Luminescence solar concentrators (LSCs) have been showing renewed interest as part of a portfolio of solutions for Building-Integrated Photovoltaics. LSCs operate by absorbing a fraction of the sunlight and concentrating it to photovoltaic modules installed at its edges through luminescence downshifting. Divalent Thulium (Tm²⁺, Z=69) has sparked a particular interest. Tm²⁺ has a wide overlap with the solar spectrum while showing virtually no self-absorption when doped into inorganic hosts. These characteristics are generated by the electric-dipole allowed, broad, and intense 4f→5d absorption transitions covering the UV to NIR. Allied to their high absorption cross-sections, reaching over 10^-18 cm², Tm²⁺-activated LSCs in thin-film form becomes possible, thus receiving attention recently [1,2].

Quantum efficiency (QE) is an important parameter to evaluate the potential of new material as an LSC because QE is proportional to LSC efficiency. Until recently, not much information was available on the QE of the Tm²⁺'s 4f→4f emission. Previously, the QE was pinpointed to the Tm²⁺ excited-states dynamics [3]. Surprisingly, Tm²⁺ PL of NaCl:Tm phosphors showed rise times, as evidenced by temperature-dependent time-resolved photoluminescence (TRPL) studies and the following temperature dependence. At 20K, up to four distinct Tm²⁺ emissions were observed corresponding to four different transitions: three out of the 5d band structure to the ²F_7/2 state and the 4f→4f (²F_5/2→²F_7/2) transition. As the temperature increases to 100 K, those 5d→4f transitions undergo quenching via 5d→5d multi-phonon relaxation emission. As the temperature increases further one additional process, interband crossing, is thermally activated so only the 4f→4f emission remains.

The objective of this work is to present recent progress towards optimization of Tm²⁺ doped NaCl thin-films grown by gradient RF-sputtering [4] . In this work, we present the concentration-dependent relaxation dynamics of Tm²⁺ luminescence in NaCl thin films through TRPL in a wide Tm concentration range. A rise time in TRPL is also observed and it is attributed to 5d→4f non-radiative relaxation. These results are interpreted as a combined effect between multi-phonon and interband crossing. They are compared to the model used for temperature-dependent studies on phosphors [3].

References

[1] M. Suta and C. Wickleder, “Synthesis, spectroscopic properties and applications of divalent lanthanides apart from Eu2+,” J. Lumin., vol. 210, no. February, pp. 210–238, Jun. 2019.

[2] G. B. F. Bosco et al., “Metal-insulator multilayer precursors for enhanced Sm2+ absorption and Tm2+ luminescence in SiAlO thin films prepared by magnetron sputtering,” J. Lumin., vol. 241, p. 118421, Jan. 2022.

[3] M. P. Plokker et al., “Experimental and Numerical Analysis of Tm2+ Excited-States Dynamics and Luminescence in CaX2 (X = Cl, Br, I),” J. Phys. Condens. Matter, Apr. 2021.

[4] Merkx, E. P. J., Plokker, M. P., van der Kolk, E. (2021). "The potential of transparent sputtered NaI:Tm2+, CaBr2:Tm2+, and CaI2:Tm2+ thin films as luminescent solar concentrators." Solar Energy Materials and Solar Cells, 223, 110944.