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J. Phys. Chem. B 2002, 106, 4396-4404
Electron Transfer from the Singlet and Triplet Excited States of Ru(dcbpy)2(NCS)2 into Nanocrystalline TiO2 Thin Films Jani Kallioinen,† Ga´ bor Benko1 ,‡ Villy Sundstro1 m,‡ Jouko E. I. Korppi-Tommola,† and Arkady P. Yartsev*,‡ Department of Chemistry, UniVersity of JyVa¨ skyla¨ , P.O. Box 35, FIN-40351 JyVa¨ skyla¨ , Finland, and Department of Chemical Physics, Lund UniVersity, P.O. Box 124, S-22100 Lund, Sweden ReceiVed: NoVember 29, 2001; In Final Form: January 26, 2002
Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpy)2(NCS)2 (RuN3) into titanium dioxide (TiO2) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of ∼30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs-1 for ET and 1/75 fs-1 for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from ∼1 to ∼60 ps.
1. Introduction Sensitization of nanocrystalline, wide band gap semiconductor films with light-harvesting dyes extends the photoresponse of molecular-based photovoltaic devices into the visible wavelength region.1-3 The Gra¨tzel-type dye sensitized solar cell (DSSC)4,5 is an important example of such a device. At the heart of the DSSC, there is a few-micrometers-thick semiconductor film consisting of a network of nanoparticles. Its porous structure gives it a very high internal surface area to which a large number of dye molecules may bind via carboxyl, phosphonate, etc. linking groups; consequently, a high optical density of the sensitized film is obtained. The role of the dye molecule in light-energy conversion is to act as an antenna; it captures the energy of sunlight, which initiates a long-lived charge-separated state when an electron from the electronically excited state of the dye is injected into the conduction band of the semiconductor (eqs 1 and 2). hν
ET
Dye-SC 98 Dye**-SC 98 Dye+-SC(e-) relax
ET
Dye**-SC 98 Dye*-SC 98 Dye+-SC(e-)
(1) (2)
The two different electron transfer (ET) pathways indicate the possibility of electron injection from the fully relaxed excited state (Dye*) as well as from nonrelaxed higher-lying excited states (Dye**). When an electron is injected, the dye cation (Dye+) is formed together with an electron in the conduction band of the semiconductor (SC(e-)). In the DSSC, the electrons are further transported through the network of semiconductor nanoparticles into an outer electrical circuit to perform work. The neutral dye is regenerated by a redox system in contact with the adsorbed dye to sustain a cyclic process. The original ground state of the dye can be restored also by back electron * Corresponding author. E-mail:
[email protected]. † University of Jyva ¨ skyla¨. ‡ Lund University.
transfer (BET) from the conduction band/trap states of the semiconductor to the dye cation (eq 3). BET
Dye+-SC(e-) 98 Dye-SC
(3)
In this case, the photon energy is not used to perform work in the DSSC. Consequently, for an efficient light-to-current conversion process, it is necessary to establish conditions of both fast electron injection and slow BET. To characterize the pathways and rates of the interfacial ET reactions between the dye and the semiconductor, it is not necessary to investigate a functioning DSSC but just the dyesensitized semiconductor film. Each of the species involved in the interfacial ET (see eqs 1-3) has a characteristic absorption spectrum, which can be used in transient absorption spectroscopy to follow the photoinduced dynamics of the dye-semiconductor system. The time constants of the ET process can be resolved by monitoring the rise of the ET product absorption, the dye cation, or the injected electrons in the conduction band of the semiconductor. The other possibility is to monitor the decay of the reactant absorption, excited state absorption (ESA), or in some cases, stimulated emission (if electrons are injected from singlet states). The loss of the ground-state absorption of the dye is not directly connected to the electron-injection process because the ground state is not an electron-donating state, but the recovery of the ground-state bleaching can be used to monitor the kinetics of the BET (eq 3). Transition metal compounds have been the most promising molecules in DSSC, and about 10% light-to-energy conversion efficiency has been obtained by using Ru(dcbpy)2(NCS)2 [dcbpy ) 4,4′-dicarboxy-2,2′-bipyridine] attached to nanocrystalline TiO2 thin films.1-4 The lowest excited state of Ru(dcbpy)2(NCS)2 (i.e., RuN3) lies about 0.3 V above the conductionband edge of TiO2,1-4,6 allowing electron injection from both the singlet and triplet excited states of RuN3 (RuN3*). Regardless of the nature of the electron donor states, it is known that the electron-injection process proceeds with a nearly unity
10.1021/jp0143443 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/05/2002
Electron Transfer in RuN3-TiO2 Film overall quantum yield4,7 and occurs on the femtosecond and picosecond time scales. BET occurs nonexponentially on the microsecond to millisecond time scale with only a negligible contribution from picosecond/nanosecond components.8,15,19-23 The several orders of magnitude difference between the time constants of electron injection and BET is one of the important properties that makes the RuN3-TiO2 system one of the most efficient light-to-energy converters available for DSSC.1-3 Photoinduced electron injection from RuN3 into nanocrystalline TiO2 thin films has been intensively studied by transient absorption spectroscopy in the visible, near-IR, and mid-IR regions.8-18 In general, the large number of previous studies on RuN3-TiO2 reflects the complexity of the observed dynamics. Many of the studies were performed in the visible and nearIR spectral regions, and it has been shown that spectral overlap of the ground state, excited states, dye cation (RuN3+), and absorption of injected electrons in the conduction band of TiO2 results in complex kinetics. In the first femtosecond transient absorption measurements by Durrant et al.,8 photoinduced electron injection of RuN3-sensitized TiO2 films in an ethylene carbonate/propylene carbonate (1:1) solution was reported to be biphasic, with time constants and amplitudes (in parentheses) of
