Redox mediator


In DSC, the redox mediator, which can be a liquid or gel electrolyte or a solid hole conductor, plays the important role to regenerate the oxidized dye and transport the hole towards the cathode, where a catalyst (usually metallic platinum) regenerates the oxidized electrolyte or hole conductor, closing the circuit. Record DSC efficiencies exceeding 11%, have been obtained using Ru(II)-dyes on TiO2 in combination with a liquid electrolyte based on the I-/I3- couple in volatile organic solvents. Despite the high efficiency of the I-/I3- electrolyte, this redox couple poses several problems in relation to its  volatility and corrosive nature towards most metals. Even though a quasi-solid gel I-/I3- electrolyte has shown to considerably enhance the DSC lifetime, partly solving the electrolyte leakage problem and enhancing the cell temporal stability other redox couples are emerging as powerful alternatives to iodide-based liquid electrolytes.

We have focued our theoretical studies on liquid I-/I3- electrolyte and other redox couples in close interplay with experiment.

Regarding the I-/I3-, we have investigated the spectroscopic signature of oxidized dyes and of their complexes with I occurring in the oxidation/regeneration process in DSSC. In particular, we have reported a study on [RuII(dcbpy)2(X)2] complexes, where dcbpy = 4,40-dicarboxy-2,20-bipyridyl and X = NCS and CN, and of their adducts with I. Focusing on the interaction between the oxidized dyes and iodide and perform DFT/TDDFT calculations using a continuum model of solvation on the complexes of interest, we have provided a picture of the electronic structure of the oxidized dyes and their adducts with I. We finded a good agreement between the simulated and the experimental absorption spectra, including the absorption band in the IR region that is experimentally assigned to the [dye+:I-] complexes. In line with the experiments, we finded such band in the IR to overlap substantially with that of the dye cation for X =NCS, whereas it is readily identified for X =CN, for which the LMCT band of the cation has negligible intensity. Our results confirmed the assignment of the IR band to a charge-transfer transition occurringwithin the [dye+:I-] complex, corresponding to shift of electron density fromthe Ru-X HOMOs to the LUMO based on the I atom.


Despite the high efficiency of the I-/I3- electrolyte, this redox couple poses several problems in relation to its volatility and corrosive nature towards most metals.
Research efforts have been focused on replacing liquid DSC electrolytes by solid state hole conductor materials. Record efficiencies for solid-state DSCs (SDSCs) have been obtained using 2,2’7,7’–tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene (spiro-MeOTAD) as hole transport material (HTM). The molecular structure, electronic and optical properties of spiro-MeOTAD in different oxidation states have been investigated by means of DFT/TDDFT methods. The spiro-MeOTAD radical cation shows a long-term stability, even though the II and IV formal oxidation states are accessible. DFT and TDDFT allow us to characterize the excited states involved in the absorption processes of the spiro-MeOTAD derived cations, an important aspect considering that the oxidized species absorb in the visible region. The excellent agreement between theory and experiment both for the neutral spiro-MeOTAD and its oxidized forms opens the possibility to individuate the features which make it an efficient HTM thus to help the design of chemically modified or substituted spiro-bifluorenes.



Lobello, M. G.; Fantacci, S.; De Angelis, F. J. Phys. Chem. C 2011, 115, 18863 – 18872.

Fantacci, S.; De Angelis, F.; Nazeeruddin, M. K.; Graetzel, M. J. Phys. Chem. C  2011, 115, 23126 – 23133.