Photoluminescence And Excited States Dynamics Of Tm2+-Doped Csca(Cl/Br)(3) And Csca(Br/I)(3) Perovskites

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+ 4f(12)5d(1)-levels as relative to the two 4f(13) levels. At low temperatures up to five distinct Tm2+ 4f(12)5d(1)-> 4f(13) emissions and the 4f(13)-> 4f(13) emission can be observed. As the temperature increases, most of the 4f(12)5d(1)-> 4f(13) emissions undergo quenching via multi-phonon relaxation (MPR) and at room temperature only the lowest energy 4f(12)5d(1)-> 4f(13) and the 4f(13)-> 4f(13) emission remains. For all compositions a 4f(13)-> 4f(13) risetime phenomenon is then observed whose duration matches the 4f(12)5d(1)-> 4f(13) decay time. It shows the feeding of the 4f(13) state after 4f(12)5d(1) excitation. Surprisingly, the feeding time becomes longer from Cl -> Br -> I, while the related 4f(12)5d(1)-4f(13) energy gap becomes smaller. The temperature dependence of the 4f(12)5d(1)-> 4f(13) and 4f(13)-> 4f(13) 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 4f(12)5d(1)-4f(13) 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 4f(12)5d(1)-> 4f(13) 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 4f(13)-> 4f(13) emission, due to the presence of fluctuations that are likely caused by intrinsic differences in sample quality. Nevertheless, a 4f(13)-> 4f(13) QE of up to 70% has been observed and the materials can therefore be used in luminescence solar concentrators.