Experimental and numerical analysis of Tm²⁺excited-states dynamics and luminescence in CaX₂(X= Cl, Br, I)

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.