Terry E. Haas

Investigation of electrochromic properties of nanocrystalline tungsten oxide thin film

Thin films of tungsten oxide were grown by organometallic chemical vapor deposition (OMCVD) using tetra(allyl)tungsten, W(η3-C3H5)4. X-Ray diffraction (XRD) analyses showed amorphous films at substrate temperatures (Ts) 350°C. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed grain sizes in the range 20–40 nm. In situ electrochemical reduction of WO3.2/ITO (2.0 M HCl) produced a faint blue color in less than 1 s. The maximum coloration efficiency (CE) was found to be 22 cm2/mC at 630 nm. The density of the films decreases from 4.53 to 4.29 g/cm3 after annealing. An optical bandgap (Eg) of ∼3.2 eV was estimated for both as-deposited and annealed films.

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.

Twisting and piezochromism of phenylene-ethynylenes with aromatic interactions between side chains and main chains

This paper describes a series of three-ring phenylene-ethynylenes (PEs) in which specific, non-covalent arene–arene interactions control conformation in the solid-state. As determined by single crystal X-ray structures, edge-face interactions between benzyl ester side chains and conjugated main chains are observed. In contrast, perfluorobenzyl ester side chains interact cofacially with main chains, resulting in ∼60° torsional angles between neighboring aryl rings in crystalline PEs. Absorbance and fluorescence spectra of films of these compounds reflect these conformational effects, with the spectra of perfluorobenzyl-substituted compounds shifting hypsochromically from solution- to solid-state. In a demonstration of how balancing non-covalent interactions can open the way to new responsive materials, a main chain twisted derivative with octyloxy substituents displayed significant piezochromic behavior.

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/.

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.