TiO2 hollow spheres as effective additives in oligomer electrolytes for dye-sensitized solar cells

Materials Letters 65 (2011) 2506–2509

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

TiO2 hollow spheres as effective additives in oligomer electrolytes for dye-sensitized solar cells Jong Hyuk Park, Sun Young Jung, A Rim Yu, Sang-Soo Lee ⁎ Polymer Hybrids Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

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Article history: Received 18 February 2011 Accepted 26 April 2011 Available online 4 May 2011 Keywords: Hollow sphere TiO2 additive Oligomer electrolyte Dye-sensitized solar cell

a b s t r a c t TiO2 hollow spheres are employed as an additive of oligomer electrolytes for dye-sensitized solar cells (DSSCs). The measurement of steady-state currents confirms that introducing TiO2 hollow spheres can facilitate ionic diffusion in oligomer electrolytes. Even compared with conventional nanoparticle additives, the hollow spheres are more favorable to increase the diffusion coefficients of I − and I3− in oligomer electrolytes. Furthermore, the hollow structure with a submicron size is effective to scatter incident light and thereby enhance the light-harvesting efficiency of DSSCs. The energy-conversion efficiency of the DSSCs with TiO2 hollow sphere additives significantly improves up to 7.22% due to the facilitated ionic diffusion and the enhanced light-harvesting efficiency. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cell (DSSC) has been considered as a promising alternative to conventional solar cells due to its low production cost and high energy-conversion efficiency [1]. The high efficiency, however, has been achieved by employing only volatile electrolytes with a risk of evaporation and leakage. For the practical application of DSSCs, their stability as well as high efficiency should be guaranteed. Non-volatile electrolytes for satisfying this request have been extensively investigated [2–6]. In particular, long-term stable DSSCs with oligomer electrolytes have been demonstrated [5,6]. Despite the promising potential of oligomer electrolytes, high efficiency is hard to obtain when employing oligomer electrolytes. This inferiority is mainly due to slow ionic diffusion in oligomer electrolytes [7]. Consequently, oligomer electrolytes with an enhanced diffusion rate have been highly demanded. One well-known method to facilitate ionic diffusion is introducing additives into electrolytes [8–10]. The additives are also useful in converting liquid electrolytes into gel ones without leakage. However, conventional additives, typically nanoparticles with a much smaller size than the wavelength of sunlight, cannot effectively scatter incident light. Moreover, the aggregation of the nanoparticles may bring about phaseseparation in electrolytes and hamper ionic diffusion. To address this issue, TiO2 hollow spheres with a submicron size are utilized as an additive of oligomer electrolytes for DSSCs. The change in diffusion coefficients of I − and I3− due to the hollow sphere

⁎ Corresponding author. Tel.: + 82 2 958 5356; fax: + 82 2 958 5309. E-mail address: [email protected] (S.-S. Lee). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.096

additives is measured and the results are compared with conventional nanoparticle additives. In addition, the effect of the hollow sphere additives on the light-harvesting efficiency and the photovoltaic characteristics of DSSCs is investigated. 2. Experimental The detailed method for preparing TiO2 hollow spheres has been explained previously [11]. Polystyrene (PS) spheres were synthesized by emulsion polymerization and utilized as templates of hollow spheres. TiO2 shell on PS spheres was built up by the hydrolysis reaction of TiCl4. To construct the TiO2 shell, PS spheres were mixed with 0.2 M TiCl4 solution at 53 °C for 90 min. After rinsing several times via centrifuging, the product was coated again in the same way to increase shell thickness. Through annealing at 500 °C, PS templates were removed and a hollow structure was constructed. Poly(ethylene glycol) dimethyl ether (PEGDME, MW = 250 g mol− 1) containing 0.8 M 1-butyl-3-methylimidazolium iodide, 0.1 M iodine and 0.1 M N-methylbenzimidazole was used as a reference oligomer electrolyte. To prepare composite electrolytes, the different amounts (5, 10, and 15 wt.%) of TiO2 hollow spheres were mixed with the reference electrolyte. For comparison with conventional additives, TiO2 nanoparticles (anatase, ~25 nm) were employed as a reference additive. The preparation of photo and counter electrodes for DSSCs has been described in references [5,7]. TiO2 paste (Dyesol, 18NR-T) was cast on transparent fluorine-doped SnO2 (FTO) glass (Pilkinton, TEC-8, 8 Ω sq − 1). The photoelectrode with a 12.0 μm thick TiO2 layer was obtained by sintering at 500 °C for 15 min. The photoelectrode was sensitized with purified N719 dye solution. A Pt layer on the counter electrode was prepared by spin-coating H2PtCl6 solution on FTO glass and annealing at 400 °C for 20 min. After injecting the prepared

J.H. Park et al. / Materials Letters 65 (2011) 2506–2509

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Fig. 1. SEM images of (a) synthesized PS spheres, (b) TiO2-coated PS spheres and (c) TiO2 hollow spheres, (d) TEM image and (e) XRD pattern of TiO2 hollow spheres.

electrolytes, the photo and counter electrodes were assembled using a 60 μm thick thermal adhesion film. The active area of the DSSCs was about 0.24 cm 2. Steady-state currents were measured using a scanning electrochemical microscope to obtain the diffusion coefficients of I − and I3− in electrolytes [2,7]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to observe the morphology of TiO2 hollow spheres. The crystallinity of the

hollow spheres was investigated by X-ray diffraction (XRD). Incident photon-to-current conversion efficiency (IPCE) of the prepared DSSCs from 400 nm to 800 nm was obtained by using a monochromatic beam generated from a 75 W Xenon lamp. The photovoltaic characteristics of DSSCs including short-circuit current (JSC), opencircuit voltage (VOC), fill factor (FF) and energy-conversion efficiency (η) were measured under 1 sun condition (100 mW cm − 2, AM 1.5) using Keithley model 2400 and a 1000 W Xenon lamp. 3. Results and discussion

Fig. 2. Steady-state voltammograms of composite electrolytes with different hollow sphere contents measured at a scan rate of 5 mV s− 1.

TiO2 hollow spheres were successfully prepared via TiO2 coating on PS templates [11]. To scatter sunlight effectively, the size of hollow spheres was designed to be submicron. Fig. 1a shows the synthesized PS spheres with a diameter of about 580 nm. The hexagonal packing implies their uniform size and shape. To build up robust TiO2 shell, the PS spheres were coated twice with TiCl4 solution. After the coating process, the size of the spheres increased up to 650 nm (Fig. 1b). Burning out the coated particles can eliminate inner PS templates and construct a hollow structure. Fig. 1c and d show the prepared TiO2 hollow spheres with a diameter and a shell thickness of about 600 and 40 nm, respectively. The crystalline phase of the hollow spheres was anatase as shown in Fig. 1e. Diffusion coefficients of I − and I3− can largely affect the performance of DSSCs. In particular, a slow diffusion rate of ions in oligomer electrolytes has restricted the energy-conversion efficiency [7]. To facilitate ionic diffusion, nanoparticle additives have been introduced into the electrolytes [8–10]. However, TiO2 hollow spheres can provide a better ionic diffusion path than simple nanoparticles because the hollow structure can prevent particle aggregation and phase-separation in electrolytes (Fig. S1). The ionic diffusion coefficients in the composite

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Table 1 Diffusion coefficients of active species in composite electrolytes and photovoltaic characteristics of DSSCs employing the electrolytes. Electrolyte

Oligomer Oligomer + hollow sphere 10 wt.% Oligomer + nanoparticle 10 wt.%

Diffusion coefficient (10−7 cm2 s− 1)

Cell performance

I−

I3−

VOC (V)

JSC (mA cm− 2)

FF

η (%)

4.22 5.50 4.91

2.02 4.12 3.46

0.70 0.73 0.72

11.07 15.48 12.11

0.61 0.64 0.63

4.75 7.22 5.48

electrolytes with hollow spheres can be evaluated from steady-state current (ISS) using the following equation: ISS = 4nFDCr; where n is the electron number per molecule, F is the Faraday constant, D is the ionic diffusion coefficient, C is the bulk concentration of electroactive species, and r is the radius of the Pt ultramicroelectrode (5 μm). We assumed that the concentration change due to introducing additives is negligible. Steady-state voltammograms of the reference and composite electrolytes are shown in Fig. 2. The composite electrolytes have much higher steady-state currents than the reference electrolyte. In particular, the composite electrolyte containing 10 wt.% hollow spheres exhibited two times larger diffusion coefficient of I3− (Table 1). Because the rate of redox reactions in DSSCs is typically determined by the diffusion rate of I3−, the large increase in the diffusion coefficient of I3− due to hollow sphere additives will be helpful in improving cell performance. Even compared to nanoparticle additives, the hollow spheres are more

favorable to facilitate ionic diffusion. Since the hollow spheres have a much larger volume compared with the nanoparticles, they can be uniformly distributed in the entire electrolytes (Fig. S1) and thereby provide effective ionic diffusion paths. On the contrary, the nanoparticles require a large amount of additives to connect ionic diffusion paths due to their aggregation. It may restrict the increase of an ionic diffusion rate in the electrolyte with nanoparticle additives. IPCE spectra from the DSSCs employing composite electrolytes reveal interesting features. The DSSCs with either of two additives have higher IPCE values than those with the reference electrolyte (Fig. 3a). This may be due to the larger diffusion coefficient of ions in composite electrolytes [8–10]. For the same reason, the DSSCs with hollow sphere additives can exhibit higher IPCE than those with nanoparticle additives. The former has higher IPCE values especially in the long wavelength range of over 600 nm because the hollow spheres with a uniform size of visible wavelengths can more effectively scatter incident light than the simple nanoparticles. This high IPCE value will result in increasing light-harvesting efficiency and JSC of DSSCs. Fig. 3b describes the photovoltaic characteristics of the DSSCs employing composite electrolytes. The DSSC with hollow sphere additives shows the highest JSC due to the enhanced ionic diffusion rate and light scattering effect. In addition, the VOC and FF of DSSCs also improve when employing either of two additives. This can be explained by the reduced charge recombination at the interface between sensitized photoelectrode and electrolytes [9,10]. Free I3− near the photoelectrode can cause the charge recombination; since I3− is immobilized on the surfaces of additives, employing additives can lower the concentration of free I3− and reduce the charge recombination. Due to these merits of TiO2 hollow sphere additives, the efficiency of the DSSC significantly increased from 4.75 to 7.22% (Table 1).

4. Conclusions TiO2 hollow spheres were employed as an additive of oligomer electrolytes for DSSCs. TiO2 hollow sphere additives can largely increase diffusion coefficients of ions in oligomer electrolytes. Even compared with conventional nanoparticle additives, they are more effective in facilitating ionic diffusion. Furthermore, the hollow spheres in oligomer electrolytes can enhance the light-harvesting efficiency, JSC, VOC and FF of DSSCs. Introducing TiO2 hollow sphere additives is an excellent method to improve the efficiency of the DSSCs with oligomer electrolytes.

Acknowledgments This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Korea.

Appendix A. Supplementary Data

Fig. 3. (a) IPCE spectra and (b) J–V curves of DSSCs employing composite electrolytes.

Supplementary data to this article can be found online at doi:10.1016/j. matlet.2011.04.096.

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