Molecular design of ruthenium(II) polypyridyl photosensitizers for efficient nanocrystalline TiO2 solar cells

Transition metal complexes using polypyridine ligands are finding increasing use in the dye sensitized solar cells. To improve further the efficiency of this cell, an enhanced spectral response of the sensitizer in the lower energies is required. In this paper we review our effort in the molecular design of the ruthenium polypyridyl complexes for nanocrystalline TiO2-based solar cells. The poor cell efficiency in Ru(4,4′-dicarboxy-2,2′-biquinoline)2(NCS)2/TiO2 system may be ascribed to the low excited-state oxidation potential, which plays a crucial role in the electron-transfer process. Ru(2-(2-(4-carboxypyridyl))-4-carboxyquinoline)2(NCS)2, when anchored to nanocrystalline TiO2 films, achieves efficient sensitization over the whole visible range extending up to 900 nm, yielding incident photon-to-current conversion efficiency (IPCE) of 55%. The low cell efficiency of Ru(4-4′-dicarboxy-2,2′-bypyridine)2(ethyl-2-cyano-3,3-dimercaptoacrylate) may be due to slow regeneration of the dye by electron donation from iodide following charge injection into the TiO2. Tuning of HOMO and LUMO energy level show that an efficient sensitizer should possess ground-state and excited-state redox potentials of 0.5 and −0.8 V vs. SCE, respectively. Transient absorption studies of Ru phenanthroline complexes show that the efficiency of electron injection is strongly affected by the number of carboxyl groups of the sensitizing dye. The β-diketonate complex [Ru(4-4′-dicarboxy-2,2′-bypyridine)2(acetylacetonato)]Cl, when anchored to nanocrystalline TiO2 films, achieves very efficient sensitization across the entire visible region, yielding 60% IPCE. A new series of panchromatic sensitizers of Ru(tricarboxyterpyridine)(β-diketonato)(NCS) type have been developed. Ru(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)(1,1,1-trifluoropentane-2,4-dionato)(NCS) achieved an efficient sensitization of nanocrystalline TiO2 solar cells over the whole visible range extending into near IR region and displaying a maximum around 600 nm, where IPCE approaches a high value of 70%.