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⁺ 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.