Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells

A new series of panchromatic ruthenium(II) sensitizers derived from carboxy lated terpyridyl complexes of tris-thiocyanato Ru(II) have been developed. Black dye containing different degrees of protonation {(C2H5)(3)NH}[Ru(H(3) tcterpy)(NCS)(3)] 1, {(C4H9)(4)N}(2)[Ru(H(2)tcterpy)(NCS)(3)] 2, {(C4H9)(4) N}(3)[RU(Htcterpy)(NCS)(3)] 3, and {(C4H9)(4)N}(4)[Ru(tcterpy)(NCS)(3)] 4 ( tcterpy = 4,4',4 " -tricarboxy-2,2':6',2 " -terpyridine) have been synthesi zed and fully characterized by UV-vis, emission, IR, Raman, NMR, cyclic vol tammetry, and X-ray diffraction studies. The crystal structure of complex 2 confirms the presence of a Ru(II)N6 central core derived from the terpyrid ine ligand and three N-bonded thiocyanates. Intermolecular H-bonding betwee n carboxylates on neighboring terpyridines gives rise to 2-D H-bonded array s. The absorption and emission maxima of the black dye show a bathochromic shift with decreasing pH and exhibit pH-dependent excited-state lifetimes. The red-shift of the emission maxima is due to better pi -acceptor properti es of the acid form that lowers the energy of the CT excited state. The low -energy metal-to-ligand charge-transfer absorption band showed marked solva tochromism due to the presence of thiocyanate ligands. The Ru(II)/(III) oxi dation potential of the black dye and the ligand-based reduction potential shifted cathodically with decreasing number of protons and showed more reve rsible character. The adsorption of complex 3 from methoxyacetonitrile solu tion onto transparent TiO2 films was interpreted by a Langmuir isotherm yie lding an adsorption equilibrium constant, K-ads, of (1.0 +/- 0.3) x 10(5) M -1. The amount of dye adsorbed at monolayer saturation was (n(alpha) = 6.9 +/- 0.3) x 10(-8) mol/mg of TiO2, which is around 30% less than that of the cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)-ruthenium(II) co mplex. The black dye, when anchored to nanocrystalline;TiO2. films achieves very efficient sensitization over the whole visible range extending into t he near-IR region up to 920 nm, yielding over 80% incident photon to-curren t efficiencies (IPCE). solar cells containing the black dye were subjected to analysis by a photovoltaic calibration laboratory (NREL, U.S.A.) to dete rmine their solar-to-electric conversion efficiency under standard AM 1.5 s unlight. A short circuit photocurrent density obtained was 20.5 mA/cm(2), a nd the open circuit voltage was 0.72 V corresponding to an overall conversi on efficiency of 10.4%.