Increases in solar conversion efficiencies of the ZrO2 nanofiber-doped TiO2 photoelectrode for dye-sensitized solar cells
© Wang et al; licensee Springer. 2012
Received: 9 September 2011
Accepted: 2 February 2012
Published: 2 February 2012
In this paper, in order to improve the efficiency of dye-sensitized solar cells, we introduced zirconia [ZrO2] nanofibers into a mesoporous titania [TiO2] photoelectrode. The photoelectrode consists of a few weight percent of ZrO2 nanofibers and a mesoporous TiO2 powder. The mixed ZrO2 nanofibers and the mesoporous TiO2 powder possessed a larger surface area than the corresponding mesoporous TiO2 powder. The optimum ratio of the ZrO2 nanofiber was 5 wt.%. The 5 wt.% ZrO2-mixed device could get a short-circuit photocurrent density of 15.9 mA/cm2, an open-circuit photovoltage of 0.69 V, a fill factor of 0.60, and a light-to-electricity conversion efficiency of 6.5% under irradiation of AM 1.5 (100 mW/cm2).
Keywordszirconia nanofiber titania DSSC
Dye-sensitized solar cells [DSSCs] have generated a considerable research interest because of their high-energy conversion efficiency (approximately 11%) and low production costs [1–3]. A typical DSSC device contains a light-harvesting layer on a photoelectrode and a Pt-coated layer on a counter electrode; both electrodes are made of a transparent conducting oxide substrate; an iodine-based electrolyte fills the space between the photoelectrode and the counter electrode to serve as a redox mediator in a sandwich-type structure. Performance of the DSSC depends on many factors such as the TiO2 surface morphology, particle size, thickness of the photoelectrode, nature of the dye, etc. [4–10].
A high light-to-electricity conversion efficiency results from a large surface area of the mesoporous TiO2 photoelectrode, on which the dyes can be sufficiently adsorbed. In this study, we introduced zirconia [ZrO2] nanofibers into the mesoporous titania [TiO2] photoelectrode. The ZrO2 nanofibers are prepared by electrospinning. The TiO2 film composite with ZrO2 nanofibers creates a larger surface area than the single TiO2 film, in which case the amount of dye loading was increased and short-circuit photocurrent density and solar conversion efficiency are also increased.
The TiO2 paste was prepared by mixing TiO2 with Degussa P-25, polyethylene glycol, acetyl acetone, distilled water, triton X-100, HNO3, and ZrO2 nanofibers. The concentrations of ZrO2 nanofibers were 0, 3, 5, and 7 wt.%. The mixed solutions were ball milled at 100 rpm for 10 h. The photoelectrode was fabricated using a clean fluorine-doped tin dioxide [FTO] (approximately 8 Ω/cm2, Pilkington conductive glass, Seoul, South Korea) by squeeze printing. The coated photoelectrode was heat treated at 450°C for 30 min with a heating rate of 5°C/min. The obtained photoelectrode was immersed into the ethanol solution containing [cis-diisothiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II) bis(tetrabutylammonium)] (N719 dye, Solaronix, Aubonne, Switzerland) for 24 h. The active area of the photoelectrode was 0.5 × 0.5 cm2. On the other hand, the counter electrode was prepared similar to the photoelectrode preparation. Pt-Sol (Pt catalyst/SP, Solaronix) was coated onto the FTO glass by the squeeze printing method. The coated paste was heat treated at 450°C for 30 min with a heating rate of 5°C/min.
The electrolyte solution consisted of 0.3 M 1,2-dimethyl-3-propylimidazolium iodide, 0.5 M Li(I), 0.05 M I2, and 0.5 M 4-t-butylpyridine in 3-methoxypropionitrile between the two electrodes. The dye-coated photoelectrode and the Pt-coated counter electrode were sandwiched using a 60-μm-thick hot-melt sealing foil (SX 1170-60, Solaronix).
The field-emission scanning electron microscope [FE-SEM] (S-4700, Hitachi, Seoul, South Korea) and BET were used to examine the morphology and the pore distribution volume of the TiO2 film. In order to investigate the physical and optical characteristics of the dye-adsorbed TiO2 films, the UV-visible [UV-Vis] spectrum measurement was performed. The photovoltaic properties were investigated by measuring the photocurrent-voltage characteristics under illumination with air mass [AM] 1.5 (100 mW/cm2) simulated sunlight.
Results and discussion
Photocurrent-voltage characteristics of DSSCs using TiO2 with different amounts of ZrO2 nanofibers
3 wt.% ZrO2 nanofiber-doped TiO2
5 wt.% ZrO2 nanofiber-doped TiO2
7 wt.% ZrO2 nanofiber-doped TiO2
In summary, a ZrO2 nanofiber-doped TiO2 film was used as a photoelectrode in DSSCs. The ZrO2 nanofiber-doped TiO2 films had a larger surface area than the pure TiO2 film, in which case the amount of dye loading was increased, and Jsc and η were also increased. The optimum ratio of the ZrO2 nanofiber was 5 wt.%. The DSSC with the 5 wt.% ZrO2 nanofiber photoelectrode provided the highest η of 6.5%, Jsc of 15.9 mA/cm2, Voc of 0.69 V, and FF of 0.60 under AM 1.5 (100 mW/cm2) simulated sunlight illumination. Therefore, ZrO2 fibers are a promising additive for the realization of high-efficiency DSSCs.
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