Lithium ion effect on electron injection from a photoexcited coumarin derivative into a TiO2 nanocrystalline film investigated by visible-to-IR ultrafast spectroscopy

The dynamics of ultrafast electron injection from a coumarin derivative (NKX-2311), which is an efficient photosensitizer for dye-sensitized solar cells, into the conduction band of TiO(2) nanocrystalline films have been investigated by means of femtosecond transient absorption spectroscopy in a wide wavelength range from 600 nm to 10 mum. In the absence of Li(+) ions, electron injection into the TiO(2) conduction band occurred in about 300 fs. In the presence of Li(+) ions, however, electron injection occurred within approximately 100 fs, and the oxidized dye generated was found to interact with nearby Li(+) ions. Possible positions of Li(+) ion attachment to the dye molecule were examined by means of semiempirical molecular orbital calculations. The electron injection efficiency was found to increase by a factor of 1.37 in the presence of Li(+) ions. The effects of Li(+) ions on the energy of the TiO(2) conduction band and the electronic interaction between the dye molecule and Li(+) ions are discussed, and the major cause for the acceleration of electron injection was suggested to be a conduction-band shift of TiO(2).     

Interfacial electron transfer in dye sensitised nanocrystalline TiO2 films

Sub-picosecond transient absorption study has been employed to study the electron transfer kinetics in the dye-sensitized TiO₂ films used in commercial photovoltaic devices. The electron injection in these dye sensitized films occurs on an ultrafast time scale with two components, 150 fs and 1·2 ps.     

Modulation of Electron Injection Dynamics of Ru‐Based Dye/TiO2 System in the Presence of Three Different Organic Solvents: Role of Solvent Dipole Moment and Donor Number

In the present work, femtosecond transient absorption spectroscopy (fs‐TAS) has been employed to investigate the electron injection efficiency (EIE) both from the singlet and triplet excited states of a well‐known ruthenium dye (N719) to the conduction band (CB) of nanostructured TiO₂ in presence of three different organic solvents [γ‐butylactone (GBL), 3‐methoxypropionitrile (MPN), and dimethylformamide (DMF)] with different donor numbers (DNs) and dipole moments (DMs). The DM and DN of a solvent modulates the CB edge energy of TiO₂, and this effect reflects well in the fs‐TAS results, which shows an EIE trend following the order GBL≥MPN?DMF, that is, highest in GBL and lowest in DMF solvent environments. Fs‐TAS results indicate a lower contribution of electron injection from both the singlet and triplet states in DMF, for which the dominant adsorption of DMF molecules on the TiO₂ surface seems to play an important role in the mechanism.     

Calculated structural and electronic interactions of the ruthenium dye N3 with a titanium dioxide nanocrystal

Structural and electronic properties of a small anatase TiO2 nanocrystal sensitized by the ruthenium dye N3 (Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2) have been investigated using density functional theory (DFT) with support from Hartree-Fock (HF) and time dependent DFT (TD-DFT) calculations. Significant structural adjustments of both the dye and the nanocrystal are predicted to be induced by the strain imposed by the simultaneous formation of multiple dye-surface bonds. Electronic properties of the combined dye-nanocrystal system have also been calculated, including information about interfacial orbital mixing and the lowest excited singlet states. Ultrafast photoinduced electron transfer processes across the dye-nanoparticle interface in dye-sensitized solar cells are finally discussed in view of estimated electronic coupling strengths. The calculations predict injection times on the order of 10 fs for MLCT excitations to the ligand pi* levels that interact most strongly with the TiO2 conduction band, and an order of magnitude increase in the injection times for excitations to dye levels with poor spatial or energetic overlaps with the substrate conduction band.     

Atomistic mechanism of charge separation upon photoexcitation at the dye-semiconductor interface for photovoltaic applications

Charge separation in excited states upon visible light absorption is a central process in photovoltaic solar cell applications. Employing state-of-the-art first principles calculations based on time-dependent density functional theory (TDDFT), we simulate electron-hole dynamics in real time and illustrate the microscopic mechanism of charge separation at the interface between organic dye molecules and oxide semiconductor surfaces in dye-sensitized solar cells. We found that electron-hole separation proceeds non-adiabatically on an ultrafast timescale <100 fs at an anthocyanin/TiO(2) interface, and it is strongly mediated by the vibrations of interface Ti-O bonds, which anchor the dye onto the TiO(2) surface. The obtained absorption spectrum and electron injection timescale agree with experimental measurements.     

Oligothiophene-containing coumarin dyes for efficient dye-sensitized solar cells

We have developed oligothiophene-containing coumarin dyes fully functionalized for dye-sensitized nanocrystalline TiO(2) solar cells (DSSCs). DSSCs based on the dyes gave good performance in terms of incident photon-to-current conversion efficiency (IPCE) in the range of 400-800 nm. A solar energy-to-electricity conversion efficiency (eta) of 7.4% was obtained with a DSSC based on 2-cyano-3-[5'-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-[2,2']bithiophenyl-5-yl]acrylic acid (NKX-2677) under simulated AM 1.5G irradiation (100 mW cm(-2)) with a mask: short-circuit current density (J(sc)) = 13.5 mA cm(-2); open-circuit voltage (V(oc)) = 0.71 V; fill factor (FF) = 0.77. Transient absorption spectroscopy measurements indicated that electron injection from NKX-2677 to the conduction band of TiO(2) is very rapid (<100 fs), which is much faster than the emission lifetime of the dye (1.0 ns), giving a highly efficient electron injection yield of near unity.     

Rationale for kinetic heterogeneity of ultrafast light-induced electron transfer from Ru(II) complex sensitizers to nanocrystalline TiO2

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 of vibration-mediated electron transfer kinetic models and might be controlled by the electron dephasing in the solid.     

A photo-induced electron transfer study of an organic dye anchored on the surfaces of TiO2 nanotubes and nanoparticles

We report on femtosecond-nanosecond (fs-ns) studies of the triphenylamine organic dye (TPC1) interacting with titania nanoparticles of different sizes, nanotubes and nanorods. We used time-resolved emission and absorption spectroscopy to measure the photoinduced dynamics of forward and back electron transfer processes taking place in TPC1-titania complexes in acetonitrile (ACN) and dichloromethane (DCM) solutions. We observed that the electron injection from the dye to titania occurs in a multi-exponential way with the main contribution of 100 fs from the hot excited charge-transfer state of anchored TPC1. This process competes with the relaxation of the excited state, mainly governed by solvation, that takes place with average time constants of 400 fs in ACN and 1.3 ps in DCM solutions. A minor contribution to the electron injection process takes place with longer time constants of about 1-10 ps from the relaxed excited state of TPC1. The latter times and their contribution do not depend on the size of the nanoparticles, but are substantially smaller in the case of nanotubes (1-3 ps), probably due to the caging effect. The contribution is also smaller in DCM than in ACN. The efficient back recombination takes place also in a multi-exponential way with times of 1 ps, 15 ps and 1 ns, and only 20-30% of the initial injected electrons in the conduction band are left within the first 1 ns after excitation. The faster recombination rates are suggested due to those originating from the free electrons in the conduction band of titania or the electrons in the shallow trap states, while the slower recombination is due to the electrons in the deep trap states. The results reported here should be relevant to a better understanding of the photobehaviour of an organic dye with promising potential for use in solar cells. They should also help to determine the important factors that limit the efficiency of solar cells based on the triphenylamine-based dyes for solar energy conversion.     

Strongly coupled ruthenium-polypyridyl complexes for efficient electron injection in dye-sensitized semiconductor nanoparticles

Dynamics of interfacial electron transfer (ET) in the ruthenium-polypyridyl complex [{bis(2,2'-bpy)-(4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)} ruthenium(II) hexafluorophosphate] (Ru-cat)-sensitized TiO(2) nanoparticles has been investigated using femtosecond transient absorption spectroscopy detecting in the visible and near-infrared region. It has been observed that Ru-cat is coupled strongly with the TiO(2) nanoparticles through its pendant catechol moiety. Electron injection has been confirmed by direct detection of electrons in the conduction band, cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy. A single-exponential and pulse width limited (<100 fs) electron injection has been observed, and the origin of it might have been from the nonthermalized excited states of the Ru-cat molecule. The result gave a strong indication that the electron injection competes with the thermalization of the photoexcited states due to large coupling elements for the forward ET reaction. Back-ET dynamics has been determined by monitoring the decay kinetics of the cation radical and injected electron and also from recovery kinetics of the bleach of the adsorbed dye. It has been fit with a multiexponential function, where approximately 30% of the injected electrons are recombined with a time constant of <2 ps, again indicating large coupling elements for the charge recombination reaction. However, our results have shown relatively long-lived charge separation in the Ru-cat/TiO(2) system as compared to other organic dye-sensitized TiO(2) nanoparticles with similar interactions.     

High-order harmonic generation in rare gases: a new source in photoionization spectroscopy

We demonstrate that we can use the extreme ultraviolet radiation produced by high order harmonic generation to perform photoionization experiments. With harmonics from the 11th to the 69th of a 140 fs Cr:LiSAF laser operating at 825 nm, we measure the relative photoionization cross sections of xenon, krypton, argon and neon over the range 10 to 110 eV. With narrow bandwidth harmonics produced by a tunable, 1 ps dye laser, we observe the autoionizing states between the 4p ⁵ ionization thresholds in krypton.     

Photosensitive function of encapsulated dye in carbon nanotubes

Single-wall carbon nanotubes (SWCNTs) exhibit resonant absorption localized in specific spectral regions. To expand the light spectrum that can be utilized by SWCNTs, we have encapsulated squarylium dye into SWCNTs and clarified its microscopic structure and photosensitizing function. X-ray diffraction and polarization-resolved optical absorption measurements revealed that the encapsulated dye molecules are located at an off center position inside the tubes and aligned to the nanotube axis. Efficient energy transfer from the encapsulated dye to SWCNTs was clearly observed in the photoluminescence spectra. Enhancement of transient absorption saturation in the S1 state of the semiconducting SWCNTs was detected after the photoexcitation of the encapsulated dye, which indicates that ultrafast (<190 fs) energy transfer occurred from the dye to the SWCNTs.     

苯并噻二唑衍生物作为铆接基团提高染料敏化太阳能电池效率

This work describes the characteristics of benzobisthiadiazole analogues with different heteroatom substitution patterns as electronwithdrawing anchoring groups in dye-sensitized solar cells (DSSCs). In order to provide a systematic analysis of the effect of the designed anchoring groups, the widely used anchor cyanoacrylic acid was used as the reference. Theoretical calculations show that the newly designed anchors are capable of displaying a decent level of light absorption covering the entire visible range up to the near-IR region of 1000 nm. More importantly, an ultrafast electron injection is observed from the dyes SPN and SPS into the TiO₂ surface. The quantum dynamics of the interfacial electron transfer (IET) reveal that SPN and SPS anchors provide efficient IET performance. About 90% of the electron injection occurs in the first 15 fs, and is complete after ~100 fs. Furthermore, the pathway of electron injection is direct, leading to very efficient transfer of the wavepacket through the TiO₂ semiconductor. Therefore, the performances of both the anchors, SPN and SPS, are equivalent and even superior to that of cyanoacrylic acid. These findings are important in the context of providing guidelines for the design of metal-free organic dye sensitizers for high efficient DSSCs.     

Molecular Vibronic Structures of HDITC in Solutions Studied by Femtosecond Wavelength-Resolved Pump-Probe Spectroscopy

Wavelength-resolved pump-probe measurements using 11 fs duration pulses were performed to study the vibronic structures of HDITC, a cyanine dye, in ethylene glycol. Ten vibrational modes were observed in the forms of quantum beats. The frequencies of these ten vibrational modes ranged from 135 cm^?1 to 1300 cm^?1. The relative potential displacements along these ten vibrational coordinates were estimated by comparison with theoretical calculations.     

Electronic and Vibrational Coherence Dynamics in a Cyanine Dye Studied Using a Few‐Cycle Pulsed Laser

We report the relaxation times of electronic and vibrational coherence in the cyanine dye 1,1′,3,3,3′,3′‐hexamethyl‐4,4′,5,5′‐dibenzo‐2,2′‐indotricarbocyanine, measured using a 7.1 fs pulsed laser. The vibrational phase relaxation times are found to be between 380 and 680 fs in the ground and lowest excited singlet states. The vibrational dephasing times of the 294, 446, and 736 cm^?1 modes are relatively long among the six modes associated with excited‐state wave packets. The slower relaxations are explained in terms of a coupled triplet of vibrational modes, which preserves coherence by forming a tightly bound group to satisfy the condition of circa conservation of vibrational energy. Using data from the negative‐time range (i.e., when the probe pulse precedes the pump pulse), the electronic phase relaxation time is found to be 31±1 fs. The dynamic vibrational mode in the excited state (1171 cm^?1), detected in the positive‐time range, is also studied from the negative‐time traces under the same experimental conditions.     

Ultrafast Photodynamics of the Indoline Dye D149 Adsorbed to Porous ZnO in Dye‐Sensitized Solar Cells

We investigate the ultrafast dynamics of the photoinduced electron transfer between surface‐adsorbed indoline D149 dye and porous ZnO as used in the working electrodes of dye‐sensitized solar cells. Transient absorption spectroscopy was conducted on the dye in solution, on solid state samples and for the latter in contact to a I^?/I₃^? redox electrolyte typical for dye‐sensitized solar cells to elucidate the effect of each component in the observed dynamics. D149 in a solution of 1:1 acetonitrile and tert‐butyl alcohol shows excited‐state lifetimes of 300±50 ps. This signature is severely quenched when D149 is adsorbed to ZnO, with the fastest component of the decay trace measured at 150±20 fs due to the charge‐transfer mechanism. Absorption bands of the oxidized dye molecule were investigated to determine regeneration times which are in excess of 1 ns. The addition of the redox electrolyte to the system results in faster regeneration times, of the order of 1 ns.     

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.     

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.     

Photoinduced charge injection from excited triplet hypocrellin B into TiO_2 colloid in ethanol

Photosensitization of TiO2 colloid by hypocrellin B (HB),a natural photodynamic pigment with extremely high photosta-bility,has been studied by surface enhanced Raman spec-troscopy (SERS),laser flash photolysis and electron paramagnetic resonance (EPR) techniques.The photoseiisitization of TiO2 occurred practically from the excited triplet dye and the electron injection rate constant is 1.3×106 s-1.The influences of donor and acceptor on the electron injection were investigated.