Photoluminescence and excited states dynamics of Tm²⁺-doped CsCa(Cl/Br)₃ and CsCa(Br/I)₃ perovskites

Abstract

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.

Evaluating the Tm2+ 4f125d1 → 4f13 and 4f13 → 4f13 Luminescence and Quenching Dynamics in Orthorhombic BaCl2

“…Up to three distinct Tm 2+ emissions can be observed: two broad and intertwined 4f 12 5d 1 → 4f 13 emissions, positioned close to 735 and 815 nm, and a sharp 4f 13 → 4f 13 line emission near 1140 nm that corresponds to the 2 F 5/2 → 2 F 7/2 transition. This latter emission has been reported many times before, − and compared to the other two broad 4f 12 5d 1 → 4f 13 emissions, its intensity in orthorhombic BaCl 2 appears to be relatively high at low temperatures.…”

“…In addition, the intensity of the two 4f 12 5d 1 → 4f 13 emissions appears to be relatively low at 20 K as compared to the 4f 13 → 4f 13 line emission (see Figure ). In contrast to other Tm 2+ -doped halides, the spin-allowed ( 3 H 6 ,5d 1 ) S =1/2 → 2 F 7/2 emission in orthorhombic BaCl 2 :Tm 2+ appears to be much stronger than both the ( 3 H 6 ,5d 1 ) S =1/2 → 2 F 7/2 and ( 3 H 6 ,5d 1 ) S =3/2 → 2 F 7/2 emissions ,,,,− and the spin-forbidden ( 3 H 6 ,5d 1 ) S =3/2 → 2 F 7/2 emission. The very small bump located at around 615 nm cannot be linked to a Tm 2+ transition and is likely to be related to defect emission or an artifact caused by the sample holder.…”

“…As such, orthorhombic BaCl 2 :Tm 2+ is the very first chloride compound in which the Tm 2+ spin-allowed ( 3 H 6 ,5d 1 ) S =1/2 → 2 F 7/2 emission is identified. Up to now, it has only been identified in iodide and bromide host lattices: AI 2 :Tm 2+ (A = Ca and Sr), RbCaI 3 :Tm 2+ , CsCaX 3 :Tm 2+ , and NaX:Tm 2+ (X = Br and I). ,,,,− With this classification of emissions, the Tm 2+ LS Stokes shift in orthorhombic BaCl 2 then amounts to 3940 cm –1 . Often the Stokes shift in a compound appears to be the same for different lanthanides, and we may compare the Stokes shift for Tm 2+ with that for Sm 2+ and Eu 2+ in orthorhombic BaCl 2 .…”

“…The crossing point between the ( 3 H 6 ,5d 1 ) S =3/2 and 2 F 5/2 states exemplifies the quenching process of interband crossing, which is activated at a specific temperature when enough thermal energy ε is available for reaching the crossing point. At an even higher temperature, the crossing point between the ( 3 H 6 ,5d 1 ) S =3/2 and 2 F 7/2 ground state will be thermally provoked. ,,, …”

“…At the time, Grimm and Beurer from Bern University studied the Tm 2+ 4f 12 5d 1 → 4f 13 and 4f 13 → 4f 13 temperature-dependent luminescence in predominantly CsCaX 3 (X = Cl, Br, and I), RbCaI 3 :Tm 2+ , and ACl 2 (A = Ca, Sr, and Ba) halides, obtaining valuable information on the 4f 12 5d 1 -4f 13 quenching dynamics. − A decade later, in 2015, ten Kate et al discovered the potential suitability of Tm 2+ -doped halides for luminescence solar concentrators. In the wake of this discovery, we reinvestigated the Tm 2+ thermal quenching dynamics and studied it in several host lattices, such as NaX, CaX 2 (X = Cl, Br, and I), and SrI 2 , and in addition, in halide solid solutions of CsCa­(Cl/Br) 3 and CsCa­(Br/I) 3 . − In view of these recent results, the previous study of Grimm et al . on BaCl 2 :Tm 2+ with an orthorhombic structure raises several open questions.…”