Insights into OLED functioning through coordinated experimental measurements and numerical model simulations

We present some typical applications of our device simulation model "MOLED" which has proven to be a very useful tool for obtaining insight into the functioning of multi-layer organic light-emitting devices (OLED's). Our general approach consists of combining experimental measurement and numerical device modelling in a deliberate and coordinated way. Taking a step-by-step, evolutive approach, starting with simple single layer devices and gradually moving up to more complex multi-layer architectures with a partially doped emission layer, we could progressively determine unknown parameters in the model and at the same time shed light on specific aspects of OLED functioning. For example, electrode characterization was done by measuring and modeling single layer devices with different electrode materials, while the simulation of time of flight signals gave us insight into the transient state and the effects of correlated disorder on the macroscopic mobility. The internal electric field in multi-layer OLED's could also be investigated by varying the thicknesses of the hole transport and electron transport layers. Applying this method to the standard OLED device structure that has received broad attention in the literature, we have found a number of surprising results. From our experiments, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. The device simulations fully clarify the situation, giving insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we found that there is a leakage of unrecombined holes towards the cathode at low voltages. We also found a strong variation of the electric field in the Alq3 layer due to space charge effects. By using the laser dye derivatives DCM-TPA with electron trapping capabilities and DCM-II with both electron and hole trapping capabilities as dopants in a standard OLED architecture, we could study the effect on transport and emission characteristics. In the case of the exclusively electron trapping dopant, a blue-shift of the emission color with increasing bias is observed which we find is due to a splitting of the recombination zone

Approches numériques du transport de charges dans les semiconducteurs organiques

Hocine Houili

Using plastics or dyes like molecular semiconductors to make optoelectronic devices at low cost is a project in industry that is of great interest to the "Information Community" of this beginning of the 21st century. Some applications of this technology have already been realized including flexible flat-panel displays made with organic light-emitting diodes, organic transistors for active matrices, organic solar cells and sensors. In all these applications it is essential to deduce and understand the laws that determine the charge transport and the light emission in these materials. The goal of this thesis is to contribute to the theoretical models that describe these transport processes. In order to predict the behaviour of charge carriers in a given device, one can choose two methods. One can either numerically solve the master equations that describe the general behaviour of charges and currents in each part of the device, or one can simulate the detailed behaviour of each charge in the device by fixing the conditions of its movement using a procedure known as "Monte Carlo". In this thesis I addressed the two approaches. I have been interested successively in the prediction of the electric properties of multilayer light-emitting devices, considered as a whole. Then I was able to understand in detail the processes accruing at the organic-organic interfaces by using the Monte Carlo method. My last project involved the study of the channel of an organic transistor. In each one of these cases I took account of the characteristic features of small molecule organic materials in order to develop models for charge transport for the study and optimization of two important types of organic devices : organic light-emitting diodes and field-effect transistors. Amorphous organic semiconductors are mainly used for the fabrication of organic diodes. They are characterized by an energetically disordered density of states that is assumed to be Gaussian. The transport of charges in these materials occurs via hopping from one molecule to another in this density of states. The correlations between the energies of the molecular sites have important effects on transport ; the dependence of mobility on the applied electric field in particular. If the energetic disorder is due to the random orientations of the permanent dipoles of the molecules, then the correlations between the orientations of these dipoles can profoundly change the spatial configuration of the energetic disorder and consequently the charge transport. Due to the fact that organic diodes often comprise of multiple organic layers, understanding device behaviour at the organic-organic interfaces is of critical importance. Electrons and holes accumulate close to these interfaces and they are therefore more likely to recombine and emit light. The Coulomb interactions between the charge carriers and the energy disorder are at the origin of complex processes taking place at these accumulation regions. I studied these processes through a 3D multi-particle Monte Carlo simulation. The short-range Coulomb repulsion forbids the double-occupancy of the sites and shifts the energy level for injection of carriers across the interface upward, thus increasing the electric current of the diode. The long-range Coulomb interactions increase the electric field and the current at the interface although this increase is less important than suggested by 1D models. Effects related to the hopping process such as the effect of the exact form of the hopping law and backward or return hop at the interface are clearly discussed. The Monte Carlo simulations are cross-compared with an analytical model for hopping transport across the interface. This comparison made it possible to understand other aspects of the physical processes at organic-organic interface and supports the Monte Carlo simulations. The study of the electron-hole recombination showed a decrease of the cross-section with an increase of the applied electric field, but this decrease is less than what can be expected by the classical Langevin theory. In spite of their efficiency, Monte Carlo simulations are not attractive when modelling complex multi-layer diodes. The 1D models are better suited in this case. A 1D model called MOLED, which was developed at Laboratoire d'Optoélectronique des Matériaux Moléculaires (LOMM) for the simulation of the organic diodes, is used and described in detail in this thesis. We used MOLED to explore the effect of the energy correlations on the electric field dependence of the mobility. In fact we simulated the time-of-flight experiment, a common method used to determine of the carrier mobility. The crystalline organic semiconductors achieve mobility performances close or even better than amorphous silicon. Their use for the fabrication of field-effect transistors is thus a good choice. The models for charge transport in crystalline organic materials differ much from their counterparts for amorphous organic materials. Indeed band models are more appropriate to describe charge transport in crystalline organic materials. However it is very important to take into account the effects of electronic polarization in these models. I developed a method of calculation of these effects of polarization in the bulk of the organic semiconductor and I extended it to the case of the interface with a dielectric insulator, a situation encountered in the channel of the organic transistor. These calculations showed that when the polarity of the dielectric increases the inter-molecular transfer integral decreases. This is confirmed by experimental measurements showing increasingly low values of mobility when the permittivity of the insulator increases.

EPFL2006

Internal electric field and charge distribution in multilayer organic light-emitting diodes

Detlef Berner, Eduard Tutis, Libero Zuppiroli

The internal electric field in multilayer organic light-emitting diodes (OLEDs) is investigated using a combination of experimental measurement and numerical device modeling. This approach results in a detailed understanding of the functioning of a multilayer OLED. The method is applied to a standard device structure that has received broad attention in literature. From the experimental part, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. Device simulations fully clarify the situation, giving an insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we find that there is a leakage of unrecombined holes towards the cathode at low voltages. We also find a strong variation of the electric field in the tris(8-hydroxyquinolinato)aluminum layer

AIP2003

Astrophysical applications of gravitationally lensed quasars

Alexander Eigenbrod

Gravitational lensing describes how light is deflected as it passes in the vicinity of a mass distribution. The amplitude of the deflection is proportional to the mass of the deflector, called "gravitational lens", and is generally weak, even for large masses. The faintness of this phenomenon explains why gravitational lensing remained essentially unobserved until the late 1970s (only gravitational lensing by the Sun has been observed during the solar eclipse of 1919). Before that time, gravitational lensing was considered merely as a theoretical curiosity. However, the situation dramatically changed with the discovery of the first extragalactic gravitational lens in 1979. Since then, together with the technological progress of astronomical instruments, gravitational lensing has turned from a curiosity into a powerful tool to address important astrophysical and cosmological questions. The present thesis focuses on applications related to gravitationally lensed quasars. Quasars are active galactic nuclei, where matter is heated up as it spirals down onto the central supermassive black hole. When a galaxy is located on the line of sight to a distant quasar, it acts as a gravitational lens and produces multiple images of this background source. The light of the quasar follows different paths for each of its images. Thus, variations of the intrinsic quasar luminosity are observed at different times in each image. The time delays between the images can be used to determine the Hubble constant H0, because they are inversely proportional to H0. This constant describes the current expansion rate of the Universe, and is one of the fundamental parameters of cosmological models. Many efforts have been spent over the years to determine H0, but its value is still poorly constrained. Gravitational lensing has the potential to noticeably decrease the uncertainty of H0. In practice, this requires regular and long-term monitoring of lensed quasars. We have run a series of numerical simulations to both optimize the available telescope time, and measure the time delays with an accuracy of a few percent. The results of these simulations are presented in the form of compact plots to be used to optimize the observational strategy of present and future monitoring programs. Once the time delays are measured, one can infer estimates of H0, provided several other observational constraints are available. A key element to accurately convert time delays into H0 is the redshift of the lensing galaxy. These redshift measurements are difficult because lensing galaxies are generally hidden in the glare of the much brighter quasar images. As a consequence, lens redshifts are often poorly constrained or even completely unknown. We have acquired spectroscopic data of sixteen lensing galaxies with the Very Large Telescope located in Chile. In combination with a powerful deconvolution algorithm, we determine the redshift of these sixteen lensing galaxies, which represents about 25% of all currently known quasar lensing galaxies. These results are useful for both H0 determinations and statistical studies of gravitational lenses, which can be used to provide new constraints on cosmological parameters. While the first part of this thesis focuses on the acquisition of observational constraints for the lens models, the main part consists in using the phenomenon of microlensing to determine the energy profile (or spatial structure) of quasar accretion disks. Microlensing is produced by the stars located in the lensing galaxy. These stars act as secondary lenses, and are called microlenses. Since the stars are moving in the galaxy, they induce flux and color variations in the images of the lensed quasar. These effects can be used as a natural telescope to probe the still mysterious inner structures of quasars with a spatial resolution about ten thousand times better than the capacities of current astronomical instruments, including the Very Large Telescope Interferometer. We present a three-year long spectrophotometric monitoring of the lensed quasar QSO 2237+0305, also known as the Einstein Cross, conducted at the Very Large Telescope. This monitoring reveals significant microlensing-induced variations in the spectra of the quasar images. In a subsequent analysis, we find that the source responsible for the optical and ultraviolet continuum has an energy profile well reproduced by a power-law R α λζ with ζ = 1.2 ± 0.3, where R is the size of the source emitting at wavelength λ. This agrees with the predictions of the standard thin accretion disk model and is, so far, the most accurate determination of a quasar energy profile. As a complement to our microlensing study, we have obtained high spectral and spatial resolution observations of the lensing galaxy of QSO 2237+0305. Our spectroscopic data are acquired with the SINFONI, FLAMES, and FORS2 spectrographs of the Very Large Telescope. We describe the reduction of these data, and provide the currently best and most complete determination of the kinematics of a gravitational lens. The comparison of our data with previously published dynamical models suggests that those may have overestimated the mass of the galaxy bulge. Thus, new and more sophisticated models are required. These models, combined with gravitational lensing, will provide two independent constraints on the mass distribution. This will allow to better determine the quantity and distribution of dark matter in this lensing galaxy, and especially in its extended halo.