Thiocyanate-free cyclometalated ruthenium(II) sensitizers for DSSC: A combined experimental and theoretical investigation

In an effort to bring out efficient thiocyanate-free dyes for dye sensitized solar cells (DSSC) we have designed, synthesized and characterized four novel cyclometalated ruthenium(II) dyes (M1 to M4) with superior photochemical properties. All dyes contain terpyridyl ligands (TPY) with carboxylic acids as anchoring groups and cyclometalated ligand (TPY-C) with substituents for fine tuning the electronic properties. We obtain a broad absorption band which extends up to 725 nm due to metal to ligand charge transfer (MLCT) when donating groups are used, which slightly blue-shifts when a withdrawing group is used. In addition to the CT, small HOMO-LUMO gaps are obtained from electrochemical measurements which indicate characteristics of an ideal sensitizer. All four dyes were used as sensitizers for DSSC and photoelectrochemical measurements were carried out. Reasonably good efficiency (7.1%) has been achieved for M3. We have carried out periodic-DFT studies of these dyes adsorbed on the (TiO2)(38) cluster. They revealed that, in bidentate bridging mode the dyes preferably bind with the help of two carboxylic groups onto the TiO2. To the best of our knowledge we are the first to do DFT studies of thiocyanate free cyclometalated ruthenium(II) dyes tethered to TiO2.