Terahertz Time-Domain Spectroscopy Study of the Conductivity of Hole-Transporting Materials

Some redox-active ionic liquids, organic amorphous solids containing electron-donating moieties, and conductive polymers can efficiently transport positive electrical charges. These hole-conducting media find increasing applications in unconventional solar cells and organic light-emitting diodes. Appropriate methods are required to unravel the detailed conduction and trapping mechanisms in these materials as well as to fully understand the interplay of molecular vibrations and charge transport processes. Here we present terahertz time-domain spectroscopy (THz-TDS) as a powerful technique, that allows the direct determination of the complex conductivity of hole transporting materials in a contactless, purely optical manner. Beyond the measurement of the conductivity of solid and liquid materials, THz (far infrared) spectroscopy also provides direct information of the librational and vibrational modes coupled to charge transport processes and therefore is invaluable in the study of the mechanism of polaronic transport in matter. Application of this technique is illustrated by examples provided by the study of the ionic liquid 3-methyl-1-propylimidazolium bis(trifluoromethane) sulfonimide (PMIT FSI) and that of the molecular liquid hole conductor 10-methylphenoxazine. Both systems are of particular interest, as both types of holetransporting media are successfully used as alternatives to solvent-based electrolytes in dye sensitized solar cells

Linear and Time-Resolved THz Spectroscopy of Photonic and Charge Transporting Systems

Jan Cornelius Brauer

Photo-induced charge separation and charge transport are fundamental processes in many energy conversion techniques, where light is converted into chemical energy (PEC), into electrical energy (solar cells) or in the opposite sense if electrical energy is converted into light (LEDs). In all cited examples, charge carriers have either to be transported towards an interface where they can undergo radiative recombination, trigger a chemical reaction, or they have to be evacuated from the interface as free charge carriers to be able to perform work in an external electrical circuit. In this work the charge generation dynamics and transport is studied using THz time domain spectroscopy and complimentary techniques on systems finding application in dye sensitized solar cells and PEC cells for water splitting. The functioning of a dye sensitized solar cell is based on the kinetic competition between several reactions, i.e. electron injection, dye-regeneration, electron back- reaction and charge extraction. Accordingly the rates of all of these reactions have to be optimized in order to optimize the cell performance. This requires a good understanding of the underlying mechanism of these processes. In photochemical cells, the encountered processes are in a way similar. After initial excitation, the generated charges have to be separated and have to reach either the interface to the electrolyte or the external circuit before being annihilated by recombining. Knowing the diffusion length and carrier lifetime in a specific material might therefore be helpful to optimize the film morphology, i. e. the aspect ratio. Chapter 1 gives an overview over the functioning of DSCs and PEC cells and summarizes some theoretical aspects of the processes encountered in these devices. Chapter 2 is devoted to the description of the experimental tools used and built in this work. In chapter 3, charge transport in a liquid molecular hole conductor, 10- methylphenoxazine, is investigated. The transport is rationalized using a model taking into account ionic diffusion and intermolecular hole transfer, i. e. hopping. It is found that the conduction occurs principally by hopping, as the HTM cation seems to be mainly present in an ion-paired form with the anion from the doping agent. Hopping of charge requires molecular vibration to bring acceptor and donor molecule in electronic resonance. Two vibrational modes of the HTM cation and neutral molecules that might be coupled to charge transport are observed. In chapter 4, electron injection from a dye into the conduction band of TiO2 films is investigated by monitoring the appearance of the free electron. This signal is compared to the signal arising from direct UV excitation of the TiO2. In addition to previous studies done in the group, the effect of dye aggregation leading to decelerated electron injection is investigated. Comparing the THz signal amplitude to the injected charge carrier concentration, the THz mobility in TiO2 films was found to be 0.1-0.15 cm2V-1s-1 and constant over the investigated charge carrier concentrations. Furthermore the influence of sintering temperature of the TiO2 film has been addressed in a preliminary study. In chapter 5, the influence of liquid electrolytes, of a solid state hole conducting material and of the commonly used additives Li+ and tBP on the charge generation dynamics have been investigated. While in this work the presence of an electrolyte/HTM has not shown any influence on the electron injection dynamics, the generation of the free electron seems to be strongly dependent on the molecular surrounding. This discrepancy is discussed in terms of a coulombic bound electron- hole pair as an intermediate state in the electron injection dynamics. Chapter 6 contains a study on the charge carrier dynamics in cuprous oxide, being used as material for photoelectrochemical water splitting. The wavelength and fluence dependence on the charge carrier lifetime, mobility and diffusion length was studied. The strong dependence of the charge carrier lifetime is discussed in terms of trapping mediated recombination.

EPFL2012

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

Exciton Dynamics in 2D Organic Assemblies for Realization of Next-generation Optoelectronic Devices

Surendra Babu Anantharaman

Supramolecular assemblies of cyanine dye molecules formed by weak van der Waals forces have gained immense attention for their distinct optical properties (high extinction coefficient, narrow absorption and photoluminescence line-width and small Stokes shift) compared to their monomer counterparts. Strong light-matter interaction in the J-aggregates causes coherent exciton migration and high color purity emission. This motivates to fabricate J-aggregate thin films for different opto-electronic devices such as solar cells, where J-aggregates serve as exciton transport channels, organic light emitting diodes (OLEDs) and polariton lasers. Exciton migration and photoluminescence quantum yield (PLQY) of the J-aggregates in thin films dictate the efficiency of energy harvesting and light emitting devices, respectively. The main focus of this thesis is the development of a new J-aggregate growth route which favors the formation of large, coherent domains of 2D J-aggregate thin films and to investigate the exciton dynamics. Furthermore, non-radiative decay channels suppressing the PLQY of J-aggregates in thin film and in solution were identified. The coherent crystalline domain size in thin films is typically limited to nanoscale length because dye molecules spontaneously self-assemble already in solution. This can be overcome by maintaining equilibrium between monomers and the critical nucleus of a J-aggregate in solution. This approach is shown here as a successful route to achieve large coherent domains on a functionalized substrate. The growth model was further confirmed with small-angle x-ray scattering (SAXS) studies. Using time-resolved photoluminescence (TRPL) spectroscopy, non-radiative decay channels limiting the exciton migration were identified. The results suggest that the increase in domain size and order is directly proportional to the increased radiative decay of the excitons. Furthermore, the PLQY of J-aggregates in water at room temperature is typically ~5%. Upon addition of alkylamine, the quantum yield was drastically improved while maintaining line-width and peak position. Using small-angle neutron scattering (SANS), a two-phase region was identified in the water-dye-alkylamine ternary phase diagram in which high PLQY J-aggregates form. From TRPL studies, an increase in radiative lifetime is in agreement with the increase in PLQY. As an application, ultra-narrowband photodetectors were explored owing to the narrow line-width of J-aggregate absorption. The concept proved advantageous over other strategies for narrowing the response width, with competing figure of merits. Realization of an inkjet printed prototype with higher frequency response compared to spin-coated devices highlights their potential for industrial applications. Excitonic channels using J-aggregate nanowires were formed by complexing J-aggregates with dendronized polymers. SAXS studies revealed a core-shell configuration of the DP-J-aggregate hybrid structure. A correlation between polymer conformation and J-aggregates allowed to assemble J-aggregate nanowires as thin films. Centrosymmetric cyanine dimers are known for their non-luminescent nature. Twisted dimer packing of dye molecules in a polymer matrix are confirmed from circular dichroism spectroscopy studies. This explains a strongly red-shifted photoluminescence with high quantum yield in the thin film.

EPFL2019