Optical Properties Band Structure Electrochromic WO. Thin Films for Smart windows sCattering at an interface between two materials e Electrochromic Eilm different n (index of refraction) mUltilayer stacks that behave like batteries ◆ Absorption eVisible indication of their electrical charge eLectron excitation to defect levels in the band eFully charged: opaque aPartially charged: partially transparent .Electron excitation across the band gap sFully discharged; transparent empty empty rechargeable solid state Metal catalyst,… Electrochromic glass Why the Color Change? Electrochromic glas CB Iws(d咧 ating a electrochromic layer which changes color from (d) smission into the building. When there is little sunlight he glass brightens, so that the need for the artificial light is wo, M+e/?Mwo, e -+ey i wo, Wide band Narrow band gap Metallic W6++ Wi+ CO ORS in Solid Materials Impurities in Ceramics oCO ORS cOlor is the result of the combination of 刽 a Ruby, sapphire, topaz, wavelengths that are transmitted ure form electrons drop back into original positions e AT within the band gap leads to color very pure single crystal colorless add Cr, O, to ALOs will become deep red. RUBY2 Optical Properties & Band Structure Reflection Scattering at an interface between two materials with different n (index of refraction) Absorption Electron excitation to defect levels in the band gap Electron excitation across the band gap Metal filled empty Insulator filled empty filled empty Semiconductor Valence Band Conduction Band Electrochromic WO3 Thin Films for Smart Windows Uses: mirrors, displays, rechargeable solid state batteries, pH-sensitive electrochemical transistors or displays, chemical sensors, solar cells, selective oxidation catalyst, … e- into Conducting Bond of WVIO3 M+ into hole Electrochromic Film: Multilayer stacks that behave like batteries Visible indication of their electrical charge Fully charged: opaque Partially charged: partially transparent Fully discharged: transparent Electrochromic glass is an energy-saving component for buildings that can change color on command. It works by passing low-voltage electrical charges across a microscopically-thin coating on the glass surface, activating a electrochromic layer which changes color from clear to dark. The electric current can be activated manually or by sensors which react to light intensity. Glass darkening reduces solar transmission into the building. When there is little sunlight, the glass brightens, so that the need for the artificial light is minimized. Electrochromic Glass OFF ON Why the Color Change? WO3 Wide band gap insulator MxWO3 Narrow band gap semiconductor x(M+ + e- ) MxWO3 Metallic x(M+ + e- ) VB [O2- (2pp)] CB [W6+(d0)] Localized VB [W5+(d1)] Delocalized VB [W5+(d1)] W5+ + W6+ ® W6+ + W5+ COLORS in Solid Materials COLORS! Color is the result of the combination of wavelengths that are transmitted Absorbed radiation can be reemitted as excited electrons drop back into original positions Þ not necessarily the same frequency as that absorbed Specific impurities can introduce electron levels within the band-gap - leads to color e.g. Al2O3 - very pure - single crystal - colorless add Cr2O3 to Al2O3 will become deep red - RUBY Impurities in Ceramics Corundum:Al2O3 Ruby, sapphire, topaz, amethyst Colorless in pure form Impurities result in different colors