Zirconium(IV) and Hafnium(IV) Porphyrin and Phthalocyanine Complexes as New Dyes for Solar Cell Devices

Metalloporphyrin and metallophthalocyanine dyes ligating Hf­(IV) and Zr­(IV) ions bind to semiconductor oxide surfaces such as TiO₂ via the protruding group IV metal ions. The use of oxophilic metal ions with large ionic radii that protrude from the macrocycle is a unique mode of attaching chromophores to oxide surfaces in the design of dye-sensitized solar cells (DSSCs). Our previous report on the structure and physical properties of ternary complexes wherein the Hf­(IV) and Zr­(IV) ions are ligated to both a porphyrinoid and to a defect site on a polyoxometalate (POM) represents a model for this new way of binding dyes to oxide surfaces. The Zr­(IV) and Hf­(IV) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) with two ligated acetates, (TPP)­Hf­(OAc)₂ and (TPP)­Zr­(OAc)₂, and the corresponding metallophthalocyanine (Pc) diacetate complexes, (Pc)­Hf­(OAc)₂ and (Pc)­Zr­(OAc)₂, were evaluated as novel dyes for the fabrication of dye-sensitized solar cells. Similarly to the ternary complexes with the POM, the oxide surface replaces the acetates to affect binding. In DSSCs, the Zr­(IV) phthalocyanine dye performs better than the Zr­(IV) porphyrin dye, and reaches an overall efficiency of ∼1.0%. The Hf­(IV) dyes are less efficient. The photophysical properties of these complexes in solution suggested energetically favorable injection of electrons into the conduction band of TiO₂ semiconductor nanoparticles, as well as a good band gap match with I₃^–/I^–pair in liquid 1-butyl-3-methyl imidazolium iodide. The combination of blue absorbing TPP with the red absorbing Pc complexes can increase the absorbance of solar light in the device; however, the overall conversion efficiency of DSSCs using TiO₂ nanoparticles treated with a mixture of both Zr­(IV) complexes is comparable, but not greater than, the single (Pc)­Zr. Thus, surface bound (TPP)Zr increases the absorbance in blue region of the spectra, but at the cost of diminished absorbance in the red in this DSSC architecture