Constance Magne

Electrodeposited Nanoporous versus Nanoparticulate ZnO Films of Similar Roughness for Dye-Sensitized Solar Cell Applications

We present a comparative study of two different ZnO porous film morphologies for dye-sensitized solar cell (DSSC) fabrications. Nanoparticulate ZnO was prepared by the doctor-blade technique starting from a paste containing ZnO nanoparticles. Nanoporous ZnO films were grown by a soft template-assisted electrochemical growth technique. The film thicknesses were adjusted at similar roughness of about 300 in order to permit a worthy comparison. The effects on the cell performances of sensitization by dyes belonging to three different families, namely, xanthene (eosin Y) and indoline (D102, D131, D149 and D205) organic dyes as well as a ruthenium polypyridine complex (N719), have been investigated. The mesoporous electrodeposited matrix exhibits significant morphological changes upon the photoanode preparation, especially upon the dye sensitization, that yield to a dramatic change of the inner layer morphology and increase in the layer internal specific surface area. In the case of indoline dyes, better efficiencies were found with the electrodeposited ZnO porous matrixes compared to the nanoparticulate ones, in spite of significantly shorter electron lifetimes measured by impedance spectroscopy. The observation is interpreted in terms of much shorter transfer time in the oxide in the case of the electrodeposited ZnO films. Among the tested dyes, the D149 and D205 indoline organic dyes with a strong acceptor group were found the most efficient with the best cell over 4.6% of overall conversion efficiency.

Effects of ZnO film growth route and nanostructure on electron transport and recombination in dye-sensitized solar cells

The photovoltaic performances of ZnO-based dye-sensitized solar cells (DSSCs) have been studied for ZnO porous films prepared by different techniques. A comparison is made between nanoparticle (NP) films prepared by a sol–gel method and two different electrodeposited (ED) nanoporous films. The D149 indoline dye/ZnO with ZnO prepared by electrochemistry at rather high overvoltage has been found to be the best system. The cell functioning has been studied in-depth by electrochemical impedance spectroscopy (EIS) measurements made over a large potential range, in the dark and under illumination. It is shown that a much deeper sub-conduction band edge density of states (DOS) exists in the case of NP cells compared to ED cells. The electron dynamics have been analysed by determining the charge carrier lifetimes and transport/collection times. In the case of NP films, the charge carrier behaviour is characterized by rather long lifetimes and transport times with a marked voltage response typical of conduction that is governed by trapping–detrapping events. On the contrary, in the case of electrodeposited films, these two parameters are much shorter and the transport is characterized by a straight charge transport. Using photoluminescence data, the charge carrier lifetimes have been related to the surface defects of the ZnO nanostructures. The deduced electron collections were found to be very efficient with values greater than 90%. We have also determined that the ZnO film conductivities were higher than for anatase TiO2, and that the electron diffusion coefficient was higher in the ED film compared to the NP film.

Brookite TiO2 Nanoparticle Films for Dye-Sensitized Solar Cells

Brookite TiO(2) nanoparticles have been synthesized at low temperature by a soft solution growth method and have been used as building blocks to prepare pure brookite nanoparticle porous films. The film brookite structure was confirmed by XRD and Raman spectroscopy. By spectrophotometry, it was shown that the films had a direct band gap of 3.4 eV. After sensitization by the N719 dye, efficient cells have been produced. A best overall conversion efficiency of 5.97%, without a scattering layer, was found for the larger TiO(2) starting nanoparticles. The cell open-circuit voltage was improved compared with that of anatase cells and a lower electron diffusion coefficient was found in the photoanodes made of smaller brookite particles. Lanthanum-doped brookite nanoparticle films were also studied. They showed a marked decreased in the amount of dye loading, and hence, the solar cells had a reduced current density that was not compensated for by the increased open-circuit voltage of the cells.

Enhancement of photovoltaic performances in dye-sensitized solar cells by co-sensitization with metal-free organic dyes

A one-step dye cocktail strategy is proposed for the efficient sensitization of a ZnO photoelectrode in dye-sensitized solar cells. A high IPCE plateau is obtained between 400 and 600 nm by using two complementary indoline metal-free organic dyes. The roles of the co-adsorbents are also investigated and the cells are shown to be highly stable upon aging.

Amphiphilic acids as co-adsorbents of metal-free organic dyes for the efficient sensitization of nanostructured photoelectrode

Photoelectrode sensitization is an important step of metal-free organic dye-sensitized solar cell (DSSC) preparation. We describe an easy and flexible means for improving organic dye content of ZnO electrodes and avoiding the formation of aggregates by the use of amphiphilic fatty acids as co-adsorbents. A series of fatty acids with variable hydrophobic alkyl chain length has been studied and the D149 indoline dye loading and the recombination step have been optimized. The best performances were measured with butyric acid and octanoic acid. Computing and experimental results show that they present an alkyl chain length compatible with the dye molecule size and structure and provide an efficient re-generation of the oxidized dye by iodide. We also demonstrate that they act as efficient recombination barriers. Moreover the DSSC durability was markedly improved with octanoic acid compared to solar cells without co-adsorbent or with the common cholic acid co-adsorbent. The carrier lifetime increased upon the aging process irrespective of the co-adsorbant due to the increase of the photoelectrode chemical capacitance.