Kwok-Keung Wong

Thin film solid state ionic materials for electrochromic smart windowTM glass

Abstract The optical transmissivity (and especially the reflectivity) of a multilayer electrochromic smart winow TM glass structure can be reversibly modulated by an electrical current pulse; and the modulation is spectrally selective. This is important forbuilding and vehicle windows and for other electro-optical applications. Thin film solid state ionic materials are the key elements of such a structure. Their roles and some of their current states of development at Tufts (and particularly the state of developent of the electrochromic layer) are discussed. Also discussed is the state of development of prototype all solid electrochemically reversible, five-layer window devices. These devices employ: a lithium-insertable rf sputter-deposited polycrystalline WO 3 film that exhibits reflection modulation as the electrochromic layer, a lithium-inserted rf sputter deposited In 2 O 3 film or a thermally deposited V 2 O 5 film as the counterelectrode layer; an rf sputter deposited LiNbO 3 film as the ion conducting (solid electrolyte) layer; and rf sputter deposited ITO (Sn-doped In 2 O 3 ) or undoped In 2 O 3 films as the transparent conducting layers. The devices exhibit useful operating characteristics and indicate that robust and economical practical smart window TM glass could soon become a commercial reality.

Recent research related to the development of electrochromic windows

Abstract The results of recent research on each of the layers of a solid-state multilayer structure for electrochromic windows are reviewed. This includes a review of the requirements of the multilayer structure for building windows and especially the need to have an electrochemically balanched system. The results indicate that excess free electron scattering in polycrystalline WO 3 films (the electrochromatic layer) is not only the source of lower than desired reflectivity modulation, but it is also the major source of higher than desired absorptivity modulation. Research on LiAlF 4 indicates that it is a viable candidate for the ion conducting layer, being a good lithium ion conductor and a good electronic insulator. Finally, we have recently discovered that tin-doped and undoped In 2 O 3 films are mixed conductors, exhibiting lithium ion injection/extraction. This is significant since such films could possibly serve in the dual capacity of counterelectrode and transparent conductor, thereby reducing the complexity, the technical problems, and the cost of electrochromic window assemblies.

Electrochromic behavior in ITO and related oxides.

Electrochromic properties, are reported for thin films of the transparent conducting oxides In/sub 2/OCd3:Sn are doped SnO/sub 2/.

Further evidence for free electrons as dominating the behavior of electrochromic polycrystalline WO3 films

The results of two types of experiments on electrochromic (poly)crystalline films of rf sputtered WO3 are presented as additional new evidence that the behavior (especially scattering) of the free electrons plays a dominant role in the electrochromic properties of such films.

Near‐infrared reflectance modulation with electrochromic crystalline WO3 films deposited on ambient temperature glass substrates by an oxygen ion‐assisted technique

Electrochromic, crystalline WO3 films have been deposited on glass substrates at ambient temperature by an oxygen‐ion‐assisted technique using oxygen ion energies ≥300 eV and oxygen ion to vapor molecule (WO3) ratios, γ≥2.5. After lithiation, the resulting LixWO3 films exhibited >50% reflectivity in the near infrared, and the reflectivity dispersion was fit by a Drude free‐electron model, yielding the Drude parameters: plasma energy, Ep=3.3 eV; and the loss (damping) parameter, EΓ=1.0 eV. (The bound electron permittivity, eb, was fixed at 4.0.) These values are comparable to those obtained with WO3 films rf sputter deposited onto substrates at temperatures >420 °C. During the ion‐assisted deposition the substrate temperature reached approximately 90 °C, caused primarily by radiation from the WO3 evaporant source. This indicates that economical low‐temperature substrates, such as plastics, could be used. These results suggest that practical electrochromic smart windows for energy‐efficient buildings might ...

Nuclear reaction analysis profiling as direct evidence for lithium ion mass transport in thin film ‘‘rocking‐chair’’ structures

A nuclear reaction analysis technique using the p,γ reaction, 7Li(p,γ)8Be, occurring at approximately 440 keV, (half‐width≊12 keV), has been utilized to determine the lithium concentration profiles in multilayer electrochromic window (‘‘smart window’’)/rechargeable battery cells when in their ‘‘colored’’/charged and ‘‘bleached’’/discharged states. The lithium profiles have been observed to shift according to the cells’ states, thereby providing direct experimental evidence for the so‐called rocking‐chair model for such structures.

Optical frequencies free electron scattering studies on electrochromic materials for variable reflectivity windows

Abstract Electrically variable reflectivity electrochromic windows have the potential to significantly reduce heating and cooling loads in buildings. The importance of reducing the optical frequencies free electron scattering in the variable reflectivity electrochromic layer will be shown. Based on spectroscopic ellipsometry studies and an analysis of the dynamical resistivity of polycrystalline films of colored WO3 and of single crystals of NaxWO3, it is tentatively concluded that the principal source of free electron scattering in WO3 films is structural disorder, and probably crystallographic shear planes which act as monopolar dislocations. This is in contrast to the much higher reflectivity of single crystal tungsten bronzes for which the more dominant scattering mechanism appears to be that associated with ionized impurities. We therefore conclude that it should be possible to improve the reflectivity modulation in films of electrochromic WO3, and consequently that a practical variable reflectivity electrochromic window should be achievable.

Completely solid lithiated smart windows

The optical transmissivity (and especially the reflectivity) of a multilayer electrochromic smart window® glass structure (patents applied for) can be reversibly modulated by an electrical current pulse; and the modulation is spectrally selective. This is important for building and vehicle windows and for other electro-optical applications. Discussed is the state of development of each of the layers for, and the state of development of, prototype all-solid electrochemically reversible, 5-layer Smart Window devices. These devices employ: a lithium-insertable RF sputter-deposited polycrystalline WO3 film that exhibits reflection modulation as the electrochromic layer; a lithium-insertable RF sputter-deposited In2O3 film or a thermally deposited V2O5 film as the counterelectrode layer; an RF sputter-deposited LiNbO3 film as the ion conducting (solid electrolyte) layer; and RF sputter-deposited ITO (Sn-doped In2O3) or undoped In2O3 films as the transparent conducting layers. The devices exhibit useful operating characteristics and indicate that robust and economical practical smart window® glass could soon become a commercial reality.

Some lessons learned from research on a thin film electrochromic window

Abstract Five lessons learned during research on a variable reflectivity, completely inorganic, thin film electrochromic window were: (i) the “rocking-chair” model is valid, and has been, and is expected to be, useful in guiding the design, development and production of the window; (ii) with appropriate processing, charge transport through the six interfaces can be managed properly; (iii) nearly complete stress-change compensation during switching can be achieved for the window; (iv) electrochromic materials of opposite carrier types are desirable; and (v) one can fabricate, routinely by a production-type process, completely inorganic EC windows which are electrically, mechanically, and optically robust.

Prototype All-Solid Lithiated Smart Window Devices

The optical transmissivity (and especially the reflectivity) of a ultilayer electrochvomic smart windowTM glass structure can be reversibly modulated by an electrical current pulse; and the modulation is spectrally selective. This is important for building and vehicle windows and for other electro-optical applications. Discussed is the state of development of each of the layers for, and the state of development of, prototype all-solid electrochemically reversible, 5-layer smart window devices. These devices employ: a lithium-insertable rf sputter-deposited polycrystalline WO3 film that exhibits reflection modulation as the electrochromic layer; a lithium-insertable rf sputter deposited In203 film or a thermally deposited V205 film as the counterelectrode layer; an rf sputter deposited LiNbO3 film as the ion conducting (solid electrolyte) layer; and rf sputter deposited ITO (Sn-doped In203) or undoped In203 films as the transparent conducting layers. The devices exhibit useful operating characteristics and indicate that robust and economical practical smart windowTM glass could soon become a commercial reality.