Film Counter Electrode Research Articles

Observation of gap structures in La 1.85Sr 0.15CuO 4− δ single crystals by tunneling spectroscopy

We studied La 1.85Sr 0.15CuO 4− δ single crystals by tunneling spectroscopy both in planar and in break junction configurations. In the c-axis oriented planar junctions, with natural barriers and evaporated Pb film counterelectrodes, we observed at low-temperature gap-like structures around ±7–10 meV. In the a, b oriented break junctions we found conductance structures at ±15–20 mV, an increasing background at high biases and a conductance peak at zero voltage. The presence of this peak can be interpreted in terms of d-wave component for the order parameter in this material.

Spectroelectrochemical studies of V/Ti-, V/Ti/Zr- and V/Ti/Ce-oxide counter-electrode films

V/Ti-, V/Ti/Zr- and V/Ti/Ce-oxide films were prepared using the sol–gel route and deposited by dip-coating. After heating at 300 and 400°C the films showed mixed anodic–cathodic electrochromism similar to V 2O 5 films. In situ UV–VIS spectroelectrochemical measurements revealed a small optical modulation in the films and evaluated their appropriateness for counter electrodes in electrochromic devices. Additionally, a thorough analysis of the IR vibrational spectra (4000–400 cm −1) of amorphous (300°C) and crystalline (400°C) V/Ti-oxide films was made. We investigated the initial, charged and discharged states of films with a molar ratio V:Ti in alkoxide precursors 3:1 and 1:1. We found that Ti formed a solid solution with V when the films were annealed at 300°C. The characteristic V–O–Ti stretching appeared at 790 cm −1. This mode showed a modulation of intensity with charging/discharging indicating the contribution of V and Ti species to the insertion processes. Alternatively, annealing at 400°C leads to a segregation of phases into V 2O 5 and TiO 2 and a decrease in cycling stability.

Mixed cerium/titanium and cerium/zirconium oxides as thin film counter electrodes for all solid state electrochromic transmissive devices

Optically passive thin films of mixed cerium/titanium and cerium/zirconium oxides have been prepared by electron beam reactive evaporation. Different oxidation levels have been achieved using different oxygen flows in the deposition process. The samples have been optically and electrochemically characterized. Performances are discussed in view of a future utilization in a electrochromic device as thin film counter electrode. Different materials have been tested to identify the best solution to be used as an optically passive ion storage layer in an existing electrochromic device in alternative to an optically active V 2O 5 counter electrode. Process conditions and materials performances are reported, related to each other and discussed. Development studies are still in progress towards the optimization of the device and its future scaling up.

Comparative studies of “all sol–gel” electrochromic devices with optically passive counter-electrode films, ormolyte Li + ion-conductor and WO 3 or Nb 2O 5 electrochromic films

Laminated electrochromic (EC) devices are becoming increasingly important for making “smart” windows and switchable displays. Mostly, polymeric Li + ionic conductors in combination with vacuum deposited active electrochromic and counter-electrode films are used. In this paper we report on the development of all sol–gel EC devices, that is, those where all three internal layers are prepared via the sol–gel route, including the ionically conductive inorganic–organic hybrid (ormolyte). The electrochemical and optical properties of EC devices are presented and the cycling stability and reversibility of their optical modulation assessed. The results show that WO 3/ormolyte/SnO 2 : Mo, WO 3/ormolyte/SnO 2 : Sb, WO 3/ormolyte/SnO 2 : Sb : Mo, Nb 2O 5/ormolyte/SnO 2 : Sb : Mo and WO 3/ormolyte/LiCo-oxide exhibit a transmission modulation dependent on the thickness of the active electrochromic and counter-electrode films and the thickness of the ormolyte layer. Electrochemical and optical properties of individual films are described and correlated with the stability of the all sol–gel EC devices.

Electrochromism of sol–gel derived niobium oxide films

Niobium oxide films are promising cathodic electrochromics that in many aspects can compete with the more frequently studied WO3 films. The films reported herein were prepared using the sol–gel route from a NbCl5 precursor. The electrochromic properties were pronounced for crystalline films heat-treated at 500°C and exhibited transmittance changes between coloured and bleached states of 60% in the ultraviolet (UV) and 80% in the visible (VIS) and near-infrared (NIR) regions. Improved bleaching and more reversible electrochromism of thick niobium oxide films (thickness (d)>250nm) were obtained by lithiation.Electrochromic (EC) devices were also prepared by assembling niobium oxide and lithiated niobium oxide films of different thicknesses with a hybrid inorganic/organic Li+ ionic conductor (organically modified electrolyte-ormolyte) and a molybdenum and antimony doped tin oxide (SnO2:Sb(7%):Mo(10%) counter electrode films. The EC devices exhibited adequate colouring/bleaching kinetics (<2min) and colouring/bleaching changes up to 40–50%.

Chromogenic WPA/TiO2 hybrid gels and films

A novel low-temperature electrochromic and photochromic material in the form of gels and thin solid films composed of H3PW12O40·nH2O (WPA) incorporated in titanium oxide gel issued of Ti(OPr i )4 with addition of acetic acid were made via the sol-gel route by the dip-coating technique. Gels in bulk and thin film forms have been studied with the help of FT-IR transmission, reflection absorption and Raman spectroscopy. Similarity of the characteristic band frequencies with pure WPA confirmed that entrapped Keggin salt is well preserved inside the gel. Photochromism of WPA/TiO2 gels which appears after short exposure to UV (λ=366 nm) radiation is related to the excitation effect of branched alcohol leading to formation of carbonyl modes due to the alcohol transformation to aldehyde. Electrochromic properties of WPA/TiO2 gel films were tested in aqueous 0.001M HClO4 and 0.1M LiClO4 electrolytes as well as in the sol-gel electrochromic device (ECD) composed of LiClO4 doped ormolyte and Sb:Mo:SnO2 counter electrode film with optically passive response. Slight improvement of the WPA/TiO2 intercalation/deintercalation kinetics was obtained with the films exhibiting one order of magnitude higher electronic conductivity which were made with addition of in situ polymerized pyrrole with Fe(NO3)3.

Counterelectrode films for electrochromic windows

Thin films examined as candidate materials for the counterelectrode in an electrochromic window exhibit ion insertion reactions which are far less ideal than those of WO 3 and related materials used for the active electrochromic electrode. The ion-insertion kinetics of ITO, Pr 6O 11, NiO and a modified form of vanadium oxide have been studied by non-aqueous electrochemical and surface analysis techniques. Short term reversibility can be obtained in many film compositions, sometimes unexpectedly due to structural changes induced by the electrochemical treatment. However, in the case of the first three films above, long term cycling shows capacity loss due to corrosion and conductivity loss. The analysis of the insertion/extraction kinetics was greatly complicated by large overpotentials and non steady-state behaviour under cycling. The modified vanadium oxide film has been shown to have a large reversible capacity at a range of potential where degradative side reactions were not detected. The kinetics of the cycling process were found to follow simple diffusion theory without complication and diffusion coefficients were calculated to be in the range of 10 −16 m 2 s −1.

Tunneling spectroscopy into substituted YBaCuO systems

We have studied the tunneling characteristics of Gd and Al substituted YBa 2Cu 3O 7−δ single crystals with T c = 90 K and 80 K respectively. The high-T c superconductos were etched 30 min in a 1% Br solution in methanol and the tunnel barriers were formed by exposing the crystals to the ambient atmosphere for about 30 min. Pb film counterelectrodes were thermally evaporated through a metallic mask. No relevant differences have been found in the differential resistance vs voltage of undoped 90 K and Gd junctions, so confirming that Gd is substituting for Y. A different behavior has been found in the Al doped junctions that show smeared structures of reduced amplitude.

Planar tunnel junctions on Fe-doped YBaCuO systems

We have measured the tunneling characteristics of planar junctions fabricated on YBa 2(Cu 1−xFe x) 3O 7 single crystals with different percentages of Fe substitutions. A liquid etched bath (1% Br in methanol, by volume) has been used to remove the degraded surfaces of the high T c superconductors. After a short air exposition, the junctions were completed by evaporating Pb thin film counterelectrodes through a metallic mask. In comparison with the undoped 90 K phase, the doped junctions show gap-like structures smeared and reduced in amplitude at about ±25 mV, while a second feature at about ±5 mV is present only for slightly doped samples. This fact seems to indicate that Fe is mainly substituting for Cu in the one-dimensional CuO chains.

Rare earth oxides in electrochromic windows

Electrochromic windows, as well as some other electrochromic devices, require a counter electrode film which can accept up to 15 mC cm −2 of charge in the form of electrons from the circuit and ions from the electrolyte. This work investigated counter electrodes of PrO 2− x prepared by metal organic chemical vapour deposition. The yellow-brown films were reduced electrochemically in an electrolyte consisting of 0.5M lithium triflate in propylene carbonate. The films were found to bleach on the passage of cathodic charge and recolour with anodic charge. Provided the switching was relatively fast, and that the film was not left in the reduced state for long periods, the charge was reversible over 500 cycles. However, the voltage-time profiles resulting from galvanostatic lithium insertion showed large overpotentials, and relaxation to a charge-independent potential. X-ray diffraction measurements supported the conclusion that the first lithium insertion was accompanied by an irreversible phase change. Preliminary results indicate that subsequent cycling is possible.

Tunneling spectroscopy in superconducting Ba1−xKxBiO3: Direct evidence for phonon-mediated coupling

Tunneling measurements have been performed on bulk samples of Ba1−xKxBiO3 (x=0.375) using the natural, surface oxide as the tunnel barrier and an In thin film counterelectrode. High bias conductance and second derivative measurements show reproducible features in the energy range 10 to 65 meV which are characteristic of phonon effect as seen in conventional superconductors. The structures appear at energies which correspond to peaks in the phonon density of states as measured by inelastic neutron scattering and Raman spectroscopy. Estimates of the electron-phonon coupling strength, λ, and the average phonon frequency, <ω > , from the tunneling data indicate that phonon mediated pairing is responsible for superconductivity in this compound. Strong lifetime broadening effects are observed in the gap-region conductance of these junctions indicating gapless superconductivity for samples with concentration, x=0.375.

Low-loss lumped-element capacitors for superconductive integrated circuits

Low-loss lumped-element capacitors for superconductive circuits were fabricated using sputter-deposited Nb electrodes. The dielectric layer was formed by partially anodizing the Nb base electrode. The deposition technique for the counterelectrode strongly affected the parasitic shunt conductance of the capacitors. It was found that this conductance could be reduced by depositing the Nb counterelectrode films by DC magnetron sputtering at a low rate and at a high Ar pressure. By optimizing these process parameters, capacitors with breakdown voltages greater than 85% of the anodic oxide formation voltage and loss tangents less than 0.003 at 10 MHz were fabricated. These capacitors were integrated with Nb thin-film inductors to produce L-C resonators with quality factors greater than 400. >

TUNNELLING JUNCTIONS OF SINGLE CRYSTAL BCSCO/SN FILM

The single particle tunnelling junctions consisting of single crystal BCSCO and Sn film counter electrode have been prepared. The I-V and dI/dV-V characteristics have been measured. Observed value of the energy gap is 23 meV which corresponds to 2Δ/kT c of 6.0, much larger than the BCS weak-coupling limit of 3.52.

Thin Film Cadmium Chalcogenide/Aqueous Polysulfide Photoelectrochemical Solar Cells with In‐Situ Tin Storage

A photoelectrochemical system is presented which converts solar energy to both electrical and electrochemically stored energy. A decrease in cell illumination initiates spontaneous reconversion of electrochemical to electrical energy, providing nearly constant (light insensitive) electrical power for an external applied load. The cell contains a thin film Cd(Se,Te) photoelectrode and a thin film counterelectrode immersed in an aqueous polysulfide electrolyte separated by cation selective membrane from a tin electrode immersed in an aqueous sulfide electrolyte (Sn/S=). Cyclic voltammetry indicates that unlike tin/alkali systems, the Sn/S= half‐cell does not undergo passivation at storage potentials. Measured overpotentials in Sn/S= indicate large relative surface area tin electrodes can minimize polarization losses. Characterization of a variety of membranes indicates that a sulfonated polyethylene membrane minimizes problems of polysulfide passage into the tin/aqueous sulfide half‐cell. Photo and storage cell voltage compatibility is achieved by electrolyte modification and employing two photocells in series per tin half‐cell. A storage efficiency >90% can be achieved, and cell operation is limited by photo and not storage conversion efficiency. Two weeks outdoor operation yield an overall (conversion plus storage) solar to electrical efficiency of 2.7%.