TiO2 Solar Cells Research Articles

Impedance Analysis of CdSe Quantum Dot-Sensitized TiO2 Solar Cells Decorated with Au Nanoparticles and P3OT

Electrochemical impedance spectroscopy was used to analyze 10 μm thickness TiO2 films containing 250 nm TiO2 nanoparticles (NPs) with anatase crystalline phase deposited on FTO substrates. The TiO2 films were sensitized with CdSe quantum dots (QDs) (4.5 nm) and decorated with Au nanoparticles (NPs) (7 nm) and poly-3-octylthiophene (P3OT) in three different configurations, namely TiO2/QDs, TiO2/Au/QDs, and TiO2/Au/QDs/P3OT. The characteristic absorption bands for each component, TiO2, P3OT, CdSe QDs, and Au NPs, are centered at 338 nm (3.67 eV), 459 nm (2.7 eV), 552 nm (2.24 eV), and 517 nm (2.40 eV), respectively. The UV-Vis spectrum of the composite film TiO2/Au/QDs/P3OT shows clear absorption peaks from 338 to 700 nm. Equivalent circuits were proposed for the different configuration interfaces to fit experimental impedances. The results suggest that the introduction of Au NPs into the TiO2/Au/QDs films strongly reduces resistance to the electron flux from QDs to TiO2 and increases the overall charge transport. Meanwhile, P3OT in the TiO2/Au/QD/P3OT films promotes hole transport and increases the capacitance with the electrolyte, reducing the electron leakage and improving the fill-factor and the final light energy conversion efficiency.

Research Update: Doping ZnO and TiO2 for solar cells

ZnO and TiO2 are two of the most commonly used n-type metal oxide semiconductors in new generation solar cells due to their abundance, low-cost, and stability. ZnO and TiO2 can be used as active layers, photoanodes, buffer layers, transparent conducting oxides, hole-blocking layers, and intermediate layers. Doping is essential to tailor the materials properties for each application. The dopants used and their impact in solar cells are reviewed. In addition, the advantages, disadvantages, and commercial potential of the various fabrication methods of these oxides are presented.

Synthesis and Characterization of Cds Sensitized TiO2 Solar Cell

Synthesis and Characterization of Cds Sensitized TiO2 Solar Cell

Conversion efficiency enhancement of CdS quantum dot-sensitized electrospun nanostructured TiO2 solar cells by organic dipole treatment

Here we fabricated quantum dot-sensitized solar cells (QDSCs) based on electrospinning of unique TiO2 nanostructures and subsequent CdS QDs deposition via successive ionic layer adsorption and reaction (SILAR) followed by dipole treatment (DT). It was found that by treating with 4-methoxy benzenethiol for 24h, an overall conversion efficiency of 1.17% was achieved under AM1.5 illumination which corresponds to a dramatic 100% enhancement compared with that of untreated cells. The significant photovoltaic improvement was attributed to the upward shifting of CdS QDs energy levels for efficient charge separation and effective electron transport in electrospun TiO2 nanostructures which is confirmed by electrochemical impedance spectroscopy.

Co-Sensitized Mesoporous TiO2Solar Cells: Hybrid Sensitizer of SILAR-Grown CdS Quantum Dot (QD) and Molecular Dye (Z907) with a Metal Oxide Interlayer

E-mail: dwchang@cu.ac.krReceived May 18, 2013, Accepted August 16, 2013Key Words : CdS Quantum dot, Metal oxide interlayer, Hybrid sensitizer, Solar cellDuring the past two decades, mesoporous metal oxidefilm-based solar cells sensitized by molecular dyes orinorganic semiconductors have advanced greatly in terms ofpower conversion efficiencies and stability.

A perspective of mesoscopic solar cells based on metal chalcogenide quantum dots and organometal-halide perovskites

We review two types of inorganic nanomaterials—metal chalcogenide quantum dots (QDs) and lead halide perovskites—that serve as prospective light harvesters in hybrid mesoscopic solar cells. Metal chalcogenide QDs are introduced in three parts: chalcogenides of cadmium (CdS, CdSe and CdTe), chalcogenides of lead (PbS and PbSe) and chalcogenides of antimony (Sb2S3 and Sb2Se3). The devices made using these chalcogenide QDs in a liquid-type electrolyte showed the best cell efficiencies, ranging from 3 to 6%. For solid-state QD-sensitized solar cells (QDSCs), the device performances were generally poor; only devices made of Sb2S3 and PbS QDs attained cell efficiencies approaching ∼7%. In contrast, nanocrystalline lead halide perovskites have emerged since 2009 as potential photosensitizers in liquid-type sensitized TiO2 solar cells. In 2012, the efficiencies of the all-solid-state perovskite solar cells were enhanced to 9.7 and 10.9% using anodes of TiO2 and Al2O3 films, respectively, with 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) as a hole-transporting material. In 2013, the performance of a TiO2 solar cell sensitized with lead iodide perovskite (CH3NH3PbI3) was optimized further to attain an overall power conversion efficiency η=15%, which is a new milestone for solar cells of this type having a device structure similar to that of a dye-sensitized solar cell. In light of growing energy demands, the depletion of fossil fuels and the damaging environmental effects of their use, efficient energy generation from renewable sources must be attained. Because of the abundance of solar energy, photovoltaic devices have attracted a great deal of attention as they offer a promising solution to contemporary energy challenges. Eric Wei-Guang Diau and co-workers from the National Chiao Tung University in Taiwan review recent research progress on two types of inorganic photosensitizers for mesoscopic solar cells: metal chalcogenide MmXn quantum dots — where M represents cadmium, lead or antimony and X denotes sulfur, selenium or tellurium — and organometal halide perovskite nanocrystals, which contain halogens rather than the oxygens of conventional perovskites. In mesoscopic solar cells, these materials serve as light-harvesting layers where capture and charge separation occur. Diau and co-workers summarize their synthesis, characterization and photovoltaic properties and introduce their recent progress trends on device performance for all-solid-state solar cells. Metal chalcogenide quantum dots (QDs) and lead halide perovskites are two types of prospective light harvesters for mesoscopic solar cells. The two most promising QD sensitizers are PbS and Sb2S3, for which the PCEs of their corresponding QDSCs have attained ∼7% in 2012. In 2013, the performance of a TiO2 solar cell sensitized with lead-iodide perovskite (CH3NH3PbI3) was optimized to attain an overall power conversion efficiency of 15%, which is a new milestone for solar cells of this type, with a device structure similar to that of a dye-sensitized solar cell.

Facile Synthesis of Poly(3,4-ethylenedioxythiophene) Film via Solid-State Polymerization as High-Performance Pt-Free Counter Electrodes for Plastic Dye-Sensitized Solar Cells

A high-performance Pt-free counter electrode (CE) based on poly(3,4-ethylenedioxythiophene) (PEDOT) film for plastic dye-sensitized solar cells (DSCs) has been developed via a facile solid-state polymerization (SSP) approach. The polymerization was simply initiated by sintering the monomer, 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT), at the temperature of 80 °C, which can be applied on the plastic substrate. The cyclic voltammetry measurements revealed that the catalytic activity of the SSP-PEDOT CE for triiodide reduction is comparable with that of the Pt CE. Under optimized conditions, the power conversion efficiency of a DSC with a N719-sensitized TiO2 photoanode and the SSP-PEDOT CE is 7.04% measured under standard 1 sun illumination (100 mW cm(-2), AM 1.5), which is very close to that of the device fabricated under the same conditions with a conventional thermally deposited Pt CE (7.35%). Furthermore, taking advantage of the compatibility of the SSP-PEDOT with the plastic substrates, a full plastic N719-sensitized TiO2 solar cell was demonstrated, and an efficiency of 4.65% was achieved, which is comparable with the performance of a plastic DSC with a sputter-deposited Pt CE (5.38%). These results demonstrated that solid-state polymerization initiated at low temperature is a facile and low-cost method of fabricating the high-performance Pt-free CEs for plastic DSCs.

On the behavior of the carboxyphenylterpyridine(8-quinolinolate) thiocyanatoruthenium(II) complex as a new black dye in TiO2 solar cells modified with carboxymethyl-beta-cyclodextrin

Abstract A terpyridine ligand encompassing a terminal 4-carboxyphenyl group (cptpy), was employed in a new Ru(II) black dye, in the presence of 8-quinolinolate (Q) and SCN − as ancillary ligands. Such compound, here referred as [Ru(cptpy) (Q) (NCS)], was designed aiming its inclusion into carboxymethyl-beta-cyclodextrin, anchored on TiO 2 . This host–guest strategy was employed to prevent the formation of aggregates and protect the photoinjecting moiety against parallel deactivation events. Such expectation has indeed been fulfilled by the system. On the other hand, 8-quinolinolate as a strong electron donor ligand, effectively enhanced the light harvesting behavior of the dye, shifting and spreading the IPCE peaks over the entire visible region. Unfortunately, the red shift of visible charge-transfer bands was compensated by a decrease of the Ru(III)/(II) potentials, slowing down the electron transfer kinetics with the I 3 /I − redox mediator. Therefore, the observed counterbalance between charge transfer energies and redox potentials imposes a critical limit in the design of better mononuclear ruthenium-polypyridine dyes.

Syntheses and structural characterization of new heteroleptic 1,1′-bis(diphenylphosphino)ferrocene-dithio complexes of Ni, Pd and Pt: Their uses as sensitizers in TiO2 dye sensitized solar cells

A series of new heteroleptic complexes of the form [M(dppf)L] [where dppf = 1,1′-bis(diphenylphosphino)ferrocene; M = Ni(II) (1), Pd(II) (2), Pt(II) (3) and L = p-tolylsulfonyl dithiocarbimate, p-CH3C6H4SO2NCS2; M = Ni(II) (4), Pd(II) (5), Pt(II) (6) and L = p-chlorobenzene sulfonyl dithiocarbimate, p-ClC6H4SO2NCS2; M = Ni(II) (7), Pd(II) (8) and L = p-bromobenzene sulfonyl dithiocarbimate, p-BrC6H4SO2NCS2; M = Ni(II) (9), Pd(II) (10) and L = 1-ethoxycarbonyl-1-cyanoethylene-2,2-dithiolate (ecda), C2H5OCO(CN)CCS2] have been synthesized and characterized by elemental analysis, spectroscopy (IR,1H, 13C and 31P NMR and UV–Vis.), cyclic voltammetry and single crystal X-ray diffraction and their light harvesting properties have been investigated by using them as photosensitizers in dye sensitized TiO2 solar cells. In these complexes the metal lies at the center of a distorted square planar environment; the distortion varies in the order Pd ≈ Pt > Ni. Among these, the nickel complexes showed light-to-electrical energy conversion efficiencies close to that shown by Ru dye N719 whereas the corresponding palladium and platinum complexes show inferior efficiencies under similar experimental conditions.

Effect of cation size on the performance of dye sensitized nanocrystalline TiO2 solar cells based on quasi-solid state PAN electrolytes containing quaternary ammonium iodides

The performance of dye sensitized nanocrystalline TiO2 solar cells based on quasi-solid state PAN electrolytes containing a series of six quaternary ammonium iodide salts NH4I, Eth4NI, Prop4NI, But4NI, Pent4NI, Hex4NI with different cation sizes has been evaluated. Even though the total ionic conductivity decreases with increasing cation size from NH4I to Hex4NI, the iodide ion conductivity in the electrolytes increases with cation size. The open circuit photovoltage (Voc) of the dye sensitized solar cells also increases with cation radius. The efficiency of the DSSCs varies with cation size and exhibits a maximum for Prop4NI salt. The short circuit photo current density (Jsc) follows a similar variation with cation size, suggesting a strong correlation between the cation size dependence of efficiency and Jsc. The DSSC fabricated with tetrapropylammonium iodide exhibited the maximum efficiency of 4.30% with maximum Jsc of 10.78mAcm−2 and a Voc of 0.746V. The results have been interpreted on the basis of “the electrolyte effects” where Jsc is determined essentially by the iodide ion conductivity and “the electrode effects” where the Jsc and Voc are determined by the cation adsorption in the dyed TiO2 electrode and resulting electron dynamics at the semiconductor/electrolyte interface. For DSSCs with small cations, electrolyte effects appear to be dominating and rate determining while for DSSCs with larger cations, electrode effects appear to be dominating and rate determining.

Enhanced Photovoltaic Effect in BiVO4 Semiconductor by Incorporation with an Ultrathin BiFeO3 Ferroelectric Layer

The photovoltaic effect of BiVO4 semiconductor was investigated by incorporating an ultrathin BiFeO3 ferroelectric layer. It is found that the ultrathin ferroelectric layer with strong self-polarization and high carrier density is desirable to enhance the photovoltaic effect and to manipulate the photovoltaic polarity of the semiconductors. The photovoltage increases by 5-fold to 1 V, and the photocurrent density increases by 2-fold to 140 μA/cm(2), in which the photovoltage is the highest compared with the reported values in polycrystalline and epitaxial ferroelectric thin film solar cells. The mechanism for the observed effect is discussed on the basis of a polarization-induced Schottky-like barrier at the BiFeO3/fluorine doped tin oxide interface. Our work provides good guidance for fabrication of cost-effective semiconductor photovoltaic devices with high performance, and this kind of ultrathin ferroelectric film may also have promising applications in copper indium gallium selenide solar cell, dye-sensitized TiO2 solar cell, lighting emitting diode, and other photoelectron related devices.

Computational Insight on the Working Principles of Zinc Porphyrin Dye-Sensitized Solar Cells

A step-by-step theoretical protocol based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) at both the molecular and periodic levels have been performed to study a zinc porphyrin complex (named YDoc) sensitized TiO2 solar cell including dye excitations, electron injection, the regeneration of photooxidized dyes and the effect of electrolyte additives. Our study reveals the possibility of a favorable electron transfer from the excited dye to the semiconductor conduction band (CB) and suggests three possible pathways of the electron injection from the dye to the nanoparticle (TiO2)38. One is the direct one-step injection by photoexcitation, and the other two are from the different parts of the excited dye to the nanoparticle. The influence of the electrolyte composition on the geometric and electronic features of the dye/TiO2 system has also been studied. It is found that, with the additive of the lithium ion, the energy gap between the LUMO of dye and the TiO2 CB edge increases, which subsequently increases the driving force for the ultrafast excited-state electron injection, contrary to the effect of 4-tert-butylpyridine additive. The computational results of the oxidized dye interacting with I– and I2– reveal that there are a few possible mechanisms for the regeneration of oxidized dye. The effective mechanisms of the regeneration are suggested.

Efficiency enhancement in dye sensitized solar cells based on PAN gel electrolyte with Pr4NI + MgI2 binary iodide salt mixture

The effect of using a binary iodide salt mixture in N719 dye-sensitized TiO2 solar cells (DSSCs) is investigated. The cells use tetrapropylammonium iodide (Pr4NI) and magnesium iodide (MgI2) in a plasticized polyacrylonitrile gel in glass/FTO/nano-porous TiO2/gel, I-2/Pt/FTO/glass solar cell structure. The salt composition in the gel electrolyte is varied to optimize the efficiency of DSSCs. The DSSCs with MgI2 or Pr4NI as the only iodide salt showed the efficiencies 2.56 and 4.16 %, respectively, under AM 1.5 (100 mW cm(-2)) illumination while the DSSC with mixed cations with 18.4:81.6 MgI2:Pr4NI molar ratio shows the highest efficiency of 5.18 %. Thus the efficiency enhancement, relative to the high efficiency end member is about 25 %. DC polarization measurements establish the predominantly ionic behavior of the electrolytes, and show that the variation of efficiency with salt composition correlates with the change in short circuit photocurrent density (J (sc)), which appears to be governed by the iodide ion conductivity. It is also found that J (sc) correlates with the iodide ion transference number estimated from DC polarization data taken with non-blocking iodine electrodes. This study suggests that binary iodide mixtures may be used to obtain efficiency enhancement in different types of DSSCs based on polymeric, gel, or solvent electrolytes.

Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light-Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells

The origin of simultaneous improvements in the short-circuit current density (Jsc) and open-circuit voltage (Voc) of porphyrin dye-sensitized TiO2 solar cells following white light illumination was studied by systematic variation of several different device parameters. Reduction of the dye surface loading resulted in greater relative performance enhancements, suggesting open space at the TiO2 surface expedites the process. Variation of the electrolyte composition and subsequent analysis of the conduction band potential shifts suggested that a light-induced replacement of surface-adsorbed lithium (Li+) ions with dimethylpropylimidazolium (DMPIm+) ions was responsible for an increased electron lifetime by decreasing the recombination with the redox mediator. Variation of the solvent viscosity was found to affect the illumination time required to generate increased performance, while similar performance enhancements were not replicated by application of negative bias under dark conditions, indicating the lig...

Fabrication of thin film nanocrystalline TiO2 solar cells using ruthenium complexes with carboxyl and sulfonyl groups

Two ruthenium complexes with carboxyl and sulfonyl groups have been synthesized, [RuII(L1)2(NCS)2] RuIIbis(4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt)-di(thiocyanate) [K313], [RuII(L1)2(dcbpy)] RuII bis(4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt)(4,4′-dicarboxy-2,2′-bipyridyl) [K314] as photosensitizers. UV–vis, fluorescence emission, AFM and CV measurements are also supplied for ruthenium complexes. Photovoltaic properties of dye sensitized nanocrystalline semiconductor solar cells based on Ruthenium complexes which bear carboxyl and sunfonyl groups have been tested under standard AM 1.5 sunlight. Under the standard global AM 1.5 solar conditions, K314 and K313-sensitized solar cells demonstrate short circuit photocurrent densities of 14.92mA/cm2 and 11.23mA/cm2 and overall conversion efficiencies of 5.09% and 4.02%, respectively.

Effect of different compositions of ethylene carbonate and propylene carbonate containing iodide/triiodide redox electrolyte on the photovoltaic performance of DSSC

The effect of different compositions (in weight percent) of ethylene carbonate (EC) and propylene carbonate (PC) containing iodide/triiodide redox electrolyte on the photoelectrochemical performance of N719-sensitized nanocrystalline TiO2 solar cell was studied. The cells consisted of 0.6 M 1-hexyl-2,3-dimethylimidazolium iodide, 0.1 M LiI, 0.05 M I2 and 0.5 M 4-tert-butylpyridine in different compositions such as 1:1, 1:2, and 2:1 wt% of EC and PC. In 1:1 wt% of EC and PC containing redox electrolyte, short circuit photocurrent density (Jsc) increased and open circuit voltage (Voc) decreased. But in 1:2 and 2:1 wt% of EC and PC containing redox electrolytes, Voc increased and Jsc decreased but fill factor remained relatively constant. Dye-sensitized solar cells (DSSCs) prepared using these electrolytes give a short circuit photocurrent densities of 16.86, 12.71, and 12.09 mA/cm2; an open circuit voltages of 0.73, 0.78, and 0.79 V; fill factors of 0.63, 0.64, and 0.64; and an overall conversion efficiencies of 7.76, 6.34, and 6.13 % at an incident light of 100 mWcm−2 for 1:1, 2:1, and 1:2 wt% of EC/PC containing redox electrolytes, respectively. The incident photon-to-current conversion efficiency was higher in the case of 1:1 wt% of EC and PC containing redox electrolyte than 1:2 and 2:1 wt% of EC and PC containing redox electrolyte. It revealed that 1:1 wt% of EC and PC containing iodide/triiodide redox electrolyte is an effective electrolyte system for the fabrication of long-term stable DSSC.

Influence of organic nitrogenous compounds phenothiazine and diphenyl amine in poly(vinylidene fluoride) blended with poly(ethylene oxide) polymer electrolyte in dye-sensitized solar cells

The present study reports on the comparison of the performance of organic nitrogenous compounds namely, phenothiazine (PT) and diphenyl amine (DPA) in poly(vinylidene fluoride) (PVdF)–poly(ethylene oxide) (PEO) blend polymer electrolyte containing potassium iodide and iodine and its application in cis-dithiocyanato-bis (2,2′bipyridyl 4,4′dicarboxylic acid) ruthenium (II) complex (N3 dye) sensitized nanocrystalline TiO2 solar cells. PT and DPA have simple structures and possess lone pair electrons from nitrogen atoms; these electrons coordinate with iodine in the redox couple (I−/I3−) and tend to increase the conductivity of PVdF-PEO blend polymer electrolyte. On comparison of the electrical performance of PT and DPA in DSSC, the PT doped polymer blend electrolyte was observed to have an enhanced conductivity of 3.5×10−4Scm−1. The DSSC fabricated using the PT doped polymer blend electrolyte demonstrated open-circuit voltage (Voc), short-current (Jsc), fill factor (ff) and conversion efficiency (η) values of 850mV, 13.2mAcm−2, 0.53 and 8.5% respectively, under illumination of 70mWcm−2. The above results reveal that the electron density of PT appears to contribute to relatively higher interaction with iodine in the redox couple when compared to that in the DPA doped PVdF-PEO/KI/I2 polymer blend electrolyte. The structural modifications in the doped polymer blend electrolytes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and differential scanning calorimetry (DSC). Furthermore, electrical conductivity was measured by impedance analysis and the iodide concentration was analyzed using UV–vis spectroscopy whereas current–voltage (I–V) curves were characterized using a potentiostat.

Photocurrent activity of light-harvesting complex II isolated from spinach and its pigments in dye-sensitized TiO2 solar cell

Light-harvesting complex II (LHCII) was used as sensitizer in biosolar cell. To improve the cell performance, we studied the adsorbing conditions and compared the photocurrent activity of LHCII and its pigments. The maximum adsorbances were achieved at the concentration over 80μg chl/ml with 96h incubation for LHCII and at the concentration over 120μg chl/ml with 24h incubation for pigments. The initial performance of pigment-sensitized solar cell was superior to that of LHCII-sensitized solar cell. With the increase of adsorbance, both the short-circuit current and efficiency increased for LHCII-sensitized solar cell and pigment-sensitized solar cell. The best short-circuit current, open-circuit voltage, fill factor and efficiency were 2.26mA/cm2, 0.53V, 0.69 and 0.79% for pigment-sensitized solar cells and 0.80mA/cm2, 0.59V, 0.58 and 0.27% for LHCII-sensitized solar cells, respectively. The adsorption models of LHCII and pigments on TiO2 are proposed, and the strategies for further study are briefly discussed.

Photovoltaic Characteristics of Hybrid MEH-PPV and TiO<sub>2</sub> Nanoparticle Based Organic Solar Cells

The photovoltaic properties of organic solar cells based on hybrid poly [2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylen) (MEH-PPV) and anatase titanium dioxide (TiO2) nanoparticles as a function of TiO2 concentration were investigated. Synthesis of TiO2 nanoparticles was performed by sol-gel immerses heated method and been used as a filler in MEH-PPV polymer matrix. The hybrid MEH-PPV: TiO2 solar cells exhibited increased in light absorption and power conversion efficiency than the pristine organic solar cell. By further optimizing the concentration of TiO2 nanoparticles, the short-circuit current of the hybrid MEH-PPV: TiO2 was reached up to 0.004823 (mA/cm2) and the corresponding power conversion efficiency was 0.000378% was obtained under Air Mass 1.5 illumination which was more than 80% higher compared to the device without TiO2 nanoparticles. This indicates by embedded TiO2 nanoparticles in MEH-PPV matrix encouraging the charge transportation in the active layer of organic solar cells device.

Alternation of Charge Injection and Recombination in Dye-Sensitized Solar Cells by the Addition of Nonconjugated Bridge to Organic Dyes

Most metal-free organic dyes for dye-sensitized solar cells are designed by following a donor conjugated-bridge acceptor structure with a carboxyl acid as an anchoring unit. In this work, we examined the influence of a nonconjugated methylene unit between the cyano group and carboxyl acid by applying it to a previously reported carbazole dye, MK-2. Two dyes, MKZ-35 and -36, were synthesized with glycine and β-alanine, respectively. Dye-sensitized TiO2 solar cells (DSSCs) with MKZ-35 and -36 showed smaller values in the short-circuit current (Jsc) and higher values in open-circuit voltage (Voc) compared with the values with MK-2. The lower Jsc was due to less injection efficiency and fast geminate recombination while the higher Voc was attributed to longer lifetime of the injected electrons in the DSSCs. DFT calculations showed that MKZ-35 dyes interact with each other. One possible explanation for the longer electron lifetime is that the interacted molecules may act as a 3D enlarged dimer molecule or form an induced quadrupole, reducing the interaction between the dyes and acceptor species. On the other hand, the longer electron lifetime with MKZ-36 than that with MK-2 seems to be due to the longer distance between the TiO2 surface and conjugated framework of the dye.