Optical indices of lithiated electrochromic oxides

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Abstract

Optical indices have been determined for thin films of several electrochromic oxide materials. One of the most important materials in electrochromic devices, WO3, was thoroughly characterized for a range of electrochromic states by sequential injection of Li ions. Another promising material, Li0.5Ni0.5O, was also studied in detail. Less detailed results are presented for three other common lithium-intercalating electrochromic electrode materials: V2O5, LiCoO2, and CeO2–TiO2. The films were grown by sputtering, pulsed laser deposition (PLD) and sol–gel techniques. Measurements were made using a combination of variable-angle spectroscopic ellipsometry and spectroradiometry. The optical constants were then extracted using physical and spectral models appropriate to each material. Optical indices of the underlying transparent conductors, determined in separate studies, were fixed in the models of this work. The optical models frequently agree well with independent physical measurements of film structure, particularly surface roughness by atomic force microscopy. Inhomogeneity due to surface roughness, gradient composition, and phase separation are common in both the transparent conductors and electrochromics, resulting sometimes in particularly complex models for these materials. Complete sets of data are presented over the entire solar spectrum for a range of colored states. These data are suitable for prediction of additional optical properties such as oblique transmittance and design of complete electrochromic devices.

Introduction

The complex refractive index of electrochromic materials and other materials used in electrochromic devices is needed for design and optimization. Although reports on radiometric properties of electrochromic materials, primarily visible transmittance, are extensive, complete sets of optical constants are scarce. Even for WO3, the most widely used and studied electrochromic material, available information on optical indices has been incomplete. For all other materials the volume of data falls off rapidly. The most common limitations of existing data are in spectral and electrochemical range. The situation is further complicated by the fact that there are notable variations in properties for nominally similar materials. These variations may be due to composition, density, surface roughness and even thickness.

In this work, we survey materials that intercalate Li ions including WO3. We not only characterize the extreme bleached and colored states, but also a range of intermediate charge states for several of these materials. This is very important in design and optimization studies because of the need to achieve a balance between cooling and lighting [1]. The spectral range of our measurements typically spans the entire solar spectrum from 300 to 2500 nm. We attempt to make the models of structure and dispersion used to analyze the optical data consistent with the physical nature of the materials. A subtheme of this work is the effect of surface morphology which is an outcome of our attempts to validate the models with direct AFM observations of roughness. We recently made a first attempt to model a complete device including transparent conductors [2].

Section snippets

Methods

The films studied in this paper were grown by sputtering, pulsed laser deposition and sol–gel techniques under a wide variety of conditions. Specific details are noted for each section. PLD has been an especially useful technique because samples can be grown rapidly and the targets are small and thus easy to fabricate. The films are usually dense and smooth. Perhaps most important, control of stoichiometry is easier than with sputtering, where various ion and neutral-beam effects complicate the

Transparent conductors

In order to model complete electrochromic devices, we must of course have the properties of the transparent conducting layers, typically SnO2·F (we use Tech15TM made by Libbey–Owens–Ford) or In2O3 : Sn (ITO). These materials have recently been studied in sufficient detail to make full solar spectral calculations 5, 6, 7. Furthermore, we could not put the electrochromic materials of this paper into colored states without depositing them on transparent conductors. So we must have good values of

Conclusions

Using relatively simple models of dispersion and structure, optical indices can be determined over a wide spectral range for a variety of electrochromic materials in different electrochromic states. Surface roughness can be accurately predicted as part of the optical model for most cases. At least for PLD films surface roughness is usually correlated with poor electrochromic performance and stability. Complete sets of optical indices are presented, suitable for design of advanced electrochromic

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technologies, Building Systems and Materials Division of the US Department of Energy under Contract No.DE-AC03-76SF00098.

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