The potential of SiO2:Al3+,Eu2+ blue phosphor coatings in greenhouse application

Solar spectral conversion by a low-cost luminescent coating for greenhouse applications increases crop yield and can contribute to addressing the food crisis. A luminescent coating based on cheap SiO₂ particles doped with Eu²⁺ and Al³⁺ demonstrated extra photosynthetic active radiation (PAR) in this work. To optimize the efficiency of this phosphor for greenhouse applications, three phosphor series with varying Al/Eu content in SiO₂ were synthesized via a sol–gel approach and characterized by luminescence decay time, absorption, luminescent excitation, emission, and quantum yield measurements. With increasing the Eu%, at a fixed Al%, the decay time and quantum yield decreased while the emission shifted to the red. The effect can be explained by a more and more efficient resonance energy transfer to lower energy Eu²⁺ ions and quenching sites. While increasing the Al% at a fixed Eu%, the decay time and quantum yield increased, and the red-shift was reduced. Both effects can be explained by an enhanced Eu²⁺ solubility (reduced Eu clustering) through the Al³⁺ co-doping, causing the average Eu²⁺-Eu²⁺ distance to be longer and the onset of concentration quenching to shift to a higher Eu%. Specifically, we found that for 1 mol% Eu²⁺, a minimum of 4 mol% Al³⁺ was required to avoid concentration quenching. Two indicators were developed to quantify the UV to PAR converting efficiency and to quantify the PAR transmission enhancement. Both indicators were determined in a real coating sample based on the optimized phosphor. The result showed an additional PAR was provided by our luminescent coating. A general discussion about all factors that can bring the conversion efficiency of a phosphor coating closer to the theoretical maximum will be presented.