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Large-area, uniform white light LED source on a flexible substrate

Chin-Wei Sher, Kuo-Ju Chen, Chien-Chung Lin, Hau-Vei Han, Huang-Yu Lin, Zong-Yi Tu, Hsien-Hao Tu, Keiji Honjo, Hsin-Yi Jiang, Sin-Liang Ou, Ray-Hua Horng, Xiuling Li, Chien-Chung Fu, and Hao-Chung Kuo

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Abstract

This study demonstrates the flexible white LED structure with high lumen efficiency and uniform optical performance for neutral white and warm white CCT. Flip-chip LEDs were attached on a polyimide substrate with copper strips as electrical and thermal conduction paths. Yellow phosphors are mixed with polydimenthysiloxane (PDMS) to provide mechanical support and flexibility. The light efficiency of this device can reach 120 lm/W and 85% of light output uniformity of the emission area can be achieved. Moreover, the optical simulation is employed to evaluate various designs of this flexible film in order to obtain uniform output. Both the pitch between the individual devices and the thickness of the phosphor film are calculated for optimization purpose. This flexible white LED with high lumen efficiency and good reliability is suitable for the large area fixture in the general lighting applications.

© 2015 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 Schematic illustration of process flow of flexible white LED structure

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Fig. 2
Fig. 2 (a) LED array under optical microscope; (b) SEM of the flexible white LED; (d) Cross section between LED and PI substrate; (e) the metal particle in silicone-based anisotropic conductive adhesive bonding material (c)(f) The energy dispersive spectrometer (EDS) of silicone-based anisotropic conductive adhesive showing the different compositions in the substance.

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Fig. 3
Fig. 3 (a)(d) Scanning electron microscope (SEM) images of the cross section of the phosphor film structure at 5000 K and 3000 K. (b)(e) The EDS spectra for phosphor layers of 5000 K and 3000 K samples, respectively. (c)(f) The pictures of the phosphor film for neutral and warm white at 5000 K and 3000 K color temperature.

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Fig. 4
Fig. 4 (a)(b) the pictures of flexible blue LED substrate (c) the EL spectra from 0.09A to 0.54A of injection currents (d) the forward voltage and luminous flux vs. injection currents (from 0.1 to 0.55 A).

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Fig. 5
Fig. 5 (a) the flexible white LED’s emission spectra in neutral white and warm white. (b)(c) the luminous flux of the flexible white LED structures at 5000K/3000K driven at currents from 0 to 1000 mA.

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Fig. 6
Fig. 6 (a) The picture and (b) the luminous flux and voltage of the flexible white LED for different bending diameters

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Fig. 7
Fig. 7 Thermal resistance of flexible white LED

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Fig. 8
Fig. 8 the life time test of flexible white LED device by 1000 hours

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Fig. 9
Fig. 9 (a) The simulated results at different chip to chip pitches and (b) at different silicon glue thicknesses.

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Equations (1)

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U n i f o r m i t y = ( L min / L max ) x 1 00 %
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