Exploring the Role of Central Metals in Bulky Phthalocyanines for Dye-Sensitized Solar Cells.

Two series of metallo-(Zn(II), Mg(II), and Ru(II)) and free-base phthalocyanines (Pcs) with a carboxyl anchoring group and well-established bulky peripheral substituents (either tert-butyl or bulky 2,6-diisopropylphenoxy) were synthesized and tested as sensitizers in dye-sensitized solar cells (DSSCs). The trend of photovoltaic efficiencies (PCEs) for free-base and metallo Pcs followed the order Zn(II)Pc > Mg(II)Pc >> H2Pc ≈ Ru(II)Pc regardless of the peripheral substitution. Higher efficiencies (4.95 versus. 3.63 for the Zn(II) derivatives) were achieved with Pcs bearing the bulkier 2,6-diisopropylphenoxy group, indicating a lower aggregation and more suitable HOMO-LUMO levels. Furthermore, these derivatives showed a more relevant influence of the metal on the PCE (from the highest value of 4.95 for the Zn(II)Pc to the lowest value of 0.23 for the Ru(II)Pc. In both series, the best PCEs observed with the Zn(II) derivatives were mainly due to their highest Jsc values. The lowest efficiencies found for the free-bases and Ru(II) derivatives were attributed to a mismatch between their LUMO levels and the conduction band of the TiO2 and lower light-harvesting capabilities, respectively. In conclusion, Zn(II) derivatives are still the best Pc candidates to use as sensitizers in molecular photovoltaics.