A bioinspired solution for spectrally selective thermochromic VO<sub>2</sub> coated intelligent glazing

Alaric Taylor; Ivan Parkin; Nuruzzaman Noor; Clemens Tummeltshammer; Mark S Brown; Ioannis Papakonstantinou

doi:10.1364/OE.21.00A750

Optics Express
Vol. 21,
Issue S5,
pp. A750-A764
(2013)
•https://doi.org/10.1364/OE.21.00A750

A bioinspired solution for spectrally selective thermochromic VO₂ coated intelligent glazing

Alaric Taylor, Ivan Parkin, Nuruzzaman Noor, Clemens Tummeltshammer, Mark S Brown, and Ioannis Papakonstantinou

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Alaric Taylor, Ivan Parkin, Nuruzzaman Noor, Clemens Tummeltshammer, Mark S Brown, and Ioannis Papakonstantinou, "A bioinspired solution for spectrally selective thermochromic VO₂ coated intelligent glazing," Opt. Express 21, A750-A764 (2013)

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Abstract

We present a novel approach towards achieving high visible transmittance for vanadium dioxide (VO₂) coated surfaces whilst maintaining the solar energy transmittance modulation required for smart-window applications. Our method deviates from conventional approaches and utilizes subwavelength surface structures, based upon those present on the eyeballs of moths, that are engineered to exhibit broadband, polarization insensitive and wide-angle antireflection properties. The moth-eye functionalised surface is expected to benefit from simultaneous super-hydrophobic properties that enable the window to self-clean. We develop a set of design rules for the moth-eye surface nanostructures and, following this, numerically optimize their dimensions using parameter search algorithms implemented through a series of Finite Difference Time Domain (FDTD) simulations. We select six high-performing cases for presentation, all of which have a periodicity of 130 nm and aspect ratios between 1.9 and 8.8. Based upon our calculations the selected cases modulate the solar energy transmittance by as much as 23.1% whilst maintaining high visible transmittance of up to 70.3%. The performance metrics of the windows presented in this paper are the highest calculated for VO₂ based smart-windows.

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Fig. 1 Spectra used to weight the transmittance functions in Eqs. 1. The photopic luminous efficiency of the human eye [23], ȳ(λ), and the AM_1.5 solar irradiance spectrum [24].

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Fig. 2 The optical model for VO₂ as used in our FDTD simulations.

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Fig. 3 FDTD simulations of planar VO₂ window systems with different thin-film thicknesses showing the associated changes in

T_{lum}^{cold}

and ΔT_sol. The transmittance color in both the hot and cold phases are shown in the upper bars.

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Fig. 4 Side (a) and top (b) elevations of a nanotextured surface with hexagonally arranged circular paraboloid cones. Pitch P, base-width W and VO₂ coating thickness C. As in (b), when

W = P \sqrt{\frac{4}{3}} - 2 C

both the peak-to-trough height difference and the areal density at the air-VO₂ interface are maximized.

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Fig. 5 FDTD parameter search for hexagonally arranged VO₂ coated paraboloid nanostructures in which

W = \sqrt{\frac{4}{3}} P - 2 C

. Contours lines of T_lum are overlaid upon a heat-map of ΔT_sol and vice-versa. Six parameter sets are chosen [A–F] (detailed in Table 3)

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Fig. 6 [C] Normal incidence transmittance, absorptance and reflectance spectra for a moth-eye smart-window, case C: H = 700 nm, C = 8 nm, P = 130 nm and W = 134 nm.

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Fig. 7 Wide-angle FDTD simulations of the transmittance through (a) a moth-eye smart-window surface (case C) and (b) a planar 50 nm thick VO₂ smart-window with a 40 nm TiO₂ antireflection coating applied to the surface.

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Tables (3)

Table 1 A comparison of smart-window transmittance metrics for a variety of VO₂ based multilayer smart-window systems. Eqs. 1 and 2 are used to calculate the reported metrics using broadband transmittance data extracted from the cited publications where appropriate. All dimensions are in nanometers, T_sol, T_lum and ΔT_sol are all presented as a percentage of energy transmittance.

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Table 2 Calculations of the maximum pitch for moth-eye surface nanostructures that satisfy Eq. 3 over different wavelength intervals and angles of incidence. The moth-eye surface is assumed to be in air (n₀ = 1). Values for n₁ are calculated as the maximum refractive index for metallic and semiconductor VO₂ within the wavelength range. All angles are in degrees, wavelengths and pitches are quoted in nanometers

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Table 3 Smart-window transmittance metrics for selected moth-eye VO₂ smart-windows systems. Equations 1 and 2 are used to calculate the reported metrics. All dimensions are in nanometers, T_sol, T_lum and ΔT_sol are all presented as a percentage of energy transmittance.

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Equations (4)

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T_{lum}^{σ} = \frac{\int_{λ = 380 nm}^{780 nm} \bar{y} (λ) T^{σ} (λ) d λ}{\int_{λ = 380 nm}^{780 nm} \bar{y} (λ) d λ} T_{sol}^{σ} = \frac{\int_{λ = 300 nm}^{2500 nm} {AM}_{1.5} (λ) T^{σ} (λ) d λ}{\int_{λ = 300 nm}^{2500 nm} {AM}_{1.5} (λ) d λ}

Δ T_{lum} = T_{lum}^{cold} - T_{lum}^{hot} Δ T_{sol} = T_{sol}^{cold} - T_{sol}^{hot}

P \leq \frac{λ_{min}}{n_{1} + n_{0} sin (θ_{i})}

W = P \sqrt{\frac{4}{3}} - 2 C

			hot state			cold state
Citation	Top/.../Bottom	Thickness (nm)	T_sol	T_lum	$T_{RGB}^{hot}$	T_sol	T_lum	$T_{RGB}^{cold}$	ΔT_sol
[31]	TiO₂/VO₂	40/50	37.1	45.5	CBB064	45.0	49.9	D5B774	7.9
[5]	TiO₂/VO₂/TiO₂	25/50/25	40.8	54.9	D7C277	44.9	57.9	D7C886	4.1
[7]	ZrO₂/VO₂	56/50	37.9	48.5	CCB76B	43.6	51.3	D0BC79	5.7
[28]	a-Si/VO₂/a-Si	25/100/25	18.0	6.9	594805	26.9	10.0	6C550C	8.9
[28]	a-SiO_0.3/VO₂/a-SiO_0.3	25/100/25	31.4	34.9	B99A61	35.0	35.6	B99C66	3.6
[6]	TiO₂/VO₂/TiO₂/VO₂/TiO₂	130/40/130/40/130	40.4	43.1	BDAF7C	52.1	45.3	C5B182	11.8

λ_min	$\int_{λ_{min}}^{780} \bar{y} (λ) d λ$	Angle of incidence, θ_i
λ_min	$\int_{λ_{min}}^{780} \bar{y} (λ) d λ$	0°	20°	40°	60°	80°

525.2	80%	183.0	163.5	149.5	140.6	136.3
508.6	90%	177.2	158.4	144.8	136.1	131.9
460.0	99%	160.3	143.2	131.0	123.1	119.3	P > 150 nm
429.8	99.9%	149.8	133.8	122.4	115.1	111.5	P < 130 nm

					hot state			cold state
	Height	Base-width	VO₂ thickness	Pitch	T_sol	T_lum	$T_{RGB}^{hot}$	T_sol	T_lum	$T_{RGB}^{cold}$	ΔT_sol
A	1150	132	9	130	23.8	33.6	A49D73	46.9	39.4	C3A36A	23.1
B	850	132	9	130	33.4	44.1	B7B28D	55.4	49.5	D1B686	21.9
C	700	134	8	130	44.5	55.1	C9C5A7	63.9	59.9	DDC8A1	19.4
D	500	135	7.5	130	57.0	66.6	D8D6BF	72.5	70.3	E7D8BB	15.5
E	340	112	19	130	35.1	45.3	B9B48E	54.9	50.4	D1B887	19.8
F	250	70	40	130	22.5	31.5	9F996E	42.3	37.0	BD9F65	19.8

			hot state			cold state
Citation	Top/.../Bottom	Thickness (nm)	T_sol	T_lum	$T_{RGB}^{hot}$	T_sol	T_lum	$T_{RGB}^{cold}$	ΔT_sol
[31]	TiO₂/VO₂	40/50	37.1	45.5	CBB064	45.0	49.9	D5B774	7.9
[5]	TiO₂/VO₂/TiO₂	25/50/25	40.8	54.9	D7C277	44.9	57.9	D7C886	4.1
[7]	ZrO₂/VO₂	56/50	37.9	48.5	CCB76B	43.6	51.3	D0BC79	5.7
[28]	a-Si/VO₂/a-Si	25/100/25	18.0	6.9	594805	26.9	10.0	6C550C	8.9
[28]	a-SiO_0.3/VO₂/a-SiO_0.3	25/100/25	31.4	34.9	B99A61	35.0	35.6	B99C66	3.6
[6]	TiO₂/VO₂/TiO₂/VO₂/TiO₂	130/40/130/40/130	40.4	43.1	BDAF7C	52.1	45.3	C5B182	11.8

λ_min	$\int_{λ_{min}}^{780} \bar{y} (λ) d λ$	Angle of incidence, θ_i
λ_min	$\int_{λ_{min}}^{780} \bar{y} (λ) d λ$	0°	20°	40°	60°	80°

525.2	80%	183.0	163.5	149.5	140.6	136.3
508.6	90%	177.2	158.4	144.8	136.1	131.9
460.0	99%	160.3	143.2	131.0	123.1	119.3	P > 150 nm
429.8	99.9%	149.8	133.8	122.4	115.1	111.5	P < 130 nm

Abstract

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Figures (7)

Tables (3)

Equations (4)

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