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

Abstract

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 [1-3]. 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.

Experimental details

The ZrO₂ 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.

Figure 1.

Preparation of the ZrO₂ nanofiber.

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

Figure 2 shows the FE-SEM images of the TiO₂ film's surface and the 5 wt.% ZrO₂ nanofiber-added TiO₂ film's surface. In the ZrO₂ nanofiber-added TiO₂ film in Figure 2b, the ZrO₂ nanofiber was shown at the surface of the TiO₂ film, or the TiO₂ film was studded with the ZrO₂ nanofiber. The TiO₂ film's surface area was increased, and the dye adsorption contents became larger by the addition of the ZrO₂ nanofiber. So, we can forecast that the TiO₂ electrode is able to obtain high conversion efficiency.

Figure 2.

FE-SEM images of (a) TiO₂ film and (b) 5 wt.% ZrO₂ nanofiber-doped TiO₂ film.

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.% ZrO₂ nanofiber at around 30 nm was observed. Compared to the pure TiO₂ film, the 5 wt.% ZrO₂ nanofiber-added TiO₂ films show a significant change in the pore size distribution. A large pore volume in BJH was observed on the TiO₂ film with 5 wt.% ZrO₂ nanofibers, which is in agreement with the results of the FE-SEM image.

Figure 3.

BJH curve of TiO₂ film (triangle) and 5 wt.% ZrO₂ nanofiber-doped TiO2 film (square).

Figure 4 shows the UV-Vis absorption spectra of the dye-adsorbed TiO₂ film and the 3, 5, and 7 wt.% ZrO₂ nanofiber-doped TiO₂ films. From the results, the absorption spectra increased at around 538 nm with added ZrO₂ nanofibers doped in the TiO₂ film and also enhanced the amount of dye loading. So, the 5 wt.% ZrO₂ nanofiber-doped TiO₂ film had the best dye loading, and also, its solar conversion efficiency was the best among the samples.

Figure 4.

UV-Vis spectrum of TiO₂ film and ZrO₂ nanofiber-doped TiO₂ film after 24-h dye absorption.

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 TiO₂/dye/electrolyte interface, and the Warburg diffusion process of I^-/I₃^- [9,10]. As shown in Figure 5, the second semicircle is the resistance (R₂) related to the electron transport in the TiO₂/dye/electrolyte interface which is reduced. It can be seen that the TiO₂ film and the 3, 5, and 7 wt.% ZrO₂ nanofiber-doped TiO₂ films are 13.2, 10.3, 9.6, and 11.9 Ω, respectively.

Figure 5.

Nyquist plots of the DSSCs using TiO₂ film and ZrO₂ nanofiber-doped TiO₂ film. Black circle, 0 wt.%; red diamond, 3 wt.%; cross mark, 5 wt.%; green triangle, 7 wt.%.

Figure 6 shows photocurrent-voltage characteristics of the DSSCs with the ZrO₂ nanofiber-doped TiO₂ film. The open-circuit photovoltage was almost the same, and the short-circuit photocurrent density increased with the added amount of ZrO₂ nanofibers and had reached the maximum at 5 wt.% of ZrO₂ nanofiber (15.9 mA/cm²). The open-circuit photovoltage [V_oc], the short-circuit photocurrent density [J_sc], the fill factor [FF], and the light-to-electricity conversion efficiency [η] at 5 wt.% added ZrO₂ nanofiber were 0.69 V, 15.9 mA/cm², 0.60, and 6.5%, respectively, as shown in Table 1. From the results, we can realize that the insertion of ZrO₂ nanofibers creates a larger surface area and reduces the resistance of the photoelectrode, especially for the optimal amount of ZrO₂ contents (7 wt.%) of the photoelectrode in DSSCs.

Figure 6.

Photocurrent-voltage curve of the TiO₂ film and the TiO₂ films with ZrO₂ nanofibers.

Table 1.

Photocurrent-voltage characteristics of DSSCs using TiO₂ with different amounts of ZrO₂ nanofibers

Sample	Voc (V)	Jsc (mA/cm2)	FF	η (%)
Pure TiO2	0.68	11.5	0.62	4.9
3 wt.% ZrO2 nanofiber-doped TiO2	0.69	14.9	0.60	6.2
5 wt.% ZrO2 nanofiber-doped TiO2	0.69	15.9	0.60	6.5
7 wt.% ZrO2 nanofiber-doped TiO2	0.69	14.7	0.59	6.0

V_oc, open-circuit photovoltage; J_sc, short-circuit photocurrent density; FF, fill factor; η, light-to-electricity conversion efficiency; TiO₂, titania; ZrO₂, zirconia.

Conclusions

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 J_sc 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%, J_sc of 15.9 mA/cm², V_oc 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.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

JW fabricated the DSSCs and UV-Vis analysis. EMJ was the paper chaser and performed the analysis of photocurrent-voltage characteristics and impedance. WLW performed the BET analysis. J-YP prepared the ZrO₂ nanofibers. XGZ performed the FE-SEM analysis. H-BG was thesis director. All authors read and approved the final manuscript.