No description available.

No description available.

No description available.

Ultrafast charge and electron transfer, primary events in artificial photosynthesis, are key in solar energy harvesting. This dissertation provides insight into photo-induced charge and electron transfer in the donor and acceptor constructs built using a range of donor and acceptor entities, including transition metal dichalcogenides (TMDs, molybdenum disulfide (MoS2), and tungsten disulfide (WS2)), N-doped graphene, diketopyrrolopyrrol (DPP), boron-dipyrromethene (BODIPY), benzothiadiazole (BTD), free base and metal porphyrins, zinc phthalocyanine (ZnPc), phenothiazine (PTZ), triphenylamine (TPA), ferrocene (Fc), fullerene (C60), tetracyanobutadiene (TCBD), and dicyanoquinodimethane (DCNQ). The carefully built geometries and configurations of the donor and (D), acceptor (A), with a spacer in these constructs promote intramolecular charge transfer, and intervalence charge transfer to enhance charge and electron transfer efficiencies. Steady-state UV-visible absorption spectroscopy, fluorescence and phosphorescence spectroscopies, electrochemistry (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)), spectroelectrochemistry (absorption spectroscopy under controlled potential electrolysis), transient absorption spectroscopy, and quantum mechanical calculations (density functional theory, DFT) are used to probe ground and the excited state events as well as excited state charge separation resulting in cation and anion species. The current findings are useful for the increased reliance on renewable energy resources, especially solar energy.

No description available.

No description available.

<p> Excitons represent the electronic excited state of organic semiconductor and many low-dimensional inorganic semiconductors. In solar energy conversion systems, exciton transport affects how fast the charges reach the electrodes thus governs the performance of photovoltaic cells. In optoelectronic applications such as semiconductor lasers and light-emitting diodes, exciton radiative rate determines the efficiency of luminescence in competition to various nonradiative processes. Therefore, understanding how exciton migrates over space as well as its decay dynamics are vital for the design of highly efficient optoelectronic devices. To interrogate these photophysical processes requires experimental tools with simultaneous high temporal and spatial resolution. In this thesis, I introduce two transient imaging systems (photoluminescence imaging with 300 ps time resolution, and transient absorption microscopy with 200 fs time) that are innovative tools to directly probe excited state dynamics and transport in sub-μm domains. The techniques were applied to a type of promising semiconductor, perovskites, including surface-passivated hybrid perovskite and 2D layered perovskites to explore the fundamental mechanisms that affect exciton transport. The fundamental understanding of excitons shed light on the underlying physics such as exciton delocalization, exciton-exciton interaction, and how these properties affected by the static and dynamic disorders of the material. We further demonstrated a novel twisted superlattice using ultrathin perovskites that confines excitons due to increased density of state from the moiré flat bands. In addition, excitons can be accelerated by strongly interacts photons, forming polariton quasiparticles that possess small effective mass. This is demonstrated by coupling 2D layered perovskites to a plasmonic array. We further showcase the formation of bulk polaritons without an external optical cavity in a self-assembled organic aggregate. Experimental investigation into these intriguing phenomena provide an approach to study fundamental processes such as many-body interaction and quantum coherence. </p>

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

In this dissertation, we investigate two emerging strategies for enhancing the performance of organic photovoltaics. The first takes advantage of a process called singlet exciton fission, and the second embodies an exodus from the fullerene electron acceptors prominent in organic solar cells. Indeed, this versatile class of tunable small molecules are aptly termed nonfullerene acceptors. However, both strategies would benefit from a greater understanding of underlying principles. Singlet exciton fission is a photon-multiplying process in which a singlet exciton from a high-energy absorbed photon splits into two triplet excitons. The process could significantly reduce energy lost to heat in photovoltaic devices, but its mechanisms are still misunderstood. One model involves direct coupling between the singlet and triplet states, and another model involves an intermediate charge transfer state. Transient absorption spectroscopy allowed us to examine singlet fission in films of pentacene, fluorinated pentacene, and coevaporated blends of various mixing ratios. We directly observe an intermolecular charge transfer state during singlet fission in solid films of coevaporated pentacene and peruoropentacene, which supports the model of charge transfer state-mediated singlet fission. Furthermore, we successfully induced singlet fission in one blend by directly exciting the charge transfer state below the bandgap. We use various types of steady state and time-resolved spectroscopy to characterize two types of nonfullerene electron acceptors. The first type is a group of tetraazabenzodiuoranthene diimide (BFI) dimers and a BFI monomer. The BFI dimers were designed to have twisted, nonplanar 3-dimensional structures and have helped achieve power conversion efficiencies of over 8% in organic solar cells. The other type of nonfullerene acceptor is a calamitic small molecule, and we consider the BAF-4CN electron acceptor, which has also been used in a solar cell whose efficiency exceeded 8%. Spectroscopic studies give insight into the performances of these nonfullerene devices in relation to fullerene-derivative counterparts. We find that the nonfullerene blends suffer from more geminate charge recombination. However, despite this drawback, in some cases, slower rates of nongeminate recombination may lead to successful power conversion efficiencies in nonfullerene solar cells.