Abstract
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 CaX 2: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.
Highlights
luminescence solar concentrators (LSCs) can become one of many building-integrated photovoltaic (BIPV) solutions that contribute to more sustainable buildings
Not much is known about the quantum efficiency (QE) of the 4f–4f emission after 4f–5d excitation. This parameter is directly proportional to the overall LSC efficiency and is mainly determined by the Tm2+ excited-states dynamics [1, 2]
In our previous study on NaX:Tm2+ (X = Cl, Br, I) mono-halides [8] we investigated the excited-states dynamics and measured the QE, but we were unable to provide a full quantitative description of important processes such as: the quenching mechanism related to the lowest energy 5d–4f emission, the presence of the 4f–4f emission at 20 K, and the efficient non-radiative 5d–4f ground state route
Summary
LSCs can become one of many building-integrated photovoltaic (BIPV) solutions that contribute to more sustainable buildings. LSCs make use of a special luminescent coating that absorbs sunlight and emits light of different energy towards solar cells mounted in the edges of a window. Not much is known about the QE of the 4f–4f emission after 4f–5d excitation This parameter is directly proportional to the overall LSC efficiency and is mainly determined by the Tm2+ excited-states dynamics [1, 2]. A qualitative description of the excited-states dynamics is provided for all three materials, followed by a quantitative modelling on CaBr2:Tm2+ of time- and temperature-dependent 5d–4f and 4f–4f intensities and an overall discussion is provided focusing on the recorded QE-values
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