Effect of coadsorbent on the photovoltaic performance of squaraine sensitized nanocrystalline solar cells.

The effect of chenodeoxycholic acid as the coadsorbent with a squaraine sensitizer on TiO(2) nanocrystalline solar cells was investigated, and it was found that the coadsorbent prevents the squaraine sensitizer from aggregating on the TiO(2) nanoparticles but reduces dye loading leading to an interdependent photovoltaic performance. Analysis of the absorption spectra, and incident monochromatic photon-to-current conversion efficiency data showed that the load of squaraine sensitizer as well as the appearance of H-aggregates is strongly dependent on the molar concentration of chenodeoxycholic acid coadsorbent. The open circuit voltage of the solar cells with chenodeoxycholic acid increases due to the enhanced electron lifetime in the TiO(2) nanoparticles coupled with the band edge shift of TiO(2) to negative potentials.

Highly phosphorescence iridium complexes and their application in organic light-emitting devices.

A new series of iridium(III) mixed ligand complexes TBA[Ir(ppy)(2)(CN)(2)] (1), TBA[Ir(ppy)(2)(NCS)(2)] (2), TBA[Ir(ppy)(2)(NCO)(2)] (3), and [Ir(ppy)(2)(acac)] (4) (ppy = 2-phenylpyridine; acac = acetoylacetonate, TBA = tetrabutylammonium cation) have been developed and fully characterized by UV-vis, emission, IR, NMR, and cyclic voltammetric studies. The lowest energy MLCT transitions are tuned from 463 to 494 nm by tuning the energy of the HOMO levels. These complexes show emission maxima in the blue, green, and yellow region of the visible spectrum and exhibit unprecedented phosphorescence quantum yields, 97 +/- 3% with an excited-state lifetimes of 1-3 micros in dichloromethane solution at 298 K. The near-unity quantum yields of these complexes are related to an increased energy gap between the triplet emitting state and the deactivating e(g) level that have been achieved by meticulous selection of ligands having strong ligand field strength. Organic light-emitting devices were fabricated using the complex 4 doped into a purified 4,4'-bis(carbazol-9-yl)biphenyl host exhibiting a maximum of the external quantum efficiencies of 13.2% and a power efficiency of 37 lm/W for the 9 mol % doped system.

Novel ruthenium sensitizers containing functionalized hybrid tetradentate ligands: synthesis, characterization, and INDO/S analysis.

Ruthenium sensitizers of the type trans-[Ru(L(1))(X)(2)], trans-[Ru(L(2))(X)(2)], trans-[Ru(L(3))(X)(2)], and trans-[Ru(L(4))(X)(2)] (where L(1) = 6,6'-bis(1-H-benzimidazol-2-yl)-4,4'-bis(methoxycarbonyl)-2,2'-bipyridine, L(2) = 4,4' "-bis(tert-butyl)-4',4' '-bis[p-(methoxycarbonyl)phenyl]-2,2':6',2' ':6' ',2' "-quaterpyridine, L(3) = 4',4' '-bis[3,4-(dimethoxy)phenyl]-2,2':6',2' ':6' ',2' "-quaterpyridine, and L(4) = 4',4' '-diethoxycarbonyl-2,2':6',2' ':6' ',2' "-quaterpyridine; X = Cl(-), NCS(-)) were synthesized and characterized by CV, NMR, and UV-vis absorption and emission spectroscopy. The trans-dichloro and dithiocyanate complexes show MLCT transitions in the entire visible and near-IR region. The lowest energy metal-to-ligand charge-transfer transition band of the trans-dichloro complexes is around 14 300 cm(-1) in DMF solution, and these complexes show weak and broad emission signals with onset at above 10 500 cm(-1). The absorption and emission maxima of the trans-dithiocyanate complexes are blue-shifted compared to those of its trans-dichloro analogues because of the strong pi acceptor property of the NCS(-) compared to the Cl(-). The electronic spectra of trans-[Ru(L)(X)(2)] complexes were calculated by INDO/S and compared with the experimental data. The extent of mixing between metal 4d and ligand pi orbitals is discussed. Extensive pi-back-donation is observed. The panchromatic response of these novel complexes renders them as suitable sensitizers for solar energy conversion applications based on titanium dioxide mesoporous electrodes. Preliminary results using the trans-[Ru(L(4))(NCS)(2)] complex show 75% incident photon-to-current efficiencies (IPCE), yielding 18 mA/cm(2) current density under standard AM 1.5 sunlight.

Engineering of efficient panchromatic sensitizers for nanocrystalline TiO(2)-based solar cells.

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 [(C(2)H(5))(3)NH][Ru(H(3)tcterpy)(NCS)(3)] 1, [(C(4)H(9))(4)N](2)[Ru(H(2)tcterpy)(NCS)(3)] 2, [(C(4)H(9))(4)N](3)[Ru(Htcterpy)(NCS)(3)] 3, and [(C(4)H(9))(4)N](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 Ru(II)N6 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 pi-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 TiO(2) films was interpreted by a Langmuir isotherm yielding 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 TiO(2), 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 TiO(2) 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/cm(2), and the open circuit voltage was 0.72 V corresponding to an overall conversion efficiency of 10.4%.

Acid-Base Equilibria of (2,2'-Bipyridyl-4,4'-dicarboxylic acid)ruthenium(II) Complexes and the Effect of Protonation on Charge-Transfer Sensitization of Nanocrystalline Titania.

The ruthenium complexes [Ru(dcbpyH(2))(2)(Cl)(2)] (1), [Ru(dcbpyH(2))(2)(NCS)(2)] (2), (Bu(4)N)(4)[Ru(dcbpy)(2)(NCS)(2)] (3), and (Bu(4)N)(2)[Ru(dcbpyH)(2)(NCS)(2)] (4) were synthesized and characterized by cyclic voltammetry, UV-vis absorption, and emission, IR, Raman, and NMR spectroscopy. The absorption and emission maxima of these complexes red shifted with decreasing pH, and showed pH-dependent excited-state lifetimes. The ground-state pK(a) values were determined by spectrophotometeric methods, and the dissociation of protons was found to occur in two steps (pK(a) = 3 and 1.5). The Ru(II)/(III) couple in the complex (Bu(4)N)(4)[Ru(dcbpy)(2)(NCS)(2)] is shifted ca. 290 mV negatively with regard to that of the complex [Ru(dcbpyH(2))(2)(NCS)(2)] due to the replacement of H(+) by tetrabutylammonium cation. The negative shift for the dcbpy-based reduction potential is even larger, i.e., about 600 mV compared to that of the complex [Ru(dcbpyH(2))(2)(NCS)(2)]. The effect of deprotonation on the performance of these complexes as photosensitizers for nanocrystalline titania was investigated.

Molecular Engineering of Photosensitizers for Nanocrystalline Solar Cells: Synthesis and Characterization of Ru Dyes Based on Phosphonated Terpyridines.

We report the results of an investigation on the preparation, spectral, and photoelectrochemical properties of Ru(II)-polypyridyl complexes containing a new phosphonated terpyridine (P-terpy) ligand: [Ru(H(2)P-terpy)(2)] and [Ru(HP-terpy)(Me(2)bpy)(NCS)]. Resonance Raman spectral and luminescence studies allow probing into the nature of the low-energy MLCT transitions observed in these complexes. The crystal and molecular structure of the mixed-ligand complex [Ru(HP-terpy)(Me(2)bpy)(NCS)] based on X-ray diffraction study is reported. This complex appears to be a promising candidate as a photosensitizer in dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO(2).