This study expands the role of polythiophenes as an electron donating chromophore within energy harvesting milti-modular donor-acceptor systems. The polythiophene moiety would act as an electron donating spacer group between the donor and acceptor entities, viz., phenothiazine and fulleropyrrolidine, respectively, in the newly synthesized supramolecular triads. The triads 10-{[2,2';5',2"] terthiophene-5-fulleropyrrolidine} phenothiazine and 10-{[2,2'] bithiophene-5-fulleropyrrolidine} phenothiazine were synthesized and characterized through electrochemical and spectroscopic methods to ascertain their structural integrity. the componets of the triads were selected for their established redox parameters. Phenothiazine would act as a secondary donor and would facilitate hole-transfer from the polythiophene primary electron donor, due to its ease of oxidation and yield a long-lived charge separated state. Fulleropyrrolidine would act as an acceptor for ease of reductive capabilities and its ability to hold multiple charges. Finally, occurrence of photoinduced electron transferleading to the anticipated charge separated states is established from advanced transient spectroscopic techniques on these novel supramolecular systems.

The efficiency of photoconversion systems, such as organic photovoltaic (OPV) cells, is largely controlled by a series of fundamental photophysical processes occurring at the interface before carrier collection. A profound understanding of ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) is the key determinant to improving the overall performances of photovoltaic devices. The discussion in this dissertation primarily focuses on the relevant parameters that are involved in photon absorption, exciton separation, carrier transport, carrier recombination and carrier collection in organic photovoltaic devices. A combination of steady-state and femtosecond broadband transient spectroscopies was used to investigate the photoinduced charge carrier dynamics in various donor-acceptor systems. Furthermore, this study was extended to investigate some important factors that influence charge transfer in donor-acceptor systems, such as the morphology, energy band alignment, electronic properties and chemical structure. Interestingly, clear correlations among the steady-state measurements, time-resolved spectroscopy results, grain alignment of the electron transporting layer (ETL), carrier mobility, and device performance are found. In this thesis, we explored the significant impacts of ultrafast charge separation and charge recombination at donor/acceptor (D/A) interfaces on the performance of a conjugated polymer PTB7-Th device with three fullerene acceptors: PC71BM, PC61BM and IC60BA. Time-resolved laser spectroscopy and high-resolution electron microscopy can illustrate the basis for fabricating solar cell devices with improved performances. In addition, we studied the effects of the incorporation of heavy metals into π-conjugated chromophores on electron transfer by monitoring the triplet state lifetime of the oligomer using transient absorption spectroscopy, as understanding the mechanisms controlling intersystem crossing and photoinduced electron transfer dynamics is required to improve the device performance of solar cells. Here, we evaluated the effects of incorporating Pt(II) on intersystem crossing and photoinduced electron transfer by comparing and analyzing the photoexcited dynamics of DPP-Pt(II)(acac) and metal-free DPP with different acceptors such as TCNE, TMPyP, and TPyP.

Molecular engineering of donor-acceptor dyads and multimodular systems to control the yield and lifetime of charge separation is one of the key goals of artificial photosynthesis for harvesting sustainably solar energy. The design of the donor-acceptor systems mimic a part of green plants and bacterial photosynthetic processes. The photochemical events in natural photosynthesis involve the capturing and funneling of solar energy by a group of well-organized chromophores referred to as an ‘antenna' system causing an electron transfer into the ‘reaction center,' where an electron transfer processes occur resulting a long-lived charge separated state. Over the last two to three decades, many efforts have been directed by the scientific community designing of multi-modular systems that are capable of capturing most of the useful sunlight and generating charge separated states of prolonged lifetimes with adequate amounts of energy. In this dissertation, we report on the design and synthesis of donor–acceptor conjugates with the goal of modulating the yield and lifetime of their charge separated states and hence, improving the conversion of light energy into chemical potential. In simple donor-acceptor systems, generally, the energy and electron transfer events originate from the singlet excited state of the donor or acceptor and can store the greatest amount of energy but must be fast to out compete intersystem crossing. To address this limitation, we have designed novel donor –acceptor conjugates that use high-energy triplet sensitizers in which electron transfer is initiated from the long lived triplet state of the donor. The triplet photosensitizers used were palladium(II) porphyrin and platinum(II) porphyrin. Heavy metal effect in these porphyrins promoted intersystem crossing and the energies of their excited state was quite high. For the case of palladium (II) porphyrin the energy stored was found to 1.89 eV and that of platinum(II) porphyrin 1.84 eV. In addition to using triplet photosensitizers as donors, we have used donors that are difficult to oxidize and hence producing long lived charge separated states with adequate amount of stored energy. The system that was used for this study is zinc porphyrin with meso-aryl pentafluorophenyl substituents and fullerene, C60 as the acceptor. The presence of fluorine substituents on zinc porphyrin makes it harder to undergo oxidation. When this high potential donor-acceptor system undergoes a photoinduced charge-separation, the estimated energy stored was found to be 1.70 eV, one of the highest reported in literature so far. To further extend the lifetime of the charge separated states generated in this high-potential zinc porphyrin-fullerene dyad a pyridine functionalized tetrathiafulvalene was axially coordinated to the Zn metal producing a supramolecular triad capable of producing long-lived charge separated state. In a subsequent study, a multi-modular donor-acceptor system composed of a porphyrin, fullerene (C60) and a BF2-chelated dipyrromethene (BODIPY) with a supramolecular arrangement in the form of porphyrin-BODIPY-C60, one of the few reported in literature. By selectively exciting BODIPY and ZnP moieties, efficient singlet-singlet energy transfer from 1BODIPY * to ZnP in toluene was observed in the case of the dyad ZnP-BODIPY. However, when ZnP is excited, electron transfer occurred with the formation ZnP.+-BODIPY-C60.- charge separated state persisting for microseconds.

No description available.

Block copolymer (BCP) nanocomposites are an important class of hybrid materials in which the BCP guides the spatial location and the periodic assembly of the additives. High loadings of well-dispersed nanofillers are generally important for many applications including mechanical reinforcing of polymers. In particular the composites shown in this work might find use as etch masks in nanolithography, or for enabling various phase selective reactions for new materials development. This work explores the use of hydrogen bonding interactions between various additives (such as homopolymers and non-polymeric additives) and small, disordered BCPs to cause the formation of well-ordered morphologies with small domains. A detailed study of the organization of homopolymer chains and the evolution of structure during the process of ordering is performed. The results demonstrate that by tuning the selective interaction of the additive with the incorporating phase of the BCP, composites with significantly high loadings of additives can be formed while maintaining order in the BCP morphology. The possibility of high and selective loading of additives in one of the phases of the ordered BCP composite opens new avenues due to high degree of functionalization and the proximity of the additives within the incorporating phase. This aspect is utilized in one case for the formation of a network structure between adjoining additive cores to derive mesoporous inorganic materials with their structures templated by the BCP. The concept of additive-driven assembly is extended to formulate BCP-additive blends with an ability to undergo photo-induced ordering. Underlying this strategy is the ability to transition a weakly interacting additive to its strongly interacting form. This strategy provides an on-demand, non-intrusive route for formation of well-ordered nanostructures in arbitrarily defined regions of an otherwise disordered material. The second area explored in this dissertation deals with the incorporation of additives into photoresists for next generation extreme ultra violet (EUV) photolithography applications. The concept of hydrogen bonding between the additives and the polymeric photoresist was utilized to cause formation of a physical network that is expected to slow down the diffusion of photoacid leading to better photolithographic performance (25-30 nm resolution obtained).

As part of the study, photosynthetic system constructs based on BF2-chelated dipyrromethene (BODIPY), BF2-chelated azadipyrromethene (AzaBODIPY), porphyrin, phthalocyanine, oxasmaragdyrin, polythiophene, fullerene (C60), single-walled carbon nanotube and graphene are investigated. Antenna systems of BODIPY dyads and oligomers having BODIPY as an excitation energy donor connected to different acceptors including BODIPY, azaBODIPY, oxasmaragdyrin and aluminum porphyrin are studied. Different synthetic methodologies are used to afford donor-acceptor systems either directly linked with no spacer or with short spacers of varying length and orientation. The effect of donor orientation, donor optical gap as well as nature of donor-acceptor coupling on the donor-acceptor spectral overlap and hence the rate of excitation energy transfer is investigated. In all these systems, an ultrafast energy transfer followed by electron transfer is observed. In particular, in a directly connected BODIPY-azaBODIPY dyad an unusually ultrafast energy transfer (~ 150−200 f) via Förster mechanism is observed. The observation of energy transfer via Förster instead of Dexter mechanism in such closely coupled donor-acceptor systems shows the balance between spatial and electronic coupling achieved in the donor-acceptor system. Moreover, in donor-acceptor systems involving semiconducting 1D and 2D materials, covalently functionalized single-walled carbon nanotubes via charge stabilizing (TPA)3ZnP and noncovalently hybridized exfoliated graphene via polythiophene chromophores are studied for their charge transportation functions. In both cases, not only an ultrafast charge transfer in the range of (~ 2−5 p) is observed but also the charge-separated states were long lived implying the potential of these functionalized materials as efficient charge transporting substrates with organic chromophores for photovoltaic and optoelectronic applications where ultrafast intercomponent charge transfer is vital. In addition, as a final part of this dissertation, the mechanisms of electron injection and back electron transfer in heterogeneous systems involving supramolecularly anchored high potential chromophores on TiO2 film are studied by femtosecond transient absorption spectroscopy. In this study, not only are important insights gained on the utilization of supramolecular anchoring of chromophores such as porphyrins, phthalocyanines, and their perflorinated high potential analogues, chromophores currently showing promise as highly efficient sensitizers in dye sensitized solar cells, on TiO2 film but also on the effect of anchor length and sensitizer orientation on the rates of electron injection and back electron transfer at the sensitizer-TiO2 interface.

No description available.

The relentless commercial drive for smaller, faster, and cheaper semi-conductor devices has pushed the existing patterning technologies to their limits. Photolithography, one of the crucial processes that determine the feature size in a microchip, is currently facing this challenge. The immaturity of next generation lithography (NGL) technology, particularly EUV, forces the semiconductor industry to explore new processing technologies that can extend the use of the existing lithographic method (i.e. ArF lithography) to enable production beyond the 32 nm node. Two new resolution enhancement techniques, double exposure lithography (DEL) and pitch division lithography (PDL), were proposed that could extend the resolution capability of the current lithography tools. This thesis describes the material and process development for these two techniques. DEL technique requires two exposure passes in a single lithographic cycle. The first exposure is performed with a mask that has a relaxed pitch, and the mask is then shifted by half pitch and re-used for the second exposure. The resolution of the resulting pattern on the wafer is doubled with respect to the features on the mask. This technique can be enabled with a type of material that functions as optical threshold layer (OTL). The key requirements for materials to be useful for OTL are a photoinduced isothermal phase transition and permeance modulation with reverse capabilities. A number of materials were designed and tested based on long alkyl side chain crystalline polymers that bear azobenzene pendant groups on the main chain. The target copolymers were synthesized and fully characterized. A proof-of-concept for the OTL design was successfully demonstrated with a series of customized analytical techniques. PDL technique doubles the line density of a grating mask with only a single exposure and is fully compatible with current lithography tools. Thus, this technique is capable of extending the resolution limit of the current ArF lithography without increasing the cost-of-ownership. Pitch division with a single exposure is accomplished by a dual-tone photoresist. This thesis presents a novel method to enable a dual-tone behavior by addition of a photobase generator (PBG) into a conventional resist formulation. The PBG was optimized to function as an exposure-dependent base quencher, which mainly neutralizes the acid generated in high dose regions but has only a minor influence in low dose regions. The resulting acid concentration profile is a parabola-like function of exposure dose, and only the medium exposure dose produces a sufficient amount of acid to switch the resist solubility. This acid response is exploited to produce pitch division patterns by creating a set of negative-tone lines in the overexposed regions in addition to the conventional positive-tone lines. A number of PBGs were synthesized and characterized, and their decomposition rate constants were studied using various techniques. Simulations were carried out to assess the feasibility of pitch division lithography. It was concluded that pitch division lithography is advantageous when the process aggressiveness factor k₁ is below 0.27. Finally, lithography evaluations of these dual-tone resists demonstrated a proof-of-concept for pitch division lithography with 45 nm pitch divided line and space patterns for a k₁ of 0.13. / text

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The demand for renewable energy sources has grown out of the humanity’s increasing need for electricity as well as depleting fossil fuel reserves. Organic-dye sensitised solar cells were developed as a green, cost-effective alternative to the market-dominating silicon solar cell technology. The field of photovoltaic devices and organic-DSSCs is interesting because we want to develop better, more efficient cells at lower costs using environmentally friendly materials. By studying the fundamental physics and chemistry processes occurring during and after the interaction of light with these devices, we create a window into the mechanism of photosynthesis. Our DSSCs were prepared by sensitisation of highly porous ZnO with different indoline dyes containing the same chromophore, but different alkyl chain lengths bonded to one of two carboxyl anchors as: DN91 (1 C) < DN216 (5 C) < DN285 (10 C). The role of the dye molecules is to absorb photons and donate electrons to the ZnO which acts as the charge acceptor, at the dye|ZnO interface. Through photoelectrochemical characterisation it was found that the structure of the dyes has an effect on the maximum current (JSC) produced by the cells: the shorter the alkyl chain, the higher the JSC. This macroscopic investigation was complimented by microscopic measurements in the form of transient absorption spectroscopy. This allows us to follow, in real time, the photoinduced oxidation of the dye and its regeneration occurring through desired and undesired pathways. It was found that the injection efficiencies of the dye molecules were directly responsible for the trend in the short circuit currents. / AFRIKAANSE OPSOMMING: Die aanvraag na die ontwikkeling van herwinbare energie bronne spruit voort uit die voorsienbare uitputting van fossiel brandstof bronne sowel as die groeiende behoefte om aan die mensdom se elektrisiteit behoeftes te voldoen. Kleurstof gesensitiseerde sonselle is ontwikkel as ’n groen, koste-effektiewe alternatief tot die silikon sonsel tegnologie wat die mark domineer. Die fotovoltaïse toestel veld, spesifiek organiese kleurstof gesensitiseerde sonselle is interessant omdat daar ruimte bestaan vir die ontwikkeling van beter meer effektiewe selle in terme van vervaardigings koste en prosesse wat omgewingsvriendelik is. Deur die fundamentele fisika en chemiese prosesse wat plaas vind tydens en na lig interaksie met hierdie selle te bestudeer gee dit insig oor die werkingsmeganisme van fotosintese. Ons kleurstof gesensitiseerde sonselle is voorberei deur sensitasie van hoogs poreuse ZnO met verskillende indolien kleurstowwe wat dieselfde kromofoor bevat wat met verskillende alkiel ketting lengtes verbind is aan een van twee karboksiel ankers as: DN91 (1 C) < DN216 (5 C) < DN285 (10 C). Die rol van die kleurstof molekules is om fotone te absorbeer en elektrone te doneer aan die ZnO wat as die lading akseptor dien by die kleurstof|ZnO intervlak. Deur fotoelektrochemiese karakterisasie is bevind dat die struktuur van die kleurstof ’n effek het op die maksimum stroom (JSC) wat die selle produseer: hoe korter die die akiel ketting, hoe hoër die JSC. Hierdie makroskopiese ondersoek is voltooi deur mikroskopiese metings in die vorm van tydopgelosde absorpsiespektroskopie. Dit laat ons toe om die fotogeinduseerde oksidasie asook regenerasie van die kleurstof te volg soos wat dit plaas vind deur gewenste sowel as ongewenste roetes. Dit is bevind dat die inspuitings effektiwiteit van die kleurstof molekules direk verantwoordelik is vir die waarneembare trajek in die kortsluitings stroom.

O objetivo deste trabalho é o estudo das Mudanças Estruturais Fotoinduzidas em filmes do sistema vítreo [Sb(PO3)3]n &#8722;Sb2O3 bem como a produção e caracterização do sistema vítreo em questão. Vidros de x[Sb(PO3)3]n&#8722;(100&#8722;x)Sb2O3 (5 &#8804; x &#8804; 40) foram produzidos pela fusão dos precursores Sb2O3 e [Sb(PO3)3]n a 900°C em cadinhos de carbono vítreo e em seguida vertidos e resfriados rapidamente em moldes de aço inox. Dessa forma foi possível obter vidros homogêneos e estáveis com dimensões de 1,0 x 1,0 x 0,5 cm3. A caracterização do sistema vítreo e parâmetros como região de formação vítrea e temperaturas características foram obtidos por meio de técnicas como DSC, Difração de Raios&#8722;X, FTIR, e MAS&#8722;NMR. A região de formação vítrea neste sistema ocorre para composições entre 10 &#8804; x &#8804 25 sendo que para x = 5, x = 30, e x = 35 forma&#8722;se um vidro com pequena fração de microcristalinidade que apresenta coloração amarelada sendo visualmente transparente, já para x = 40 forma&#8722;se um vidro com maior fração de microcristalinidade e totalmente opaco. Este sistema vítreo possui interessantes propriedades como larga janela de transmitância (&#8764; 0,4 a 8 &#181;m), alto índice de refração (n &#8764; 2,0), baixa temperatura de fusão (Tf&#8764;900 &#176;C) e baixa temperatura de transição vítrea, (Tg&#8764;300 &#176;C). Filmes de x[Sb(PO3)3]n&#8722;(100&#8722;x)Sb2O3 foram produzidos por evaporação térmica dos vidros através de canhão de elétrons (EB&#8722;PVD) em um sistema não comercial de evaporação desenvolvido em nosso laboratório, onde conseguimos produzir de maneira eficiente e com boa qualidade óptica filmes de até 10 &#181;m de espessura. Estes filmes apresentam um atípico fenômeno de fotocontração em torno de 8 % da espessura do filme, acompanhado de fotoclareamento após irradiação com laser UV em 350,7 nm. Além da caracterização a nível macroscópico do fenômeno de fotocontração (comportamento em função da potência e tempo de irradiação com laser UV) nossa atenção ficou voltada para o estudo desse efeito a nível estrutural, através de técnicas como Difração de Raios&#8722;X, XANES, FTIR, RPE, RBS e Holografia Óptica. Além da ampla caracterização mostramos a possibilidade de aplicação do material para holografia ou armazenamento óptico, redes de difração, e matrizes de microlentes. O efeito de fotocontração nos filmes foi notado devido ao fotoclareamento observado visualmente após a exposição ao UV e foi confirmado por medidas de perfilometria da região exposta ao UV. O efeito de fotoclareamento (simultâneo a fotocontração da superficie) ocorre gerando deslocamento de &#8764; 30 nm do bandgap (3,58 eV, 347 nm) para maiores energias (4,01 eV, 310nm). Também observamos um pequeno deslocamento do bandgap para irradiações com &#955;.=482 e 460 nm. A fotocontração evolui não linearmente com o tempo e potência de irradiação. Depende também do tempo de exposição e da potência do laser, tendendo a saturação após 4 horas de exposição a 5 W&#8260;cm2 (100 mW) ou 2 horas de exposição a 15 W&#8260;cm2 (300 mw). O efeito de fotocontração tem forte dependência com a concentração de polifosfato de antimônio na composição dos filmes do sistema vítreo x[Sb(PO3)3]n&#8722;(100&#8722;x)Sb2O3. Filmes de 1,0 &#181; de espessura irradiados com 100 mW (5,0 W&#8260;cm2) por 4 horas (região de saturação) apresentam fotocontração (&#8722;&#916;V&#8260;V) de até 8,0 % da espessura do filme (para x = 25) quando irradiados com laser UV (&#955;.=350 nm) Para x = 10 a contração é menor que 2,0 %, e para concentrações de polifosfato acima de 25 % efeito é diminuído drasticamente. A atmosfera na qual o filme é irradiado tem grande influência no efeito de fotocontração. Filmes de 20 % [Sb(PO3)3]n&#8722;80 % Sb2O3 (espessura: 1,0 &#181;m)irradiado com laser UV por 4 horas a 100 mW (5,0 W&#8260;cm2) apresentaram fotocontração (&#8722;&#916;V&#8260;V) de &#8764; 8 % (&#8764; 50 % maior) em atmosfera de O2, 5 % no ar, e menor que 2 % para o vácuo, N2 e He. Apesar de se partir da evaporação dos vidros para produção dos filmes, a principal diferença de estrutura entre vidro e filme é que estes últimos apresentam estrutura com grande número de vazios intersticiais ao redor de unidades estruturais, o que não ocorre para os vidros. Nossos resultados mostraram efetivamente que o efeito de fotocontração em filmes irradiados com UV está relacionado com mudanças de estrutura de grupos fosfatos e efeitos fototérmicos cooperativos. Na parte de aplicações gravamos redes holográficas de relevo nos filmes estudados. Utilizando uma montagem tipo espelho de Lloyd para holografia foi possível gravar redes com período de 600 nm até 20 &#181;m e medimos a eficiência de difração em tempo real utilizando um feixe de prova de laser He&#8722;Ne. As redes gravadas em filmes de filmes de 20% [Sb(PO3)3]n&#8722;80 % Sb2O3 com período de 20 &#181;m de espessura apresentam eficiência de difração de 3 a 10,5 % para aproximadamente 1 hora de gravação e potencia variando de 50 a 600 mW (2,5 a 30 W&#8260;cm2), respectivamente. / The aim of this work is the study of the Photoinduced Structural Changes (PSC) in films of the [Sb(PO3)3]n &#8722;Sb2O3 glassy system, as well as the production and characterization of the glassy system. The glass samples x[Sb(PO3)3]n&#8722;(100&#8722;x)Sb2O3 (5 &#8804; x &#8804; 40) were produced by the melt&#8722;quenehing technique of suitable quantities of the precursors Sb2O3 and [Sb(PO3)3]n, fused at 900&#176;C in glassy carbon crucibles and verted into stainless steel moulds. Homogeneous and stable glass plates of 1.0 x 1.0 x 0.5 cm3 were obtained. The characterization of the glass system and the obtaining of parameters such as the glass forming region and characteristic temperatures were possible through DSC, X&#8722;ray Diffraction, FTIR and MAS&#8722;NMR techniques. The studied glass system possess remarkable properties such as broad transmittance window (from 0.4 to 8.0 &#181;m), high index of refraction (n &#8764; 2.0), low melting temperature (Tf&#8764;900 &#176;C) and low glass transition temperature (Tg&#8764;300 &#176;C). The films were produced by electron beam physical vapor deposition (EB&#8722;PVD) of the glasses, in a non&#8722;commercial evaporation system developed in our laboratory, where is possible to produce high quality films with thickness of up to 10 &#181;m. The films samples presented an atypical photocontraction behavior that consists of a decrease of up to 10 % in the thickness, accompanied of photobleaching after UV irradiation using a 350.7 nm laser line. Besides the macroscopic characterization of the photocontraction effect (behavior as function of the UV laser power density and irradiation time), our attention was focused on the study of the effect at structural level using techniques such as X&#8722;ray diffraction, XANES, FTIR, EPR, RBS and Optical Holography. In addition to the wide characterization we shown in this work the possibilities of application of this material for optical storage and holography, diffraction gratings, micro-lens array.