Improving scattering layer through mixture of nanoporous spheres and nanoparticles in ZnO-based dye-sensitized solar cells
© Kim et al.; licensee Springer. 2014
Received: 4 March 2014
Accepted: 28 May 2014
Published: 11 June 2014
A scattering layer is utilized by mixing nanoporous spheres and nanoparticles in ZnO-based dye-sensitized solar cells. Hundred-nanometer-sized ZnO spheres consisting of approximately 35-nm-sized nanoparticles provide not only effective light scattering but also a large surface area. Furthermore, ZnO nanoparticles are added to the scattering layer to facilitate charge transport and increase the surface area as filling up large voids. The mixed scattering layer of nanoparticles and nanoporous spheres on top of the nanoparticle-based electrode (bilayer geometry) improves solar cell efficiency by enhancing both the short-circuit current (Jsc) and fill factor (FF), compared to the layer consisting of only nanoparticles or nanoporous spheres.
Dye-sensitized solar cells (DSSCs) have shown promising potential as an alternative to Si thin-film solar cells because of low fabrication cost and relatively high efficiency [1, 2]. Efficient utilization of sunlight is greatly important in photovoltaic systems for high efficiency. Therefore, there have been many studies on the scattering layer to fully utilize incident light inside solar cells by using different morphologies and sizes of scatterers in TiO2-based DSSCs [3–10]. However, few studies for the scattering layer exist in ZnO-based DSSCs [11–13], despite the advantages of ZnO such as higher carrier mobility and fabrication easiness for various nanostructures [14, 15].
Among various nanostructures, hundred-nanometer-sized nanoporous spheres provide both effective light scattering and large surface area . X. Tao's group and W. Que's group have reported on the scattering layer consisting of nanoporous spheres [17, 18]. While they have shown improvements on the scattering effect, large voids between spheres leave the possibility of providing more available surface area where dye can be attached, and better charge transport by improved percolation of large-sized spheres should be achieved.
In this paper, we report the improvements of scattering layers using a mixture of nanoparticles and nanoporous spheres. Nanoporous spheres act as effective light scatterers with the large surface area, and nanoparticles favor both efficient charge transport and an additional surface area.
The ZnO nanoporous spheres were synthesized by using zinc acetate dihydrate (0.01 M, Zn(CH3COO)2 · 2H2O, Sigma-Aldrich, St. Louis, MO, USA) and diethylene glycol ((HOCH2CH2)2O, Sigma-Aldrich) in an oil bath at 160°C for 6 h . After washing with ethanol, the as-synthesized ZnO nanoporous spheres (NS) and ZnO nanoparticle (NP) (721085, Sigma-Aldrich) were mixed to the weight ratios of NP to NS of 10:0, 7:3, 5:5, 3:7, and 0:10. To fabricate bilayer-structured electrodes, a paste consisting of only ZnO nanoparticles (NP/NS = 10:0) was first spread on a fluorine-doped tin oxide substrate (FTO, TEC 8, Pilkington, St. Helens, UK) covered with a dense TiO2 blocking layer by sputtering. After solvent evaporation, the mixed pastes with various ratios of NS and NP were spread on top of the nanoparticle film by a doctor blade method. The active area was 0.28 cm2, and the as-deposited films were subsequently annealed at 350°C for 1 h.
The films were sensitized with 0.5 mM of N719 dye (RuL2(NCS)2:2TBA, L = 2,2′-bipyridyl-4,4′-dicarboxylic acid, TBA = tetrabutylammonium, Solaronix, Aubonne, Switzerland) for 30 min at RT. The sensitized electrode and platinized counter electrode were sealed with thermoplastic foil (25 μm, DuPont, Wilmington, DE, USA), and the gap between the two electrodes was filled with an iodide-based redox electrolyte (AN-50, Solaronix).
X-ray diffraction (XRD; M18XHF-SRA, Mac Science, Tokyo, Japan) was employed to analyze the crystal structure of the ZnO electrodes, and field emission scanning electron microscopy (FE-SEM; SU70, Hitachi, Tokyo, Japan) was used to observe the morphology of the bilayer-structured electrodes. The electrochemical properties were analyzed by a solar cell measurement system (K3000, McScience, Suwon, South Korea) under a solar simulator (xenon lamp, air mass (AM) 1.5, 100 mW cm−2). The extinction and diffused reflectance spectra were recorded on a UV/Vis spectrophotometer (Cary 5000, Agilent Technologies, Santa Clara, CA, USA), and incident photon-to-current conversion efficiency (IPCE) spectra were measured by an IPCE measurement system (K3100, McScience). Electrochemical impedance spectra (EIS) were taken by using a potentiostat (CHI 608C, CH Instrumental Inc., Austin, TX, USA) to analyze the kinetic parameters in the DSSCs [19–21].
Results and discussion
Furthermore, after dye adsorption, the NP/NS = 3:7 film shows the highest extinction (Figure 3b). Especially when compared to the NP/NS = 0:10 film, the higher extinction near the dye absorption peak is clear . The results indicate an optimum condition for the surface area between void filling by nanoparticles and primary nanoporous spheres. The notable change in the curve shape for the NP/NS = 0:10 film (Figure 3a,b) means that light scattering plays a role considerably for the adsorbed dye molecules .
Characteristics of photocurrent-voltage curves and charge transfer resistances ( R ct ) for ZnO/electrolyte interfaces
5.98 ± 0.25
0.56 ± 0.01
0.67 ± 0.01
2.25 ± 0.15
30.7 ± 0.3
6.64 ± 0.30
0.55 ± 0.01
0.65 ± 0.02
2.36 ± 0.17
33.1 ± 0.2
7.45 ± 0.13
0.56 ± 0.01
0.68 ± 0.03
2.81 ± 0.14
29.8 ± 0.2
7.47 ± 0.24
0.58 ± 0.01
0.67 ± 0.01
2.91 ± 0.13
31.6 ± 0.2
7.28 ± 0.18
0.56 ± 0.01
0.64 ± 0.02
2.60 ± 0.09
34.5 ± 0.3
If charge collection probabilities are similar among the cells, quantum efficiency depends on the light trapping inside the solar cell [34–37]. The NP/NS = 3:7 cell exhibits the highest IPCE values in the whole visible region (Figure 4b), and this IPCE trend is consistent with the extinction data (Figure 3b). Therefore, the enhanced light-harvesting capability (i.e., Jsc) by the mixed scattering layer is attributed to efficient light scattering and increased surface area.
To improve the utilization of scattering layer in ZnO-based DSSCs, nanoparticles and nanoporous spheres are mixed with various ratios. The nanoporous spheres play an important role in the scattering effect with the large surface area but possess disadvantages of large voids and point contacts between spheres. Nanoparticles clearly advance facile carrier transport with the additional surface area, thereby improving the solar cell efficiency by the enhanced short-circuit current (Jsc) and fill factor (FF).
This research was supported by the National Research Foundation of Korea (NRF): 2013R1A1A2065793 and 2010–0029065.
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