Ronald B. Goldner

High near‐infrared reflectivity modulation with polycrystalline electrochromic WO3 films

A near‐infrared reflectivity exceeding 60% at 2.5‐μm wavelength has been observed for a polycrystalline, rf sputter‐deposited electrochromic (EC) WO3 film in a deeply colored state. This reflectivity is considerably higher than that previously reported for a thermally evaporated EC‐WO3 film that was crystallized by a post‐deposition thermal anneal. The shapes of the x‐ray spectra of the two films are also different. The results of ellipsometry measurements of the optical constants provide convincing evidence for the validity of a free‐electron Drude model to explain the reflectivity modulation observed in polycrystalline EC‐WO3.

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.

A study of electronic shorting in IBDA-deposited Lipon films

Abstract Because a near term goal of our research is to obtain optimal performance LiCoO 2 /lithium phosphorus oxynitride (Lipon)/C thin film batteries, and due to the major importance of the electrolyte in any battery, we have recently been attempting to better understand the causes of electronic shorting in our Lipon electrolyte films. After studying the residual and temperature-dependent stress of these films and observing cracking after they had undergone a temperature change from 300°C to room temperature, we adopted a model in which the thermal expansion coefficient mismatch between Lipon and our glass substrates accounted for the cracking and therefore led to the shorting. This model was also supported by evidence that Al films (which had thermal expansion coefficients close to that of Lipon and proved to act as “buffer layers” by preventing cracking of Lipon when glass/Al/Lipon structures were cooled from 300°C to room temperature) were successfully used to produce short-free Al/Lipon/Al devices.

Thin films of lithium cobalt oxide

Abstract Thin films of lithium cobalt oxide have been prepared by rf sputtering from crystalline LiCoO2. Chemical analysis shows that the films as deposited are deficient in lithium. X-ray diffraction suggests that the films retain the basic rhombohedral structure of LiCoO2. A plausible structure defect model for these lithium deficient films is that of a disordered α-NaFeO2 structure. Little absorption and moderate reflectivity are found in the transmission and reflectance spectra in the infrared range. The visible absorption corresponds to a direct allowed optical transition with an energy gap of 2.2 eV. Electrical conductivity measurements of the films show that at 300 K the films behave as semiconductors, with an activation energy of 0.12 eV. Ionic conductivity does not contribute significantly to the measured conductivity. Thermopower measurements indicate that the films are p-type conductors and suggest a narrow conduction band.

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.

Density of States Calculations of Small Diameter Single Graphene Sheets

The densities of states for the {pi}-band of single graphene sheets with small diameters were calculated by employing a linear combination of atomic orbital approach using as the basis set the carbon p{sub z} atomic orbitals together with a modified Hueckel approximation wherein the overlap integrals out to the fourth nearest neighbors set were included. These densities of states were used to predict the voltage of lithiated carbon vs. lithium metal, an important characteristic for disordered carbon used as the negative electrode in rechargeable lithium-ion batteries. Calculations were made for isolated single graphene sheets, C{sub n}, with n = 24, 54, 96, 150, and 216. The results suggested that the lowest voltage should occur for lithiated carbon electrodes composed of single graphene sheets with the smallest diameter ({approx} 0.7 nm for C{sub 24}).

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/.

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 ...

Rapidly Grown IBAD LiPON Films with High Li-Ion Conductivity and Electrochemical Stability

LiPON films have been produced by nitrogen ion beam assisted deposition (IBAD) of thermally evaporated Li 3 PO 4 . This process allowed us to rapidly deposit X-ray amorphous LiPON films with densities close to that of the handbook value for Li 3 PO 4 , high ionic conductivity (1.6 X 10 - 6 S/cm), electronic conductivity smaller than 8 X 10 - 1 3 S/cm, and an electrochemical stability window of over 6.0 V. These properties, combined with the high deposition rates that can be achieved with this process (>1.0 nm/s), make it an attractive candidate for industrial production of LiPON thin films for Li-ion batteries.

Electrochromic materials for controlled radiant energy transfer in buildings

Windows and building envelopes with an externally variable light transmission can be used to reduce significantly heating and cooling loads. Thin film, large area electrochromic structures are conceptualized which would permit reversible aperture control through application of small dc electrical currents. The structures have the generalized geometry TEC‖EC1‖FIC‖EC2‖TEC, where TEC is a thin transparent electrical conductor, EC1 and EC2 are complementary electrochromic layers, and FIC is a fast ion conductor. Measurements of the optical properties of the polycrystalline electrochromic oxide HxWO3 show large increases in reflectivity on increasing x, an effect not pronounced in the more frequently investigated amorphous material. The variation of the optical constants with x are used to demonstrate the dominance of free electron scattering in determining the optical properties. It is therefore concluded that electrochromic windows based on polycrystalline electrochromic layers may be operated in a reflectivity mode, which has several important advantages over variable absorption operations.