Unfolding the Excited States Dynamics of Tm²⁺-doped Halides: In Prospect of Novel Luminescence Solar Concentrators

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.RST/Luminescence Material