Nanostructuration of YAG:Ce Coatings by ZnO Nanowires: A Smart Way to Enhance Light Extraction Efficiency

Abstract

In this study, we report on the enhancement of the light extraction efficiency of sol–gel-derived Y3Al5O12:Ce3+ (YAG:Ce) coatings using ZnO nanowire (NW) arrays. The ZnO NWs were grown by hydrothermal synthesis from a ZnO seed layer directly deposited on a YAG:Ce coating. Highly dense and vertically aligned ZnO NW arrays were evidenced on the top of the YAG:Ce coating by electron microscopy. A photoluminescence study showed that this original design leads to a different angular distribution of light together with an increase in emission efficiency of the YAG:Ce coating upon blue excitation, up to 60% more efficient compared to a non-structured YAG:Ce coating (without NWs). These improvements are ascribed to multi-scattering events for photons within the structure, allowing them to escape from the phosphor layer by taking optical paths different from those of the non-structured coating. This strategy of light extraction enhancement appears to be very promising, since it uses soft chemical processes and cheap ZnO NWs and is applicable to any sol–gel-derived luminescent coating.

Enhanced Spontaneous Light Emission of ZnO Nanowire-Based Gratings

“…For NWs with a diameter of 40 nm and a density of approximately 200 NW/μm 2 , which are the features determined by the morphological study, the resulting equivalent optical index n h is approximately 1.65. A similar value was obtained from the analysis of the SEM images of similar coatings . This homogenized model enables us to assimilate the ZnO coating to a waveguide on top of a quartz substrate and the periodic pattern of the ZnO NWs to a grating made of ribbons of width W ZnO and period D as depicted in Figure .…”

“…A similar value was obtained from the analysis of the SEM images of similar coatings. 32 This homogenized model enables us to assimilate the ZnO coating to a waveguide on top of a quartz substrate and the periodic pattern of the ZnO NWs to a grating made of ribbons of width W ZnO and period D as depicted in Figure 4. Consequently, the ZnO coating exhibits a refractive index higher than that of the quartz substrate (n Q = 1.45).…”