Ti Foil-based Flexible Dye–sensitized Solar Cells: Photophysical and Photoelectrochemical Studies
Date Issued
2012
Date
2012
Author(s)
Lin, Lu-Yin
Abstract
There are three parts in this dissertation aim to investigate the Ti foil-based photoanodes for back-illuminated dye-sensitized solar cells (DSSCs).
In the first part, the fundamental researches in this system was made to improve the cell performance and the durability. The effects of Pt sputtering periods on the counter electrode, sintering temperatures for TiO2/Ti foil based photoanodes, thickness of Ti foils, and the composition of the electrolyte were investigated. A solar-to-electricity conversion efficiency (η) of 5.95% was obtained after optimizations of these parameters. Meanwhile, the improved
durability was found for back-illuminated DSSCs, due to the absorption of UV-rays by the iodine in the electrolyte. The back-illuminated DSSC shows a good durability even under
continuous illumination of 100 mW/cm2 light intensity least for 500 h.
The main challenges in this system lie on the formation of amorphous metal oxide between the TiO2 semiconductor layer and the Ti foil with sintering process to hinder the transfer of electrons and the lower incident light caused by back illumination, resulting in less dye-excitation and reduction of the photocurrent density. In the second part, strategies to solve the challenges of formation of amorphous metal oxide and back illumination are proposed. A binder-free TiO2 paste was used for the TiO2/Ti foil based photoanode with a low temperature sintering process to minimize the formation of amorphous metal oxides. It cannot
be realized with a TiO2 paste with binder, because the residual of the organic binder in the TiO2 films of the photoanodes fabricated in a low sintering temperature would act as obstacles for electron transportation and reduce cell performance. It was evidenced that the DSSC with the TiO2 film fabricated with a binder-free TiO2 paste sintered at a relatively low temperature shows competitive performance (350 oC, η = 4.34%) to the cell with a TiO2 paste with bindersintered at higher temperature (450 oC, η = 4.33%). Therefore, the energy consumption can be reduced with this low temperature sintering process. Net-like Pt counter electrodes were made to increase the incident light illuminated from the back side of the cell. Higher transmittance of an average value of 99% was obtained for net-like Pt counter electrodes, with compared to that of 92% for a bare one, under Pt sputtering current of 40 mA for 5 s. Even the catalytic ability of the Pt layer is worse for net-like Pt counter electrodes caused by less Pt deposition,
their higher transmittances lead to the better performance for the pertinent DSSC (η = 4.77%).
Other than applying TiO2 nanoparticles (TNPs) as the semiconductor layer for the studies mentioned above, the other structure with higher electron transferring rate was introduced in the last part to compensate for the lower electron transportation in TNPs caused by numerous boundaries between TNPs. One-dimensional TiO2 nanotubes (TNTs) were widely used in the recent years because it exhibits better electron transportation and can reduce the
loss of electrons by recombination. However, its smaller surface area as compared to that of TNPs limits its application. Two methods were proposed to solve this problem. One is to combine the TNTs with TNPs, and there are two ways applied in this dissertation, i.e., fabrication of a layer-by-layer structure and infiltration of TNPs into TNTs. For the layer-by-layer structure, the underlayer TNTs improves the electron transportation and increase contact points between Ti foil and TNPs, while TNPs overlayer provides larger surface area for dye adsorption. The pertinent DSSC exhibited a η of 6.68% with compared to
that of the cell with only TNPs (η = 5.55%). For the TNTs infiltrated with TNPs, this composite film possesses higher surface area and higher electrons transfer rate, and η of
6.45% and 4.21% were obtained for the DSSC with the composite film and bare TNTs on the photoanodes, respectively. The other method to solve the problem of smaller surface area of TNTs is to increase their length, but the open-circuit voltage will decrease at the same time. A core-shell structure was applied with a yttrium oxide thin film on the surface of TNTs tocompensate for the lower open-circuit voltage with longer TNTs made to increase the surface area. The η of the pertinent DSSC was improved to 6.52% with compared to that of 5.35% for the DSSC with bare TNTs.
For a flexible DSSC, not only a flexible photoanode but a flexible counter electrode is indispensable. Traditionally, the Ti foils are anodized for making TNTs and applied in a
photoanode. However, in the last chapter in the last part, the imprints of TNTs were utilized by first fabricating TNTs on Ti foils by anodization and removing TNTs completely from Ti foils by ultrasonically vibration. The resulting Ti foils with TNTs imprints was applied as the substrate of counter electrodes with highly active surface area for Pt sputtering. These counter electrodes exhibit better catalytic ability for I-/I3- redox reaction. An extremely high η of 9.35% was obtained for the DSSC with a Pt-sputtered Ti foil with TNT imprints on its surface as the counter electrode, which is much higher than that for the cell with bare Ti foil as the substrate of its counter electrode (7.81%).
Subjects
Back illumination
Dye-sensitized solar cells
Flexible
Titanium foil
TiO2 nanotubes
Type
thesis
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