Angular color shift of micro-LED displays

Fangwang Gou; En-Lin Hsiang; Guanjun Tan; Pei-Ting Chou; Yun-Li Li; Yi-Fen Lan; Shin-Tson Wu

doi:10.1364/OE.27.00A746

Optics Express
Vol. 27,
Issue 12,
pp. A746-A757
(2019)
•https://doi.org/10.1364/OE.27.00A746

Angular color shift of micro-LED displays

Fangwang Gou, En-Lin Hsiang, Guanjun Tan, Pei-Ting Chou, Yun-Li Li, Yi-Fen Lan, and Shin-Tson Wu

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Fangwang Gou, En-Lin Hsiang, Guanjun Tan, Pei-Ting Chou, Yun-Li Li, Yi-Fen Lan, and Shin-Tson Wu, "Angular color shift of micro-LED displays," Opt. Express 27, A746-A757 (2019)

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Abstract

Sidewall emission of a micro-scale light emitting diode (micro-LED) improves the light extraction efficiency, but it causes mismatched angular distributions between AlGaInP-based red micro-LED and InGaN-based blue/green counterparts due to material difference. As a result, color shift of RGB micro-LED displays may become visually noticeable. To address this issue, we first analyze the angular distributions of RGB micro-LEDs and obtain good agreement between simulation and experiment. Next, we propose a device structure with top black matrix and taper angle in micro-LEDs, which greatly suppresses the color shift while keeping a reasonably high light extraction efficiency.

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Fig. 1 SEM image of RGB micro-LEDs arrays after transfer.

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Fig. 2 Measured emission spectra of (a) red, (b) green, and (c) blue micro-LEDs at different viewing angles. Red and black dashed lines indicate the central wavelength and FWHM at each viewing angle.

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Fig. 3 Measured and simulated far-field radiation patterns of RGB micro-LEDs. Dots are experimental data and lines are simulation results.

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Fig. 4 Simulated total, top, and sidewall emissions of (a) red, (b) green, and (c) blue micro-LEDs at different viewing angles.

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Fig. 5 (a) Top view of micro-LED chip with a point-like source located at (x, y). (b, c) Side views of light emission from the point source with emission angle θ_i: (a) θ_i < θ_c: top emission; (c) θ_i > 90°−θ_c: sidewall emissions.

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Fig. 6 10 reference colors in CIE1976 color space, with D65 white point and RGB micro-LEDs primary colors.

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Fig. 7 Simulated color triangle of the RGB micro-LED display system and the CIE coordinates of 10 reference colors from 0° to 80° viewing angle. The arrows indicate the color shift as viewing angle changes.

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Fig. 8 RGB spectra of our and Osram’s micro-LEDs.

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Fig. 9 RGB micro-LED display with top black matrix.

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Fig. 10 Light intensity enhancement ratio normalized to bare RGB micro-LEDs (90° taper angle and without black matrix) as taper angle α changes.

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Fig. 11 Simulated color shifts of 10 reference colors from 0° to 80° viewing angle for RGB micro-LED display with top black matrix and 120° taper angle.

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Fig. 12 Simulated color shifts of the first 18 colors in Macbeth ColorChecker from 0° to 80° viewing angle for RGB micro-LED display with top black matrix and 120° taper angle.

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Fig. 13 Simulated radiation patterns of (a) red, (b) green, and (c) blue micro-LEDs with top black matrix and 120° taper angle at different viewing angle θ and azimuthal angle φ.

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Tables (2)

Table 1 Optical parameters of commonly used AlGaInP-based red micro-LED and InGaN-based blue and green micro-LEDs adopted in simulations.

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Table 2 Simulated and calculated sidewall emission ratio for a RGB micro-LED display with different chip size.

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Equations (4)

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θ_{c} = \sin^{- 1} (\frac{n_{a i r}}{n}) .

η = \frac{Ω}{4 π},

Ω = \int_{0}^{2 π} d ϕ \int_{0}^{θ_{c}} \sin θ d θ .

η_{n} = \frac{1}{2} e^{- 2 n α d} R_{s}^{n} \int_{θ_{n - 1}}^{θ_{n}} \sin θ d θ .

Red Layers	Thickness (µm)	n	k	Green/Blue Layers	Thickness (µm)	n		k
Red Layers	Thickness (µm)	n	k	Green/Blue Layers	Thickness (µm)	Green	Blue	Green	Blue
ITO	0.1	2.07	0	n-GaN	2.5	2.38	2.43	4e-5	4e-5
p-GaP	1.0	3.33	0	MQW	0.1	2.41	2.49	2e-2	2e-2
p-AlGaInP	0.1	3.30	0	p-AlGaN	0.05	2.31	2.35	6e-4	7e-4
p-AlInP	0.6	3.21	0	p-GaN	0.3	2.38	2.43	4e-5	4e-5
MQW	0.4	3.60	0.16	n/p-Metal	0.5	0.44	1.43	2.29	1.85
n-AlInP	0.3	3.21	0	-	-	-	-	-	-
n-AlGaInP	1.2	3.30	0	-	-	-	-	-	-
n/p-Metal	0.5	0.15	3.52	-	-	-	-	-	-

Chip size (μm²)	Red		Green		Blue
Chip size (μm²)	Sim.	Cal.	Sim.	Cal.	Sim.	Cal.
15 × 30	10.8%	11.0%	58.9%	58.5%	62.2%	65.7%
35 × 60	5.3%	5.6%	52.4%	54.2%	55.1%	56.7%
50 × 100	3.8%	4.0%	47.2%	49.6%	50.1%	52.9%

Red Layers	Thickness (µm)	n	k	Green/Blue Layers	Thickness (µm)	n		k
Red Layers	Thickness (µm)	n	k	Green/Blue Layers	Thickness (µm)	Green	Blue	Green	Blue
ITO	0.1	2.07	0	n-GaN	2.5	2.38	2.43	4e-5	4e-5
p-GaP	1.0	3.33	0	MQW	0.1	2.41	2.49	2e-2	2e-2
p-AlGaInP	0.1	3.30	0	p-AlGaN	0.05	2.31	2.35	6e-4	7e-4
p-AlInP	0.6	3.21	0	p-GaN	0.3	2.38	2.43	4e-5	4e-5
MQW	0.4	3.60	0.16	n/p-Metal	0.5	0.44	1.43	2.29	1.85
n-AlInP	0.3	3.21	0	-	-	-	-	-	-
n-AlGaInP	1.2	3.30	0	-	-	-	-	-	-
n/p-Metal	0.5	0.15	3.52	-	-	-	-	-	-

Chip size (μm²)	Red		Green		Blue
Chip size (μm²)	Sim.	Cal.	Sim.	Cal.	Sim.	Cal.
15 × 30	10.8%	11.0%	58.9%	58.5%	62.2%	65.7%
35 × 60	5.3%	5.6%	52.4%	54.2%	55.1%	56.7%
50 × 100	3.8%	4.0%	47.2%	49.6%	50.1%	52.9%

Abstract

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Figures (13)

Tables (2)

Equations (4)

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