Charge-transfer studies of iron cyano compounds bound to nanocrystalline TiO(2) surfaces

Nanocrystalline (anatase) titanium dioxide films have been sensitized to visible light with K(4)[Fe(CN)(6)] and Na(2)[Fe(LL)(CN)(4)], where LL = bpy (2,2'-bipyridine), dmb (4,4'-dimethyl-2,2'-bipyridine), or dpb (4,4'-diphenyl-2,2'-bipyridine). Coordination of Fe(CN)(6)(4-) to the TiO(2) surface results in the appearance of a broad absorption band (fwhm approximately 8200 cm(-1)) centered at 23800 +/- 400 cm(-1) assigned to an Fe(II)-->TiO(2) metal-to-particle charge-transfer (MPCT) band. The absorption spectra of Fe(LL)(CN)(4)(2-) compounds anchored to TiO(2) are well modeled by a sum of metal-to-ligand charge-transfer (MLCT) bands and a MPCT band. Pulsed light excitation (417 or 532 nm, approximately 8 ns fwhm, approximately 2-15 mJ/pulse) results in the immediate appearance of absorption difference spectra assigned to an interfacial charge separated state [TiO(2)(e(-)), Fe(III)], k(inj) > 10(8) s(-1). Charge recombination is well described by a second-order equal concentration kinetic model and requires milliseconds for completion. A model is proposed wherein sensitization of Fe(LL)(CN)(4)(2-)/TiO(2) occurs by MPCT and MLCT pathways, the quantum yield for the latter being dependent on environment. The solvatochromism of the materials allows the reorganization energies associated with charge transfer to be quantified. The photocurrent efficiencies of the sensitized materials are also reported.     

High molar extinction coefficient heteroleptic ruthenium complexes for thin film dye-sensitized solar cells

Two novel heteroleptic sensitizers, Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-hexyloxystyryl)-2,2-bipyridine)(NCS)2 and Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-methoxystyryl)-2,2'-bipyridine) (NCS)2, coded as K-19 and K-73, respectively, have been synthesized and characterized by 1H NMR, FTIR, UV-vis absorption, and emission spectroscopy and excited-state lifetime and spectroelectrochemical measurements. The introduction of the alkoxystyryl group extends the conjugation of the bipyridine donor ligand increasing markedly their molar extinction coefficient and solar light harvesting capacity. The dynamics of photoinduced charge separation following electronic excitation of the K-19 dye was scrutinized by time-resolved laser spectroscopy. The electron transfer from K-19 to the conduction band of TiO2 is completed within 20 fs while charge recombination has a half-life time of 800 s. The high extinction coefficients of these sensitizers enable realization of a new generation of a thin film dye sensitized solar cell (DSC) yielding high conversion efficiency at full sunlight even with viscous electrolytes based on ionic liquids or nonvolatile solvents. An unprecedented yield of over 9% was obtained under standard reporting conditions (simulated global air mass 1.5 sunlight at 1000 W/m2 intensity) when the K-73 sensitizer was combined with a nonvolatile "robust" electrolyte. The K-19 dye gave a conversion yield of 7.1% when used in conjunction with the binary ionic liquid electrolyte. These devices exhibit excellent stability under light soaking at 60 degrees C. The effect of the mesoscopic TiO2 film thickness on photovoltaic performance has been analyzed by electrochemical impedance spectroscopy (EIS).     

Amphiphilic ruthenium sensitizers and their applications in dye-sensitized solar cells

Amphiphilic ligands 4,4'-bis(1-adamantyl-aminocarbonyl)-2,2'-bipyridine (L(1)), 4,4'-bis[5-[N-[2-(3beta-cholest-5-en-3-ylcarbamate-N-yl)ethyl]aminocarbonyl]]-2,2'-bipyridine (L(2)), 4,4'-bis[5-[N-[2-(3beta-cholest-5-en-3-ylcarbamate-N-yl)propyl]aminocarbonyl]]-2,2'-bipyridine (L(3)), and 4,4'-bis(dodecan-12-ol)-2,2'-bipyridine (L(4)) and their heteroleptic ruthenium(II) complexes of the type [Ru(II)LL(1)(NCS)(2)] (5), [Ru(II)LL(2)(NCS)(2)] (6), [Ru(II)LL(3)(NCS)(2)] (7), and [Ru(II)LL(4)(NCS)(2)] (8) (where L = 4,4'-bis(carboxylic acid)-2,2'-bipyridine) have been synthesized starting from dichloro(p-cymene)ruthenium(II) dimer. All the ligands and the complexes were characterized by analytical, spectroscopic, and electrochemical techniques. The performance of these complexes as charge-transfer photosensitizers in nanocrystalline TiO(2)-based solar cells was studied. When complexes 5-8 anchored onto a 12 + 4 microm thick nanocrystalline TiO(2) films, very efficient sensitization was achieved (85 +/- 5% incident photon-to-current efficiencies in the visible region, using an electrolyte consisting of 0.6 M butylmethylimidazolium iodide, 0.05 M I(2), 0.1 M LiI, and 0.5 M tert-butyl pyridine in 1:1 acetonitrile + valeronitrile). Under standard AM 1.5 sunlight, the complex 8 yielded a short-circuit photocurrent density of 17 +/- 0.5 mA/cm(2), the open-circuit voltage was 720 +/- 50 mV, and the fill factor was 0.72 +/- 0.05, corresponding to an overall conversion efficiency of 8.8 +/- 0.5%.     

Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers

We report a combined experimental and computational study of several ruthenium(II) sensitizers originated from the [Ru(dcbpyH(2))(2)(NCS)(2)], N3, and [Ru(dcbpyH(2))(tdbpy)(NCS)(2)], N621, (dcbpyH(2) = 4,4'-dicarboxy-2,2'-bipyridine, tdbpy = 4,4'-tridecyl-2,2'-bipyridine) complexes. A purification procedure was developed to obtain pure N-bonded isomers of both types of sensitizers. The photovoltaic data of the purified N3 and N621 sensitizers adsorbed on TiO(2) films in their monoprotonated and diprotonated state, exhibited remarkable power conversion efficiency at 1 sun, 11.18 and 9.57%, respectively. An extensive Density Functional Theory (DFT)-Time Dependent DFT study of these sensitizers in solution was performed, investigating the effect of protonation of the terminal carboxylic groups and of the counterions on the electronic structure and optical properties of the dyes. The calculated absorption spectra are in good agreement with the experiment, thus allowing a detailed assignment of the UV-vis spectral features of the two types of dyes. The computed alignments of the molecular orbitals of the different complexes with the band edges of a model TiO(2) nanoparticle provide additional insights into the electronic factors governing the efficiency of dye-sensitized solar cell devices.     

The influence of dye structure on charge recombination in dye-sensitized solar cells

Replacing the nonyl groups on the solar cell dye Ru(4,4'-carboxylic acid-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(NCS)(2) (Z-907) with amino groups results in a marked decrease in solar cell performance. This is despite the fact that the amino derivative (Z-960) has more favourable light absorption characteristics than Z-907 when used with thick nanocrystalline TiO(2) layers. Electron transfer to the electrolyte from the exposed fluorine-doped tin oxide (FTO) substrate is particularly fast in cells employing the Z-960 dye if a compact TiO(2) blocking layer is not used. The kinetics of electron transfer from the nanocrystalline TiO(2) layer in DSCs employing Z-960 are comparable to those of bare TiO(2) and ca. 2 to 5 times faster than for cells employing Z-907. The faster charge recombination in cells employing Z-960 lowers open-circuit photovoltage and results in very significant charge collection losses that lower short-circuit photocurrent. Voltammetric measurements show that surface modification of FTO electrodes with Z-960 results in slightly more facile charge transfer to acceptor species in triiodide/iodide electrolytes in the dark. A simpler molecule, p-aminobenzoic acid, more dramatically catalyses this charge transfer reaction. Conversely, chemical modification of FTO electrodes with Z-907 or p-toluic acid retards charge transfer kinetics. Similar results are obtained for nanocrystalline TiO(2) electrodes modified with these benzoic acid derivatives. These results strongly imply that surface adsorbed molecules bearing amino groups, including dye molecules, can catalyse charge recombination in dye-sensitized solar cells.     

Long-wavelength sensitization of TiO2 by ruthenium diimine compounds with low-lying π* orbitals

The role of low-lying π* orbitals in dye-sensitized solar cells based on mesoporous thin films of anatase TiO(2) nanocrystallites remains unknown. Herein we report three ruthenium compounds, cis-Ru(dcbq)(2)(NCS)(2), cis-Ru(dcbq)(bpy)(NCS)(2), and cis-Ru(dcb)(bq)(NCS)(2), where bpy is 2,2'-bipyridine, dcb is 4,4'-(CO(2)H)(2)-2,2'-bipyridine, bq is 2,2'-biquinoline, and dcbq is 4,4'-(CO(2)H)(2)-2,2'-biquinoline, that were synthesized, characterized, and contrasted with the well-known N3 compound (i.e., cis-Ru(dcb)(2)(NCS)(2)) in dye-sensitized solar cells. These compounds maintain the same cis-Ru(NCS)(2) core with a systematic variation in the energy of the π* orbitals of the diimine ligand: bpy > dcb > bq > dcbq. The lowered π* orbitals resulted in enhanced red absorption relative to N3. With HCl pretreated TiO(2) in regenerative solar cells, sensitization from 400 to 900 nm was realized with cis-Ru(dcb)(bq)(NCS)(2) and global power conversion efficiencies as high as 6.5% were achieved under 1 sun of AM 1.5 irradiation. The energy conversion efficiency was found to be acutely sensitive to the presence of p-tert-butylpyridine (TBP) in a 0.5 M LiI/0.05 M I(2) acetonitrile electrolyte. Nanosecond transient absorption studies revealed that the addition of TBP decreased the excited-state injection yield for the compounds with biquinoline ligands. Spectro-electrochemical studies showed that the HCl pretreatment lowered the effective density of TiO(2) acceptor states and confirmed that the presence of TBP raised them toward the vacuum level. There was no spectroscopic data to support the hypothesis that the π* levels of the diimine ligand mediate back-electron transfer to the oxidized dye or the redox mediator was found.     

Comparison of electron injection dynamics from re-bipyridyl complexes to TiO2 nanocrystalline thin films in different solvent environments

Factors that control photoinduced interfacial electron transfer (ET) between molecular adsorbates and semiconductor nanoparticles have been intensely investigated in recent years. In this work, the solvent dependence of interfacial ET was studied by comparing ET rates in dye sensitized TiO2 nanocrystalline films in different solvent environments. Photoinduced ET rates from Re(LA)(CO)3Cl [LA=dcbpy=4,4'-dicarboxy-2,2'-bipyridine] (ReC1A) to TiO2 nanocrystalline thin films in air, pH buffer, MeOH, EtOH, and DMF were measured by femtosecond transient IR spectroscopy. The ET rates in these solvent environments were noticeably different. However, differences between the rates in pH buffer and nonaqueous solvents (MeOH, EtOH, and DMF) were much smaller than the values expected from much more negative TiO2 conduction band-edge positions in the latter solvents under anhydrous conditions. It was suggested that the presence of adsorbed water, which was evident in FTIR spectra, lowered the band edge of TiO2 in these solvents and reduced the rate differences. The important effect of adsorbed water was verified by comparing two samples of Re(LP)(CO)3Cl [LP=2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] sensitized TiO2 in DMF, in which the presence of a trace amount of water was found to significantly increase the injection rate.     

Phenomenally high molar extinction coefficient sensitizer with "donor-acceptor" ligands for dye-sensitized solar cell applications

A phenomenally high molar extinction coefficient heteroleptic ruthenium(II) complex [Ru(4,4'-carboxylic acid-2,2'-bipyridine)(4,4'-(4-{4-methyl-2,5-bis[3-methylbutoxy]styryl}-2,5-bis[3-methylbutoxy]-2,2'-bipyridine)(NCS) 2] ( DCSC13) was synthesized by incorporating donor-acceptor ligands. The absorption spectrum of the DCSC13 sensitizer is dominated by metal-to-ligand charge-transfer transitions (MLCT) in the visible region, with absorption maxima appearing at 442 and 554 nm. The lowest MLCT absorption bands are red-shifted, and the molar extinction coefficients of these bands are significantly higher at 72,100 and 30,600 M(-1) cm(-1), respectively, when compared to those of the analogous [Ru(4,4'-carboxylic acid-2,2'-bipyridine)(4,4'-dimethyl-2,2'-bipyridine)(NCS)2] (N820) sensitizer. The DCSC13 complex, when anchored on nanocrystalline TiO 2 films, exhibited increased short-circuit photocurrent and consequent power-conversion efficiency when compared with the N820 sensitizer.     

Influence of ionic liquids bearing functional groups in dye-sensitized solar cells

Ionic liquids containing the nitrile and vinyl functional groups attached to imidazolium cations combined with various anions, e.g., iodide, bis[(trifluoromethyl)sulfonyl]imide ([TFSI]-), or dicyanamide ([N(CN)2]-), have been prepared and characterized. These ionic liquids have been successfully used as electrolytes for dye-sensitized solar cells based on nanocrystalline TiO2 with the amphiphilic ruthenium sensitizer [ruthenium (4,4'-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-hexyloxystyryl)-2,2'-bipyridine)][NCS]2 (coded K-19). The iodide salt was used in 3-methoxypropionitrile-based electrolytes, and the performances of both types of devices were evaluated on the basis of their photocurrent density-voltage characteristics and dark current measurements, demonstrating that the functional groups do not exert a detrimental effect on the performance. The solid-state structure of the nitrile-functionalized salt [C1C3CN(im)]I has also been established by single-crystal X-ray diffraction, revealing extensive hydrogen bonding between the cation protons and the iodide.     

Panchromatic sensitization of nanocrystalline TiO2 with cis-Bis(4-carboxy-2-[2'-(4'-carboxypyridyl)]quinoline)bis(thiocyanato-N)ruthenium(II)

We compared the spectral (IR and Raman), electrochemical, and photoelectrochemical properties of nanocrystalline TiO(2) sensitized with the newly synthesized complex [NBu(4)](2)[cis-Ru(Hdcpq)(2)(NCS)(2)] (1; [NBu(4)](+) = tetrabutylammonium cation; H(2)dcpq = 4-carboxy-2-[2'-(4'-carboxypyridyl)]quinoline) with those of TiO(2) sensitized with [NBu(4)](2)[cis-Ru(Hdcbpy)(2)(NCS)(2)] (2; H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and [NBu(4)](2)[cis-Ru(Hdcbiq)(2)(NCS)(2)] (3; H(2)dcbiq = 4,4'-dicarboxy-2,2'-biquinoline). Complex 1 achieved efficient sensitization of nanocrystalline TiO(2) films over a wide visible and near-IR region, generating a large short-circuit photocurrent. The absorbed photon-to-current conversion efficiency decreased in the order 2 > 1 > 3 with the decrease in the free energy change (-Delta G(inj)) of the electron injection from the ruthenium complex to TiO(2). The open-circuit photovoltages (V(oc)'s) of dye-sensitized solar cells decreased in the order 2 > 1 > 3 with the increase in the dark current resulting from reverse electron transfer from TiO(2) to I(3)(-). The sensitizer-dependent V(oc) value can be interpreted as a result of reverse electron transfer through the sensitizing dye molecules.     

Characterization of photoinduced self-exchange reactions at molecule-semiconductor interfaces by transient polarization spectroscopy: lateral intermolecular energy and hole transfer across sensitized TiO2 thin films

Transient anisotropy measurements are reported as a new spectroscopic tool for mechanistic characterization of photoinduced charge-transfer and energy-transfer self-exchange reactions at molecule-semiconductor interfaces. An anisotropic molecular subpopulation was generated by photoselection of randomly oriented Ru(II)-polypyridyl compounds, anchored to mesoscopic nanocrystalline TiO(2) or ZrO(2) thin films, with linearly polarized light. Subsequent characterization of the photoinduced dichromism change by visible absorption and photoluminescence spectroscopies on the nano- to millisecond time scale enabled the direct comparison of competitive processes: excited-state decay vs self-exchange energy transfer, or interfacial charge recombination vs self-exchange hole transfer. Self-exchange energy transfer was found to be many orders-of-magnitude faster than hole transfer at the sensitized TiO(2) interfaces; for [Ru(dtb)(2)(dcb)](PF(6))(2), where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine and dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, anchored to TiO(2), the energy-transfer correlation time was θ(ent) = 3.3 μs while the average hole-transfer correlation time was <θ(h+)> = 110 ms, under identical experimental conditions. Monte Carlo simulations successfully modeled the anisotropy decays associated with lateral energy transfer. These mesoscopic, nanocrystalline TiO(2) thin films developed for regenerative solar cells thus function as active "antennae", harvesting sunlight and transferring energy across their surface. For the dicationic sensitizer, [Ru(dtb)(2)(dcb)](PF(6))(2), anisotropy changes indicative of self-exchange hole transfer were observed only when ions were present in the acetonitrile solution. In contrast, evidence for lateral hole transfer was observed in neat acetonitrile for a neutral sensitizer, cis-Ru(dnb)(dcb)(NCS)(2), where dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine, anchored to TiO(2). The results indicate that mechanistic models of interfacial charge recombination between electrons in TiO(2) and oxidized sensitizers must take into account diffusion of the injected electron and the oxidized sensitizer. The phenomena presented herein represent two means of concentrating potential energy derived from visible light that could be used to funnel energy to molecular catalysts for multiple-charge-transfer reactions, such as the generation of solar fuels.     

Effect of sensitizer adsorption temperature on the performance of dye-sensitized solar cells

Employing a mesoscopic titania photoanode whose bilayer structure was judiciously selected to fit the optoelectronic characteristics of the Ru-based heteroleptic complex Na-cis-Ru(4,4'-(5-hexyltiophen-2-yl)-2,2'-bipyridine)(4-carboxylic-acid-4'-carboxylate-2,2'-bipyridine)(thiocyanate)(2), coded as C101, we investigated the effect of temperature for dye adsorption on the photovoltaic performance of dye-sensitized solar cells (DSCs). We found a significant efficiency enhancement upon lowering the temperature applied during the sensitizer uptake from solution. When the dye adsorption was performed at 4 °C, the photovoltaic performance parameters measured under standard reporting conditions (AM1.5 G sunlight at 1000 W/m(2) intensity and 25 °C), i.e., the open circuit voltage (V(oc)), the short circuit photocurrent density (J(sc)), the fill factor (FF), and consequently the power conversion efficiency (PCE), improved in comparison to cells stained at 20 and 60 °C. Results from electrochemical impedance spectroscopy (EIS) and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) show that the self-assembled layer of C101 formed at lower temperature impairs the back-electron transfer from the TiO(2) conduction band to the triiodide ions in the electrolyte more strongly than the film produced at 60 °C. Profiting from the favorable influence that the low-temperature dye uptake exerts on photovoltaic performance, we have realized DSCs showing a power conversion efficiency of 11.5%.     

Synthesis and characterization of tris(heteroleptic) Ru(II) complexes bearing styryl subunits

We have developed and optimized a well-controlled and refined methodology for the synthesis of substituted π-conjugated 4,4'-styryl-2,2'-bipyridine ligands and also adapted the tris(heteroleptic) synthetic approach developed by Mann and co-workers to produce two new representative Ru(II)-based complexes bearing the metal oxide surface-anchoring precursor 4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine. The two targeted Ru(II) complexes, (4,4'-dimethyl-2,2'-bipyridine)(4,4'-di-tert-butyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (1) and (4,4'-dimethyl-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dnbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (2) were obtained as analytically pure compounds in high overall yields (>50% after 5 steps) and were isolated without significant purification effort. In these tris(heteroleptic) molecules, NMR-based structural characterization became nontrivial as the coordinated ligand sets each sense profoundly distinct magnetic environments greatly complicating traditional 1D spectra. However, rational two-dimensional approaches based on both homo- and heteronuclear couplings were readily applied to these structures producing quite definitive analytical characterization and the associated methodology is described in detail. Preliminary photoluminescence and photochemical characterization of 1 and 2 strongly suggests that both molecules are energetically and kinetically suitable to serve as sensitizers in energy-relevant applications.     

Calculated structural and electronic interactions of the ruthenium dye N3 with a titanium dioxide nanocrystal

Structural and electronic properties of a small anatase TiO2 nanocrystal sensitized by the ruthenium dye N3 (Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2) have been investigated using density functional theory (DFT) with support from Hartree-Fock (HF) and time dependent DFT (TD-DFT) calculations. Significant structural adjustments of both the dye and the nanocrystal are predicted to be induced by the strain imposed by the simultaneous formation of multiple dye-surface bonds. Electronic properties of the combined dye-nanocrystal system have also been calculated, including information about interfacial orbital mixing and the lowest excited singlet states. Ultrafast photoinduced electron transfer processes across the dye-nanoparticle interface in dye-sensitized solar cells are finally discussed in view of estimated electronic coupling strengths. The calculations predict injection times on the order of 10 fs for MLCT excitations to the ligand pi* levels that interact most strongly with the TiO2 conduction band, and an order of magnitude increase in the injection times for excitations to dye levels with poor spatial or energetic overlaps with the substrate conduction band.     

Effects of tethering alkyl chains for amphiphilic ruthenium complex dyes on their adsorption to titanium oxide and photovoltaic properties

Ruthenium (II) complex dye, Ru(4,4'-dicarboxyl-2,2'-bipyridine)(4-nonyl-2,2'-bipyridine) (NCS)(2), (denoted as RuC9) tethering single alkyl chain was synthesized and well characterized. Its adsorption behavior onto the mesoporous TiO(2) and photovoltaic properties were compared with Z907 which has similar chemical structure but tethers two alkyl chains. RuC9 dyes tend to aggregate into vesicles in the acetonitrile/t-butanol co-solvent as a result of the amphiphilic structure, whereas Z907 dyes aggregate into lamellae. The dye-sensitized solar cell (DSSC) with RuC9 dye showed higher short-circuit photocurrent than that with Z907, attributing to its higher molar optical extinction coefficient and more adsorption amount onto the mesoporous TiO(2). However, the DSSC with Z907 dye has higher open-circuit photovoltage and power conversion efficiency, presumably due to the fact that Z907 with more alkyl chains formed a molecular layer with higher hydrophobicity. It reduced the charge recombination in the interface between the dye-sensitized mesoporous TiO(2) and electrolyte as verified by the electrochemical impedance spectroscopy and intensity modulated photocurrent and photovoltage spectroscopies.     

Electrochemical and phosphorescent properties of new mixed-ligand Ir(II) complexes coordinated with both terpyridine and various bipyridine derivatives.

Seven useful mixed-ligand complexes in the form of [Ir(terpy)(L)Cl]2+ were prepared and their spectroscopic and electrochemical properties were investigated. The ligands used were terpy = 2,2':6',2'-terpyridine, L = 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, 4,4'-diphenyl-2,2'-bipyridine, 1,10-phenanthroline, 5-phenyl-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, 2,3-bis(2-pyridyl)pyrazine. Synthetic methods were developed by a sequential ligand-replacement which occurred in the reaction vessel using a microwave oven. All complexes showed that LUMOs are based on the pi-system contribution of the terpyridine ligand for [Ir(terpy)(bpy)Cl]2+, [Ir(terpy)(dmbpy)Cl]2+, [Ir(terpy)(dpbpy)Cl]2+, [Ir(terpy)(phen)Cl]2+, [Ir(terpy)(dpphen)Cl]2+ and [Ir(terpy)(phphen)Cl]2+. On the other hand, the LUMO in the [Ir(terpy)(bppz)Cl]2+ complex is localized on the pi-system of the bppz ligand, whereas the HOMOs in the iridium complexes are localized on the terpyridine ligand. It was found that Ir(terpy)(L)Cl emits in a fluid solution at room temperature. The ancillary ligands, such as terpy and bpy, have been explored to extend the lifetime of the triplet 3(pi-pi') excited states of Ir(III) terpyridine complexes. Ir(III) terpyridine units with an electron donor (dmbpy) or electron acceptor substituents (terpy, dpbpy, phphen, dpphen and bppz) are found to decrease the energy of the 3LC states for use as photosensitizer molecular components in supramolecular devices. The spectroscopic and electrochemical details are also reported herein.     

Heteroleptic ruthenium complexes containing uncommon 5,5'-disubstituted-2,2'-bipyridine chromophores for dye-sensitized solar cells

Four new heteroleptic ruthenium sensitizers [Ru(4,4'-carboxylic acid-2,2'-bipyridine)(L)(NCS)(2)] (L = 5,5'-bis(4-octylthiophen-2-yl)-2,2'-bipyridine (1), 5,5'-bis(N,N-diphenyl-4-aminophenyl)-2,2'-bipyridine (2), 5,5'-bis(5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2'-bipyridine (3) and 5,5'-bis(4-octyl-5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2'-bipyridine (4)) were synthesized, characterized by physicochemical and computational methods, and utilized as photosensitizers in nanocrystalline dye-sensitized solar cells (DSSCs). The λ(max) of the metal-to-ligand charge transfer (MLCT) absorption of these four ruthenium dyes (527 nm for 1, 535 nm for 2, 585 nm for 3 and 553 nm for 4) can be tuned by various structural modifications of the ancillary ligand and it was shown that increasing the conjugation length of such ligand reduces the energy as well as the molar absorption coefficient of the MLCT band. The maximum incident photon to current conversion efficiency (IPCE) of 41.4% at 550 nm, 38.6% at 480 nm, 39.4% at 470 nm and 31.1% at 480 nm for 1-, 2-, 3- and 4-sensitized solar cells were obtained. Respectable power conversion efficiencies of 3.00%, 2.51%, 2.00% and 2.03% were realized, respectively, when the sensitizers 1, 2, 3 and 4 were used in DSSCs under the standard air mass (AM) 1.5 sunlight illumination (versus 5.9% for standard N719).     

Electron-rich heteroaromatic conjugated bipyridine based ruthenium sensitizer for efficient dye-sensitized solar cells

A novel heteroleptic ruthenium complex carrying a heteroaromatic-4,4'-pi-conjugated 2,2'-bipyridine [Ru(II)LL'(NCS)(2)] (L = 4,4'-bis[(E)-2-(3,4-ethylenedioxythien-2-yl)vinyl]-2,2'-bipyridine, L' = 4,4'-(dicarboxylic acid)-2,2'-bipyridine) was synthesized and used in dye-sensitized solar cells, yielding photovoltaic efficiencies of 9.1% under standard global AM 1.5 sunlight.     

Measurements and modeling of recombination from nanoparticle TiO2 electrodes

Electron-transfer reactions from nanoparticle TiO(2) films to outer-sphere redox shuttles were investigated. Steady-state dark current density versus applied potential and open circuit voltage decay measurements were employed to determine the rates of recombination to cobalt(III) tris(4,4'-dimethyl-2,2'-bipyridyl), [Co(Me(2)bpy)(3)](3+), and ruthenium(III) bis(2,2'-bipyridyl)-bis(N-methylimidozole), [Ru(bpy)(2)(MeIm)(2)](3+). A striking difference in the magnitude as well as the shape of the electron lifetimes for TiO(2) electrodes in contact with these two redox shuttles is observed. A model based on Marcus theory is developed to describe recombination, including contributions from conduction band electrons and surface states. Excellent agreement was found between the modeled and measured lifetimes. The model allows for identification of each contributing component of electron transfer to the measured lifetimes. Comparison of the different components of the modeled lifetimes to the measured lifetimes provides clear evidence for recombination mediated through surface states.     

Ultrafast electron transfer between molecule adsorbate and antimony doped tin oxide (ATO) nanoparticles

Ultrafast transient IR spectroscopy has been used to examine the effect of doping on interfacial electron transfer (ET) dynamics in Re(dpbpy)(CO)(3)Cl (dpbpy = 4,4'-(CH(2)PO(OH)(2))2-2,2'-bipyridine) (ReC1PO(3)) sensitized ATO (Sb:SnO(2)) nanocrystalline thin films. In films consisting of particles with 0%, 2% and 10% Sb dopant, the rates of electron injection from the adsorbate excited state to ATO were independent of and the rates of the recombination increased with the doping level. The observed similar forward electron injection rates were attributed to negligible changes of available accepting states in the conduction band at the doping levels studied. The dependence of the recombination rate on conduction band electron density and a possible mechanism for the recombination process were discussed.