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Title
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Characterization of novel materials for thin film photovoltaics by electron paramagnetic resonance and optical spectroscopic methods
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Author
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Abstract
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| The high potential of light-weight and flexible solar cells for future built-in and even wearable applications has driven research efforts towards thin-film photovoltaics. Such devices feature an active-layer thickness of only a few 100 nm, which is substantially less than the 200 μm thick wafers typically employed in Si-based solar cells. This thesis focuses on two emerging thin film technologies: organic solar cells (OSCs), which are based on molecular semiconductors, and perovskite solar cells (PeSCs) of which the active component is a lead halide with the perovskite crystal structure. High-performance OSCs rely on a bulk heterojunction (BHJ) between an electron donor and acceptor for efficient photogeneration via charge transfer (CT). Hence, an important research objective is to identify the fundamental energy losses associated with the CT process and the main mechanisms governing charge recombination. This work contributes to this research area with an in-depth spectroscopic study of recombination via triplet excitons in two BHJ blends with a novel non-fullerene acceptor based on the 2,5-dithienylthiazolo[5,4-d]thiazole (DTTzTz) unit. Combining photo-induced absorption and luminescence spectroscopy, the photo-physical pathways of non-radiative recombination via triplets were determined, providing valuable insight into the possible suppression of triplet-related losses via molecular design. Over the past decades, electron paramagnetic resonance (EPR) has played a crucial role in the study of OSCs because of its selectiveness in detecting the positive and negative charge carriers created in the BHJ blend under illumination. The spectral filtering methods developed in this work enable the unambiguous identification and characterization of these species when their spectra are strongly overlapping, which is often the case for fullerene-free blends. The control of structural defects is essential for the further development of any semiconductor technology. Hence, one of the main objectives of this PhD research was to identify the dominant intrinsic defects in paradigm PeSC absorber methylammonium lead iodide. Hereto I have used a combination of continuous-wave and pulsed EPR and corresponding parameter computations in order to derive microscopic models for the relevant defects. While no signatures of paramagnetic defects could be observed by EPR in standard CH3NH3PbI3, this work presents the first experimental observation of light-induced polaronic states in related two-dimensional perovskites. As a first step towards future predictive computations of the magnetic resonance parameters of polaronic defects in perovskites using density functional theory (DFT), the thesis concludes with a case study of the well-known self-trapped electron in PbCl2, a closely-related lead halide material. |
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Language
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English
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Publication
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Antwerpen
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Universiteit Antwerpen, Faculteit Wetenschappen, Departement Fysica
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2020
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Volume/pages
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168 p.
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Note
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Goovaerts, Etienne [Supervisor]
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Van Doorslaer, Sabine [Supervisor]
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Full text (open access)
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