Nanopatterning of sol-gel derived luminescent thin films as a promising tool to enhance extraction efficiency in LEDs devices

Energy-saving lighting devices based on Light-Emitting Diodes (LEDs) combine a semiconductor chip emitting in the ultraviolet or blue wavelength region to one or more phosphor(s) deposited in the form of coatings. The most... [ view full abstract ]

Energy-saving lighting devices based on Light-Emitting Diodes (LEDs) combine a semiconductor chip emitting in the ultraviolet or blue wavelength region to one or more phosphor(s) deposited in the form of coatings. The most common ones combine a blue LED with the yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) and a red phosphor. Even if these devices are characterized by satisfying photometric parameters (Color Rendering Index, Color Temperature) and good luminous efficiencies, further improvements can be carried out to enhance light extraction efficiency (increase in phosphor forward emission). One of the possible strategies is to pattern the phosphor coating. Most of the papers on the topic use layers of foreign materials as photonic crystals (SiO₂, TiO₂, SiN_x) since YAG matrix is very harsh to pattern because of its good mechanical strength. These foreign materials could be eroded or fall off sooner or later, reducing the lifetime of these nanostructures. The strategy we have developed allows direct nanopatterning the YAG matrix by using the colloidal lithography combined with the Langmuir-Blodgett technique. The main interest of this procedure is that it can be applied to all luminescent matrices derived from the sol-gel process (oxides, fluorides …). As a model system, nanopatterned YAG:Tb luminescent coatings have been elaborated. The obtained nanostructuration is a hexagonal network of YAG:Tb³⁺ film (Figure 1) capable of modifying the light path, avoiding energy loss by the phenomenon of internal reflection within a given substrate. To determine the benefit of nanopatterning, conventional and angular-resolved photoluminescence were investigated on both unpatterned and patterned samples. Thanks to the nanostructuration, the extraction efficiency has been improved by 26 % and 131% depending on the sample nature (Figure 2). Noticeably, nanostructuration was found to have an influence on the angular distribution of photoluminescence whose intensity has been evaluated to its maximum at 0° with respect to the incident light. This work opens up the possibility either to reduce the phosphor quantity for the same device quality (reduction of rare-earth consumption) or to improve the light efficiency using the same quantity of phosphors.