Improved Colored State Reflectivity In Lithiated WO 3 Films

Improved colored state reflectivity in in the Drude model there are 3 important lithiated WO3 films parameters; Ep (eV), the plasma energy =(h/2n)\l [ne] q2/fm*e0 ], (h=Planck's constant,R.B.Goldner, G.Seward, K.Wong. G.Berera, Cne3 = free electron density, q = electronic T.Haas and P.Norton charge, m* = electron effective mass, eQ =free space dielectric constant); Er (eV), theElectro-Optics Technology Center, Tufts loss parameter (also known as the damping orUniversity, Medford, MA 02155 scattering parameter); and e^ = the bound"'"Honeywell Electro-Optics Center, Lexington, electron contribution to the dielectricMA 02173 response function. Ideally, to obtainreflectivities > 1, for photon energies belowABSTRACT E , one needs Er ^ 0. Also, for Er » 0 theresulting spectral dependence for thePolycrystalline W03 films have been reflectivity is very "step-like," and the prepared which exhibit high near infrared optical absorption > O.6 Furthermore, a low reflectance (-69% at 2.5ym wavelength) when Er leads to a considerably reduced switching lithiated. This is a significant increase in energy requirement for smart windows. This is near infrared reflectance over what has been because the switching energy is proportional previously reported for lithiated WO3. The to the square of transferred charge and films also exhibit a decrease in the Drude the transferred charge required to achieve scattering parameter and a reversible a desired reflectivity decreases with Ep and structure change to higher symmetry consequently film thickness, [e.g^, for Er A (monoclinic to cubic) with increased .05eV and Ep a. 5eV, films of 80%.] Thus, identifying the significance for smart window glass will be source(s) of free electron scattering and discussed. reducing the scattering is important for thedesign of smart windows. By comparing ourreflectivity results for Lix WO3 films with the reflectivity of single crystals of1. INTRODUCTION Na^WO^, we concluded that there was anadditional major source of electronThere are a number of important scattering in our films besides ionized applications for which it is highly desirable impurity scattering, which most likely to have spectrally selective reflection results from the insertion of H"1" or Li"*") . On modulation, or tuning. One such application the basis of a spectroscopic ellipsometry is that of smart window glass, which could study, we showed that the major additional be used for significantly increasing the source of electron scattering was energy efficiency of buildings and vehicles, scattering associated with extended defects, especially by controlling solar gain.1 which acted like monopole line charges.tt Reflection modulation is desirable for at Furthermore, we attributed these extended least 3 reasons: 1) to avoid excess strain defects crystallographic shear planes and consequent degradation associated with primarily arising from oxygen deficiency. We heat-producing optical absorption; 2) to also ruled out phonon scattering as dominant avoid a decrease in thermal efficiency by obtaining no measurable change in the near associated with the radiative emission infrared reflectivity as we lowered the arising from the heat-producing optical temperature to close to that of liquid absorption; and 3) spectrally-selective nitrogen (~ 80°K). In this paper we wish to modulation (filtering) can be more readily report having achieved in polycrystalline achieved than with bound electron absorption films of WO3 a lithiated state reflectivity modulation. Consequently, one of our efforts at \ = 2.5 ym of approximately 69%, a has been directed towards developing an significant improvement over previous electrochromic layer for which a high degree results, and we wish to discuss this as well of reflection modulation can be achieved. We as related results, have concentrated much of that effort on attempting to model and optimize thereflection modulation in polycrystalline 2. SAMPLE PREPARATION films of lithium-insertable WO3. We havedemonstrated that the Drude quasi-free Thin films were prepared by rf sputter electron model applies to polycrystalline deposition in an oxygen atmosphere. An MRC films of HXWO3 and LixWO3. ~4 Previously we rf sputter deposition system (Model SM 8500) reported attaining a significant increase in with a 5 inch diameter WO3 target (purchased reflection modulation for polycrystalline from Cerac, Inc.) was used. The substrates films of H^WOo over what had been reported by were either bare or ITO-coated microscope Schirmer et.al.5 In particular we obtained glass slides and were heated to temperatures protonated (or "colored") state reflectivity in excess of 400°C. Lithiation was done in at X= 2.5ym (X = wavelength) of approximately either of two ways: a) for bare glass 60% as compared to approximately 35%, substrates, the RCA technique (previously obtained by Schirmer et.al. described)10 was used - i.e., a metal