Ruthenium Sensitizer Functionalized by Acetylacetone Anchoring Groups for Dye-Sensitized Solar Cells

Abstract

A new ruthenium sensitizer Ru-acac (cis-bis(thiocyanato)(2,2′-bipyridyl-4,4′-di(3-pentane-2,4-dione)(4,4′-dinonyl-2′-bipyridyl)ruthenium(II)) functionalized by acetylacetone (acac) anchoring groups was synthesized and studied in ZnO-based dye-sensitized solar cells (DSCs). In comparison with the parent ruthenium complex bearing classical carboxylic acid groups (coded Z907), Ru-acac exhibits blue-shifted metal-to-ligand charge transfer (MLCT) absorption and emission bands as well as a cathodic shifted oxidation potential (E(RuIII/II)). Quantum chemistry time-dependent density functional theory (TD-DFT) calculations revealed that the acac groups are less conjugated with the pyridine units than carboxylic acid groups, entailing a lower electronic coupling with the semiconductor (SC) conduction band. Finally, the photovoltaic performances of both ruthenium complexes were investigated in DSCs by recording the J/V characteristics under solar spectrum simulation (AM1.5), the photoaction spectra, and by electrochemical impedance measurements. We showed that Z907 display higher photoconversion efficiency than Ru-acac owing to a lower electronic coupling with the SC and a different bending of the conduction band. Interestingly, the acac group provides a covalent binding to the ZnO surface but does not etch the structure as carboxylic acid usually does.