New Ru(II) phenanthroline complex photosensitizers having different number of carboxyl groups for dye-sensitized solar cells

Abstract

We have prepared and characterized four carboxylated Ru(II) phenanthroline complexes with different number of carboxyl groups, cis-dithiocyanato bis(4,7-dicarboxy-1,10-phenanthroline) ruthenium(II) [Ru(dcphen)2(NCS)2, DCP2], cis-dithiocyanato bis(4-monocarboxy-1,10-phenanthroline) ruthenium(II) [Ru(mcphen)2(NCS)2, MCP2], cis-dithiocyanato (4,7-dicarboxy-1,10-phenanthroline) (1,10-phenanthroline) ruthenium(II) [Ru(dcphen)(phen)(NCS)2, DCPP], and cis-dithiocyanato (4-monocarboxy-1,10-phenanthroline)(1,10-phenanthroline) ruthenium(II) [Ru(mcphen)(phen)(NCS)2, MCPP], as photosensitizers for nanocrystalline dye-sensitized solar cells. All complexes exhibit a broad MLCT absorption band around 500 nm in ethanol and an emission band around 700 nm in ethanol–methanol (4:1) at 77 K. The excited state lifetime of these complexes at 77 K was 1.5–2.9 μs and the oxidation potential, 0.90–1.12 V vs. NHE, was obtained by cyclic voltammetry in solution. The best solar-energy-to-electric conversion efficiency under AM 1.5 irradiation was obtained for DCPP which has two carboxyl groups on one phenanthroline ligand. The performance of photosensitizer of MCPP which has only one carboxyl group is lower than that for the other complexes, indicating that the number of carboxyl groups as anchor is very important for efficient solar cell performance.