TSPcZn as sensitizer in transport-optimized films

Although the film with C343 as SDA has an even improved electron transport property, however, such better characteristics do not directly lead to a further enhanced photoelectrochemical performance. It seems that the photoelectrochemical performance of the films with C343 as SDA is reduced by a relatively lower electron injection efficiency from the sensitizer to the semiconductor. The adsorption of the sensitizer to the 002 plain of ZnO might not be appropriate for the injection of the excited electrons. Or relatively higher amount of the sensitizer molecules compared to the surface area of the film with C343 as SDA found by BET measurements leads to the ineffective sensitizer molecules in the film and hence lower electron injection efficiencies. If the latter was the reason, increasing the surface area and optimizing the adsorption condition for sensitizers will improve the photoelectrochemical performance.

The absorption spectra of the films are shown in Fig. 5.56. The absorption peaks which are corresponding to the Q- band of TSPcZn can be seen from the spectra. A rather strong light scattering was seen for re-ad TSPcZn / ZnO (EY as SDA), whereas re-ad TSPcZn / ZnO (C343 as SDA) was rather transparent. The amount of dye loaded in the film and the film thickness are almost identical for both films and hence is the dye concentration in the films. (Table 7) However, the surface area of the bare ZnO (C343 as SDA) was found smaller than the one of the films with EY as SDA^１７３ It implies more concentrated TSPcZn molecules on the inner surfaces of the film with C343 as SDA fortunately not leading, however, to increased aggregation as clearly seen by quite constant Uv-Vis spectra.

Table 7: Dye content, average film thickness, and dye concentration of the investigated films Dye content

/ 10^-9 mol cm^-2

Film thickness / µm

Dye concentration / 10^-5 mol cm^-3 re-ad TSPcZn / ZnO (EY as SDA) 4.46 2.46 1.81 re-ad TSPcZn / ZnO (C343 as SDA) 4.51 2.60 1.74

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.00

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 re-ad TSPcZn/ZnO (EY as SDA)

Photocurrent / mA cm-2

Photocurrent / mA cm-2

Time / s

re-ad TSPcZn/ZnO (C343 as SDA)

Fig. 5.57; Time-resolved photocurrents measured by using a red LED for re-ad TSPcZn / ZnO (EY as SDA) (solid line) and re-ad TSPcZn / ZnO (C343 as SDA) (dashed line).

The time- resolved photocurrents measured for re-ad TSPcZn / ZnO (EY as SDA) and re-ad TSPcZn / ZnO (C343 as SDA) are shown in Fig. 5.57. The re-ad TSPcZn / ZnO (EY as SDA) showed a rapid increase of the photocurrent as the illumination started and an overshoot was observed. Indicating the recombination reactions which are caused by slow hole transfer to the redox electrolyte. Such phenomenon can be seen when the electron collection in the electrode is so rapid.^１８５ For re-ad TSPcZn / ZnO (C343 as SDA), it took a rather long time of about 300 ms to reach the steady- state photocurrent speaking for a

large number of traps in the film. An active role of the sensitizer in the different ZnO matrices as already suggested in the last section is thereby shown and the complexity of the optimization of such electrodes is seen.

1E14 1E15

1E-3 0.01 0.1

-0.78 -1.03 -0.37

-0.46 Electron transit time / s Electron lifetime / s

Photon flux / cm^-2 s^-1

transit time EY as SDA C343 as SDA lifetime

EY as SDA C343 as SDA

Fig. 5.58; Electron transit times τD and lifetimes τn for re-ad TSPcZn / ZnO (EY as SDA) (τD = at -0.2 V vs. Ag/Ag⁺, τn = ) and re-ad TSPcZn / ZnO (C343 as SDA) (τD = at -0.2 V vs. Ag/Ag⁺, τn = ) at the different light intensities. The numbers described on the figure are the slope of the line.

The electron transit times and the electron lifetimes for these films were also obtained by IMPS and IMVS measurements and the time constants are shown in Fig. 5.58. As it was observed for other films, the electron transit time becomes shorter with increasing light intensity, which is caused by the higher electron density in the conduction band of the ZnO to fill the deep traps. Similarly the electron lifetime becomes shorter as the light intensity becomes higher. Higher electron density in the conduction band of ZnO leads to the higher possibility for the photogenerated electrons to recombine with the redox electrolyte.

Significantly shorter electron transit time, the difference of approximately one order, for re-ad TSPcZn / ZnO (EY as SDA) compared to re-ad TSPcZn / ZnO (C343 as SDA) was found, whereas the electron lifetime was in similar time range for both films. The electron transit time of re-ad TSPcZn / ZnO (EY as SDA) is more than one order shorter than the electron lifetime in the film. So, efficient collection efficiency could be expected. However, it was observed clearly in the time- resolved photocurrent that there is an evidence of the recombination; a peak and a relaxation following the illumination starts. And the sign of the recombination was also seen in IMPS (not shown) as the responses in (+, +) quadrant.

A relatively small difference of the electron transit time and the electron lifetime was found for re-ad TSPcZn / ZnO (C343 as SDA). Significantly steep slope implies the high density of traps in the ZnO matrix. These obtained results for both films again indicate the active role of the dye / ZnO interplay. One of the reasons is that the

photoelectrochemical properties of TSPcZn / ZnO strongly depend on the plain of ZnO where TSPcZn molecules adsorb or the role that it plays in suppressing the back reactions or compensating the trap levels. However, since a typical shape of the time- resolve photocurrent for re-ad TSPcZn / ZnO (EY as SDA) is shown in Fig. 5.16, the shape of the time- resolved photocurrent shown here is untypical.

Fig. 5.59; Effective electron diffusion coefficients Dn as the function of the dc photocurrents (a) and the photovoltages (b) for re-ad TSPcZn / ZnO (EY as SDA) () and re-ad TSPcZn / ZnO (C343 as SDA) () obtained by fitting the IMPS response. The symbol (+) is the diffusion coefficient obtained when the absorption coefficient is set at 1000 for re-ad TSPcZn / ZnO (C343 SDA). The parameters used for the fitting and the values obtained by fitting are indicated in Appendix 10.

The electron diffusion coefficients were obtained for these films. (Fig. 5.59) Since these films have a similar dye content, film thickness and electron lifetime, the difference of the electron transit time reflects the differences in their electron diffusion coefficients. Again, the effective absorption coefficient had to be modified in order to obtain reasonable plots.

(t in Fig. 5.59) First of all, the calculation of the absorption coefficient from the dye concentration in the film will lead to an unavoidable deviation since TSPcZn molecules form the aggregation. And moreover, since the parameters like the dye content and the film thickness are almost same for these films (Table 7), the routine calculation for determination of the absorption coefficient gives almost similar values. However it is obvious from their absorption spectra (Fig. 5.56) that the absorption profile for those films is completely different by the light scattering. Nevertheless, larger diffusion coefficients were found for re-ad TSPcZn / ZnO (EY as SDA) than for re-ad TSPcZn / ZnO (C343 as SDA). This result stays opposite to the results in the last section and indicates the complicated role of the sensitizer both in the back reactions and the trap compensation, caused, for example, by adsorption of the same TSPcZn sensitizer on different plains of the two ZnO matrices. It should be noted, however, these films were prepared and SDA molecules were removed in Gifu University and only the adsorption of TSPcZn was carried out in Giessen. There were some weeks between the extraction of SDA molecules and the adsorption of the sensitizer. Normally, these procedures were carried within just two days.

Then, such unusual interval could change their surface condition.

0.01 0.1 1 0.0

2.0x10^-4 4.0x10^-4 6.0x10^-4 8.0x10^-4 1.0x10^-3 1.2x10^-3 1.4x10^-3 1.6x10^-3 1.8x10^-3 2.0x10^-3

Diffusion length / cm

dc photocurrent / mA cm^-2

Fig. 5.60; Calculated diffusion lengths L as the function of the dc photocurrents for re-ad TSPcZn / ZnO (EY as SDA) () and re-ad TSPcZn / ZnO (C343 as SDA) () obtained from Dn and τn. The symbol (+) is the diffusion length obtained when the absorption coefficient is set at 1000 for re-ad TSPcZn / ZnO (C343 SDA).

The diffusion lengths were also calculated for these films. (Fig. 5.60) It was also shown here that the diffusion length is weakly dependent on the light intensity due to the compensation between the diffusion coefficient and the electron lifetime which changes in opposite sense to the light intensity. The relatively large diffusion coefficients of re-ad TSPcZn / ZnO (EY as SDA) reflect also to the diffusion length, obtained values exceeded significantly its films thickness, whereas the diffusion lengths for re-ad TSPcZn / ZnO (C343 as SDA) exceeded its film thickness only when the light intensity is strong, otherwise they are smaller than the film thickness, already indicating an inefficient collection.

Consequently, better photoelectrochemical properties were found for the film with EY as SDA when TSPcZn was used as sensitizer. It indicates that TSPcZn might have a preferable plain to adsorb and the difference of the adsorbed plain might significantly influence the electron transport property of the ZnO matrix. To compare the different oriented ZnO films further, other sensitizers should be tested.