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.
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.
Electrochromism in order to be well understood, requires an in-situ study during changes of color. The usual analytical techniques, even if they give excellent information, are not generally able to be used during the polarization of a sample in an electrolytic medium. Three techniques will be described here, none of which have been commonly used up to now, but all of which can have a noticeable effect in the field of electrochromism.
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