Showing papers by "Michael Grätzel published in 2005"
TL;DR: Developing solar cells that are based on the sensitization of mesoscopic oxide films by dyes or quantum dots, and the examples for the first outdoor application of such solar cells will be provided.
Abstract: 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 wi...
3,214 citations
TL;DR: 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.
Abstract: We report a combined experimental and computational study of several ruthenium(II) sensitizers originated from the [Ru(dcbpyH2)2(NCS)2], N3, and [Ru(dcbpyH2)(tdbpy)(NCS)2], N621, (dcbpyH2 = 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 TiO2 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 ...
2,660 citations
TL;DR: 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.
Abstract: 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.
1,875 citations
TL;DR: In this paper, the authors developed a new type of solar cell that is based on a photoelectrochemical process, where the light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor.
Abstract: During the past five years, the authors have developed in their laboratory a new type of solar cell that is based on a photoelectrochemical process. The light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor (i.e., titanium oxide (TiO{sub 2})). When excited by a photon, the dye has the ability to transfer an electron to the semiconductor. The electric field that is inside the material allows extraction of the electron, and the positive charge is transferred from the dye to a redox mediator that is present in solution. A respectable photovoltaic efficiency (i.e., 10%) is obtained by the use of mesoporous, nanostructured films of anatase particles. The authors show how the TiO{sub 2} electrode microstructure influences the photovoltaic response of the cell. More specifically, they focus on how processing parameters such as precursor chemistry, temperature for hydrothermal growth, binder addition, and sintering conditions influence the film porosity, pore-size distribution, light scattering, and electron percolation and consequently affect the solar-cell efficiency.
1,552 citations
TL;DR: The ruthenium complex bis-tetrabutylammonium cis-dithiocyanato-N,N'-bis-2,2'-bipyridine-4-carboxylic acid, 4'-carboxylate ruthene(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
596 citations
TL;DR: 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, showing enhanced solar conversion efficiency by about 50% compared to that of traditional films of the same thickness made from randomly oriented anatase nanocrystals.
Abstract: 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-μm-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.
532 citations
TL;DR: In this article, the authors used an organic metal free indoline dye as a sensitizer in a solid state solar cell with a hole conductor and achieved an efficiency of 4 % over the solar spectrum.
Abstract: Dye sensitized solar cells are an interesting low cost alter native to conventional solar cells. Efficiencies over 10 % have been achieved. [1,2] Advantageous is the replacement of the liquid electrolyte in these devices with a solid charge carrier material to avoid any sealing and long term stability prob lems. In 1998 Bach et al. [3] demonstrated that the electrolyte can be replaced by a hole conductor. Here we report a very efficient solid state solar cell with the amorphous organic hole transport material 2,2¢,7,7¢ tetrakis (N,N di p methoxyphenyl amine)9,9¢ spirobifluorene (spiro OMeTAD) as hole conduc tor and for the first time an organic metal free indoline dye as sensitizer. Record efficiencies for this type of cell of over 4 % over the solar spectrum were reached. Until now, sensitization using organic dyes has not been as efficient as using ruthenium dyes, which have been success fully applied previously. Metal free dyes such as perylene de rivatives, [4] coumarin dyes, [5] porphyrin dyes, [6] and cyanine and merocyanine dyes [7] have been used as sensitizers, but did not achieve the same solar conversion efficiency as ruthenium dyes. The efficiencies achieved in this work with the indoline dye (D102) are even higher than the highest currently report ed values for ruthenium dye sensitized solid state cells. In ad dition, this dye has the advantage that it can be produced at low cost, because it does not contain the expensive rare metal ruthenium and it is easy to synthesize. Up to now the best results reported with spiro OMeTAD as a hole conductor are efficiencies of 3.2 %. [8] In that work the dye uptake and open circuit voltage were optimized by a sil ver complexation. In the case of the indoline dye we were able to reach an efficiency of just over 4 % without further optimi zation. This shows the extremely high potential of the indoline dye as a sensitizer in solid state dye sensitized solar cells. In doline dyes have previously been used in dye sensitized solar cells with a liquid electrolyte. [9,10] There also they showed a good performance of up to g = 6.1 % conversion efficiency compared to 6.3 % for a N3 dye sensitized cell. [9] Solid state devices usually have a lower performance, which is not the case here with an extraordinary efficiency of 4 %, which ap proaches that of their liquid …
525 citations
TL;DR: An amphiphilic heteroleptic polypyridyl ruthenium complex with a high molar extinction coefficient was synthesized and demonstrated as an efficient, thermostable sensitizer in nanocrystalline dye-sensitized solar cells.
Abstract: An amphiphilic heteroleptic polypyridyl ruthenium complex with a high molar extinction coefficient was synthesized and demonstrated as an efficient, thermostable sensitizer in nanocrystalline dye-sensitized solar cells.
514 citations
TL;DR: In this article, the mesoscopic α-Fe2O3 layers produced by USP consist mainly of 100 nm-sized platelets with a thickness of 5−10 nm.
Abstract: α-Fe2O3 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 α-Fe2O3 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 ...
481 citations
TL;DR: Density functional theory (DFT) and time-dependent DFT (TDFT) calculations show that key molecular orbitals 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.
Abstract: 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 π-conjugation, causing enhancement and red shifts of visible transitions and increasing the possibility of electron transfer from the substituent. The porp...
422 citations
TL;DR: 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.
Abstract: 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.
TL;DR: The quality of human life depends to a large degree on the availability of energy sources as mentioned in this paper and the present annual worldwide energy consumption has already attained a level of over 400 exajoules and is...
Abstract: The quality of human life depends to a large degree on the availability of energy sources. The present annual worldwide energy consumption has already attained a level of over 400 exajoules and is ...
TL;DR: The mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, alters the open-circuit photovoltage and short-circuits current of dye-sensitized solar cells was investigated and evidence for weakly and strongly adsorbed forms of the dye resulting from either different binding conformations or aggregates is found.
Abstract: The objective of this research is to determine the operational characteristics key to efficient, low-cost, stable solar cells based on dye-sensitized mesoporous films (in collaboration with DOE's Office of Science Program). Toward this end, we have investigated the mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, improves the open-circuit photovoltage (VOC) and short-circuit photocurrent density (JSC). We find that adding chenodeoxycholate not only shifts the TiO2 conduction-band edge to negative potentials but also accelerates the rate of recombination. The net effect of these opposing phenomena is to produce a higher photovoltage. It is also found that chenodeoxycholate reduces the dye loading significantly but has only a modest effect on JSC. Implications of these results to developing more efficient cells are discussed.
TL;DR: In this article, the performance of K8 and K9 complexes as charge transfer photosensitizers in nanocrystalline TiO2-based solar cells was investigated, and the results showed that the K9-sensitized solar cell gave a photocurrent density of 16.5 ± 0.5%.
TL;DR: Results indicate that GBA shifted the conduction band of TiO(2) toward a more negative potential and reduced the interfacial charge-transfer reaction from conductionBand electrons to triiodide in the electrolyte (also known as the back reaction).
Abstract: Dye-sensitized solar cells based on nanocrystalline TiO2 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 ∼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 TiO2 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 approxima...
TL;DR: 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, suggesting the process is beyond the scope of vibration-mediated electron transfer kinetic models and might be controlled by the electron dephasing in the solid.
Abstract: 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 o...
TL;DR: In this article, the authors reported a solid-state-dye-sensitized solar cell with an efficiency of 4% over the standard air mass 1.5 spectrum (100mW∕cm2) using an amphiphilic dye with hydrophobic spacers.
Abstract: We report a solid-state-dye-sensitized solar cell with an efficiency of 4% over the standard air mass 1.5 spectrum (100mW∕cm2). This was made possible by using an amphiphilic dye with hydrophobic spacers. We attribute the performance to the self-assembly of the dye to a dense layer on the TiO2 surface with its carboxylate groups as anchors and with its hydrophobic isolating chains as blocking layer between hole conductor and TiO2. In addition we studied the dependence of the thickness of the nanoporous TiO2 layer and the device performance. These results show the high potential for solid-state-dye-sensitized solar cells to compete with amorphous silicon cells as low-cost alternative.
TL;DR: The dye-sensitized solar cell (DSSC) as mentioned in this paper provides a technically and economically viable alternative concept to present-day p-n junction photovoltaic devices.
Abstract: The dye-sensitized solar cell (DSSC) provides a technically and economically viable alternative concept to present-day p-n junction photovoltaic devices. In contrast to conventional silicon systems, where the semiconductor assumes both the task of light absorption and charge carrier transport, these two functions are separated in DSSCs. The use of sensitizers having a broad absorption band in conjunction with wide-bandgap semiconductor films of mesoporous or nanocrystalline morphology permits harvesting a large fraction of sunlight. There are good prospects that these devices can attain the conversion efficiency of liquid-electrolyte-based dye-sensitized solar cells, which currently stands at 11 %. In this article, we present the current state of the field, the realm of our review being restricted to the discussion of organic molecular hole conductors, which have demonstrated the best performance to date.
TL;DR: The performance of the K8 complex as a charge transfer photosensitizer in nanocrystalline TiO(2) based solar cells was studied and very efficient sensitization was achieved yielding 77 +/-5% incident photon-to-current efficiencies (IPCE) in the visible region.
Abstract: A novel ligand 4,4‘-bis(carboxyvinyl)-2,2‘-bipyridine (L) and its ruthenium(II) complex [Ru(II)L2(NCS)2] (K8) were synthesized and characterized by analytical, spectroscopic, and electrochemical techniques. The performance of the K8 complex as a charge transfer photosensitizer in nanocrystalline TiO2 based solar cells was studied. When the K8 complex anchored onto a nanocrystalline TiO2 film, we achieved very efficient sensitization yielding 77 ± 5% incident photon-to-current efficiencies (IPCE) in the visible region using an electrolyte consisting of 0.6 M methyl-N-butyl imidiazolium iodide, 0.05 M iodine, 0.05 M LiI, and 0.5 M 4-tert-butylpyridine in a 50/50 (v/v) mixture of valeronitrile and acetonitrile. Under standard AM 1.5 sunlight, the complex K8 gave a short circuit photocurrent density of 18 ± 0.5 mA/cm2, and the open circuit voltage was 640 ± 50 mV with fill factor of 0.75 ± 0.05, corresponding to an overall conversion efficiency of 8.64 ± 0.5%.
TL;DR: In this article, the authors demonstrate a ⩾8% efficient nanocrystalline dye-sensitized solar cell retaining over 98% of its initial performance after 1000 h of accelerated tests subjected to thermal stress at 80 °C in the dark.
Abstract: We demonstrate a ⩾8% efficient nanocrystalline dye-sensitized solar cell retaining over 98% of its initial performance after 1000 h of accelerated tests subjected to thermal stress at 80 °C in the dark. Device degradation was also negligible following 1000 h of visible light soaking at 60 °C. This high performance and stable device was realized by using a robust electrolyte of low volatility in conjunction with the 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, which was grafted together with 1-decylphosphonic acid on the mesoporous titania film acting as photoanode.
TL;DR: It is 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, with higher currents and open-circuit voltages up to a limiting chain length.
Abstract: 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.
TL;DR: This poster presents a probabilistic procedure to characterize the response of the immune system to laser-spot assisted, 3D image analysis of central nervous system injury.
Abstract: Reference LPI-ARTICLE-2005-001doi:10.1002/cphc.200500147View record in Web of Science Record created on 2006-02-21, modified on 2016-08-08
TL;DR: This poster presents a probabilistic simulation of the response of the immune system to EMTs to treat central nervous system injuries and shows clear patterns of decline in the number of immune checkpoints.
Abstract: Reference LPI-ARTICLE-2005-018doi:10.1002/anie.200502009View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12
TL;DR: TaON was found to be ca.
TL;DR: In this article, two high boiling point nitrile-based electrolytes, namely, 3-ethoxypropionitrile (CH 3 CH 2 OCH 2 CH 2 CN, EPN)/1 M LiTFSI and 3-(2,2, 2-trifluoro)ethoxy-propionic acid (THPPA), were investigated.
TL;DR: In this article, it was shown that dye aggregation at the interface is actually responsible for the slow kinetic component of the interfacial electron transfer, and a thorough control of the dissolution of the dye and of its adsorption onto nanocrystalline oxide films allowed the reduction of the portion of dye excited states that react within the slow compartment and even made the latter completely disappear.
Abstract: Interfacial electron transfer from a molecular dye to a semiconductor is a keystone process in the conversion of light into electricity in dye-sensitized solar cells. The most successful devices developed so far are basedon the sensitization of nanocrystalline titanium dioxide by ruthenium polypyridyl complexes. The ultrafast electron injection from the widely used Ru I I (dcbpy) 2 (NCS) 2 dye in particular has been intensely studied. Several research groups, including ours, have found that this reaction apparently proceeds with a fast sub-100 fs phase, followed by a slower kinetic component with a time constant of 0.7-100 ps and accounting for 16-65% of the total yield. No convincing explanation has been provided for a clear understanding of the origin of this non-exponential kinetic behavior. In this contribution we show that aggregation of dye molecules at the interface is actually responsible for the slow kinetic component of the interfacial electron transfer. A thorough control of the dissolution of the dye and of its adsorption onto nanocrystalline oxide films allowed the reduction of the portion of dye excited states that react within the slow compartment and even made the latter completely disappear. In the absence of dye aggregates, femtosecond pump-probe studies of the sensitizer's oxidized state appearance yielded a rate constant for charge injection >5×10 1 3 s - 1 , corresponding to an electron transfer time of less than 20 fs.