Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye−Semiconductor (TiO<sub>2</sub>) Interface

Rehm, Julie M.; McLendon, George; Nagasawa, Yutaka; Yoshihara, Keitaro; Moser, Jacques-E.; Grätzel, Michaël

doi:10.1021/jp960155m

Cited by 419 publications

(390 citation statements)

References 12 publications

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“…Our conclusion that electron injection occurs on a picosecond and subpicosecond time scales is consistent with conclusion of Willig and coworkers that electron injection in Ru(dcbpy) 2 (H 2 O) 2 2+ -sensitized nanocrystalline TiO 2 films occurs in <7 ps. Similar subpicosecond excited-state quenching has also been reported for adsorption of coumarin dye onto nanocrystalline TiO 2 films 27 and has been inferred from fluorescence quenching experiments for an oxazine dye into SnS 2 . 28 This result is however at variance the results of Kamat and co-workers, which indicate that electron injection from Ru(dcbpy)(bpy) 2 2+ into a range of different semiconductor films occurs in 2-10 ns.…”

Section: Discussionsupporting

confidence: 79%

Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

et al. 1996

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We have employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of the ruthenium dye Ru II (2,2′-bipyridyl-4,4′-dicarboxylate) 2 (NCS) 2 (1) adsorbed onto the surface of nanocrystalline titanium dioxide (TiO 2 ) films. This sensitizer dye is of particular interest as it is the most efficient sensitizer dye reported to date and is receiving considerable attention for applications in photoelectrochemical solar energy conversion. Transient data collected for 1 adsorbed onto TiO 2 films were compared with those obtained for control dye-coated ZrO 2 films, as the high conduction band edge of ZrO 2 prevents electron injection. Adsorption of the dye onto the TiO 2 film was found to result in a rapid (<500 ps) quenching of the dye excited-state luminescence. Absorption difference spectra collected for the two dye-coated films were assigned by comparison with the spectroscopy of the dye excited and cation states in solution. These transient absorption data indicated that electron injection in these films occurs in e10 -12 s. Detailed analysis indicates the injection is at least biphasic, with ∼50% occurring in <150 fs (instrument response limited) and 50% in 1.2 ( 0.2 ps. These ultrafast electron injection kinetics are contrasted with the charge recombination reaction, which occurs on the microsecond-millisecond time scales. The ultrafast rate of electron injection observed here is critical both for the high energy conversion efficiencies obtained with this sensitizer dye, and for the excellent long-term stability of this dye in photoelectrochemical solar cells.

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Section: Discussionsupporting

confidence: 79%

Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

et al. 1996

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“…There is now a growing consensus that the electron injection process, in dye-sensitized nanocrystallineTiO 2 films, can proceed on subpicosecond time scales for a range of different sensitizer dyes. [4][5][6][7][8][9][10][11] We recently found that these injection kinetics are moreover rather insensitive to variations in experimental conditions (e.g., electrolyte composition and application of electrical bias 6,11 ). In contrast, the corresponding charge recombination reaction between injected electrons and dye cations has been reported to exhibit time constants ranging from picoseconds to milliseconds.…”

Section: Introductionmentioning

confidence: 99%

Parameters Influencing Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

et al. 1999

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Optical excitation of Ru II (2,2′-bipyridyl-4,4′dicarboxylate) 2 (NCS) 2 -sensitized nanocrystalline TiO 2 films results in injection of an electron into the semiconductor. This paper addresses the kinetics of charge recombination which follows this charge separation reaction. These charge recombination kinetics were found to be strongly dependent upon excitation intensity, electrolyte composition, and the application of an electrical bias to the TiO 2 film. For excitation intensities resulting in less than one excited dye molecule/TiO 2 particle, the recombination kinetics were independent of excitation intensity. Increasing the excitation intensity above this level resulted in a rapid acceleration in the charge recombination kinetics. Similarly, for positive electrical potentials applied to the TiO 2 electrode, the recombination kinetics were independent of applied potential. If the applied potential was more negative than a threshold potential V kin , a rapid acceleration of the charge recombination kinetics was again observed, for example from ∼1 ms at +0.1 V vs Ag/AgCl to ∼3 ps at -0.8 V (∼10 8 fold increase in the rate). Moreover, at a constant applied potential the charge recombination kinetics were found to be strongly dependent upon electrolyte composition (up to 10 6 -fold change in rate). This strong dependence upon the electrolyte composition was found to be associated with shifts in the threshold potential V kin . Spectroelectrochemical measurements were used to monitor the shift in the trap/conduction band density of states induced by the electrolyte composition. A direct correlation was observed between the threshold voltage V kin observed from kinetic measurements, and the threshold voltage for electron occupation of conduction band/trap states of the TiO 2 observed from spectroelectrochemical measurements. This direct correlation was observed for a wide range of electrolyte compositions including protic and aprotic solvents and the addition of Li + ions and 4-tert-butylpyridine. We conclude that the charge recombination kinetics in such dye-sensitized films are strongly dependent upon the electron occupation in trap/conduction band states of the TiO 2 film. This occupation may be modulated by variations in light intensity, applied electrical potential, and electrolyte composition. These results are discussed with relevance to the function of dye-sensitized photoelectrochemical devices.

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“…If this state lies energetically above the conduction band edge of the colloid, electron injection to the semiconductor can occur on a fast or ultrafast time scale. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] The resulting charge-separated system was reported to undergo relaxation and recombination processes, with typical time constants in the range from 10 fs up to 500 µs, 9,12,21,24,25 dependent on the dye/semiconductor system. According to Marcus theory one of the main parameters for the electron injection rate is the difference in free energy ∆G between the donor and acceptor states.…”

Section: Introductionmentioning

confidence: 99%

The Role of Surface States in the Ultrafast Photoinduced Electron Transfer from Sensitizing Dye Molecules to Semiconductor Colloids

et al. 2000

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Investigations on the ultrafast electron injection mechanism from the dye alizarin to wide band gap semiconductor colloids in aqueous medium are presented, combined with detailed studies on population, depopulation, and relaxation phenomena in trap states and their influence on the injection process. Because of the very strong electronic coupling between dye and semiconductor in an alizarin/TiO 2 system, a very fast electron injection from the excited dye to the conduction band of TiO 2 is expected. Our measurements show an injection time τ inj < 100 fs, suggesting that the electron transfer follows an adiabatic mechanism. Furthermore, we present experiments over a wide spectral range on the recombination reaction of the electron in the conduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constants from 400 fs to the nanosecond time scale. The nonexponential character of the recombination reaction is attributed to fast relaxation processes. The crucial contribution of surface trap states and their influence on the observed dynamics was investigated with alizarin adsorbed on the insulating substrate ZrO 2 . Since the conduction band edge lies far above (≈1 eV) the S 1 state of alizarin, the electron injection into this band is completely suppressed. Despite this fact our spectroscopic investigations show that on ultrafast time scales the formation of an alizarin cation occurs. This observation, is explained by fast electron injection into surface trap states near the docking site on the colloid. For the alizarin/ZrO 2 system the time scale for the injection into these traps is determined to be faster than 100 fs. The relaxation processes in the traps and the repopulation of the S 1 state occur within 450 fs, the subsequent ground-state relaxation takes 160 ps. The ultrafast injection dynamics into the traps, recorded for alizarin/ ZrO 2 , underlines the importance of surface states for the initial charge separation also for systems with a lower band edge such as TiO 2 . We show that in the dye/ZrO 2 system the process of electron injection is not suppressed but "stopped" after the ultrafast transition into trap states. It is therefore a valuable system for probing the electron dynamics in surface states.

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Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye−Semiconductor (TiO₂) Interface

Cited by 419 publications

References 12 publications

Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

Parameters Influencing Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

The Role of Surface States in the Ultrafast Photoinduced Electron Transfer from Sensitizing Dye Molecules to Semiconductor Colloids

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Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye−Semiconductor (TiO2) Interface

Cited by 419 publications

References 12 publications

Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

Parameters Influencing Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

The Role of Surface States in the Ultrafast Photoinduced Electron Transfer from Sensitizing Dye Molecules to Semiconductor Colloids

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Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye−Semiconductor (TiO₂) Interface