Enhanced Electron Collection in TiO2 Nanoparticle-Based Dye

Oct 15, 2010 - Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States, and Materials Science Di...
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J. Phys. Chem. C 2010, 114, 19151–19156

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Enhanced Electron Collection in TiO2 Nanoparticle-Based Dye-Sensitized Solar Cells by an Array of Metal Micropillars on a Planar Fluorinated Tin Oxide Anode Zhenzhen Yang,†,‡ Tao Xu,*,†,‡ Shanmin Gao,† Ulrich Welp,‡ and Wai-Kwong Kwok‡ Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115, United States, and Materials Science DiVision, Argonne National Laboratory, Argonne, Illinois 60439, United States ReceiVed: September 14, 2010

Charge collection efficiency exhibits a strong influence on the overall efficiency of nanocrystalline dyesensitized solar cells. It highly depends on the quality of the TiO2 nanoparticulate layer in the photoanode, and hence most efforts have been directed on the improvement and deliberate optimization of the quality the TiO2 nanocrystalline layer. In this work, we aim to reduce the electron collection distance between the place of origin in the TiO2 layer to the electron-collecting TCO anode as an alternative way to enhance the charge collection efficiency. We use an array of metal micropillars on fluorine-doped tin oxide (FTO) as the colleting anode. Under the same conditions, the Ni micropillar-on-FTO-based dye-sensitized solar cells (DSSCs) exhibit a remarkably enhanced current density, which is approximately 1.8 times greater compared with the bare FTO-based DSSCs. Electron transport was investigated using the electrochemical impedance spectroscopy technique. Our results reveal that the electron collection time in Ni micropillar-on-FTO-based DSSCs is much shorter than that of bare FTO-based DSSCs, indicating faster electron collection due to the Ni micropillars buried in TiO2 nanoparticulate layer that serve as electron transport shortcuts. As a result, the charge collection efficiency was enhanced by 15-20% with respect to that of the bare FTO-based DSSCs. Consequently, the overall energy conversion efficiency was found to increase from 2.6% in bare FTO-based DSSCs to 4.8% in Ni micropillar-on-FTO-based DSSCs for a 6 µm-thick TiO2 NP film. Introduction Nanocrystalline dye-sensitized solar cells (DSSCs) are appealing devices for solar-electric energy conversion due to their potentially low cost and simple device processing. In a classically configured DSSC, the incident solar photons first penetrates the device through a piece of transparent conducting oxide (TCO) glass, on which a thick layer of interconnected semiconducting nanoparticles (NPs) such as TiO2 is coated to provide a large internal surface area for anchoring the light-harvesting dye molecules. Photoelectrons from excited states of the dyes are injected to the conduction band of the TiO2 NPs and subsequently collected by the TCO anode. Concurrently, the dye+ cations are reduced (regenerated) by redox mediators, traditionally via dye+ + I-f dye + I• and 2I• +I- f I3- in the electrolyte that permeates the TiO2 NP network. Through mass flow, the I3- diffuses back to the cathode, typically, a platinized TCO, on which I3- is reduced to I- using Pt as a catalyst to complete the circuit.1-3 The overall energy conversion coefficient of DSSCs is a product of the integrated efficiencies (with respect to the wavelength of incident photons) of three critical cell processes, including light harvesting efficiency (LHE), charge injection efficiency (CIE), and charge collection efficiency (CCE).1,4 The LHE is mainly determined by the optical and electronic nature of the dye molecules, as well as dye loading amount on the TiO2 NP layer. For a DSSC consisting of a 12-15 µm thick TiO2 NP layer (∼15 nm in diameter) loaded with commercial state-of-the-art dyes, e.g., N719 and N3 dyes, its LHE can approach nearly unity at the peak absorbance of the dye * To whom correspondence should be addressed. Email: [email protected]. † Northern Illinois University. ‡ Argonne National Laboratory.

molecules with assistance by a light-reflecting layer atop the TiO2 NP layer.5 CIE describes the injection efficiency of the photoelectrons from the excited dyes to the conduction band of the dye-anchored TiO2 NPs. In general, for N719 and N3 dyes, CIE also approaches unit in the majority of the dyes’ absorption spectra.4,6,7 The CCE, however, exhibits strong dependence on the quality of the TiO2 NP network. In the simplest linear model, CCE in a TiO2 NP-based DSSC can be approximately estimated as CCE ) 1 - τd/τn.8,9 Here, τn is the electron recombination time constant, i.e., electron lifetime in the TiO2 NP network, which is mainly associated with the kinetics and energetics of the redox shuttles; τd is the electron collection time constant and can be estimated as τd ) de2/D0,10-12 where d is the electron transport distance from its place of origin to the TCO collecting anode and D0 is the electron diffusion coefficient in the TiO2 NP network. D0 is mainly determined by the quality of the TiO2 NP network; thus, a high quality TiO2 layer with less defective sites gives rise to greater D0, leading to shorter τd, and consequently to greater CCE.13 In practical applications, however, various defects in the TiO2 NP network such as impurities, vacancies, lattice dislocations (crystal distortion), etc., either at the particle-particle interface, grain-grain boundary, and in the bulk of the particles, are inevitably present and can readily deteriorate D0 of the TiO2 NP film.14-17 This effect is inherent to the nature of the electron transport in the TiO2 NP network; that is, unlike the drift transport found in most solid-state photovoltaic cells, the strong coupling of the moving electrons with counterions in the electrolyte screens off any macroscopic electric fields necessary for drift transport.11,12,18 Except at the TCO/TiO2 interface (∼30 nm thick), where there is an interfacial built-in potential that can sweep photoelectrons from TiO2 to TCO,6,19,20 there is no

10.1021/jp108761k  2010 American Chemical Society Published on Web 10/15/2010

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Yang et al. walking electrons in the TiO2 NP layer so that they do not have to be collected by the planar TCO. The net effect is that the electron transport distance is shortened by the presence of these micropillars embedded in the TiO2 NP layer. Meanwhile, due to the microscale size of the micropillars, they only slightly decrease the transparency and the effective volume (thus the effective surface roughness) of the TiO2 NP layer necessary for dye loading. In this way, the inverse effect of the thickness of the TiO2 NP layer on LHE and on CCE can be resolved to some degree. As a proof-of-concept, we established metallic microarrays on a planar FTO (fluorinated tin oxide) glass as a photoanode to experimentally explore their effectiveness and to understand the basic science related with this strategy. Experimental Section

Figure 1. (a) Schematic diagram of the designed square array of micropillars (topview). (b) Schematic diagram of dye-sensitized solar cells with metallic Ni micropillars embedded in the TiO2 NP layer.

macroscopic electric field across most of the TiO2 NP network. Rather, the electron transport in a majority of the wet and illuminated TiO2 NP network takes place via ambipolar diffusion, which is driven by the concentration gradient of free electrons. Consequently, a moving electron, on average, encounters one million trapping/detrapping events at the defect sites in the TiO2 NP film21,22 and spends milliseconds up to seconds to percolate through the NP film prior to reaching the TCO anode, i.e., τd. Thus, the delay of electron transport by the trap states significantly favors the recombination of electrons in TiO2 with oxidizing species in the surrounding electrolyte, such as I3- and/or dye+. Therefore, uncertainty in the defectrelated trap states in the TiO2 NP layer has a profound effect on D0 and consequently on the CCE of the devices. As a matter of fact, only a handful of groups have reported DSSCs based on TiO2 NPs (12-15 µm thick) with overall device efficiency exceeding 10% using I-/I3- as a redox shuttle.5,23,24 In these reports, deliberate optimization of the quality of the TiO2 NP layer was critical for achieving high electron diffusion coefficient (D0 > 10-5 cm2/s) so that the CCE can reach above 90%.5,13,24 Hence, TiO2 NP layer with high quality is crucial for achieving similar efficiency comparable to the best reported DSSCs. On the other hand, however, it is worth mentioning that according to eq 2, the electron collection time, τd, has a greater dependence on de, i.e., square dependence, than on the electron diffusion coefficient D0.10-12 Thus, reducing de would be a more effective way to shorten the electron transport time in the TiO2 NP layer and thus to enhance the CCE. However, the challenge lies in the dilemma that shorter de inevitably requires thinner TiO2 NP film, leading to low dye-loading amount and consequently to low LHE.25-29 In other words, the thickness of the TiO2 NP layer has an inverse effect on LHE and CCE. Instead of focusing on incrementally improving the quality of the TiO2 NP layer, in this work, we use an alternative approach, that is, to shorten the electron transport distance in the TiO2 NP layer without causing significant loss in the roughness factor of the TiO2 layer. As illustrated in Figure 1, our strategy is to increase the interface area between the electroncollecting anode and the TiO2 NP layer by introducing an array of conducting micropillars on the planar TCO. This array of micropillars is in equipotential with the planar TCO. Hence, the extra anode-TiO2 interface, mainly contributed from the sidewalls of these micropillars, can help to collect the random-

Fabrication of Microarrays by Photolithography. The FTO glass (Solaronix) was cleaned in acetone under ultrasonication prior to the spin-coating of photoresist (Shipley 1827) at 2500 rpm for 40 s to form a ∼4 µm thick film. The photoresist-coated FTO glass was then soft baked at 110 °C for 1 min to evaporate off any excessive solvents. After prebaking, the desired photolithographic pattern, typically a square array of circular microwells (pattern area ) 0.7 cm × 0.7 cm) was obtained using a UV laser writer (Microtech LW405) and developed in developer (Shipley 351: DI water, 1v:3v) for one minute. Electrochemical Deposition of Nickel Micropillars. The growth of Ni microantennas was guided by two-electrode electrochemical deposition. The FTO glass with photolithographic pattern was used as a cathode and a graphite plate was used as a counterelectrode. Electrodeposition was carried out under room temperature at a cathodic potential of -1.8 V in Watts solution (a mixture of 300 g/L NiSO4.6H2O, 45 g/L NiCl2 · 6H2O, 45 g/L H3BO3, pH ) 4.5). Lift-off was carried out by immersing the sample in acetone for 5 min. The resulting slightly conic shaped Ni pillars are ascribed to the inevitable intensity gradient of UV light in the photoresist during the exposure. Pretreatment of the FTO Glass. To suppress the shunt leak current at the FTO/electrolyte and metal/electrolyte interfaces, a thin layer of about 20 nm Ti was deposited to all surfaces by magnetron sputtering. The samples were tilted at all angles during the sputtering deposition process to minimize the shadow effect so that the sidewalls of the pillars were also coated. This thin layer of Ti was converted to a compact layer of TiO2 by heating the substrate at 400 °C for one hour.30,31 Electrode Preparation and Device Assembling. The cathodes were prepared by dropping the precursor (5 mM H2PtCl6 in isopropanol) onto the FTO glass substrates and subsequently firing them at 400 °C in open air for 15 min. In a typical photoanode preparation process, a slurry solution of TiO2 nanoparticle was prepared by grinding a mixture of 150 mg TiO2 (anatase nanopowder, ∼25 nm, Aldrich), 450 µL of water, 50 µL of acetylacetone, and 2.5 µL of Triton X-100. The resulting paste was uniformly spread onto the entire FTO substrate including the areas with and without Ni micropillars. Scotch tape was used to define the area to be coated with TiO2 film. A doctor blade was used to scratch off excessive paste above the scotch tape, and the TiO2 paste film was vertically pressed with a press to ensure uniform thickness on all sample areas. The sample was then cut into two pieces, one with Ni micropillars and the other one without Ni micropillars (control sample). This pair of samples was dried at room temperature for 30 min prior to sintering at 450 °C for 60 min (temperature rising rate ) 1 °C/minute). This process yielded an ap-

TiO2 Nanoparticle-Based DSSCs proximately 6 µm thick TiO2 film confirmed by scanning electron microscope. After cooling the samples to 80 °C to minimize the rehydration of TiO2 due to the moisture in the air, the pair of photoanodes were immediately soaked overnight in a dye solution containing 0.3 mM cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(Π) bis-tetrabutylammonium (N719) in absolute ethanol in the dark. The samples were then rinsed with ethanol for 30 min to remove nonchemisorbed dye molecules and kept in the dark. The DSSCs were assembled by sandwiching the TiO2 NP coated TCO photoanode with the 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 4-tertbutylpyridine in 3-methoxypropionitrile) by capillary force. A black mask with a window area of 0.25 cm2 was applied on the photoanode side to define the same active area for both devices. Device Characterization and Measurement. The device structure was characterized using scanning electron microscopy (SEM, TeScan Vega II SBH). The J-V curves were measured by a potentiostat (Gamry Reference 600) at one Sun 1.5 a.m. G provided by a solar simulator (Photo Emission Inc. CA, model SS50B). The Gamry Reference 600 potentiostat was equipped with an EIS300 software to conduct the electrochemical impedance spectroscopy (EIS) study. The EIS spectra were obtained by applying open circuit voltage as forward bias potentials in a frequency range from 0.06 to 60 kHz with an AC amplitude of 10 mV. The dye loading amount was measured by immersing the sensitized photoanodes into excess KOH solution (3.00 mL, 10 mM KOH) for 30 min to desorb and completely deprotonate the dye.32 The absorbance of the resulting light pink solutions and three calibration solutions with known dye concentrations were measured under a UV-vis spectrophotometer (Ocean Optics Inc.). According to Beer’s law, the concentration of the target solution can be obtained by fitting the absorbance at 308 nm (with respect to blank solvent) to the calibration curve. Optical transmission of the photoelectrodes was also measured using the UV-vis spectrometer. Results and Discussion Figure 2a shows the SEM topview of a square array of metallic micropillars. The coverage of cylindrical metallic micropillars on FTO (x) is given by x ) π × d2/4(l + d)2, where d is the diameter of the pillars and l is the edge-to-edge distance of the micropillars (refer to Figure 1a). For our pattern with d ) 3 µm and l ) 10 µm, the Ni micropillar array occupies only ∼4.3% of the total FTO area, therefore causing only negligible loss in the optical transparency of the photoanode as shown in Figure 2 (b). Figure 2c is the SEM image obtained at a tilted angle to delineate the side of the Ni micropillars. Approximately 5 µm tall Ni micropillars were formed by filling up the 4 µm deep microwells followed by a small overgrowth out of the well to gain an extra ∼1 µm in height, which led to the formation of slightly hemispherical heads at the top of the pillars. Thus, by taking the side area of the pillars into account, the conducting surface area is estimated to be ∼1.4 times of the projected area. To minimize the shunt leak current at the FTO/electrolyte and/or metal/electrolyte interfaces, the entire FTO substrate including the areas with and without Ni micropillar arrays were all passivated by a thin and compact film of TiO2 as an electron

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Figure 2. (a) SEM topview image of square array of Ni micropillars on FTO glass. (b) UV-vis transmittance of a FTO with Ni micropatterns (red) in comparison to a bare FTO (blue). (c) Tilted SEM image of the Ni micropillars on FTO glass showing the side walls of the micropillars.

blocking layer by sputtering a thin layer of Ti, followed by its oxidation to form a compact TiO2 thin film as electron blocking layer.30,31 Next, a film of TiO2 NPs (diameter ∼25 nm) was homogeneously applied on the patterned FTO substrates. For the purpose of comparison, the area without micropillars on the same FTO substrate was also applied with the TiO2 NPs layer simultaneously to serve as the corresponding control samples. The TiO2 paste was vertically pressed by a flat surface to ensure the uniformity of the thickness of the TiO2 NP layer. Figure 3a is a low magnification SEM image showing the cross-section of the sintered film by breaking the substrate. Note that the burrs on the edge are unavoidable due to the fracture of the film when it was broken. Nonetheless, it can be seen that the thickness of the TiO2 layer is quite uniform and is about 6 µm thick, comparable to the height of the micropillars. Figure 3b shows an SEM image of a section in the absence of the burrs, which clearly shows the good uniformity of the thickness of the TiO2 NP film. Figure 3c shows the magnified SEM cross-section image of a Ni micropillar buried in a ∼6 µm thick TiO2 NP layer. DSSCs were constructed by anchoring N719 dye molecules on the TiO2 NPs, followed by the assembly of spacer, I-/I3- electrolyte and Pt-FTO as cathode. Figure 4 demonstrates the best J-V characteristics for the prepared DSSCs with Ni micropillars embedded in the TiO2 NP layer obtained under one Sun (AM 1.5 G). For a fair comparison, a conventional DSSC, i.e., a sample without micropillars that was simultaneously prepared on the same FTO substrate was also measured. Table 1 summarized the key parameters of the cells, including open circuit voltage (Voc),

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Figure 3. (a) Cross-section SEM image of the Ni micropillars buried in a TiO2 NP layer with uniform thickness of ∼6 µm; (b) Magnified cross-section SEM image of the TiO2 layer showing the uniform thickness. (c) Magnified cross-section SEM image of an individual Ni micropillar in the TiO2 layer.

Figure 4. JV curves of DSSCs with and without Ni micropillars on FTO under AM 1.5 G illumination. The area of both devices is 0.25 cm2.

TABLE 1: Average Photovoltaic Parameters of DSSCs based on Eight Pairs of Samples, Including Ni Micropillars on FTO-Based DSSCs and Their Corresponding Bare FTO-Based DSSCs with Equal Dye Loading Ni micropillars bare FTO

Jsc (mA/cm2)

Voc (mV)

FF

η (%)

12.0 ( 0.60 6.4 ( 0.40

700 ( 15 705 ( 15

0.59 ( 0.04 0.61 ( 0.03

4.9 ( 0.2 2.7 ( 0.2

short-circuit current (Jsc), energy conversion coefficient (η), and fill factor (FF). It is clear that the samples with Ni micropillars exhibit a significantly higher current density than the corresponding control samples by a factor of 1.8 in Jsc. There is no enhancement in Voc, indicating that the energetics of the device is not affected by these Ni micropillars. As a result, for 6 µm-thick TiO2 NP films, the average energy conversion efficiency was found to increase from 2.6% in bare FTO-based DSSCs to 4.8% in Ni micropillar-on-FTO-based DSSCs (Table 1). When benchmarked with the highly optimized TiO2 nanocrystallinebased DSSCs with similar thickness,28,29 our Ni micropillaron-FTO-based DSSCs exhibit even better efficiencies. Additionally, Figure 4 shows that the dark current of the Ni micropillar-on-FTO-based DSSC exhibits a slightly lower onset voltage than that of the corresponding control sample, namely the bare FTO-based DSSCs. This effect is likely due to the more surface roughness of the FTO with Ni micropillars, which increases the chance of back electron transfer through anode/

Figure 5. Equivalent circuit used to fit the impedance measurements on the DSSCs. Rs is the series resistance. RF and CF are the charge transfer resistance and corresponding capacitance at the interface between uncovered FTO layer and electrolyte. RFT and CFT are the resistance and interface capacitance at the FTO-TiO2 contact. W is the electrolyte diffusion impedance. Rpt and Cpt are the charge transfer resistance and interface capacitance at the Pt counter electrode-redox electrolyte. An extended distributed element (DX) is assigned to present the interfacial charge separation across a heterogeneous junction in the porous film in the DSSCs, in which Rtr and Rct are the resistances for electrons transport through TiO2 within the film, charge recombination process between electrons in the TiO2 nanoparticles, and holes in electrolyte, and Cµ is the chemical capacitance, respectively.

electrolyte interface. Therefore, high-quality passivation of the surface of Ni micropillars as well as the supporting FTO by a compact TiO2 layer is important for obtaining devices with good performance. To identify the causes of the improved performance in the DSSCs using Ni-micropillars-on-FTO as the photoanode, we first conducted the dye loading measurement on the photoelectrodes. The measured dye loading amounts for the Ni-micropillars-on-FTO-based DSSCs is around 1.1 × 10-7 mol/cm2, while the dye loading for the bare FTO-based DSSCs is about 1.3 × 10-7 mol/cm2. The slightly less dye-loading amount in the Ni-micropillars-on-FTO-based DSSCs than that in the bare FTO-based DSSCs is perhaps due to the volume occupied by the Ni micropillars. Nonetheless, the slight variation in dyeloading amount in the two kinds of samples indicates that dyeloading amount does not account for the relatively enhanced current density found in the Ni-micropillars-on-FTO-based DSSCs. Since there is no significant discrepancy in the dye-loading amount between the corresponding two kinds of samples, we assume that LHE and CIE are nearly identical. Therefore, the enhanced Jsc in the presence of Ni micropillars is likely due to the high CCE in the Ni-micropillars-on-FTO-based DSSCs. Thus, we further investigated the time constants of the two kinds of DSSCs using electrochemical impedance spectroscopy (EIS). EIS is an effective technique for elucidating the competition between the electron lifetime, (i.e., recombination kinetics of electrons in TiO2 NP network with oxidizing species in the surrounding electrolyte, e.g., I-3 and dye+) and the electron diffusion kinetics to the collecting TCO anode.26,33,34 As a useful point of reference, the most efficient (i.e., 10-11% power efficiency) DSSCs to date have been characterized using EIS. Similar to this and other reports.26,35,36 A geometrically appropriate equivalent circuit of our DSSCs, which contains three interfaces formed by TCO (with or without Ni pillars)|TiO2,TiO2|dye|electrolyte and electrolyte|Pt-FTO is illustrated in Figure 5. The FTO|Ni junction is not treated as a capacitive interface because FTO are highly doped n-type semiconductors, and they are generally considered as metals. Thus, the Ni pillar and FTO are in equipotential, and they are considered as a whole component. The FTO|TiO2 interface is denoted by RFT and CFT.

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Figure 6. Nyquist plots of representative EIS data at 650 mV forward bias in the dark condition (a) for Ni micropillar-FTO-based DSSC (red, closed circle) and bare-FTO-based DSSC (blue, closed triangle) and their magnified part at high frequency (b) for Ni micropillar-FTO-based DSSC (red, closed circle) and bare-FTO-based DSSC (blue, closed triangle).

Figure 7. (a) Charge lifetime (τn) and charge collection time (τd) in a Ni micropillars-on-FTO-based DSSC (red) and a bare FTO-based DSSC (blue). (b) Charge collection efficiency at different bias voltage for Ni micropillar-on-FTO-based DSSC (red) and for bare-FTO-based DSSC (blue).

The circuit also considers possible communication between the FTO anode (with or without Ni pillars) with electrolyte as depicted by RF and CF. Rs is a lumped series resistance for the transport resistance of FTO and all resistances out of the cell. Rpt and Cpt are the charge transfer resistance and interfacial capacitance at the platinized-FTO|electrolyte interface, respectively. W is the impedance of diffusion of the redox species in the electrolyte. An extended distributed element (DX) is assigned to represent the interfacial charge separation across a heterogeneous junction in the TiO2 NP network, in which rtr is the resistance for electrons transport through each TiO2 NP; rct is the microscopic charge transfer resistance, reflecting the resistance against the charge recombination events from the phase of high (in TiO2) to the phase of low Fermi levels (in electrolyte); Cµ is the chemical capacitance,37 accounting for the energy storage by virtue of carrier injection, respectively. A representative Nyquist plot, Figure 6, shows the characteristic impedance features of a Ni micropillars-on-FTO-based DSSC at -650 mV forward bias (defined as when the FTO electrode is negatively biased and the counter platinized FTO electrode is positively biased). The impedance spectra at voltage between -0.7 and -0.3 V (this region correspondingly reflects the region of about 0-0.4 V in the J-V curve under illumination) are fitted based on the suggested equivalent circuit according to Bisquert’s diffusion-recombination model. Good fits to the equivalent circuit model were obtained at all applied potentials investigated. Estimated parameters of the electron transport in the device were extracted from the Nyquist plots according to the procedure proposed by Adachi et al.26 The lowfrequency semicircle in Figure 6a is assigned to the macroscopic charge transfer resistance, Rct related to the macroscopic recombination of electrons (Rct ) rct/L) at the TiO2|electrolyte interface, where L is the thickness of the TiO2 layer. The very small semicircle magnified in Figure 6b at high frequency is

assigned to the resistance of Pt and capacitance Cpt of the electrolyte|Pt cathode interface. In addition, the short linear section at middle frequency reflects the macroscopic electron transport resistance Rtr (Rtr ) rtrL) in TiO2.37,38 If the capacitance Cµ is taken to be strictly “chemical” in nature (reflecting density of states), it is rational to assume a multiple trapping diffusion interpretation in which τd ) RtrCµ and τn ) RctCµ.34 Derived from the EIS measurements, Figure 7(a) exhibits the comparison in electron collection time (τd) and electron lifetime (τn) for a Ni-micropillars-on-FTO-based DSSC and its corresponding bare FTO-based DSSC. Several features of the charge dynamics in the two kinds of devices are noteworthy in Figure 7a. First, a roughly linear trend in τd and τn with increasing applied potential is observed, in agreement with the results reported by Hsiao et al.39 Second, the similar charge lifetime (τn) were observed for both the Ni-micropillarson-FTO-based DSSC and its corresponding bare FTO-based DSSC. This agrees with the fact that electron lifetime is mainly determined by the kinetics of the redox species. Therefore, the observation of similarity in τn for both kinds of samples is reasonable since both samples used the same I-/I3- electrolytes. It is worthwhile to note that Figure 7a also shows that τd for the Ni-micropillars-on-FTO-based DSSC is notably faster than its corresponding bare FTO-based DSSC, by a factor of 2-8, depending on the applied voltages. Furthermore, CCE under applied bias was calculated based on a simple linear model, that is, CCE ) 1 - τd/τn, and the results are plotted in Figure 7b. On average, CCE of the Nimicropillars-on-FTO-based DSSC is enhanced by 15-20% with respect to that of the corresponding bare FTO-based DSSC. It is thus clear that the addition of the Ni micropillars on a planar FTO provide more collection area inside the nanoparticulate layer than a bare FTO. Hence, electrons in TiO2 do not have to percolate all the way to the planar FTO if they can be collected midway to the FTO by the protruding micropillars.

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Conclusions In summary, this study suggests that enhancing the roughness of electron-collecting TCO anode using an array of metallic micropillars is an effective approach to reduce the electron collecting distance in the TiO2 NP network without a significant loss of dye-loading amount. Moreover, the reduced electron collecting distance effectively improves the electron collection efficiency of the trap-limited electron diffusion in the TiO2 nanoparticulate layer. In perspective, enhanced electron collecting kinetics via our strategies may potentially allow the use of a diverse range of alternative redox systems possessing faster reduction kinetics but less overpotential (a necessary driving force for dye regeneration under slow kinetics but a major energy loss in current state-of-the-art DSSCs) than traditional I-/I3- so as to attain higher Voc. In addition, taller, slimmer, and therefore denser pillars may further enhance the charge collection efficiency, which can possible be obtained by using negative photoresists. Acknowledgment. We thank the U.S. Department of Energy for financial support under Contract No. DE-AC02-06CH11357 and the NIU-Argonne Graduate NanoScience Fellowship through InSET. We also acknowledge help from Dr. Ralu S. Divan at Center for Nanoscale Materials, Argonne National Laboratory. T.X. is grateful for the stimulating discussions with Dr. Alex B. F. Martinson at the Materials Science Division, Argonne National Laboratory. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740. (2) Gra¨tzel, M. Nature 2001, 414, 338–344. (3) Hamann, T. W.; Jensen, R. A.; Martinson, A. B. F.; van Ryswyk, H.; Hupp, J. T. Energy EnViron. Sci. 2008, 1, 66–78. (4) Nazeeruddin, M. K.; Kay, A. R. I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (5) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gratzel, M. G. J. Am. Chem. Soc. 2005, 127, 16835–16847. (6) van de Lagemaat, J.; Park, N.-G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044–2052. (7) Franco, G.; Gehring, J.; Peter, L. M.; Ponomarev, E. A.; Uhlendorf, I. J. Phys. Chem. B 1999, 103, 692–698. (8) Schlichtho¨rl, G.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 1999, 103, 782–791. (9) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69–74.

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