LETTER pubs.acs.org/JPCL
Tunneling-Type Charge Recombination in Nanocrystalline TiO2 Films at Low Temperature Ryuzi Katoh*,† and Akihiro Furube National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ABSTRACT: The effect of temperature on the charge recombination (CR) rate in bare and dye-sensitized nanocrystalline TiO2 films was studied by means of transient absorption spectroscopy. For bare TiO2 films, the CR rate was not sensitive to temperature, indicating that CR occurs by means of tunneling and without trapping of charges. By contrast, for dye-sensitized TiO2 films, the CR rate was sensitive to temperature at around room temperature, suggesting that the barrier produced by dye adsorption on the TiO2 surface had been overcome. At lower temperature, the CR rate of the dye-sensitized films was not sensitive, again suggesting a tunneling CR process. These results suggest that slow CR is not caused by charge traps but is instead caused by a barrier between a positive charge and an electron. SECTION: Nanoparticles and Nanostructures
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or photoenergy conversion systems based on photoinduced electron transfer reactions, a long-lived charge separated state is essential to permit subsequent reactions that utilize the charged species as starting materials. Such long-lived charge separated states, which require suppression of charge recombination (CR), can be achieved by two possible routes: either spatial separation between a positive and a negative charge, or energetic matching of CR reactions in desirable electron transfer kinetic regimes. In the field of molecular photochemistry, both of these routes have been demonstrated for various types of specifically engineered supramolecular systems, some of which have shown remarkable properties.13 In such systems, spatial separation of charges is achieved by use of electron relay systems, and suppression of CR reactions can be realized by energy-level matching to ensure that the free energy change of the reaction is in the Marcus-inverted region. Dye-sensitized solar cells (DSSCs) are one example of photoenergy conversion devices based on molecular photochemical processes.4,5 It is widely agreed that the long-lived charge separated state plays an essential role in high-performance DSSCs.6 Accordingly, the CR process in dye-sensitized TiO2 nanocrystalline films, which form the photoactive electrodes for DSSCs, has been extensively studied by means of transient absorption (TA) spectroscopy. Early TA studies reported observed CR times that varied from picoseconds to milliseconds. However, it has since been observed that CR rates are strongly dependent on the excitation density employed, accelerating nonlinearly at high photoinduced electron densities such that the recombination does not obey conventional second order kinetics7 and saturating under weak excitation conditions where CR occurs within the millisecond time region.7,8 Such weak excitation conditions are most suited to discuss the origin of CR process in DSSCs. Under these conditions, geminate r 2011 American Chemical Society
recombination, where an injected electron recombines with a parent cation in a TiO2 nanoparticle, is the dominant CR mechanism. According to previous studies, the CR kinetics can be fitted phenomenologically by a stretched exponential function.9 Ne ðtÞ expð ðkrec tÞR Þ N0
ð1Þ
where N0 and Ne(t) are the number of electrons initially produced and the number produced at time t, respectively; krec is the CR rate constant; and R is the constant used to characterize deviation from simple exponential kinetics. To rationalize such dispersive CR kinetics, theoretical models based on multiple trapping of electrons have been presented.10,11 These models explain decay profiles as well as the observed electron density dependence of CR. In such models, the CR rate is assumed to be limited by thermally induced detrapping, suggesting that decay kinetics is sensitive to temperature. It is therefore likely that the role of the charge trapping process in CR could be clarified from the observed temperature dependence of the CR rate. Previous work on the temperature dependence of CR between cations and electrons was carried out in nanocrystalline TiO2 films at higher temperature12 and for TiO2 nanoparticles dispersed in solution;13 however, there remains substantial scope for detailed studies in this area. Figure 1a shows typical decay profiles of TA signals in bare TiO2 films excited at 355 nm and normalized to each signal’s peak intensity. The excitation light intensity was Iex = 0.024 mJ cm2 . Actual absorbance was about 5 10.5 Received: June 21, 2011 Accepted: July 14, 2011 Published: July 14, 2011 1888
dx.doi.org/10.1021/jz2008424 | J. Phys. Chem. Lett. 2011, 2, 1888–1891
The Journal of Physical Chemistry Letters
Figure 1. (a) Decay profiles of TA signals obtained for bare TiO2 films at various temperatures. Excitation and probe wavelengths were 355 and 830 nm, respectively. The solid lines show fits to eq 1 with an assumed value of R = 0.45. (b) Recombination rate constants krec as a function of the inverse temperature obtained by fitting eq 1.
The probe wavelength was 830 nm, a region in which both electrons and holes can be detected.14 Since the charge separation yield of TiO2 under ultraviolet (UV) excitation was found to be almost unity,15 the number of electronhole pairs per particle can be estimated to be 0.2 particle1 from the value of the penetration depth of the excitation light (2 μm). This calculated density of electronhole pairs indicates that the CR process observed here was geminate recombination within individual particles. The decay profiles are well fitted by eq 1 with an assumed value of R = 0.45. Figure 1b shows krec obtained as a function of temperature in bare TiO2 films. It is obvious that the CR rates are not sensitive to temperature over the measured range. This observation is not consistent with models in which CR requires thermal trapping of an electron. Alternatively, these results are consistent with models in which CR occurs by means of tunneling through the barrier between a hole and an electron with temperatureindependent mobility of charges. We note that mobility of electrons in bare TiO2 films can be evaluated through timeresolved microwave conductivity (TRMC),1517 and the tunneling model is consistent with the recent experimental observation that electron mobility in nanocrystalline TiO2 films does not exhibit temperature dependence.16 The absolute value of the mobility of electrons in nanocrystalline TiO2 films at room temperature has been estimated to be
LETTER
Figure 2. (a) Decay profiles of TA signals obtained for N719-sensitized TiO2 films at various temperatures. Excitation and probe wavelengths were 532 and 830 nm, respectively. The solid lines show fits to eq 1 with an assumed value of R = 0.45. (b) Recombination rate constants krec as a function of the inverse temperature obtained by fitting eq 1.
μ = 0.01 cm2 V1 s1 through TRMC measurements.15 Holes in these films are trapped at the surface of the particle, as confirmed by the observation that holes in TiO2 react very quickly (
