Polycarpos Falaras, Katerina Chryssou, Thomas Stergiopoulos, Ioannis Arabatzis, Georgios Katsaros, Vincent Catalano, Raif Kurtaran, Anne Hugot-Le Goff, Marie-Claude Bernard
New dyes of the type Ru(II)(bdmpp)(bpy) [where bdmpp is 2,6-bis(3,5-dimethyl-N-pyrazoyl)pyridine and bpy is 2,2'-bipyridine-4,4'-dicarboxylic acid] are prepared and characterized by infra-red (IR), mass (MS) and electrospray mass spectroscopy (ES-MS) as well as 1H NMR (1D and 2D) spectroscopies. The compounds present broad and very high intensity MLCT absorption bands in the visible and can be chemically anchored on TiO2 films via ester-like linkage involving carboxylato groups. These complexes have been tested with success as potential molecular antennas in dye-sensitized solar cells. Both opaque and transparent nanocrystalline TiO2 thin film electrodes obtained by a doctor blade technique sensitized by these complexes were incorporated in a sandwich type regenerative photoelectrochemical solar cell containing 0.1M LiI +0.01M I2 in propylene carbonate as well as a platinized conductive glass counter electrode. The cell was characterized by Raman spectroscopy under anodic and cathodic bias. Two new vibration bands were observed in the lower frequency region. The first one at 112 cm-1 is due to tri-iodide formed on the photoactive electrode, and the second one at 167 cm-1 is a sign of the dye/iodide interaction and corresponds to a vibration in a chemically stable "DI" intermediate species. Under direct sunlight illumination (solar irradiance of 60 mW/cm2) by using a composite polymer solid state electrolye, the cell ITO/TiO2/(Ru(II)(bdmpp)(bpy)(NCS))(PF6)/electrolyte/Pt-ITO produced a continuous photocurrent as high as 4.29mA/cm2, and gave IPCE values about half of the corresponding values obtained by the standard N3 dye under the same conditions. The photovoltage is about 600 mV and the overall energy conversion cell’s efficiency is as high as 1.72%.
Raman spectroscopy (RS) was used to study the interfacial species due to the presence of a redox couple in the electrolyte during the operation of dye-sensitized solar cells (DSSC). Two bands appear in the Raman spectra of polypyridinium (ppy) dyes adsorbed on nanocrystalline anatase. They can be directly connected to the presence of iodine/iodide couple because they are not present when hydroquinone/quinone (HQ/Q) redox couple is used. The band at 112 cm-1 has already be assigned to the presence of tri-iodide; it disappears only at very high cathodic polarization. The band at 167 cm-1 is due to the formation of an intermediate compound between the oxidized form of the dye , D+, and iodine; it involves a Py-I bonding. The use of different focusing proves that this compound is located below the adsorbed tri-iodide. Electrochemical Impedance Spectroscopy (EIS) allows to determine the charge transfer kinetics at the three main interfaces of the cell and to characterize the diffusion of tri-iodide species. Three different cell functioning regimes (photocurrent plateau, recombination, accumulation) were described. The Raman intensities of the two iodide-connected bands are investigated in each of the three regimes. The “Py-I” band is strong in the photocurrent plateau range and disappears in the recombination range.
In the present work, the effects of nanosecond pulsed laser irradiation on oxidized metallic surfaces were investigated, in order to estimate the possibility of using laser techniques for cleaning applications. After oxidation, the oxide layer developed on the metallic surfaces depends on the composition of the substrate and also on oxidation conditions. In this study a modeling system: pure Fe oxidized at 500 degrees C, has been investigated. The influence of the optical properties of the oxide films on the removal mechanisms and the cleaning efficiency of the laser technique was illustrated by performing specific electrochemical treatments.
The interesting optical properties of polyaniline (PANI) allowed its utilization in all solid-state electrochromic devices. Using a sulfonic acid polymer as solid electrolyte gave to PANI an unusual optical behavior leading to electrochromic properties very superior to the properties that it has in any liquid inorganic electrolyte. The improved conductivity of PANI doped with AMP-sulfonate is displayed by the presence of a free-carriers tail even at pH as high as 4.5. The free-carriers tail is studied using UV/vis/near IR spectroscopy, and the kinetics of coloration/bleaching are studied using Optical Multichannel Analysis in the 1.5 - 3 eV range. The modifications of the PANI optical features by solid-state doping are examined. The possibility to still improve the performances of these devices--in particular their rate of color change--by using `secondarily doped' PANI is investigated, which requires a preliminary spectrochemical analysis of PANI films doped with camphorsulfonic acid and treated in m-cresol during their electrochemical polarization.
The Raman spectra of polyaniline (PANI) doped by functionalized sulfonic acids present very few differences with the spectra obtained after doping by inorganic anions, in spite of the noticeable differences displayed elsewhere by their optical spectra: the 3 eV (localized polaron) as well as 1.5 eV bands are stronger and sharper in presence of sulfonate. In fact, during the study of the different PANI forms, it was observed that the connections between Raman and optical features are far from being straight; a simple analysis in terms of the resonance in either benzoid or quinoid units is not enough. To arrive to a better understanding of these connections, we have compared the Raman spectra obtained with blue (localized polaron excitation), red (excitonic resonance) and near-IR (intrachain excitations resonance) lines; with the green laser line, PANI is out of resonance. Spectra were also obtained on samples treated to present a `secondary doping', and therefore a strong IR absorption. The use of red and IR lights allows to study the splitting of the C equals N peak, which can clearly be decomposed in two sub-bands, as well as the splitting of the 1330 cm-1 peak (assumed to be due to the stretching of C - N+$CCL).
The association of polyaniline, PANI, (colored in anodic polarization) and tungsten oxide (colored in cathodic polarization), separated by a solid electrolyte (PAMPS) led to interesting all solid-state electrochromic devices (ASSED). Here, significant improvements are brought to the different ASSED elements, concerning in particular the compositions of the cathodic material and of the solid electrolyte, the pH of which (in the range 0.65 - 7.65) as well as the water content (in the range 0 - 5.2%) are varied. The optimization of the thickness of the PANI layer is discussed. Another crucial point is the choice of the coloration potential (1 V or 1.2 V) allowing to associate a high coloration efficiency with an excellent stability. One knows that the blue colored PANI form appearing at high pH, which is optically the most suited to solar application, with its exciton absorption band at 2 eV, does not allow stable cycling in liquid inorganic electrolyte. On the contrary, with PAMPS, it is possible to use high pH, keeping the reversibility up to 20,000 cycles, probably in reason of the presence of free-carriers at pH as high as 4.5, which also is discussed.
The degradation mechanisms in polyaniline (PANI) thin films polymerized on SnO2 after a large number of coloration/bleaching voltammetric cycles were investigated using optical and spectroscopic techniques, allowing us to follow the chemical changes related to the degradation. HCl solutions with different pH (0 and 3) were used and the role of the coloration potential was studied. The number and the nature of the polarons in the colored form play a leading part in the polymer stability, and they are modified by the cycling. This point was particularly emphasized here.
The adsorption on a titanium dioxide substrate of organic monolayers used in the nanocrystalline dye sensitized solar cells was investigated by Raman spectroscopy, owing to the high resonance effect in these molecules. During the polarization of TiO2 modified by Ru-bi or ter pyridinium compounds in a photoelectrochemical cell, an enhancement affect appeared, allowing us to scrutinize the part of the complex which is in contact with the substrate. This affect remains to a large extent unexplained; it could be attributed to SERRS (surface enhanced Raman resonance). Noticeable differences appeared in the function of the nature of the pyridil ligands. In the case of bipy, in addition to the `normal' (ground state) Raman bands, a new series of bands appeared which correspond to the particular ligand exchanging electrons with the substrate titanium atoms. The Raman intensity of these new peaks is directly related to the electric field (i.e., to the potential magnitude independently of its cathodic or anodic nature). In the case of terpy, the enhancement mechanism seems different. The similarity of the molecular configuration created by the adsorption with the radical anion formed by the excitation of the metal ligand charge transfer complex has to be emphasized. Very interesting prospects for the understanding of the adsorption mechanisms are therefore opened.
Mixed hydroxides films with various Mn/Ni ratio (Ni concentration of 26, 46, 68, 86, and 90%) were obtained by cathodic precipitation starting with a mixture of manganese and nickel nitrates. Their electrochromic properties in 0.1 M KOH solution were studied using in situ Raman spectroscopy (RS) which gives us the chemical identification of the films in their bleached and colored states, UV/visible spectroscopy, and optical multichannel analysis (OMA) which allows us to obtain, besides the optical properties of the material, the kinetics of the color changes. A large difference is observed between the films containing a nickel percentage <EQ 50 or >= 70%. The addition of manganese does not improve the electrochromic properties of nickel oxide, except for the kinetic point of view, since the coloration times are reduced by about one order of magnitude passing from Ni(OH)2 to Mn(OH)2.
Optical multichannel analysis (OMA) was used to follow the kinetics of color changes in electrochromic polyaniline (PANI) films when they are polarized in an electrolytic medium at different potentials; these color changes which are related to the transition from one polymer form to another one are very fast. For the setting of electrochromic devices, PANI must be used in acid electrolytic media. In these media, the color change mechanisms are essentially related to the chains protonation/deprotonation. The study is now extended to different pH media (neutral and alkaline) in order to obtain a better understanding of the chemical mechanisms of passage from one form to another one, and particularly of the oxidation of phenyl in quinone rings. OMA allowed us to also obtain an apparent value of the films absorption coefficient. In the investigated range (300/800 nm), several absorption phenomena characteristic of the different polymer forms can be displayed. The range in which two different forms coexist can then be characterized by the deconvolution of the optical spectra. Finally, the electrochromism in polymer is characterized by the change of optical density (Delta) DO directly obtained from the OMA experimental spectra.
Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XI: Chromogenics for Smart Windows
19 May 1992 | Toulouse-Labege, France
Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XI: Photovoltaics, Photochemistry, and Photoelectrochemistry
19 May 1992 | Toulouse-Labege, France
Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XI: Selective Materials, Concentrators and Reflectors, Transparent Insulation and Super Windows
18 May 1992 | Toulouse-Labege, France
Optical Materials Technology for Energy Efficiency and Solar Energy
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