| 1. |
- Canton, Sophie, et al.
(författare)
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Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts
- 2013
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:11, s. 1972-1976
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Tidskriftsartikel (refereegranskat)abstract
- Building a detailed understanding of the structure function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 +/- 0.03 angstrom. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.
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| 2. |
- Christensson, Niklas, et al.
(författare)
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Four-wave-mixing spectroscopy of peridinin in solution and in the peridinin-chlorophyll-a protein
- 2010
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Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 373:1-2, s. 15-22
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Tidskriftsartikel (refereegranskat)abstract
- A model for the third order optical response of carotenoids is used to analyse transient grating and pump-probe data of peridinin in solution and bound in the peridinin-chlorophyll protein (PCP). For peridinin in solution, the transient grating signal detected at 505 nm exhibits a bi-exponential recovery whose fast phase is assigned to relaxation from the S-2 state that has a lifetime of 75 +/- 25 fs. The slower, solvent-dependent rise component is assigned to equilibration of the (S-1/ICT) state, taking place on a time scale of 0.6 and similar to 2.5 ps in acetontrile and benzene, respectively. These dynamics match those obtained from pump-probe measured in the spectral region of the ICT state, implying that the ICT state contributes to the signal at 505 nm. In PCP, the transient grating signal shows distinctly different kinetics, and the signal shows no recovery. This difference is explained by energy transfer from peridinin to chlorophyll-a. (C) 2009 Elsevier B. V. All rights reserved.
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| 3. |
- Dietzek, Benjamin, et al.
(författare)
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Optimal control of peridinin excited-state dynamics
- 2010
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Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 373:1-2, s. 129-136
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Tidskriftsartikel (refereegranskat)abstract
- Optimal control is applied to study the excited-state relaxation of the carbonyl-carotenoid peridinin in solution. Phase-shaping of the excitation pulses is employed to influence the photoinduced reaction dynamics of peridinin. The outcome of various control experiments using different experimentally imposed fitness parameters is discussed. Furthermore, the effects of pump-wavelength and different solvents on the control efficiency are presented. The data show that excited-state population within either the S-1 or the ICT state can be reduced significantly by applying optimal control, while the efficiency of control decreases upon excitation into the low-energy side of the absorption band. However, we are unable to alter the ratio of S-1 and ICT population or increase the population of either state compared to excitation with a transform-limited pulse. We compare the results to various control mechanisms and argue that characteristic low-wavenumber modes are relevant for the photochemistry of peridinin. (C) 2010 Elsevier B. V. All rights reserved.
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| 4. |
- Hansen, Thorsten, et al.
(författare)
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Orbital Topology Controlling Charge Injection in Quantum-Dot-Sensitized Solar Cells
- 2014
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 5:7, s. 1157-1162
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Tidskriftsartikel (refereegranskat)abstract
- Quantum-dot-sensitized solar cells are emerging as a promising development of dye-sensitized solar cells, where photostable semiconductor quantum dots replace molecular dyes. Upon photoexcitation of a quantum dot, an electron is transferred to a high-band-gap metal oxide. Swift electron transfer is crucial to ensure a high overall efficiency of the solar cell. Using femtosecond time-resolved spectroscopy, we find the rate of electron transfer to be surprisingly sensitive to the chemical structure of the linker molecules that attach the quantum dots to the metal oxide. A rectangular barrier model is unable to capture the observed variation. Applying bridge-mediated electron-transfer theory, we find that the electron-transfer rates depend on the topology of the frontier orbital of the molecular linker. This promises the capability of fine tuning the electron-transfer rates by rational design of the linker molecules.
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| 5. |
- Imahori, Hiroshi, et al.
(författare)
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Photoinduced Charge Carrier Dynamics of Zn-Porphyrin-TiO(2) Electrodes: The Key Role of Charge Recombination for Solar Cell Performance (†).
- 2011
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Ingår i: Journal of physical chemistry. A. - : American Chemical Society (ACS). - 1520-5215 .- 1089-5639. ; 115:16, s. 3679-3690
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Tidskriftsartikel (refereegranskat)abstract
- Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.
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| 6. |
- Kurotobi, Kei, et al.
(författare)
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Highly Asymmetrical Porphyrins with Enhanced Push-Pull Character for Dye-Sensitized Solar Cells
- 2013
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Ingår i: Chemistry: A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 19:50, s. 17075-17081
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Tidskriftsartikel (refereegranskat)abstract
- A porphyrin -system has been modulated by enhancing the push-pull character with highly asymmetrical substitution for dye-sensitized solar cells for the first time. Namely, both two diarylamino moieties as a strong electron-donating group and one carboxyphenylethynyl moiety as a strong electron-withdrawing, anchoring group were introduced into the meso-positions of the porphyrin core in a lower symmetrical manner. As a result of the improved light-harvesting property as well as high electron distribution in the anchoring group of LUMO, a push-pull-enhanced, porphyrin-sensitized solar cell exhibited more than 10% power conversion efficiency, which exceeded that of a representative highly efficient porphyrin (i.e., YD2)-sensitized solar cell under optimized conditions. The rational molecular design concept based on highly asymmetric, push-pull substitution will open the possibilities of further improving cell performance in organic solar cells.
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| 7. |
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| 8. |
- Polivka, Tomas, et al.
(författare)
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Carotenoid-protein interaction alters the S-1 energy of hydroxyechinenone in the Orange Carotenoid Protein
- 2013
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728. ; 1827:3, s. 248-254
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Tidskriftsartikel (refereegranskat)abstract
- The Orange Carotenoid Protein (OCP) is a photoactive water soluble protein that is crucial for photoprotection in cyanobacteria. When activated by blue-green light, it triggers quenching of phycobilisome fluorescence and regulates energy flow from the phycobilisome to the reaction center. The OCP contains a single pigment, the carotenoid 3'-hydroxyechinenone (hECN). Binding to the OCP causes a conformational change in hECN leading to an extension of its effective conjugation length. We have determined the S-1 energy of hECN in organic solvent and compared it with the S-1 energy of hECN bound to the OCP. In methanol and n-hexane, hECN has an S-1 energy of 14,300 cm(-1), slightly higher than carotenoids with shorter conjugation lengths such as zeaxanthin or beta-carotene; this is consistent with the proposal that the presence of the conjugated carbonyl group in hECN increases its Si energy. The S-1 energy of hECN in organic solvent is independent of solvent polarity. Upon binding to the OCP, the S-1 energy of hECN is further increased to 14,700 cm(-1), underscoring the importance of protein binding which twists the conjugated carbonyl group into s-trans conformation and enhances the effect of the carbonyl group. Activated OCP, however, has an S-1 energy of 14,000 cm(-1), indicating that significant changes in the vicinity of the conjugated carbonyl group occur upon activation. (C) 2012 Elsevier B.V. All rights reserved.
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| 9. |
- Ponseca, Carlito, et al.
(författare)
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Electron and Hole Contributions to the Terahertz Photoconductivity of a Conjugated Polymer:Fullerene Blend Identified
- 2012
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 3:17, s. 2442-2446
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved terahertz spectroscopy was employed for the investigation of charge-transport dynamics in benzothiadiazolo-dithiophene polyfluorene ([2,7-(9,9-dioctylfluorene)-alt-S,S-(4',7'-di-2-thieny1-2',1',3'-benzoth iadiazole)]) (APFO-3) polymers with various chain lengths and in its monomer form, all blended with an electron acceptor ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). Upon photo-excitation, charged polaron pairs are created, negative charges are transferred to fullerenes, while positive polarons remain on polymers/monomers. Vastly different hole mobility in polymer and monomer blends allows us to distinguish the hole and electron contributions to the carrier mobility.
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| 10. |
- Ponseca, Carlito, et al.
(författare)
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Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-Long Balanced Mobilities, and Slow Recombination
- 2014
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 136:14, s. 5189-5192
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Tidskriftsartikel (refereegranskat)abstract
- Organometal halide perovskite-based solar cells have recently been reported to be highly efficient, giving an overall power conversion efficiency of up to 15%. However, much of the fundamental photophysical properties underlying this performance has remained unknown. Here, we apply photoluminescence, transient absorption, time-resolved terahertz and microwave conductivity measurements to determine the time scales of generation and recombination of charge carriers as well as their transport properties in solution-processed CH3NH3PbI3 perovskite materials. We found that electron-hole pairs are generated almost instantaneously after photoexcitation and dissociate in 2 ps forming highly mobile charges (25 cm(2) V-1 s(-1)) in the neat perovskite and in perovskite/alumina blends; almost balanced electron and hole mobilities remain very high up to the microsecond time scale. When the perovskite is introduced into a TiO2 mesoporous structure, electron injection from perovskite to the metal oxide is efficient in less than a picosecond, but the lower intrinsic electron mobility of TiO2 leads to unbalanced charge transport. Microwave conductivity measurements showed that the decay of mobile charges is very slow in CH3NH3PbI3, lasting up to tens of microseconds. These results unravel the remarkable intrinsic properties of CH3NH3PbI3 perovskite material if used as light absorber and charge transport layer. Moreover, finding a metal oxide with higher electron mobility may further increase the performance of this class of solar cells.
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| 11. |
- Qenawy, Mohamed, et al.
(författare)
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Balancing Electron Transfer and Surface Passivation in Gradient CdSe/ZnS Core-Shell Quantum Dots Attached to ZnO
- 2013
-
Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:11, s. 1760-1765
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Tidskriftsartikel (refereegranskat)abstract
- Core-shell (CS) quantum dots (QDs) are promising light absorbers for solar cell applications mainly because of their enhanced photostability compared with bare QDs. Moreover, the superb photostability can be combined with a low number of defects by using CSQDs with a gradient composition change from the core to the shell. Here, we study electron injection from the gradient CSQDs to ZnO nanoparticles. We observe the typical exponential injection rate dependence on the shell thickness (beta = 0.51 angstrom(-1)) and discuss it in light of previously published results on step-like CSQDs. Despite the rapid drop in injection rates with shell thickness, we find that there exists an optimum thickness of the shell layer at similar to 1 nm, which combines high injection efficiency (>90%) with a superior passivation of QDs.
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| 12. |
- Qenawy, Mohamed, et al.
(författare)
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Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar Cell Performance
- 2014
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:44, s. 25802-25808
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Tidskriftsartikel (refereegranskat)abstract
- The performance of the current quantum dot (QD) solar cells is limited by several deficiencies. One of them is the existence of surface traps, especially hole traps, which are blocking the hole injection into the electrolyte. The trapping can be efficiently suppressed by growing a shell of wider band gap material around the core dot. Optimum parameters of such a shell layer for photovoltaic applications are, however, not established. We study effects of the shell formation on the ultrafast carrier dynamics and the performance of QD-sensitized solar cells. We can disentangle electron and hole dynamics and demonstrate that the QD shell diminishes surface hole trapping. By combining the knowledge about the hole trapping and electron injection into metal oxide we can clearly correlate the electron and hole dynamics with the solar cell efficiency as a function of the shell thickness. We conclude that the optimal shell thickness is 1.3 nm for this system.
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| 13. |
- Shao, Shuyan, et al.
(författare)
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Optimizing ZnO nanoparticle surface for bulk heterojunction hybrid solar cells
- 2013
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier. - 0927-0248 .- 1879-3398. ; 118, s. 43-47
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Tidskriftsartikel (refereegranskat)abstract
- The performance of hybrid solar cells composed of polymer and ZnO is mainly hindered by the defects of ZnO. Here, we investigate the effects of ZnO nanoparticle surface modification with poly(ethylene oxide) (PEO) on the performance of bulk heterojunction hybrid solar cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and ZnO nanoparticles. The reference device using ZnO nanoparticles as electron acceptor shows an open-circuit voltage (VOC) of 0.83 V, a short-circuit current (JSC) of 3.00 mA/cm2, a fill factor (FF) of 0.46 and a power conversion efficiency (PCE) of 1.15%. After modification with very small amount of PEO, the PCE will be enhanced, which is attributed to less surface traps of ZnO nanoparticles with PEO modification. With optimized PEO (0.05%) modified ZnO nanoparticles as electron acceptors, the device typically shows a VOC of 0.86 V, a JSC of 3.84 mA/cm2, a FF of 0.51 and a PCE of 1.68% due to less recombination loss of carriers, smaller series resistance, and improved electrical coupling between ZnO nanoparticle and MEH-PPV. However, further increase of PEO content to 0.3% will deteriorate device performance.
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| 14. |
- Slouf, Vaclav, et al.
(författare)
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Photoprotection in a purple phototrophic bacterium mediated by oxygen-dependent alteration of carotenoid excited-state properties
- 2012
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Ingår i: Proceedings of the National Academy of Sciences. - : Proceedings of the National Academy of Sciences. - 1091-6490 .- 0027-8424. ; 109:22, s. 8570-8575
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Tidskriftsartikel (refereegranskat)abstract
- Carotenoids are known to offer protection against the potentially damaging combination of light and oxygen encountered by purple phototrophic bacteria, but the efficiency of such protection depends on the type of carotenoid. Rhodobacter sphaeroides synthesizes spheroidene as the main carotenoid under anaerobic conditions whereas, in the presence of oxygen, the enzyme spheroidene monooxygenase catalyses the incorporation of a keto group forming spheroidenone. We performed ultrafast transient absorption spectroscopy on membranes containing reaction center-light-harvesting 1-PufX (RC-LH1-PufX) complexes and showed that when oxygen is present the incorporation of the keto group into spheroidene, forming spheroidenone, reconfigures the energy transfer pathway in the LH1, but not the LH2, antenna. The spheroidene/spheroidenone transition acts as a molecular switch that is suggested to twist spheroidenone into an s-trans configuration increasing its conjugation length and lowering the energy of the lowest triplet state so it can act as an effective quencher of singlet oxygen. The other consequence of converting carotenoids in RC-LH1-PufX complexes is that S-2/S-1/triplet pathways for spheroidene is replaced with a new pathway for spheroidenone involving an activated intramolecular charge-transfer (ICT) state. This strategy for RC-LH1-PufX-spheroidenone complexes maintains the light-harvesting cross-section of the antenna by opening an active, ultrafast S-1/ICT channel for energy transfer to LH1 Bchls while optimizing the triplet energy for singlet oxygen quenching. We propose that spheroidene/spheroidenone switching represents a simple and effective photoprotective mechanism of likely importance for phototrophic bacteria that encounter light and oxygen.
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| 15. |
- Ye, Shen, et al.
(författare)
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Role of Adsorption Structures of Zn-Porphyrin on TiO2 in Dye-Sensitized Solar Cells Studied by Sum Frequency Generation Vibrational Spectroscopy and Ultrafast Spectroscopy
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:12, s. 6066-6080
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Tidskriftsartikel (refereegranskat)abstract
- Several Zn-porphyrin (ZnP) derivatives were designed to build highly efficient dye-sensitized solar cells (DSC). It was found that solar cell efficiencies normalized for surface coverage (eta(rel)) are affected by the molecular spacer connecting the porphyrin core to the TiO2 surface, the sensitization conditions (solvent and time), and, to a lesser extent, the nature of the terminal group of the ZnP. Ultrafast transient absorption spectroscopy shows that electron transfer rates are strongly dependent on spacer and sensitization conditions. To understand this behavior at a molecular level, surface-sensitive vibrational spectroscopy, sum frequency generation (SFG), has been employed to investigate the adsorption geometries of these ZnP derivatives on the TiO2 surface for the first time. The average tilt angles and adsorption ordering of the ZnP molecules on the TiO2 surface were measured. A simple linear correlation between adsorption geometry of the adsorbed ZnP molecules, eta(rel), and the concentration of long-lived electrons in the conduction band of TiO2 was shown to exist. The more perpendicular the orientation of the adsorbed ZnP (relative to the TiO2 surface), the higher the concentration of long-lived electrons in the conduction band, which contributes to the increase of photocurrent and solar cell efficiency. This result indicates that the electron transfer between ZnP and TiO2 occurs "through-space" rather than "through the molecular spacer". It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Aggregation effects, which were observed for the longer sensitization times, are also discussed in relation to adsorption geometry and radiationless quenching processes. With the work reported here we demonstrate a novel strategy for DSC material characterization that can lead to design and manufacturing of photoactive materials with predictable and controlled properties.
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| 16. |
- Zheng, Kaibo, et al.
(författare)
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Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond the Point Dipole Approximation
- 2014
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 136:17, s. 6259-6268
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Tidskriftsartikel (refereegranskat)abstract
- Understanding of Forster resonance energy transfer (FRET) in thin films composed of quantum dots (QDs) is of fundamental and technological significance in optimal design of QD based optoelectronic devices. The separation between QDs in the densely packed films is usually smaller than the size of QDs, so that the simple point dipole approximation, widely used in the conventional approach, can no longer offer quantitative description of the FRET dynamics in such systems. Here, we report the investigations of the FRET dynamics in densely packed films composed of multisized CdSe QDs using ultrafast transient absorption spectroscopy and theoretical modeling. Pairwise interdot transfer time was determined in the range of 1.5 to 2 ns by spectral analyses which enable separation of the FRET contribution from intrinsic exciton decay. A rational model is suggested by taking into account the distribution of the electronic transition densities in the dots and using the film morphology revealed by AFM images. The FRET dynamics predicted by the model are in good quantitative agreement with experimental observations without adjustable parameters. Finally, we use our theoretical model to calculate dynamics of directed energy transfer in ordered multilayer QD films, which we also observe experimentally. The Monte Carlo simulations reveal that three ideal QD monolayers can provide exciton funneling efficiency above 80% from the most distant layer. Thereby, utilization of directed energy transfer can significantly improve light harvesting efficiency of QD devices.
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| 17. |
- Zheng, Kaibo, et al.
(författare)
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Effect of metal oxide morphology on electron injection from CdSe quantum dots to ZnO
- 2013
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 102:16
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Tidskriftsartikel (refereegranskat)abstract
- Performance of quantum dot sensitized solar cells relies on a rapid electron injection from quantum dot to metal oxide. We studied the injection process in CdSe-ZnO system by ultrafast time-resolved absorption spectroscopy for two types of acceptor morphologies-nanowires and nanoparticles' films. Based on comparison between experimental data and Marcus theory, we demonstrate that the acceptor morphology has a significant impact on electron injection due to (i) change in material permittivity and (ii) different density of the band-edge states. The results open a reference to improve injection efficiency in quantum dot-metal oxide system by selection of the acceptor morphology. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4803173]
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| 18. |
- Zheng, Kaibo, et al.
(författare)
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Fast Monolayer Adsorption and Slow Energy Transfer in CdSe Quantum Dot Sensitized ZnO Nanowires
- 2013
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society. - 1089-5639 .- 1520-5215. ; 117:29, s. 5919-5925
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Tidskriftsartikel (refereegranskat)abstract
- A method for CdSe quantum dot (QD) sensitization of ZnO nanowires (NW) with fast adsorption rate is applied. Photoinduced excited state dynamics of the quantum dots in the case of more than monolayer coverage of the nanowires is studied. Transient absorption kinetics reveals an excitation depopulation process of indirectly attached quantum dots with a lifetime of similar to 4 ns. Photoluminescence and incident photon-to-electron conversion efficiency show that this process consists of both radiative e-h recombination and nonradiative excitation-to-charge conversion. We argue that the latter occurs via interdot energy transfer from the indirectly attached QDs to the dots with direct contact to the nanowires. From the latter, fast electron injection into ZnO occurs. The energy transfer time constant is found to be around 5 ns.
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| 19. |
- Zheng, Kaibo, et al.
(författare)
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Ultrafast Charge Transfer from CdSe Quantum Dots to p-Type NiO: Hole Injection vs Hole Trapping
- 2014
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:32, s. 18462-18471
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Tidskriftsartikel (refereegranskat)abstract
- Semiconductor quantum dot (QD) to metal oxide electron injection dynamics is well documented in the scientific literature. In contrast to that, not much is known so far about hole injection time scales in such systems. The current study fills this gap. We investigate photocathodes consisting of CdSe QDs and p-type NiO to study hole injection dynamics from the valence band of the QDs to NiO. The combination of two complementary techniques, ultrafast time-resolved absorption and fluorescence spectroscopies, enabled us to distinguish between hole trapping and injection. A kinetic component on the time scale of a few hundreds of picoseconds was identified as hole injection. By changing the size of the QDs, the driving force of the hole injection was tuned and we demonstrated that the hole injection rates are well described by the Marcus theory of charge transfer. In order to enhance the overall hole injection efficiency, we have passivated the CdSe QDs by a gradient ZnS shell. The core-shell QDs show significantly slower hole injection; still, since trapping was almost eliminated, the overall hole injection efficiency was greatly enhanced.
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| 20. |
- Zidek, Karel, et al.
(författare)
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Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study
- 2012
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 134:29, s. 12110-12117
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Tidskriftsartikel (refereegranskat)abstract
- Photoinduced electron injection dynamics from CdSe quantum dots to ZnO nanowires is studied by transient absorption and time-resolved terahertz spectroscopy measurements. Ultrafast electron transfer from the CdSe quantum dots to ZnO is proven to be efficient already on a picoseconds time scale (tau = 3-12 ps). The measured kinetics was found to have a two-component character, whose origin is discussed in detail. The obtained results suggest that electrons are injected into ZnO via an intermediate charge transfer state.
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| 21. |
- Zidek, Karel, et al.
(författare)
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Quantum dot photodegradation due to CdSe-ZnO charge transfer: Transient absorption study
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:24
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Tidskriftsartikel (refereegranskat)abstract
- We study changes in ultrafast transient absorption due to photodegradation of quantum dots attached to ZnO nanowire. The time-resolved measurements reveal impact of photodegradation on three distinct kinetic components present in transient absorption tau similar to 7 ps, 80 ps, and 7.5 ns). In addition, we observe superlinear dependence of photodegradation rate on concentration of excited electrons. The data are used to evaluate the mean electron back-recombination time of similar to 1 mu s. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729382]
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| 22. |
- Zidek, Karel, et al.
(författare)
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Simultaneous Creation and Recovery of Trap States on Quantum Dots in a Photoirradiated CdSe-ZnO System
- 2014
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:47, s. 27567-27573
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Tidskriftsartikel (refereegranskat)abstract
- We study changes in the steady-state absorption and ultrafast transient absorption kinetics of the photoirradiated CdSe quantum dotZnO system. The changes enable us to reconstruct kinetics of trap creation, which are analyzed with respect to three possible models: trap creation without recovery, trap creation and recovery, and trap creation with an upper limit for trap number accommodated on a quantum dot. We demonstrate that only the model of parallel trap creation and recovery can explain our experimental data. The evidence points toward oxygen generating trapping sites on QD surface and simultaneously passivating the trapping sites by their oxidation.
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| 23. |
- Zidek, Karel, et al.
(författare)
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Ultrafast Dynamics of Multiple Exciton Harvesting in the CdSe-ZnO System: Electron Injection versus Auger Recombination.
- 2012
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:12, s. 6393-6399
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Tidskriftsartikel (refereegranskat)abstract
- We study multiple electron transfer from a CdSe quantum dot (QD) to ZnO, which is a prerequisite for successful utilization of multiple exciton generation for photovoltaics. By using ultrafast time-resolved spectroscopy we observe competition between electron injection into ZnO and quenching of multiexcitons via Auger recombination. We show that fast electron injection dominates over biexcitonic Auger recombination and multiple electrons can be transferred into ZnO. A kinetic component with time constant of a few tens of picoseconds was identified as the competition between injection of the second electron from a doubly excited QD and a trion Auger recombination. Moreover, we demonstrate that the multiexciton harvesting efficiency changes significantly with QD size. Within a narrow QD diameter range from 2 to 4 nm, the efficiency of electron injection from a doubly excited QD can vary from 30% to 70% in our system.
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