Charge Transport in Photoanodes Constructed with Mesoporous TiO2

Feb 24, 2014 - Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Victoria 3010, Australia. §. CSIRO Energy Tech...
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Charge Transport in Photoanodes Constructed with Mesoporous TiO2 Beads for Dye-Sensitized Solar Cells Alexander R. Pascoe,† Dehong Chen,‡ Fuzhi Huang,† Noel W. Duffy,§ Rachel A. Caruso,‡ and Yi-Bing Cheng*,† †

Department of Materials Engineering, Monash University, Victoria 3800, Australia Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Victoria 3010, Australia § CSIRO Energy Technology, Clayton, Victoria 3169, Australia ‡

S Supporting Information *

ABSTRACT: Mesoporous TiO2 beads exhibit the beneficial properties of enhanced light scattering and fast charge transport in a single nanostructured assembly, which makes them ideal for dye-sensitized solar cell applications. However, their unique geometry gives rise to charge transport behaviors that are particular to the beads themselves. This study examined charge transport in TiO2 beads for dye-sensitized solar cell applications on both plastic and glass substrate devices. Through small perturbation and transient techniques, two effective diffusion rates within the film were observed due to the contrast between the intrabead and interbead connections. The dip in diffusion rates away from their typical exponential behavior at high charge densities could be attributed to the poor electrical contact between the TiO2 beads and the conductive oxide substrate. By the application of a small nanoparticle under-layer, the high contact resistance was overcome while maintaining relatively high diffusion rates. The identification of these charge transport issues and their causes provides an important step toward the optimized deployment of mesoporous TiO2 beads for solar cell applications.

1. INTRODUCTION Since the dye-sensitized solar cell (DSSC) was first reported,1 significant efforts have been focused toward the improved light harvesting and increased collection efficiency of the device. A large portion of this work has focused on novel photoanode materials and architectures. One-dimensional nanostructured materials such as TiO2/ZnO nanowires2−7 and nanotubes8−11 have been employed to accelerate the transport of conductive charges throughout the semiconductor film. Greater absorption of the solar spectrum has also been achieved through the use of larger diameter TiO2 nanoparticles,12 which scatter incident light as predicted by Mie’s solution of Maxwell’s equations. Mesoporous TiO2 beads present the ability to simultaneously increase charge diffusion rates within the photoanode and enhance light harvesting due to their strong scattering abilities.13−19 These beneficial properties have prompted the use of mesoporous beads to replace large diameter TiO2 nanoparticles as scattering layers for highefficiency DSSCs.16,20−23 The high level of control achieved during the synthesis of these structures afford precise nanoparticle and pore sizes within the beads, which facilitates tailored applications where electrolyte infiltration is of key concern.18,24 Mixtures of nanostructured geometries incorporated into mesoporous beads exhibit the fast transport properties characteristic of onedimensional materials as well as the high dye-loadings seen in a nanoparticle film.25,26 In summary, TiO2 beads offer the benefits of improved photon collection and charge transport packaged into a single nanoparticle assembly. © 2014 American Chemical Society

Mesoporous TiO2 beads also present considerable promise for application in DSSCs deposited on plastic substrates. The inability to sinter a TiO2 nanoparticle film deposited on a flexible plastic substrate limits the necessary necking formed between nanoparticles, and greatly inhibits the overall performance of the device. A variety of different remedies have been trialed to overcome this low temperature limitation, none more effective than the mechanical compression of the nanoparticle film.27−31 However, this technique mainly achieves physical contacts between nanoparticles, and thus still offers electron diffusion lengths that are shorter than those observed in a sintered film. The ability to presinter the TiO2 beads means that favorable chemical necking between nanoparticles within the beads, prior to the deposition of the film, can be obtained. In conjunction with the cold isostatic press (CIP) technique,28 these presintered bead photoanodes have yielded efficiencies on plastic substrates above 7%.13 Additionally, the beads permit TiCl4 treatments and the dye sensitization of the TiO2 prior to the film deposition, which boost cell efficiencies and accelerate device fabrication times, respectively. Despite the immense advantages offered by mesoporous TiO2 beads, their geometries present unique concepts for Special Issue: Michael Grätzel Festschrift Received: December 23, 2013 Revised: February 19, 2014 Published: February 24, 2014 16635

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Figure 1. Tracer diffusion coefficients as measured through photocurrent transients. The diffusion coefficients were given by D ≅ ((4d2)/π2)(j(t)/Qt(t)), where d is the film thickness, j(t) is the current density and Qt(t) is the charge remaining in the film. 830, 550, and 350 nm diameter beads films were printed on a PET-ITO substrate and compressed through the CIP method.

Figure 2. Chemical diffusion coefficients as measured through IMPS and small perturbation step transients (Step).

2. EXPERIMENTAL METHOD Synthesis of Mesoporous TiO2 Beads. Spherical mesoporous anatase assemblies of diverse diameters (830 ± 40, 550 ± 50, and 320 ± 50 nm) were fabricated using a sol− gel and solvothermal crystallization method, followed by a heattreatment at 500 °C for 2 h, as described previously.24 For simplicity, the resulting spherical anatase assemblies were named as 830, 550, and 350 nm beads throughout this report. Fabrication of DSSCs. Solar cell working electrodes were fabricated on both fluorine doped tin-oxide (FTO)-coated glass substrates (Dyesol 9 Ω/□) and indium doped tin-oxide (ITO) polyethylene terephthalate (PET) substrates (Oike 15 Ω/□). The FTO coated glass substrates were cleaned in three stages using Hellmanex solution, distilled water, and ethanol. The ITO covered PET substrates were sonicated in ethanol for approximately 10 min. A mesoporous TiO2 bead slurry of

charge transport that are not seen in traditional nanoparticle films. The point-like connection between beads has been identified as an impediment for electron diffusion, as a large volume of charges are required to move through a very limited contact area.17 This study provides a novel analysis of charge diffusion within mesoporous TiO2 beads, confirming the bimodal electron diffusion rates between beads and within the beads on both plastic and glass substrate DSSCs. It is worth noting that thin photoanode films were used in this study, as the purpose of this work was not to report record efficiencies, but to accurately characterize electron diffusion in TiO2 bead-based DSSCs. These results provide an important understanding of electron transport in mesoporous TiO2 beads, which is necessary for the optimized application of these nanostructures. 16636

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approximately 20 wt % in ethanol was formed, and films were printed using the doctor blade technique on both glass and plastic substrates. Plastic films were then compressed using the CIP method at pressures of 25 and 75 MPa, and glass substrate films were sintered at either 150 or 300 °C for 30 min. Films were soaked in an N719 dye solution of 1:1 acetonitrile and propanol for an approximate duration of 18 h. Glass substrate cells were formed using a 25 μm Bynel gasket and a pyrolyzed 10 mM chloroplatinic acid (H2PtCl6) solution in isopropanol on FTO glass as a counter electrode. Plastic substrate cells were formed using a 25 μm Bynel gasket and a commercially sourced Pt-ITO-PET counter electrode (Oike 20−30 nm Pt). Plastic substrate cells used an electrolyte of 0.4 M LiI, 0.4 M tetrabutylammonium iodide, 0.04 M I2, and 0.3 M N-methylbenzimidazole in a mixture of acetonitrile/3-methoxypropionitrile (9:1). Glass substrate cells used an electrolyte of 0.6 M 1-butyl-3methylimidazolium iodide (BMII), 0.03 M I2, 0.1 M guanidinium thiocyanate, and 0.5 M tert-butylpyridine in acetonitrile/ valeronitrile (1:1). Synthesis of Nanoglue Solution. The nanoglue solution was formed by mixing 2.86 g HNO3 (70 wt %, Aldrich) with 50 mL ethanol (99.7%, Merck), followed by the addition of 10.78 g of titanium butoxide (TIB, 97% Aldrich) at room temperature. After stirring for 2 h, 4.56 g of water was added to the solution and stirred for an additional hour. The resulting solution was kept static overnight, and then ethanol and distilled water (1:2) were added to the solution. Doctor bladed films were dipped in the nanoglue solution for a duration of 10 s, and were then heated at 150 °C for 30 min. Characterization Techniques. The film thickness of each working electrode was measured using a Veeco Dektak 150 profilometer prior to dye soaking. The current−voltage response of the completed cells was characterized using an arc xenon lamp with an AM1.5 spectral filter. Small perturbation measurements were made using a Zahner Zennium Electrochemical Workstation ECW IM6 as a frequency response analyzer. Electrochemical impedance spectroscopy (EIS) was performed using a 10 mV applied perturbation in the 100 mHz to 500 kHz frequency range at open-circuit under illumination. Intensity modulated photocurrent spectroscopy (IMPS) was performed using a < 5% perturbation of the steady state illumination. Photocurrent transients and charge extraction measurements were performed in accordance with previous experimentation.32,33 Small perturbation step transients were performed using a white bias LED with a 630 nm pulse LED. Impedance and IMPS data were analyzed using Zview equivalent circuit modeling software (Scribner).

the PET substrates and compressed using the CIP method. The current−voltage performance of the flexible substrate devices is reported in Figure S1. The cells were measured using transient and small perturbation techniques to establish electron diffusion rates within the beads film. These two techniques measure the tracer diffusion coefficient D and the chemical diffusion coefficient Dn, respectively, which differ due to the high concentration of trapping states within the TiO2 band gap. The tracer diffusion coefficient describes the average diffusion rate of a single electron, while the chemical diffusion coefficient describes the effective diffusion of a population of electrons.34 Figure 1 displays the tracer diffusion coefficients measured for three different bead diameters, while Figure 2 shows the chemical diffusion coefficients for the same devices. A strong correlation between electron diffusion rates and the bead diameter is observed in both the photocurrent transient (Figure 1) and small perturbation (Figure 2) measurements. To the best of our knowledge, small perturbation step transients have not been previously reported for the characterization of mesoporous TiO2

3. RESULTS AND DISCUSSION Mesoporous TiO2 Beads on Plastic Substrates. The use of mesoporous TiO2 beads in conjunction with the CIP compression method has proven to be an effective way to deliver high efficiencies on plastic substrate DSSCs.13 Previous studies have shown charge transport in TiO2 beads to be significantly faster than a P25 nanoparticle film.21 However, there are two types of bonds in the bead film, namely, a physical contact between the beads due to the compression and a chemical bonding between nanoparticles inside the beads due to sintering at 500 °C prior to the film formation. The different nature of the bondings between bead-to-bead and nanoparticleto-nanoparticle contacts could result in two different diffusion rates operating within the film. To investigate the electron transport rates in TiO2 beads for flexible substrate applications, 830, 550, and 350 nm diameter bead films were deposited on

Figure 3. (a) Tracer diffusion coefficients for 830, 550, and 350 nm beads films and a P25 film printed on a FTO-glass substrate. Photocurrent transient were started after an illumination at an intensity of approximately two suns. (b) Slope of the tracer diffusion data in (a) given by ∂(logeD)/∂n. Due to the high noise in the diffusion measurements at low charge densities, a fitted dotted line has been used to represent the trend of the data. The decrease in the log− linear slope clearly shows that the large particles incur the bottleneck at lower charge densities relative to the smaller diameter particles. 16637

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Figure 4. Impedance spectra measured at one sun illumination for the four glass substrate cells. The large high frequency feature corresponds to the contact resistance Rcontact.35,36 The inset shows the small contribution of the counter electrode resistance RPt at the very high frequencies, which was not typically seen due to the large Rcontact feature.

beads films. It is worth noting the good agreement between the IMPS measurements and the small perturbation step transients apparent in Figure 2, which validates the accuracy of the step transient technique. Further information describing the small perturbation measurements is presented in Figures S2−S5 in the Supporting Information. The link between the bead diameter and the diffusion rates indicates that the charge transport in the photoanode is limited by the density of bead-to-bead contacts in the film established through the CIP method. In the larger diameter bead films, diffusion paths are characterized by a high ratio of sintered (within the beads) to nonsintered (bead-to-bead) nanoparticle connections, which permits faster transport times. For a film of the same thickness, electrons diffusing through the smaller diameter bead films are required to pass through a greater number of inhibiting bead-to-bead contacts (i.e., nonsintered contacts), effectively slowing their movement. The transport limiting bead-to-bead contact is also observed as a function of the applied CIP pressure, as shown in Figure S6. This data confirms the bimodal diffusion rates of electrons traveling within the TiO2 beads versus electrons traveling between the TiO2 beads. To make best use of the fast intrabead component of this bimodal diffusion rate, it seems appropriate to construct flexible substrate films using the largest diameter TiO2 beads practical. However, the faster transport rates gained from a large diameter bead film may be at the cost of the poorer light scattering properties and packing density of a > 1 μm bead diameter film. It is the careful combination of charge transport rates, light scattering properties and charge accumulation in TiO2 beads that can be used to produce highly efficient plastic substrate DSSCs. Mesoporous TiO2 Beads on Glass Substrates. In the section above, it was shown that the mechanical compression of the beads film elicited a relatively poor electrical contact between beads (interbead) compared to the chemical connections within beads (intrabead). The fabrication of a glass substrate film typically involves the sintering of the TiO2 nanoparticles, which is a vastly different process to the mechanical compression. It is foreseeable that the interbead connections resulting from a sintering step

could be significantly different from those arising from mechanical compression, and hence the charge transport across these connections would also differ. Glass substrate devices were constructed in order to ascertain whether the contrast between interbead and intrabead connections is characteristic of both compressed and sintered beads films. Cells on glass substrates were also measured to investigate transport effects at high carrier concentrations. Previous work has identified a reduction in the bead transport at high electron densities, above which conventional nanoparticle films exhibit faster diffusion rates than a presintered bead film.17 This bottleneck was previously attributed to the bead-to-bead contact; however, there is no direct evidence to support this hypothesis. Figure 3a shows the tracer diffusion rates for three beads films and a P25 film printed on glass and then sintered at a reduced temperature of 300 °C. The photocurrent decay was started after an illumination intensity of approximately 2 suns in order to observe transport rates at high electron concentrations. It is worth noting that the diffusion coefficients of the glass substrate devices exhibited the same dependence on the bead diameter as what was shown in the plastic substrate devices. This information reveals that the sintering step produces a contrast between the interbead and intrabead connections, as is seen in the mechanical compression case. At the high carrier densities (greater than 4 × 1017 cm−3 for the 830 and 550 nm beads, and greater than 6 × 1017 cm−3 for the 350 nm beads and P25 films) it is possible to see the diffusion rates of each film dip away from the theoretically predicted exponential behavior. The departure from the predicted exponential behavior is characteristic of a conductive bottleneck, which impedes the electron transport during high charge concentrations. If the slope of this log−linear plot is calculated, as shown in Figure 3b, the larger diameter particles are seen to suffer this bottleneck at comparatively lower carrier densities. However, Figure 3b does not give us any information concerning the location of this bottleneck. To identify the origin of the transport limiting bottleneck, electrochemical impedance spectroscopy was used to measure 16638

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the glass substrate films. By analyzing the impedance spectra of the four cells shown in Figure 4, it becomes apparent that the origin of this bottleneck is not due to the bead-to-bead contacts, but rather due to the contact resistance between nanoparticles on the conductive oxide layer.35,36 This resistance is revealed as a high-frequency feature in the impedance spectra, which is more prominent in the case of the larger diameter beads compared to the P25 and 350 nm beads films. If the bottleneck was caused by the interbead connections, the impedance spectra would reveal an increase in the transport resistance Rt for the larger diameter beads compared to the smaller diameter beads. However, this is not observed, and the main difference seen between different bead diameter films is the contact resistance Rcontact. The limited contact area between the large diameter beads and the conductive oxide layer leads to a separation of charge, and presents an additional resistive barrier that must be overcome by all conductive electrons if they are to be successfully extracted to the cell contacts. The discovery that this charge transport bottleneck is caused by the film contact with the conductive oxide layer prompts the use of an under-layer to bridge the resistance at this interface. Previous studies have shown an increase in the device efficiency of TiO2 beads cells using a nanoparticle under-layer, despite an associated reduction in the charge transport rates.21,22 These findings complement our results regarding the high contact resistance at the bead-FTO interface, as a high contact underlayer may reduce the total series resistance and boost the overall device efficiency. Two prime candidates for an underlayer, which comprise relatively small nanoparticles, include a P25 under-layer and a nanoglue37,38 treated P25 under-layer. These TiO2 materials were used in this study to increase contact between the FTO and an 830 nm beads film that was deposited directly on the under-layer. Figure 5 shows the contact resistance and recombination resistance plotted as a function of the chemical capacitance Cμ. The three films measured included a uniform 830 nm beads film, an 830 nm beads film with a P25 under-layer, and an 830 nm beads film with a nanoglue treated P25 under-layer. The chemical capacitance gives us an indication of the density of states, and can be likened to the charge density at a given potential.39 Therefore, a comparison between cells at the same Cμ is analogous to analyzing cells at an equivalent electron density. As predicted by our previous results, the use of a small nanoparticle under-layer greatly reduces the contact resistance of the TiO2−FTO interface at high charge concentrations. The beads films using both a P25 and a nanoglue under-layer displayed lower contact resistances with the TCO substrate than a uniform beads film without an under-layer. The nanoglue treated under-layer showed the lowest relative contact resistance as it utilized both the small diameter particles of the P25 solution as well as an amorphous TiO2 nanoglue coating, which further enhance the electrical contact with the substrate. The nanoglue treatment also creates a core−shell structure of amorphous-anatase TiO2, which acts to block recombination across the surface of the nanoparticle, resulting in a higher recombination resistance compared to the bare TiO2 nanoparticles (Figure 5b). This reduction in recombination offered by a nanoglue treatement of the under-layer also provides a small benefit to the total electron lifetime in the film. Given that the core−shell structured under-layer only constitutes a small fraction of the film thickness, the gain in the recombination resistance is only slight and is not as effective as if the nanoglue treatment was applied to the entire film.

Figure 5. (a) Contact resistance Rcontact at the TiO2−FTO interface as a function of the chemical capacitance Cμ. Plotting the resistance relative to the chemical capacitance is analogous to plotting the resistance as a function of the charge density n. (b) Recombination resistance Rr as a function of the chemical capacitance. The small increase in the Rr for the nanoglue (NG) + 830 nm beads film is due to the inhibited back-reaction as a result of the nanoglue posttreatment.

Although the use of the composite under-layer/beads film cannot match the transport rates of a beads film alone, it does offer transport rates greater than a P25 nanoparticle film as well overcoming the transport bottleneck at the higher electron densities. This information is reflected in Figure 6, which shows the tracer diffusion rates as a function of the charge density. Both P25 and nanoglue treated P25 underlayer films show electron diffusion coefficients that lie between the fast diffusing beads film and the slow diffusing P25 film. In this regard, the composite under-layer/beads film presents transport rates that are nearly as fast as a uniform beads film, with the enhanced substrate contact of a small nanoparticle film. At an illumination intensity equal to or below 1 sun, our results showed no evidence of a crossover point where the low contact resistance films present faster diffusion rates than the uniform beads film. However, this effect has been shown in previous work,17 and it is expected that with future improvements to the cell’s charge generation capabilities, a nanoparticle under-layer will prove to be a prerequisite for TiO2 beads DSSCs. An interesting 16639

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Figure 6. Tracer diffusion coefficients for the 830 nm beads film and the under-layer +830 nm beads films.

Figure 7. Performance data for the 830 nm beads films and the under-layer +830 nm beads films measured at 1 sun. Two cells for each configuration were measured using a solar simulator before the best performing cell from each pair was selected for further characterization. All films were sintered at 150 °C for 30 min.

charge generation rates of the nanoglue under-layer also account for the higher short circuit current densities, despite the reduction in the electron diffusion rate. Although there is a reduction in the open-circuit potential, most likely due to the higher density of trap states in the low-temperature sintered P25 and nanoglue particles,40 the performance data shows that there is the possibility of an increase in the total efficiency with the addition of a nanoparticle under-layer.

observation presented in Figure 6 is that faster charge diffusion rates are observed using a P25 under-layer than using a nanoglue treated under-layer, despite strong evidence showing that electron transport is improved after a nanoglue treatment.37,38 It is first important to note that the use of a nanoglue treatment has been shown to provide a significant increase in the density of states (see Figure S7), effectively raising the electron population in the under-layer. In this case, even though the beads still possess the well sintered pathways to facilitate fast charge transport, the high charge densities present in the under-layer have removed some of the driving force for electrons within the beads to diffuse down the concentration gradient. This reduction in the driving force results in comparatively slower charge diffusion rates for the nanoglue under-layer relative to what is achievable on a P25 nanoparticle under-layer. However, the diffusion rates alone do not guarantee a high efficiency cell, and as shown in previous studies, the charge transport rates can be successfully sacrificed to achieve a boost in the overall efficiency.21,22 Figure 7 shows that a reduction in the Rcontact due to the presence of an under-layer (which produces an associated reduction in the series resistance RS) significantly improves the fill factor of the device. The high

4. CONCLUSION Mesoporous TiO2 beads present the benefits of enhanced light scattering, improved charge generation and fast electron transport all packaged into a single nanostructure. Characterization of TiO2 beads deposited on plastic substrate DSSCs showed a bimodal diffusion rate, where the bead-to-bead transport proved the limiting pathway for the extraction of excited carriers. The results also indicated that the dip in diffusion rates observed at high electron densities was caused by a bottleneck at the bead-TCO interface, and the ensuing high resistances caused by this impediment. It is worth noting that because of the CIP compression used to form plastic substrate devices, the bead-TCO contact only proved a concern on the glass substrate cells, where the electrical contact after printing is 16640

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(10) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Highly-Ordered TiO2 Nanotube Arrays up to 220 μm in Length: Use in Water Photoelectrolysis and Dye-Sensitized Solar Cells. Nanotechnology 2007, 18, 65707−65707. (11) Shankar, K.; Bandara, J.; Paulose, M.; Wietasch, H.; Varghese, O. K.; Mor, G. K.; LaTempa, T. J.; Thelakkat, M.; Grimes, C. A. Highly Efficient Solar Cells Using TiO2 Nanotube Arrays Sensitized with a Donor-Antenna Dye. Nano Lett. 2008, 8, 1654−1659. (12) Zhang, Z.; Ito, S.; O’Regan, B.; Kuang, D.; Zakeeruddin, S. M.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Péchy, P.; et al. The Electronic Role of the TiO2 Light-Scattering Layer in DyeSensitized Solar Cells. Z. Phys. Chem. 2007, 221, 319−327. (13) Huang, F.; Chen, D.; Li, Q.; Caruso, R. A.; Cheng, Y.-B. Construction of Nanostructured Electrodes on Flexible Substrates Using Pre-Treated Building Blocks. Appl. Phys. Lett. 2012, 100, 23102−23102. (14) Chen, Y.; Huang, F.; Chen, D.; Cao, L.; Zhang, X. L.; Caruso, R. A.; Cheng, Y. B. Effect of Mesoporous TiO2 Bead Diameter in Working Electrodes on the Efficiency of Dye-Sensitized Solar Cells. ChemSusChem 2011, 4, 1498−1503. (15) Huang, F.; Chen, D.; Cao, L.; Caruso, R. A.; Cheng, Y.-B. Flexible Dye-Sensitized Solar Cells Containing Multiple Dyes in Discrete Layers. Energy Environ. Sci. 2011, 4, 2803−2806. (16) Huang, F.; Chen, D.; Zhang, X. L.; Caruso, R. A.; Cheng, Y.-B. Dual-Function Scattering Layer of Submicrometer-Sized Mesoporous TiO2 Beads for High-Efficiency Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2010, 20, 1301−1305. (17) Sauvage, F.; Chen, D.; Comte, P.; Huang, F.; Heiniger, L.-P.; Cheng, Y.-B.; Caruso, R. A.; Graetzel, M. Dye-Sensitized Solar Cells Employing a Single Film of Mesoporous TiO2 Beads Achieve Power Conversion Efficiencies Over 10%. ACS Nano 2010, 4, 4420−4425. (18) Chen, D.; Huang, F.; Cheng, Y.-B.; Caruso, R. A. Mesoporous Anatase TiO2 Beads with High Surface Areas and Controllable Pore Sizes: A Superior Candidate for High-Performance Dye-Sensitized Solar Cells. Adv. Mater. (Weinheim, Ger.) 2009, 21, 2206−2210. (19) Chen, D.; Caruso, R. A. Recent Progress in the Synthesis of Spherical Titania Nanostructures and Their Applications. Adv. Funct. Mater. 2013, 23, 1356−1374. (20) Ke, C.-R.; Ting, J.-M. Anatase TiO2 Beads Having Ultra-Fast Electron Diffusion Rates for Use in Low Temperature Flexible DyeSensitized Solar Cells. J. Power Sources 2012, 208, 316−321. (21) Yan, K.; Qiu, Y.; Chen, W.; Zhang, M.; Yang, S. A Double Layered Photoanode Made of Highly Crystalline TiO2 Nanooctahedra and Agglutinated Mesoporous TiO2 Microspheres for High Efficiency Dye Sensitized Solar Cells. Energy Environ. Science 2011, 4, 2168− 2176. (22) Park, Y.-C.; Chang, Y.-J.; Kum, B.-G.; Kong, E.-H.; Son, J. Y.; Kwon, Y. S.; Park, T.; Jang, H. M. Size-Tunable Mesoporous Spherical TiO2 as a Scattering Overlayer in High-Performance Dye-Sensitized Solar Cells. J. Mater. Chem. 2011, 21, 9582−9586. (23) Ke, C.-R.; Chen, L.-C.; Ting, J.-M. Photoanodes Consisting of Mesoporous Anatase TiO2 Beads with Various Sizes for HighEfficiency Flexible Dye-Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 2600−2607. (24) Chen, D.; Cao, L.; Huang, F.; Imperia, P.; Cheng, Y. B.; Caruso, R. A. Synthesis of Monodisperse Mesoporous Titania Beads with Controllable Diameter, High Surface Areas, and Variable Pore Diameters (14−23 nm). J. Am. Chem. Soc. 2010, 132, 4438−4444. (25) Chen, D.; Huang, F.; Cao, L.; Cheng, Y. B.; Caruso, R. A. Spiky Mesoporous Anatase Titania Beads: A Metastable Ammonium Titanate-Mediated Synthesis. Chemistry 2012, 18, 13762−13769. (26) Liao, J.-Y.; Lei, B.-X.; Kuang, D.-B.; Su, C.-Y. Tri-Functional Hierarchical TiO2 Spheres Consisting of Anatase Nanorods and Nanoparticles for High Efficiency Dye-Sensitized Solar Cells. Energy Environ. Science 2011, 4, 4079−4085. (27) Yamaguchi, T.; Tobe, N.; Matsumoto, D.; Nagai, T.; Arakawa, H. Highly Efficient Plastic-Substrate Dye-Sensitized Solar Cells with Validated Conversion Efficiency of 7.6%. Sol. Energy Mater. Sol. Cells 2010, 94, 812−816.

comparatively poor. This was overcome through the use of a nanoparticle under-layer, which bridged the high contact resistance while still maintaining relatively fast diffusion rates. The bead-to-bead contact and bead-TCO contact issues identified in this study present an interesting insight into the application of TiO2 beads for DSSCs. This information, in association with future work, will be useful in the optimization of DSSCs employing TiO2 beads, ensuring that they are best used to exploit their impressive properties.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data including the current−voltage performance of the plastic cells, the analysis of the small perturbation spectra, TiO2 bead diffusion rates as a result of different CIP compressions, as well as a description of the nanoglue treatment are presented in the Supporting Information. This material is available free of charge via the Internet at http:// pubs.acs.org.

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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 research was supported by the Victorian Organic Solar Cell Consortium, the Australian Renewable Energy Agency (ARENA) and the Australian Government Department of Industry. RAC acknowledges an Australian Research Council Future Fellowship (FT0990583).

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4125606 | J. Phys. Chem. C 2014, 118, 16635−16642