Components of smart windows : Investigations of electrochromic films, transparent counter electrodes and sputtering techniques
Abstract: In smart windows, the transmittance of visible and near infrared radiation can be reversibly and persistently modulatcd by an applied voltage pulse (~2 V). The micrometer-thick multi-layer structure contains two electrodes capable of charge insertion. One of these electrodes must be electrochromic; usually it consists of tungsten oxide. The other, i.e., the counter electrode, can either be a complementary coloring electrochromic material or a material remaining transparent upon charge insertion. The lack of a suitable counter electrode has been an obstacle for commercialization of electrochromic devices.This thesis investigates the optical modulation mechanisms and deterioration of electrochromic materials, including oxidas of W, Mo, Ti, La, Ce, Pr, Hf, Zr, Ni, Cr, and their mixtures. New counter electrodes have been developed. The materials were deposited by multi-target sputtering, which is a method well suited for operation in metallic target-oxidized substrate mode. The materials were characterized by physical, optical and electrochemical methods.To study the interface roughness, light scattering was measured with a specially designed total integrated scattering instrument. Experiments on tungsten-oxide-based films show that optical modulation and electrochemical degradation do not change the interface topography. Using electrochromic films in light scattering measurements offers a new possibility to study cross-correlation since the contribution to the scattering from different interfaces can be separated.The coloration mechanisms are discussed in terms of the electron structure of the materials. Optical and electrochemical measurements indicate that the cerium 4f states can be filled without causing optical absorption in oxides containing cerium. For nickel oxide and chromium oxide it is suggested that the metallic non-bonding character of electron states at the top of the valence band is essential for the anodic electrochromism of these materials.Devices exhibiting good performance were assembled using tungsten oxide, various counter electrodes, and electrolytes of lithium polymer and proton composites. Transparent nickel-oxide-containing devices can be assembled without pretreatment of the components. Zirconium-cerium oxide is fully transparent throughout the whole visible range and can be employed in devices where a high transmittance is essential.
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