J. Godlewski

Injection-limited currents in insulators

A general treatment of the injection-limited currents (ILC) at the insulator intersurface, based on the continuity equation including source current, is presented. Various approximations are discussed of which the one-dimensional Onsager model is a particular case for a weak gradient source current or the injection distance approaching zero. A coordinate-dependent geminate recombination is introduced into this model. Formal requirements leading to analytical, tractable solutions divide the electric field (F) range into two regimes: low-field regime (LFR) (F 5.2 eT2 V/m). At room temperature T=293 K and with typical dielectric permittivity e=4, FLFR 106 V/m. In the approximations discussed the current-field dependence is linear or sublinear in the LFR and follows the function j ∞F3/4 exp (aF1/2) (a=constant) in the HFR. Comparison of the theory with the experimental data shows that the current changes at the predicted division value of the field and shows good agreement in the HFR. In the LFR, experimentally observed ILC follow the function j ∞exp (-bF-1/2) (b=constant). It is shown that it is caused by surface-trap limitation of the injection currents (STLIC).

Photoelectric properties of heterojunctions formed from di-( pyridyl ) -perylenetetracarboxylic diimide and copper phthalocyanine or pentacene

The origin of conductivity in di-(pyridyl)-perylenetetracarboxylic diimide (Py-PTC) is investigated. The obtained results allow us to classify Py-PTC as n-type material. Heterojunctions formed from Py-PTC and copper phthalocyanine or pentacene reveal reasonable photovoltaic properties.

Macrotrap model for charge-carrier transport in low-mobility solids

The neutral macrotrap model of charge carrier trapping has been developed to explain the electric field (F) and temperature (T) dependence of the effective charge carrier mobility (µeff) in low mobility solids. The potential barrier for the carriers localized in the neutral macrotraps (spatially extended trapping domains) can be effectively lowered by an external electric field accessible in the experiment, making the mobility and its thermal activation field dependent. The potential of a neutral macrotrap is approximated by Φ(r)=(3kT/eσ) ln (r0/r), where r is the distance from the center of the macrotrap, r0 stands for its radius, e is elementary charge and σ is a characteristic parameter of the exponential energy distribution of point traps composing the macrotrap. The lowering of the barrier is proportional to F at low fields and to ln F at high fields. The field behavior of the effective mobility is governed by this lowering of the barrier and by its field-dependent position which, therefore, can be reached by the thermally activated carriers with a field-dependent probability in the process of carrier diffusion. The latter introduces a factor exp (-3kT/σelDF) (lD-diffusion length of the carriers) responsible for the self-consistency of the field dependence of the mobility and the Arrhenius temperature term exp [-E(F, T)/kT]. This model provides a consistent description of representative experimental data reported for low-carrier mobility crystalline and disordered solids.

Asymmetries in the optical properties of vacuum-deposited organic films illuminated at the substrate and non-substrate surfaces

Abstract 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.

Electroluminescence in Tetracene Crystals

Abstract dc and pulsed electroluminescence (EL) in tetracene crystals have been studied at room temperature. The crystals are provided with Na/K alloy electron injecting contact and Au semitransparent layer as a hole injecting contact. dc EL intensity shows a superlinear relationship with crystal current. EL decay indicates that the electroluminescence in tetracene consists of two components: one prompt and a second delayed component. The results can be understood if the delayed component depends upon crystal current and upon some characteristics of the active traps in the crystal. The spectral distribution of the electroluminescence as compared to photoluminescence spectrum reveals the intensive long wavelength emission. It is presumably due to trapped excitons.

Spatial Extension of Charge Traps: Consequences for Space-Charge-Limited Currents

Abstract It is proposed that charge carriers are trapped by spatially extended domains (macrotraps) produced by physical perturbations of crystal lattice. These extended domains consist of local traps (microtraps) with energy (E) distributed in space (r) such that E = (3kT/[sgrave])ln(ro/r), where [sgrave] is a characteristic parameter of the exponential energy distribution function, and r0 is the radius of the macrotrap. The steady-state space-charge-limited currents (SCLC) are studied and interpreted by invoking the macrotrap concept. The results indicate that the potential of the macrotraps can be effectively modulated by an external electric field accessible in experiment and from the power law j ∼ Un (n > 2) of the measured current (j versus voltage (U) [sgrave] can be determined (n = 2 + 3/[sgrave]). In particular, we examined high-quality anthracene crystals for which two distinct distributions of microtraps have been found with [sgrave]1 = 1 ± 0.2 and [sgrave]2 = 0.3 ±; 0.05. Typical values of the...

High-field recombination electroluminescence in vacuum-deposited anthracene and doped anthracene films

The electroluminescence (EL) study of anthracene and tetracene-doped anthracene films deposited by a vacuum evaporation onto a substrate at temperatureT0=−60 °C is reported. Such films show high resistence to electrical breakdown so that their EL behaviour at high electric fields up to above 106 V/cm could be investigated. The high-field EL is interpreted in terms of electron-hole recombination mechanisms with creation of emitting singlet excitons; the electron and hole concentration being limited by injection at metal contacts and by field-independent carrier velocities. The theoretical expression for current density follows the experimental data for fields above 4×105 V/cm at room temperature and the EL intensity is predicted to be a power function of the measured current. It is suggested that in pure anthracene layers the power is determined by the contact injection conditions, i.e. by barrier heights and the effective masses of holes and electrons. In the doped films a guest molecules-induced energy trap distribution leads to a decrease of the power as confirmed by the experimental plots of the host EL intensity versus current density.

Spatial Distribution of the Distribution of the Electroluminescence and the Recombination Process in Tetracene Single Crystals

Abstract The spectral dependence of the recombination electroluminescence (REL) to photoluminescence (PH) ratio has been used as an experimental probe for studying the physical characteristics of the recombination in tetracene. A method is developed for determination of REL spatial distribution. In allcrystals studied, under conditions where two injecting contacts are used, the light emission is concentrated in front of the electrodes. At a sufficiently high voltage applied, a splitting up of the light emission zone in two layers is found in front of the anode. These experimental results can be explained qualitatively assuming appropriate gradients of charge densities, charge-quenching of the delayed component of the REL and charge densitydependence of the probability of a recombining electron-hole pair to yield a singlet exciton.

Photoenhanced current in thin organic layers

Experimental dependences of photoenhanced current (PEC) on wavelength, voltage, light intensity and temperature carried out on polycrystalline tetracene, pentacene and perylene layers are presented. As it is shown, the description of PEC in thin organic layers requires introducing the Gaussian trap distribution as well as diffusion of both excitons and charge carriers into the classical model of PEC.

Electric field dependence of the bimolecular recombination rate of the charge carriers

The dependence of the bimolecular rate constant on the external electric field in organic systems characterised by low charge carrier mobility was investigated. From analytical approximations and numerical calculations, the bimolecular rate constant is found to decrease with increasing electric field and depends on the charge carrier concentration.