The ultrafast charge dynamics in a number of nanostructured materials relevant to the production of renewable energy are investigated using ultrafast transient absorption spectroscopy. The materials include mercury telluride and cadmium mercury telluride quantum dots, and gold nanoparticles loaded on titanium dioxide colloidal spheres. The analysis of the resultant pump-induced transmittance change spectra and transients allow the determination of charge relaxation routes including multiple exciton generation, trion formation and direct-surface trapping. The investigation of HgTe QDs passivated with thioglycerol, mercaptopropionic acid and dodecanethiol ligands suggests that mercaptopropionic acid ligand results in better passivation of HgTe QDs due to its carboxylic acid group. It allows more electron density donation to the QD surface to passivate the traps related with unsaturated Hg bonds and hence supresses the associated non-radiative processes. The decay lifetimes of the thioglycerol/dodecanethiol-capped QDs in addition to the photo-induced absorption feature in their spectra, are found to be consistent with surface charge trapping observed in CdSe QDs. In comparison, the transients obtained for mercaptopropionic acid passivated QDs coupled with the pump-induced transmittance change spectrum show no sign of any surface-related processes. Therefore, our analyses allow the determination of multiple exciton generation for the first time in these QDs with a quantum yield of 1.36 ± 0.04 when photo-exciting with photons of energy 3.1 times the band gap. Such result should turn researchers’ attention to those ligands which could improve the QD solar cell field. The study of exciton dynamics in CdxHg(1-x)Te alloy QDs is also presented here. Their pump-induced transmittance change spectrum show two bleaches: at the shoulder position of the steady state absorption and at the PL peak. The exciton dynamics of these materials are studied using four different wavelengths, two of them are above the MEG threshold. The resultant transmittance transients and the pump-induced transmittance change spectrum are free of any photo-induced absorption or long-lived surface trapping. Hence, the decay of the transients obtained above the MEG threshold for well-stirred samples at low pump fluences is attributed to biexciton recombination. The assessment of multiple exciton generation reveals a quantum yield value of 1.12 ± 0.01 when photo-exciting with 2.6 times the band gap. Finally, the investigation of the recovery of the plasmon bleach in TiO2 colloidal spheres decorated with different sizes of Au NPs is presented in this thesis. The pump-induced transmittance change spectra obtained for two different wavelengths show bleaches at the plasmon band maximum superimposed with two wings of absorption features at shorter and longer wavelengths. The resultant transmittance transients for these samples are well-described by bi-exponential decay with a very quick decline of a few ps associated with electron–phonon scattering, followed by a slower decay over a few 10s of ps associated with heat dissipation. Only the heat dissipation rate is found to be dependent on the size of the Au NPs as it rises from 49 ± 3 ps to 128 ± 6 ps when the diameter of the Au NPs is increased from 12.2 ± 2.2 nm to 24.5 ± 2.8 nm, respectively.

A proposta desse estudo é investigar as propriedades de migração do éxciton, transferência de energia e transporte de cargas em heteroestruturas orgânicas ultrafinas compostas pela integração de um polímero semicondutor com moléculas de clorofila. A sintonização dos estados eletrônicos desses materiais torna possível a obtenção de heteroestruturas com modulação energética capazes de aprisionar éxcitons e cargas apresentando potencialidade de aplicação em Diodos Orgânicos Emissores de Luz (OLEDs). Para tal filmes de polifuoreno (PFO) (camada transportadora de carga) totalmente amorfo e filmes de clorofila (camada ativa na forma de poço de potencial) foram preparados utilizando a técnica e automontagem (LBL) combinada com spin-coating, caracterizados por microscopia confocal por varredura a laser e técnicas espectroscópicas de absorção e emissão. Investigou-se os processos fotofísicos utilizando microscopia confocal e de tempo de vida. Os resultados foram interpretados com base no modelo de transferência de energia de Förster combinado com as taxas de Miller-Abrahams e com a equação de difusão excitônica. Com essa abordagem, obteve-se uma relação entre a migração do éxciton no PFO e a transferência de energia não radiativa deste polímero para as moléculas de clorofila. Observou-se uma eficiente transferência de energia igual a 94% no regime de filmes ultrafinos. Para compreender os mecanismos de transporte de carga, implementamos e validamos o método de simulação de Monte Carlo para o transporte de carga em sistemas orgânicos desordenados. Com essa abordagem investigou-se a dinâmica das cargas em filmes poliméricos desordenados com e sem a camada de poço de potencial. Propriedades elétricas, tais como, mobilidade elétrica e coeficiente de difusão, foram obtidas e estão de acordo com os reportados na literatura. Obteve-se uma taxa de preenchimento de cargas no poço de potencial igual a 1010 buracos/s para campo elétrico de 1 MV/cm e constatou-se que a taxa aumenta com o campo elétrico. Tal abordagem apresenta-se como uma alternativa interessante para auxiliar o planejamento experimental de OLEDs baseados em heteroestruturas orgânicas. / In this study the exciton migration, energy transfer and charge transport in ultrathin organic heterostructure formed by semiconductor polymer and chlorophyll molecules were investigated. The energetic tuning between these materials promotes organic heterostructures with energetic modulation capable of trapping excitons and charges showing an application potential in Organic Light Emitting Diodes (OLEDS). Amorphous polyfluorenes (PFO) and chlorophyll a (chla) were prepared using self-assembly combined with spin-coating methods and characterized by confocal laser scanning microscopy and spectroscopic techniques. Photophysical processes were investigated using confocal and life-time microscopy and the results interpreted from the model of Förster energy combined with the Miller-Abrahams rate as well as the exciton diffusion equation. These results provided a relationship between the exciton migration in the PFO film and the non-radiative energy transfer from polymer to chla molecules. An efficient transfer of energy equal to 94% was observed. Method of the Monte Carlo simulation were implemented to investigate the charge transport in this disordered organic system. Using this method, the charge dynamics with and no potential well layer was studied. Electrical properties obtained, such as electric mobility and diffusion coefficient, are in agreement with literature. It was estimated a charge fill rate in the potential well equal to 1010 holes/s for 1 MV/cm and this parameter increases with the electric field. This approach has been shown to be an interesting alternative for the experimental design of OLEDs composed by organic heterostructure.

A proposta desse estudo é investigar as propriedades de migração do éxciton, transferência de energia e transporte de cargas em heteroestruturas orgânicas ultrafinas compostas pela integração de um polímero semicondutor com moléculas de clorofila. A sintonização dos estados eletrônicos desses materiais torna possível a obtenção de heteroestruturas com modulação energética capazes de aprisionar éxcitons e cargas apresentando potencialidade de aplicação em Diodos Orgânicos Emissores de Luz (OLEDs). Para tal filmes de polifuoreno (PFO) (camada transportadora de carga) totalmente amorfo e filmes de clorofila (camada ativa na forma de poço de potencial) foram preparados utilizando a técnica e automontagem (LBL) combinada com spin-coating, caracterizados por microscopia confocal por varredura a laser e técnicas espectroscópicas de absorção e emissão. Investigou-se os processos fotofísicos utilizando microscopia confocal e de tempo de vida. Os resultados foram interpretados com base no modelo de transferência de energia de Förster combinado com as taxas de Miller-Abrahams e com a equação de difusão excitônica. Com essa abordagem, obteve-se uma relação entre a migração do éxciton no PFO e a transferência de energia não radiativa deste polímero para as moléculas de clorofila. Observou-se uma eficiente transferência de energia igual a 94% no regime de filmes ultrafinos. Para compreender os mecanismos de transporte de carga, implementamos e validamos o método de simulação de Monte Carlo para o transporte de carga em sistemas orgânicos desordenados. Com essa abordagem investigou-se a dinâmica das cargas em filmes poliméricos desordenados com e sem a camada de poço de potencial. Propriedades elétricas, tais como, mobilidade elétrica e coeficiente de difusão, foram obtidas e estão de acordo com os reportados na literatura. Obteve-se uma taxa de preenchimento de cargas no poço de potencial igual a 1010 buracos/s para campo elétrico de 1 MV/cm e constatou-se que a taxa aumenta com o campo elétrico. Tal abordagem apresenta-se como uma alternativa interessante para auxiliar o planejamento experimental de OLEDs baseados em heteroestruturas orgânicas. / In this study the exciton migration, energy transfer and charge transport in ultrathin organic heterostructure formed by semiconductor polymer and chlorophyll molecules were investigated. The energetic tuning between these materials promotes organic heterostructures with energetic modulation capable of trapping excitons and charges showing an application potential in Organic Light Emitting Diodes (OLEDS). Amorphous polyfluorenes (PFO) and chlorophyll a (chla) were prepared using self-assembly combined with spin-coating methods and characterized by confocal laser scanning microscopy and spectroscopic techniques. Photophysical processes were investigated using confocal and life-time microscopy and the results interpreted from the model of Förster energy combined with the Miller-Abrahams rate as well as the exciton diffusion equation. These results provided a relationship between the exciton migration in the PFO film and the non-radiative energy transfer from polymer to chla molecules. An efficient transfer of energy equal to 94% was observed. Method of the Monte Carlo simulation were implemented to investigate the charge transport in this disordered organic system. Using this method, the charge dynamics with and no potential well layer was studied. Electrical properties obtained, such as electric mobility and diffusion coefficient, are in agreement with literature. It was estimated a charge fill rate in the potential well equal to 1010 holes/s for 1 MV/cm and this parameter increases with the electric field. This approach has been shown to be an interesting alternative for the experimental design of OLEDs composed by organic heterostructure.

Abstract: Understanding and controlling the ultrafast charge carrier and exciton dynamics at the interface of semiconductor nanocrystals (NCs) offer an excellent opportunity to improve the charge collection and the overall performance of many optoelectronic and energy-based devices. In this dissertation, we study how interfacial engineering of these materials can have a direct influence on controlling the charge transfer and the nonradiative losses in different donor-acceptor systems. The first introductory chapter provides an overview of all the fundamental photophysical processes controlling the interfacial phenomena. Then, the second chapter highlights all the chemicals and synthesis methods employed during this thesis. The subsequent two chapters discuss the detailed experimental studies and observations related to different materials and interfaces. First, it describes how we can dramatically tune the intersystem crossing (ISC) rate, the triplet state lifetime, turn on/off the electron injection at the CdTe-Prophyrin interface via tuning either the quantum dot size or the porphyrin molecular structure. Also, how the intermolecular distances, electronic coupling, and subsequently, the photoinduced charge transfer can be controlled by the interfacial electrostatic interactions at CdTe-Fullerene interfaces. Second, due to the promise that of perovskite NCs holds for improving many solar cell and optoelectronic applications, chapter 3 highlights the tremendous effect that the shape of perovskite nanocrystals has on the rate and the mechanism of charge transfer at the MAPbBr3- TCNE interface. Besides, it demonstrates how the confinement effect brought by changing the dimensionality influence the charge transfer dynamics at the MAPbBr3-BQ interface. Finally, it explains how the effective passivation of the surface defects and the subsequent suppression of the formation of surface nonradiative recombination centers in CsPbCl3 NCs controls the photoluminescence quantum yield and the photodetector performance.

In this thesis, we investigate the collective electron dynamics in single and coupled superlattice systems under the influence of a DC electric field. Firstly, we illustrate that Bloch oscillations suppress electron transport and the resulting charge domains form self-sustained current oscillations. Upon the application of a tilted magnetic field, stochastic web structures are shown to form in the phase space of the electron trajectory. This occurs only when the Bloch and cyclotron frequencies are commensurate allowing the electrons to demonstrate chaotic unbounded trajectories, leading to an increase in transport. The charge domain dynamics also present additional peaks during such resonances. The rapid changes in the dynamical states found is an example of non-KAM chaos. We show then the amplitude and frequency of current oscillations in a single superlattice can be controlled. Secondly, two models are designed to mutually couple two semiconductor superlattices by a common resistive load. We examine the effects of coupling strength and frequency detuning on the collective current dynamics. The devices are considered to be arranged together on a single substrate as well as on individual substrates. Large AC power is witnessed during anti-phase and in-phase synchronization between current oscillations. Finally, two superlattices are coupled through a resonance circuit incorporating single mode resonances from external influences in the circuit. In this system, chaotic current dynamics are induced with regions of chaos separating different regions of synchronization. High frequency oscillations with minimal phase difference cause the largest power generation. In all three coupling models high frequency components are found in the Fourier power spectra. The power generated in the coupled systems is found greater and at times more than double the power generated in the autonomous superlattice. Thus this thesis provides innovative methods of enhancing and controlling powerful high-frequency signals. This effectively gives manipulation over the intensity of the electromagnetic radiation produced by the superlattice.

Ce travail étudie le rôle de la densité de trous à l’équilibre sur la dynamique des charges et de la norme de l’aimantation de (Ga,Mn)As pour des densités de manganèse et d’impuretés fixées indépendamment. Des expériences « pompe-sonde » mettent en relation les dynamiques de réflectivité et d’angle de rotation Kerr. Deux relaxations sont mises en évidence. La première traduit un échauffement variable du gaz de trous entre 1ps et 100ps. La seconde traduit une diffusion-recombinaison des charges entre 100ps et 1500ps et évolue en fonction du rapport entre extension spatiale d’états d’impuretés, piégeant les électrons photo générés, et vitesse de Fermi. Pour compléter l’approche, une étude numérique de l’état fondamental des échantillons par la théorie de la fonctionnelle de la densité relie aimantation, température et densité de trous. Elle interprète la dynamique de la norme de l’aimantation à partir d’un diagramme de phase statique correspondant aux données publiées pour (Ga,Mn)As, qui est fonction de la température et de la densité de trous. Cette dynamique se ramène à celle de la réflectivité. Ceci permet de préciser les contributions de la norme et de l’orientation de l’aimantation dans le signal dynamique de rotation Kerr. / The effects of the background hole density on the charge and magnitude of the magnetization dynamics in (Ga,Mn)As grown with independently fixed manganese and impurity densities. A pump and probe experiment monitored simultaneously the reflectivity and Kerr angle dynamics. Two relaxation steps are highlighted. First the cooling down of the charge clouds between 1ps and 100ps and second the carrier’s diffusion-recombination between 100ps and 1.500 ns. The latter depends on the ratio between the spatial extent of impurity states, which trap the photo electrons, and the Fermi velocity. To complete these experimental results, a numerical study of the ground state of the samples, using a density functional theory, relates the magnitude of the magnetization, the temperature of the carriers and the density of holes. Phase diagram are computed, and compared to already published results. We show that the magnitude of the magnetization dynamics can be fully determined from the reflectivity measurements. We conclude that it is possible to distinguish the dynamics of the magnetization magnitude and direction using the Kerr angle dynamical signal.