Doping a TiO2 Photoanode with Nb5+ to Enhance Transparency and

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J. Phys. Chem. C 2010, 114, 15849–15856

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Doping a TiO2 Photoanode with Nb5+ to Enhance Transparency and Charge Collection Efficiency in Dye-Sensitized Solar Cells Aravind Kumar Chandiran,† Fre´de´ric Sauvage,*,† Montse Casas-Cabanas,‡ Pascal Comte,† S. M. Zakeeruddin,† and Michael Graetzel*,† Laboratoire de Photonique et Interfaces, Institut des Sciences et Inge´nierie Chimiques, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland, and Laboratoire CRISMAT/ ENSICAEN, 6 Bd du Mare´chal Juin, 14050 Caen, France ReceiVed: July 1, 2010; ReVised Manuscript ReceiVed: August 10, 2010

The optoelectronic properties of our benchmark nanocrystalline anatase TiO2 photoanode were modified by means of aliovalent doping with Nb5+. Even for a low doping level, the charge collection efficiency can be noticeably improved as a result of a higher electron lifetime when using the heteroleptic Ru(+II) C101 dye. Particularly of interest, while this was only possible by adding additives in the electrolyte, the doping concentration allows tuning of the energetic of the trap state distribution; parameter particularly crucial for the injection rate and charge collection efficiency. This improvement brings the power conversion efficiency of a 7 µm thick transparent photoanode to 8.7% while intensifying the electrode’s transparency. Introduction Owing to its high chemical stability, low toxicity, and ideal position of the conduction band edge, the anatase polymorph of TiO2 gathers intensive research attention for photoelectrochemical applications spanning from photocatalysis and water splitting1-6 to dye-sensitized solar cells (DSC).7 In addition, the particular electronic structure of TiO2, where the Ti4+ adopts a 3d0 configuration, paves the way toward a broad multifaceted defect chemistry since this latter can be strongly influenced by the introduction of new donor levels into the band gap either by doping, by substitution of the cationic site with a higher d electron count, or by the formation of oxygen vacancies. Among the different possibilities to alter the intrinsic TiO2’s optoelectronic properties, doping by Nb5+ has rapidly raised interest as this element exhibits a similar ionic radius as Ti4+ (r(Ti4+) ) 0.605 Å vs r(Nb5+) ) 0.64 Å)8 while forming strongly hybridized 4d orbitals with the 3d orbitals of Ti.9 This particularity is at the origin of the discovery made by Furubayashi et al., who credited the Nb-doped anatase TiO2 to be a plausible lowercost candidate to ITO as a new TCO by reporting roomtemperature resistivity in the range of 2-3 × 10-4 Ω · cm while transmitting more than 90% of the visible light.10 In DSC, the TiO2 bears three major roles, (i) to provide a substrate for the dye monolayer self-assembly, (ii) to accept electrons from the chromophore’s excited state, and (iii) to transport the electron toward the front current collector. The particle size, morphology, and aggregation are the parameters that directly influence the dye loading, I3-/I- mass transport within the mesopores, and also the internal light containment, where a high electrode haze seems to be beneficial to reach maximal photovoltaic performances.11,12 The DSC principle, demonstrating above 11% certified power conversion efficiency (PCE),12 rests on a sequence of charge-transfer steps occurring on an extended time scale ranging from femtoseconds to seconds for charge injection * To whom correspondence should be addressed. E-mail: frederic.sauvage@ epfl.ch (F.S.); [email protected] (M.G.). † Ecole Polytechnique Fe´de´rale de Lausanne. ‡ Laboratoire CRISMAT/ENSICAEN.

and tri-iodide diffusion, respectively.13,14 Within the millisecond time domain, two opposing electron paths are in rivalry, therefore affecting the charge collection efficiency, the charge transport toward the current collector and the charge recombination with an oxidized species (Ru+III and/or I3-).15 This work aims at modifying the intrinsic optoelectronic properties of the anatase TiO2 in order to promote the charge transport in the transparent layer while strengthening its transparency, a factor particularly noteworthy for tandem cells, bifacial application, or DSC panels integrated on smart windows. For this, our approach was to incorporate Nb5+ in the anatase lattice to create donor intermediate levels below the conduction band. Besides the description of a synthetic method to effectively dope the benchmark mesoscopic anatase TiO2 photoanode by a low concentration of niobium, we also report on the relationship between niobium content and photovoltaic characteristics. For this, we have used the new heteroleptic C101 dye (Na-cisRu(4,4′-(5-hexyltiophen-2-yl)-2,2′-bipyridine)(4-carboxylic-acid4′-carboxylate-2,2′-bipyridine) (thiocyanate)2), which displays a higher molar extinction coefficient and red-shifted MLCT as compared to the benchmark N719 or Z907 dyes.16 The doping effect on trap state distribution, transport, and recombination rates will be discussed on the basis of photocurrent and photovoltage transient decay analysis. Experimental Section (a) Synthesis of TiO2 and Nb-Doped TiO2. Titanium isopropoxide (97%), niobium pentachloride (99.9%), and terpineol were obtained from Aldrich. Acetic acid, nitric acid (65%), ethyl cellulose (viscosity: 5-15 mPa · s; 30-50 mPa · s), and ethanol were purchased from Fluka. All reagents and solvents were used as received. An equimolar (0.2 mol) proportion of acetic acid (12 g) was added to titanium isopropoxide (58.6 g) under constant stirring. The dopant precursor, corresponding to a level of 0.5, 1, or 2% of the doping, was added dropwise under stirring. The intermediate product was then transferred into a conical flask containing 350 mL of water. A white precipitate was formed immediately due to hydrolysis of titanium isopropoxide. The solution was kept under vigorous

10.1021/jp106058c  2010 American Chemical Society Published on Web 08/31/2010

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Figure 1. Superposition of the X-ray diffractograms for undoped TiO2 and the different Nb-doped TiO2 materials. The variation of the cell parameters as a function of dopant concentration is shown in the inset.

stirring for 1 h. The particles were peptized using 4 mL of concentrated HNO3 (65%) and the solution heated to 78 °C for 90 min. Prior to hydrothermal reaction, this latter was concentrated to 150 g using a rotovap. The solution was then autoclaved at 250 °C for 12 h (reactor’s volume: 210 mL). After cooling down to room temperature, 1 mL of concentrated HNO3 was again added to the colloidal solution and then dispersed using a titanium probed ultrasonicator for 2 min that was programmed to follow the sequence of 2 s pulse and 2 s of waiting time. This process was repeated three times with alternate stirring to attain homogeneity in the particle dispersion. The suspension was again concentrated to 20% by weight of TiO2 while the remaining water, alcohols, and nitric acid were removed by centrifugation in ethanol. The procedure for screen printing paste preparation is detailed in ref 17. (b) Material Characterization. The structural characteristics of TiO2 and different Nb-doped TiO2 samples were analyzed by X-ray diffraction using a Bruker D8 discover diffractometer in a (θ-2θ) configuration with Cu KR1 radiation (λ ) 1.54056 Å). The surface area and the porosity of the films were evaluated by the BET method using the N2 gas sorption technique at 77 K in a Micrometrics ASAP2000 apparatus. Prior to the measurement, the sample was degassed at 250 °C under vacuum for 4 h. The thickness of the printed semiconductor films was measured using a KLA Tencor alpha-step 500 surface profiler. The optical properties of TiO2 and Nb-TiO2 films were evaluated using the Cary 5 UV-visible-NIR spectrophotometer. SEM micrographs and EDX quantifications were done on screen-printed films of TiO2 and Nb-TiO2 using an FEI XLF30FEG microscope. The valence state of the metal ions and their atomic concentrations were probed using X-ray photoelectron spectroscopy (XPS/ESCA KRATOS AXIS ULTRA). The amount of dye uptake on the semiconductor films was measured by dye desorption in DMF containing tetrabutyl ammonium hydroxide (the film surface area was 0.159 cm2). The absorbance of the resulting solution was measured by UV-visible spectrophotometry (model: Hewlett-Packard 8452A diode array spectrophotometer). (c) Device Fabrication. The photoanode films were prepared by screen printing onto NSG10 FTO glass. Prior to screen printing, the glass was chemically treated in a 40 mM TiCl4 solution at 70 °C for 30 min. The mesoporous films were sensitized with the new heteroleptic ruthenium polypyridyl C101

complex for 14 h at 4 °C. The dye solution was composed of 300 µM dye and a 1/4 ratio of dineohexyl phosphinic acid (DINHOP) dissolved in an equivolume of acetonitrile and tertbutanol. After sensitization, the electrode was washed with acetonitrile and assembled. The counter electrode was made of TEC15 glass coated with Pt using a 5 mM H2PtCl6 solution and thermal treatment to 410 °C for 15 min. The two electrodes were assembled using a 25 µm thick Surlyn polymer film. The electrolyte used was composed of 1 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butyl pyridine, and 0.1 M GuNCS in a 85% acetonitrile and 15% valeronitrile mixture. This latter was injected by vacuum backfilling through a hole, sand blaster at the side of the counter electrode. (d) Photovoltaic Characterization. A 450 W xenon lamp (Oriel, U.S.A.) with an intensity of 100 mW · cm-2 was used as a light source. The spectral output of the lamp was filtered using a Schott K113 Tempax sunlight filter (Pra¨zisions Glas & Optik GmbH, Germany) to reduce the mismatch between the simulated and actual solar spectra to less than 2%. The current-voltage characteristics of the cell were recorded with a Keithley model 2400 digital source meter (Keithley, U.S.A.). The photoactive area of 0.159 cm2 was defined by a black metal mask. Incident photon-to-current conversion efficiency measurements were realized using a 300 W xenon light source (ILC Technology, USA). A Gemini-180 double monochromator Jobin Yvon Ltd. (U.K.) was used to select and increment wavelength radiation to the cell. The monochromatic incident light was passed through a shutter set at 1 Hz frequency. Monochromatic light was superimposed by a white light bias corresponding to 5% solar intensity. (e) Determination of Electron Recombination, Transport, and the Distribution of Trap States. The electron recombination and transport in the TiO2 semiconductor film was measured by transient photovoltage and photocurrent decay measurements, respectively. The white light was generated by an array of LEDs, while a pulsed red light (0.05 s square pulse width) was controlled by a fast solid-state switch to ensure light perturbation. The current and voltage decays were recorded on a Macinterfaced Keithley 2602 source meter. Results and Discussion (a) Structural and Physical Characterization. Figure 1 compares the X-ray diffractogram collected for the standard

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Figure 2. SEM micrographs showing (a) TiO2 and (b) 2% Nb-doped TiO2 particles.

TiO2 and the various Nb-doped TiO2 samples synthesized until a 2% atomic concentration of niobium in initial solution. Regardless of initial stoichiometry, the anatase crystal structure is maintained without evidence of niobium oxide phases crystallized as part as TiO2. We only noted a minor presence of the rutile polymorph in the undoped material, marked out by the extra reflection at 27.4° corresponding to the (110) plane. However, it is interesting to note that the addition of NbCl5 along titanium isopropoxide prevents the formation of rutile while the initial pH of the solution is unaffected. Ahmad et al. reported a similar observation when using Nb and Sc as the dopant18 and provided an explanation on the basis of an augmentation of the activation energy for the anatase to rutile phase transformation when TiO2 becomes doped. Using Fullprof software,19 we have refined the lattice cell parameters as a function of the amount of Nb in the lattice (Figure 1 inset). Note here that we have reported the dopant concentration successfully introduced in the lattice as determined by EDX spectroscopy. In reality, the EDX analysis pointed out a noticeable discrepancy in the Nb/Ti content of about 30-40% compared to the initial amount of dopant introduced in solution. The remaining stayed unreacted in solution. The linear evolution of the lattice cell parameters as a function of the niobium content verifies Vegard’s law.20 This testifies to the successful incorporation of the dopant in the lattice and to its good solubility since its limit has not been reached in that range of dopant concentrations. The refined lattice cell parameters are a ) 3.7856(1) Å and c ) 9.5013(3) Å for our standard TiO2. This is in good agreement with the values reported in the literature.21,22 The incorporation of Nb induces a slight expansion of the lattice in both directions. For comparison, the parameter refined for the 2% Nb-doped sample attains a ) 3.7888(3) Å and c ) 9.5052(8) Å. This evolution confirms the observations of Y. Furubayashi et al.,23 which could be attributed to the larger ionic radius of Nb5+. On the other hand, the growth of the particles is affected by the doping. It entails a modification of the morphology of the particles and a reduction of their size (Figure 2). In agreement, the diffractograms feature a slight broadening of the diffraction peaks upon doping. At first approximation using the empirical Scherrer equation, we have calculated the crystallite size along the two perpendicular [200] and [004] directions. For TiO2, this estimate yields a size of ∼25 and 15.4 nm, respectively (Figure 3). At a low Nb dopant level, the crystallite size decreases relatively linearly before stabilizing at 20 and 11.4 nm for the higher doping levels. This suggests a lengthening of the particles from the aspect ratio. We pursued this

characterization in more detail on the two end members (TiO2 and 2 mol % Nb-TiO2) by analyzing the whole powder pattern fitting without instrumental and microstrain broadening effects. After subtraction of the instrumental contribution to line broadening (obtained from an Al2O3 standard, U ) 0.065963 deg, V ) -0.058749 deg2, W ) 0.014681 deg2), the profile of the peaks was modeled with analytical approximations that have proven to be very useful to treat anisotropic and strain broadening.24 Size broadening was thus modeled with linear combinations of spherical harmonics and strain broadening with a quartic form in reciprocal space. With this method, the average crystallite sizes were found to be 13 nm along the [200] direction and 16 nm along the the [004] direction of TiO2, while 11 and 14 nm were calculated for 2 mol % Nb-TiO2. These values are lower than those obtained with the Scherrer equation but more reliable since the contributions of instrumental and microstrain origin to line broadening were herein considered. With the obtained results, the average morphology of the particles was reconstructed with the GFourier program25 (Figure 3). The 2% Nb-doped TiO2 sample was found to exhibit a more elongated and bulged elliptical shape in comparison to the undoped sample. A small amount of microstrain was also found in both samples, that is, 1.3 × 10-3 for the nondoped sample and 2.0 × 10-3 for the 2% Nb-doped sample (expressed in terms of ∆d/d, d being the interplanar distance). The more significant amount of microstrain for this latter indicates larger fluctuations in cell parameters and is most likely a consequence of doping. This morphological/textural modification therefore comes with an increase of the BET surface area based on N2 desorption from 78 to 113 m2/g with a type-IV isotherm characteristic of mesopores. The obtained values for surface area, pore size, and film porosity are tabulated before and after the TiCl4 posttreatment (Table 1). This post-treatment leaves nanocrystals of 3-4 nm in size on the film, which reduces noticeably the electrode’s surface area, most likely as a result of a pore clogging up deposition. The doping also decreases the pore size of the electrode from 24 to 18 nm, with the exception of the 2% Nb sample, which contains aggregated particles. In additional to this textural aspect, the incorporation of niobium modifies not surprisingly the optical band gap of the material. It results in the color modification of the obtained powder, which turns from white to blue, subsequent to the hydrothermal process. This color shift is attributed to the formation of intermediate donor levels below the conduction band.26 However, we experienced this state to be metastable, that is, the color of the powder turned back to white either by heat treating the material to 500 °C (step required to remove

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Figure 3. Evolution of the crystallite size as a function of doping level along the [200] and [004] directions. The simulated shape of the particles from X-ray refinement is given for TiO2 and 2% Nb-doped TiO2.

TABLE 1: Evolution of Particle Size, Porosity, and BET Surface Area for TiO2 and Nb-Doped TiO2, before and after TiCl4 Post-Treatment sample TiO2 0.5Nb-TiO2 1Nb-TiO2 2Nb-TiO2

BET surface area (m2/g)

porosity (%)

pore size (nm)

77.6 70.1 90.6 75.6 101.1 77.3 113.1 94.4

69.5 56.9 67.2 58.0 67.0 54.4 69.2 62.7

24.1 23.9 23.1 18.4 18.4 18.5 22.9 18.8

no TiCl4 post treatment TiCl4 post-treatment no TiCl4 post-treatment TiCl4 post-treatment no TiCl4 post-treatment TiCl4 post-treatment no TiCl4 post-treatment TiCl4 post-treatment

terpineol and ethyl cellulose from the screen-printed paste and prior to sensitization) or leaving the material under air for about six months. The refinement of the XRD patterns before and after such treatment shows (i) no phase separation and (ii) an increase of the lattice parameters for both cases, which could be explained by an oxygen uptake in the lattice. This latter statement is consistent with the elemental analysis carried out by XPS, which shows a noticeable increase of oxygen content (O/(Nb + Ti) ratio ) 2.00 f 2.14). Figure 4 shows the absorption edge of the screen-printed films for TiO2 and the Nb-doped TiO2 samples. The representation (Rhν)0.5 as a function of energy highlights an upshift of the absorption edge from 3.22 to 3.28 eV ascribed to the Burstein-Moss effect.27,28 This higher band gap together with the smaller particle size offers a greater electrode transparency (Figure S1, Supporting Information). To gain better insight into the doping mechanism, because the latter is particularly dependent on the synthetic procedure and the type of precursors, we have analyzed the XRD diffractograms recorded by Rietveld refinement and also used XPS spectroscopy to probe the Ti-Nb valence states. The Rietveld refinements on the two end members were carried out using the FullProf program19 by means of the pseudo-Voigt profile function of Thompson, Cox, and Hastings.29 Cell

dimensions, atomic positions, isotropic thermal displacements, scale factor, zero points, and profile parameters were refined. Background points were modeled using a six-coefficient polynomial function. In the starting model of the 2% Nb-TiO2 sample, it was assumed that niobium replaces titanium in the structure, as already suggested in the literature; therefore, atomic occupancies for niobium and titanium were constrained to the theoretical values. Despite that no convergence was reached when their relative occupancies were left to vary due to the low amount of niobium in the sample, the structural model fits the data well, as shown in the Rietveld refined profile of Figure S2 (Supporting Information). Refinement details for both samples are shown in Table 2, and the resulting M-O distances are gathered in Table 3. The M-O atomic distances are somewhat larger in the 2% Nb-TiO2 sample, according to the larger ionic radius of Nb5+ (0.64 Å), when compared to that of Ti4+ (0.605 Å), further confirming the replacement of titanium by niobium in the structure. Note that refinements carried out on other positions for Nb5+ did not yield good convergence. The XPS spectra for both the pristine and doped photoanode samples consist of a Ti 2p transition at 458.7 eV with ∆E(2p1/2-2p3/2) ) 5.8 eV characteristics of the profile of a Ti4+ valence state.30 The niobium is present uniquely in its pentavalent state (Nb 3d5/2 and 3d3/2 transitions at 207.5 and 210.2 eV,

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Figure 4. An (Rhν)0.5 as a function of energy plot for TiO2 and Nb-doped TiO2 samples. The band gap for different samples are expressed from the x-intercept.

TABLE 2: Rietveld Refinement Details For TiO2 and 2% Nb TiO2 Samples I 41/a m d; a ) 3.78646(5) Å; c ) 9.5026(1) Å; Rp: 8.67; Rwp: 7.33; Rexp: 3.07; χ2: 5.70

TiO2 Atom type

Wyckoff site

x

y

z

B (Å2)

Ti O

4b 8e

0 0

1/4 1/4

3/8 0.16753 (9)

0.46 (1) -0.09 (3)

I 41/a m d; a ) 3.78888(1) Å; c ) 9.5095(1) Å; Rp: 10.3; Rwp: 8.77; Rexp: 4.62; χ2: 3.60

2% Nb TiO2 atom type

Wyckoff site

x

y

z

B (Å2)

occupancy

Ti Nb O

4b 4b 8e

0 0 0

1/4 1/4 1/4

3/8 3/8 0.1677(1)

0.57 (2) 0.57 (2) -0.10 (4)

0.98 0.02 1.00

TABLE 3: Atomic Distances for TiO2 and 2% Nb TiO2 Samples TiO2

distance (Å)

2% Nb TiO2

distance (Å)

Ti-O

1.9359(2) × 4 1.971(1) × 2

Ti/Nb-O

1.9374(2) × 4 1.972(1) × 2

respectively).31 Note a very small shoulder on the Ti 2p transition for the unsintered 2% Nb-TiO2 sample, which could be attributed to a small presence of Ti3+. By contrast to EDX, the elemental analysis conducted by XPS on the 2% Nb-TiO2 sample indicates the accurate Nb/Ti ratio of 2 at the surface, while only 1.4% was measured by EDX. These conflicting results can be reconciled if a gradient of Nb concentration from the shell to the core of the particles occurs. This possibility has already been evoked and discussed by Ruiz et al.32 On the basis of these results, using our synthetic procedure, we hypothesize that the supplementary charge introduced into the lattice by doping with Nb5+ is compensated by the creation of Ti vacancies accordingly to the following Kro¨ger-Vink notation

decline to 14.4 mA/cm2. The fill factor decreases upon addition of niobium from 74.6 to 69%. All of cell characteristics are gathered in Table 4. The increased current for low doping level is consistent with the IPCE comparison between undoped TiO2 and the 0.5% doping level, where this latter shows superior conversion efficiency between 390 to 650 nm (Figure 6). Interestingly, there is a domain corresponding to a low level of doping which appears particularly beneficial as the short-circuit density can be greatly enhanced while the Voc is not noticeably

1 ′′′′ 5+ x • Nbsolvated + TiTi (TiO2) f NbTi + VTi (TiO2) 4

[

]

(b) Photovoltaic Characteristics. Figure 5 shows the remarkable influence of doping exerted on the (J-V) characteristics of the photoanode sensitized with C101 dye. Increasing the concentration of Nb5+ gives rise to an important drop of VOC from 721 to 656 mV and an increase of JSC from 13.6 to a maximum of 15.2 mA/cm2 for a 0.5% doping level before a

Figure 5. (J-V) curves measured under 1 equiv. of sunlight illumination (AM 1.5G) for TiO2 and Nb-doped TiO2 photoanodes sensitized with C101 dye.

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TABLE 4: Evolution of Cell Characteristics for Standard TiO2 and Nb-doped TiO2 Films, with and without TiCl4 Post-Treatment TiO2

0.5Nb-TiO2

1Nb-TiO2

2Nb-TiO2

sample

no TiCl4 treatment

TiCl4 post treatment

no TiCl4 treatment

TiCl4 post treatment

no TiCl4 treatment

TiCl4 post treatment

no TiCl4 treatment

TiCl4 post treatment

VOC(mV) JSC(mA/cm2) ff (%) η (%)

721.4 13.6 74.6 7.4

740.2 15.3 73.2 8.4

721.0 15.2 73.5 8.1

735.4 16.3 71.5 8.7

674.6 15.0 72.9 7.6

718.4 16.0 67.8 8.0

656.4 14.4 69.0 6.6

697.4 16.0 61.9 7.1

affected. This enables improvement of the PCE from 7.4 to 8.1% before a decline to 7.6% and then 6.6% for 2Nb-TiO2. In order to rule out any textural effect, we have compared those results with 17 nm based undoped TiO2 particles, which display a similar BET surface area, pore size, and porosity as our Nbdoped TiO2. For this sample, the PV characteristics were JSC ) 14.2 mA/cm2, VOC ) 733.6 mV, and ff ) 76.8%, leading to η ) 8.0%. The observed trend is thus not a result of the smaller size of the particles. To these values, the TiCl4 post-treatment of the photoanode also provides a positive effect on doped TiO2 samples. It affords a raise of the PCE to a maximum of 8.7% (vs 8.4% in the case of undoped TiO2) as a result of both an increase of VOC and JSC at the expense of the fill factor. Nevertheless, the effect of TiCl4 post-treatment in terms of PCE becomes smaller with an increase of the doping level. This is mainly attributed to a significant drop of the fill factor from 74.6 to 73.6% for undoped TiO2 versus 69.0 to 61.9% for 2Nb-TiO2. The amount of dye anchored onto the different mesoporous films has been evaluated by UV-vis spectroscopy, probing the MLCT transition of the dye desorbed in DMF containing tetrabutyl ammonium hydroxide. As expected from the particle size decrease, the dye uptake increases by almost 30% from 6.8 × 10-8 to 8.9 × 10-8 mol/cm2 for 1Nb-TiO2 before declining to 7.1 × 10-8 mol/cm2 for the 2% Nb-doped sample (Table 5). This drop reflects the onset of particle agglomeration. For comparison, the undoped 17 nm TiO2 sample adsorbs 10.2 × 10-8 mol/cm2 of C101 dye. This discrepancy suggests that the acid-base property of the anatase surface is also considerably modified by Nb5+ doping. Deduced from charge extraction measurements,33 Figure 7 compares the energy position as a function of the distribution of traps for the different samples with and without TiCl4 post-treatment. Nb5+ doping entails a shift of the density of states to deeper levels below the conduction band. The deviation from a single exponential

Figure 6. IPCE spectra for a 7 µm thick electrode composed of 20 nm TiO2 particles and 0.5% Nb-doped TiO2 particles.

TABLE 5: Dye Uptake for TiO2 and Different Nb-Doped Photoanodes Evaluated from a Spot Electrode of Area 0.283 cm2 and 7 µm Thickness sample

dye concentration (×10-8 mol/cm2); no TiCl4 post-treatment

TiO2 0.5Nb-TiO2 1Nb-TiO2 2Nb-TiO2

6.8 8.2 8.9 7.1

distribution observed for the 2Nb-TiO2 sample witnesses the creation of intermediate donor levels below the conduction band. This could explain to some extent the observed VOC drop. By comparison, the TiCl4 post-treatment alleviates the surface defects of particles, resulting in a slight upshift of the trap distribution, in good agreement with the observations reported by B. O’Regan et al.34 In part, due to the down shift of the conduction band edge, the doping with niobium enables to drastically alleviate the dynamics of electron recombination with I3-, which results in an increase of the electron lifetime by almost 1 order of magnitude (Figure 8a). Note that the TiCl4 post-treatment acts also to increase the electron lifetime, as O’Regan et al. have pointed out already.34 However, not shown here for brevity reasons, we found the difference of electron lifetime between TiCl4 post-treated and untreated samples to become narrower when the TiO2 particles were doped. Note also here and for the following that the particle size reduction driven by the doping cannot explain the trend experienced for the electron lifetime, transport rate, and therefore collection efficiency. Unfortunately, the gain achieved for the electron lifetime is partially compensated for by a loss of electron transport, while the rate constant decreases significantly (Figure 8b). As a result, the electron diffusion coefficient is reduced by about four times,

Figure 7. Trap state distribution for TiO2 and Nb-doped TiO2 (a) without and (b) with TiCl4 post-treatment.

Doping a TiO2 Photoanode with Nb5+

J. Phys. Chem. C, Vol. 114, No. 37, 2010 15855 to strengthen the particles’ transparency to visible light and to enhance the charge collection efficiency. To prepare such particles, we have used a hydrothermal approach to produce nanocrystalline anatase TiO2 particles containing different doping levels of Nb5+. The effectiveness of doping was notably demonstrated by a X-ray diffraction study, from which the cell parameter refined verifies Vegard’s law. Rietveld analysis of the patterns in combination with XPS suggests that Nb5+ comes as a substitution into the Ti4+ site. In this work, we also showed that the introduction of niobium influences the particle size and their shape, with an increased aspect ratio. For a low doping level (0.5% of niobium in solution), we achieved an increase in the power conversion efficiency from 7.4 to 8.1%, resulting mainly from a noticeable increase in the short-circuit current density. A maximum efficiency of 8.7% has been reached by treating the particles with TiCl4(aq). On the basis of photovoltage and photocurrent transient measurements, we attribute this improvement to a better charge collection efficiency due to a noticeable retardation of the dynamics of electron back reaction with tri-iodide and the increased band gap of the material. Acknowledgment. A.K.C., F.S., P.C., and M.G. wish to thank Dr. S. M. Zakeeruddin and Pr. Peng Wang for providing us with the C101 dye solution and electrolyte and Dr. R. HumphryBaker for fruitful discussion. The authors also acknowledge financial support of this work by EU project ROBUST DSC Grant Agreement Number 212792. A.K.C. is indebted to EACEA, Brussels, for the financial support of the ErasmusMundus Master Course MESC program (Materials for Energy Storage and Conversion). Supporting Information Available: Optical transmittance of TiO2 and the different Nb-doped TiO2 photoanodes and Rietveld refinement of the 2% Nb-doped TiO2 sample. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 8. Evolution of (a) electron lifetime, (b) transport rate, and (c) charge collection efficiency as a function of charge density for TiO2 and Nb-doped TiO2 photoanodes.

from 8 × 10-5 to 2 × 10-5 cm2/s, at 1018 #/cm3 charge density. The TiCl4 post-treatment also induces a loss of electron transport. These two characteristics are rationalized by the collection efficiency, corresponding to the ratio between the transport rate and the sum of recombination + transport rates. The result demonstrates the doping to be an efficient approach for augmenting the charge collection efficiency via the electron lifetime in DSC (Figure 8c). Conclusion We have modified the optoelectronic properties of anatase TiO2 by means of aliovalent doping using Nb5+, with the aim

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