Study of Triplet Exciton Dynamics in Small Organic Molecule Films Using Time Resolved Optical Spectroscopy

In recent years it has become clear that knowledge of triplet transport in single layer and multilayer films can be crucial in improving the efficiency of organic light emitting devices and solar cells. This thesis reports an investigation of triplet exciton dynamics in small organic molecule single and multilayer layer films using optical time resolved nanosecond spectroscopy. A diligent step by step approach, leading towards the investigation of complex donor/spacer/acceptor multilayer structures is used. First of all, single layer films to be a constituents of multilayer structures were studied by measuring fluorescence, delayed fluorescence and phosphorescence. 4,4’-N,N’-dicarbazolyl-1,1’-biphenyl (CBP) widely used in organic light emitting diodes is characterized. Evidence is provided that in some of these spectra emission comes from trap states rather than the CBP molecule itself. Also N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl-4,4’’-diamine (NPB) has been investigated. Results indicate that bimolecular triplet recombination is dominant and that triplet transport has dispersive features even at room temperature in NPB films. Then films of heavy metal iridium complexes fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) and iridium(III)tris(1-phenylisoquinoline) (Ir(piq)3) are put into the spotlight. New states previously not reported are identified and decay with the slope -1 characteristic of more than one iridium complex and previously not published in literature is observed. Triplet interface sites in bilayer Ir(piq)3/NPB films obstructing triplet migration are determined and triplet movement across interface is experimentally captured for the first time. The origin of these interface trap states is suggested. Then this system is upgraded into Ir(ppy)3/NPB/Ir(piq)3 and triplet transfer from Ir(ppy)3 to Ir(piq)3 via NPB is investigated. A model of triplet exciton dynamics in Ir(piq)3/NPB films using classical diffusion equations is presented with interface sites included. Computer simulations were performed and the results are in very good agreement with the experimental ones. Finally problems encountered are identified and main guidelines on how to do research in complicated multilayer structures are set

Immigrant entrepreneurship

The interplay of migration and entrepreneurship has been shifted to a significant research area by the continuous increase of immigrants as a share in the population of countries, that are considered to be having a high attractiveness towards migrants. The United Kingdom has been pronounced as one of the countries receiving the biggest flows of immigrants. It has led to increasing immigrant self-employment, particularly in the areas with a high immigrant concentration. Considerable numbers of Lithuanian immigrants have established themselves in rural areas in the East of England region, and some of them eventually got engaged in entrepreneurial activities. Despite the country's business-friendly environment, immigrant entrepreneurs still encounter challenges that significantly constraint their business, both establishment and performance. Challenges and constraints that immigrant entrepreneurs face in a host country are caused by the interplay of various contexts such as social, economic, cultural, institutional, etc. that immigrant entrepreneurs are embedded in. Furthermore, entrepreneurship in a rural context is respectively associated with even more obstacles. There are less developed and available local services and infrastructure compared to urban areas, long distances, which eventually lead to limited access to essential resources and capital, necessary for a viable business. In this thesis, I analyse Lithuanian immigrant entrepreneurship in the UK and contexts mentioned above, which impact immigrant business performance and can be perceived as push and pull factors. Through semi-structured interviews with Lithuanian immigrant entrepreneurs, I explore the perceptions of Lithuanian immigrants' experience in their entrepreneurial activities based in the East of England region through the lens of mixed embeddedness concept. More specifically, I investigate different social, economic, and institutional contexts that Lithuanian immigrants are embedded in. In order to illustrate the mixed embeddedness concept, which aims to incorporate co‐ethnic social networks and links between immigrant entrepreneurs and the economic and institutional context in the host society, I present a case study of Lithuanian immigrant entrepreneurs in rural East of England. The research study was conducted during the pre- Brexit period when immigration to the UK was seen as one of the central factors in the Brexit debate

Race, imaginaire biologique et identité dans A la recherche du temps perdu

This thesis examines the influence on Proust of nineteenth-century racialist and anti-Semitic writers, and of natural scientists such as Darwin, Haeckel or Lamarck, in order to understand the concept of race in Proust's novel, with a particular focus on the representation of Jewishness and homosexuality

Transformisme et création littéraire chez Marcel Proust

Although several studies have explored the role of science in Proust’s works, few have focused on the aesthetics of biology in "À la recherche du temps perdu". This article looks at the influence of transformist theories (Darwinism or Lamarckism) in Proust’s novel. It shows that naturalists such as Cuvier or Geoffroy Saint-Hilaire, who did not support the theory of the transmutation of species, are missing from the novel. Furthermore, the article analyses the themes of metamorphosis and hybridity in order to show how, in Proust, transformism - and specifically Lamarckism -, is linked with mythology and literary creation

Homlokzat és tetőszerkezet közötti tűzterjedés problémái : A homlokzati tűzterjedés speciális esete = Problems of Fire Spreading Between Facades and Roof

A worldwide problem facing the design of buildings is how to prevent the spread of fire from a buildings’ elevation into the roof space. Analysis of how the eaves to a building are designed can be critical in preventing loss of lives and extensive damage to a building's fabric. It has been found that not only the use of materials can result in different outcomes, also the geometric arrangement of elements, distance of the eaves from the wall and even the depth at which openings are placed within a wall are all valid factors. The overall aim being to reduce potential for fire to spread by reducing potential for fires to reach uncontrollable temperatures

Highly Efficient TADF OLEDs: How the Emitter–Host Interaction Controls Both the Excited State Species and Electrical Properties of the Devices to Achieve Near 100% Triplet Harvesting and High Efficiency

New emitters that can harvest both singlet and triplet excited states to give 100% internal conversion of charge into light, are required to replace Ir based phosphors in organic light emitting diodes (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence (TADF). Here, it is shown that a D–A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on a previously studied D–A–D molecule in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices, it is unambiguously established that the triplet states are harvested via TADF, but more interestingly, these results are found to be independent of whether the emitter is the ICT state or the D–A–D/host exciplex

Triplet Harvesting with a Simple Aromatic Carbonyl

The efficiency of organic light-emitting diodes crucially depends on triplet harvesters. These accept energy from triplet correlated electron hole pairs and convert it into light. Here, experimental evidence is given that simple aromatic carbonyls, such as thioxanthone, could serve this purpose. In these compounds, the emissive 1ππ* excitation may rapidly equilibrate with an upper triplet state (3nπ*). This equilibrium may persist for nanoseconds. Population of the 3nπ* state via energy transfer from an electron hole pair should result in fluorescence emission and thereby triplet harvesting. To demonstrate the effect, solutions of 1,4-dichlorobenzene (triplet sensitizer) and thioxanthone (harvester) were excited at 266 nm with a nanosecond laser. The emission decay reveals a 100 ns decay absent in the thioxanthone only sample. This matches predictions for an energy transfer limited by diffusion and gives clear evidence that thioxanthone can convert triplet excitations into light

Two-photon exchange effect on deuteron electromagnetic form factors

Corrections of two-photon exchange to proton and neutron electromagnetic form factors are employed to study the effect of two-photon exchange on the deuteron electromagnetic form factors. Numerical results of the effect are given. It is suggested to test the effect in the measurement of $P_z$ in a small angle limit.Comment: 12 pages 9 figure