Increases in solar conversion efficiencies of the ZrO2 nanofiber-doped TiO2 photoelectrode for dye-sensitized solar cells

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).


Introduction
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][2][3]. A typical DSSC device contains a light-harvesting layer on a photoelectrode and a Ptcoated 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 TiO 2 surface morphology, particle size, thickness of the photoelectrode, nature of the dye, etc. [4][5][6][7][8][9][10].
A high light-to-electricity conversion efficiency results from a large surface area of the mesoporous TiO 2 photoelectrode, on which the dyes can be sufficiently adsorbed. In this study, we introduced zirconia [ZrO 2 ] nanofibers into the mesoporous titania [TiO 2 ] photoelectrode. The ZrO 2 nanofibers are prepared by electrospinning. The TiO 2 film composite with ZrO 2 nanofibers creates a larger surface area than the single TiO 2 film, in which case the amount of dye loading was increased and short-circuit photocurrent density and solar conversion efficiency are also increased.

Experimental details
The ZrO 2 nanofiber additives were prepared by electrospinning method. At first, mixed together, 6 ml zirconium acetate, 12 ml acetic acid, 12 ml ethanol, and 50 g poly(methyl methacrylate) were stirred for 24 h; then, the compounds were sintered at 700°C for 4 h. A detailed process is displayed in Figure 1.
The TiO 2 paste was prepared by mixing TiO 2 with Degussa P-25, polyethylene glycol, acetyl acetone, distilled water, triton X-100, HNO 3 , and ZrO 2 nanofibers. The concentrations of ZrO 2 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/cm 2 , 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 cm 2 . 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 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 TiO 2 film. In order to investigate the physical and optical characteristics of the dyeadsorbed TiO 2 films, the UV-visible [UV-Vis] spectrum measurement was performed. The photovoltaic properties were investigated by measuring the photocurrentvoltage characteristics under illumination with air mass [AM] 1.5 (100 mW/cm 2 ) simulated sunlight.
Results and discussion Figure 2 shows the FE-SEM images of the TiO 2 film's surface and the 5 wt.% ZrO 2 nanofiber-added TiO 2 film's surface. In the ZrO 2 nanofiber-added TiO 2 film in Figure 2b, the ZrO 2 nanofiber was shown at the surface of the TiO 2 film, or the TiO 2 film was studded with the ZrO 2 nanofiber. The TiO 2 film's surface area was increased, and the dye adsorption contents became larger by the addition of the ZrO 2 nanofiber. So, we can forecast that the TiO 2 electrode is able to obtain high conversion efficiency. Figure 3 shows the pore distributions calculated from adsorption data using the Barrett-Joyner-Halenda [BJH] method. As shown in Figure 3, a broad peak was found at around 25 nm, and an added 5 wt.% ZrO 2 nanofiber at around 30 nm was observed. Compared to the pure TiO 2 film, the 5 wt.% ZrO 2 nanofiber-added TiO 2 films show a significant change in the pore size distribution. A large pore volume in BJH was observed on the TiO 2 film with 5 wt.% ZrO 2 nanofibers, which is in agreement with the results of the FE-SEM image. Figure 4 shows the UV-Vis absorption spectra of the dye-adsorbed TiO 2 film and the 3, 5, and 7 wt.% ZrO 2 nanofiber-doped TiO 2 films. From the results, the absorption spectra increased at around 538 nm with added ZrO 2 nanofibers doped in the TiO 2 film and also enhanced the amount of dye loading. So, the 5 wt.% ZrO 2 nanofiber-doped TiO 2 film had the best dye   loading, and also, its solar conversion efficiency was the best among the samples.
In order to determine the factors that influence the stability of the DSSCs, electrochemical impedance spectroscopy [EIS] was performed. Figure 5 shows that the Nyquist plot of EIS of the DSSCs exhibits semicircles, which are assigned to the electrochemical reaction at the Pt counter electrode, the charge transfer at the TiO 2 /dye/electrolyte interface, and the Warburg diffusion process of I -/I 3 - [9,10]. As shown in Figure 5, the second semicircle is the resistance (R 2 ) related to the electron transport in the TiO 2 /dye/electrolyte interface which is reduced. It can be seen that the TiO 2 film and the 3, 5, and 7 wt.% ZrO 2 nanofiber-doped TiO 2 films are 13.2, 10.3, 9.6, and 11.9 Ω, respectively. Figure 6 shows photocurrent-voltage characteristics of the DSSCs with the ZrO 2 nanofiber-doped TiO 2 film. The open-circuit photovoltage was almost the same, and the short-circuit photocurrent density increased with the added amount of ZrO 2 nanofibers and had reached the maximum at 5 wt.% of ZrO 2 nanofiber (15.9 mA/ cm 2 ). The open-circuit photovoltage [V oc ], the short-circuit photocurrent density [J sc ], the fill factor [FF], and the light-to-electricity conversion efficiency [h] at 5 wt. % added ZrO 2 nanofiber were 0.69 V, 15.9 mA/cm 2 , 0.60, and 6.5%, respectively, as shown in Table 1. From the results, we can realize that the insertion of ZrO 2 nanofibers creates a larger surface area and reduces the resistance of the photoelectrode, especially for the optimal amount of ZrO 2 contents (7 wt.%) of the photoelectrode in DSSCs.

Conclusions
In summary, a ZrO 2 nanofiber-doped TiO 2 film was used as a photoelectrode in DSSCs. The ZrO 2 nanofiber-doped TiO 2 films had a larger surface area than the pure TiO 2 film, in which case the amount of dye loading was increased, and J sc and h were also increased. The   optimum ratio of the ZrO 2 nanofiber was 5 wt.%. The DSSC with the 5 wt.% ZrO 2 nanofiber photoelectrode provided the highest h of 6.5%, J sc of 15.9 mA/cm 2 , V oc of 0.69 V, and FF of 0.60 under AM 1.5 (100 mW/cm 2 ) simulated sunlight illumination. Therefore, ZrO 2 fibers are a promising additive for the realization of high-efficiency DSSCs.