Photocurrent transient and IMPS

Photocurrent transient and IMPS were measured for ZnO films prepared in the presence of either EY or C343 and those SDA molecules were extracted by dipping the film into an aqueous KOH (pH:10.5). The photocurrent transient measurement is useful to see the non steady-state photocurrent response for the illumination. In this study, the measurements were carried out in the ms range. 0.5 M TBAI and 0.05 M I2 in a mixed solvent of ethylene carbonate / acetonitrile (4:1 by volume) was used as electrolyte. An UV-LED was used for light source and the electrodes were illuminated from the substrate side for 50 ms. Pt and Ag/Ag⁺ were used as counter electrode and reference electrode respectively. The potential of -0.2 V vs. Ag/Ag⁺ was applied to the electrode during the measurement.

0.00 0.02 0.04 0.06 0.08

0.00 0.02 0.04 0.06 0.08 0.10

0.00 0.05 0.10 0.15 0.20 0.25

Current / mA cm-2 Current / mA cm-2

ZnO (C343 as SDA)

ZnO (EY as SDA)

Time / s

Fig. 5.41; Time-resolved photocurrents measured for ZnO films which have different orientation.

The results of photocurrent transients are shown in Fig. 5.41. The stable photocurrent of 0.225 and 0.009 mA cm^-2 were obtained from a ZnO with EY as SDA and a ZnO with C343 as SDA respectively. It takes about 30 ms or less to reach the steady- state photocurrents for both films. Such times are necessary for filling the traps in the ZnO matrix. Although the magnitude of the photocurrent was not same; the photocurrent measured for the film with C343 as SDA was higher than the film with EY as SDA, a clearly faster response for the illumination was observed from the film with C343 as SDA (dotted line in Fig. 5.41).

And also the faster decrease of the photocurrent was observed when the illumination was cut off. Those results suggest the faster electron transport property in ZnO with C343 as SDA or less number of traps in ZnO with C343 as SDA. In both cases, it shows that films with C343 as SDA have nicer photoelectrochemical properties.

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.00

0.02 0.04 0.06 0.08 0.10

Current / mA cm-2

Time / s

ZnO (EY as SDA) ZnO (C343 as SDA)

Fig. 5.42; Time-resolved photocurrents measured for ZnO films which have different orientations.

Photocurrents were adjusted to the similar magnitude by changing the light intensity.

However, when the photocurrent is higher, it can be expected that the concentration of the electrons in the conduction band is also higher than the other. In such case, the traps in ZnO matrix can be filled quickly by a higher number of electrons in the conduction band.

It makes these different characteristic films harder to compare. Then time-resolved photocurrents were measured at a similar magnitude of the photocurrent. The photocurrents were adjusted by adjusting the light intensity. The result of such time- resolved photocurrents is shown in Fig. 5.42.

Even when the photocurrents were adjusted to the similar magnitude, it was seen from the plots that the response of the film with C343 as SDA was faster than that of the film with EY as SDA. And when the illumination was cut off, a faster decrease of the photocurrent was observed for the film with C343 as SDA than for the film with EY as SDA, clearly speaking for either faster electron transport or less number of traps in the film.

To see the difference of the electron transport properties of those films in more detail, IMPS measurements were carried out to obtain the electron transit time in such ZnO films by using an UV-LED. The result is shown in Fig. 5.43. It should be noted that these measurements were carried out with the previous setting (see Appendix 9) and without a bias illumination. The results below 10 Hz were removed.

-1 0 1 2 3 4 5 6 7 8 9 -6

-5 -4 -3 -2 -1 0 1

F_min 140 Hz F_min

65 Hz

Im (I photo / 10-6 A cm-2 )

Re (I_photo / 10^-6A cm^-2)

ZnO (EY as SDA) ZnO (C343 as SDA)

Fig. 5.43; IMPS plots of a ZnO with EY as SDA and a ZnO with C343 as SDA measured by using an UV-LED from the substrate side. 0.5 M TBAI and 0.05 M I2 in a mixed solvent of ethylene carbonate / acetonitrile (4:1 by volume) was used as electrolyte. Note that these measurements were carried out with the previous setting without a bias illumination. (See Appendix 9)

Rather flat shapes of IMPS responses were obtained for both films. 65 and 140 Hz of fmin

values were found from the ZnO with EY as SDA and the ZnO with C343 as SDA respectively. Since the average electron transit time can be calculated from the value, fmin,

by using the formula; τ_D=1(2⋅π⋅f_min), the transit times of the electrons for those films are found at 2.4 x 10^-3 s and 1.1 x 10^-3 s for the ZnO with EY as SDA and the ZnO with C343 as SDA, respectively. The larger photocurrent and the faster response for the ZnO with C343 as SDA were clearly confirmed by these IMPS data.

Such improved electron transport in porous semiconductor electrodes has become an important for the further optimization of dye- sensitized solar cells.^{７８,２０７,２０８} Generally, the injection time of the electron from LUMO to the conduction band of the semiconductor is much faster than the electron transit time in the nanocrystalline semiconductor matrix.

２００ And it leads to a higher concentration of the electrons in the conduction band and the probability for the recombination will increase.^１９７ A faster electron transport property in the semiconductor is one of the solutions to reduce the recombination since the electrons injected to the conduction band of the semiconductor can be transported quickly to the external circuit before the recombination takes place. The results obtained in this section indicate that films with C343 as SDA can be a candidate for such a solution.

5.3.2. Characterization of re-ad (Eosin Y or C343) / ZnO (Eosin Y or C343 as SDA) films

In the last section, the electron transport properties of the ZnO films prepared in the presence of either EY or C343 as SDA were studied and a faster electron transport in the films with C343 as SDA was shown by the direct excitation of ZnO and hence the creation of electrons and holes. In dye- sensitized solar cells, however, there are no holes in the semiconductors since the excited electrons are created at the sensitizers and only those electrons are injected to the semiconductors. So the condition for the electron transport is different when the electrons are injected from sensitizers.

The photoelectrochemical characterizations were carried out here for the films which have different orientations and the sensitizer molecules adsorbed on the surface. EY and C343 were used as sensitizer. The films were investigated by measurements such as absorption spectrum, photocurrent transient, IMPS and IMVS. Electron transit times and lifetimes were estimated for such films. And moreover the diffusion coefficients of the electron and the diffusion lengths were obtained from IMPS results and electron lifetimes measured in IMVS.