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

Abstract

A new series of panchromatic ruthenium(II) sensitizers derived from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) have been developed. Black dye containing different degrees of protonation {(C2H5)3NH}[Ru(H3tcterpy)(NCS) 3] 1, {(C4H9)4N}2[Ru(H2 tcterpy)(NCS)3] 2, {(C4H9)4N}3[Ru(Htcterpy)(NCS) 3] 3, and {(C4H9)4N}4[Ru(tcterpy)(NCS) 3] 4 (tcterpy = 4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine) have been synthesized and fully characterized by UV-vis, emission. IR, Raman, NMR, cyclic voltammetry, and X-ray diffraction studies. The crystal structure of complex 2 confirms the presence of a RuIIN6 central core derived from the terpyridine ligand and three N-bonded thiocyanates. Intermolecular H-bonding between carboxylates on neighboring terpyridines gives rise to 2-D H-bonded arrays. 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 π-acceptor properties 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 solvatochromism due to the presence of thiocyanate ligands. The Ru(II)/(III) oxidation potential of the black dye and the ligand-based reduction potential shifted cathodically with decreasing number of protons and showed more reversible character. The adsorption of complex 3 from methoxyacetonitrile solution onto transparent TiO2 films was interpreted by a Langmuir isotherm yielding an adsorption equilibrium constant, Kads, of (1.0 ± 0.3) × 105 M-1. The amount of dye adsorbed at monolayer saturation was (na = 6.9 ± 0.3) × 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) complex. The black dye, when anchored to nanocrystalline TiO2 films achieves very efficient sensitization over the whole visible range extending into the near-IR region up to 920 nm, yielding over 80% incident photon-to-current efficiencies (IPCE). Solar cells containing the black dye were subjected to analysis by a photovoltaic calibration laboratory (NREL, U.S.A.) to determine their solar-to-electric conversion efficiency under standard AM 1.5 sunlight. A short circuit photocurrent density obtained was 20.5 mA/cm2, and the open circuit voltage was 0.72 V corresponding to an overall conversion efficiency of 10.4%.