Efficient ruthenium sensitizer with an extended π-conjugated terpyridine ligand for dye-sensitized solar cells.

A ruthenium sensitizer with an extended π-conjugated terpyridine (TUS-42) has been synthesized for dye-sensitized solar cells (DSCs). Upon extension of the π-conjugated system of the terpyridine ligand, the conversion efficiency of the DSC with TUS-42 improved successfully to 10.7%, which is almost comparable to that of one of the most efficient ruthenium sensitizers (Black dye). Interestingly, an extremely large short-circuit current density (Jsc) value (22.7 mA/cm(2)) was obtained in the DSC with TUS-42 even though the amount of dye adsorption to the TiO2 photoelectrode is relatively small.

Dependence of the efficiency improvement of black-dye-based dye-sensitized solar cells on alkyl chain length of quaternary ammonium cations in electrolyte solutions.

Dependence of the suppression of the backward electron transfer reaction from the TiO2 photoelectrode to I3(-) in the electrolyte on the alkyl chain length of the quaternary ammonium cation has been investigated for further efficiency improvement of high-performance cosensitized dye-sensitized solar cells (DSCs). The tetraheptylammonium cation was found to be more effective than the tetraethylammonium and tetrabutylammonium cations for the suppression of the backward electron transfer reaction without changing the conduction band energy of TiO2. 12.0% conversion efficiency, which is the second highest efficiency for DSCs based on ruthenium sensitizers, was achieved in the cosensitized DSC with Black dye and D131 by using an electrolyte solution containing a moderate concentration of tetraheptylammonium iodide.

Novel ruthenium sensitizers having different numbers of carboxyl groups for dye-sensitized solar cells: effects of the adsorption manner at the TiO2 surface on the solar cell performance.

Two novel ruthenium sensitizers having multiple carboxyl groups ((TBA)[Ru{4'-(3,4-dicarboxyphenyl)-4,4″-dicarboxyterpyridine}(NCS)3] (TUS-21) and (TBA)[Ru{4'-(3-carboxyphenyl)-4,4″-dicarboxyterpyridine}(NCS)3] (TUS-37); TBA = tetrabutylammonium) have been synthesized as improved model sensitizers for the previously reported ruthenium sensitizer TUS-20 ((TBA)[Ru{4'-(3,4-dicarboxyphenyl)terpyridine}(NCS)3]). The absorption maxima of two MLCT bands and the absorption onsets of TUS-21 and TUS-37 were shifted to longer wavelengths of about 30 nm in comparison to those of TUS-20 by introducing a carboxyl group to the each terminal pyridine ring of the terpyridine ligand. TUS-21 and TUS-37 showed quite similar adsorption behaviors to the TiO2 surface, and this adsorption behavior was found to be different from that of TUS-20. ATR-IR measurements revealed that TUS-21 and TUS-37 bind to the TiO2 surface by using two carboxyl groups at the 3-position of the phenyl ring and at one of the terminal pyridine rings of the terpyridine ligand, while TUS-20 is reported to bind by using two carboxyl groups at the 3,4-dicarboxyphenyl unit. The dye-sensitized solar cell (DSC) with TUS-21 exhibited 10.2% conversion efficiency, which is much higher than that of the DSC with TUS-20 (7.5%), under AM 1.5 (100 mW/cm(2)) irradiation.

Characterization of photovoltaic performance of the dye-sensitized solar cell with a novel ruthenium complex having a bisdemethoxycurcumin as a ligand.

The first example of a ruthenium sensitizer (TUS-22) having a natural dye, bisdemethoxycurcumin, as a ligand has been synthesized. The dye-sensitized solar cell based on this novel dye showed 5.8% conversion efficiency under AM 1.5 (100 mW/cm(2)) irradiation.

Highly efficient plastic substrate dye-sensitized solar cells using a compression method for preparation of TiO(2) photoelectrodes.

The efficiency of a plastic-substrate dye-sensitized solar cell was much improved by a new method consisting of a press method without heat treatment, light confinement effect of TiO(2) film and water-based TiO(2) paste; this device shows the highest light-to-electrical energy conversion efficiency based on plastic-substrate dye-sensitized solar cells, 7.4% under 100 mW cm(-2) (1 sun) AM1.5 illumination.

Lithium ion effect on electron injection from a photoexcited coumarin derivative into a TiO2 nanocrystalline film investigated by visible-to-IR ultrafast spectroscopy.

The dynamics of ultrafast electron injection from a coumarin derivative (NKX-2311), which is an efficient photosensitizer for dye-sensitized solar cells, into the conduction band of TiO(2) nanocrystalline films have been investigated by means of femtosecond transient absorption spectroscopy in a wide wavelength range from 600 nm to 10 mum. In the absence of Li(+) ions, electron injection into the TiO(2) conduction band occurred in about 300 fs. In the presence of Li(+) ions, however, electron injection occurred within approximately 100 fs, and the oxidized dye generated was found to interact with nearby Li(+) ions. Possible positions of Li(+) ion attachment to the dye molecule were examined by means of semiempirical molecular orbital calculations. The electron injection efficiency was found to increase by a factor of 1.37 in the presence of Li(+) ions. The effects of Li(+) ions on the energy of the TiO(2) conduction band and the electronic interaction between the dye molecule and Li(+) ions are discussed, and the major cause for the acceleration of electron injection was suggested to be a conduction-band shift of TiO(2).

Photophysical and (photo)electrochemical properties of a coumarin dye.

A new coumarin dye, cyano-{5,5-dimethyl-3-[2-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)vinyl]cyclohex-2-enylidene}-acetic acid (NKX-2753), was prepared and characterized with respect to photophysical and electrochemical properties. It was employed as a dye sensitizer in dye-sensitized solar cells and showed efficient photon-to-electron conversion properties. The photocurrent action spectrum exhibited a broad feature with a maximum incident photon-to-electron conversion efficiency (IPCE) of 84% at 540 nm, which is comparable to that for the famous red dye RuL2(NCS)2 (known as N3), where L stands for 2,2'-bipyridyl-4,4'-dicarboxylic acid. The sandwich-type solar cell with NKX-2753, under illumination of full sun (AM1.5, 100 mW cm(-2)), produced 16.1 mA cm(-2) of short-circuit photocurrent, 0.60 V of open-circuit photovoltage, and 0.69 of fill factor, corresponding to 6.7% of overall energy conversion efficiency using 0.1 M LiI, 0.05 M I2, 0.1 M guanidinium thiocyanate, and 0.6 M 1,2-dimethyl-3-n-propyl-imidazolium iodide in dry acetonitrile as redox electrolyte. In comparison with its analogue NKX-2586 (Langmuir 2004, 20, 4205), NKX-2753 with an extra side ring on the alkene chain produced much higher IPCE values at the same conditions. The side ring acted as a spacer to efficiently prevent dye aggregation when adsorbed on the TiO2 surface, resulting in significant improvements of short-circuit photocurrent, open-circuit photovoltage, and fill factor compared with NKX-2586 that aggregated on the TiO2 surface.

Ruthenium sensitizers with a hexylthiophene-modified terpyridine ligand for dye-sensitized solar cells: synthesis, photo- and electrochemical properties, and adsorption behavior to the TiO2 surface.

Two novel ruthenium sensitizers with a hexylthiophene-modified terpyridine ligand (TUS-35 and TUS-36) were synthesized to improve the molar absorptivity of the previously reported ruthenium sensitizer (TBA)[Ru{4'-(3,4-dicarboxyphenyl)-4,4″-dicarboxyterpyridine}(NCS)3], TBA = tetrabutylammonium (TUS-21). A relatively strong absorption appeared at ∼380 nm, and the molar absorption coefficient at the metal-to-ligand charge transfer (MLCT) band decreased in TUS-35 by introducing a 2-hexylthiophene unit to the 5-position of the terpyridine-derived ligand. For comparison, a relatively strong absorption was observed at ∼350 nm without decreasing the molar absorption coefficient at the MLCT band in TUS-36 by introducing a 2-hexylthiophene unit to the 4-position of the terpyridine-derived ligand. On the other hand, the energy levels of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of these two sensitizers were found to be almost equal to those of TUS-21. The adsorption behavior of TUS-35 and TUS-36 was similar to that of (TBA)[Ru{4'-(3,4-dicarboxyphenyl)terpyridine}(NCS)3] (TUS-20), which binds to the TiO2 surface by using the 3,4-dicarboxyphenly unit, rather than that of TUS-21, which adsorbs to the TiO2 photoelectrode using one of the carboxyl groups at the terminal pyridines of the terpyridine-derived ligand. Therefore, TUS-35 and TUS-36 are considered to bind to the TiO2 surface by using the 3,4-dicarboxyphenly unit just like TUS-20. The dye-sensitized solar cells (DSCs) with TUS-35 and TUS-36 showed a relatively lower conversion efficiency (6.4% and 5.7%, respectively) compared to the DSC with TUS-21 (10.2%). Open-circuit photovoltage decay and electrochemical impedance spectroscopy measurements revealed that the promoted charge recombination and/or charge transfer of the injected electrons in the TiO2 photoelectrode is a main reason for the inferior performances of TUS-35 and TUS-36.

Novel near-infrared carboxylated 1,3-indandione sensitizers for highly efficient flexible dye-sensitized solar cells.

Three novel metal-free organic dyes (DN458, DN475 and DN484) were designed for use in plastic-substrate dye-sensitized solar cells (PDSCs). The photoelectric conversion region of DN475 was successfully expanded into the near-infrared region. As a result, an energy conversion efficiency of 5.76% was achieved.

Novel polyene dyes for highly efficient dye-sensitized solar cells.

We have developed an efficient and novel polyene-dye-sensitized nanocrystalline TiO2 solar cells producing a 6.8% solar energy-to-electricity conversion efficiency (eta) under AM 1.5 irradiation (100 mW cm(-2)): short-circuit current density (Jsc), 12.9 mA cm(-2), open-circuit photovoltage (Voc), 0.71 V, fill factor (ff), 0.74.

Photoelectrochemical decomposition of water on nanocrystalline BiVO4 film electrodes under visible light.

The nanocrystalline BiVO4 film electrode on conducting glass showed an excellent efficiency (IPCE = 29% at 420 nm) for the decomposition of water under visible light.

Ruthenium sensitizers having an ortho-dicarboxyl group as an anchoring unit for dye-sensitized solar cells: synthesis, photo- and electrochemical properties, and adsorption behavior to the TiO2 surface.

A novel ruthenium sensitizer ((TBA)[Ru(3',4'-dicarboxyterpyridine)(NCS)3], TBA = tetrabutylammonium, TUS-28) has been synthesized as an improved model sensitizer for (TBA)[Ru(4'-(3,4-dicarboxyphenyl)terpyridine)(NCS)3] (TUS-20). The molar absorptivity of TUS-28 in the whole visible region was smaller than that of TUS-20 due to the absence of the phenyl ring at the terpyridine ligand. On the other hand, the energy levels of HOMO and LUMO of TUS-28 were still suitable for effective electron transfer reactions in dye-sensitized solar cells (DSCs). TUS-28 did not show a superior adsorptivity to the TiO2 surface just like TUS-20, even though TUS-28 has an ortho-dicarboxyl group which is reported to be an efficient anchoring unit. ATR-IR measurements revealed that both carboxyl groups of TUS-28 participate in binding to the TiO2 surface just like TUS-20. Therefore, steric hindrance between the hydrogen atom at the 4-position of 3',4'-dicarboxyterpyridine and the OH group at the TiO2 surface seems to retard the rapid adsorption, and to weaken the binding strength of TUS-28. The DSC with TUS-28 showed 8.2% conversion efficiency, which is higher than that of TUS-20 (7.5%). The J(sc) value of the DSC with TUS-28 was larger than that of TUS-20 even though the amount of dye adsorption of TUS-28 was smaller than that of TUS-20. A more effective electron injection reaction is considered to contribute mainly to the efficiency improvement of TUS-28 since the path length of the electron injection from TUS-28 to the conduction band of the TiO2 is shorter than that of TUS-20 due to the absence of the phenyl ring.

A new cosensitization method using the Lewis acid sites of a TiO2 photoelectrode for dye-sensitized solar cells.

Co-sensitized dye-sensitized solar cells using black dye and a pyridine-anchor dye (NI5 or YNI-2) showing site-selective adsorption behaviour at the TiO2 surface have been prepared for the first time to reduce the competitive adsorption between the two dyes.

Novel ruthenium sensitizers with a dianionic tridentate ligand for dye-sensitized solar cells: the relationship between the solar cell performances and the electron-withdrawing ability of substituents on the ligand.

Five novel ruthenium sensitizers (TUS sensitizers) with a dianionic tridentate ligand (pyridine-2,6-dicarboxyamidato and its derivatives) have been synthesized for application to dye-sensitized solar cells (DSCs). These TUS sensitizers have much larger molar absorption coefficients in the wavelength range below 600 nm compared with those of Black dye which is a structural analog and a highly efficient ruthenium sensitizer. The energy levels of HOMOs and LUMOs of TUS sensitizers shifted to the positive direction with increasing the electron-withdrawing ability of the substituents on the dianionic tridentate ligand. The energy levels of HOMO and LUMO showed linear correlation with respect to the Hammett constant of the substituents. The DSCs with TUS sensitizers showed much lower performances than that of Black dye. Both inferior adsorptivity on the TiO2 surface and unfavorable energy levels of HOMOs and LUMOs for the effective electron transfer reactions in the DSCs are considered to be the main reasons for the much lower performances of TUS sensitizers. The conversion efficiency of the DSC with a TUS sensitizer increased with increasing the electron-withdrawing ability of the substituents on the dianionic tridentate ligand. The observed linear relationship between the conversion efficiency and the driving force of the reduction process of the oxidized form of dyes by I(-) suggests that the dye regeneration process is a rate-determining step in the DSCs with TUS sensitizers.

Effective enhancement of the performance of black dye based dye-sensitized solar cells by metal oxide surface modification of the TiO2 photoelectrode.

Effective enhancement of the performance of black dye based dye-sensitized solar cells has been achieved by MgO or Al(2)O(3) surface modification of the TiO(2) photoelectrode. The conversion efficiency was improved from 10.4% to 10.8% due to the blocking effect of the thin overlayer at the TiO(2) surface.

Effects of dye-adsorption solvent on the performances of the dye-sensitized solar cells based on black dye.

The effects of the dye-adsorption solvent on the performances of the dye-sensitized solar cells (DSSCs) based on black dye have been investigated. The highest conversion efficiency (10.6 %) was obtained in the cases for which 1-PrOH and the mixed solvent of EtOH and tBuOH (3:1 v/v) were employed as dye-adsorption solvents. The optimized value for the dielectric constant of the dye-adsorption solvent was found to be around 20. The DSSCs that used MeOH as a dye-adsorption solvent showed inferior solar-cell performance relative to the DSSCs that used EtOH, 1-PrOH, 2-PrOH, and 1-BuOH. Photo- and electrochemical measurements of black dye both in solution and adsorbed onto the TiO(2) surface revealed that black dye aggregates at the TiO(2) surface during the adsorption process in the case for MeOH. Both the shorter electron lifetime in the TiO(2) photoelectrode and the greater resistance in the TiO(2)-dye-elecrolyte interface, attributed to the dye aggregation at the TiO(2) surface, cause the decrease in the solar-cell performance of the DSSC that used MeOH as a dye adsorption solvent.

Synthesis of novel beta-diketonate bis(bipyridyl) Os(II) dyes for utilization of infrared light in dye-sensitized solar cells.

Bis(bipyridyl) osmium(II) dyes having a beta-diketonate ligand were synthesized in order to utilize visible and infrared light for dye-sensitized solar cells. Compared to black dye, under AM 1.5 irradiation (100 mW cm(-2)), DSCs using our novel Os dyes exhibited a better spectral response in the near-IR region (by 1050 nm) and showed a higher J(SC) value of 23.7 mA cm(-2).

Synthesis of tubular titanate via a self-assembly and self-removal process.

Novel morphology titanate tubes were successfully synthesized via a self-assembly and self-removal process. After the product was treated by calcinating and washing, crystalline TiO2 tubes were obtained. In this study, two new concepts are applied to design the synthetic route: (i) titanium glycolate rods obtained from an ethylene glycol-mediated process and titanate sheets synthesized using the hydrothermal process were used as the template and the precursor, respectively, and ii) the template was directly removed in the reaction without posttreatment. Furthermore, a possible mechanism was proposed for the formation of tubular structures.

Significant efficiency improvement of the black dye-sensitized solar cell through protonation of TiO2 films.

This paper describes the influence of acid pretreatment ofTiO2 mesoporous films prior to dye sensitization on the performance of dye-sensitized solar cells based on [(C4H9)4N]3[Ru(Htcterpy)(NCS)3] (tcterpy = 4,4',4"-tricarboxy- 2,2',2"-terpyridine), the so-called black dye. The HCl pretreatment caused an increase in overall efficiency by 8%, with a major contribution from photocurrent improvement. It is speculated, from the analysis of incident photon-to-electron conversion efficiency, UV-vis absorption spectra, redox properties of the dye and TiO2, and the impedance spectra of the dye-sensitized solar cells, that photocurrent enhancement is attributed to the increases in electron injection and/or charge collection efficiency besides the improvement of light harvesting efficiency upon HCl pretreatment. Open-circuit photovoltage (V(oc)) remained almost unchanged in the case of significant positive shift of flat band potential for TiO2 upon HCl pretreatment. The suppression of electron transfer from conduction band electrons to the I3- ions in the electrolyte upon HCl pretreatment, reflected by the increased resistance at the TiO2/dye/electrolyte interface and reduced dark current, resulted in a V(oc) gain, which compensated the V(oc) loss due to the positive shift of the flat band. Using the HCl pretreatment approach, 10.5% of overall efficiency with the black dye was obtained under illumination of simulated AM 1.5 solar light (100 mW cm(-2)) using an antireflection film on the cell surface.

Synthesis of single-crystal manganese dioxide nanowires by a soft chemical process.

α- and ß-MnO(2) single-crystal nanowires have been successfully synthesized through a soft chemical process, which involves no catalysts or templates. The diameters of the nanowires are about 10-40 nm and the lengths up to several tens of micrometres. Experimental results showed that MnO(2) polymorphic forms can be synthesized by selected-control reaction temperatures and their crystal phases in the nanowires were confirmed by XRD and TEM measurement. Further, the formation process of nanowires is discussed.