Spin dependent recombination in conjugated polymers: an optically detected magnetic resonance study

This work describes and discusses Photoluminescence (PL), and X-band Photoluminescence Detected Magnetic Resonance (PLDMR) of poly(3-alkylthiophene) (P3AT) films and solutions, poly(para-phenylenevinylene) (PPV), poly(2,5-dihexoxyparaphenylenevinylene) (PDHOPV), poly(2,5-dioctoxyparaphenylenevinylene) (PDOOPV), blends of poly(2-methoxy,5-(2[superscript]\u27ethylhexoxy)paraphenylenevinylene) (PMOEHOPV) with polyethylene (PE) films both unoriented and oriented, and poly(2,5-dibutoxyparaphenyleneacetylene) (PDBOPA) films. The Electroluminescence Detected Magnetic Resonance (ELDMR) and Conductivity Detected Magnetic Resonance (CDMR) narrow quenching signal in PPV is also reported and discussed.;The PLDMR of all polymers exhibit the following PL enhancing features: (i) a narrow resonance at g ≅ 2.003, (ii) a broad triplet powder pattern around g ~ 2, and (iii) the [delta]m[subscript] s = 2 transition of this triplet at g ≅ 4.07. In P3AT and thermally cycled PPV derivatives, a broad PL-quenching resonance is present around g ~ 2. The narrow resonance, symmetric in PPV, PDHOPV, PDOOPV, and PDBOPA but not in P3AT, is attributed to distant polaron recombination, indicating charge conjugation symmetry violation is stronger in the latter system. The spectral dependance of the narrow resonance in P3AT, PPV and PDHOPV which is similar to the PL spectrums suggests that the polaron levels are close to the band edges. Two distinct excitons are observed in P3AT at low and high temperatures. The low temperature exciton is suspected to be weakly pinned to the thiophene rings. The high temperature exciton, observed in all polymer films and powders measured except PPA, which has different symmetry, is believed to be a mobile intrinsic feature in nondegenerate ground state (semi)conducting polymers. PPA is an exception probably due to a high degree of disorder in the samples studied. A broad PL-quenching PLDMR about g ~ 2, is observed in P3HT and PDHOPV at low temperature, probably due to geminate singlet decay. Its FWHM of 300 G, assuming a lifetime broadened resonance, correlates with measured PL lifetimes of ~400 ps.;ELDMR measurements on PPV also yielded an enhancing half field triplet powder pattern similar to that observed in the PLDMR. However, a much stronger narrow EL-quenching resonance is observed at g ≅ 2.0023, probably due to non-radiative decay of two like charged polarons to a bipolaron. The narrow conductivity-quenching CDMR is attributed to the same decay channel, as the mobility of a bipolaron is believed to be lower than that of a polaron

Tailoring optical properties and stimulated emission in nanostructured polythiophene

Polythiophenes are the most widely utilized semiconducting polymers in organic electronics, but they are scarcely exploited in photonics due to their high photo-induced absorption caused by interchain polaron pairs, which prevents the establishment of a window of net optical gain. Here we study the photophysics of poly(3-hexylthiophene) configured with different degrees of supramolecular ordering, spin-coated thin films and templated nanowires, and find marked differences in their optical properties. Transient absorption measurements evidence a partially-polarized stimulated emission band in the nanowire samples, in contrast with the photo-induced absorption band observed in spin-coated thin films. In combination with theoretical modeling, our experimental results reveal the origin of the primary photoexcitations dominating the dynamics for different supramolecular ordering, with singlet excitons in the nanostructured samples superseding the presence of polaron pairs, which are present in the disordered films. Our approach demonstrates a viable strategy to direct optical properties through structural control, and the observation of optical gain opens the possibility to the use of polythiophene nanostructures as building blocks of organic optical amplifiers and active photonic devices

Understanding electron flow in conducting polymer films : injection, mobility, recombination and mesostructure

We survey the current state of models for electronic processes in conducting polymer devices, especially light-emitting diodes. We pay special attention to several processes that have been somewhat neglected in the previous literature: charge injection from electrodes into a polymer sample, mobility of charge-or energy-carrying defects within a single molecule and (more briefly) transfer of carriers between molecules and the interaction between the charge transport and the mesostructure of the polymer. Within all these areas substantial. progress has been made in recent years in elucidating the important physics, but further progress is needed to make quantitative contact with experiment.Engineering and Physical Sciences Research Council (EPSRC)

Generation and recombination of excited states in organic semiconductors

Tese (doutorado)—University of Brasília, Institute of Physics, 2015.Desde de a descoberta das propriedades de emissão de luz atribuídas aos semicondutores orgânicos baseados em grupos fenil na década de 90, esforços consideráveis têm sido empregados no estudo de propriedades fotofísicas desses materiais. Vantagens interessantes como facilidade de síntese, flexibilidade, baixo custo de produção e vasta área de aplicação tornam os semicondutores orgânicos atrativos para a indústria de eletrônicos, particularmente quando a fabricação de uma nova tecnologia de telas é considerada. Existem várias aplicações em potencial tais como células solares orgânicas, transistores de filme–fino e diodos emissores de luz orgânicos (OLDEs). A dinâmica de recombinação entre quase–partículas, de maneira a formar espécies excitadas, tem sido identificada como um processo importante nessas aplicações. Um dos desafios relevantes na ciência e tecnologia de semicondutores orgânicos é a caracterização dos efeitos de temperatura e impureza sobre a geração e recombinação entre quase–partículas. Com isso, o entendimento de como tais efeitos desempenham o papel de alterar a dinâmica de formação de novos produtos é de fundamental importância para o desenvolvimento de dispositivos optoeletrônicos mais eficientes. Nesta tese, a dinâmica de recombinação entre quase–partículas em semicondutores orgânicos é investigada numericamente sob influência de impurezas, interações coulombianas, temperatura e campo elétrico externo. O objetivo desse trabalho é fornecer um panorama físico dos produtos e seus respectivos rendimentos resultantes da recombinação entre diferentes tipos de quase–partículas em semicondutores orgânicos, principalmente quando os efeitos mencionados acima são levados em consideração, contribuindo para a compreensão deste processo, que pode fornecer orientações para aprimorar, por exemplo, a eficiência de processos que envolvam eletroluminescência em dispositivos optoeletrônicos tais como OLEDs.Since the discovery of light emitting properties on phenyl–based organic semiconductors in the 90’s, considerable efforts have been devoted to study the photophysical applications of conjugated polymers for development of new technologies in organic optoelectronic devices. The potential advantages in terms of ease of synthesis, flexibility, low cost, and large-area capability make the organic semiconductors attractive for the electronics industry, particularly when it comes to the promising development of a new display technology. There are many potential applications such as Organic Solar Cells, Thin–Film Transistors, and Organic Light Emitting Diodes (OLEDs). The recombination dynamics between quasi–particles, in order to generate excited species, has been identified as an important process in these applications. Furthermore, the products arising from the recombination mechanism as well as their final yields, have been shown to be directly related to the devices performance. One of the significant challenges in the science and technology of organic semiconductors is the characterization of the temperature and impurity effects on the generation and recombination of quasi–particles. Thus, understanding how such effects play the role of changing the formation dynamics of new products from the recombination process is of fundamental importance to the development of more efficient optoelectronic devices. In this thesis, the recombination dynamics between quasi–particles in organic semiconductors is numerically investigated under the influence of impurities, Coulomb interactions, temperature and an external electric field. The aim of this work is to give a physical picture of the products and their yields derived from the recombination of different kinds of quasi–particles in organic semiconductors, when the above–mentioned effects are considered and contribute to the understanding of these important processes, which may provide guidance to improve, for example, the electroluminescence yields in OLEDs

THEORY OF DEFECTS IN CONDUCTING POLYMERS .2. APPLICATION TO POLYACETYLENE

We exploit the approach of a previous paper, based on self-consistent quantum-chemical molecular dynamics, to investigate the energetics and dynamics of excitations in conducting polymers. The predictions include the formation energies of solitons and polarons, the phenomenon of doping by alkali atoms, luminescence quenching in cis-polyacetylene, the soliton mobility in trans-polyacetylene and the non-existence of breathers in cis-polyacetylene

Magnetic field effects in organic semiconductors : theory and simulations

Organic semiconductors are a promising class of materials, offering several advantages over inorganic semiconductors. They are light, flexible, easy and cheap to produce, and easily chemically tunable. Organic semiconductors are currently used for lighting applications and in the displays of some smartphones and televisions. Exciting magnetic field effects have been observed in the current through and light production of organic semiconductors. A magnetoconductance and magnetoelectroluminescence of up to 30% have been measured in organic semiconductor films. Recently, an even larger magnetoconductance with a magnitude of 93% was found in molecular wires. The magnetic field effects are remarkably independent of the specific material used, always having either a Lorentzian or a so-called non-Lorentzian lineshape with a width of a few millitesla. The hyperfine interaction between nuclear magnetic moments and the spins of the particles—electrons, hole, excitons, bipolarons, etc.— in an organic semiconductor can be approximated by an effective magnetic field that acts on the spins. The difference in those so-called hyperfine fields experienced by two particles leads to spin mixing that can be suppressed by applying an external magnetic field. Magnetic field effects arise because quantities like the current and light output depend on processes that are spin-dependent and are thus affected by the amount of spin mixing. The goals of this thesis are to explain experimentally observed magnetic field effects and to make predictions for obtaining even larger effects. This is done analytically using stochastic Liouville equations as well as using Monte Carlo simulations. A much-discussed question that is related to magneto-electroluminescence is whether the statistical ratio of one singlet to three triplets can be violated in exciton formation. We have studied this question using a two-site model in Chapter 3. We found that, if the singlet- and triplet-exciton formation rates differ, the statistical singlet-to-triplet exciton ratio of 1:3 is violated when hopping is slower than or comparable to the hyperfine frequency—the precession frequency of an electron spin due to the hyperfine field. Furthermore, for those hopping rates, we found a magnetic field dependence of the singlet fraction—a measure of the electroluminescence—if and only if singlet- and triplet-exciton formation rates differ. We also found that an ultra-small-magnetic-field effect that is sometimes observed can result from the increase in spin mixing when a magnetic field is applied that is comparable in magnitude to the hyperfine fields. In Chapter 4 we found that the violation of the statistical ratio and its magnetic field effect occur at hopping rates that are several orders of magnitude higher than the hyperfine frequency when Coulomb interaction and energetic disorder are present. In addition, a violation of the statistical ratio can be found even in the fast hopping limit, because electron-hole pairs can split up after which both can recombine with another electron and hole. This violation of the singlet fraction does not depend on the magnetic field. The main conclusion of Chapter 5 is that very large magnetic field effects in both the current and diffusion constant can be obtained in doped polymers. The onedimensionality of the charge transport through the polymer leads to effective spin blocking at dopant sites. This blocking occurs even in absence of an electric field and is amplified when the charge concentration is increased. A huge magnetoconductance has been measured in molecular wires embedded in a zeolite L crystal. We have modeled the conduction through those wires using a chain of sites in Chapter 6. We conclude that a similar mechanism as in the doped polymers leads to spin blocking in the wires, where trapped electrons instead of dopant sites lead to spin blocking. We suggest that the potassium ions that are present in the zeolite lead to the necessary trapping, because trapping by just the energetic disorder results in neither the right magnitude nor in the right electric field dependence of the magnetoconductance as compared to the experiment. When all effective magnetic fields are aligned, the amount of spin mixing depends on the hopping rate and the difference in magnitude of the effective magnetic fields felt by two particles on different sites. The effective magnetic fields vary from site to site due to the random nature of the hyperfine fields. However, in Chapter 7 we show that additional spin mixing happens when an external magnetic field is present that varies more strongly as a function of position than the hyperfine fields do. We conclude that the magnetoconductance that was measured in a device with a single magnetic electrode is the result of this kind of spin mixing. The magnetization of the magnetic layer changes as a function of the applied magnetic field. The resulting change in the fringe fields throughout the organic semiconductor leads to a change in spin mixing and thus in a change in the current. Finally, the main conclusions of this thesis are summarized and an outlook on the future of modeling of magnetic field effects in organic semiconductors is given in Chapter 8

Microstructure, morphology and device physics of gravure printed and solution processed organic field-effect transistors

This thesis explores the relationship between microstructure, morphology and device physics in gravure printed and solution processed organic field-effect transistors (OFETs). Chapter 1 introduces the key concepts encountered in this work: the properties of organic semiconductors and OFETs; the use of printing techniques in organic electronics; and the relationship between microstructure and OFET performance in poly(3-hexylthiophene) (P3HT). Chapter 2 details the materials and experimental techniques used in this thesis. In Chapter 3, gravure printing is demonstrated for high throughput fabrication of OFETs. Printed devices are achieved with typical saturated mobility of 0.03cm2/Vs and on/off ratio in the range 103:9-4:6, which exceeds that achieved with spin coated devices with the same material system and geometry. Chapter 4 presents a systematic comparison of the microstructure and OFET characteristics of gravure printed and spin coated P3HT thin films. First light scattering is used to understand the conformation of P3HT chains in various solvents, then grazing incidence wide angle X-ray scattering (GIWAXS), absorption characteristics and atom force microscopy (AFM) are used to characterise the microstructure of the P3HT lms. In turn, this is compared to OFET performance. In Chapter 5 two solvent based techniques are investigated as alternatives to thermal annealing as methods to enhance microstructure. A blend of a high and low boiling point solvent is first examined as the casting solvent for P3HT and is found to moderately improve P3HT field-effect mobility. Secondly, solvent vapour treatment (SVT) - exposing a P3HT film to a solvent vapour after spin coating - is studied by in-situ GIWAXS. The time resolved measurement of interchain and interlamella distances allowed the dynamics of SVT to be investigated. SVT was found to decrease P3HT crystallinity, although AFM showed it lead to smoother films. In Chapter 6 two emerging materials are investigated for use in OFETs. Preliminary work on fabricating OFETs with single crystal copper phthalocyanine is presented. Finally, work towards a metal-free OFET is described in which the source and drain electrodes are formed of high conductivity PEDOT deposited by vapour phase polymerisation

Efficient multiconfigurational time-dependent simulation of conjugated polymers

Conjugated polymers have become an important class of functional materials for a wide range of optoelectronic applications, from which polymer-based solar cells stand out as one of the most promising new devices. While experimental progress has been made at a good pace over the last couple of decades, the fundamental processes governing the photophysics of conjugated polymers are not yet completely understood. A theoretical description is challenging, since these systems exhibit both strong electron-electron and electron-nuclear interactions. A detailed understanding of the photoexcitation process, and of the steps following photoexcitation, requires a nonadiabatic treatment of the electron-nuclear dynamics, and a proper description of the excited electronic states and interchain interactions, for which many-body effects are important. Some of these ingredients have often been neglected in dynamical calculations. In particular, most studies which include electron-electron interactions have ignored the singlet character of the photoexcited state, by restricting the wavefunction to the form of a single Slater determinant. In this thesis, we develop a nonadiabatic molecular dynamics method which allows for the coupled evolution of the nuclear degrees of freedom and of multiconfigurational electronic wavefunctions. The proposed scheme effectively establishes a compromise between efficiency and accuracy, which enables the study of large systems. Furthermore, it is designed to take into account the appropriate spin symmetry of the electronic wavefunction, thus allowing us to distinguish between singlet and triplet excited states, which exhibit quite different properties. The formalism is applied to semiempirical single- and double-strand models of a prototypical conjugated polymer, in order to investigate the effects of Coulomb interactions and interchain coupling on the dynamics of low-lying excitations. The nature of the photoexcited states and the issue of charge photogeneration in conjugated polymers are also addressed, as well as the charge transfer process at donor/acceptor interfaces

Doctor of Philosophy

dissertationThe present dissertation is the result of our studies of the optical and electrical properties of self-assembled monolayer (SAM) diodes and bulk heterojunction organic photovoltaic (BOPV) devices. In our studies of SAM diodes, we fabricated solid-state mixtures of two dierent kinds of molecules; 1,4 benzene-dimethane-thiol (MeBDT) and 1-pentanethiol (PT). By varying the concentration r of MeBDT with respect to PT, we can go from a regime of isolated molecular wires (10ˉ8 10ˉ3). For r = 0, we found that a potential barrier dominated the transport properties of the device. In the isolated molecules regime, the conductance of MeBDT dominates the transport. In this regime, because of the linearity of the conductance with respect to r, we were able to obtain a \single molecule resistance" at V = 0:1V of RM = 6 10ˉ9. In the aggregated molecules regime, an ohmic response in the current-voltage (I-V) characteristics was observed for bias voltages 0:5V with the appearance of a new band in the dierential conductance around V = 0 along with a new double band in the optical gap at 2:4eV resulting in yellow/red photoluminescence emission. Opto-electrical studies of BOPV devices reveal that there are very few similarities between these types of solar cells and conventional solar cells. From simulations and experiemental measurements of the I-V characteristics, we found that while the open voltage circuit (Voc) is important for engineers, it carries no intrinsic information of the device. It cannot exceed the built-in potential of the device (Vbuiltˉin ). The later origin was found to be dependent on electrode work function dierence for a non-Ohmic contact conguration and on the active layer's blend in an Ohmic contact conguration. In a bid to improve BOPV device performance, we added to the blend spin 1=2 radical molecules. At concentration ( 2%), an increase in device performance was observed. The principal cause for this increase was the increase in the carrier's mobility as a function of the concentration of radicals