| 1. |
- Benko, Gabor
(författare)
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Dynamics of Photoinduced Processes in Dye-Sensitized Nanocrystalline Semiconductor Films--Reactions in the Photoactive Part of the Grätzel Solar Cell
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The key materials used to convert solar energy into electricity in the Grätzel-type solar cells are dye molecules and titanium dioxide particles attached to each other. The former are similar to those in common products like purple pigments in blueberry juice, while the latter are like sunscreen, white paint, or toothpaste. Their size, measured in nanometers, is a hundred thousand times smaller than a strand of hair. This thesis presents a series of investigations that attempt to give a better understanding of how the dye molecules, energized by sunlight, inject electrons into the titanium dioxide particles, which is an essential reaction in the functioning of the solar cell. As this reaction is one of the fastest ever studied, we have to use the world’s fastest camera. This camera uses laser flashes as short or even shorter in duration as the speed of the reaction, which occurs already in femtoseconds. One femtosecond is 10-15 seconds, that is, 0.000000000000001 seconds, which is to a second as a second is to 32 million years. With such a camera we are able to monitor the track of the electron in slow motion. We have seen for several dye molecule-titanium dioxide particle couples how the electron leaves the molecule and arrives into the particle as early as femtoseconds after laser flash illumination. For certain systems, the electron goes back to the dye molecule immediately after being injected in the titanium dioxide particle, without performing work in an outer electrical circuit. Obviously, those dyes are not suitable for the solar cell. The ultimate goal is to fully understand the mechanism of the electron transfer reaction, and according to the findings modify the materials to show better performance in the solar cell.
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| 2. |
- Benko, Gabor, et al.
(författare)
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Interligand electron transfer determines triplet excited state electron injection in RuN3-sensitized TiO2 films
- 2004
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:9, s. 2862-2867
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Tidskriftsartikel (refereegranskat)abstract
- Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 2,2'-bipyridine-4,4'-dicarboxylate) into a titanium dioxide nanoparticle film occurs along two pathways. The dominating part of the electron injection proceeds from the initially excited singlet state of the sensitizer into the conduction band of the semiconductor on the sub-hundred-femtosecond time scale. The slower part of the injection occurs from the thermalized triplet excited state on the picosecond time scale in a nonexponential fashion, as was shown in a previous study (Benko, G.; et al. J. Am. Chem. Soc. 2002, 124, 489). Here we show that the slower channel of injection is the result of the excited state being localized on a ligand of the sensitizer that is not attached to the semiconductor; hence, the electron cannot be injected directly from such an excited state into the semiconductor. Before being injected, it has to be transferred from the non-surface-attached ligand to the attached one. The results show that the interligand electron-transfer time is on the picosecond time scale, depends on the relative energies of the two ligands, and controls the electron injection from the excited triplet state of the sensitizer. The findings provide information relevant to the design of molecular-based assemblies and devices.
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| 3. |
- Benko, Gabor, et al.
(författare)
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Nonergodic dye-to-nanocrystalline semiconductor film electron transfer
- 2002
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Ingår i: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
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Konferensbidrag (refereegranskat)abstract
- The dominating part of electron injection from the transition metal complex Ru(dcbpy)2(NCS)2 into a titanium dioxide nanocrystalline film proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state of the molecule. The results are especially relevant to the understanding and design of molecular-based photovoltaic devices and artificial photosynthetic assemblies
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| 4. |
- Benko, Gabor, et al.
(författare)
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Nonergodic dye-to-semiconductor electron transfer
- 2002
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Ingår i: Thirteenth International Conference on Ultrafast Phenomena. Technical Digest (TOPS Vol.72). - 1557527032 ; , s. 434-435
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Konferensbidrag (refereegranskat)abstract
- The dominating part of electron injection from the transition metal complex Ru(dcbpy)2(NCS)2 into a titanium dioxide nanocrystalline film proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state of the molecule. Ultrafast transient absorption measurements with laser pulses of the order of ~30 fs have been performed
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| 5. |
- Benko, Gabor, et al.
(författare)
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Particle size and crystallinity dependent electron injection in Fluorescein 27-sensitized TiO2 films.
- 2003
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:6, s. 1370-1375
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Tidskriftsartikel (refereegranskat)abstract
- Influence of processing parameters, such as autoclaving and firing temperature, on the optical properties of nanocrystalline anatase TiO2 film and on the process of electron injection from the dye fluorescein 27 to the as-prepared films is studied. Transmission electron microscopy and steady-state and time-resolved femtosecond spectroscopy measurements indicate that the larger the TiO2 particle and the better its overall crystallinity, the faster the process of electron injection. Unraveling factors that control the properties of the sub-20-nm sized semiconductor particles, and by this the electron injection to them, is important for understanding the process of interfacial electron transfer from the dye to the semiconductor, as well as future optimization of the function of the photoelectrochemical cell based on dye-sensitized TiO2 films.
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| 6. |
- Benko, Gabor, et al.
(författare)
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Photoinduced Electron Injection from Ru(dcbPY)2(NCS)2 to SnO2 and TiO2 Nanocrystalline Films.
- 2003
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 125:5, s. 1118-1119
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Tidskriftsartikel (refereegranskat)abstract
- Photoinduced electron injection from the sensitizer Ru(dcbpy)2(NCS)2 (RuN3) into SnO2 and TiO2 nanocrystalline films occurs by two distinct channels on the femto- and picosecond time scales. The faster electron injection into the conduction band of the different semiconductors originates from the initially excited singlet state of RuN3, and occurs in competition with intersystem crossing. The rate of singlet electron injection is faster to TiO2 (1/55 fs-1) than to SnO2 (1/145 fs-1), in agreement with higher density of conduction band acceptor states in the former semiconductor. As a result of competition between the ultrafast processes, for TiO2 singlet, whereas for SnO2 triplet electron injection is dominant. Electron injection from the triplet state is nonexponential and can be fitted with time constants ranging from ~1 ps (2.5 ps for SnO2) to ~50 ps for both semiconductors. The major part of triplet injection is independent of the semiconductor and is most likely controlled by intramolecular dynamics in RuN3. The overall time scale and the yield of electron injection to the two semiconductors are very similar, suggesting that processes other than electron injection are responsible for the difference in efficiencies of solar cells made of these materials.
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| 7. |
- Benko, Gabor, et al.
(författare)
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Photoinduced Ultrafast Dye-to-Semiconductor Electron Injection from Nonthermalized and Thermalized Donor States.
- 2002
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 124:3, s. 489-493
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Tidskriftsartikel (refereegranskat)abstract
- Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) into a titanium dioxide nanocrystalline film occurs on the femto- and picosecond time scales. Here we show that the dominating part of the electron transfer proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state, prior to electronic and nuclear relaxation of the molecule. The results are especially relevant to the understanding and design of molecular-based photovoltaic devices and artificial photosynthetic assemblies.
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| 8. |
- He, Jianjun, et al.
(författare)
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Modified phthalocyanines for efficient near-IR sensitization of nanostructured TiO2 electrode
- 2002
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 124:17, s. 4922-4932
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Tidskriftsartikel (refereegranskat)abstract
- A zinc phthalocyanine with tyrosine substituents (ZnPcTyr), modified for efficient far-red/near-IR performance in dye-sensitized nanostructured TiO2 solar cells, and its reference, glycine-substituted zinc phthalocyanine, (ZnPcGly), were synthesized and characterized. The compounds were studied spectroscopically, electrochemically, and photoelectrochemically. Incorporating tyrosine groups into phthalocyanine makes the dye ethanol-soluble and reduces surface aggregation as a result of steric effects. The performance of a solar cell based on ZnPcTyr is much better than that based on ZnPcGly. Addition of 3alpha,7alpha-dihydroxy-5beta-cholic acid (cheno) and 4-tert-butylpyridine (TBP) to the dye solution when preparing a dye-sensitized TiO2 electrode diminishes significantly the surface aggregation and, therefore, improves the performance of solar cells based on these phthalocyanines. The highest monochromatic incident photo-to-current conversion efficiency (IPCE) of similar to24% at 690 nm and an overall conversion efficiency (eta) of 0.54% were achieved for a cell based on a ZnPcTyr-sensitized TiO2 electrode. Addition of TBP in the electrolyte decreases the IPCE and eta considerably, although it increases the open-circuit photovoltage. Time-resolved transient absorption measurements of interfacial electron-transfer kinetics in a ZnPoTyr-sensitized nanostructured 702 thin film show that electron injection from the excited state of the dye into the conduction band of TiO2 is completed in similar to500 fs and that more than half of the injected electrons recombines with the oxidized dye molecules in similar to300 ps. In addition to surface aggregation, the very fast electron recombination is most likely responsible for the low performance of the solar cell based on ZnPcTyr.
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| 9. |
- Kallioinen, Jani, et al.
(författare)
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Electron transfer from the singlet and triplet excited states of Ru(dcbpy)(2)(NCS)(2) into nanocrystalline TiO2 thin films
- 2002
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:17, s. 4396-4404
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpY)(2)(NCS)(2) (RuN3) into titanium dioxide (TiO2) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of similar to30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs(-1) for ET and 1/75 fs(-1) for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from similar to1 to similar to60 ps.
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| 10. |
- Kallioinen, J, et al.
(författare)
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Photoinduced ultrafast dynamics of Ru(dcbpy)(2)(NCS)(2)-sensitized nanocrystalline TiO2 films: The influence of sample preparation and experimental conditions
- 2004
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:20, s. 6365-6373
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Tidskriftsartikel (refereegranskat)abstract
- In most of the previous ultrafast electron injection studies of Ru(dcbpy)(2)(NCS)(2)-sensitized nanocrystalline TiO2 films, experimental conditions and sample preparation have been different from study to study and no studies of how the differences affect the observed dynamics have been reported. In the present paper, we have investigated the influence of such modifications. Pump photon density, environment of the sensitized film (solvent and air), and parameters of the film preparation (crystallinity and quality of the film) were varied in a systematic way and the obtained dynamics were compared to that of a well-defined reference sample: Ru(dcbpy)(2)(NCS)(2)-TiO2 in acetonitrile. In some cases, the induced changes in the dynamics were uncorrelated to the electron injection process. High pump photon density (not in the linear response region) and exposure of the sensitized film to air altered the picosecond-time- scale kinetics considerably, and the changes were attributed mostly to degradation of the dye. In other cases, changes in the measured kinetics were related to the electron injection processes: reducing the firing temperature of the nanocrystalline film or making the film via electron beam evaporation (EBE) resulted in a decrease of the overall crystallinity of the film, and the electron injection slowed. In the sensitized EBE films, in addition to an increased contribution of triplet excited-state electron injection, a new electron transfer (ET) process with a time constant of 200 fs was observed.
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