Mathieu Urien

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.

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.