Photoinduced Ultrafast Dynamics of Ru(dcbpy)2(NCS)2-Sensitized Nanocrystalline TiO2 Films: The Influence of Sample Preparation and Experimental Conditions

Reductive quenching of photosensitizer [Ru(bpy)3]2+ reveals the inhomogeneous distribution of sites in PAN polymer nanofibers for light-driven redox catalysis

Integration of molecular photocatalysts into redox-inert polymers constitutes a path towards photocatalytically active, lightweight materials. In particular, electrospun polymer fibers hold potential due to their favorable surface-to-volume ratio and their straightforward fabrication. This study focuses on the polyacrylonitrile (PAN) fibers, into which the prototype photosensitizer (PS) ruthenium tris(bipyridine) [Ru(bpy)3]2+, has been embedded by electrospinning. Studying the interaction between the optically excited [Ru(bpy)3]2+ with a non-redox inert solvent within the nanofibers, we resolve a distribution of microenvironments, which differ by the extent to which the photosensitizer is exposed to the solvent. This results in a non-exponential decay of the complex's emission and pronounced differences in the transient absorption signals.

Vibronic coherence evolution in multidimensional ultrafast photochemical processes

The complex choreography of electronic, vibrational, and vibronic couplings used by photoexcited molecules to transfer energy efficiently is remarkable, but an unambiguous description of the temporally evolving vibronic states governing these processes has proven experimentally elusive. We use multidimensional electronic-vibrational spectroscopy to identify specific time-dependent excited state vibronic couplings involving multiple electronic states, high-frequency vibrations, and low-frequency vibrations which participate in ultrafast intersystem crossing and subsequent relaxation of a photoexcited transition metal complex. We discover an excited state vibronic mechanism driving long-lived charge separation consisting of an initial electronically-localized vibrational wavepacket which triggers delocalization onto two charge transfer states after propagating for ~600 femtoseconds. Electronic delocalization consequently occurs through nonadiabatic internal conversion driven by a 50 cm^-1 coupling resulting in vibronic coherence transfer lasting for ~1 picosecond. This study showcases the power of multidimensional electronic-vibrational spectroscopy to elucidate complex, non-equilibrium energy and charge transfer mechanisms involving multiple molecular coordinates.

Ultrafast photoinduced energy and charge transfer

After presenting the basic theoretical models of excitation energy transfer and charge transfer, I describe some of the novel experimental methods used to probe them. Finally, I discuss recent results concerning ultrafast energy and charge transfer in biological systems, in chemical systems and in photovoltaics based on sensitized transition metal oxides.

Ultrafast interligand electron transfer in cis-[Ru(4,4'-dicarboxylate-2,2'-bipyridine)2(NCS)2]4- and implications for electron injection limitations in dye sensitized solar cells

Interligand electron transfer (ILET) of the lowest metal-to-ligand charge transfer (MLCT) state of N712 (cis-[Ru(dcb)2(NCS)2]4-, where dcb = 4,4'-dicarboxylate-2,2'-bipyridine) in a deuterated acetonitrile solution has been studied by means of femtosecond transient absorption anisotropy in the mid-IR. Time-independent B3LYP density functional calculations were performed to assign vibrational bands and determine their respective transition dipole moments. The transient absorption spectral band at 1327 cm-1, assigned to a symmetric carboxylate stretch, showed significant anisotropy. A rapid anisotropy increase (τ 1 ≈ 2 ps) was tentatively assigned to vibrational and solvent relaxation, considering the excess energy available after the excited singlet-triplet conversion. Thereafter, the anisotropy decayed to zero with a time constant τ 2 ≈ 240 ps, which was assigned to the rotational correlation time of the complex in deuterated acetonitrile. No other distinctive changes to the anisotropy were observed and the amplitude of the slow component at time zero agrees well with that predicted for a random mixture of MLCT localization on either of the two dcb ligands. The results therefore suggest that MLCT randomization over the two dcb ligands occurs on the sub-ps time scale. This is much faster than proposed by previous reports on the related N3 complex [Benkö et al., J. Phys. Chem. B, 2004, 108, 2862, and Waterland et al., J. Phys. Chem. A, 2001, 105, 4019], but in agreement with that found by Wallin and co-workers [J. Phys. Chem. A, 2005, 109, 4697] for the [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) complex. This suggests that electron injection from the excited dye into TiO2 in dye-sensitized solar cells is not limited by ILET.

Long-lived charge separation in dye–semiconductor assemblies: a pathway to multi-electron transfer reactions

Key approaches to achieve long-lived charge separation and promote conduction band mediated electron transfer in dye-sensitized semiconductor assemblies.

Interfacial Electron Injection Probed by a Substrate-Specific Excitonic Signature

Ultrafast interfacial electron transfer in sensitized solar cells has mostly been probed by visible-to-terahertz radiation, which is sensitive to the free carriers in the conduction band of the semiconductor substrate. Here, we demonstrate the use of deep-ultraviolet continuum pulses to probe the interfacial electron transfer, by detecting a specific excitonic transition in both N719-sensitized anatase TiO₂ and wurtzite ZnO nanoparticles. Our results are compared to those obtained on bare nanoparticles upon above-gap excitation. We show that the signal upon electron injection from the N719 dye into TiO₂ is dominated by long-range Coulomb screening of the final states of the excitonic transitions, whereas in sensitized ZnO it is dominated by phase-space filling. The present approach offers a possible route to detecting interfacial electron transfer in a broad class of systems, including other transition metal oxides or sensitizers.

Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO₂ by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO₂ reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO₂ reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

In situ spectroelectrochemical and theoretical study on the oxidation of a 4H-imidazole-ruthenium dye adsorbed on nanocrystalline TiO2 thin film electrodes

Terpyridine 4H-imidazole-ruthenium(II) complexes are considered promising candidates for use as sensitizers in dye sensitized solar cells (DSSCs) by displaying broad absorption in the visible range, where the dominant absorption features are due to metal-to-ligand charge transfer (MLCT) transitions. The ruthenium(III) intermediates resulting from photoinduced MLCT transitions are essential intermediates in the photoredox-cycle of the DSSC. However, their photophysics is much less studied compared to the ruthenium(II) parent systems. To this end, the structural alterations accompanying one-electron oxidation of the RuIm dye series (including a non-carboxylic RuIm precursor, and, carboxylic RuImCOO in solution and anchored to a nanocrystalline TiO2 film) are investigated via in situ experimental and theoretical UV-Vis absorption and resonance Raman (RR) spectroelectrochemistry. The excellent agreement between the experimental and the TDDFT spectra derived in this work allows for an in-depth assignment of UV-Vis and RR spectral features of the dyes. A concordant pronounced wavelength dependence with respect to the charge transfer character has been observed for the model system RuIm, and both RuImCOO in solution and attached on the TiO2 surface. Excitation at long wavelengths leads to the population of ligand-to-metal charge transfer states, i.e. photoreduction of the central ruthenium(III) ion, while high-energy excitation features an intra-ligand charge transfer state localized on the 4H-imidazole moiety. Therefore, these 4H-imidazole ruthenium complexes investigated here are potential multi-photoelectron donors. One electron is donated from MLCT states, and additionally, the 4H-imidazole ligand reveals electron-donating character with a significant contribution to the excited states of the ruthenium(III) complexes upon blue-light irradiation.

Zinc oxide nanocrystal quenching of emission from electron-rich ruthenium-bipyridine complexes

A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods. Each compound incorporated one electron withdrawing 4,4'-dicarboxylic acid-2,2'-bipyridine and two bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4' positions. The electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the amine substituent increased. Density functional calculations established that the HOMO was delocalized over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine. The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data, the excited state reduction potentials were estimated to range from -0.93 to -1.03 V vs. NHE. Monodispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs on the small end of the particle size distribution had electron transfer rates that were uncompetitive with radiative and nonradiative decay mechanisms.

Monitoring the intramolecular charge transfer process in the Z907 solar cell sensitizer: a transient Vis and IR spectroscopy and ab initio investigation

The excited state dynamics of Z907 in solution and on semiconductor substrates has been studied with ultrafast UV/Vis and IR spectroscopy and DFT/TDDFT calculations.

Phonon mode of TiO2 coupled with the electron transfer from N3 dye

Low frequency vibrational spectra of submonolayer N3 dye (Ru(4,4(')-dicarboxy-2,2(')-bipyridine)2(NCS)2) adsorbed on TiO2 (110) were reported by using fourth-order coherent Raman spectroscopy, which is interface-sensitive vibrational spectroscopy. Most of the peaks observed in the experiment were at the same frequency as that of Raman and infrared spectra of the dye and TiO2. Two interfacial modes at 640 and 100 cm(-1) and one resonantly enhanced phonon at 146 cm(-1) appeared in addition to the pure TiO2 and N3 spectra. Adsorption of N3 dye on TiO2 contributed to the enhancement of 100 and 146 cm(-1) mode. The results not only reported interfacial low-frequency vibrations of TiO2 (110) with N3 dye adsorption but also suggested the coupling between the surface vibrations of TiO2 and charge transfer between N3 dye and TiO2 on the surface.

Photoinduced Interfacial Electron Injection Dynamics in Dye-Sensitized Solar Cells under Photovoltaic Operating Conditions

We report a pump-probe spectroscopy study of electron injection rates in dye-sensitized solar cell (DSSC) devices. We examine the case of working devices employing an N719 ruthenium sensitizer and an iodide electrolyte. Electron injection is found to occur mainly on a sub-100 fs time scale, followed by a slower component with a lifetime of 26.9 ps, in accordance with previous reports on model samples. The amplitude of this latter component varies with electrolyte composition from 25 to 9%. The appearance of slower components in the electron injection dynamics may be attributed to an aggregated or weakly bound state of the surface-adsorbed N719 sensitizer. Further measurements are reported varying the cell light bias and load conditions, revealing no influence on electron injection dynamics. No other electron injection event is found to occur up to 1 ns. These results show no evidence for a slowdown of electron injection under working conditions compared to model systems for the electrolytes examined in this study.

Long-wavelength sensitization of TiO2 by ruthenium diimine compounds with low-lying π* orbitals

The role of low-lying π* orbitals in dye-sensitized solar cells based on mesoporous thin films of anatase TiO(2) nanocrystallites remains unknown. Herein we report three ruthenium compounds, cis-Ru(dcbq)(2)(NCS)(2), cis-Ru(dcbq)(bpy)(NCS)(2), and cis-Ru(dcb)(bq)(NCS)(2), where bpy is 2,2'-bipyridine, dcb is 4,4'-(CO(2)H)(2)-2,2'-bipyridine, bq is 2,2'-biquinoline, and dcbq is 4,4'-(CO(2)H)(2)-2,2'-biquinoline, that were synthesized, characterized, and contrasted with the well-known N3 compound (i.e., cis-Ru(dcb)(2)(NCS)(2)) in dye-sensitized solar cells. These compounds maintain the same cis-Ru(NCS)(2) core with a systematic variation in the energy of the π* orbitals of the diimine ligand: bpy > dcb > bq > dcbq. The lowered π* orbitals resulted in enhanced red absorption relative to N3. With HCl pretreated TiO(2) in regenerative solar cells, sensitization from 400 to 900 nm was realized with cis-Ru(dcb)(bq)(NCS)(2) and global power conversion efficiencies as high as 6.5% were achieved under 1 sun of AM 1.5 irradiation. The energy conversion efficiency was found to be acutely sensitive to the presence of p-tert-butylpyridine (TBP) in a 0.5 M LiI/0.05 M I(2) acetonitrile electrolyte. Nanosecond transient absorption studies revealed that the addition of TBP decreased the excited-state injection yield for the compounds with biquinoline ligands. Spectro-electrochemical studies showed that the HCl pretreatment lowered the effective density of TiO(2) acceptor states and confirmed that the presence of TBP raised them toward the vacuum level. There was no spectroscopic data to support the hypothesis that the π* levels of the diimine ligand mediate back-electron transfer to the oxidized dye or the redox mediator was found.

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).

ZnO nanotube based dye-sensitized solar cells

We introduce high surface area ZnO nanotube photoanodes templated by anodic aluminum oxide for use in dye-sensitized solar cells (DSSCs). Atomic layer deposition is utilized to coat pores conformally, providing a direct path for charge collection over tens of micrometers thickness. Compared to similar ZnO-based devices, ZnO nanotube cells show exceptional photovoltage and fill factors, in addition to power efficiencies up to 1.6%. The novel fabrication technique provides a facile, metal-oxide general route to well-defined DSSC photoanodes.

Comparison of Electron Injection Dynamics from Re-bipyridyl Complexes to TiO2 Nanocrystalline Thin Films in Different Solvent Environments

Factors that control photoinduced interfacial electron transfer (ET) between molecular adsorbates and semiconductor nanoparticles have been intensely investigated in recent years. In this work, the solvent dependence of interfacial ET was studied by comparing ET rates in dye sensitized TiO2 nanocrystalline films in different solvent environments. Photoinduced ET rates from Re(LA)(CO)3Cl [LA=dcbpy=4,4'-dicarboxy-2,2'-bipyridine] (ReC1A) to TiO2 nanocrystalline thin films in air, pH buffer, MeOH, EtOH, and DMF were measured by femtosecond transient IR spectroscopy. The ET rates in these solvent environments were noticeably different. However, differences between the rates in pH buffer and nonaqueous solvents (MeOH, EtOH, and DMF) were much smaller than the values expected from much more negative TiO2 conduction band-edge positions in the latter solvents under anhydrous conditions. It was suggested that the presence of adsorbed water, which was evident in FTIR spectra, lowered the band edge of TiO2 in these solvents and reduced the rate differences. The important effect of adsorbed water was verified by comparing two samples of Re(LP)(CO)3Cl [LP=2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] sensitized TiO2 in DMF, in which the presence of a trace amount of water was found to significantly increase the injection rate.

Comparison of Interfacial Electron Transfer through Carboxylate and Phosphonate Anchoring Groups†

The effects of anchoring groups on electron injection from adsorbate to nanocrystalline thin films were investigated by comparing injection kinetics through carboxylate versus phosphonate groups to TiO2 and SnO2. In the first pair of molecules, Re(LA)(CO)3Cl (ReC1A) and Re(Lp)(CO)3Cl (ReC1P), [LA=2,2'-bipyridine-4,4'-bis-CH2-COOH, Lp=2,2'-bipyridine-4,4'-bis-CH2-PO3H2], the anchoring groups were insulated from the bipyridine ligand by a CH2 group. In the second pair of molecules, Ru(dcbpyH2)2(NCS)2 (RuN3) and Ru(bpbpyH2)2(NCS)2 (RuN3P), [dcbpy=2,2'-bipyridine-4,4'-biscarboxylic acid, bpbpy=2,2'-bipyridine-4,4'-bisphosphonic acid], the anchoring groups were directly connected to the bipyridine ligands. The injection kinetics, as measured by subpicosecond IR absorption spectroscopy, showed that electron injection rates from ReC1P to both TiO2 and SnO2 were faster than those from ReC1A. The injection rates from RuN3 and RuN3P to SnO2 films were similar. On TiO2, the injection kinetics from RuN3 and RuN3P were biphasic: carboxylate group enhances the rate of the <100 fs component, but reduces the rate of the slower components. To provide insight into the effect of the anchoring groups, the electronic structures of Re-bipyridyl-Ti model clusters containing carboxylate and phosphonate anchoring groups and with and without a CH2 spacer were computed using density functional theory. With the CH2 spacer, the phosphonate group led to a stronger electronic coupling between bpy and Ti center than the carboxylate group, which accounted for the faster injection from ReC1P than ReC1A. When the anchoring groups were directly connected to the bpy ligand without the CH2 spacer, such as in RuN3 and RuN3P, their effects were 2-fold: the carboxylate group enhanced the electronic coupling of bpy pi* with TiO2 and lowered the energy of the bpy orbital. How these competing factors led to different effects on TiO2 and SnO2 and on different components of the biphasic injection kinetics were discussed.

Calculation of the vibrationally non-relaxed photo-induced electron transfer rate constant in dye-sensitized solar cells

In this paper we shall show how to calculate the single vibronic-level electron-transfer rate constant, which will be compared with the thermal averaged one. To apply the theoretical results to the dye-sensitized solar cells, we use a simple model to describe how we model the final state of the electron-transfer process. Numerical calculations will be performed to demonstrate the theoretical results.

Control of electron transfer pathways in a dye-sensitized solar cell

Using shaped laser pulses, we increase the yield of ultrafast electron injection from the sensitizer to TiO2 nanocrystals in the core part of a dye-sensitized solar cell. The temporal structure of the optimized excitation pulse is in clear correlation with nuclear oscillations in the impulsively excited dye molecule. From DFT structure optimization and normal mode analyses we identified the modes which are responsible for the oscillations. The best pulse shape suggests Impulsive Stimulated (anti-Stokes) Raman scattering as a key process of optimization.

Inhomogeneity of Electron Injection Rates in Dye-Sensitized TiO2: Comparison of the Mesoporous Film and Single Nanoparticle Behavior

As it has been shown by pump-probe experiments electron injection at the interface between a dye molecule and mesoporous TiO2 proceeds with rates exceeding 1 x 10(13) s(-1). However, similar dye-TiO2 systems exhibit residual dye emission with lifetimes extending into the long nanosecond range. To address this inhomogeneity of injection rates time-correlated single photon counting microscopy was used to compare the emission behavior of dye-sensitized mesoporous films of TiO2 with that of individual anatase nanoparticles that had undergone extensive dialysis. The sensitized films produce intense residual emission with multiexponential decay components as long as 220 ns. The channels of mesoporous films contain physisorbed and trapped dye, which is the dominant source of the emission. It is likely that the wide range of lifetimes reflects the distribution of mean free paths experienced by the loose dye molecules diffusing within the film prior to undergoing oxidative quenching. In contrast, the intensity of emission from individual nanoparticles from which the loose dye was removed by dialysis is orders of magnitude lower. The lifetimes obtained from such particles are much shorter, with the primary component on a sub-nanosecond time scale. The presence of residual emission with a 230 ps lifetime shows that even on the surfaces of dialyzed nanoparticles there is a fraction of sensitizer molecules that do not inject electrons with the same high rate as is observed in ultrafast pump-probe experiments on films. Since the physisorbed dye was removed from these samples by dialysis, the residual emission is likely to originate from dye molecules bound to surface defects. Unusual collective emission bursts were observed in some of the measurements on sensitized nanoparticles. We attribute this behavior to stimulated emission from individual nanocrystallites.

Theoretical treatments of ultrafast electron transfer from adsorbed dye molecule to semiconductor nanocrystalline surface

In studying ultrafast electron transfer from a dye molecule to a nanosized semiconductor particle, pump-probe experiments are commonly used. In this system the electron transfer (ET) rate is faster than vibrational relaxation so that the ET rate should be described by a single-level rate constant and the probing signal (often in the form of time-resolved spectra) contains the contribution from the dynamics of both population and coherence (i.e., wave packet). In this paper, we shall present the theoretical treatments for femtosecond time-resolved pump-probe experiment and the dynamics of population and coherence by the density matrix method, and the calculation of single-level ET rate constant involved in a pump-probe experiment. As an application, we show the theoretical results using parameters extracted from experiments on a specific dye/semiconductor system.

ZnO−Al2O3 and ZnO−TiO2 Core−Shell Nanowire Dye-Sensitized Solar Cells

We describe the construction and performance of dye-sensitized solar cells (DSCs) based on arrays of ZnO nanowires coated with thin shells of amorphous Al(2)O(3) or anatase TiO(2) by atomic layer deposition. We find that alumina shells of all thicknesses act as insulating barriers that improve cell open-circuit voltage (V(OC)) only at the expense of a larger decrease in short-circuit current density (J(SC)). However, titania shells 10-25 nm in thickness cause a dramatic increase in V(OC) and fill factor with little current falloff, resulting in a substantial improvement in overall conversion efficiency, up to 2.25% under 100 mW cm(-2) AM 1.5 simulated sunlight. The superior performance of the ZnO-TiO(2) core-shell nanowire cells is a result of a radial surface field within each nanowire that decreases the rate of recombination in these devices. In a related set of experiments, we have found that TiO(2) blocking layers deposited underneath the nanowire films yield cells with reduced efficiency, in contrast to the beneficial use of blocking layers in some TiO(2) nanoparticle cells. Raising the efficiency of our nanowire DSCs above 2.5% depends on achieving higher dye loadings through an increase in nanowire array surface area.

Electron-Transfer Dynamics from Ru Polypyridyl Complexes to In2O3 Nanocrystalline Thin Films

Photoinduced electron injection dynamics from Ru(dcbpy)(2)(X)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X(2) = 2SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470) to In(2)O(3) nanocrystalline thin films were studied using ultrafast transient IR absorption spectroscopy. After 532 nm excitation of the adsorbates, the dynamics of electron injection from their excited states to In(2)O(3) were studied by monitoring the IR absorption of the injected electrons in the semiconductor. The injection kinetics were non-single-exponential. For samples exposed to air, the half rise times, defined as the time of 50% injection yield, were 5 +/- 0.8, 85 +/- 20, and >200 ps for RuN3, Ru505, and Ru470, respectively. For samples in pH 2 buffer, the corresponding half time for injection from these complexes became 6 +/- 1, 105 +/- 20, and 18 +/- 5 ps. The injection kinetics from RuN3 to In(2)O(3) was found to be similar to that to SnO(2). These kinetics traces showed a negligible <100 fs injection component and were very different from those to TiO(2). The dependences of the injection kinetics on adsorbate energetics and the nature of the semiconductors are discussed.

Solid-State Photochromic Device Based on Nanocrystalline TiO2 Functionalized with Electron Donor−Acceptor Species

In this Communication, we report on a new type of solid-state photochromic device, S-TiO(2)-A, which is based on electron-transfer processes involving a molecular sensitizer (S) and an electron acceptor (A) coadsorbed onto the surface of nanocrystalline TiO(2). The electron-transfer process is mediated by the semiconductor conduction band and leads to a long-lived charge-separated state S(+)/A(-). The lifetime of this state can be controlled by oxygen diffusion through a polymeric coating deposited on the device.

Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized p-Type-Nanostructured NiO Films

Photoinduced electron transfer from the valence band of nanocrystalline NiO, a p-type semiconductor, to an excited bound dye, coumarin 343, and the subsequent recombination have been measured by femtosecond transient absorbance spectroscopy probing with white light. It was found that both processes are nonexponential. The photoinduced electron transfer from the semiconductor to the excited bound dye has an ultrafast component (approximately 200 fs), which is comparable to the time constants measured for photoinduced electron injection in C343-TiO2 colloid solutions. The process is very efficient and constitutes the main path of deactivation of the excited dye. Back electron transfer is also remarkably fast, with the main part of the recombination process happening with a time constant of approximately 20 ps. Dye-sensitized nanostructured p-type semiconductors are attractive materials due to their potential use as photocathodes in dye-sensitized solar cells and solid electrolytes in solid-state dye-sensitized solar cells. To our knowledge, this is the first time that the photoinduced electron-transfer kinetics of a sensitized p-type semiconductor has been studied.