Approches numériques du transport de charges dans les semiconducteurs organiques

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.

Interface issues in organic semiconductor based devices

Mauro Castellani

The present doctoral thesis aims to contribute to the field of organic semiconductor physics and technology, both of which have become fast growing disciplines. Two technological applications are emerging from these research efforts: Organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). OLEDs are already being fabricated on an industrial level today, while some fundamental issues still remain to be resolved for OFETs. During the last ten years, research interest has been continuously shifting from the bulk properties of organic semiconductors to their interfaces. Such issues are in fact addressed in this thesis work and include different interactions of thin organic layers with their environments. In the field of OLEDs, a multitude of issues have been investigated here. Most importantly and in contrast to most publications in this field, OLED's with complex device architectures that fully compete with standard devices performancewise have been fabricated and characterized. An oxygen plasma treatment of indium-tin-oxide anode has been shown to reduce the injection performance while enhancing the quantum efficiency. Two different mechanisms are proposed to explain this. Going a step further, the life-time behavior of such devices has been examined. Different aging mechanisms were identified: in short term and intermediate term aging, an oxygen plasma treatment of anode clearly reduces the operational voltage. Bulk degradation of the organic electroluminescent layer then becomes predominant for long term aging. Another important finding concerns the formation of exciplexes at organic/organic heterojunctions: In devices comprising a high hole injection barrier between the hole-transport and the electron-transport layer, the dominant formation of exciplexes is identified by a red-shifted shoulder in the electroluminescent spectrum and a low quantum yield. In contrast to that, the quantum efficiency is increased markedly by minimizing this hole barrier. In such devices, the formation of exciplexes is beneficial for device performance, since it facilitates hole transfer across the heterojunction. For the first time, the operational currents could be divided into different radiative and non-radiative recombination channels of exciplexes and excitons respectively. This quantification was further developed to simulate current-versus-voltage behavior and quantum efficiency. The work on pentacene based devices was initiated by fabricating ultrathin pentacene films on polished sapphire and by characterizing them with a temperature variable four probe setup. Oxygen plasma treatment of the substrate introduces oxygen radicals onto the surface and subsequent pentacene deposition leads to electron extraction from the pentacene molecules to defects at the oxide surface. Such an increase of charge carrier density within the semiconducting layer enhances its conductivity. Using a thick, aliphatic self-assembled monolayer (SAM) as an intermediate layer, both the activation energy and the film resistance increase by at least an order of magnitude compared to devices without SAM. Hence, charge transfer to the substrate is inhibited. The OFETs fabricated for this thesis work are all based on pentacene as a semiconductor and the gate interface was the focus of interest. Two different device types were fabricated: One device category comprises ultra-thin film OFETs (10nm). In such devices contact resistance is negligible. This gives rise to a dependence of device behavior on the gate interface. For this reason, the dielectric constant of the gate insulator was varied by changing the gate materials: the hole mobility is enhanced four-fold by using an aliphatic SAM at the gate interface instead of an oxygen plasma treated alumina (Al2O3). This effect has been interpreted in terms of self-trapping at the interface and its reduction by a low-k dielectric gate. In addition the on-off ratio was enhanced by a factor of 20, thanks to the SAM-inhibited pentacene doping due to a reduced semiconductor/substrate interaction. The second OFETdevice category was based on thicker (80nm) pentacene films. Their behavior is injection controlled. This finding originates in the application of a top contact configuration which implies that the carriers cross the granular material to reach the channel. Impedance spectroscopic measurements on different metal-insulator-semiconductor diodes unveiled a morphology dependent transveral mobility, which is linked to the degree of structural disorder in the film. In conclusion: Understanding of the various physical processes occurring at the different interfaces (anode and organic/organic interfaces in OLEDs, gate interface in OFETs) has considerably improved the performance of organic semiconductor based devices.

EPFL2006

Dynamics of Electronic and Ionic Charges in Cyanine Organic Semiconductor Devices

Sandra Christina Jenatsch

Organic semiconductor materials have been widely applied in optoelectronic devices to replace their inorganic counterparts and explore new fields of applications. Due to their high extinction coefficients, chemical tunability and solubility, cyanine dyes are an interesting class of molecules to be employed in organic semiconductor devices. Their beneficial properties have been demonstrated in organic solar cells (OSC), photodetectors and recently in organic light-emitting electrochemical cells (OLEC). The focus of this thesis is to analyse different aspects regarding the dynamics of electronic and ionic charges in cyanine based devices. Models and conclusions are not restricted to cyanine based devices but can potentially be applied in various systems where combined ionic and electronic charges are present. In order to focus on electronic processes in OSCs, a combination of steady-state, transient and impedance analysis is used. It is resolved that trapped charges can be introduced at the dye/hole extracting layer interface by using specific solvents for the cyanine layer fabrication. These measurements also reveal the low hole mobility of cyanine films which in turn limits the possible upper layer thickness in OSCs. A frequently used way to enhance the conductivity of organic semiconductors is the addition of extra charges by chemical doping. This concept is employed for a near-infrared absorbing dye which is used as donor in a bilayer solar cell. The dependence of all figures of merit versus doping concentration and layer thickness is studied in detail. It is found that the performance of the optimised cell is not further improved by doping, but that it presents a good option if thicker layers are required, e.g. on rough substrates. Furthermore, the stability and degradation pathway of the p-doped species is analysed. This information is of general relevance as they also occur after charge transfer at the heterojunction interface of an OSC and during operation of OLECs. The dynamics of ionic charge carriers is separated into two contributions. In a first step, the steady-state diffusion profile of the counter ion within a cyanine/fullerene bilayer structure is scrutinised using different profiling techniques. An unexpected constant ion profile is found throughout the C60 layer. Simultaneously, the potential within the acceptor is measured by the Kelvin probe technique. A mechanism of combined electron and ion transfer from the C60 to the dye and vice versa is proposed which is in agreement with the experimental results. A prerequisite for OLEC operation is the mobility of ionic charge carriers. Their dynamics in cyanine salts under the effect of an external field is therefore studied in such devices. Several methods are developed and applied to characterise the dynamic p-i-n structure in detail, i.e. the position and width of the intrinsic region as well as the doping concentrations in p- and n-doped regions. The combination of all these methods helps to understand the transient behaviour of these cyanine devices but can also be adapted to other OLEC systems. A major drawback of cyanine dyes as emissive material is their low fluorescence quantum yield in solid films. To overcome this restriction and improve the OLEC efficiency, host-guest systems with various visibly emitting dyes are studied. The gain in fluorescence quantum yield in the film can directly be transferred to an increase in external quantum efficiency of the device.

EPFL2017

Absorption enhancement in thin-film silicon solar cells

Ali Naqavi

Thin-film silicon technology is a major candidate to comply with the ever-increasing global energy demand. The small thickness of silicon allows high industrial throughput and low material usage and subsequently opens new avenues to mass-production of low-cost solar cells. This small thickness, together with the indirect bandgap of silicon and thus its relatively weak absorption of long-wavelength light, requires methods to improve absorption. This is often called light trapping. Light absorption can be enhanced in a thin film solar cell by introducing light scattering functionalities, for example textured interfaces. Of common interest is to know: 1) how far the photocurrent and efficiency of a cell can be improved by applying these geometrical changes, 2) which geometries can satisfy the criterion of high photocurrent generation, and 3) how much the incident angle affects the two previous points. This thesis addresses the mentioned questions. Specifically, amorphous silicon (a-Si) cells are the main subject of focus in the thesis, however, the results and conclusions are applicable to other types of solar cells, and similar structures such as light-emitting diodes. The first part of the thesis is devoted to the electromagnetic theory for thin film multilayers and the numerical methods which were used for optical simulations during the PhD work. These methods are described and compared, and some common sources of numerical error in them are identified. To address the first and the third questions, the limits of light absorption enhancement, photocurrent generation and efficiency in thin-film solar cells are studied. As a result, we obtain the limits of absorption enhancement in thin films with periodic texture, over a wide angular and wavelength range. More specifically, first we extend the statistical temporal coupled-mode theory to the case of thin films with wavelength-scale grating couplers. Then, we use this theory to study the effect of the incident angle and the grating period on the absorption enhancement in an idealized thin film with a thickness of 200 nm and refractive index n=4. We show that absorption in a thin-film solar cell depends strongly on the grating period and angle of incidence; therefore, consideration of oblique incident of light in these cells is a necessity. We provide guidelines for the design of thin-film solar cells with periodic texture. Afterwards, we obtain the limit of absorption enhancement for different structures including a full thin-film a-Si solar cell stack for different grating geometries. We show that for thin-films, hexagonal gratings enhance absorption more significantly compared to square gratings. We identify parasitic absorption as a major bottleneck for photocurrent generation. To deal with realistic cases, we investigate the guided modes of thin-film a-Si solar cells by rigorous simulations (the second and the third question). First, we extract the guided modes of the cells and study them in an equivalent planar model. We show that a plasmonic mode exists for very thin buffer layers. Then, we focus on the effect of texture geometry over a broad angular range by comparing the short-circuit current density (Jsc) of the cells for different grating patterns. We find that based on the cell configuration, the optimal texture may be symmetric or asymmetric. We show that TM polarized light produces higher photocurrent at large incident angles regardless of the texture geometry. In the final part of the thesis, we study two novel configurations for thin-film solar cells. First a plasmonic a-Si cell is considered which does not include a buffer layer. We demonstrate that the Jsc of the plasmonic cell is sensitive to the thickness of the n-doped silicon layer and we find that for an n-Si thickness of less than 10 nm, the plasmonic cell outperforms a conventional a-Si cell. Then we simulate an a-Si cell with a periodic array of ZnO nanowires inside the active layer. Our simulations indicate that assuming a periodicity of around 500 nm, the Jsc is highest for a nanowire diameter of about 300 nm.

EPFL2013