The Potential Of Transparent Sputtered Nai:Tm2+, Cabr2:Tm2+, And Cai2:Tm2+ Thin Films As Luminescent Solar Concentrators

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 4f(13) -> 4f(12)d(1) 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 4500 K and 6000 K. Under optimal conditions, thin-films constrained to a thickness of 10 mu m 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.