Ru-based Dye Research Articles

Anchoring Modes of Ru-Based N719 Dye onto Titania Substrates

Anchoring Modes of Ru-Based N719 Dye onto Titania Substrates

Homo-tandem-bifacial dye-sensitized solar cell: A new paradigm to boost photoconversion efficiency above limit

Over the last three decades, dye-sensitized solar cells (DSSCs) have received a lot of attention, reaching record efficiencies under AM 1.5G illumination slightly over 10%. However, these values are lower than other competing photovoltaic systems. The performance of DSSCs has to be further enhanced in order to have a substantial influence and application potential in the field of photovoltaics. Herein we propose a new intriguing strategy to take advantage of two concepts well known in the field of photovoltaics, such as the tandem configuration and the bifaciality, never applied at the same time to DSSCs. Thanks to the combination of these strategies, we are noteworthy able to overcome the maximum efficiency ever obtained for a traditional DSSC based on iodide/triiodide redox couple and Ru-based dye, as high as 14.96%. Our findings open the way to further enhancement with optimized DSSCs configuration.

Hybrid solar cells of Ru-based dye complexes as interfacial modification layers: Energy level alignment and photoelectric properties improvement

The chemical incompatibility at the organic-inorganic heterojunction interface in hybrid solar cells is a serious issue that cannot be ignored. In this work, four Ru-based dye complexes (D-series organic dyes) were employed for the interfacial properties modulation. The results showed that the introduction of organic molecules can not only improve the compatibility between the organic and inorganic materials, but also provide an appropriate energy levels alignment of the materials in the hybrid films. A notable power conversion efficiency of 2.91% was achieved. The intrinsic factors that affecting the cells performance were investigated by electrochemical impedance and open circuit voltage decay measurements.

Inhibiting formation of Zn2+/N179 molecules insulation layer and degradation of ZnO-based dye-sensitized solar cells via quasi-solid-state electrolytes

Abstract ZnO dye-sensitized solar cells (DSSCs) have been deemed as one of promising solar devices. However, ZnO DSSCs with liquid electrolyte always surfer from dissolution of ZnO film and formation of ZnO2+/Ru-based dye molecules insulation layer, thus weakens conversion efficiency and long-tern stability of the devices. To overcome these obstacles, quasi-solid-state iodine-based electrolytes based on PVDF-HFP, and filled with functionalized multi-walled carbon nanotube (FMWCNT) improve its ionic conductivity via increasing charge transport channels and free volume of iodine/tri-iodine. It is found that optimal conversion efficiency of 3.87% was achieved in ZnO nanosheets (NSs) DSSC with 0.5 wt%-FMWCNT quasi-solid-state electrolyte. Namely, it achieves approximate conversion efficiency of the DSSC with typical liquid iodine-based electrolyte (3.94%) under ~ half of ionic conductivity of liquid electrolyte. Moreover, this device remains 86.65% of original conversion efficiency after 1008 h, which is higher than that of the device with liquid electrolyte (50.37%). The result confirms that quasi-solid-state electrolyte inhibits dissolution of ZnO film and formation of ZnO2+/N719 molecules insulation layer.

Metal–Phosphate Bilayers for Anatase Surface Modification

Compared to many other metal oxides, anatase TiO2 shows relatively lower reactivity toward carboxylic acid anchor groups. The latter is crucial for applications, for example, in dye-sensitized solar cells (DSSCs), where the most used dyes bind to the metal oxide surface through carboxylic acid terminations. To improve the surface reactivity, metal-phosphate bilayers of Ni or Co were synthesized on anatase TiO2 compact oxide and nanotubes. In both cases, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) results showed that the bilayers were successfully formed and that the phosphate layer works as an intermediate between TiO2 and the other species. ToF-SIMS depth profiles of modified nanotubes showed that Ni and Co are present through the whole tube length and reduce in content after heat treatment, in agreement with XPS results. Phosphate groups, on the other hand, are more present in the tubes' depth, and their content on the surface is reduced upon exposure to temperature. The reactivity of the modified surfaces toward carboxylic acid-terminated molecules, as stearic acid and Ru-based N719 dye, was evaluated. Contact angle measurements together with dye desorption experiments demonstrated that the Co-phosphate bilayers heat-treated at 300 °C resulted in the largest enhancement compared to the reference. Bilayer-modified compact anatase TiO2 and anatase TiO2 nanotubes were utilized as photoanodes in DSSCs. An increase in efficiency was observed for all modified electrodes with phosphate-Co treatment, leading to the highest JSC values and an efficiency improvement of 48%.

Comparative spectroscopic approach for the dye loading optimization of sheet-like ZnO photoanodes for dye-sensitized solar cells

The peculiarly critical sensitization of zinc oxide with Ru-based dyes is the serious problem that prevents this semiconductor oxide from achieving photoconversion efficiencies in dye-sensitized solar cells (DSCs) comparable to those obtained with TiO2. Within this framework, methodologies for the optimization of the sensitization procedure of this material in acidic environments prove to be essential. In this paper, in view of streamlining the critical sensitization procedure of sheet-like zinc oxide-based DSC photoanodes with Ru-based dye, a comparative study on active and passive evaluations of N719 dye loading on sheet-like ZnO nanostructures are reported. For the active in-situ spectroscopic approach, a home-made set-up has been appropriately designed and developed for real time monitoring the dye adsorption on the ZnO-based photoanodes, whereas the standard passive dye loading measurements were performed after the electrodes had been sensitized for different time periods in the dye solutions of different concentrations. The dye adsorption results are carefully analyzed and correlated to the photovoltaic conversion efficiency and the incident photon-to-electron conversion efficiency (IPCE) of the corresponding ZnO-based DSCs, fabricated by employing our customized microfluidic architecture. The obtained results show that the real time monitoring of the dye absorption can be successfully employed for optimizing the sensitization conditions of ZnO-based photoanodes for DSCs. When applied online in a large scale production system for dye-sensitized solar cells, the proposed active method will permit to save both time and materials.

Microscopic observation of dye molecules for solar cells on a titania surface.

The lateral distribution and coverage of Ru-based dye molecules, which are used for dye-sensitized solar cells (DSCs), were directly examined on a titania surface using high-resolution scanning transmission electron microscopy (STEM). The clean surface of a free-standing titania nanosheet was first confirmed with atomic resolution, and then, the nanosheet was used as a substrate. A single dye molecule on the titania nanosheet was visualized for the first time. The quantitative STEM images revealed an inhomogeneous dye-molecule distribution at the early stage of its absorption, i.e., the aggregation of the dye molecules. The majority of the titania surface was not covered by dye molecules, suggesting that optimization of the dye molecule distribution could yield further improvement of the DSC conversion efficiencies.

High performance dye-sensitized solar cell from a cocktail solution of a ruthenium dye and metal free organic dye

A Ru-based dye K-60 and a metal free D–A organic dye Y1 have been employed for the fabrication of a mixed dye sensitized solar cell (MDSSCs) system.

Novel D-π-A Organic Dyes with Thieno[3,2-b]thiophene-3,4-ethylenedioxythiophene Unit as a π-Bridge for Highly Efficient Dye-Sensitized Solar Cells with Long-Term Stability

This paper reports on new D-π-A organic dyes for application in dye-sensitized solar cells (DSSCs), which were developed by incorporating thieno[3,2-b]thiophene-thiophene (M9) and thieno[3,2-b]thiophene-EDOT (M10) as π-bridges. These dyes exhibited relatively small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps in spite of the short π-conjugation lengths, resulting in broad spectral responses. As photosensitizers in DSSCs, M10 showed a broader spectral response than M9, leading to a greater short-circuit photocurrent (Jsc). In addition, M10 exhibited higher open-circuit voltage (Voc) compared to M9, because of the greater electron lifetime of the photoanode. The impedance analysis revealed that the greater electron lifetime of the photoanode with M10 was attributed to the lower electron recombination rate caused by the blocking effect of the bulky EDOT unit. As a result, M10 showed much higher conversion efficiency (η = 7.00%) than M9 (η = 5.67%) under one sun condition (AM 1.5 G, 100 mW/cm(2)). This conversion efficiency was comparable to that of the conventional Ru-based dye N719 (η = 7.24%) under the same condition. In addition, M10 exhibited a remarkable long-term stability, i.e., 95% of the initial conversion efficiency was maintained after light soaking for 45 days (1080 h).

Covalent Modification of Photoanodes for Stable Dye-Sensitized Solar Cells

This paper describes the surface modification of TiO2 with 3-aminopropyltriethoxysilane (APTES) followed by covalent attachment of Ru-based N719 dye molecules to TiO2 through an amide linkage for use as photoanodes (PAs) in dye-sensitized solar cells (DSSCs). Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) confirms the surface chemistry between the TiO2 and dye. The photovoltaic efficiency of DSSCs with covalently linked dye is very similar (6-7%) to that of traditionally prepared DSSCs prepared by direct immersion when both have similar dye coverage. Importantly, the efficiency of PAs with covalently linked dye did not change after storage for more than 60 days in air, whereas the traditionally prepared PAs decreased dramatically after 1 day and lost most of their efficiency after a week. FTIR and UV-vis characterization of the dye suggests that covalent linkage improves stability by preventing the loss of the thiocyanato ligands and/or tetrabutylammonium cations on the dye. PAs with covalently linked dye are also more stable toward water, acid, heat, and UV light compared to traditionally prepared PAs and are more stable compared to other modified PAs with dye attached through electrostatic or hydrogen-bonding interactions.

Electronic structure of Fe- vs. Ru-based dye molecules

In order to explore whether Ru can be replaced by inexpensive Fe in dye molecules for solar cells, the differences in the electronic structure of Fe- and Ru-based dyes are investigated by X-ray absorption spectroscopy and first-principles calculations. Molecules with the metal in a sixfold, octahedral N cage, such as tris(bipyridines) and tris(phenanthrolines), exhibit a systematic downward shift of the N 1s-to-π* transition when Ru is replaced by Fe. This shift is explained by an extra transfer of negative charge from the metal to the N ligands in the case of Fe, which reduces the binding energy of the N 1s core level. The C 1s-to-π* transitions show the opposite trend, with an increase in the transition energy when replacing Ru by Fe. Molecules with the metal in a fourfold, planar N cage (porphyrins) exhibit a more complex behavior due to a subtle competition between the crystal field, axial ligands, and the 2+ vs. 3+ oxidation states.

ZnO Nanostructured Diodes - Enhancing Energy Generation through Scavenging Vibration

ABSTRACTRecent developments on the use of the piezoelectric effect in ZnO nanorod-based p-n junctions for energy harvesting applications are presented. Two types of junctions are used. The first is a hybrid p-n device combining the semiconducting polymer poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) with ZnO nanorods. The second type of junction is an all-inorganic junction between n-type ZnO nanorods and p-type CuSCN. It is shown that both these diodes can be produced on flexible plastic substrates, which generate a voltage output when bent. The voltage output of the ZnO/PEDOT:PSS diodes are measured across a range of resistive loads while bending to find a maximum power point of 12 μWcm-2 at 4 kΩ. It is shown that a voltage output is also generated when this structure is vibrated acoustically. The ZnO/CuSCN diode is sensitized to sunlight with a Ru-based dye to form a photovoltaic device. It is shown that the device efficiency can be increased by application of acoustic vibrations. This is attributed to the electric field generated by the piezoelectric effect in ZnO affecting the charge-carrier recombination at the ZnO surface.

Gallium-doped indium oxide nanoleaves: Structural characterization, growth mechanism and optical properties

The novel two-dimensional (2-D) Ga-doped In2O3 nanoleaves are synthesized by a simple one-step carbonthermal evaporation method using Cu–Sn alloy as the substrates. Two basic parts construct this leaf-like nanostructure: a long central trunk and two tapered nanoribbons in symmetric distribution in relation to the trunk. The Ga–In–O alloy particles are located at or close to the tips of the central trunks and serve as catalysts for the central trunk growth by the self-catalytic vapor–liquid–solid (VLS) mechanism. And the homoepitaxial growth of tapered nanoribbon on the surface of the central trunk can be explained by vapor–solid (VS) mechanism. The room-temperature photoluminescence (PL) measurement of this nanoscaled Ga-doped In2O3 transparent conducting oxide (TCO) detected two blue peaks located at 432nm and 481nm, respectively, which can be used by Ru-based dye and indicates potential application in dye-sensitized solar cells (DSSCs). The successful preparation of this novel 2-D Ga-doped In2O3 nanoleaves not only enriches the synthesis of TCO materials, but also provides new blocks in future architecture of functional nano-devices.

Effect of ethylene glycol on the growth of hexagonal SnS 2 nanoplates and their optical properties

Hexagonal SnS 2 particles were synthesized via a solvothermal method using a mixture of ethylene glycol (EG) and distilled water as solvent. Hexagonal SnS 2 nanoplates of more regular morphology were obtained when the volumetric ratio of EG to distilled water (EG:H 2O) decreased from 4:1 to 1:4. The effect of EG on the growth of hexagonal SnS 2 nanoplates was investigated and a growth restraint mechanism in [0 0 1] was proposed. A large band gap of 3.52 eV of the hexagonal SnS 2 nanoplates may facilitate electron injection from photo-excited dye molecules in dye-sensitized solar cells (DSSCs). A photoluminescence (PL) peak at 761 nm under excitation at 507 nm was also observed in the hexagonal SnS 2 nanoplates. The 761 nm emission, which is within the absorption band of the Ru-based dye, is expected to make sufficient utilization of solar energy in DSSCs.

Pseudo First-Order Adsorption Kinetics of N719 Dye on TiO2 Surface

We have investigated the adsorption kinetics of Ru-based N719 dye on TiO(2) surface in dye-sensitized solar cell using 0.5 mM and 5 mM dye solutions. The amount of adsorbed dye on TiO(2) surface of ca. 5 μm-thick film was measured as a function of immersion (adsorption) time. The amount of adsorbed dye increases with increasing the adsorption time and keeps constant after saturation. Completion of dye adsorption is found to be more than 5 times faster in 5 mM than in 0.5 mM. Since the change of dye concentration is negligible compared to that of number of TiO(2) adsorption site, reaction order and rate constant can be estimated from a pseudo reaction. Among the zeroth-, first-, and second-order simulation, the observed data follow first order reaction for both 0.5 mM and 5 mM cases. The rate constant is estimated to be 0.504 min(-1) for 5 mM and 0.094 min(-1) for 0.5 mM, which indicates that completion of dye adsorption is about 5 times shorter in 5 mM than in 0.5 mM. This is consistent with the observed adsorption time difference. Except for the difference in adsorption kinetics, best cell efficiency is similar regardless of dye solution concentration.

Flexible dye sensitized solar cells using TiO2 nanotubes

The growth of TiO2 nanotube arrays on plastic flexible substrates is researched. The approach uses anodization of a titanium thick film for obtaining nanotubes directly on poly(ethylene terephthalate) (PET) and Kapton HN substrate. The morphological features of the tubes can be finely tuned by varying the preparation conditions, and tube morphology affects the functional properties of the nanotube array. Crystallization of the anatase phase in nanotubes on Kapton HN substrate is obtained via post growth annealing. The nanotube arrays have been dye-sensitized using the commercial Ru-based N719 dye. The system was tested as photoanode in a flexible dye sensitized solar cell. Photoconversion efficiency of 3.5% was obtained.

Low-temperature TiO2 Films for Dye-sensitized Solar Cells: Factors Affecting Energy Conversion Efficiency

Semiconductor films prepared by electrostatic layer-by-layer deposition can be used to fabricate dye-sensitized solar cells after low-temperature treatment (150 °C). However, the resulting photocurrent is much less than when the film is sintered at 500 °C. The difference in short-circuit current is a factor of 2.2 with the Ru-based dye N719 and is 3.5 with the organic dye D5. The photocurrent at a given wavelength is proportional to the light-harvesting efficiency, charge injection effciency, and charge collection efficiency. Sintered films take up more than 60% more of either dye than unsintered films and therefore absorb more photons. Electron injection is hindered in unsintered films due to a conduction band edge potential 100 mV more negative than in a sintered electrode. Additional injection effects could be due to adsorption of the dye to polymer rather than to TiO2 in unsintered films, although our measurements were inconclusive on this point. Kinetic studies show electron transport times (τtr) an order of magnitude faster then electron lifetimes (τe) in both sintered and unsintered electrodes. Furthermore, a Li+ insertion experiment shows that both films have good electrical connectivity between TiO2 nanoparticles. Unsintered films thus exhibit efficient charge transport despite the presence of polymer and the lack of heat treatment to induce necking.

Fabrication of MgO-coated TiO2 nanotubes and application to dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) are more spotlighted than conventional photovoltaic devices due to their relatively low cost, easy fabrication and high efficiency. However, there are limitations to increase the conversion efficiency of DSCs. The limiting factors are the quantity of dye adsorption and charge recombination between TiO2 electrode and electrolyte. Coating other materials such as high energy band gap insulators or semiconductors on the TiO2 electrode enhances dye adsorption and reduces charge recombination. We fabricated DSCs based on bare TiO2 nanotube arrays and 0.02 and 0.04 M MgO coated TiO2 nanotube arrays. MgO layer increased the photovoltage and photocurrent. The overall conversion efficiency of DSCs using 0.02 M MgO coated TiO2 nanotubes was 1.61%. MgO formed insulating layers between TiO2 nanotube array electrode and electrolyte. Charge recombination was inhibited at the interfaces of TiO2 nanotube array electrode and electrolyte by MgO insulating layers. MgO coating also improved dye adsorption because iso-electric point (IEP) of MgO was larger than TiO2. When the IEP of coating material is larger than TiO2, the chemical attraction between the electrode surface and Ru-based dye molecule is increased.

Charge Dynamics following Dye Photoinjection into a TiO2 Nanocrystalline Network

Electrons can be injected into TiO2 nanocrystals by a surface-adsorbed Ru-based dye in a well-known photosensitization process. Here we present time-resolved microwave conductivity measurements of the decay of electrons out of an electrically interconnected network of such TiO2 nanocrystals, following charge injection, and in the presence of an electron donor to the dye cation that prevents direct recombination. The time scale for this decay process is hundreds of milliseconds to seconds and explains the high current collection efficiency in devices based on these materials, since the long time scale allows for slow charge transport through the network.

Mechanisms of Instability in Ru-Based Dye Sensitization Solar Cells

In-situ infrared studies performed with operating Ru-complex-sensitized wet solar cells using a total reflection technique reveal that the ruthenium complex (both tri- and mononuclear) attached to TiO2 is photoelectrochemically transformed and irreversibly consumed under conditions of insufficient regeneration by iodide or from the oxide within the nanocrystalline TiO2 pores. The sensitizer [(Ru(bpy)2(CN)2)2Ru(bpca)2]2- (bpy is 2,2‘-bipyridine, bpca is 2,2‘-bipyridine-4,4‘-dicarboxylate) decomposes into fragments; one of them was identified to be Ru(bpy)2(CN)2. For the sensitizer Ru(bpca)2(SCN)2, it is shown that a molecular fragment (absorbing at 2013 cm-1) is generated which is diffusing out of the nanostructured TiO2 layer. Due to its correlation with the photocurrent density, it is identified as a product of the oxidized sensitizer. Due to a high serial resistance introduced by the total reflection element and the resulting low fillfactor of the sensitization cell during in-situ measurements, only sma...