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J. Phys. Chem. C 2011, 115, 2–10
Photoinitiated Electron Transfer Dynamics of a Terthiophene Carboxylate on Monodispersed Zinc Oxide Nanocrystals Adam S. Huss, Julia E. Rossini, Darren J. Ceckanowicz, Jon N. Bohnsack, Kent R. Mann, Wayne L. Gladfelter, and David A. Blank* Department of Chemistry, UniVersity of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States ReceiVed: August 24, 2010; ReVised Manuscript ReceiVed: NoVember 29, 2010
Photophysical data for solution phase mixtures of a new terthiophene based organic dye, 3′,4′-dibutyl-2phenyl-2,2′:5′,2′′-terthiophene-5′′-carboxylic acid, and size selected, well-dispersed zinc oxide nanocrystals are reported. Time-resolved fluorescence and time- and frequency-resolved pump-probe spectroscopy confirm and characterize electron injection from the dye to the semiconductor nanocrystals (NCs) in room temperature ethanol dispersions at a series of dye:ZnO NC concentration ratios. The spectrum of the oxidized dye was determined by spectroelectrochemistry. The singlet excited state of the dye (190 ps lifetime in ethanol) is quenched almost exclusively by electron transfer to the ZnO NC, and the electron transfer dynamics exhibit a single time scale of 3.5 ( 0.5 ps at all concentration ratios. In the measured transient responses at different dye:ZnO NC ratios, gain in the amplitude of the electron injection component is anticorrelated with loss of amplitude from unperturbed excited state dye molecules. The dependence of this amplitude on dye:ZnO NC ratio deviates significantly from the prediction of a standard Stern-Volmer model. This observation is in agreement with the static quenching studies presented in the companion manuscript (DOI: 10.1021/jp1080143). By identifying electron transfer as the quenching mechanism at all ratios, the work presented here helps to exclude concentration quenching as the basis for the complicated quenching results, and supports the model proposed in the companion work that incorporates competitive binding between ZnO NC s and free Zn2+ cations in solution. I. Introduction First reported by Gra¨tzel and co-workers in 1991, dyesensitized solar cells (DSSCs) are typically made using nanocrystalline titanium dioxide films and an inorganic bipyridine based sensitizer.2 Conversion efficiencies have been reported in excess of 10%, suggesting that this technology has the potential to become competitive with standard p-n junction devices in some applications.3 Zinc oxide has been considered as a potential alternative component in DSSCs due to the similar energetic alignment of the conduction band, higher electron mobility, and flexibility in terms of synthesis and film morphology.4-8 However, DSSCs utilizing ZnO typically show poorer conversion efficiencies and slower dye to nanocrystal electron transfer rates compared to analogous TiO2 systems.3,9,10 Several possible reasons for the reduced performance with ZnO have been hypothesized, including problems related to chemical stability of the ZnO and differences in donor/acceptor electronic coupling.9,11 Electron transfer dynamics are complicated on nanocrystalline surfaces, where the heterogeneous dye environment exhibits multiple injection time scales for a given sensitizer. Reported time constants range from 50 ps, to the single exponential decay and static offset on the right side of eq 2. The data fit well to a single exponential, and the time constant from both experiments, τlt ) 200 ps, indicates that this is the excited singlet lifetime component from unperturbed dye molecules. The unperturbed lifetime fit was subtracted from the raw data and the remaining residual, shown in Figure 4b, was fit to the stretched exponential in the first term of eq 2. This component is assigned to electron transfer. The optimized fitting parameters are listed in Table 2 and demonstrate excellent agreement between dynamics measured by the two different methods. Following subtraction of the long time tail, the early time dynamics are virtually indistinguishable, as shown in Figure 4b. We note that good fits to the residuals could also be obtained using a biexponential decay with time constants of ∼3 and ∼15 ps in place of the stretched exponential decay. The stretched exponential was selected because no discernible improvements in the fits were evident with the addition of a fourth adjustable parameter in the case of the biexponential decay. Figure 5 shows TA at 1.65 eV for a series of dye:ZnO NC ratios. These data were fit with eq 2 following the same procedure outlined above, and the optimized parameters are listed in Table 3. At the lower dye:ZnO NC ratios there is a clear ∼3.5 ps ET component, and the amplitude of this component is anticorrelated with the amplitude of the unperturbed lifetime component. The quality of the data following
Figure 5. Transient absorption at 1.65 eV for the free dye and a series of dye:ZnO NC ratios: (a) the full transients and fits; (b) the residual following subtraction of the exponential lifetime component. Points are raw data and lines are fits as described in the text. Fit parameters are given in Table 3.
subtraction of the unperturbed lifetime component is highest for the 8:1 and 1:2 ratios. At 50:1 the relative amplitude of the electron injection component has been reduced and there is additional scatter due to lower solubility; however, these data still exhibit a clear 3 ps ET component. The 250:1 sample is dominated by the unperturbed lifetime component, but there is evidence of an intermediate decay with an approximately 17 ps time scale left in the residual. The low signal-to-noise left in the residual at 250:1 reduces confidence in the fit and precludes additional analysis. The maximum relative amplitude for the ET component compared with the unperturbed lifetime component is found at the 8:1 dye:ZnO NC ratio, suggesting this to be the cleanest measure of the ET dynamics in the series. Having the maximum contribution from ET at a dye:ZnO NC ratio of 8:1 (rather than 1:2 for example) is consistent with the static binding studies1 and will be considered in more detail in the discussion section. The ET time scales for the three lowest ratios are nearly identical, and the two highest quality residual data sets at 8:1 and 1:2 are practically indistinguishable, as shown in Figure 5b. Time-dependent increase in the cation was probed by TA at 2.06 eV for the 8:1 dye:ZnO NC sample, and this is shown in Figure 6 along with the same experiment on a solution of the
Photoinitiated Electron Transfer Dynamics
J. Phys. Chem. C, Vol. 115, No. 1, 2011 7
TABLE 3: Fit Parameters for Decay of the Transient Absorption at 1.65 eV for a Series of Dye:ZnO NC Ratiosa
a
dye:ZnO NC
τet (ps)
γet
A
τlt (ps)
B
C
1:2 8:1 50:1 240:1
3.2 ( 0.2 2.8 ( 0.2 3.1 ( 0.3 17 ( 2
0.76 ( 0.03 0.70 ( 0.02 1.3 ( 0.2 0.69 ( 0.1
0.5 ( 0.02 1.01 ( 0.02 0.21 ( 0.02 0.23 ( 0.02
191 ( 2 193 ( 5 205 ( 2 190 ( 2
0.25 ( 0.01 0.06 ( 0.01 0.69 ( 0.01 0.73 ( 0.01
0.29 ( 0.01 0.07 ( 0.01 0.06 ( 0.01 0.03 ( 0.01
Errors are reported as the 95% confidence interval for the nonlinear regression fit. Data and fits are shown in Figure 5.
Figure 6. Transient absorption at 2.06 eV for the dye alone and the 8:1 dye:ZnO NC ratio. Points are raw data and the line is the fit as described in the text. Fit parameters are given in Table 4.
TABLE 4: Fit Parameters for Transient Absorption at 2.06 eV for the 8:1 Samplea τa (fs)
a
τet (ps)
γet
A
