Drift Transport in Al2O3-Sheathed 3-D Transparent Conducting Oxide

Apr 25, 2014 - Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States. § Advanced Photon ...
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Drift Transport in Al2O3‑Sheathed 3‑D Transparent Conducting Oxide Photoanodes Observed in Liquid Electrolyte-Based Dye-Sensitized Solar Cells Fa-Qian Liu,† Kai Zhu,*,‡ Tao Li,§ and Tao Xu*,† †

Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States § Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

ABSTRACT: It has long been taken for granted that electron transport in liquid-electrolyte-based dye-sensitized solar cells (DSSCs) undergoes an ambipolar diffusive transport due to the strong coupling between electrons in the photoanode and the nearby mobile cations in liquid electrolyte, which, therefore, screens off any electric field in the photoanodes and consequently eliminates the possibility for drift transport. In this work, we demonstrate the existence of drift transport in liquid electrolyte-based DSSCs using a thin Al2O3-sheathed 3-dimentional (3-D) fluorinated tin oxide (FTO), as photoanodes. The electron diffusion rate in such 3-D TCO based DSSC exhibits a striking enhancement to the value of ∼10−2 cm2/s, about 104 times faster than that of the TiO2 nanoparticle-based DSSCs. The electron diffusion coefficient is independent of the photoelectron density, while intensity modulated photocurrent spectroscopy (IMPS) suggests that the time constants of electron transport exhibit a linear dependence on the bias voltage, a strong indication of drift transport behavior in this 3-D FTO hollow nanobeads-based DSSC, despite the use of liquid I−/I3− electrolyte.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have been the subject of substantial academic research and commercial development over the past 20 years due to their potential low cost and simple fabrication procedures in comparison to Si-based solar cells.1 Extensive progress has been made in enhancing the efficiency, stability, and processing of this technology. It is reported that the electron transport in the network of interconnected TiO2 nanoparticles undergoes an ambipolar diffusion mechanism, in which electrons always coupled with cations such as Li+ in electrolyte, screening off any potential gradient in the body of the TiO2 nanoparticle network.2 Thus, the electrons have to take random walk, i.e., to diffuse through the TiO2 nanoparticle network that contains vast amount of shallow and deep defects on surface and in bulk phase, resulting in the so-called “sticky” electrons transport, in which electrons cripple through the photoanode via frequent (over 1 million times) trapping and detrapping events.1,3 That being said, however, a surprising fact is that a tiny portion of the photoanode, located at the interface between the transparent conducting oxide (TCO) glass and TiO2, undergoes drift transport, driven by the Fermi-level difference between TCO and TiO2 that leads to a potential gradient across the TCO−TiO2 interface.4−7 Nonetheless, this tiny section (only a few tens of nanometers) of drift transport is negligible compared to the over 10 um long diffusive transport in the thick TiO2 layer. Therefore, introducing the more © 2014 American Chemical Society

effective drift-assisted transport in photoanode can be a potential remedy to speed up electron extraction in the photoanode, which may eventually allow the use of much faster redox shuttles (relative to I−/I3−) with less overpotential (relative to the highest occupied molecular orbital (HOMO) of dyes) for a higher attainable photovoltage.8,9 Currently, effort has primarily been focused on exploring new photovoltaic materials or structures with faster charge transport properties.10−15 However, most photovoltaic materials are not good conductors. Even for single crystal anatase TiO2 or ZnO, the conductivity is only a few S/cm.16−20 In contrast, TCO electrodes can reach a high conductivity over >1 × 103 S/cm, (1 × 107 times greater than anatase TiO2 nanoparticular films) due to its high carrier concentration (>1 × 1020/cm3)21 and carrier mobility(65 cm2 V−1 s−1).22,23 However, the drift transport at the TCO/TiO2 interface has received much less attention in terms of its role in electron extraction in a photoanode. This is because in conventional DSSCs, planar TCO electrodes are used to allow light penetration and to collect charges at the very end of the electron transport pathways, far away from the interfacial charge separation or redox sites, where instantaneous charge Received: March 4, 2014 Revised: April 15, 2014 Published: April 25, 2014 9951

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Figure 1. Schematic view of the synthesis of a 3-D FTO hollow nanobead photoanode. The 3-D FTO structure can be fabricated by self-assembling PS nanobeads on a planar FTO electrode, followed by infiltration of FTO precursory solution, gelation, and desiccation to form the 3-D FTO hollow nanobeads film. Then, a thin insulating Al2O3 layer is conformally coated on all surface of the FTO by ALD method.

collection by the TCO electrode is desired.24−29 Therefore, to fully exploit the function of TCOs as an electrode material, it is a viable strategy to transform the traditional 2-D conducting oxide films to 3-D nanostructured matrix so as to maximize the interface area between the conducting oxides and the active materials (e.g., dye molecules and electrolytes) where charge separation and/or redox reactions occur.8,30−32 To date, only a handful of innovations on the 3-D TCOs as fast electron extraction photoanodes were reported.33,34 Herein, as schematically shown in Figure 1, we report a 3-D fluorinated tin oxide (FTO) hollow nanobeads electrode fabricated using polystyrene (PS) nanobeads as a template. A thin layer of Al2O3, acting as a tunneling barrier, is conformally deposited on all surfaces of this 3-D FTO nanobead electrode with a controlled thickness by use of an atomic layer deposition (ALD) technique. Upon dye-sensitization, this Al2O3-sheathed 3-D FTO hollow nanobead film is used as a photoanode in further DSSC studies with the prospect of enhanced charge transport kinetics via drift transport in FTO.

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total active electrode area was 0.25−0.50 cm2. Traditional TiO2 nanoparticle-based DSSCs are also prepared for comparison as needed. The current density−voltage (J−V) curves were collected using a Source Measure Unit (Keithley 2400) at one Sun 1.5 air mass global (AM G) spectrum provided by a solar simulator (Photo Emission Inc. CA, model SS50B). Electrochemical impedance spectroscopy (EIS) measurements were performed with a potentiostat/frequency analyzer (PARSTAT 2273) using a two-electrode configuration. The modulation frequencies range from 50 mHz to 100 kHz. The amplitude of the modulation voltage was 10 mV. Z-view 2.9c (Scribner Associates) was used to fit the EIS spectra to the equivalent circuit based on the transmission line model. The measurement of the dye loading amount was conducted by soaking an N-719 sensitized sample (area = 0.5 cm2) in 3 mL 10 mM KOH aqueous soluton for 2 h (target solution). The UV−vis spectra of the target solution and three solutions with known dye concentrations (calibration solutions) were measured. On the basis of Beer’s law, the concentration of the target solution can be obtained by fitting the absorbance at 522 nm (with respect to blank solvent) to the calibration curve.

EXPERIMENTAL SECTION

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The FTO nanobead electrodes were prepared by a morphology-controllable and template-assisted evaporative coassembly method reported in our previous work using sulfonated polystyrene nanobeads with diameters of ∼197 nm and ∼526 nm.34 Ultrathin layers of Al2O3 were deposited using trimethylaluminum (TMA) and water as precursors. The layers were deposited at 200 °C using exposure times of 8 s for TMA and for water, with 30 s of nitrogen between each pulse. After ALD, electrodes were heated to 200 °C and were immediately soaked in an ethanolic solution of 0.5 mM (Bu4N)2 [Ru(4,40dicarboxy-2,20-bipyridine)2-(NCS)2] (“N719”, Dyesol) overnight. The samples were then rinsed with ethanol and dried with a N2 stream. The dye-sensitized solar cell was assembled by coupling the dye sensitized Al2O3 shealthed 3-D FTO nanobead anode with a Pt-coated FTO cathode using a piece of hot melt Surlyn (25 μm thick, Solaronix) as a spacer. The internal space of the cell was filled with an electrolyte (0.5 M LiI, 50 mM I2, 0.5 M 4tertbutylpyridine in 3-methoxypropionitrile) by capillary force through a small predrilled hole in the counter electrode. The

RESULTS AND DISCUSSION As schematically shown in Figure 1, a 3-D FTO hollow electrode can be fabricated by simply mixing the sulfonated PS beads suspension with precursors (SnCl2·2H2O and NH4F) at room temperature to form a templated sol−gel along the surface of the PS nanobeads. Upon desiccation, the precursory gel formed a homogeneous compact gel layer around each PS bead. Further calcination results in the solid FTO nanobead structure (Figure 1). Finally, a thin layer of Al2O3 was conformally coated on all surfaces of the 3-D FTO hollow nanobeads as well as the planar FTO substrate with controlled thickness by the ALD method. This thin Al2O3 layer reduces the electron back transfer from FTO (both 3-D FTO hollow nanobeads and the planar FTO film underneath) to electrolyte (shunt leakage) in DSSCs.35 ALD technique, as a layer-by-layer deposition technique, has been used to achieve a compact ultrathin barrier layer on various morphologies with angstromscale control over the coating thicknesses.33,36 Upon dye9952

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Figure 2. Morphologies of the representative FTO hollow nanobeads. (a) Scanning electron microscopy (SEM) image of 546 nm monodispersed hollow FTO nanobeads. scale bar = 500 nm. (b) Transmission electronmicroscopy (TEM) image of the 546 nm hollow FTO nanobeads, scale bar = 500 nm. (c) The cross-sectional SEM image of a 6-μm thin film consisting of the 200 nm hollow FTO nanobeads bar = 2 μm. (d) SEM image of 200 nm monodispersed hollow FTO nanobeads, scale bar = 200 nm. (e) TEM image of 200 nm hollow FTO nanobeads, scale bar = 100 nm. (f) XRD pattern of 200 nm FTO hollow nanobeads and the standard diffraction of SnO2 (JCPDS 5-0467).

sheet resistance of the 3-D FTO hollow nanobeads films is ∼27 Ω/square, indicating the excellent conductivity of our 3-D FTO hollow nanobeads electrodes, compared to the MΩ/square level for TiO2 nanoparticle electrodes. Furthermore, as an effort to suppress recombination, five ALD cycles of Al2O3, acting as the barrier to suppress back electron transfer, were deposited on all surface of the 3-D FTO hollow nanobeads electrodes using trimethylaluminum (TMA) and water as precursors with the growth per cycle of ∼1 Å.38 For photovoltaic study, DSSCs are constructed using our Al2O3 sheathed 3-D FTO hollow nanobead film as the photoanode, N719 dye as sensitizer, and I−/I3− as redox mediator. For comparison, two kinds of Al2O3 sheathed FTO hollow nanobeads films were used photoanodes. The 546 nm hollow FTO nanobeads film is termed as ASFHNB-546 and the 200 nm FTO nanobeads film is termed as ASFHNB-200. The dye-loading amount was also measured to be 0.7 × 10−8 mol/ cm2 for 6 μm-thick ASFHNB-546 film and 2.7 × 10−8 mol/cm2 for 6 μm-thick ASFHNB-200 film. Figure 3a shows the J−V curves of the cells based on the two kinds of photoanodes. The key photovoltaic parameters are summarized in Table 1. The sample ASFHNB-200 exhibits a short-circuit current density of 7 mA/cm2, a factor of 1.64 higher than that of ASFHNB-546, due to the higher electrode surface area of ASFHNB-200 (Roughness factor ∼180) that leads to more dye-loading amount. The Voc for both electrodes is ∼520 mV. This value is about 120 mV higher than the typical Voc values for nanoparticle SnO2 electrodes reported previously (Table 1).39 The energy gap (∼0.6 eV) between the excited dyes and the Fermi level of FTO is a necessary driving force to enable efficient electron injection from the exited dyes to the FTO via tunneling through the Al2O3 layer. Several factors can

sensitization, the Al2O3-sheathed 3-D FTO hollow nanobeads is used as the photoanodes in further DSSCs studies. Figure 2a shows the SEM image of the resulting hollow FTO nanobeads with a diameter of 546 nm, slightly larger than that of the starting sulfonated PS beads (526 nm). In addition, on the surface of each hollow FTO bead, an aperture (∼100 nm in diameter) is formed due to the gas evacuation (resulting from the thermal decomposition of PS beads) during calcination. The TEM image (Figure 2b) clearly confirms the hollow structure with a wall thickness of ∼25 nm. The inner cavity diameter is ∼492 nm. The cross-sectional SEM image (Figure 2c) of the hollow FTO nanobead film confirms that the hollow nanobead morphology is omnipresent in the film. In addition, by using smaller PS nanobeads, such as the 194 nm PS nanobeads as templates, the size of the resulting FTO hollow nanobeads decreased to ∼200 nm (Figure 2d, with the wall thickness of ∼14 nm shown in Figure 2e). The XRD measurements (Figure 2f) indicate that the as-synthesized FTO nanobeads have a polycrystalline configuration.37 All peaks in the XRD spectra are characteristic for SnO2 (JCPDS Powder Diffraction File Card 5-0467). Our previous N2 adsorption/desorption isotherms study shows that the BET surface area for 200 nm FTO hollow nanobeads and for 546 nm FTO hollow nanobeads is 53 m2/g and 21 m2/g, respectively.34 Thus, the surface roughness factor (SRF, i.e., the ratio of effective surface area of the FTO nanobeads film to the projected substrate area) of the 3-D FTO hollow nanobeads films can be calculated based on the BET surface areas, and the thickness of the FTO hollow nanobeads film. For a 6-μm thick film consisting of 546 nm 3-D FTO hollow nanobeads, its SRFs is ∼50; and for a 6-μm thick film consisting of 200 nm 3-D FTO hollow nanobeads, its SRFs is ∼180. The 9953

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Figure 3. Typical J−V curves of DSSCs based on Al2O3 sheathed 3-D FTO hollow nanobead photoanodes under AM 1.5 G illumination (a) and under dark (b), respectively. The area of both devices is 0.36 cm2.

Table 1. Photovoltaic Parameters of DSSCs with Same Thickness and Dye Loading Amount sample

Jsc (mA/cm2)

Voc (V)

FF

η (%)

ASFHNB-200 ASFHNB-546 nano-SnO239

7.084 4.268 4.9

0.525 0.520 0.401

0.413 0.425 0.509

1.536 0.943 1.0

Figure 4. Dependence of the electron diffusion coefficient on the photoelectron density at short circuit for DSSCs composed of 3-D FTO photoanodes and traditional TiO2 photoanode.

sufficiently faster than the traditional TiO2 nanoparticle photoanode. ASFHNB-200 exhibits a very fast diffusion coefficient of ∼10−2 cm2/s, which is ∼104 times quicker than the traditional TiO2 nanoparticle-based DSSCs. Figure 4 also indicates that electrons move faster as the surface area decreases or, equivalently, as the roughness factor decreases, in agreement with previous study for TiO2 based DSSCs.43 The time constant of electron transport (τd) is derived from the IMPS data measured under short circuit according to eq 1:

be attributed to the observed Voc and Jsc enhancement in comparison to the SnO2 nanoparticle electrodes. First, Voc is determined by the energy gap between the quasi Fermi level of the photoanode under illumination and the redox potential of the I−/I̅3.40 With heavy doping of F, the Fermi level of FTO rises compared to SnO2 due to Burstein−Moss shift,41 leading to a higher Voc. Second, the introduction of Al2O3 compact layer was believed to effectively reduce the charge recombination at the FTO-electrolyte, hence to improve the photocurrent density. Finally, we assume this enhancement is due to the optimized electron transport in our Al2O3 sheathed 3-D FTO hollow nanobead photoanode, which is further investigated by intensity modulated photocurrent spectroscopy (IMPS). Figure 3b shows the dark I−V curve comparing 3D FTO electrodes to a traditional TiO2 nanoparticle photoelectrode. The ASFHNB546 electrode shows a higher onset voltage, associated with a smaller dark current than those of the ASFHNB-200 electrode, presumably resulting from a smaller electrode surface area that has less leaks due to imperfect covering of the ALD Al2O3 layer. However, both of the FTO nanobeads electrodes exhibit higher dark current than a traditional TiO2 electrode, which is due to the imperfectness of the ALD Al2O3, where there must be a considerable amount of exposed FTO without being covered by Al2O3. This is also the reason for the lower FF found in our FTO nanobeads electrode than traditional TiO2 nanoparticle electrodes. Improvement of the compactness of ALD layer can possibly be achieved via fine-tuning of the reaction conditions, including the nature of the precursory gases, their duration times, temperature, cycling numbers, and so forth, which will become our future effort in order to enhance the device performance. Figure 4 shows the electron diffusion coefficient (D) as a function of photoelectron density (ne) in the photoande. For TiO2 films, D displays a power-law dependence on ne. For DSSCs, this nonlinear dependence has typically been attributed to electrons that undergo multiple trapping and detrapping during their transit through the TiO2 network.4243 However, all the two 3-D FTO nanobeads photoanode-based DSSCs show no dependence on the photoelectron density, and the diffusion coefficient in the 3-D FTO nanobeads photoanodes is

τd = 1/2πfd

(1)

where fd is the characteristic frequency minimum of the IMPS. We then fit τd to the following:, τd = (R 0 + R S)*C

(2)

where τd is the electron transport time constant, R0 is the series resistance in photoanode, Rs is the externally connected series resistance, and C is the double layer capacitance at the electrode/electrolyte interface.44 This yields approximately C = 4 × 10−4 F and R0 = 15 ohm. Figure 5 shows that at Rs = 0

Figure 5. Time constants measured from IMPS as a function of externally connected series resistance.

ohm, τd of ASFHNB-200 based DSSC is ∼5 ms, faster than ∼15 ms of bare SnO2 NPs photoanode reported before,45 implying that the former has a faster electron transport rate compared to that of the latter, due to the efficient, 3-D interconnected transport pathways in the FTO hollow nanobeads. The fact that this simple model fits the data very well suggests that the internal FTO resistance is less than 9954

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externally connected resistance. The R0 value, which is the sum of planar FTO glass resistance and the 3-D FTO nanobeads film resistance, is nearly 2 orders of magnitude lower than that of conventional TiO2 NP-based photoanode, indicating that FTO is a superior material for electron extraction. The much lower dependence of electron diffusion coefficient on photoelectron density in our Al2O3-sheathed FTO nanobead photoanodes than that of the conventional TiO2 nanoparticle electrode suggests that electron transport is preferentially dominated by a drift mechanism. In the case of pure drift transport, carrier mobility is independent of carrier density, assuming the density of state is overwhelmingly greater than carrier density, such as metals. Drift current can be expressed as follows: j = enμε

Figure 6. Transport time constants measured from IMPS as a function of bias voltage for (a) ASFHNB-200 based DSSC and (b) conventional TiO2 nanoparticle-based DSSC.

(3)

3, therefore, it is reasonable to see the transport time constants exhibit a linear dependence on the bias voltage as observed in Figure 6 a. The drift transport achieved in our Al2O3-sheathed 3-D FTO nanobead-based photoanode DSSCs is attributed to several factors including the much (1−2 orders) higher free electron density in FTO than the Li+ concentration in the electrolyte; the significantly higher carrier mobility in FTO (∼65 cm 2 V − 1 s − 1 ) 4 8 than in TiO 2 nanoparticle network (10−6cm2V−1s−1);49 and the insulating layer of Al2O3 that suppresses the back electron transfer.35 Thus, the 3-D FTO structure is quite favorable for electron extraction and can significantly reduce the transport time and prevent the exposure of photoelectrons to counter charges so as to suppress recombination. Figure 7 shows the electron recombination lifetime for different electrodes. The recombination lifetimes were calcu-

in which j is the drift current density, e is the electron charge, n is the carrier density, μ is the effective carrier mobility, and ε is the applied voltage.7,46 Herein, both n and μ are inherent properties of a material with no mutual dependence. Since FTO is generally treated as metallic behavior, it is not surprising that the photoelectron transport in FTO also behaviors in accordance with drift mechanism. Furthermore, under short circuit conditions, C originates from the double layer capacitance at the FTO/electrolyte interface.44,47 The capacitance obtained from the fitting (eq 2) is 400 μF, which is nearly 1 order of magnitude higher than the value (about 50 μF) measured for nanocrystalline TiO2 photovoltaic cells,44 showing the vast interfaces of the 3-D FTO/electrolyte interface comparing to the traditional 2-D FTO substrate. Since C is proportional to the electron density n, according to the equation, C = (e2/kT) n, where k is the Boltzmann constant, T is the temperature, and e is the electron charge, the increase of the capacitance also indicates that good electrical communication is established between the quasiFermi level in the shielding Al2O3 layer and the conducting electron collector, i.e., the 3-D FTO nanobeads core. This phenomenon indicates that there is significantly higher electron accumulation in the FTO scaffold in our 3-D FTO nanobeads electrode, which agrees with the high density of states in FTO (>1021/cm3, or 10−2 mol/cm3),33 1−2 orders of magnitude greater than the counterion (e.g., Li+) concentration in electrolyte (typically at the level of 5 × 10−4 mol/cm3). Such accumulation allows the establishment of potential gradient in the scaffold of our FTO nanostructure, leading to the observed drift transport even in liquid electrolyte. Figure 6a shows the effect of bias voltage on the electron transport time constants for the 3-D FTO hollow nanobeadsbased DSSC, and Figure 6b shows the same study for the TiO2 DSSC. The transport time constants for conventional TiO2 DSSC shows no dependence on the bias voltage, which is expected as the electric field is screened off in liquid electrolyte, so that there is no field effect on transport. Strikingly, the transport time constants for the 3-D FTO hollow nanobead (ASFHNB-200)-based DSSC show a clear dependence on the bias voltage, that is, the transport time constant decreases linearly with the increase of the bias voltage. In other words, there is an unambiguous field effect that governs the electron transport in our Al2O3-sheathed FTO nanobeads photoanode. As the electron transport time constants in FTO reflect the exposed time of electrons in electrolyte, and in drift transport mode, the current, thus the average drifting speed of electrons exhibits linear dependence on applied voltage as indicated in eq

Figure 7. Electron recombination lifetimes as a function of voltage.

lated from the open circuit potential decay curves. The values were obtained from eq 4,50 −1 ⎛ kT ⎞⎛ dV ⎞ τr = −⎜ ⎟⎜ OC ⎟ ⎝ e ⎠⎝ dτr ⎠

(4)

It is also observed that the recombination time constants for Al2O3 sheathed 3-D FTO hollow nanobead electrodes are very close to that of pure TiO2 based DSSCs. The ASFHNB-200 FTO electrode shows faster recombination than the ASFHNB546 FTO sample, resulting from the difference of the surface area of FTO electrodes. In general, larger surface area leads to faster recombination. 9955

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Nanoporous Semiconductor Electrodes. J. Solid State Electrochem. 1999, 3, 337−347. (7) Rühle, S.; Dittrich, T. Investigation of the Electric Field in TiO2/ FTO Junctions Used in Dye-Sensitized Solar Cells by Photocurrent Transients. J. Phys. Chem. B 2005, 109, 9522−9526. (8) Hamann, T. W.; Jensen, R. A.; Martinson, A. B. F.; Van Ryswyk, H.; Hupp, J. T. Advancing Beyond Current Generation Dye-Sensitized Solar Cells. Energy Environ. Sci. 2008, 1, 66−78. (9) Spokoyny, A. M.; Li, T. C.; Farha, O. K.; Machan, C. W.; She, C.; Stern, C. L.; Marks, T. J.; Hupp, J. T.; Mirkin, C. A. Electronic Tuning of Nickel-Based Bis(dicarbollide) Redox Shuttles in Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2010, 49, 5339−5343. (10) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. ZnO Nanotube Based Dye-Sensitized Solar Cells. Nano Lett. 2007, 7, 2183−2187. (11) Wang, Q.; Zhu, K.; Neale, N. R.; Frank, A. J. Constructing Ordered Sensitized Heterojunctions: Bottom-Up Electrochemical Synthesis of p-Type Semiconductors in Oriented n-TiO2 Nanotube Arrays. Nano Lett. 2009, 9, 806−813. (12) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Use of Highly-Ordered TiO2 Nanotube Arrays in Dye-Sensitized Solar Cells. Nano Lett. 2006, 6, 215−218. (13) Ito, S.; Zakeeruddin, S. M.; Comte, P.; Liska, P.; Kuang, D.; Grätzel, M. Bifacial Dye-Sensitized Solar Cells Based on an Ionic Liquid Electrolyte. Nat. Photonics 2008, 2, 693−698. (14) Kim, D.; Ghicov, A.; Albu, S. P.; Schmuki, P. Bamboo-Type TiO2 Nanotubes: Improved Conversion Efficiency in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 16454−16455. (15) Yang, Z.; Xu, T.; Y, I.; Welp, U.; Kwok, W. K. Enhanced Electron Transport in Dye-Sensitized Solar Cells Using Short ZnO Nanotips on A Rough Metal Anode. J. Phys. Chem. C 2009, 113, 20521−20526. (16) Kavan, L.; Grätzel, M.; Gilbert, S. E.; Klemenz, C.; Schell, H. J. Electrochemical and Photoelectrochemical Investigation of SingleCrystal Anatase. J. Am. Chem. Soc. 1996, 118, 6716−6723. (17) Forro, L.; Chauvet, O.; Emin, D.; Zuppiroli, L.; Berger, H.; Lévy, F. High Mobility N-type Charge Carriers in Large Single Crystals of Anatase (TiO2). J. Appl. Phys. 1994, 75, 633. (18) Wagner, P.; Helbig, R. Hall Effect and Anisotropy of the Mobility of the Electrons in Zinc Oxide. J. Phys. Chem. Sol. 1974, 35. (19) Grimes, C. A. Synthesis and Application of Highly Ordered Arrays of TiO2 Nanotubes. J. Mater. Chem. 2007, 17, 1451−1457. (20) Fessenden, R. W.; Kamat, P. V. Rate Constants for Charge Injection from Excited Sensitizer into SnO2, ZnO, and TiO2 Semiconductor Nanocrystallites. J. Phys. Chem. 1995, 99, 12902− 12906. (21) Wu, H.; Hu, L.; Carney, T.; Ruan, Z.; Kong, D.; Yu, Z.; Yao, Y.; Cha, J. J.; Zhu, J.; Fan, S.; Cui, Y. Low Reflectivity and High Flexibility of Tin-Doped Indium Oxide Nanofiber Transparent Electrodes. J. Am. Chem. Soc. 2010, 133, 27−29. (22) Calnan, S.; Tiwari, A. N. High Mobility Transparent Conducting Oxides for Thin Film Solar Cells. Thin Solid Films 2010, 518, 1839−1849. (23) Peter, L. M. Dye-sensitized Nanocrystalline Solar Cells. Phys. Chem. Chem. Phys. 2007, 9, 2630−2642. (24) O’Brien, P. G.; Puzzo, D. P.; Chutinan, A.; Bonifacio, L. D.; Ozin, G. A.; Kherani, N. P. Selectively Transparent and Conducting Photonic Crystals. Adv. Mater. 2010, 22, 611−616. (25) Fan, Z. Y.; Ruebusch, D. J.; Rathore, A. A.; Kapadia, R.; Ergen, O.; Leu, P. W.; Javey, A. Challenges and Prospects of Nanopillar-Based Solar Cells. Nano Res. 2009, 2, 829−843. (26) Martinson, A. B. F.; Elam, J. W.; Liu, J.; Pellin, M. J.; Marks, T. J.; Hupp, J. T. Radial Electron Collection in Dye-sensitized Solar Cells. Nano Lett. 2008, 8, 2862−2866. (27) Esmanski, A.; Ozin, G. A. Silicon Inverse-Opal-Based Macroporous Materials as Negative Electrodes for Lithium Ion Batteries. Adv. Funct. Mater. 2009, 19, 1999−2010. (28) Meng, H.; Xie, F. Y.; Chen, J.; Sun, S. H.; Shen, P. K. Morphology Controllable Growth of Pt Nanoparticles/nanowires on

CONCLUSIONS In summary, we have successfully synthesized 3-D interconnected FTO photoanodes by evaporative coassembly of a colloidal template and prepared Al2O3 sheathed 3-D FTO electrodes. Due to the much greater (1−2 orders of magnitude) electron density in the FTO scaffold than the mobile cation (Li+) concentration in the electrolyte, such that the free electrons in FTO cannot be completely coupled by the mobile cations in the liquid electrolyte. As such, it becomes feasible to establish the potential gradient of the FTO scaffold, and electron transport in this 3-D FTO structure exhibits drift behavior, as evidenced by the linear dependence of electron transport time constants in FTO on the photovoltage, as well as the nearly independence of electron diffusion coefficient on the photoelectron density in IMPS study. In prospective, this work paves up a new avenue toward more efficient photovoltaic systems with much faster redox shuttles (relative to I−/I3−) but less overpotential (relative to HOMO of dyes).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from the U.S. National Science Foundation (CBET-1150617). The electron microscopy was conducted at the Electron Microscopy Center for Materials Research at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. K.Z. acknowledges support by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DEAC36-08GO28308 with the National Renewable Energy Laboratory. F.L. is also partially supported by the NSF of China (no. 21371105). We thank Dr. Alex B. F. Martinson at Materials Science Division, Argonne National Laboratory for his help with atomic layer deposition.

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