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

Abstract

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.