J. Phys. Chem. C 2009, 113, 21233–21241
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Influence of TiO2/Perylene Interface Modifications on Electron Injection and Recombination Dynamics Andreas F. Bartelt,* Robert Schu¨tz, Antje Neubauer, Thomas Hannappel, and Rainer Eichberger Helmholtz Center Berlin for Materials and Energy, Department Materials for PhotoVoltaics, E-I5, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ReceiVed: July 31, 2009; ReVised Manuscript ReceiVed: October 19, 2009
It is well-known that the efficiency of dye sensitized solar cells can be improved by controlling the interface energetics using molecular interface modifiers. Whereas this leads to a beneficial band level shifting, it also affects the interfacial electron injection and recombination dynamics. Here we demonstrate a significant retardation of the injection process and a loss of ultrafast recombination components by coadsorbing inert gases and solvents with increasing dipole moments on a TiO2/perylene interface. Model perylene dyes with different electronic couplings to the colloidal TiO2 films were subject to precisely defined chemical environments, and the electron transfer dynamics was investigated with femtosecond transient absorption spectroscopy. The coadsorption of N2 and Ar doubled the injection times compared to the ultrafast sub-60 fs electron injection in vacuum. The introduction of solvents led to injection retardations by up to 2 orders of magnitude. This slow-down correlates well with the degree of polarity of the chemical species and is consistent with a calculated electronic shift of the oxide conduction band relative to the injecting molecular level. The ultrafast component of the nonexponential back electron transfer was significantly reduced with coadsorbent polarity. 1. Introduction Dye-sensitized solar cells (DSC) operate by injecting electrons from the excited state of a light-absorbing molecular dye into the continuum of conduction band states of a wide bandgap semiconductor, mostly colloidal TiO2.1 The injected electrons are separated from the holes by charge transport through the semiconductor. The neutral dye is regenerated by dye cation reduction, often via the redox pair I-/I3-. The DSC performance is governed by a competition between beneficial and detrimental charge transfer processes. Efficient dye regeneration is obtained for slow back-electron transfer of electrons from the semiconductor to the dye cation, compared to the rate of regeneration. For efficient injection, the electron injection process needs to be faster than the competing excited state decays. Fast electron injection is obtained for molecular injection levels positioned sufficiently high above the TiO2 conduction band minimum. The position of the TiO2 conduction band also determines the opencircuit voltage VOC of a dye sensitized solar cell. Since 1993, many attempts have been made to modulate the TiO2 band levels and increase the performance, using different additives to the electrolyte.2-5 Significant improvements were achieved with 4-tert-butylpyridine (4-TBP) as additive in DSCs operated with Ru-bipyridyl-sensitized TiO2 films and I-/I3- electrolytes. The VOC was increased by the induced energetic conduction band shift and the reduced interface recombination of the injected electron with the triiodide due to the passivating effect of the 4-TBP adsorption.3,6,7 Other used additives were, e.g., deoxycholic acid (DCA)4 and chenodeoxycholate.5 Although the effect of adsorbate-induced level-shifting on the electron lifetime and recombination dynamics has been investigated, less attention was paid to the effect on the injection * To whom correspondence should be addressed. E-mail: andreas.
[email protected].
dynamics. Some TiO2/metal-organic-dye systems were investigated with respect to charge injection under specific environmental conditions, e.g., in solvents of different pH,8-10 which shift the TiO2 conduction band by 60 mV per pH unit,11 or for different solvent environments10 and/or 4-TBP.12 However, the complex absorption structure and injection dynamics of the widely used Ru-bipyridyl-sensitized TiO2 films significantly complicates a comprehensive analysis. Recent results using 4-TBP additives suggest that the induced conduction band shift can reduce the charge injection rate, thereby harming the electron injection efficiency.13 On the other hand, additives like, e.g., DCA improved the short circuit current JSC, and indications for an enhanced injection yield due to coadsorbents were found.4,14 These examples illustrate that electron injection can still be a key limiting factor in device performance, and a detailed understanding of the injection dynamics in the presence of additives as well as the ramifications on electron recombination processes are needed. The electron injection from a photoexcited chromophore into the wide bandgap semiconductor of a DSC is mostly governed by (a) the electronic coupling H between injecting dye states and the accepting semiconductor states, and (b) the energy difference between the injecting dye state and the TiO2 conduction band minimum, which we denote with E*. The dependence on E* is mostly due to the density of states in the conduction band, which increases toward more negative energies. Previously, the dependence of the electron injection on the coupling strength H was investigated using two perylene derivatives which only differed in their anchor/bridge groups such that they exhibited different electronic couplings to the TiO2 surface. The injection rates were significantly different and could be well correlated to the different coupling strengths, i.e., the strong coupling injection was more than 5 times faster than the weak coupling injection.15,16 The dependence of the injection
10.1021/jp907386y CCC: $40.75 2009 American Chemical Society Published on Web 11/18/2009
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J. Phys. Chem. C, Vol. 113, No. 50, 2009
rate on the density of states (DOS) of the accepting semiconductor conduction band states was also investigated,17 using the same perylene derivatives and comparing the charge injection dynamics into TiO2 with colloidal ZnO electrodes. While TiO2 and ZnO have almost the same band gap and conduction band position, the DOS of ZnO is significantly lower. Consequently, a smaller injection rate into ZnO was found which could be well correlated with the lower ZnO DOS.17 Here, we analyze the dependence of the electron injection and early recombination rates on the dipolar strength and concentration of coadsorbing species, affecting the relative energetics between molecular donor level and TiO2 conduction band. In the case of a negative TiO2 band shift, the density of accepting TiO2 states becomes smaller, leading to retardation in the injection dynamics. The electron transfer dynamics was investigated on a heterojunction system comprised of the same two perylene derivatives as in the above-mentioned investigations attached to colloidal TiO2 and subject to various chemical environments. The focus of this work is to identify the effect of coadsorbents on charge injection and recombination dynamics, which is achieved by investigating the nanostructured interfaces under clean and defined conditions. Perylene dyes were chosen because the participating ground, excited, and cationic states can be separately spectroscopically addressed. Furthermore, they exhibit monoexponential injection dynamics characterized by the absence of triplet state contributions. Apart from the effect on the electron injection dynamics, the influence of the coadsorbed species on the recombination dynamics of injected electrons with the dye cation is analyzed. This issue is currently shifting into the focus of investigation as both new organic dyes with broader absorption range and higher extinction coefficients as well as new electrolytes and solid-state hole conductors are being developed. For all systems, a reduced back electron transfer rate is crucial. Interface modifiers can play an important role in achieving the desired back reaction slow-down18 and have been tested both in dye sensitized and TiO2/polymere solar cells.19 However, some organic dyes show ultrafast recombination rate components with time scales 2 ps is due to the spectral overlap with the cationic signal. The overlap is stronger for perylene 2.17
injection dynamics traces were recorded which indicated progressive injection retardation with increasing toluene partial vapor pressure. At saturation pressure, toluene condensation was observed, which coincided with maximal injection retardation. The toluene traces shown in Figure 2 were recorded with toluene condensation on the sample, representing the injection dynamics under liquid toluene environment. After evacuation and renewed chamber purge with 200 mbar of N2, THF was inserted into the chamber. Due to the six times higher THF vapor pressure compared to toluene, the THF partial pressure inside the chamber increased more rapidly, which was monitored in realtime by an increase of the injection retardation, until saturation set in. Shown in Figure 2 are exemplary injection dynamics traces for THF partial vapor pressures below the saturation pressure point and under liquid THF environment. The latter were recorded on samples immersed in the THF cuvette located inside the measurement chamber. As can be seen, for both perylene 1 and 2 the THF traces clearly exhibit lower injection rates than the corresponding toluene traces. Note, that the THF vapor injection traces are slower for perylene 2 than for perylene 1. This is a consequence of the fact that the transient absorption measurement of perylene 2 was taken about 2 h after the measurement of perylene 1. During this time, the THF vapor pressure had increased. We verified the injection rates by detecting the transient absorption decay of the neutral perylene 1 and 2 excited states at 710 nm (Figure 3), which is another measure of the injection dynamics and could be performed readily one after the other. Here, a slightly slower injection dynamics of perylene 1 in comparison to perylene 2 was found for a given THF partial vapor pressure (see Figure 3 and Table 1). Note that the remaining signal for longer times is due to the small spectral overlap of excited and cationic states, which leads to longer-lasting tail-edge absorption of the cationic state even at 710 nm. Due to the red-shift of the perylene 2 cation absorption band compared to perylene 1 this remaining signal is stronger for perylene 2 (see also reference17). In order to obtain quantitative comparisons of the injection dynamics under the applied chemical environments, the time constants were extracted from the pump-probe signals by fitting the data with a convoluted single component rate equations model. The simple model describes the electron injection as a monoexponential decay of the photoexcited state into the
τinj(1), fs
τinj(2), fs
τinj(1)/τinj(2)b
52 116 122 453 860 2415
2 ps.
cationic state. Competing decay processes can be neglected, as the excited state decays of the nonadsorbed perylenes have been determined to be at least 3 orders of magnitude slower. In the fitting model, a monoexponential cationic state decay due to recombination is also included. The time dependence of the cationic population Ncat(t) is then given by
Ncat(t) ) N0
τrec (e-t/τrec - e-t/τinj) τrec - τinj
(1)
Here, τinj is the time constant of injection, and τrec is the much larger constant for recombination. To obtain the injection times τinj, the experimental data is fitted with the above expression, convoluted with the system response function G(t) at 570 nm (perylene 1) and 590 nm (perylene 2), which was determined from the experimental cross-correlation to be typically ∼ 90 fs. Using this simple rate equation model, the injection times given in Table 1 were determined. Note, that the monoexponential decay is sufficient due to the small time window investigated here compared to the recombination analysis given below. For the experimental data obtained in UHV, Ar, and N2 environments, fitting the signal with a two-component rate equation did not significantly improve the fit, so a monoexponential injection was assumed. Fitting the toluene, THF vapor, and THF liquid injection traces using a two-component rate equation resulted in minor improvements but were not considered for clarity purposes. Note that the injection times listed in Table 1 for THF vapor environment were obtained from fitting the excited state decay traces using a biexponential decay function which accounts for the additional tail-edge absorption of the cationic state at 710 nm. The obtained injection times do not reflect the THF vapor traces shown in Figure 2 but were recorded at a lower THF vapor pressure. For measuring the injection dynamics of perylene 2 in UHV, the time resolution of the current experimental setup was insufficient. However, previous measurements employing a better time resolution had found an injection time of 10 fs16 which was used to calculate the injection ratio given in Table 1. As can be seen from Table 1, introducing different chemical environments by going from inert atomic and molecular gases to solvents of different polarity, the ultrafast sub-60 fs injection process found in UHV was subsequently retarded up to 2 orders of magnitude. Thereby, the injection dynamics correlates with the degree of polarity and the density of the chemical species: The more polar and the denser the adsorbing species, the slower the injection process. In the third column of Table 1, the
Electron Injection and Recombination Dynamics
J. Phys. Chem. C, Vol. 113, No. 50, 2009 21237
injection times ratio Rinj ) τinj(1)/τinj(2) is listed. In the case of UHV, the large ratio (Rinj ) 5.55) is due to the extremely fast injection for TiO2/perylene 2 that had previously been explained by a strong direct coupling of perylene 2 to the TiO2 surface resulting in a partial charge-transfer state16,17 which is absent in the case of perylene 1. For Ar and N2 environments, the ratio Rinj is more than three times smaller, and in the solvent environments, Rinj ) 1.18 was found. Discussions. In the previous section, we demonstrated that introducing coadsorbents to the organic-inorganic heterointerface slowed down the electron injection process. This depends on both polarizability and dipole moment of the coadsorbates. Polarizable inert gases Ar and N2 with no permanent dipole moment retarded the injection by about a factor of 2. Dipolar solvent coadsorbents increased this effect significantly, with THF retarding by almost 2 orders of magnitude. The rate kinj ) (τinj)-1 of heterogeneous electron injection from the excited state of an adsorbed dye molecule into the conduction band of the semiconductor electrode can be expressed as30
kinj ∝
∫ H2(1 - f(E, Ef))g(E)e-(E* + λ - E) /4λk T dE 2
B
(2) where H is the electronic coupling between the dye excitedstate and the semiconductor conduction band states, E* is the excited-state energy of the dye molecule with respect to the semiconductor conduction band minimum, g(E) is the normalized density of states of the conduction band, f(E, Ef) is the Fermi occupancy factor, kB is the Boltzmann constant, T is the temperature, λ is the reorganization energy associated with the electron injection, and E is the energy of the conduction band states. As can be seen from eq 1, the injection rate depends on the dye excited-state energy level E* due to the density of unoccupied acceptor states available for electron acceptance. It also depends on the reorganization energy λ, as electron injection occurs optimally to conduction band states lying λ below the dye excited-state energy (i.e., when the exponential term in eq 2 is maximal). For both perylene derivatives, a reorganization energy of λ ) 187 meV has been calculated.16 The influence of different λ due to different coadsorbents on the injection dynamics is assumed to be small as long as the injection dynamics are ultrafast.16,31 A thorough treatment of the effect of different λ due to different coadsorbents is beyond the scope of the current analysis and would need theoretical analysis. A central prediction of eq 2 is that as the TiO2 conduction band is raised toward the dye excited-state potential, the rate of electron injection is retarded. This retardation arises from the reduction in unoccupied acceptor states available for electron injection as the density of states decreases toward the conduction band minimum. For perylene 1 and 2, the excited-state levels E* had been estimated to be about 570 and 510 meV above the TiO2 Fermi level, respectively, while the Fermi level was estimated to be about 100 meV below the conduction band minimum (CBM).32 Hence, the perylene excited states E* are energetically located above the TiO2 CBM. In the vacuum ultrafast injections with injection times of 57 fs (perylene 1) and 10 fs (perylene 2) were observed.16 The slowing down of the injection observed here with different chemical environments is associated with a decreased energy separation between E* and the CBM, which is assumed to be mostly caused by a shift of the TiO2 conduction band as a result of the dipolar field induced by the coadsorbents. Since there is only a marginal steady-state absorption band shift (
