Copper-based Redox Systems for Dye-sensitized Solar Celles

Sammanfattning: In order to take on the challenge of the world energy issues, there is a necessity to identify alternative energy sources to the fossil fuels. The sun represents the most abundant source of energy, and one way to harvest it is by photovoltaic devices directly converting sunlight into electricity. Among all photovoltaic devices, the dye-sensitized solar cells have attracted much interest because of the potential to be commercialized thanks to the cheap materials and the low-cost production processes. However, there are still issues to overcome, mainly related to the stability of the devices relating to the electrolyte components. In this project, a new type of electrolyte systems based on copper complex redox couples were synthetized and applied to DSSCs. Different monodentate ligands were investigated, in particular 2-mpy, 3-mpy and 4-mpy were coordinated by the copper metal centre. Several experimental techniques were used to characterized the products of synthesis, such as 1H NMR spectroscopy, mass spectrometry, powder X-ray diffraction and elemental analysis. The electrolyte properties were studied by UV-Vis spectrophotometry and cyclic voltammetry. The performance of the resulting solar cells was investigated by photocurrent density/photovoltage and incident photon-to-current conversion efficiency measurements and the recombination loss processes were studied by impedance spectroscopy and electron lifetime determinations. The redox couples based on 2-mpy and 3-mpy as ligands gave DSSC devices with very high open-circuit voltage up to 0.94 V and overall conversion efficiency up to 9.1% at 1 sun illumination. This is one of the highest conversion efficiencies recorded for copper based DSSC electrolytes and much higher than the solar cells containing the reference [Co(bpy)3]2+/3+ redox system. The high photo voltage, was attributed to the large resistance to  electron  recombination losses, which lead to a higher charge in the conduction band of the TiO2 moving it to more negative energies. This was also confirmed by the longer electron lifetime in the presence of the copper redox couples than the reference cobalt one. The TBP Lewis base additive was replaced by the methylpyridine ligands to mitigate ligand exchange problems. The replacement was successful using 3-mpy as additive as well as ligand to the copper ions. In continuation of this work, the other components of the device and the assembly process should be optimized. Moreover, single crystals of the copper systems should be grown to determine the exact structure and thus offer a better understanding of the processes that might take place in the device. An interesting project would be to try a one-pot electrolyte formulation, i.e. where all the reactants, solvent and additives are mixed and directly injected into the device. This is a good strategy to avoid the pre-syntheses and all the problems related to those.