Metal phthalocyanine derivatives as promising stable blue dyes for DSSC

The absorption properties of p-extended macrocycles such as porphyrins and phthalocyanines have been intensively applied to the light -harvesting process related to organic photovoltaics, with particular focus on Dye-Sensitized Solar Cells1. Even if in the last few years new emerging materials have shown very promising results in terms of efficiencies, it is still extremely important to study and test new dyes because of their potential application in building-integrated-photovoltaics. In particular, no blue or green dye have yet shown an outstanding stability once exposed to thermal stress and UV irradiation into the device. Phthalocyanines are of course great candidates because of their great chemical stability and high molar extinction coefficients (e> 105) and these properties justify the intensive study still carried on these molecules. Amongst them an interesting sterically hindered push-pullstructure, having a record efficiency value of 6.4%2 has been prepared and published. It is well known that the metal centers of these macrocyclic compounds can affect their absorption peaks, their HOMO/LUMO energy values as well as their possible charge transfer properties in a DSSC device. In this context we have chosen to investigate the performances of dyes possessing the same molecular structure but different central metal ions, and in particular we focused on copper derivatives, because of their extremely high stability and color intensity. We have synthesized the 9(10),16(17),23(24)-tri-ter-butyl-2-[acetynyl-(4-carboxy)phenyl]phthalocyaninato copper complex and its related free base and then investigated their optical and electrochemical properties. Very surprisingly, even for the copper derivative we have measured a considerable fluorescence decay that should be in principle almost completely depleted due to the spin-orbital interaction arising from its d9 electronic configuration. The photovoltaic performances as well as IPCE measurements will be presented for both the molecules and compared with those of the zinc derivative4 that has recently reached efficiency values around 2%.