Article pubs.acs.org/Langmuir
Hierarchical Twin-Scale Inverse Opal TiO2 Electrodes for DyeSensitized Solar Cells Chang-Yeol Cho and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, South Korea S Supporting Information *
ABSTRACT: We describe the preparation of three-dimensional hierarchical twin-scale inverse opal (ts-IO) electrodes for dye-sensitized solar cells (DSSCs). The ts-IO TiO2 structure was obtained from a template fabricated via the assembly of mesoscale colloidal particles (40−80 nm in diameter) in the confined geometry of a macroporous IO structure. The photovoltaic properties of ts-IO electrodes were optimized by varying the layer thickness or the size of mesopores in the mesoscale colloidal assembly. Electron transport was investigated using impedance spectroscopy. The result showed that due to the competing effects of recombination and dye adsorption, the maximum efficiency was observed at an electrode thickness of 12 μm. The electrodes of smaller mesopores diameters yielded the higher photocurrent density due to the decrease in the electron transport resistance at the TiO2/dye interface. A maximum efficiency of 6.90% was obtained using an electrode 12 μm thick and a mesopore diameter of 35 nm.
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INTRODUCTION Dye-sensitized solar cells (DSSCs) are a promising alternative to conventional solar cells due to their lower cost and relatively high photon-to-electron conversion efficiency.1,2 Conventional DSSCs are composed of a TiO2 nanoparticle-coated transparent conducting oxide electrode, a platinized counter electrode, and a layer of electrolyte between the two. The TiO2 electrode possesses mesopores with sizes on the order of 10 nm; these mesopores are located between the particles, which are coated with ruthenium-based dye molecules. Under operation, photoexcited electrons from the dye are injected into and transported through the TiO2 electrode. At the same time, these electrons can be captured via recombination with redox ions in the electrolyte. The time scale for transport is on the order of milliseconds, which is comparable with the time scale for recombination.3 Thus, the magnitude of the electron flux in the injection/transport and recombination processesand the relative magnitudes of these fluxesdetermine the photon-toelectron conversion efficiency. Typically, a conventional electrode possesses randomly connected nanoparticles that maximize the specific area and thereby maximize the number density of the electrons injected from the excited dye under illumination; however, these connected nanoparticles are detrimental to the transport properties because random transport increases the electron path length to the transparent conducting substrate, facilitating electron loss by recombination. Recently, 3D inverse opal (IO) electrodes have shown promise, in that the organized and/or directional morphology of the unit particles improved the electron transport.4−6 Moreover, 3D connected pores can © 2012 American Chemical Society
facilitate infiltration of the electrolyte solution and promote mass transport. In particular, this type of macroporous electrode may be important for the development of DSSCs with solid-state electrolytes.7 However, the IO electrodes still have a disadvantage that must be overcome; specifically, they have a relatively smaller specific area compared with conventional electrodes, which results in lower levels of dye adsorption and therefore a lower electron injection density. To combine the positive effects of the formation of ordered structures with those of a large surface area, recent studies successfully used mixtures with nanoparticles, the surface decoration of nanoparticles, or hierarchical structures.6,8−13 For example, when a simple composite of nanotubes and nanoparticles was used, the electron current density was significantly increased, without significantly decreasing the electron transport properties.11−13 In line with these researches, we developed the fabrication of inverse opal-based electrodes with mesoporous structures. Briefly, macroporous IO were formed first and provided a confined geometry for mesoscale colloidal assemblies. A subsequent templating process (precursor infiltration and template removal) created mesoporous IO inside the cavities of the macroporous inverse opals (twin-scale inverse opal TiO2 electrodes or ts-IO TiO2 electrodes). Compared with previous methods for mesopore generation using block copolymer organization,14−16 which often requires careful control of the self-assembly or sol−gel reaction, the colloidal particle Received: April 10, 2012 Revised: May 22, 2012 Published: June 7, 2012 9372
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Scheme 1. Scheme Illustrating the Formation of Twin-Scale Inverse Opal (ts-IO) Structures: (a) Fabrication of Macroporous Inverse Opals; (b) Infiltration of Mesoscale Colloidal Particles; (c) Removal of Template To Produce Mesoporous Inverse Opals
0.5 M 4-tert-butylpyridine (Aldrich) in a solution of acetonitrile (Aldrich)) was then injected into the gap. Characterization. SEM images were obtained using a fieldemission scanning electron microscope (FE-SEM, Hitachi S-4700). The absorption spectra and diffuse reflectance spectra were measured using a UV−vis spectrophotometer (Shimadzu, UV-2550). TEM images were obtained using a high resolution transmission electron microscope (HR-TEM, JEOL JEM-3010). The amount of adsorbed dye molecules was evaluated spectrophotometrically. The dye on a known TiO2 electrode area was detached via immersion in a 0.1 M NaOH solution, and the absorption intensity of the resulting dye solution was measured using a UV−vis spectrophotometer. The measured intensity was then converted to give the concentration of adsorbed dye molecules. The photocurrent and voltage of the DSSCs were characterized using a source meter (Keithley Instruments) under simulated solar light, which was produced by a 1000 W Xe lamp (Oriel) with AM 1.5G filters. The intensity was adjusted to 100 mW cm−2 using a Si reference cell (BS-520, Bunko-Keiki). The electrochemical impedance was measured using a potentiostat (Versatat, Ametek), at an open-circuit voltage (VOC) amplitude of 10 mV, under light conditions of 1 sun and 100 mW cm−2. The incident photon-to-current conversion efficiency (IPCE) was measured using a 300 W Xe light source, (Oriel) with a monochromator and in dc mode. The incident light intensity was measured using a photodiode detector (silicon calibrated detector, Newport).
assemblies are more facile and produced fully connected mesopores. Although we have demonstrated the mesoporous IO structures defined by interference lithography patterns,17 it should be noted that the approach here utilizes a macroscale IO structure to achieve synergistic benefits. The macroporous IO structure might provide a route allowing facile electron transport as aforementioned. In particular, the formation of mesoporous IO inside macroporous IO structures yielded a larger specific area compared to the previous structure due to the dense packing of spherical mesoporous IO structures, and thereby the efficiency was much enhanced by 40%. Moreover, we demonstrated the use of smaller mesoscale colloidal templates which is, for the first time to our knowledge, successfully achieved up to 35 nm inverse opal mesopores. In this study we controlled the electrode thickness and the diameter of the mesopores and evaluated the effects of changes in these variables on the photovoltaic properties and the efficiency of DSSCs. By applying our twin-scale inverse opalbased electrodes, we obtained a maximum photon-to-electron efficiency of 6.9% under AM1.5G and 1 sun conditions.
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EXPERIMENTAL SECTION
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Preparation of Inverse Opal-Based Mesoporous TiO 2 Electrode. To create the macroscale porous structure, monodisperse spherical polystyrene (PS) particles 1.9 μm in diameter were synthesized using dispersion polymerization; this was performed in an ethanol medium (Merck) containing 0.57 M of styrene monomer, 4 mM of 2,2′-azobis(2-methylbutyronitrile) acting as a radical initiator, and 1 mM of poly(N-vinylpyrrolidone) (Junsei Chemicals Co.) acting as a stabilizer. The prepared PS colloids were assembled on the FTO substrate during drying in a convection oven at 70 °C. TiO2 nanoparticles (average diameter of 15 nm, dispersed in water, Nanoamor Inc.) were infiltrated into the cavities of the PS colloidal crystals by capillary action. To remove the PS colloidal crystal template, the sample was subsequently calcined at 500 °C for 2 h in an air atmosphere, leaving behind a macroporous structure. The thickness of the macroporous inverse opal was controlled by varying the amount of the PS colloidal solution; this produced macroporous IO electrodes with thicknesses from 8 to 18 μm. To form the mesopores, monodisperse PS colloids 40, 60, and 80 nm in diameter (purchased from Bangs Lab) were injected several times into the macroporous IO structure and spin-coated at 1500 rpm to remove excess colloids from the surface. Subsequently, a TiO2 precursor solution of 0.9 M TiCl4 in ethanol (Sigma-Aldrich) was injected and calcined under the same conditions. Excess areas of the TiO2 electrodes were scraped away to create an active electrode area with dimensions of 3.5 mm × 3.5 mm. DSSC Assembly. The mesoporous TiO2 electrodes were immersed for 24 h in a dye solution containing 0.5 mM commercial N719 dye (Solaronix) in anhydrous 2-propanol (99.5%, SigmaAldrich). The counter electrode was prepared by casting a 0.7 mM H2PtCl6 solution in ethanol on the FTO substrate. The TiO2 electrode was assembled with the counter electrode, and the gap size between the electrodes was controlled via the use of a 60 μm thick Surlyn (DuPont). The electrolyte (which was prepared using 0.7 M 1-butyl-3methylimidazolium iodide (Sigma-Aldrich), 0.03 M I2 (Yakuri), and
RESULTS AND DISCUSSION Scheme 1 illustrates the procedure used to fabricate the ts-IO TiO2 electrodes. First, TiO2 macroporous inverse opals were created using a templating process. This involved the creation of a template using monodisperse 1.9 μm diameter PS beads, infiltration of the template with a TiO2 nanoparticle solution, drying of the solvent, and subsequent removal of the PS beads using a calcination (see Scheme 1a). A mesocolloidal solution containing 40−80 nm diameter particles was then infiltrated into and assembled inside the macropores, using solvent evaporation (see Scheme 1b). This templating process produced mesoporous inverse opals inside the macropores (ts-IO structure) (Scheme 1c). The images in Figures 1a and 1b show the macroporous inverse opals and the PS templates, respectively. The macroporous inverse opals showed spherical cavities 1.7 μm in diameter, and interconnecting pores of around 200 nm in diameter, which were created by the contact between PS particles. We selected PS colloids with a diameter of 1.9 μm because, to allow facile infiltration, the interconnecting pores should typically be larger than the mesoscale colloidal particles; for PS particles smaller than 1.5 μm, the pore size was comparable to the size of the mesoscale colloids, resulting in many cavities that were incompletely filled with mesoscale particles. Mesoscale colloids with diameters of 40 nm, 60 nm (not shown in the figure), and 80 nm were infiltrated into the macropores and dried the solvent to obtain colloidal crystals. Figures 1c and 1d show SEM images of macropores obtained 9373
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Figure 2. (a) TEM image of the ts-IO TiO2. (b) High-resolution TEM image of nanocrystalline TiO2. The inset image shows SAED pattern of ts-TiO2. Scale bar: (a) 10 nm; (b) 8 nm.
(XRD) also confirmed the anatase TiO2 of ts-IO structures (Figure S2). To investigate the ts-IO structure as an electrode for DSSCs, a cell was sensitized in a 0.5 mM N719 dye solution and assembled with a Pt-coated counter electrode. The interelectrode gap was filled with a liquid electrolyte. The current− voltage (J−V) characteristics of DSSCs with different pore sizes and different thickness were measured using a source meter, under illumination of AM 1.5, with simulated sunlight at 100 mW cm−2. Table 1 lists the J−V parameters measured for the Table 1. Photovoltaic Parameters, Calculated Efficiency, and Surface Density of N719 Dye Adsorption for Various Thicknesses and Mesoscale Pore Diameters in the ts-IO TiO2 Structure
Figure 1. Scanning electron microscopy (SEM) surface images of (a) PS colloidal crystals and (b) their TiO2 IO structures. SEM surface images of the TiO2 IO filled with PS particles with diameters of (c) 40 nm and (d) 80 nm. The insets in (c) and (d) show SEM surface images of PS colloidal crystals for the samples shown in (c) and (d). The SEM surface images in (e) and (f) show the ts-IO TiO2 structures created using the templates shown in (c) and (d). The insets in (e) and (f) show SEM surface images of mesoporous inverse opal TiO2 for the samples shown in (e) and (f). Scale bars: (a, b) 5 μm; (c−f) 2.5 μm; insets in (c−f) 300 nm.
mesoscale pore diam, thickness [nm, μm] 35, 35, 35, 53, 70,
when infiltration was performed with mesoscale colloids of diameter 40 and 80 nm, respectively. Mesoscale colloids were fully infiltrated through the macroporous film as clearly shown in Figure S1. It should be noted that the macropores provided structural support. In other words, without macropores, it was almost impossible to prepare thick mesocolloid crystal films, due to the detachment of the colloidal crystal film from the substrate. These types of the detachment are induced by residual stress during evaporation-induced crystallization and are widely reported to be unavoidable in colloidal crystals; moreover, smaller particles such as those used in this work induce stronger capillary forces during evaporation-induced crystallization and form more rigid colloidal crystal films with larger residual stress.18,19 After infiltration with the TiO2 precursor and subsequent calcination at 500 °C, ts-IO TiO2 electrodes were obtained, as shown in Figures 1e and 1f. The inset images show that the mesopores were around 35 and 70 nm in size and showed shrinkage in their diameter of around 10% compared with their mesocolloidal particle templates. The transmission electron microscopy (TEM) of ts-TiO2 is shown in Figure 2. The TEM image showed interconnected nanocrystalline of TiO2 of size of 10−20 nm. The lattice spacing in high-resolution image was correspond to the (101) planes in an anatase structure. The selected area diffraction pattern (SAED) in the inset of Figure 2b confirmed the presence of polycrystalline anatase TiO2. X-ray diffraction
8 12 18 12 12
VOC [V]
JSC [mA cm−2]
FF
η [%]
dye adsorption [10−7 mol cm−2]
0.78 0.79 0.79 0.81 0.83
10.00 13.18 12.42 10.32 7.53
0.65 0.66 0.62 0.70 0.70
5.14 6.90 6.09 5.87 4.41
0.67 1.01 1.51 0.64 0.55
DSSCs, including the JSC (short-circuit current), VOC (opencircuit voltage), FF (fill factor), and the calculated overall conversion efficiency (η) by JSC × VOC × FF/(100 mW cm−2). First, we studied the effects of the ts-IO TiO2 electrode thickness on the efficiency of the DSSCs. As shown in Table 1, the ts-IO TiO2 electrode thickness was changed from 6 to 18 μm. As shown in Figure 3a and Table 1, as the thickness was increased to 12 μm, the efficiency increased from 5.14% to 6.90%; this was attributed to the increase in the JSC value from 10.00 to 13.18 mA cm−2. However, at a thickness of 18 μm, the JSC and the efficiency showed a decreased to 12.42 mA cm−2 and 6.09%. This can be explained by the fact that the specific area for dye adsorption increases linearly with the thickness, but the loss of electrons via recombination is simultaneously increased, due to the increase in the diffusion length to the FTO substrate. Moreover, the recombination is significant where the thickness of the electrode exceeds the electron diffusion length. These two compensating, counteracting effects therefore determine the optimum electrode thickness.20 Table 1 clearly shows that the dye adsorption increased proportionally with the film thickness, while Figure 3b shows that the recombination current was larger for thicker electrodes. We also investigated the effects of varying the mesopore diameter (which was defined by the diameter of the mesoscale 9374
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Figure 4. (a) Photocurrent J−V characteristics of DSSCs based on tsIO TiO2 electrodes with different pores sizes and a fixed thickness of 12 μm. (b) Dependence of the incident photon to current efficiency (IPCE) spectra of ts-IO TiO2 electrodes, shown for various mesoscale pore diameters.
Figure 3. (a) J−V characteristics and (b) dark current curves for DSSCs based on ts-IO TiO2 electrodes with a pore size of 35 nm and different thicknesses.
colloidal particles) on the efficiency of the DSSCs (Table 1 and Figure 4a). The efficiency of the ts-IO TiO2 electrodes improved from 4.41% for the 70 nm pore electrode, to 5.87% for the 53 nm pore electrode, and to 6.90% for the 35 nm pore electrode. The spectral response of the conversion efficiency is shown as incident photon to current efficiency (IPCE) (Figure 4b). The efficiency increased with decreasing mesopore diameter, and this was attributed mainly to the increase in the JSC values as shown in Table 1. It was expected that variations in the mesopore diameter would mainly affect the specific area of the electrodes and thereby the adsorption density of dye molecules. More adsorption of dye molecules might induce a higher photocurrent density under the same optical transmittance conditions in the film. Although mesopores can induce Rayleigh scatteringwhich depends on the size of the scatterer and the wavelength of lightthis could be weak compared with the scattering produced by the macroscale inverse opal structure.21 As shown in Figure S3, similar diffuse reflectances were observed for the samples with different mesopore sizes (see Figure S3). Thus, the JSC was mainly influenced by the specific area (it increased proportionally with the specific area) and the amount of adsorbed dye molecules. Assuming a close-packed structure of monodisperse spherical pores, the specific area should increase linearly with decreasing pore diameter. We also confirmed the adsorption density of dye molecules using spectroscopic analysis. This experiment showed that the specific area of the 35 nm pore was
1.58 times and 1.84 times larger than the amount of dye molecules for the 53 and 70 nm electrodes, respectively. These ratios were similar to the ratios of the JSC values for the electrodes with 53 and 70 nm mesopores (1.3) and for the electrodes with 35 and 70 nm mesopores (1.6). To explain the increase in JSC with decreasing mesopore diameter, we also analyzed the electron transport in the electrode, using electrochemical impedance analysis (EIS). EIS was used to study the various interfaces of the DSSCs.22−24 In general, three semicircle-like plots were measured for the DSSCs, as shown in Figure 5; these were assigned to interfacial resistances at the FTO/electrode, TiO2/electrolyte, and electrolyte/platinized counter electrode interfaces. Here, an equivalent circuit was introduced to estimate the charge transfer resistances at the interfaces; RS represents the ohmic resistance of the electrolyte and the FTO resistance, and RCT1 and RCT2 are the resistances of the interfacial charge transfer at the counter electrode and the TiO 2 electrode interfaces, respectively.25 The RS and RCT1 values were the similar for all samples (∼7 and ∼11 Ω, respectively). However, RCT2 decreased as the mesopore diameter decreased. The RCT2 values measured for the different pore sizes were 33.7 Ω for the 35 nm pore, 39.49 Ω for the 53 nm pore, and 48.12 Ω for the 70 nm pore. The lower resistance implied that for smaller mesopore diameters the injection density of electrons into the conduction band of TiO2 was increased.26,27 9375
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (2010-0028961). The Korea Basic Science Institute is also acknowledged for SEM measurements.
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Figure 5. Impedance spectra for DSSCs based on ts-IO TiO2 electrodes, measured with AM 1.5G illumination and an open-circuit potential. The solid lines show fitting data, and the symbol points show the measured data. The inset shows the equivalent circuit model used to evaluate the charge transfer resistances at various interfaces. RS represents the ohmic resistance of the electrolyte and the FTO, and RCT1 and RCT2 represent the interfacial charge transfer resistance at the counter electrode and TiO2 electrode interfaces, respectively.
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CONCLUSION We have demonstrated colloidal templated mesopores, which were formed in the cavities of macroporous inverse opal structures and were applied as photoanodes in dye solar cells. Most significantly, the formation of mesopores via colloidal templating provides a facile approach that allows the controlled creation of mesopores in a wide range of sizes, in a well-defined and highly connected porous structure. We controlled the thickness of the mesoporous electrode by controlling the thickness of the macroporous inverse opal support and characterized the effects of the electrode thickness on the photovoltaic properties. Because of the competing effects of recombination and dye adsorption (both of which depend on the electrode thickness), the maximum efficiency was observed at an electrode thickness of 12 μm. The diameter of the mesopores was controlled by changing the size of the templating mesoscale colloidal particles. The specific area of the electrode increased with decreasing mesopore diameter, resulting in a higher density of dye adsorption. Moreover, the decrease in the electron transport resistance at the TiO2/dye interface confirmed the higher photocurrent density values for smaller mesopore diameters. In this work, the maximum photon-to-electric efficiency (of 6.9%) was achieved using 35 nm mesopores and a 12 μm electrode thickness, under conditions of AM1.5G and 1 sun. We are currently investigating how each pore scale can be optimized, with the aim of producing more dye adsorption as well as a higher collection efficiency. We believe that the facile control of porous structure via colloidal templating will be useful for optimizing the efficiency.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of TiO2 IO structures filled with PS particles and XRD patterns and diffuse-reflectance spectra of macroporous IO TiO2 and ts-IO TiO2. This material is available free of charge via the Internet at http://pubs.acs.org. 9376
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