Heteroleptic Ruthenium Sensitizer Research Articles

Heteroleptic Ruthenium Sensitizers with Hydrophobic Fused‐Thio­phenes for Use in Efficient Dye‐­Sensitized Solar Cells

AbstractTwo fused‐thiophene‐containing ruthenium sensitizers, [Ru(Hdcbpy)(ottip)(NCS)2]–·0.5[H]+·0.5[N(C4H9)4]+ [Z1, H2dcbpy = 4,4′‐dicarboxy‐2,2′‐bipyridine, ottip = 2‐(5‐octylthieno[3,2‐b]thiophen‐2‐yl)‐1H‐imidazo[4,5‐f][1,10]phenanthroline] and [Ru(Hdcbpy)(odttip)(NCS)2]–·0.5[H]+·0.5[N(C4H9)4]+ {Z2, odttip = 2‐[6‐octyldithieno(3,2‐b:2′,3′)thiophen‐2‐yl]‐1H‐imidazo[4,5‐f][1,10]phenanthroline}, were designed for use in dye‐sensitized solar cell (DSSC) applications. The fused‐thiophene‐based Z1 and Z2 have stronger light‐harvesting capabilities compared with a non‐fused‐thiophene analog, [Ru(Hdcbpy)(otip)(NCS)2]–·0.5[H]+·0.5[N(C4H9)4]+ [JF‐2t, otip = 2‐(5‐octylthiophen‐2‐yl)‐1H‐imidazo[4,5‐f][1,10]phenanthroline], and a standard N3 dye. The power‐conversion efficiency of the Z1‐ and Z2‐sensitized solar cells were superior (8.70 % and 8.15 %) to JF‐2t‐ and N3‐sensitized solar cells (7.85 and 7.40 %, respectively) under standard AM1.5G full sunlight irradiation. The photovoltaic performance of Z1 was better than Z2, primarily owing to the fact that Z1‐based cells have a higher level of dye loading and less charge recombination, as seen from the electrochemical impedance spectroscopy data. This study not only demonstrates the excellent power‐conversion efficiency of new fused‐thiophene ruthenium sensitizers, but also points to the promise that fused‐thiophene antennas might be a useful molecule in the engineering of ruthenium dyes for DSSCs.

Novel heteroleptic ruthenium sensitizers containing carbazole linked 4,5-diazafluorene ligand for dye sensitized solar cells

Two novel heteroleptic ruthenium (II) sensitizers S1 and S2 containing an ancillary ligand 9,9-bis(9-ethylcarbazol-3-yl)-4,5-diazafluorene and 9,9-bis(9-phenylcarbazol-3-yl)-4,5-diazafluorene respectively, have been synthesized and characterized by 1H NMR, UV–Vis absorption spectroscopy, mass spectrometry, elemental analysis and cyclic voltammetry, and used as sensitizers in dye sensitized solar cells. The results show that for the two ruthenium sensitizers, the introduction of carbazole via sp3 hybridized C9 atom of the 4,5-diazafluorene unit produces significant changes in their optical absorption spectra and subsequently alters the intra-molecular charge transfer processes that influence the photovoltaic performance of the solar cell device. The power conversion efficiency of dye sensitized solar cells fabricated using sensitizers S1 and S2 are 0.36% and 0.42% respectively, while N719 exhibits 3.6% under the same device fabrication and measuring conditions.

A Ruthenium‐Based Light‐Harvesting Antenna Bearing an Anthracene Moiety in Dye‐Sensitized Solar Cells

AbstractA heteroleptic ruthenium sensitizer (DV56) bearing a terpyridine ligand with an appended phenylanthracene moiety was synthesized and extensively characterized. The physicochemical properties of the light‐harvesting antenna were studied in solution and on sensitized TiO2 thin film electrodes using a variety of spectroscopic and photoelectrochemical techniques. Contact angle measurements suggest that the DV56‐sensitized TiO2 films are hydrophilic. Raman measurements indicate chemisorption of the dye on the semiconducting surface. Electrochemical studies of the dye show that its first oxidation potential lies well below the I−/I3− redox potential, allowing easy regeneration. Moreover, its excited state oxidation potential lies above the TiO2 conduction band edge, permitting efficient electron injection from the photosensitizer into the semiconductor. I−/I3−‐based liquid redox electrolyte dye‐sensitized solar cells incorporating DV56‐sensitized TiO2 photoelectrodes afford an overall power conversion efficiency of 3.75 %.

A New Heteroleptic Ruthenium Sensitizer for Transparent Dye‐Sensitized Solar Cells

AbstractA new heteroleptic ruthenium complex, coded CYC‐B19, incorporating an ancillary ligand endowed with hexylthio‐bithiophene segments and a conjugated anchoring ligand with vinyl groups was prepared. This new sensitizer exhibits a lower energy MLCT band centred at 562 nm with a remarkably high molar absorption coefficient of 2.97 × 104 M−1 cm−1. DFT‐TDDFT theoretical calculation revealed that insertion a vinyl group in the anchoring ligand pushes the LUMO electron locating more on the anchoring ligand. This will benefit the electron transfer from dye to TiO2 when the dye molecules were excited by light. Physicochemical measurements and the optimization of electrolyte were done to investigate the potential of CYC‐B19 in TiO2 scattering‐layer free dye‐sensitized solar cells. Not only is a good photovoltaic efficiency of 8.4% reached, but the transparent device sensitized by CYC‐B19 also presents a superior spectral response to its predecessor CYC‐B11.

Theoretical studies of the structures and spectroscopic properties of the photoelectrochemical cell ruthenium sensitizers, C101 and J13

A variety of heteroleptic ruthenium sensitizers have been engineered and synthesized because of their higher light-harvesting efficiency and lower charge-recombination possibility than the well known homoleptic N3 dye. As such, a great deal of attention has been focused on sensitizers with the general formula Ru(ancillary-ligand)(anchoring-ligand)(NCS)2, among which important examples are Ru(4,4′-bis(5-hexylthiophen-2-yl)-2,2′-bipyridine)(4,4′-carboxylic acid-4′-2,2′-bipyridine)(NCS)2 (C101) and Ru(N-(4-butoxyphenyl)-N-2-pyridinyl-2-pyridinamine)(4,4′-carboxylic acid-4′-2,2′-bipyridine)(NCS)2 (J13). In order to simulate experimental conditions with different pH values, the photosensitizing processes of these sensitizers possessing different degrees of deprotonation (2H, 1H to 0H) have been explored theoretically in this work. Their ground/excited state geometries, electronic structures and spectroscopic properties are first calculated using density functional theory (DFT) and time-dependent DFT (TDDFT). The absorption and emission spectra of all the complexes in acetonitrile solution are also predicted at the TDDFT (B3LYP) level. The calculated results show that the ancillary ligand contributes to the molecular orbital (MO) energy levels and absorption transitions. It is intriguing to observe that the introduction of a thiophene group into the ancillary ligand leads directly to the increased energy of the absorption transitions in the 380–450 nm region. The calculations reveal that although deprotonation destabilizes the overall frontier MOs of the chromophores, it tends to exert a greater influence on the unoccupied orbitals than on the occupied orbitals. Consequently, an obvious blue shift was observed for the absorptions and emissions in going from 2H, 1H to 0H. Finally, the optimal degree of deprotonation for C101 and J13 has also been evaluated, which is expected to lead to further improvements in the performance of dye-sensitized solar cells (DSSCs) coated with such sensitizers.

Effect of bulky groups in ruthenium heteroleptic sensitizers on dye sensitized solar cell performance

Two novel heteroleptic ruthenium sensitizers, TT204 and TT205, containing bulky substituents at the 4,4-positions of the ancillary 2,2-bipyridine ligand, were synthesized and characterized. They exhibit absorption maxima in the visible region at 520–530 nm due to metal-to-ligand charge transfer (MLCT) transitions. The EHOMO and ELUMO values indicate sufficient driving force for efficient dye regeneration by the iodide/tri-iodide redox electrolyte and efficient electron injection into the TiO2 conduction band following photoexcitation, respectively. The performance of these heteroleptic sensitizers in dye sensitized solar cells was investigated where TiO2 sensitization was carried out both in the presence and absence of chenodeoxycholic acid. The best efficiency among these sensitizers was recorded in the presence of chenodeoxycholic acid which generated a high short circuit current of 18.7 mA cm−2, an open circuit potential of 0.72 V and a fill factor of 73% with a resulting total power conversion efficiency of 9.81% under AM 1.5G 1 sun illumination. Comparison with the reference dye C101 indicates that though bulky groups can prevent aggregation resulting in high photocurrents even in the absence of chenodeoxycholic acid, they can also lead to lower cell voltages due to inefficient dye packing on the TiO2 surface.

Heteroleptic Ruthenium Sensitizers That Contain an Ancillary Bipyridine Ligand Tethered with Hydrocarbon Chains for Efficient Dye-Sensitized Solar Cells

New heteroleptic ruthenium complexes have been synthesized and used as the sensitizers for dye-sensitized solar cells (DSSCs). The ancillary bipyridine ligand contains rigid aromatic segments (fluorene-, carbazole-, or dithieno[3,2-b:2',3'-d]pyrrole-substituted bipyridine) tethered with a hydrophobic hexyl substituent. The conjugated aromatic segment results in significant bathochromic shift and hyperchromic effects in these complexes compared with Z907 (cis-[RuLL'(NCS)(2)]; L=4,4'-dicarboxylic acid-2,2'-bipyridine, L'=4,4'-dinonyl-2,2'- bipyridine). The long hydrocarbon chains help to suppress the dark current if appropriately disposed. DSSCs that use these complexes exhibit very impressive conversion efficiencies (5.94 to 6.91 %) that surpass that of Z907-based (6.36 %) DSSCs and are comparable with that of N719-based standard cells (7.13 %; N719=cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II) bis(tetrabutylammonium)) fabricated and measured under similar conditions (active area: 0.5×0.5 cm(2); AM 1.5 sunlight).

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).

Synthesis and characterization of novel heteroleptic ruthenium sensitizer for nanocrystalline dye-sensitized solar cells

A novel heteroleptic ruthenium complex of the type [Ru(bpin)(dcbpyH 2)Cl]Cl (where bpin is 2,6-bis(pyrazol-1-yl)isonicotinic acid and dcbpyH 2 is 4,4′-dicarboxy-2,2′-bipyridine) was synthesized and characterized for tuning the LUMO level of the ruthenium sensitizer to achieve greater stabilization in the excited state which keeps the excess energy to maintain high driving force for electron injection. The photovoltaic performance of this complex as photosensitizer in a nanocrystalline TiO 2-based solar cell was studied and its overall energy conversion efficiency was determined (1.9%).

A Dendritic Oligothiophene Ruthenium Sensitizer for Stable Dye‐Sensitized Solar Cells

We report a new type of dendritic terthiophene attached to a 2,2'-bipyridine ligand for ruthenium-based dye-sensitized solar cells. As a result of the incorporation of this electron-rich terthiophene donor unit into the heteroleptic ruthenium sensitizer [Ru(dcbpy)(3Tbpy)(NCS)(2)] (3T), the lowest-energy metal-to-ligand charge transfer (MLCT) transition is red-shifted and the molar extinction coefficient is increased compared to the analogous standard Z907Na sensitizer. A preliminary investigation of the 3T dye in combination with an iodine/iodide-based electrolyte shows an interesting photovoltaic performance, with a maximum power conversion efficiency of 7.4 % measured under air mass 1.5 global sunlight irradiation. Accelerated testing of these devices at 60 degrees C under full sunlight soaking shows a remarkable stability over 1000 h.

ChemInform Abstract: A New Heteroleptic Ruthenium Sensitizer Enhances the Absorptivity of Mesoporous Titania Film for a High Efficiency Dye‐Sensitized Solar Cell.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A new heteroleptic ruthenium sensitizer enhances the absorptivity of mesoporous titania film for a high efficiency dye-sensitized solar cell

A heteroleptic polypyridyl ruthenium complex, cis-Ru(4,4'-bis(5-octylthieno[3,2-b]thiophen-2-yl)-2,2'-bipyridine)(4,4'-dicarboxyl-2,2'-bipyridine)(NCS)2, with a high molar extinction coefficient of 20.5 x 10(3) M(-1) cm(-1) at 553 nm has been synthesized and demonstrated as a highly efficient sensitizer for a dye-sensitized solar cell, giving a power conversion efficiency of 10.53% measured under an irradiation of air mass 1.5 global (AM 1.5G) full sunlight.