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…

Diffusion-like behavior of excitons within europium doped vanadate phosphors is studied, with emphasis on the trapping potential induced by Eu3+ and VO43− luminescence. We describe the hopping, energy transfer, and decay of excitons by means of a continuous-time Markov chain distinguished by a local and delocalized trapping potential. Our model compares luminescence from decay of excitons with the trapping problem, where VO43− traps are present everywhere, while Eu3+ ions are placed randomly. We distinguish two types of trapping potentials induced by europium traps. In the localized model, simplify the lattice to Zd where the trapping potential of europium traps is only present at randomized sites in the lattice. By applying the large deviation principle for the occupation times measure, as done by Donsker and Varadhan previously, we find probability mass asymptotics of the form exp td/(d+2). For a simulation-like approach, we describe the delocalized model, where exciton movement and trapping in crystal lattices of Rb3LuV2O8:Eu3+ and YVO4:Eu3+ and is modeled using parameters such as temperature, migration rates, emission lifetimes, and europium concentration to estimate luminescent properties of the materials. Experimental data from vanadate lifetimes of Rb3LuV2O8:Eu3+ and VO43− /Eu3+ emission ratio’s of YVO4:Eu3+ are compared to the model, which agree relatively well within the confidence of the model. For YVO4:Eu3+, we have found an activation energy of Ea = (101 ± 4) meV. We discuss how our simplified model can be extended to incorporate thermal and concentration quenching, and how to account for defects in the lattice. While we discuss our findings for europium doped vanadate phosphors, the results we have found are applicable for a range of other luminescent materials.  

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.....

This study introduces a theoretical framework for assessing the impact of particle-based greenhouse spray coatings on crop yield. Greenhouses are pivotal for optimizing crop growth efficiency, and these coatings can enhance their performance. The research employs Monte Carlo ray tracing to model transmission, scattering, and luminescence processes within a coating-glass system. It investigates coatings with scattering particles (SiO2), luminescent scattering particles (SiAlO:Eu2+), and a combination of both. Key performance metrics, including hemispherical transmittance and hortiscatter, are calculated. The primary challenge is to effectively scatter incoming light while maintaining high transmission rates. Additionally, the luminescent coatings should efficiently absorb ultraviolet light without affecting photosynthetically active radiation. By utilizing the Non-dominated Sorting Algorithm II alongside Mie theory, this investigation explores the effects of varying particle size, volume mixing percentage, and coating thickness. Notably, the study identifies an optimal coating configuration that combines micrometer-sized SiO2 and nanometer-sized SiAlO:Eu2+ particles, resulting in a significant increase in crop yield (Y+ = 10.1 %). Further research is recommended to validate the crop yield formula and gain a deeper understanding of the impact of greenhouse coatings on agricultural productivity.

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.

The use of protons to treat cancer has expanded rapidly in the past two decades. For safe and effective proton therapy, the proton range in a patient’s body must be accurately determined. Current treatment planning is based on X-ray computed tomography images, which might cause uncertainty because of the different behaviour between protons and X-rays. As an alternative, proton Computed Tomography (pCT) has been proposed to directly measure the Relative Stopping Power (RSP) map in the patient and reduce this uncertainty. During a proton CT scan, a high-energy proton beam is directed at the patient. Then, the proton’s residual energy and position are measured with a detector placed behind the patient. This information is used to calculate the volumetric RSP. In the case of using a pixel based detector, a tracking algorithm is required in order to increase the proton intensity capacity of the detector. A proton track reconstruction system has been already developed by Pettersen [1], however, it has some limitations on the track density that can be reconstructed correctly. The algorithm is based on the track-following scheme, in which a growing track searches for deeper- laying activated pixels. This thesis introduces proton therapy and the advantages of pCT and proton radiography for treatment planning. Then, the main track reconstruction techniques found in the literature are reviewed. Improvements in the reconstruction process are proposed and their efficiencies are discussed. While current algorithm begins from the layer closest to the patient, in the present study a new reconstruction algorithm is developed. It differs by starting the reconstruction process from the distal end of the detector. Based on this new algorithm, studies related to its optimization are conducted. Lastly, an algorithm based on the identification of the most probable scenario is developed. The potential algorithms are evaluated on data simulated with GATE (based on Monte Carlo interactions) and PROCASIM (design to simplify the physical interactions between protons and the detector). The fraction of correctly reconstructed tracks and the computational eciency of the algorithms are analyzed to determine the most viable one. [1] H. E. S. Pettersen. A Digital Tracking Calorimeter for Proton Computed Tomography. PhD thesis, University of Bergen, Norway, February 2018.

PowerWindow is a luminescent solar concentrator which employs a Tm2+-doped material to strongly absorb sunlight and produce a sharp emission in the near-infrared spectrum for energy-harvesting purposes. The absence of self-absorption and the large spectral overlap with the solar spectrum make it an attractive addition to current building-integrated photovoltaics development. Such Tm2+-doped materials have so far been limited to halides that are susceptible to decomposition when exposed to air, which compromises its stability and limits the application. The solutions to functionalize the characteristic Tm2+ photoluminescence properties are two-fold: by an alternative inorganic host (CaSi2O2N2) that is insensitive to moisture, or by using a protective barrier (silicone) that protects halides against decomposition upon exposure to air. The proposed alternative phosphor CaSi2O2N2:Tm is synthesized by solid-state reaction and its luminescent property is examined. The Tm dopant is found to exist in trivalent state (Tm3+) and no luminescence of Tm2+ is found in the material. Besides Tm2O3, different starting powders with other oxidation states (TmI2 and Tm metal powder) were tested to reduce Tm3+ ion to Tm2+ but no change has been observed. It is concluded that the high-lying Tm2+ 4f electronic ground state in CaSi2O2N2 is susceptible to oxidization by losing electrons to its conduction band and therefore unlikely to be stabilized with this synthesis method. The protective barrier approach applied to the NaCl:Tm2+ phosphors was realized by a polydiphenylsiloxane silicone encapsulant. The precursors and the phosphors are cured together in a lamination scheme between two pieces of glass. From the optical properties derived from its absorption and transmission spectra, it is concluded that the Tm2+ valence state of NaCl:Tm2+ is stable in the polymer with no sign of Tm3+ formation at all. While its production process and the relative amount of phosphor still need to be further optimized, a proof-of concept has been attained.