Sensitizing Effect of nanocrysrtalline TiO2 films by Phthalocyanine and Porphyrins bridged Systems

The most successful charge transfer sensitizers employed so far in DSSC (Dye-sensitizer solar cell) are polypyridyl-type complexes of ruthenium yielding overall AM 1.5 solar to electric power conversion efficiency (?) of up to 10-11% and stable operation for millions of turnovers (1-3). The efficiency of these devices is, however, significantly limited by the low optical absorbance of such dyes in the red/near infrared spectral regions. Hence extensive research efforts are targeting new sensitizer dyes with stronger absorbance over this spectral region knowing that a sensitizer should fulfil other several demanding conditions: a) it must be firmly grafted the semiconductor oxide surface and inject electrons into the conduction band with a quantum yield of unity, b) its redox potential should be sufficiently high so that it can be regenerated rapidly via electron donation from the electrolyte or ___ a hole conductor , c) it should be stable enough to sustain at least 108 redox turnovers under illumination corresponding to about 20 years of exposure to natural sunlight. Both Porphiryns (Ph) and Phthalocyanines (Pc) represent attractive alternatives to the expensive and polluting pyridyl based Ru complexes , the former because of the analogy with natural photosynthetic processes, the latter because of their photochemical and thermal stability (4) including the strategy of using a combination of two dyes which complement each other in their spectral features. The result was encouraging in as much as the optical effects of the two sensitisers were found to be additive (5) opening up the way for testing other dye combination (6). In order to incorporate these essential properties we report and discuss the sensitazation of TiO2 films by ?-carbido iron phthalocyanine (FePc)2C and by ?-nitrido iron phthalocyanine-iron porphyrin [TphFe-N-FePc]. The choice of these dimeric compounds offers the advantage to attach the chromophore through axial ligands to the surface of the oxide semiconductor using the isonicotinic acid as anchoring group and in the case of mixed compound we observe two absorption in a wider range of the solar spectrum . UV-visible spectroscopy and NMR, are used to characterise the synthesised compounds while the anchoring to the surface is highlighted by infrared spectroscopy. Steady state adsorption and emission show the energy distribution of the excited states and time resolved measurements give information on the velocity of the photoinduced electron transfer and in general on the efficiency of the corresponding DSSC. REFERENCES 1. M.K.Nazeruddin et al. J.Am.Chem.Soc., 1993, 115, 6382 2. M. Gratzel , Nature, 2001, 414, 338. 3. M. Gratzel , J. Photochem. Photobiol. A, 2004, 164, 3 4. M.K.Nazeruddin et al., Chem. Commun. , 1998, 719 5. J. Fang et al., New J. Chem., 1997, 145, 270