Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the

semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction

Grätzel, Michael

Huber, Robert

Moser, Jacques E.

Spörlein, Sebastian

Wachtveitl, Josef

English

Hochschulschriftenserver - Universität Frankfurt am Main

Vol. 1 INTERNATIONAL JOURNAL OF PHOTOENERGY 1999Ultrafast photoinduced electron transfer in coumarin343 sensitized TiO2-colloidal solutionJosef Wachtveitl,1y Robert Huber,1 Sebastian Spörlein,1Jacques E. Moser,2z and Michael Grätzel21Institut für Medizinische Optik, Oettingenstr. 67, Ludwig-Maximilians-UniversitätMünchen, 80538 München, Germany2 Laboratory for Photonics & Interfaces, Department of Chemistry, EcolePolytechnique Fédérale de Lausanne, CH-1015 Lausanne, SwitzerlandAbstract. Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles isthe rst and most important reaction step for the mechanism in the so called wet solar cells [1]. The timescale between the photoexcitation of the dye and the electron injection into the conduction band of thesemiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specicelectron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and ac-ceptor molecules [210]. We show that visible pump/ white light probe is a very ecient tool to investigatethe electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the in-jection process of the electron, the cooling of the injected electron and the charge recombination reaction.1. INTRODUCTIONPhotoinduced interfacial electron transfer in dyesensitized colloidal semiconductors is a process offundamental importance. For example, the operationof photo-electrochemical devices with high conver-sion eciency is based on that principle [1]. A highquantum yield is only obtained, if the reaction ratefor charge separation is much higher than the ratesof all other competing reactions (e.g., radiative andnon-radiative deactivation processes). Therefore, itis of crucial importance that charge injection occurson an ultrafast timescale. Electron transfer reactionson the femtosecond timescale for dye/TiO2 systemshave been observed recently by ultrafast laser spec-troscopy [24, 711]. Near/mid infrared absorptionand uorescence upconversion measurements havebeen performed by several groups on dye/TiO2 sys-tems [4,8]. Here we report femtosecond time resolvedtransient absorption spectra over a wide spectral rangecovering the near UV and visible region with a hightemporal resolution using a white light continuumas probing light in an excite and probe experiment.In contrast to uorescence upconversion techniques,where only the population density of the photoex-cited state of the dye can be probed, transient ab-sorption measurements allow to distinguish betweenelectron transfer and competing quenching mecha-nisms, e.g., intermolecular energy transfer betweensensitizer molecules. So it also provides necessaryadditional information compared to near- or mid-IRinvestigations, where only the electron in the conduc-tion band of the semiconductor can be probed. In thisstudy we present data for the laser dye coumarin 343adsorbed on the surface of TiO2 nanocrystals (Fig-ure 1, inset). We are able to assign the various spectralyJosef.Wachtveitl@physik.uni-muenchen.dezje.moser@ep.chcomponents to ground state bleaching, cation forma-tion and absorption of the electron in the conductionband and report fast interfacial electron transfer on atime scale faster than 100fs. The experimental setupused for our investigations is described in [12,13].2. EXPERIMENTAL METHODSCoumarin 343 was used as obtained by Kodak. So-lutions of colloidal TiO2 were prepared as describedin [5]. The absorption spectrum of the dye was onlyslightly aected upon adsorption on the TiO2 surface(Figure 1). The sensitized colloidal solution was circu-lated through a cuvette during the measurement, toprevent accumulation of photoproducts.For the transient absorption measurements we usea home built 20Hz Ti:sapphire laser system with apulse width of about 100fs (FWHM), a pulse energy of600J and a central wavelength of 870nm. The mainpart of the energy is used for second harmonic gen-eration to provide pulses at a wavelength of 435nm.These pulses were directly used to excite the coumarinand the coumarin/TiO2 samples.Recently, spectrally tunable excitation pulsescould be obtained by pumping a nonlinear-optical-parametric-amplier (NOPA) with the 435nm pulses,that allow investigations of dye molecules in a broadspectral range (e.g., alizarin/TiO2 with a maximum ab-sorption at 495nm). Furthermore, the pulses generatedthis way can be compressed down to less than 20fs andprovide the experimental basis to observe the fast elec-tron injection processes directly [14].A white light continuum for the probing branch ofthe setup is generated by focussing the 870nm or the435nm light pulses into a 2mm sapphire plate. Thecontinuum generated with 435nm light pulses cov-ers a spectral range from 320nm to 580nm, the one2 Josef Wachtveitl et al. Vol. 10.00.20.40.60.81.0300 400 500 600Coumarin / TiO2Coumarin in MeOH  5nmTiO2Absorption[o.d.]Wavelength [nm]Figure 1. Absorption spectra of coumarin 343 in solution(methanol) and adsorbed to the surface of TiO2 nanopar-ticles. The coupling process induces only a small shift ofabout 5nm in the main absorption band of the two samples.The excitation wavelength is indicated by the arrow. Thestrong increase in the absorption for the coumarin/TiO2sample for wavelengths <350nm reects interbandtransitions from the valence- to the conduction band of thecolloid. The arrangement of the coupled coumarin is shownin the inset.generated at 870nm extends from 450nm to 1100nm.A spectral interval of about 10nm around the generat-ing wavelength of the white light can not be used due totemporal energy uctuations. In order to prevent pulselengthening by increasing chirp only mirror optics isused.The white light is split into two parts, one is used tomonitor the reference energy distribution of the whitelight, the other to probe the sample. The two parts aredetected by two independent spectrometers each witha 42 channel array of photodiodes. The arrays are readout with a computer working in single shot detectionmode. The absorption is dened by the ratio of sam-ple and reference intensity. To calculate the absolutevalue for the absorption change every fth pump pulseis blocked by a shutter and the absorption of the non-excited sample is recorded. The continuous control ofthe absorption without excitation allows the detectionof long term drift eects of the laser or degradation ofthe sample. This setup provides a high signal to noiseratio. Throughout the entire investigated spectral rangethe width (FWHM) of the cross correlation function wasless than 110fs for the 435nm pump pulses, less then85fs for the excitation pulses around 500nm gener-ated with the NOPA. The polarizations of the pump andprobe pulses were set to the magic angle.3. RESULTS AND DISCUSSIONIn the femtosecond time resolved experiments shownin Figure 2 transient absorption changes of coumarin343 in solution are compared with coumarin 343 ad-sorbed on the TiO2 surface. The transient absorptionchanges in coumarin 343 are dominated by long livedexcited state absorption and ground state bleaching.The absorption of the excited state can be observedat wavelengths < 400nm while the bleaching of theground state reects the signature of the main absorp-tion band (max  445nm) (Figure 1). The characteristicabsorption changes persist throughout the entire inves-tigated temporal range (> 3ns).In contrast to the dye in solution, the coupledcoumarin 343/TiO2 system shows an additional multi-exponential decay of the signal with a dominating fastcomponent of 350fs present in all transients recorded(Figures 2, 3, 4). A signal remains at long delay times,which resembles the characteristics of the dierencespectrum for the charge separated system, stable overmany orders of magnitude (Figures 3, 4). Figure 3 alsoshows the assignment of the various features of thedierence spectrum: cation absorption (<400nm),ground state bleaching (400nm <  < 520nm) and aweak, spectrally broad contribution of the electron inthe conduction band of TiO2 (>520nm). Due to thesimilarity of the excited state absorptionderived fromexperiments on the dye in solutionand the cationband of coumarin/TiO2, it is not possible to determinethe dynamics of the electron injection process in thisspectral region unambiguously.Therefore, two possible mechanisms can explain therecorded kinetics:−1:0−0:8−0:6−0:4−0:20.0−0.5 0.0 0.5 1.0 1.5 2.0 −0:5 0.0 0.5 1.0 1.5 2.00.00.20.40.60.81.0probe  368nmCoumarin(TiO2)Coumarin(MeOH)Coumarin(MeOH)Coumarin(TiO2)=350fsfast kineticsprobe  480nmAbsorptionChange[a.u.]Delay Time [ps] Delay Time [ps]Figure 2. Femtosecond time resolved kinetics of coumarin 343 for uncoupled dye and dye/TiO2 samples at two dierentprobing wavelengths.Vol. 1 Ultrafast photoinduced electron transfer in Coumarin ::: 3     −0:4−0:3−0:2−0:10.00.10.20.00.51.0400 500 6002s2psTransient abs.spectra atD -! D -! D e−cbh inj.D e−cb Dpump435nmcoumarin(MeOH)coumarin(TiO2)AbsorptionChange[a.u.]Absorption[OD]Wavelength [nm]  Figure 3. Top: Absorption spectra of coumarin 343 inmethanol () and adsorbed onto the surface of nanocrys-talline TiO2 (- - -). Bottom: Transient absorption spectra ofcoumarin 343 adsorbed on the surface of TiO2 nanocrys-tals after two dierent delay times and band assignment(D: cation radical, e−cb: conduction band electron) [6].−0:08−0:06−0:04−0:020.000.020.04AbsorptionChange[a.u.]360 380 460 480 500 520Wavelength [nm]Coumarin/TiO2Delay Time600fs1.6ps10ps100ps3.0nsFigure 4. Transient spectra of coumarin 343/TiO2 at vari-ous delay times.(i) The electron is injected with a time constant of350fs, indicated by the decay of the excited state andthe formation of the coumarin cation radical signal.The signal decrease with a time constant of   350fsat probe  480nm superimposed to the instantaneousbleaching of the ground state band is then caused by acontribution of photoinduced D molecules.(ii) The electron is injected on a much shorter timescale (<100fs), but the detection of this fast com-ponent is hindered by the spectral overlap of the eductand the product states. Then the observed dynamics re-ects charge recombination leading to a partial decayof the cation and the ground state features. This mech-anism is supported by the lack of spectral evolution inthe transient spectra at dierent delay times (Figure 4)and by measurements of ultrafast injection times in therange from 20fs to 180fs reported for this system byother groups [4,8].Measurements in the long wavelength region (>520nm) will allow to overcome this ambiguity, sincea direct observation of the injected electron is possi-ble. Also an improved time resolution of the setup us-ing NOPAs is necessary for the reliable detection of theelectron transfer reaction. These experiments are cur-rently underway.4. CONCLUSIONSUltrafast interfacial electron injection on the fem-tosecond time scale is observed from an electron do-nating dye molecule attached to the TiO2 surface. Theseresults allow further systematic analysis of importantelectron transfer parameters. The inuence of surface,solvent or donor-acceptor distance and eects like re-distribution of vibrational excess energy or hot electroninjection can now be addressed.REFERENCES[1] B. O'Regan and M. Grätzel, Nature 353 (1991), 737.[2] Y. Tachibana, J. E. Moser, M. Grätzel, and J. R.Klug, D. R.and Durrant, J. Phys. Chem. 100 (1996),20056.[3] B. Burfeindt, T. Hannappel, W. Storck, and F. Willig,J. Phys. Chem. 100 (1996), 16463.[4] J. M. Rehm, G. L. McLendon, Y. Nagasawa, K. Yoshi-hara, J. Moser, and M. Grätzel, J. Phys. Chem. 100(1996), 9577.[5] M. K. Nazeeruddin, P. Liska, J. E. Moser, N. Vla-chopoulos, and M. Grätzel, Helv. Chim. Acta 73(1990), 1788.[6] J. E. Moser and M. Grätzel, Chem. Phys. 176 (1993),493.[7] R. Jimenez, G. R. Fleming, P. V. Kumar, andM. Maroncelli, Nature 369 (1994), 471.[8] H. N. Gosh, J. B. Ashury, and T. Lian, J. Phys. Chem.102 (1998), 6482.[9] I. Martini, J. Hodak, and G. V. Hartland, J. Chem.Phys. 107 (1997), 19.[10] M. Hilgendor and V. Sundström, J. Phys. 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Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution

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