Alternative redox systems for the dye-sensitized solar cell

Abstract

Due to their high efficiencies and their potentially low production costs, dye-sensitized solar cells (DSSC) have attracted much attention during the last few years. The technology is based on a layer made of mesoscopic TiO2 film which significantly increases the optical path for light harvesting by the surface-anchored sensitizer molecules, whilst keeping an efficient contact with the electrolytic solution. These sensitizer molecules are often based on ruthenium polypyridyl complexes because of their high absorption coefficients in the visible, but any dye could be used to sensitize the semiconducting TiO2. The standard DSSC usually contain organic electrolytes with dissolved triiodide/iodide as the redox couple. Cells based on alternative electrolytes have been developed and were used in early DSSC designs, but showed somehow deceiving photovoltaic performances. Until now, endeavors to increase the photovoltage of dye-sensitized nanocrystalline cells by replacing iodide by a one-electron, outer-sphere redox couple with a more positive oxidation potential have not succeeded. This has been largely due to limitations imposed by the dark current and the slow diffusion of the alternate mediator. The work presented here focused on finding efficient alternative redox couples for the DSSC, able to rival the classical triiodide/iodide couple. The tested compounds were triarylamines, platinum complexes and cobalt complexes. Triarylamines and platinum complexes proved considerably less effective than the classical couple in term of photoelectrochemical efficiencies. Cobalt complexes however displayed far more interesting behavior. The experiments carried out demonstrated that the [Co(bip)2]3+/2+ complexes were the most suitable candidates to be used as redox mediators as their photoelectrochemical and kinetic behavior rivaled those of the triiodide/iodide redox couple. The system however had to be optimized to reach high efficiencies. Heteroleptic dyes incorporating long alkyl chains were found to be the most appropriate category of dye, boosting further the DSSC efficiency by reducing the dark current within the cells. Co-grafting the sensitizer and co-adsorbent led to an improved photovoltaic efficiency through the increased photocurrent. Silver addition in the dye solution also leads to a global improvement of the photovoltaic parameters. To avoid diffusion problems with the large molecules in the electrolyte, very thin nanocrystalline TiO2 layers had to be employed. A 3 μm thick nanoporous layer with a 3 μm scattering layer was found to be optimal. As cobalt complexes may undergo electrochemical processes on the SnO2 layer of the photoelectrode, a blocking layer of compact TiO2 was used to reduce the dark current. The electrochemical regeneration of the cobalt complexes with the SnO2 is not fast enough to support cell function and the counter electrode must be covered either with platinum or gold layer to ensure a good fill factor. The optimization of the electrolyte itself mainly concerned the choice of the solvent, the redox mediator concentration, the best degree of oxidation, the additives that can be used to improve the photovoltaic parameters and finally how to increase the redox couple solubility. The best efficiencies were achieved with an acetonitrile/ethylene carbonate (2:3) solvent mixture, with a total redox mediator concentration between 0.10 to 0.15 M. If 10% of this total concentration is oxidized, the current density reaches its maximum whilst having a negligible effect on the photovoltage. 4-tert-butylpyridine (TBP) and lithium perchlorate were tried as additives in the electrolyte, as TBP increases the photovoltage and Li+ the photocurrent. The optimal concentrations of these two additives was found to be around 0.05 M TBP and 0.1 M LiClO4 for an electrolyte having a total cobalt complex concentration of 0.1 M. Mass-transport within the cell is the limiting factor for high currents when using cobalt complexes as redox mediators. Attempts to reduce the solvent viscosity by using alternative solvents to the acetonitrile/ethylene carbonate mixture were not successful. One way to solve the diffusion problem was to irradiate the cell from the counter electrode side. This method works well at high irradiation intensities. Finally, reduction of the distance between the electrodes also enhances diffusion. The use of [Co(bip)2]3+/2+ complexes, with appropriate dyes and taking into account the optimizations already described above, allowed efficiencies of 8% under an irradiance of 95 W · m-2, as well as IPCEs over 80% at 540 nm to be obtained. At full sun, efficiencies of over 4% were reached, with short circuit current densities of 9.5 mA · cm-2 and open circuit voltages of the order of 860 mV.