Experimental and numerical analysis of Tm²⁺ excited-states dynamics and luminescence in CaX ₂ (X = Cl, Br, I)

Abstract

The prospect of using Tm 2+ -doped halides for luminescence solar concentrators (LSCs) requires a thorough understanding of the temperature dependent Tm 2+ 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 :Tm 2+ (X = Cl, Br, I) by temperature-and time-resolved measurements. At 20 K up to four distinct Tm 2+ 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.

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

“…The quantum efficiency (QE) of the Tm 2+2 F 5/2 → 2 F 7/2 (4f 13 → 4f 13 ) emission was then established to be 19 ± 1%. This value is in close order to that of NaCl:Tm 2+ and CaCl 2 :Tm 2+ and indicates a large loss contribution at room temperature by a possible radiationless quenching route going (in)­directly from the ( 3 H 6 ,5d 1 ) S =1/2 levels to the 2 F 7/2 ground state. , …”

“…For orthorhombic BaCl 2 :Tm 2+ , the related energy gap amounts to 7100 cm −1 and is around 1.5 times larger than in CaBr 2 :Tm 2+ , where it amounts to 4745 cm −1 . 26 However, the related Stokes shift in orthorhombic BaCl 2 :Tm 2+ is 3600 cm −1 (see Table 2) and almost 3 times larger than for CaBr 2 :Tm 2+ , 26 where it amounts to only 1245 cm −1 . So, the relatively low quenching temperature of the ( 3 H 6 ,5d 1 ) S=3/2 → 2 F 7/2 emission can be a consequence of the large Stokes shift.…”

“…This 4f 12 5d 1 → 4f 13 quenching process involves the crossing point between the ( 3 H 6 ,5d 1 ) S=3/2 and 2 F 5/2 state parabolas, as illustrated in Figure 8. 26,27,47,48 The energy of the crossing point will be situated at a relatively low energy in the case of a small energy gap between the ( 3 H 6 ,5d 1 ) S=3/2 and 2 F 5/2 levels and/or a large value for the Stokes shift of the ( 3 H 6 ,5d 1 ) S=3/2 → 2 F 7/2 emission, giving rise to efficient quenching via interband crossing. For orthorhombic BaCl 2 :Tm 2+ , the related energy gap amounts to 7100 cm −1 and is around 1.5 times larger than in CaBr 2 :Tm 2+ , where it amounts to 4745 cm −1 .…”

“…This means that the energy transfer process from the ( 3 H 6 ,5d 1 ) S=1/2 state to the ( 3 H 6 ,5d 1 ) S=3/2 state cannot take place, and consequently, a strong ( 3 H 6 ,5d 1 ) S=1/2 → 2 F 7/2 emission is observed. In the case of SrI 2 :Tm 2+ , 25 CaI 2 :Tm 2+ , 26 and RbCaI 3 :Tm 2+ , 19 the energy of the ( 3 H 6 ,5d 1 ) S=1/2 → 2 F 7/2 emission is also, but somewhat, lower than the absorption energy of the ( 3 H 6 ,5d 1 ) S=3/2 levels and we observe a relatively strong ( 3 H 6 ,5d 1 ) S=1/2 → 2 F 7/2 emission. For the other Tm 2+ -doped halides in Figure 9, the energy of the ( 3 H 6 ,5d 1 ) S=1/2 → 2 F 7/2 emission is higher than the absorption energy of the ( 3 H 6 ,5d 1 ) S=3/2 levels, resulting in a high probability of ( 3 H 6 ,5d 1 ) S=1/2 → ( 3 H 6 ,5d 1 ) S=3/2 energy transfer, and hence, the ( 3 H 6 ,5d 1 ) S=1/2 → 2 F 7/2 emission is either very weak or not observed at all.…”

“…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.…”