Chapter 4 Towards efficient tetrapod-like ZnO dye-sensitized solar cells
4.5 Ionic diffusion dynamics of ionic-liquid electrolyte
Photocurrent transient measurement
In order to understand the ionic-transport mechanism, the time-response photocurrent transients are carried out. Figure 4-10(a) is the plot of the time-response photocurrent for a 12 μm thick T-ZnO DSCs filled with IL-based electrolyte exposed under different sun light intensity with an on-off irradiation shutter. At the beginning
of the shutter opening, the higher photocurrent is obtained because there are enough I ions to provide the oxidation-reduction reaction of the dye molecule. As the time
goes, the I ions around the dye molecules diminish due to the slow diffusion of the
I
I /3 couples, so that the photocurrent decreases continuously in a few seconds.
Finally, the photocurrent comes to an equilibrium value, which depends on the ionic-diffusion ability of the ionic liquid electrolyte. As lowering the light intensity, the reduction ratio of the photocurrent is also decreasing because the lower the sunlight exposure on the device, the less I ions are needed to regenerate the excited dye molecule. So even in the viscous ionic liquid electrolyte with slow ionic-diffusion, and the diffusion of I /3 I ions to and from the counter electrode is already fast enough to provide sufficient enough redox agents that regenerate the excited dye molecules. Figure 4-10(b) shows the ratio of the final to initial photocurrent as functions of film thickness and sunlight intensity. For the IL-based DSCs with T-ZnO
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photoanode, the reduction ratio of the photocurrent increases with the film thickness
Fig. 4-10. The photocurrent transient dynamics of IL-based D149-sensitized solar cells. (a) Photocurrent dynamics obtained with the IL-based DSC device for 12μm-thick tetrapod-like ZnO (T-ZnO) photoanode; (b) Ratio of the final to the initial value of the photocurrent as a function of sun
intensity for two types of 26μm photoanodes. Open symbol represents photoanode using T-ZnO powders and solid one represents commercial ZnO (C-ZnO) powders. AN-based electrolyte (EL 1) is a
mixture of 0.5 M PMII, 0.03 M I2, and 0.5 M TBP in AN. IL-based electrolyte (EL 2) is composed by 0.2 M I2, 0.5M TBP in PMII/C8MImPF6 mixture (v/v=35:65).
and the sunlight intensity. In the DSCs of thicker photoanode, the longer effective ionic-transport pathway is obtained, so the role of the ionic diffusion is also more
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important. Moreover, the comparison of the DSCs using 26 μm thick T-ZnO and C-ZnO photoanodes clarifies that the efficient ion-transport pathway of the self-assemble photoanode by T-ZnO improves the ionic diffusion, then the higher short-circuit photocurrent of T-ZnO DSCs is achieved.
EIS analysis
The electrical impedance spectroscopy (EIS) is one of the useful methods to explore the characteristics of each component in DSC devices; in particularly, the parameters about the photoanode electrode and the electrolyte will be discussed here.
In this study, the equivalent electric circuit model [39, 66] is shown and discussed in Chapter 2. In this model, ZN symbolizes the Warburg diffusion impedance, and can be described by the following equation [67],
, (Eq. 4-4)
where RD is the dc resistance of impedance of diffusion of tri-iodide, , d is the Nernst diffusion layer thickness, and D is the diffusion coefficient of I3.
Figure 4-11(a) shows the EIS Nyquist plots of the IL-based DSC devices with 26μm C-ZnO photoanode and different thicknesses of T-ZnO photoanodes under one
sun irradiation (AM 1.5G). The corresponding Bode plots are also shown in the inset of this figure. The related fitting parameters with related photovoltaic performances
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are listed in Table 4-2. Generally, all of the DSCs exhibit three semicircles, which are commonly assigned to the electrochemical reaction of Pt counter electrode, ZnO photoanode, and Warburg diffusion process of the electrolyte from high to low frequency, respectively.
Fig. 4-11. The electrical characteristics of the IL-based D149-sensitized solar cells. (a) Cole-Cole plots for DSCs employed by commercial ZnO (C-ZnO) photoanode and different thicknesses of tetrapod-like ZnO (T-ZnO) photoanodes. The inset shows the corresponding Bode plots. The equivalent circuit model used to analyze the experimental data is in Fig 2-7. The electrolyte (EL 2) is composed by 0.2
M I2, 0.5M TBP in PMII/C8MImPF6 mixture (v/v=35:65). (b) Plot of WSC and the DSC devices with various ZnO photoanodes, and the solid line is the fitting curve of T-ZnO DSCs.
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Figure 4-11(b) shows WSC for the DSCs by using 26 μm C-ZnO photoanode and different thicknesses of T-ZnO photoanodes. Given a x -μm thick photoanode provides ax μm of effective Nernst diffusion layer in the framework pore, so the total Nernst diffusion layer in the DSCs with 30 μm spacer may be described as
) 3 0
( x
a x
d μm, and the related parameters are obtained through well fitting to data by Wsc D[ax(30x)]2. For the T-ZnO DSCs, the effective thickness thick C-ZnO photoanode is estimated to 2.1, based on the same diffusion coefficient of the electrolyte. There is no doubt that the length of the ionic diffusion pathway is one
of the influence factors to affect the performance of IL-based DSCs. Comparing to these 26μm thick DSCs, the difference of these ionic diffusion pathways is only 5%,
but the corresponding photovoltaic performance is not only 5%. So it is believed that the continuous ionic diffusion pathway assembled spontaneously by the T-ZnO provides a more efficient ionic diffusion loop than that made of C-ZnO nanopowders in conventional DSCs, due to the different pore size of these devices.
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Table 4-2. Photovoltaic performance of IL-based DSCs and corresponding properties of photoanode determined by electrochemical impedance spectroscopy (EIS) under full sunlight irradiation.
ZnO a. IL-based electrolyte (EL 2) is composed by 0.2 M I2, 0.5M TBP in PMII/C8MImPF6 mixture (v/v=35:65).
In addition, the effective rate constant for recombination (keff) varies inversely with
the thickness of the tetrapod-like DSCs, which is listed in Table 4-2. The effective rate constant for recombination is generally identified to the recombination rate of electrons in the photoanode and I3 ions in the electrolyte, which is so-called back-recombination rate. The possible interpretation of this behavior is that the thicker photoanode not only provides larger area with more recombination sites on it but also decreases the volume of the electrolyte and the mole number of I3 at the
equilibrium state. Finally, in Table 4-2,the effective electron diffusion coefficients in photoanode are also estimated by the relation equation [67],
e f f K W
e f f
R R L k D ( )
2 ,where L stands for the thickness of the photoanode, and are also consistent to the JSC of the DSCs performance shown in Fig. 4-8.
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