Photoelectrochemical and photophysical characterization of new molecular photosensitizers and electron transfer mediators for Dye-Sensitized Solar Cells

Solar energy is one of the most promising future energy resources. The direct conversion of sunlight into electricity by solar cells is of particular interest because has relevant advantages over most of the presently used electrical power generation methods: indeed, electricity is produced without the exhaust of greenhouse gases and without nuclear waste byproducts. 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 : this technology is based on a layer of mesoscopic TiO2 film (sintered on a conductive glass) which significantly increases the optical path for light harvesting by surface anchored sensitizer molecules whilst keeping an efficient contact with the electrolytic solution. These sensitizers molecules are often based on ruthenium polypyridyl complexes which couple intense visible region absorption bands to favourable ground and excited state electronic and thermodynamic properties (of their excited states) which allow, in the most favorable cases, for an almost quantitative photon to electron conversion. The standard DSSC usually contains, as electron mediator, the iodide/triiodide couple which posseses nearly ideal properties in terms of fast regeneration of the oxidized dye, very slow conduction band electron recapture and high diffusion coefficient in many solvents commonly used in the DSSCs. The main drawback of the I-/I- 3 couple arises from its high corrosivity towards most of the common higly conductive metals and poses a serious problem for the insertion in the DSSCs of metal grids necessary to increase the electron collection efficiency. It is known that transparent conductive glass exhibit large ohmic losses that are apparent when constructing cells of area exceeding few square centimeters and the only way to overcome the problem, when large surfaces are required and transparency is an issue, is to collect the electrons via metallic fingers deposited on the photoelectrode and on the counter electrode. The work presented here is largely focused on finding efficient non corrosive alternative redox couple for DSSCs based on polipyridyl Co(II) complexes and polypyridyl and pyridylquinoline Cu(I) complexes: these compounds, generally characterized by a very low visible extinction coefficient, minimize the competition with the light harvesting dye and exhibit a quasi-reversible electrochemical behaviour markedly dependent on the nature of the electrodic surface. Gold was found to be the most appropriate material to catalyze the heterogeneous electron transfer reaction at the counter electrode of the solar cells, but interesting results were also obtained with carbon coated electrodes. The best performing of these mediators when used with the best sensitizers gave photon to current conversion yields variable from 50 % to 70 %. Since the dye regeneration was found to limit the Co(II) electron mediator efficiency, Co(II) electrolyte performance has been improved by mixing kinetically fast couples and exploiting a cascade of electron transfer events. It has also been observed that copper complexes with a distorted tetragonal geometry show promise for developing alternative low cost mediators for photoelectrochemical cells. The transport of the electroactive ions is expected to play a significant role in determining DSSC efficiency: this is particularly true under strong illumination when a large number of photooxidized dye molecules are simultaneously generated at the photoanode and an efficient turnover of electron donating species is required to sustain the photocurrent. The search for suitable solid materials that can replace the liquid electrolyte is an additional interesting and active area of research. In a solid state DSSC the solid hole conducting material captures the holes and closes the circuit with the counter electrode. Conducting polymers have been studied and have shown promising results as hole conducting materials for their application to regenerative photoelectrochemical cells. In particular it has been reported that the presence of ionic liquids may improve the charge transporting capabilities of the heterointerface through screening of space charge effects. In the last part of the thesis some efforts towards the modification of the counter electrode with inexpensive and transparent materials have been described: interest in such materials is boosted by the possible realization of stacked cells, either serially or in parallel connected, in which two spectrally complementary dyes can work in their optimal absorption region, improving the spectral responsivity of the modules. In these studies it has been found that osmium complexes as well as electrodeposited conductive polymers like PEDOP an PEDOT are effective in enhancing the electrochemical response of Co(II) electron mediators at surface modified FTO. The study of Ru(II) sensitizers based on 4-4’-pyrrol (or pyrrolidine)-2,2’bipyridine have been carried out both in liquid and quasi-solid state solar cells. Sensitization of the TiO2 to the near IR region and photon to current conversion efficiencies close to 90% can be obtained with a proper synthetic design of these dyes and with a careful choice of the hole transporting electrolyte. Research on dye sensitizers has been mainly focused on transition metal complexes, but a considerable work has also been directed towards the study of organic natural sensitizers extracted from fruits and vegetables. In particular families of anthocyanin and betalain dyes have been deeply investigated as natural sensitizers for dye sensitized solar cell and, with properly engineered titania photoelectrodes, photon to current conversion efficiencies close to 70% have been reached.