This research aims to investigate the potential of utilizing particle-based luminescent coatings for light management in greenhouses. Light management includes solar spectral conversion, transmittance, and scattering. Phosphor materials perform energy conversion from high-energy radiation to low-energy radiation through photoluminescent mechanisms. These phosphor materials are applied in the form of a sprayable coating intended to be employed on greenhouse roofing. Through energy conversion, a luminescent phosphor coating can achieve spectral conversion, adding light for crop growth, and will at the same time affect both light transmittance and scattering in greenhouse roofing. The application of phosphors as luminescent coatings on greenhouse roofing is a young research topic with very few published results; therefore, it presents several research challenges. The first is the lack of practical work on luminescent greenhouse coatings, demonstrating what type of spectral conversion is beneficial to plants and even the general feasibility of phosphors for greenhouse applications. The second challenge is to define clear and standardized measurable quantities that determine the success of a greenhouse coating, for example, the coating’s UV-to-PAR conversion efficiency. The third challenge is to design and build a measuring system that can characterize these quantities, especially the hemispherical light transmittance of small-scale luminescent coating samples… ... Read more

The topic of this thesis pertains to researching novel materials for luminescent solar concentrator (LSC) applications. An LSC, as the name suggests, concentrates solar radiation incident on a large surface area to a smaller surface area. First conceptualized in the 1970’s, LSCs can be used in conjunction with solar cells to create a so-called ‘electricity generating window’. Fast-forward 50+ years and the electricity generating window is yet to be commercialised. In Chapter 1 the motivation behind developing LSCs and their operating principles is introduced along with obstacles that must be overcome in order to mass adopt this technology. For example, from basic calculations, it can be shown that in order for an electricity generating window to obtain a commercially interesting power output of >100 W/m2 it is necessary to absorb at least 50% of both the ultraviolet (UV) and visible (VIS) part of the solar spectrum. Absorbing only a part of the VIS spectrum does not only lower output but will also lead to unwanted colorization of the window. Another obstacle that is often overlooked is complying with glass-industry practises which limits synthesis methods to create LSCs. Reactive magnetron sputtering is a technique for synthesizing thin films that is up-scalable and already a standard practise in the glass industry. Sputtering is therefore the synthesis method of choice in all the work presented. The key challenge addressed in this thesis is in obtaining thin films with a thickness of a micron or less that meet the absorption and luminescence requirements for efficient LSCs..... ... Read more

Photon-counting detectors (PCD) for medical X-ray computed tomography (CT) are designed to measure the number of X-ray photons incident on a detector pixel as well as the energy of the individual X-rays. They are expected to yield improvements in image quality for a given radiation dose, and to offer opportunities for spectral imaging beyond dual-energy techniques. However, the fluence rate incident on the detector can exceed 108 mm-2 s-1 in CT, so that the detector pulses generated by the X-rays likely pile up on each other, which distorts the measurement. The semiconductors CdTe and Cd1-xZnxTe (CZT, x ≈ 0.1-0.2) are commonly considered efficient X-ray absorbers that provide sufficient rate capability (fast pulses in the order of 101 ns and a high pixel density ≥ 4 mm-2) and energy resolution (8-20% FWHM at 60 keV). In such detectors, an X-ray is converted into electron-hole pairs, which travel to (pixelated) electrodes, on which they induce a current pulse. However, the cost-effective synthesis of material of sufficient quality to make this a stable and reliable detection process appears to remain an issue. Thus, the aim of this thesis is to explore the photon-counting performance, e.g., the rate capability and energy resolution, of an alternative detector concept based on a scintillator, which converts an X-ray into a light pulse, and a silicon photomultiplier (SiPM), which detects the light. Since such a detector relies on light rather than charge transport, it may enable cost-effective manufacturing of stable and reliable PCDs...
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Tm²⁺-doped halides exhibit excellent properties for use in Luminescence Solar Concentrator (LSC) applications. Such LSCs consist of a glass waveguide with small Copper-Indium- Selenide (CIS) solar cells attached to its edges. The waveguide contains a luminescent coating based on a Tm²⁺-doped halide, whose 4f125d1 absorption bands are able to absorb a large fraction of the AM 1.5 solar spectrum. As the absorption occurs over the entire visible light range (380-750 nm) and with a largely uniform absorption strength, the coating can appear colourless and semi-transparent. Via the Tm²⁺ excited states dynamics, the absorbed sunlight will be converted into the 2F5/2→2F7/2 emission that has a wavelength of 1140 nm. Since this emission falls outside the range of the 4f125d1 absorption bands, no selfabsorption losses can occur. These generally pose a significant limitation to the overall LSC efficiency. Subsequently, the converted light is re-emitted by the coating and propagates via total internal reflection through the waveguide that directs it towards the CIS solar cells. These solar cells then photovoltaically convert it into electricity. LSC coatings based on Tm²⁺- doped halides can be applied as a sustainable window technology and, as part of Building- Integrated Photovoltaics (BIPV), can reduce the energy consumption of buildings making them self-sustaining and less dependent on fossil fuels. [1] Although the optical and luminescence properties of primarily CaF2:Tm²⁺, CsCaX3:Tm2+ (X = Cl, Br, I) and MCl2:Tm²⁺ (M = Ba, Ca, Sr) have been investigated, a substantial amount of other Tm²⁺-doped halides remain unexplored. Above all, key topics such as the internal Quantum Efficiency (QE) and Tm2+ concentration quenching in such materials remains completely unaddressed. The former property has a direct influence on the overall LSC efficiency and is governed by the Tm2+ excited states dynamics of the material. In the past, the Tm²⁺ excited states dynamics has been studied in depth for Tm²⁺-doped CsCaX3 (X = Cl, Br, I) trihalide perovskites. [2-5] However, in these works no other options beside quenching via multi-phonon relaxation has been considered and no correlation was made to QEs nor were such values ever reported. It is therefore the goal of this dissertation to investigate the Tm²⁺ excited states dynamics in various halides as a function of composition, temperature and time, and in connection to the 2F5/2→2F7/2 QE. Both a qualitative and quantitative analysis is provided on the different 4f125d1→4f125d1 and 4f125d1→4f13 nonradiative quenching processes. ... Read more

Scintillation materials convert high-energy radiation into many visible photons and, in combination with a photodetector, are used as ionizing radiation detectors. Since the discovery of ionizing radiation, there have been intensive research efforts in finding new, better performing scintillators, resulting in the development of a large variety of scintillation materials. Each scintillation material is tailored to a specific application to have the best performance. With ever-increasing material demands set by the various applications of scintillators, the search for even better performing scintillation materials remains an active field. This thesis explores newavenues of scintillation materials research in order to find the next-generation of scintillation materials. ... Read more