Les propriétés optiques linéaires et non-linéaires de nanoparticules métalliques de tailles non-négligeables comparées à celles des longueurs d’onde excitatrices sont étudiées dans cette thèse. Les informations issues de la symétrie sont mises en avant afin de décrire des nanoparticules appartenant à des groupes ponctuels. Pour cela, un formalisme totalement irréductible est mis en place afin de prendre en compte l’extension spatiale des objets étudiés. Dans ce formalisme, le tenseur de réponse non-linéaire possède un nombre fini de valeurs significatives reliant les composantes multipolaires des champs incidents et sortants. Ce formalisme est alors appliqué analytiquement à l’étude de la réponse non- linéaire du second ordre de nano-étoiles d’or en interprétant des mesures de SHG résolue en polarisation. Finalement, des expériences de spectroscopies multidimensionnelles sont utilisées dans le but de connecter les propriétés spatiales et les propriétés spectrales de ces objets. L’introduction de modes propres définis par la symétrie des objets permet encore une fois de donner un sens physique aux comportements électroniques mis en jeu / Using metallic nanoparticles with a threefold symmetry thorough the study, the impact of the symmetry on the nonlinear properties is investigated. Interpretations of polarization-resolved SHG experiments indicate the importance of multipolar resonances, in particular quadrupole and octupole, to explain the strong values of the nonlinear susceptibilities in such systems. A fully irreducible formalism is then developed to treat extended objects like nanoparticles. In this formalism, the nonlinear response tensor is a discrete set of values easily constrained by symmetries instead of a field. This formalism permits to describe simply linear and nonlinear optical response from nanoparticles. Finally, time-domain experiments are conducted with the aim to connect spatial and spectral properties. These experiments allow to interpret the spectra in terms of eigenmodes

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Harnessing the Sun’s energy via the conversion of solar photons to electricity has emerged as a sustainable energy source to fulfill our future demands. In this regard, solution-processable, size-tunable PbS quantum dots (QDs) have been identified as a promising active materials for photovoltaics (PVs). Yet, there are still serious challenges that hinder the full exploitation of QD materials in PVs. This dissertation addresses two main challenges to aid these QDs in fulfilling their tremendous potential in PV applications. First, it is essential to establish a large-scale synthetic technique which maintains control over the reaction parameters to yield QDs with well-defined shape, size, and composition. Rigorous protocols for cost-effective production on a scale are still missing from literature. Particularly, previous reports of record-performance QD-PVs have been based on small-scale, manual, batch syntheses. One way to achieve a controlled large-scale synthesis is by reducing the reaction volume to ensure uniformity. Accordingly, we design a droplet-based continuous-flow synthesis of PbS QDs. Only upon separating the nucleation and growth phases, via a dual-temperature-stage reactor, it was possible to achieve high-quality QDs with high photoluminescence quantum yield (50%) in large-scale. The performance of these QDs in a PV device was comparable to batch-synthesized QDs, thus providing a promise in utilizing automated synthesis of QDs for PV applications. Second, it is crucial to study and control the charge transfer (CT) dynamics at QD interfaces in order to optimize their PV performance. Yet, the CT investigations based on PbS QDs are limited in literature. Here, we investigate the CT and charge separation (CS) at size-tunable PbS QDs and organic acceptor interfaces using a combination of femtosecond broadband transient spectroscopic techniques and steady-state measurements. The results reveal that the energy band alignment, tuned by the quantum confinement, is a key element for efficient CT and CS processes. Additionally, the presence of interfacial electrostatic interaction between the QDs and the acceptors facilitates CT from large PbS QD (bandgap < 1 eV); thus enabling light-harvesting from the broad near-infrared solar spectrum range. The advances in this work – from automated synthesis to charge transfer studies – pave new pathways towards energy harvesting from solution-processed nanomaterials.

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.

This diploma thesis deals with the possibilities of multiparametric fluorescence spectroscopy, since the main objective of this experiment was to evaluate the possibilities of multiparametric measurements in the fluorescence spectroscopy laboratory. A suitable fluorescence probe was proposed for this type of experiment that shows high sensitivity for pH changes in the environment, SNARF-4F AM, based on a literature research. The fluorophore was dissolved in solutions of different pH and this system was examined using a time-resolved spectrofluorimeter. The method named TRES (time-resolved emission spectra) was used to obtain the emission spectra of the probe and to find the emission maximum. Fluorescence intensity decay measurements as a function of wavelengths allowed to create deconvolution of the emission spectra, which provided information about the fluorescent lifetime and the relative representation of the states of probes in the solution. Later, the probe was dissolved in solutions of different density and pH - this system served for anisotropic measurements, during which the individual correlation-rotational times of the fluorophore were obtained. The obtained results were then used as the basis for multiparametric analysis, which was performed by using a fluorescence correlation microscope and a spectrograph. This combination allows to measure the necessary fluorescence parameters in one step. A standard operating procedure was created for the spectrograph’s control. On the basis of the obtained information the suitability, accuracy and sensitivity of the multiparametric analysis were qualified.

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As the global economy searches for reliable, inexpensive and environmentally friendly renewable energy resources, energy conservation by means of photovoltaics has seen near exponential growth in the last decade. Compared to state-of-the-art inorganic solar cells, organic photovoltaics (OPVs) composed of conjugated polymers are particularly interesting because of their processability, flexibility and the potential for large area devices at a reduced fabrication cost. It has been extensively documented that the interchain and intrachain interactions of conjugated polymers complicate the fundamental understanding of the optical and electronic properties in the solid-state (i.e. thin film active layer). These interactions are highly dependent on the nanoscale morphology of the solid-state material, leading to a heterogeneous morphology where individual conjugated polymer molecules obtain a variety of different optoelectronic properties. Therefore, it is of the utmost importance to fundamentally study conjugated polymer systems at the single molecule or nanoparticle level instead of the complex macroscopic bulk level. This dissertation research aims to develop simplified nanoparticle models that are representation of the nanodomains found in the solid-state material, while fundamentally addressing light harvesting, energy transfer and interfacial charge transfer mechanisms and their relationship to the electronic structure, material composition and morphology of the nanoparticle system. In preceding work, monofunctional doped nanoparticles (polymer-polymer) were fabricated with enhanced light harvesting and Fӧrster energy transfer properties by blending Poly[(o-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)] (BPPV) and Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at various MEHPPV doping ratios. While single particle spectroscopy (SPS) reveals a broad distribution of v optoelectronic and photophysical properties, time-correlated single photon counting (TC-SPC) spectroscopy displays multiple fluorescence lifetime components for each nanoparticle composition, resulting from changing polymer chain morphologies and polymer-polymer aggregation. In addition, difunctional doped nanoparticles were fabricated by doping the monofunctional doped nanoparticles with PC60BM ([6,6]-phenyl-C61-butyric acid methyl ester) to investigate competition between intermolecular energy transfer and interfacial charge transfer. Specifically, the difunctional SPS data illustrated enhanced and reduced energy transfer mechanisms that are dependent on the material composition of MEH-PPV and PC60BM. These data are indicative of changes in inter- and intrachain interactions of BPPV and MEH-PPV and their respective nanoscale morphologies. Together, these fundamental studies provide a thorough understanding of monofunctional and difunctional doped nanoparticle photophysics, necessary for understanding the morphological, optoelectronic and photophysical processes that can limit the efficiency of OPVs and provide insight for strategies aimed at improving device efficiencies.

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