Nanocluster-TiO2 layer prepared by liquid phase deposition

The LPD process, which was developed in recent years, is a designed wet chemical film process firstly by Nagayama in 1988. Than Herbig et al. used LPD to prepare TiO2 thin film and studied its photocatalytic activity. Most vacuum-based technologies such as sputtering and evaporation are basically limited to the line-of-sight deposition of materials and cannot easily be applied to rather complex geometries. By contract, the easy production, no vacuum requirement, self-assembled and compliance to complicated geometry substrate has led many LPD applications for functional thin films. In order to directly grow nanocluster-TiO2 on ITO glass, the simplest method – LPD process was firstly considered by using H2TiF6 and H3BO3 as precursors. The reaction steps involved to obtain nanocluster-TiO2 are illustrated as followed. The H3BO3 pushes eq. (1) to form eventually Ti(OH)62- which transforms into TiO2 after thermal annealing.

(TiF6)2- + nH2O ^ m6_n(OH f – + nHF (1)

H3BO3 + 4HF ^ BF4++ H3O + 2H2O (2)

Here, the influence of deposition variables including deposition time and post-heat treatment on the microstructure of TiO2 layer and the photovoltaic property was studied. The LPD system to deposit titania film is schematically shown in Fig. 2.

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Fig. 2. Schematic diagram of LPD-TiO2 deposition system.

Figure. 3 shows the I-V characteristics of the DSSCs assembled by using TiO2 films deposited for different time, with their corresponding surface and cross sectional film morphology also shown. It was indeed capable of producing nanocluster featured TiO2 films shown in the surface morphology, regardless of the deposition time. It can also be found that the I-V characteristics are sensitive to the TiO2 film deposition time, but unfortunately non-linearly responded to the deposition time. By careful examination on the surface morphology of these TiO2 films deposited at different deposition time, the film obtained at longer period of deposition time, say 60 h presents no longer nanocluster feature, but cracked-chips feature instead. This significantly reduces the open circuit voltage (Vx) as well as the short circuit current density (Jsc). It shall be a consequence of the cracks that leads to the direct electrolyte contact to the front window layer (to reduce Voc) and the reduced specific surface area (to reduce Jsc). Further exam cross sectional morphology of the TiC>2 films as a function of deposition time, it was found that the film thickness does not linearly respond to the deposition time. This shall be the gradual loss of reactivity of the electrolyte liquid. Therefore, it is not practical to increase the film thickness by an extended deposition time. Still, we believed that by constant precursor supplement into the electrolyte liquid, it would refresh the liquid and certainly the increased film growth rate, of course with the price of process monitoring automation.

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Fig. 3. I-V characteristic of the cell assembled by LPD-TiO2 under different deposition time, with their corresponding surface and cross sectional film morphology.

Fig. 4 shows the XRD patterns of the TiO2 film with different annealing temperature. The results indicate that the as-deposited film was amorphous due to the low LPD growth temperature. Annealing provides thermal energy as a driving force to overcome activation energy that required for crystal nucleation and growth. The exact TiO2 phase to be effective for DSSC has been known to be anatase, which can found that the peak ascribed to anatase phase A(101) can only appear over 400 °C and become stronger over 600 °С, ie. better crystallinity of the film annealed at higher temperature. Over an annealing temperature of 600 °С leads to the ITC glass distortion.

The I-V characteristics of the DSSCs assembled by using TiO2 films with different annealing temperatures, with their corresponding surface and cross sectional film morphology are shown in Fig. 5. The TiO2 film surface forms numerous tiny nanocracks and needle-like structures with increasing annealing temperature. It can be found that the I-V characteristics are sensitive to the TiO2 film annealing temperatures and the Jsc increases straight up to a maximum when annealed at 600 °С. Apparently, the increase of Jsc shall be associated with the reformation of the TiO2 film morphology and the increased film crystallinity. By reforming numerous tiny nanocracks and needle-like structures, the TiO2 film has more specific surface area after post-annealing and achieves higher efficiency dye adsorbing. However, the negative effect of annealing occurred to the significant increase of the ITC electrical resistance that causes the Voc drop off as can be seen in Fig. 5. Anyhow, the overall increased photovoltaic efficiency as a function of annealing temperature is an encouraging result of this study using PLD to obtain Ti02 film and post-annealing for DSSC photoanode preparation.

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Fig. 4. XRD patterns of (a) ITO glass substrate, (b) Ti02 as-deposited specimen, and the post annealed specimens obtained at (c) 200, (d) 400 and (e) 600 °C for 30 min.

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Photovoltage (V)

Fig. 5. I-V characteristic of the cell assembled by LPD-Ti02 under different annealing temperature, with their corresponding surface and cross sectional film morphology.

1.1 Summary

In this paragraph, a LPD system is used to prepare the Ti02 layer on ITO glass at the room temperature followed by post-annealing as the photoanode in DSSC. The result is closely connected to the variation of microstructure including both the specific surface area and crystal structure. This demonstration work confirms the truth that the LPD method is capable of obtaining nanocluster Ti02 and with crystallinic anatase structure through suitable annealing treatment. Unfortunately, the unacceptable LPD-TiC>2 film growth has led some other attempts to obtain nano-structural TiO2 layer. These methods are sketched as below.