The potential of transparent sputtered NaI:Tm2+, CaBr2:Tm2+, and CaI2:Tm2+ thin films as luminescent solar concentrators

Evert P.J. Merkx*, Maarten P. Plokker, Erik van der Kolk

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

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The parameters governing the performance of a luminescent solar concentrator (LSC) are determined for sputtered thin-films of NaI:Tm2+, CaBr2:Tm2+, and CaI2:Tm2+. These parameters are determined by using six gradient thin film material libraries, combinatorially sputtered from metallic and pressed powder targets. These films show strong 4f13→4f12d1 absorption of maximally 752 cm−1 at.%−1 for NaI:Tm2+, 31 cm−1 at.%−1 for CaBr2:Tm2+, and 473 cm−1 at.%−1 for CaI2:Tm2+. This absorption covers the entire visible spectrum and does not overlap with the infrared 4f-4f emission at 1140 nm. Decay measurements are used to estimate the quantum yields of the thin-films. These quantum yields can be as high as 44 % for NaI:Tm2+, when doped with 0.3 at.% Tm. Even at doping percentages as low as 0.3 at.%, the films appear to show luminescence quenching. The concentration-dependent absorption and quantum yield are combined with the index of refraction, resolved from transmission measurements, to simulate the optical efficiency of a thin film Tm2+-doped halide LSC. These simulations show that LSCs based on Tm2+ can display excellent color rendering indices of up to 99 %, and neutral color temperatures, between 4500K and 6000K. Under optimal conditions, thin-films constrained to a thickness of 10μm and 80 % transmission of the visible spectrum, would be able to display optical efficiencies of 0.71 %. This optical efficiency compares favorably to the maximally achievable 3.5 % under these constraints. This efficiency is largely independent of the size of LSC itself.

Original languageEnglish
Article number110944
Number of pages10
JournalSolar Energy Materials and Solar Cells
Publication statusPublished - 2021


  • Building-integrated photovoltaics
  • Combinatorial science
  • Halide
  • Luminescent solar concentrator
  • Optical simulation
  • Sputter deposition


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