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This paper reports the enhanced performance of dye-sensitized solar cells (DSSCs) with microcavity-embedded nanoporous TiO2 photoanodes. For DSSCs with photoanodes composed of a stack TiO2 sublayers with microcavity concentrations arranged from low to high on the light illumination path, the short-circuit current density and the conversion efficiency were improved. A pronounced increase in optical absorption and incident monochromatic photon-to-current conversion efficiency in the long-wavelength region indicated that the enhancement of cell performance was due to the multiple scattering of light by the microcavities and the light confinement by the stack of TiO2 sublayers with a high-to-low effective index of refraction.
©2012 Optical Society of America
1. Introduction
Dye-sensitized solar cells (DSSCs), which emerged as a new generation photovoltaic device, have been studied extensively for the conversion of sunlight into electricity due to their low cost and environmental friendliness [1, 2]. To achieve high cell performance, a tremendous research effort has focused on the design and fabrication of photoanodes [3–14], such as electrodeposition of TiO2 photoanodes [3], gold nanoparticle embedded TiO2 electrodes [4], low temperature fabrication process [5], sol-gel compact TiO2 layer [6], aqueous synthesis of anatase nanocrystallites [7], layer by layer printing technique [8], nanostructured TiO2 hollow fiber electrodes [9], and inversed opal TiO2 films [10–14]. The photoanodes affect the optical path, selection of light wavelength, efficiency of dye absorption and photoelectron transport.
To enhance the light absorption by the dyes, the management of incident light becomes an important issue. For instances, the light absorption can be enhanced by increasing the light scattering or trapping in the photoanodes [15–23], including the application of ZrO2-mixed or large (300nm) TiO2 particle-embedded nanoporous TiO2 [15, 16], the implement of a light scattering layer on the top of nanocrystalline TiO2 layer [17], as well as the usage of TiO2 nanorods/nanowires or light scattering particles [18–20]. Other than the techniques of enhancing light scattering or trapping within TiO2 electrodes, there are researches concentrating on using external mirrors [24], reflectors [25], or condense lens [26] to improve the cell efficiency. In our previous study, the enhanced conversion efficiency of DSSCs with 1-μm diameter microcavity-embedded TiO2 photoanodes was demonstrated [21]. The randomly distributed microcavities were obtained simultaneously with the formation of three-dimensional interconnected nanoporous TiO2 photoanodes by sintering TiO2 paste mixed with PS microspheres [21–23]. In this study, we further investigate the effect of microcavity size and density on the conversion efficiency of DSSCs. The photoelectric performance of the devices with photoanodes composed of a stack of TiO2 sublayers with various microcavity densities is also studied. The microcavity density can be used to adjust the effective refractive index of the TiO2 sublayer. Only with proper arrangement of the microcavity densities in the TiO2 sublayers, the light trapping and the light absorption by the dyes can be enhanced and thereby improving the cell conversion efficiency.
2. Experiment
The photoanodes of DSSCs were fabricated by sintering the TiO2 paste mixed with PS microspheres. PS microspheres (1 μm, 2 μm, and 3 μm in diameter) of 10 wt.% stored in deionized (DI) water were purchased from Sigma-Aldrich. The PS microspheres were first diluted in DI water to various concentrations (2 wt.%, 5 wt.%, and 10 wt.%) before mixing with a 50 wt.% TiO2 paste (E-solar P300, Everlight Chemical Industrial Co.) diluted in ethanol at a 1:1 weight ratio. Next, the composite pastes were screen printed layer-by-layer to achieve a total thickness of 12 μm (three 4-μm-thick sublayers) on fluorine-doped tin oxide (FTO) glass, on which there was a 100 nm electron-beam evaporated TiO2 compact film. Three different structures were investigated, as shown in Fig. 1 . The monolayer structure (M) is composed of three sublayers with PS microspheres of identical size and concentration. The bilayer structure (B) comprises two sublayers with PS microspheres of identical size and concentration, denoted as the first layer, and one sublayer with PS microspheres of the same size but different concentration, denoted as the second layer. The trilayer structure (T) contains three sublayers with PS microspheres of the same size but in different concentrations.
Fig. 1 Schematic cross sections of the studied TiO2 photoelectrode structures: (a) monolayer structure of 12 μm in thickness, (b) bilayer structure with first layer of 8 μm and second layer of 4 μm in thickness, (c) trilayer structure with first, second, and third layers of 4 μm in thickness each.
The TiO2 paste was successively sintered in air at 510°C for 15 min. During sintering, the PS microspheres were calcined, leaving behind microcavities in the TiO2 film. The size and distribution of the microcavities were determined by the size and concentration of PS microspheres in each printed sublayer. The TiO2 photoanodes were subsequently treated with a 0.05 M TiCl4 solution at 70°C for 30 min before rinsing with ethanol. The microcavity-embedded porous TiO2 photoanodes were immersed in a mixed solution of acetonitrile (99.9%, J. T. Baker) and tertiary butyl alcohol (99.9%, J. T. Baker) containing 3x10−4 M of N719 dye (Solaronix) for 24 hrs. The dye-adsorbed porous TiO2 photoanodes were rinsed with ethanol and dried at room temperature. A 10 nm Pt layer was dc-sputtered onto an FTO glass substrate as the counter electrode, which was then assembled with the dye-absorbed TiO2 photoanode. Finally, liquid electrolyte based on the iodide redox in acetonitrile (E-Solar EL 100, Everlight Chemical Industrial Co.) was injected into the assembled cells. Using the same process, the counterpart DSSCs with TiO2 photoanodes produced without the application of PS microspheres in TiO2 pastes were also fabricated for comparison.
A scanning electron microscope (SEM, Hitachi S-800) was used to inspect TiO2 morphology. A surface profiler (Tencor Alpha-Step 500) was used to measure the film thickness. An ultraviolet-visible-near infrared spectrophotometer (JASCO V-570) was employed to determine the absorption of the dye-adsorbed porous TiO2 layers. A solar simulator (S2000-TBCL, Technology Bridge Corp.) equipped with a Keithley 2000 electrometer was used to evaluate the current density–voltage characteristics of cells. For the incident photon-to-current conversion efficiency (IPCE) measurement, the monochromatic light was passed through a chopper wheel with a frequency of 0.1 Hz to create a small modulated signal on top of a constant signal. The resulting modulated current was analyzed by using a lock-in amplifier. The electrochemical impedance spectra (EIS) were recorded with a potentiostat/galvanostat under a constant light illumination of 100 mW/cm2. The frequency range explored was 10 mHz to 65 kHz. The applied bias voltage and ac amplitude were set at the open-circuit voltage of the DSSCs and 10 mV between the counter electrode and the photoelectrode, starting from the short-circuit condition. The impedance spectra were then analyzed using an equivalent circuit model [5]. The illuminated cell area was 0.22 cm2.
3. Results and discussion
3.1 Morphologies of microcavity-embedded TiO2 photoanodes
Figure 2 demonstrates the SEM images for the sintered TiO2 photoanodes in a monolayer structure with the application of PS microspheres of various sizes (1 μm, 2 μm, 3 μm in diameter) and concentrations (2 wt.%, 5wt.%, 10wt.% diluted in DI water) in the TiO2 paste. Without PS microspheres, the typical pore size of the TiO2 layer is approximately 20 nm to 50 nm. For the photoanodes made with PS microspheres embedded TiO2 paste, numerous microcavities were observed. The size and density of the microcavities are determined by the size and concentration of the PS microspheres in the TiO2 paste. From the SEM images, the resulting microcavity to nanoporous TiO2 volume ratios are estimated to be 8.3%, 19.2%, and 37.1% for sintered TiO2 photoanodes made using 2 wt.%, 5 wt.%, and 10 wt.% PS microspheres diluted in DI water by quantitative metallography analysis [27].
Fig. 2 Scanning electron micrographs of TiO2 photoanodes made using (a) 2 wt.% 1 μm, (b) 2 wt.% 2 μm, (c) 2 wt.% 3 μm, (d) 2 wt.% 2 μm, (e) 5 wt.% 2 μm, and (f) 10 wt.% 2 μm PS microspheres diluted in DI water.
3.2 Photovoltaic performance of DSSCs
3.2.1 Monolayer structure
Figure 3 shows the efficiencies of DSSCs made with TiO2 photoanodes in a monolayer structure by using TiO2 paste containing PS microspheres of various sizes and concentrations under AM1.5 illumination. The photoelectric conversion efficiency initially increases and then decreases as the size and the concentration of PS microspheres increase. The cell parameters of DSSCs made without and with PS microspheres of various sizes and concentrations are listed in Table 1 . The presence of microcavities in the TiO2 photoanode may have two-fold effect: (i) enhancing the light scattering, thus increasing the photocurrent density; (ii) reducing the available surface area for dye-adsorption and the available paths for electron transportation, thus decreasing the photocurrent density and fill factor. Therefore, the efficiency declined at high PS microsphere concentrations and large microsphere sizes. For DSSCs made with TiO2 paste mixed with 2 wt.% 1 μm, 2 μm, and 3 μm PS microspheres diluted in DI water, the efficiencies are measured as 6.45%, 6.70%, and 5.95%, respectively. Compared to the cells made without PS microspheres, the conversion efficiency improves by 7.5% and 11.7% when 1 μm and 2 μm PS microspheres are introduced, but deteriorates by 0.8% when 3 μm PS microspheres are added.
Fig. 3 Conversion efficiency of DSSCs made with photoelectrodes in a monolayer structure as a function of PS microsphere concentration in DI water.
Table 1. Comparison of the cell parameters of DSSCs made without and with PS microspheres of various sizes and concentrations.
Figure 4 exhibits the absorption of light by the adsorbed dyes in the TiO2 photoanodes of a monolayer structure made with 2 wt.% PS microspheres diluted in DI water. The optical absorption of dye-adsorbed TiO2 with microcavities is slightly reduced in the short-wavelength region but significantly improved in the long-wavelength region compared to the absorption of dye-adsorbed TiO2 without microcavities. The decrease of absorption in the short-wavelength region is caused by the reduction of the surface area for dye adsorption, while the enhancement of absorption in the long-wavelength region is attributed to the increase of the optical path through multiple scatterings of light in the microcavities [15]. The optimal size and concentration of PS microspheres diluted in DI water are 2 μm and 2 wt.%, respectively.
Fig. 4 Optical absorption spectra of dye-adsorbed monolayer photoanodes made with 2 wt.% PS microspheres of various sizes (1 μm, 2 μm, and 3 μm in diameter) and without the application of PS balls during sintering.
We further investigated the IPCE of the DSSCs made with PS microspheres of optimal size and concentration. As shown in Fig. 5 , the IPCE of the DSSC made using TiO2 paste mixed with 2 wt.% PS microspheres of 2 μm in diameter diluted in DI water is significantly greater than that of the DSSC made without the application of PS microspheres in the long-wavelength region. The dramatic decrease in the IPCE of the DSSC made without the PS microspheres at a wavelength of 550 nm is ascribed to the low absorption coefficient of N719 in the red light region. The corresponding cell parameters are listed in Table 1. A significant increase of the photocurrent density was observed in the DSSC made using TiO2 pastes mixed with 2 wt.% 2 μm PS microspheres diluted in DI water. The results suggest that the increased light absorption in the long-wavelength region is accounted for the major improvement in the photocurrent, and therefore, the cell efficiency.
Fig. 5 IPCE spectra of the DSSCs made using TiO2 pastes mixed with 2 wt.% 2 μm PS microspheres diluted in DI water and without the application of PS microspheres.
3.2.2 Bilayer structure
To study the effect of multilayered structures on the performance of DSSCs, a TiO2 paste mixed with PS microspheres of the optimal size of 2 μm was used. As shown in Table 2 , DSSCs with photoanodes in a bilayer structure have a higher conversion efficiency and photocurrent density than those with photoanodes in a monolayer structure when the microcavity density of the first layer is lower than that of the second layer. The largest photocurrent density of 15.30 mA/cm2 and the highest photoelectric conversion efficiency of 7.02% were obtained when the concentrations of PS microspheres diluted in DI water used for the first layer and the second layer were 2 wt.% and 10 wt.%, respectively. Figure 6 displays the absorption of light by the adsorbed dyes in the TiO2 photoanodes of bilayer structures. Enhanced optical absorption is observed when the microcavity density of the first layer is lower than that of the second layer, that is, the effective index of refraction of the first layer is higher than that of the second layer. The results indicate that further enhancement of cell performance in addition to the light scattering in the microcavities can be attributed to the light confinement when the incident light passes from the first layer with high effective index of refraction to the second layer with low effective index.
Table 2. Comparison of the cell parameters of DSSCs with photoanodes in a bilayer structure. The diameter of PS microspheres in the TiO2 paste was 2 μm. Enhanced conversion efficiency was observed in DSSCs with photoanodes in a bilayer structure when the PS microsphere concentration of the first layer was lower than that of the second layer.
Fig. 6 Optical absorption spectra of dye-adsorbed bilayer photoanodes made with the first layers using TiO2 paste mixed with (a) 2 wt.%, (b) 5 wt.% and (c) 10 wt.% PS microspheres diluted in DI water. The size of the PS microspheres was 2 μm in diameter.
3.2.3 Trilayer structure
To further improve the cell performance by utilizing the light confinement, a trilayer structure was proposed and investigated. As shown in Table 3 , both the short-circuit current density and the conversion efficiency were enhanced when the concentration of PS microspheres in the TiO2 paste was varied from low to high. Conversely, a degradation of photoelectrical performance was noticed when the DSSCs with TiO2 photoanodes having microcavity densities from high to low. The optical absorption spectra shown in Fig. 7 again supports that both the multiple scattering of light in the microcavities and the light confinement in the multilayer structure can improve the photoelectrical performance of DSSCs. The internal resistances and the electron transport kinetics in the TiO2 were studied by electrochemical impedance spectroscopy. Figure 8 shows the Nyquist plots of the electrochemical impedance spectra of the DSSCs with photoanodes in monolayer structure (2 wt.%, 2 μm) and triplayer structures measured under light illumination of 100 mW/cm2. All the spectra exhibit three semicircles, which can be assigned to electrochemical reaction at the Pt counter electrode, charge transfer at the TiO2/dye/electrolyte and Warburg diffusion process of I-/I3- [5]. The middle characteristic frequency peaks, related to the inverse of the electron lifetime within TiO2 [28], are the same for all cases, indicating that the presence of microcavities does not influence the electron lifetime in the TiO2 film. On the other hand, the charge transport resistances at the TiO2/dye/electrolyte, Rct2, may be affected by the presence of microcavities, which can enhance the penetration of redox couples into the pores of the TiO2 but reduce the available surface area for dye-adsorption. The Rct2 were fitted to be 16.3 Ω, 11.9 Ω, 20.3 Ω, and 17.5 Ω for DSSCs with photoanodes made using TiO2 paste mixed with (2 wt.%, 2 wt.%, 2 wt.%), (2 wt.%, 5 wt.%, 10 wt.%), (10 wt.%, 5 wt.%, 2 wt.%) 2 μm PS microspheres diluted in DI water and made without the application of PS microspheres, respectively. In general, negative correlation between the Rct2 and the conversion efficiency was observed. The I-V characteristics of the best cell with photoanode made using PS microsphere concentrations of 2 wt.%, 5 wt.% and 10 wt.% diluted in DI water in the first, second, and third sublayers are shown in Fig. 9 . A short-circuit current density of 16.30 mA/cm2 and a conversion efficiency of 7.2% were obtained, which were improved by 26% and 20%, respectively, compared to those of the DSSC without microcavities.
Table 3. Comparison of the cell parameters of DSSCs with photoanodes in a trilayer structure. The diameter of PS microspheres in the TiO2 paste was 2 μm. Enhanced conversion efficiency was observed in DSSCs with photoanodes in a trilayer structure when the PS microsphere concentration was varied from low to high.
Fig. 7 Optical absorption spectra of dye-adsorbed trilayer photoanodes made with sublayers using TiO2 paste mixed with (2 wt.%, 5 wt.%, 10 wt.%) and (10 wt.%, 5 wt.%, 2 wt.%) PS microspheres diluted in DI water. The size of the PS microspheres was 2 μm in diameter.
Fig. 8 Electrochemical impedance spectra of DSSCs with photoanodes made using TiO2 paste mixed with (2 wt.%, 2 wt.%, 2 wt.%), (2 wt.%, 5 wt.%, 10 wt.%), (10 wt.%, 5 wt.%, 2 wt.%) 2 μm PS microspheres diluted in DI water and made without the application of PS microspheres. The equivalent circuit of this study is shown in the inset.
Fig. 9 Current density–voltage characteristics of DSSCs with TiO2 photoanodes in a monolayer structure, a bilayer structure, and a trilayer structure under AM 1.5 illumination. The diameter of PS microspheres in the TiO2 paste was 2 μm.
4. Conclusion
This study demonstrated dye-sensitized solar cells with photoanodes made using PS-microsphere-embedded TiO2 paste. The calcination of PS microspheres during sintering resulted in numerous randomly distributed microcavities in the TiO2 photoanodes. The light scattering in the microcavities and the light confinement in the multilayer structure with a high-to-low graded effective index of refraction enhance the dye absorption, thereby increasing the photocurrent and the cell efficiency. The short-circuit current density and conversion efficiency were improved by 26% and 20% for the DSSC with photoanode in a trilayer structure made using TiO2 paste mixed with 2 wt.%, 5 wt.% and 10 wt.% PS microspheres diluted in DI water for the first, second and third sublayers, respectively.
Acknowledgment
The author I-Chun Cheng gratefully acknowledges the financial support provided by the National Science Council of Taiwan R.O.C. under the grant numbers NSC 99-2628-E-002-203 and NSC 100-3113-E-002-012. The author Jian Z. Chen expresses gratitude toward the National Science Council of Taiwan R.O.C. for the financial support provided under the grant number NSC 99-2627-M-002-008.
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