region of the Fermi level.

A novel method of measuring the permeation of oxygen through thin films based on the oxidation of aluminium is described and results obtained for polycrystalline thin films of tetracene are reported. Thus for a tetracene film 375 nm thick the permeation was determined to be 0.39 × 1014 molecules sec-1 cm-2, a rate sufficiently large to cause the formation of additional interfaces in multilayer devices incorporating reactive materials, thereby inducing a drift in device characteristics. The results suggest that permeation is associated with the high density of grain boundaries.