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.