Taiwanese University Prototypes Flexible LED Lighting Sheet

National Chiao Tung University in Taiwan, etc prototyped a flexible white LED sheet that can be formed as a large-area sheet.

When the sheet measures 5 x 5cm, it can be bent to a curvature radius of at least 1.5cm. The maximum luminous efficiencies of sheets with color temperatures of 5,000K and 3,000K are 120lm/W and about 81lm/W, respectively. The details of the new sheet was announced in a thesis published in the Optics Express academic journal.

Large-area LED sheets have already been realized by using a technique called “COB (chip on board).” The latest sheet uses a method similar to the COB process except that it uses a flexible substrate.

Specifically, a copper (Cu) foil is attached to a polyimide (resin) substrate, and electrode patterns are formed by lithography. Then, many conventional GaN-based LED chips are surface-mounted by using a silicon resin anisotropic conductive film. In the case of the prototype, 81 LED chips measuring 1.125 x 1.125mm each were mounted on a 5 x 5cm substrate.

Moreover, to emit white-color light, yellow (Y3Al5O12:Ce3+) and red (CaAlSiN3:Eu2+) phosphors are dispersed in a silicon resin called PDMS, and a sheet thinned by spin coating method is attached to the LED chips sealed with a silicon material.

The thickness of the prototyped sheet is about 6mm, and that of the substrate and LED chips combined is 228.3μm. The thickness of the silicon material for the sealing of LED chips and light diffusion/uniformalization is several millimeters. The thickness of the phosphor film is 496μm with a color temperature of 3,000K and 187μm with a color temperature of 5,000K.

On the other hand, as for the emission lifetime of the sheet, its luminous efficiency decreased by about 2.5% after 1,000 hours of use. Based on a simple extrapolation from that, its luminous efficiency will decrease by about 22% after 10,000 hours, indicating that the emission lifetime is much shorter than that of conventional LEDs and is not superior to flexible OLED lighting sheets, which are potential competitors.

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