Rationalizing the molecular origins of Ru- and Fe-based dyes for dye-sensitized solar cells.

As part of an effort to design more efficient dyes for dye-sensitized solar cells (DSCs), structure–property relationships are established in the world's best-performing chemical series of dyes: 2,2′-bipyridyl-4,4′-carboxylatoruthenium(II) complexes. Statistical analysis, based on crystallographic data from the Cambridge Structural Database, is used to determine common structural features and the effects of structural change to its salient molecular constituents. Also included is the report of two new crystal structures for tris(2,2′-bipyridyl)dichlororuthenium(II)hexahydrate and tris(2,2′-bipyridyl)iron(II)dithiocyanate; these add to this statistical enquiry. Results show that the metal (M) core exhibits a distorted octahedral environment with M—N π-backbonding effects affording the propensity of the metal ion towards oxidation. The same characteristics are observed in iron-based analogues. The role of carboxylic groups in this series of dyes is assessed by comparing complexes which contain or are devoid of COOH groups. Space-group variation and large molecular conformational differences occur when COOH groups are present, while such structural features are very similar in their absence. The nature of the anion is also shown to influence the structure of COOH-containing complexes. These structural findings are corroborated by solution-based UV–vis absorption spectroscopy and DSC device performance tests. The presence of COOH groups in this series of compounds is shown to be mandatory for dye-uptake in TiO2 in the DSC fabrication process. Throughout this study, results are compared with those of the world's most famous DSC dye, N3 (N719 in its fully protonated form): cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II). Overall, the molecular origins of charge-transfer in these complexes are ascertained. The findings have important implications to the materials discovery of more efficient dyes for DSC technology.