Oxide Electrochromics: Advances in Mater

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Oxide Electrochromics: Advances in Materials and Devices - paper presented at MRS Spring Meeting 2011. \"This paper introduces the principles of oxide-based electrochromics and reports new data on materials and devices. Specific discussions are given on new nickel-tungsten oxide based electrochromic films with improved coloration efficiency, transparent conductors comprising ultrathin metal films, and polymer electrolytes incorporating inorg ... Shared on http://www.vacuumcoating.info

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Advances in Materials and Devices Oxide-Based Electrochromics: C.G. Granqvist The Ångström Laboratory, Uppsala University, Sweden ChromoGenics AB, Uppsala, Sweden 14 April 2011

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Environmental challenges Increasing global mean temperature & sea level Increasing trend for heavy rainfall (India) 14 April 2011

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More challenges Urban heat island (Montreal) Distribution shifts of fish 14 April 2011

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Still more challenges Wildfires (San Diego) Break-up of ice sheets 14 April 2011

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Why these problems? Increasing population Increasing energy use per person Energy production releases CO 2 14 April 2011

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Why are buildings important? • 30-40 % of primary energy is used in buildings Up to 70 % of electricity in buildings We spend 80-90 % of our time indoors 14 April 2011 • •

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It is cheap to improve buildings! Greenhouse gas abatement costs: Power generation → Buildings (negative cost!!) → 14 April 2011

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Ambient radiative properties Key features: • Spectrally selective • Angular dependent • Time-variable 14 April 2011

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The chromogenic technologies Photochromic (UV light) Thermochromic (temperature) Electrochromic (electric charge) Gasochromic (reducing/oxidizing gas) 14 April 2011

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Electrochromic (EC) device design Thin-film battery with visible charge

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Some implications of the battery model • Open circuit memory of transmittance • Intermediate transmittance levels • Non-instantaneous transmittance changes • Physical processes at the atomic scale, i.e., no haze • Color matching by two EC films • Polymer electrolyte can give added functionality (spall shielding, burglar protection, acoustic damping, thermo- chromism…) 14 April 2011

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Six challenges for EC devices • EC films: Nanocrystalline & nanoporous • Transparent conductor: ~20 Ω, T ≈ 90% • Efficient charge insertion/balancing • Elecrolyte: good ion conductivity, UV stability, (adhesion) • Long-term durability: Good strategy for voltage & current control • Large-scale manufacturability 14 April 2011

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EC oxides: The options • Cathodic (color under ion insertion) • Anodic (color under ion extraction) 14 April 2011

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EC oxides: Unifying feature The ubiquitous octahedron Implications: • Electronic structure • Ion diffusion 14 April 2011

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EC oxides: Polaron absorption Photon-induced electron transfer between neighboring metal ions ( i and j ): W i 5+ + W j 6+ + photon → W i 6+ + W j 5+ 14 April 2011

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Nanoporosity in sputter deposited films Large pressure Low temperature 14 April 2011

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Mixed EC oxides: Optical absorption Coloration efficiency: Change in optical absorption per unit of charge Enhanced coloration is possible 14 April 2011

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Transparent conductor: ITO etc • Wide band gap, heavy doping, thickness ~200nm • In 2 O 3 :Sn (ITO), ZnO:Al (AZO), ZnO:Ga (GZO), SnO 2 :F (FTO), TiO 2 :Nb (TNO)… • High transmittance • Infrared reflectance • Resistivity ~10 -4 Ω cm • Theoretically understood 14 April 2011

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Transparent conductors: Metals • Examples ZnO/Ag/ZnO TiO 2 /Au/TiO 2 ZnO/Ag/ZnO/Ag/ZnO acrylic/Ag-Au/acrylic… • Metal thickness: ~10 nm • Dielectric thickness ~100nm • Reflecting in NIR 14 April 2011

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Thin metal films: The role of the substrate t eq [nm]: 2.6 4.4 6.2 8.0 9.8 200 nm Resistivity of Au on glass and SnO 2 :In 14 April 2011

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Transparent conductors: New carbons • Graphene monolayer • C 60 ”buckyball” • Nanotubes 14 April 2011

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Carbon nanotube meshes • Good transparent conductor • No haze • Transparent in NIR • Cannot (yet) (quite) match ITO 14 April 2011

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Silver nanorod meshes • Good transparent conductors • Cheap • Some haze 14 April 2011

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Ionic Polymer development PEI-LiTFSI with SiO 2 nanoparticles • Increased ion conductivity → • No increase in light scattering → 10 -4 10 -5 10 -6 20°C 30°C 40°C 50°C 60°C 70°C 0 1 2 3 4 5 6 7 8 SiO2 concentration (wt%) 91.5 Total transmittance 3 Diffuse transmittance Diffusetransmittance(%) Total 91.0 2 550 nm 90.5 1 90.0 0 2 4 6 8 10 14 April 2011 SiO2 concentration (wt%)

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EC device designs for ”smart windows” Five-layer ”monolithic” design Laminated glass/FTO/WO 3 /electrolyte/CE/FTO/glass glass/FTO/WO 3 /PVB-type/CE/FTO/glass PET/ITO/WO 3 /electrolyte/”NiO”/ITO/PET 14 April 2011

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Electrochromic foil: General design Nickel oxide ( grain surfaces ): [Ni(OH) 2 ] bleached ↔ [NiOOH + H + + e - ] colored [NiO + Ni(OH) 2 ] bleached ↔ [Ni 2 O 3 + 2H + + 2e - ] colored Tungsten oxide ( entire film ) [WO 3 + H + + e - ] bleached ↔ [HWO 3 ] colored 14 April 2011

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Electrochromic foil: State of the art PET/ITO WO 3 ”rubber” electrolyte ”NiO” ITO/PET 5 x 5 cm 2 14 April 2011

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Electrochromic foil: Optical properties 100 90 80 70 60 50 40 30 20 10 0 Transmittance Double foil: 500 1000 1500 2000 2500 Wavelength (nm) 14 April 2011

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Foil-based Smart Window Prototype: 0.8 x 1.8 m 2 14 April 2011

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Electrochromic foil: R2R manufacturing 14 April 2011

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Electrochromic foil: Roll-to-roll First results! Proof of principle! Size: 1000 x 0.8 m 2 80 70 Transmittance 60 50 40 30 1400 cycles 6 Y After 750 cycles Bleaching time 40 s Coloring time 120 s 20 0.00 0.05 0.10 0.15 0.20 0.25 Time (s) ×10 14 April 2011

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Thermochromic VO 2 films: What are the issues? • Too high transition temperature. Dope with W! • Too low luminous transmittance. Dope with Mg! • Too low solar modulation. Use nanoparticles! 14 April 2011

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Thermochromism in VO 2 nanoparticles 1 vol% VO 2 in dielectric host • Transparent at τ < τ c • NIR absorbing at τ > τ c 14 April 2011

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Future ”superfenestration” • Electrochromic outer pane • Thermochromic inner pane • Low heat transfer (vacuum insulation, silica aerogel) • More… 14 April 2011

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14 April 2011

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14 April 2011

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Some alternative electrochromic techniques • Variable electroplating (variable reflectance) • Metal hydride (variable reflectance) • Liquid-crystal devices • Suspended particle devices 14 April 2011

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