The positioning of the Tm²⁺ 4f¹²5d¹ and 4f¹³ energy levels as relative to the conduction band of the orthorhombic BaCl₂ host lattice has been determined. Therefore, the energies of the Tm²⁺ 4f¹²5d¹-and excited 4f¹³-level were retrieved, as relative to the Tm²⁺ 4f¹³ ground state. In addition, the energy for exciton creation in the orthorhombic BaCl₂ host lattice was established, from which the bandgap energy was determined. This value was found to correspond quite well to known literature values. Furthermore, the Tm³⁺-Cl^- charge transfer transition was determined, from which the energy difference between the Tm²⁺ 4f¹³ ground state and the top of the BaCl₂ valence band was deduced. A host referred binding energy scheme deduced for BaCl₂:Tm²⁺ then showed that the lowest energy Tm²⁺ 4f¹²5d¹-levels are positioned 0.3–0.5 eV below the BaCl₂ conduction band. Room temperature photo-excitation into this level will then most likely result in thermal ionization effects that have an impact on the Tm²⁺ 4f¹²5d¹→4f¹³ and 4f¹³→4f¹³ luminescence and corresponding quantum yield.

In this study, we systematically vary the Cl/Br and Br/I ratios in CsCaX₃:Tm²⁺ (X = Cl, Br, I) and hereby gradually shift the positions of the Tm²⁺ 4f¹²5d¹-levels as relative to the two 4f¹³ levels. At low temperatures up to five distinct Tm²⁺ 4f¹²5d¹→4f¹³ emissions and the 4f¹³→4f¹³ emission can be observed. As the temperature increases, most of the 4f¹²5d¹→4f¹³ emissions undergo quenching via multi-phonon relaxation (MPR) and at room temperature only the lowest energy 4f¹²5d¹→4f¹³ and the 4f¹³→4f¹³ emission remains. For all compositions a 4f¹³→4f¹³ risetime phenomenon is then observed whose duration matches the 4f¹²5d¹→4f¹³ decay time. It shows the feeding of the 4f¹³ state after 4f¹²5d¹ excitation. Surprisingly, the feeding time becomes longer from Cl→Br→I, while the related 4f¹²5d¹-4f¹³ energy gap becomes smaller. The temperature dependence of the 4f¹²5d¹→4f¹³ and 4f¹³→4f¹³ emission intensity shows a anticorrelation as earlier observed in other systems and confirms that the feeding process is thermally stimulated. However, the thermally stimulated activation energies that control the feeding process, increase from Cl→Br→I despite our observation that the 4f¹²5d¹-4f¹³ energy gap becomes smaller. An analysis reveals that the unexpected behaviour in risetime and activation energy, as a function of composition, cannot be explained by 4f¹²5d¹→4f¹³ feeding via interband crossing, but more likely via MPR where the electron–phonon coupling strength decreases from Cl→Br→I. No strong relation was found between composition and the quantum efficiency (QE) of the 4f¹³→4f¹³ emission, due to the presence of fluctuations that are likely caused by intrinsic differences in sample quality. Nevertheless, a 4f¹³→4f¹³ QE of up to 70% has been observed and the materials can therefore be used in luminescence solar concentrators.

The prospect of using Tm²⁺-doped halides for luminescence solar concentrators (LSCs) requires a thorough understanding of the temperature dependent Tm²⁺ excited states dynamics that determines the internal quantum efficiency (QE) and thereby the efficiency of the LSC. In this study we investigated the dynamics in CaX₂:Tm²⁺ (X = Cl, Br, I) by temperature- and time-resolved measurements. At 20 K up to four distinct Tm²⁺ emissions can be observed. Most of these emissions undergo quenching via multi-phonon relaxation below 100 K. At higher temperatures, only the lowest energy 5d–4f emission and the 4f–4f emission remain. Fitting a numerical rate equation model to the data shows that the subsequent quenching of the 5d–4f emission is likely to occur initially via multi-phonon relaxation, whereas at higher temperatures additional quenching via interband crossing becomes thermally activated. At room temperature only the 4f–4f emission remains and the related QE becomes close to 30%. Possible reasons for the quantum efficiency not reaching 100% are provided.

The parameters governing the performance of a luminescent solar concentrator (LSC) are determined for sputtered thin-films of NaI:Tm²⁺, CaBr₂:Tm²⁺, and CaI₂:Tm²⁺. These parameters are determined by using six gradient thin film material libraries, combinatorially sputtered from metallic and pressed powder targets. These films show strong 4f¹³→4f¹²d¹ absorption of maximally 752 cm⁻¹ at.%⁻¹ for NaI:Tm²⁺, 31 cm⁻¹ at.%⁻¹ for CaBr₂:Tm²⁺, and 473 cm⁻¹ at.%⁻¹ for CaI₂:Tm²⁺. This absorption covers the entire visible spectrum and does not overlap with the infrared 4f-4f emission at 1140 nm. Decay measurements are used to estimate the quantum yields of the thin-films. These quantum yields can be as high as 44 % for NaI:Tm²⁺, when doped with 0.3 at.% Tm. Even at doping percentages as low as 0.3 at.%, the films appear to show luminescence quenching. The concentration-dependent absorption and quantum yield are combined with the index of refraction, resolved from transmission measurements, to simulate the optical efficiency of a thin film Tm²⁺-doped halide LSC. These simulations show that LSCs based on Tm²⁺ can display excellent color rendering indices of up to 99 %, and neutral color temperatures, between 4500K and 6000K. Under optimal conditions, thin-films constrained to a thickness of 10μm and 80 % transmission of the visible spectrum, would be able to display optical efficiencies of 0.71 %. This optical efficiency compares favorably to the maximally achievable 3.5 % under these constraints. This efficiency is largely independent of the size of LSC itself.

The concentration dependent luminescence of the SrI₂-TmI₂ system was investigated. For Tm²⁺ concentrations up to 5 mol %, the quantum efficiency (QE) of the ²F_5/2→²F_7/2 emission exhibits a constant value above 50%. The QE drops for higher Tm²⁺ concentrations, partly due to concentration quenching, as evidenced by a decreasing luminescence lifetime of the ²F_5/2→²F_7/2 emission, and partly due to the formation of a second crystal phase with CdCl₂ structure, in which the ²F_5/2→²F_7/2 emission is quenched. The temperature and time dependent relaxation dynamics were studied to identify the origin of the limited QE for Tm²⁺-doping levels below 5 mol %. An anti-correlation between the 5d-4f (³H₆,t_2g)_S=3/2→²F_7/2 and 4f-4f ²F_5/2→²F_7/2 emission intensities was found and rationalised by non-radiative, thermally stimulated, inter-configurational 5d-4f relaxation to the emitting ²F_5/2 level of Tm²⁺. Both, the rise time of the 4f-4f and the decay time of the 5d-4f emission become shorter with increasing temperature. We suggest a similar non-radiative relaxation from the 5d level towards the ²F_7/2 ground state to limit the QE below unity. This route becomes more efficient when the 5d (³H₆,t_2g)_S=3/2 state moves closer to the 4f ²F_5/2 and ²F_7/2 states, which is the case for the CdCl₂ phase with a QE close to zero.

In recent years, Thulium in its 2 + oxidation state has been identified as candidate dopant in halide hosts for luminescent solar concentrators. Yet, some of its luminescent properties with regard to these applications remain unexplored. In this study we report on the temperature dependent photo-luminescent behaviour of NaCl:Tm²⁺, NaBr:Tm²⁺, and NaI:Tm²⁺. These monohalide materials demonstrate up to five distinct emission peaks which can be attributed to the 4f¹²→4f¹² and 4f¹¹5d¹→4f¹² transitions of Tm²⁺. Their time- and temperature dependent luminescence intensity behaviours are explained by a qualitative model describing the thermally stimulated radiative- and non-radiative relaxation dynamics. The behaviour of Tm²⁺ in these monohalides proves to be similar to earlier reported findings on Tm²⁺-doped trihalide perovskites of the form CsCaX₃ (X = Cl, Br, I), however, the 4f-4f emission is by far the most dominant emission between 10 and 300 K.

In order to investigate the mechanism of the photochromic effect in yttrium oxy-hydride (YO_xH_y) thin films, Doppler broadening positron annihilation spectroscopy (PAS) was applied to probe the electronic structure and the presence of vacancies in YO_xH_y and related materials as a function of composition, UV illumination and thermal annealing. The Doppler S and W parameter depth profiles of a series of Y, yttrium di-hydride YH_1.9+δ and Y₂O₃ thin films show strong systematic changes caused by the distinct differences in electronic structure of the metals Y, YH_1.9+δ and the wide band gap insulator Y₂O₃. The Doppler broadening parameters of photochromic YO_xH_y (a semiconductor with a band gap of ~2.6eV) are intermediate to those of YH_1.9+δ and Y₂O₃. In order to probe the nanostructural changes related to the photochromic effect, the S parameter of YO_xH_y was monitored during in-situ UV illumination. A small but systematic increase of the S parameter was observed, possibly induced by generation of cation mono-vacancies or small vacancy clusters involving generated anion vacancies. The changes did not relax during bleaching under dark conditions, showing that the structural changes are not directly responsible for the photochromic mechanism. For temperatures above around 90°C, thermal annealing leads to a substantial increase in the Doppler S parameter, pointing to the formation of vacancies by local removal of hydrogen. Simultaneously, the optical band gap increases, consistent with an increase in O:H ratios.