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Michael Grätzel

Publications and source records attributed to Michael Grätzel.

At least 37 records · Page 2Linked to original sources
PubMed2006

Ion coordinating sensitizer for high efficiency mesoscopic dye-sensitized solar cells: influence of lithium ions on the photovoltaic performance of liquid and solid-state cells.

A Li+ coordinating sensitizer, NaRu(4-carboxylic acid-4'-carboxylate)(4,4'-bis[(triethylene glycol methyl ether) methyl ether]-2,2'-bipyridine)(NCS)2 (coded as K51), has been synthesized, and the effect of Li+ coordination on its performance in mesoscopic titanium dioxide dye-sensitized solar cells has been investigated. Fourier transform infrared spectra suggest that Li+ coordinates to the triethylene oxide methyl ether side chains on the dye molecules. With the addition of Li+ to a nonvolatile liquid electrolyte, we observe a significant increase in the photocurrent density, with only a small decrease in the open-circuit voltage, contrary to a non ion coordinating dye which displays a large drop in potential with the addition of Li+. For a solar cell incorporating an organic hole-transporter, we find the potential rises with increasing the Li+ concentration in the hole-transporter matrix. For the liquid electrolyte and solid-state cells, we obtain power conversion efficiencies of 7.8% and 3.8%, respectively, under simulated sunlight.

Coloring Agents↗
PubMed2006

Dye-sensitized solar cells incorporating a "liquid" hole-transporting material.

We present the first application of an amorphous "liquid" organic semiconductor in an optoelectronic device, demonstrating that it is highly suited for use as a hole-transporting material in nanostructured dye-sensitized solar cells. For such devices, we obtain power conversion efficiencies of up to 2.4% under simulated air mass 1.5 solar spectrum at 100 mWcm(-2), and incident photon-to-electron quantum efficiencies in excess of 50%.

Coloring Agents↗
PubMed2005

Effect of a coadsorbent on the performance of dye-sensitized TiO2 solar cells: shielding versus band-edge movement.

The mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, alters the open-circuit photovoltage and short-circuit current of dye-sensitized solar cells was investigated. The influence of tetrabutylammonium chenodeoxycholate on dye loading was studied under a variety of conditions in which the TiO2 films were exposed to the sensitizing dye and coadsorbent. Photocurrent--voltage measurements combined with desorption studies revealed that adding chenodeoxycholate reduces the dye loading by as much as 60% while having a relatively small effect on the short-circuit photocurrent. Calculations along with measurements showed that even at low loading, enough dye is present to absorb a significant fraction of the incident light in the visible spectrum. In concurrence with the observations of others, we find evidence for weakly and strongly adsorbed forms of the dye resulting from either different binding conformations or aggregates. The most strongly adsorbed dyes are less susceptible to displacement by chenodeoxycholate than those that are weakly adsorbed. While having no observable effect on dye coverage, multiple exposures of a TiO2 film to a dye solution substantially increased the fraction of strongly adsorbed dye as judged by the resistance of the adsorbed dye to displacement by chenodeoxycholate. Measurements of the open-circuit voltage as a function of the photocharge density, determined by infrared transmittance, showed that chenodeoxycholate not only shifts the conduction band edge to negative potentials, but also significantly increases the rate of recombination. The net effect of adding chenodeoxycholate is, however, to improve the photovoltage.

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PubMed2005

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.

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PubMed2005

Influence of 4-guanidinobutyric acid as coadsorbent in reducing recombination in dye-sensitized solar cells.

Dye-sensitized solar cells based on nanocrystalline TiO(2) have been fabricated with an amphiphilic ruthenium sensitizer [Ru (4,4'-dicarboxylic acid-2,2'-bipyridine) (4,4'-bis(p-hexyloxystyryl)-2,2'-bipyridine)(NCS)(2)], coded as K-19, and 4-guanidinobutyric acid (GBA) as coadsorbent. The cells showed a approximately 50 mV increase in open-circuit voltage and a similar current in comparison with cells without GBA cografting. The performance of both types of devices was evaluated on the basis of their photocurrent-voltage characteristics, dark current measurements, cyclic voltammetry, electrochemical impedance spectroscopy, and phototransient decay methods. The results indicate that GBA shifted the conduction band of TiO(2) toward a more negative potential and reduced the interfacial charge-transfer reaction from conduction band electrons to triiodide in the electrolyte (also known as the back reaction). In addition, the devices with GBA cografting showed an excellent stability with a power conversion efficiency of approximately 8% under simulated full sunlight (air mass 1.5, 100 mW cm(-2)) during visible light soaking at 60 degrees C.

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PubMed2005

Solar energy conversion by dye-sensitized photovoltaic cells.

The quality of human life depends to a large degree on the availability of energy. This is threatened unless renewable energy resources can be developed in the near future. Chemistry is expected to make important contributions to identify environmentally friendly solutions of the energy problem. One attractive strategy discussed in this Forum Article is the development of solar cells that are based on the sensitization of mesoscopic oxide films by dyes or quantum dots. These systems have already reached conversion efficiencies exceeding 11%. The underlying fundamental processes of light harvesting by the sensitizer, heterogeneous electron transfer from the electronically excited chromophore into the conduction band of the semiconductor oxide, and percolative migration of the injected electrons through the mesoporous film to the collector electrode will be described below in detail. A number of research topics will also be discussed, and the examples for the first outdoor application of such solar cells will be provided.

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PubMed2005

Visible light-induced water oxidation on mesoscopic alpha-Fe2O3 films made by ultrasonic spray pyrolysis.

Alpha-Fe(2)O(3) films having a mesoscopic leaflet type structure were produced for the first time by ultrasonic spray pyrolysis (USP) to explore their potential as oxygen-evolving photoanodes. The target of these studies is to use translucent hematite films deposited on conducting fluorine doped tin oxide (FTO) glass as top electrodes in a tandem cell that accomplishes the cleavage of water into hydrogen and oxygen by sunlight. The properties of layers made by USP were compared to those deposited by conventional spray pyrolysis (SP). Although both types of films show similar XRD and UV-visible and Raman spectra, they differ greatly in their morphology. The mesoscopic alpha-Fe(2)O(3) layers produced by USP consist mainly of 100 nm-sized platelets with a thickness of 5-10 nm. These nanosheets are oriented mainly perpendicularly to the FTO support, their flat surface exposing (001) facets. The mesoscopic leaflet structure has the advantage that it allows for efficient harvesting of visible light, while offering at the same time the very short distance required for the photogenerated holes to reach the electrolyte interface before recombining with conduction band electrons. This allows for water oxidation by the valence band holes even though their diffusion length is only a few nanometers. Distances are longer in the particles produced by SP favoring recombination of photoinduced charge carriers. Open-circuit photovoltage measurements indicate a lower surface state density for the nanoplatelets as compared to the round particles. These factors explain the much higher photoactivity of the USP compared to the SP deposited alpha-Fe(2)O(3) layers. Addition of hydrogen peroxide to the alkaline electrolyte further improves the photocurrent-voltage characteristics of films generated by USP indicating the hole transfer from the valence band of the semiconductor oxide to the adsorbed water to be the rate-limiting kinetic step in the oxygen generation reaction.

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PubMed2005

Rationale for kinetic heterogeneity of ultrafast light-induced electron transfer from Ru(II) complex sensitizers to nanocrystalline TiO2.

Because of their successful use in dye-sensitized solar cells, Ru(II) polypyridyl complex dyes adsorbed on nanocrystalline TiO2 films have been regarded as model systems for the experimental study of the ultrafast dynamics of interfacial light-induced electron transfer. Most studies have reported charge injection kinetics from Ru(dcbpyH2)2(NCS)2 (N3) to take place with a fast (sub-100 fs) phase, followed by a slower (0.7-100 ps) multiexponential component. This complex, multiphasic behavior observed for the electron injection process has prevented the development of a satisfying kinetic model and has led to often contradicting conclusions. Here, we show that the observed kinetic heterogeneity can result from the aggregation of sensitizer molecules on the surface. Carefully controlled deposition of Ru(II) complex dye molecules onto nanocrystalline titania consistently yields a monophasic injection dynamics with a time constant shorter than 20 fs. The latter figure suggests the process is beyond the scope of vibration-mediated electron transfer kinetic models and might be controlled by the electron dephasing in the solid.

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PubMed2005

Efficient light harvesting by using green Zn-porphyrin-sensitized nanocrystalline TiO2 films.

A series of novel zinc metalloporphyrins, cyano-3-(2'-(5',10',15',20'-tetraphenylporphyrinato zinc(II))yl)-acrylic acid (Zn-3), 3-(trans-2'-(5',10',15',20'-tetraphenylporphyrinato zinc(II))yl)-acrylic acid (Zn-5), 2-cyano-5-(2'-(5',10',15',20'-tetraphenylporphyrinato zinc(II))yl)-penta-2,4-dienoic acid (Zn-8), 4-(trans-2'-(2' '-(5' ',10' ',15' ',20' '-tetraphenylporphyrinato zinc(II))yl)ethen-1'-yl))-1,2-benzenedicarboxylic acid (Zn-11), and 2-cyano-3-[4'-(trans-2' '-(2' ''-(5' '',10' '',15' '',20' ''-tetraphenylporphyrinato zinc(II))yl) ethen-1' '-yl)-phenyl]-acrylic acid (Zn-13) were synthesized and characterized by using various spectroscopic techniques. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations show that key molecular orbitals (MOs) of porphyrins Zn-5 and Zn-3 are stabilized and extended out onto the substituent by pi-conjugation, causing enhancement and red shifts of visible transitions and increasing the possibility of electron transfer from the substituent. The porphyrins were investigated for conversion of sunlight into electricity by constructing dye-sensitized TiO(2) solar cells using an I(-)/I(3)(-) electrolyte. The cells yield close to 85% incident photon-to-current efficiencies (IPCEs), and under standard AM 1.5 sunlight, the Zn-3-sensitized solar cell demonstrates a short circuit photocurrent density of 13.0 +/- 0.5 mA/cm(2), an open-circuit voltage of 610 +/- 50 mV, and a fill factor of 0.70 +/- 0.03. This corresponds to an overall conversion efficiency of 5.6%, making it the most efficient porphyrin-sensitized solar cell reported to date.

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PubMed2005

Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells.

Electrochemical impedance spectroscopy (EIS) has been performed to investigate electronic and ionic processes in dye-sensitized solar cells (DSC). A theoretical model has been elaborated, to interpret the frequency response of the device. The high-frequency feature is attributed to the charge transfer at the counter electrode while the response in the intermediate-frequency region is associated with the electron transport in the mesoscopic TiO2 film and the back reaction at the TiO2/electrolyte interface. The low-frequency region reflects the diffusion in the electrolyte. Using an appropriate equivalent circuit, the electron transport rate and electron lifetime in the mesoscopic film have been derived, which agree with the values derived from transient photocurrent and photovoltage measurements. The EIS measurements show that DSC performance variations under prolonged thermal aging result mainly from the decrease in the lifetime of the conduction band electron in the TiO2 film.

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PubMed2005

Control of dark current in photoelectrochemical (TiO2/I--I3-)) and dye-sensitized solar cells.

The ruthenium complex bis-tetrabutylammonium cis-dithiocyanato-N,N'-bis-2,2'-bipyridine-4-carboxylic acid, 4'-carboxylate ruthenium(II), N-719, was found to block the dark current of dye sensitized solar cells (DSC), based on mesoporous TiO2 films deposited on a F-doped tin oxide electrode and the effect was compared to surface treatment by TiCl4 and the introduction of a compact TiO2 blocking layer.

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PubMed2005

Charge separation and efficient light energy conversion in sensitized mesoscopic solar cells based on binary ionic liquids.

A 7.4% power conversion efficiency at air mass (AM) 1.5 full sunlight was reached with a mesoscopic solar cell employing a new binary ionic liquid electrolyte composed of 1-propyl-3-methylimidazolium iodide and 1-ethyl-3-methylimidazolium tricyanomethanide in conjunction with the amphiphilic ruthenium complex NaRu(4-carboxylic acid-4'-carboxylate)(4,4'-dinonyl-2,2'-bipyridine)(NCS)(2), coded as Z-907Na. Ultramicroelectrode voltammetric, nanosecond laser transient absorbance, and photovoltaic measurements show that a high iodide concentration is required for dye regeneration to compete efficiently with charge recombination. A surprisingly fast reductive quenching process is turned on in pure iodide melts. This channel is unproductive, explaining the lower photocurrents observed under these conditions.

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PubMed2005

Cross surface ambipolar charge percolation in molecular triads on mesoscopic oxide films.

We report on cross surface ambipolar charge percolation within a monolayer of a molecular triad adsorbed on semiconducting or insulating mesoscopic metal oxide films. The triad consists of a triphenlyamine (TPA) donor and a perylenemonoimide (PMI) acceptor connected by a bithiophene (T2) bridge. The self-assembled PMI-T2-TPA monolayer exhibits p-type or n-type conduction depending on the potential that is applied to the conducting glass (FTO) electrode supporting the oxide films. Cross surface electron transfer is turned on at around -1.24 V (vs Fc+/Fc) where the PMI moiety is electroactive. The color of the film changes from red to blue during the reduction of the PMI. By contrast, lateral hole transfer is turned on at around 0.8 V (vs Fc+/Fc) where the TPA moiety becomes electroactive. The stepwise oxidation of the T2-TPA units at 0.79 and 1.28 V (vs Fc+/Fc) is associated with a color change of the film from red to black. Cyclic voltammetric as well as chronocoulometric and spectroelectrochemical measurements were applied to determine the percolation threshold for cross surface charge transfer and the diffusion coefficients for the electron and hole hopping process. The effect of oxide surface states on the lateral charge motion was also investigated.

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PubMed2005

Effect of hydrocarbon chain length of amphiphilic ruthenium dyes on solid-state dye-sensitized photovoltaics.

We studied the influence of the hydrophobic hydrocarbon chain length of amphiphilic ruthenium dyes on the device performance in solid-state dye-sensitized solar cells. We found that the dyes with longer hydrocarbon chains gave higher efficiency values when used as a sensitizer in solid-state dye-sensitized solar cells. With increasing chain length, we observed higher currents and open-circuit voltages up to a limiting chain length. We attribute this improvement to the expected larger distance between TiO2 and the hole conductor, which seems to suppress recombination effectively.

Electric Conductivity↗
PubMed2005

Organized mesoporous TiO2 films exhibiting greatly enhanced performance in dye-sensitized solar cells.

The Pluronic P123 templated mesoporous TiO2 film was grown via layer-by-layer deposition and characterized by a novel methodology based on the adsorption of n-pentane. Multiple-layer depositions did not perturb the mesoporous structure significantly. Our TiO2 film was sensitized by a newly developed Ru-bipyridine dye (N945) and was applied as a photoanode in dye-sensitized solar cell. The 1-microm-thick mesoporous film, made by the superposition of three layers, showed enhanced solar conversion efficiency by about 50% compared to that of traditional films of the same thickness made from randomly oriented anatase nanocrystals.

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