Study of ZnS:Ag and BaMgAl10O17:Eu2+ phosphor for UV-to-PAR spectral conversion greenhouse coatings

Abstract

Spectral conversion films and coatings based on photoluminescent materials have attracted increasing attention for greenhouse applications and are reported to provide a net benefit through spectral control. However, there is little experimental data on spectral conversion coating that quantifies their optical properties, including absorption, transmittance, scattering loss, and the influence of the phosphor intrinsic parameters. In this work, UV-to-PAR spectral conversion coatings based on two commercial phosphors ZnS:Ag and BaMgAl10O17:Eu2+ (BAM:Eu), with different particle loading and thickness, were fabricated and assessed by direct transmittance, hemispherical light transmittance (THEM), and diffuse reflectance. Transmittance decreased with increasing coating thickness and particle loading, whereas backward scattering showed the opposite trend. Performance indicators derived from direct transmittance revealed that host-absorption ZnS:Ag coatings exhibited greater UV absorption and higher PAR Enhancement than activator-absorption BAM:Eu coatings, resulting from their higher absorption coefficient. Quantum yield had a minimal impact because both phosphors displayed close PLQY values. However, the backward scattering of ZnS:Ag coatings was more pronounced than that of BAM:Eu coating due to the high refractive index of ZnS:Ag (n ≈ 2.3), as confirmed by diffuse reflectance measurements. The 18 mass%, 200 μm ZnS:Ag coating showed the highest 1.3% PAR Enhancement, but its THEM was reduced by about 45%, demonstrating that backward scattering can counteract the benefits of spectral conversion. A general discussion for phosphor selection in greenhouse applications is also provided in this work.