The potential of transparent sputtered NaI:Tm[formula omitted], CaBr[formula omitted]:Tm[formula omitted], and CaI[formula omitted]:Tm[formula omitted] thin films as luminescent solar concentrators

Evert P.J Merkx,Maarten P Plokker,Erik Van Der Kolk

doi:10.1016/j.solmat.2020.110944

Evert P.J Merkx, Maarten P Plokker + Show 1 more

Open Access

https://doi.org/10.1016/j.solmat.2020.110944

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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 752cm−1 at.%−1 for NaI:Tm2+, 31cm−1 at.%−1 for CaBr2:Tm2+, and 473cm−1 at.%−1 for CaI2:Tm2+. This absorption covers the entire visible spectrum and does not overlap with the infrared 4f-4f emission at 1140nm. 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 0.3at.% Tm. Even at doping percentages as low as 0.3at.%, 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.

Highlights

Luminescent solar concentrators (LSCs) are an electricity-generating replacement for window glass, commonly found in windows or facades
To evaluate the performance of these materials, we present a combinatorial study on the LSC characteristics of some of these Tm2+-doped halides
NaI, CaBr2, and CaI2 doped with Tm2+, that have previously been suggested as materials for a thin-film LSC, have in this article been investigated on their potential as LSCs

Summary

Introduction

Luminescent solar concentrators (LSCs) are an electricity-generating replacement for window glass, commonly found in windows or facades This type of energy-generating replacement for building materials is called building-integrated PV (BIPV). These LSCs generate electricity through a luminescent coating deposited on the window glass. This coating harvests a fraction of the incoming sunlight and converts this to luminescent light. This luminescence is isotropically emitted and remains largely trapped between the coating and the window glass, due to total internal reflection. Due to their electricity-generating capabilities, LSCs can become part of the building envelope to realize net-zero energy buildings, in line with goals of the European Union [1]

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