Dispositivi Solari a Coloranti di Nuova Generazione: Sensibilizzatori e Conduttori Nano-Ingegnerizzati (DSSCX)

The search for alternative energy sources is an important research goal; currently, harvesting energy directly from sunlight is recognized as a solution to the world’s energy problem. Up to now, commercially available photovoltaic technologies are based on silicon cells, but they are expensive and not too much efficient. However other alternatives, based on organic or coordination systems as photosensitizers are now under study. In particular the so called dye sensitized solar cells (DSSCs) are photovoltaic devices promising low cost and satisfactory efficiency thanks to the involvement of a molecular photosensitizer and a cheap and large band gap semiconductor, such as TiO₂.

Up to now bipyridine complexes of Ru(II) are among the most efficient sensitizers for DSSCs, however recently a 12.5% efficiency has been reached using a bulky Zn(II) porphyrin as sensitizer and an polypyridine Co complex as electrolyte, instead of the couple I3-/I-.

The present project will deal with the synthesis, spectroscopic and electrochemical characterisation of new bulky porphyrinic systems substituted in the pyrrolic position with a delocalised spacer having one or more carboxylic groups, as photosensitizers in DSSCs.

In particular, Zn(II) tetraphenylporphyrins having long alkoxylic chains localized in the orto or para position of 4 arylic groups of the porphyrinic cycle, will be studied. These porphyrins will be functionalized with substituents in the pyrrolic position with delocalised moieties with one or more carboxylic groups that guarantee anchoring to the TiO₂ semiconductor and electron transfer to its surface.

The presence of these long chains should increase the electron density of the porphyrin core and avoid, for steric hindrance, aggregation phenomena, typical for flat systems such as porphyrins, and in particular avoid charge recombination such as the reduction of the oxidized electrolyte on the TiO₂ photoanode.

The I₃^-/I^- couple commonly used as electrolyte in DSSCs has some disadvantages, therefore a part of the project will be dedicated to the study of new copper complexes as electron transfer mediators. In natural systems, blue copper proteins act as efficient electron transfer mediators thanks to a distorted tetragonal geometry, that minimizes the differences in geometry between Cu(I) and Cu(II). Similar copper complexes have been used in DSSCs. The Cu(II)/Cu(I) redox potential is sensitive to the tetragonal distortion and can be easily tuned by a proper ligand choice: steric hindrance and softness will favor an enhancement of the redox potential. Copper complexes, characterized by a high tetragonal distortion that allows a fast electron transfer, a high redox potential and an enhanced open circuit voltage (Voc), will be prepared. Complexes of the type: [Cu(2,9-R-1,10-phenantroline](CF₃SO₃)₂ (R=alkyl or phenyl group with various steric hindrance), with possibly electondonor or electronwithdrawing groups on the phenantroline, will be synthesized. Our attention will be focused also to new symmetrical Cu complexes with bispidines.

All the new photosensitizers and electrolytic couples will be characterized both electrochemically and photochemically to evaluate their application in DSSCs.

The syntheses will be preceded by theoretical design (CNR-ISTM UR of Perugia), while the final phase of evaluation of efficiency on small scale will be carried out in collaboration with the UR of Ferrara (UNIFE) and Milano Bicocca (MIB) and on a large scale with the collaboration of the University of Rome (UNITV).