Electrochimica Acta 182 (2015) 230–237

The Synthesis of Nb-doped TiO2 Nanoparticles forImproved-Performance Dye Sensitized Solar Cells

Haijun Sua,b,1, Yu-Ting Huanga,1, Ya-Huei Changa, Peng Zhaia, Nga Yu Haua,Peter Chun Hin Cheunga, Wei-Ting Yehc, Tzu-Chien Weid, Shien-Ping Fenga,*aDepartment of Mechanical Engineering, the University of Hong Kong, Pokfulam Rd., Hong Kongb State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR Chinac Energy Material Research Group, Eternal Chemical Corporation, Luzhu Dist., Kaohsiung City 821, TaiwandDepartment of Chemical Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan

A R T I C L E I N F O

Article history:Received 21 July 2015Received in revised form 11 September 2015Accepted 13 September 2015Available online 15 September 2015

Keywords:Titanium dioxide nanoparticlesNiobium dopingPhotovoltaicsDye sensitized solar cells

A B S T R A C T

The current co-hydrolysis used for the preparation of Nb-doped TiO2 starts from expensive and unstableniobium-based precursors, which is not suitable for mass production. In this paper, a scalable synthesis ofNb-doped TiO2 nanoparticles (NPs) made by mixing TiO2 paste with functionalized Nb2O5 sol gel isdeveloped for the photoanodes in dye-sensitized solar cells (DSSCs). By doping Nb into TiO2, the positiveshift of conduction band minimum (CBM) enhances the electron injection and the improved electronconductivity facilitates the electron transport, leading to an improved Jsc. In addition, the Nb-dopingsuppresses the surface recombination at TiO2-electrolyte interface, resulting in the increase of Voc.Therefore, the DSSCs based on 2.0 mol% Nb-doped TiO2 photoanodes improve both Jsc and Voc to achieve18.9% improvement of photoconversion efficiency as compared with the standard DSSCs. The DSSCs withNb-doped TiO2 photoanode is also shown an improved long-term stability.

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1. Introduction

Dye-sensitized solar cells (DSSCs) have received great attentionbecause of their low cost and simple manufacturing process, aswell as recent high power conversion efficiency [1,2]. A typicalDSSC comprises a dye-sensitized nanocrysalline TiO2 film coatingon a transparent conductive oxide (TCO) glass as photoanode, aliquid redox electrolyte containing an I�/I3� redox couple, and aplatinum (Pt) catalyst as counter electrode (CE) [3]. The energyconversion efficiency of DSSCs is mainly dominated by theperformance of photoanode, which the nanostructured porousTiO2 film provides an enormous surface area available for dyechemisorption [4–7]. By absorption of a photon, the dye moleculeis excited to generate electron-hole pairs where the electrons arerapidly injected into TiO2 and the holes are reacted with I� ions inthe electrolyte. The electron transport then occurs in theconduction band of the TiO2 to the TCO substrate. The commonmethod to fabricate nanoporous TiO2 film is to screen-print TiO2

NPs paste over TCO surface followed by annealing to burn out the

* Corresponding author. Tel.:+ 852 2859 2639; Fax: +852 2858 5415.E-mail address: hpfeng@hku.hk (S.-P. Feng).

1 Haijun Su and Yu-Ting Huang contributed equally to this work.

http://dx.doi.org/10.1016/j.electacta.2015.09.0720013-4686/ã 2015 Elsevier Ltd. All rights reserved.

polymer binders. The paste is made of TiO2 NPs synthesized byhydrothermal process starting with titanium-based precursor. Ingeneral, a good TiO2 photoanode should possess a high electricalconductivity and a tight chemical bonding with dye, and match thedye's lowest unoccupied molecular orbital (LUMO). To furtherimprove the performance of TiO2 electrode, the incorporation ofmetal (Er, Yb, Zn, Co, Nb) as a dopant in TiO2 has been reported touse as a favorable electron-transfer mediator in photovoltaicdevices [8–11]. Among them, Nb-doped TiO2 with its low cost,nontoxicity, earth abundance, and both thermal and chemicalstability has received great attention as a new promisingalternative to DSSCs. Lü et al. [8] and Nikolay et al. [7] havesynthesized Nb-doped TiO2 particles by co-hydrolysis of Ti and Nbprecursors. It was reported that the use of these Nb-doped TiO2

electrodes in DSSCs can improve the electron injection andtransport because of the increase of electrical conductivity and theshift of Fermi level (EF) and conduction band minimum (CBM).However, the mechanism of the effects caused by Nb doping is stillcontroversial. Lü et al. reported an improved efficiency of DSSCbased on TiO2 doped with high Nb concentration over 5 mol% whileNikolay et al. suggested a lightly Nb-doped concentration less than2.5 mol% [7,8]. Moreover, the current reported co-hydrolysis of Nb-doped TiO2 was prepared from expensive and unstable niobiumalkoxide or niobium chloride (NbCl5), which is not suitable for

Fig. 1. The photograph of the sol gel containing Nb after UV and centrifugationtreatment (inserted) and the Raman spectra of the as-prepared sol gel sample.

H. Su et al. / Electrochimica Acta 182 (2015) 230–237 231

mass production. The complicated hydrothermal process involvingtwo or more different precursors for making Nb-doped TiO2 NPsnarrowly controls high crystalline, uniform size/shape andinevitably generates the structural defects, which play a negativerole on electron injection, transfer and recombination [12]. Inaddition, most research has been conducted on the photovoltaicperformance of DSSCs using Nb-doped TiO2 photoanode, but verylittle has focused on its long-term reliability.

This paper therefore aims to develop a synthesis to prepare Nb-doped TiO2 NPs by mixing TiO2 paste with functionalized Nb2O5 solgel, which is scalable to industrial level. The 18.9% improvement ofenergy conversion efficiency of DSSCs can be achieved by using thisNb-doped TiO2 photoanode. The mechanism of the improvementcaused by Nb doping was systematically investigated based on themicrostructure, electronic structure, light absorption, and electro-chemical behaviors. For the first time, the long-term performanceof DSSCs with Nb-doped TiO2 photoanode was reported in thiswork.

2. Experimental Section

2.1. The synthesis of TiO2-Nb composite paste

All chemical materials were purchased commercially, and usedwithout further purification. TiO2 paste with 18.6 wt% NPs (20–50 nm) was provided by Eternal Chemical Co. Ltd., which wasprepared by hydrothermal synthesis using titanium isopropoxideas precursor. On the other hand, 0.08 g NbCl5 (99%, Aldrich) powderwas dissolved in 1.5 ml absolute ethanol with mechanical stirringat room temperature and then added 1.5 ml deionized water toobtain the transparent solution. After 20 min of UV irradiationfollowed by high speed centrifugation (4000 rpm, 10 min), apolymeric Nb-gel was formed. As noted, the excess water should beremoved from the Nb-gel before the next step. Then, the gels withdifferent weights corresponding to 0.5 mol%, 1 mol%, 2 mol%, 5 mol% were added into 2 g TiO2 pastes. A conditioning mixer (Are-250,Thinky) was used to obtain the composite TiO2-Nb pastes by2200 rpm high-speed mixing for 2 min and 2000 rpm deformingfor 2 min. The preparation procedure of 2 mol% composition andcalculation of XNbmol% can be referred in Fig. S1 and page S3

2.2. DSSCs cell assembly

FTO glass (10 V/&, 3.1 mm thick, Nippon Sheet Glass) wasimmersed in a 2% PK-LCG545 (Parker Corp.) at 50 �C for 30 minwith sonication to clean the surface followed by deionized waterrinse. The standard TiO2 photoanode was screen-printed by TiO2

paste (particle size 20 nm, product, Eternal) on FTO glassrepeatedly until the film thickness reached 10 mm. The compositeTiO2-Nb photoanode was screen-printed by TiO2 paste of 6 mmfollowed by TiO2-Nb paste of 4 mm. The TiO2 and TiO2-Nb film werethen sintered in furnace at 450 �C for 30 min to produce thenanoporous TiO2 and Nb-doped TiO2 anodes. Dye impregnationwas done by immersing the TiO2 anodes in a 0.4 mM N719 (D719,Everlight Chemical Industrial Corp.) ethanol solution at roomtemperature for 12 h. The effective area of the TiO2 photoanode is0.16 cm2. The FTO glass (10 V/&, 2.2 mm thick, Nippon SheetGlass) coated by Pt NPs catalyst by two-step dip-coating processwas used as the counter electrode (CE) [13]. The dye-adsorbedphotoanode and the PtNPs CE were stacked face-to-face and sealedwith a 30 mm-thick thermal-plastic Surlyn spacer (SX1170-25,Solaronix). A proper amount of liquid electrolyte (0.6 M PMII,0.05 M I2, 0.1 M LiI, 0.5 M TBP in AN/VN (85:15 = v/v)) was injectedinto the gap between the two electrodes. For the long-termstability test, all DSSC cells were sealed with silicon sealant toprevent the electrolyte leakage and then placed in the

environmental chamber (THH-80L, HongZhan Group CompanyLimited). The electrolyte used in long-term stability test was MPN-based one (0.05 M I2, 0.5 M LiI, 0.5 M TBP in 3-Methoxypropioni-trile) due to its relative high boiling point of 165 �C.

2.3. Material, electrochemical, and photovoltaic characterizations

Field-emission transmission electron microscope (FE-TEM,Tecnai G2 F20 S-TWIN, FEI), Scanning electron microscope (SEM,S-4800, Hitachi), Raman spectroscopy (inVia Reflex, Renishawwith a 514 nm Ar laser source, laser power (10 mW)), X-raydiffractometer (XRD, D8 Advance, Bruker), X-ray photoelectronspectroscopy (XPS, PHI-5400, PE) were used for material character-izations. UV–vis spectrophotometer (HP 8453) was used to obtainthe absorption spectroscopy for determining the band gap ofundoped and Nb-doped TiO2. The incident photon-to-currentconversion efficiency (IPCE) was measured by IPCE Kit equipment(PEC-S20, Peccell) in wavelength range of 300–800 nm. Thephotocurrent–voltage (J–V) curves of DSSCs were recorded witha computer-controlled digital source meter (Keithley 2400) underexposure of a standard solar simulator (PEC-L01, Peccell) under 1sun illumination (AM 1.5 G, 100 mWcm�2). The electrochemicalimpedance spectroscopy (EIS) was measured for DSSCs cell under 1sun illumination by electrochemical workstation (Reference 3000,Gamry) from 0.1 to 106Hz with 10 mV amplitude at open-circuitcondition.

3. Results and Discussion

3.1. Structural characterization of Nb-doped TiO2 NPs

The milky white Nb polymeric gel can be formed by hydrolysis/condensation processes via UV irradiation, as shown in Fig. S1. TheRaman analysis for Nb2O5 sol gel in Fig. 1 shows two peaks at883 cm�1 and 2979 cm�1, corresponding to the vibration mode ofterminal Nb¼O bond (characterization peak: Nb��O 380–500 cm�1, Nb��O��Nb 580–850 cm�1,the Nb¼O 850–910 cm�1)[14] and the stretching O��H bond [15]. This Nb2O5 sol gelconsisting of amorphous Nb2O5 and O-H functional group isfavorable to mix with the TiO2 paste from hydrophilic surface [16].(FTIR result also supports the Raman analysis as shown in Fig. S2)

Fig. 2(a) is XRD patterns of the prepared TiO2 samples withdifferent Nb contents. All the samples show the anatase phase withbody-centered tetragonal crystal structure. The (10 1) diffractionpeaks of anatase phase gradually shift to lower diffraction angleindicating increase of d-spacing when increasing Nb dopant

Fig. 2. The XRD patterns of (a) undoped and Nb-doped TiO2 nanoparticles and (b) details of the XRD patterns around 24� to 27� 2u values.

232 H. Su et al. / Electrochimica Acta 182 (2015) 230–237

content (Fig. 2(b)) because of the relative large Nb5+ radiuscompared with Ti4+ radius. The HRTEM observation in Fig. S3provides evidence of the presence of Nb dopant that the d-spacingof (10 1) lattice planes of TiO2 increases with increasing Nbcontent, which is consistent with the calculated d-spacing fromXRD results as summarized in Table S3. No other peaks correlatedto Nb, Nb2O5, NbCl5 were found, which indicates that Nb has beensuccessfully doped into the TiO2 lattice. As noted, the previouslyreported co-hydrolysis starting from Ti and Nb precursors wasdifficult to produce the Nb-doped TiO2 NPs with identical size,shape and structure so that existed the mixed nanocrystals withrod, platelet, or rhombic shapes [17]. In our work, the round-shapeNb-doped TiO2 NPs with anatase structure and uniform particlesize (�20 nm) can be achieved by a simple blending and annealingprocess.

Fig. 3 is Raman spectra of the prepared TiO2 samples withdifferent Nb contents. All the samples show the characteristicRaman modes of anatase TiO2, which is consistent with the above-mentioned XRD results in Fig. 2. In Fig. 3(a), four maincharacteristic peaks of TiO2 are centered at 146, 396, 518,639 cm�1, corresponding to Eg (v6), B1g, A1g and Eg (v1) vibrationmodes, respectively [18]. Trizio et al. [12] reported that theamorphous Nb2O5 will possibly appear if the excess Nb dopant(>10 mol%) is added. As seen, the anatase peak width at 146 cm�1

increases with increasing the Nb content, which can be explainedby the formation of Nb��O��Ti bond [16]. In Fig. 3(b)-(c), the peaksat 518 cm�1 and 639 cm�1 slightly shift toward to small wave-number, which is correlated to the change of crystalline size of TiO2

[19]. The increasing intensity of the anatase peak at 639 cm�1

results from the formation of Nb��O bond [20].

Fig. 3. The Raman spectra of (a) the Nb-doped TiO2 nanoparticles with different Nb conte550�750 cm�1.

Fig. 4 is XPS measurement for standard TiO2 and Nb-doped TiO2

samples (2 mol% and 5 mol%). Compared with the standard TiO2,Nb 3p1/2, Nb 3p3/2, and Nb 3d5/2 peaks can be found in the Nb-doped samples and their intensities increases with increasing theNb dopant. According to XPS data in Fig. 4(b), the calculated molarratio of Nb to TiO2 is 1.9 mol%, which is very close to the totaladditive amount of 2 mol% of Nb solgel. (Characterization of theNb-doped TiO2 content from ICP-OES is shown in Table S1) Fig. 5 isXPS spectra of Ti 2p, Nb 3d and O 1s for the standard TiO2 and Nb-doped TiO2 samples. The binding energies of Ti 2p3/2 (458.6 eV) andTi 2p1/2 (464.4 eV) in Fig. 5(a) correspond to Ti4+ oxidation state,meaning the pure stoichiometric TiO2 structure [21]. As seen, theNb-doping causes the peak shift of Ti 2p toward a higher bindingenergy, which is likely attributed to the larger electronegativity ofNb (1.6) than that of Ti (1.54). In Fig. 5(b), Nb 3d3/2 (209.9 eV) andNb 3d5/2 (207.3 eV) are correlated to Nb5+, which increases withincreasing Nb dopants. As noted, no Nb2+ (204 eV) and Nb4+

(205 eV) were detected in our samples [4]. The O 1s spectra inFig. 5(c) corresponds to Ti4+-O bonds, which has a similar trend ofthe peak shift because the increase of Nb5+-O bonds in TiO2 leads toan increase of lattice oxygen [22]. Fig. 5(d) shows a two-bandstructure, the main peak for the O1s electron binding energy forTiO2 (OL) and the other peak at relative high binding energyattributed to the adsorbed OH groups (OH) [23]. The Fig. S4 showsthat the position of the OH peak shifts toward a higher energy levelwhen increasing Nb dopant, which is beneficial for dye anchoringdue to the surface OH sites of TiO2 film [24]. As mentioned above,the investigations based on XRD, Raman and XPS summarized thatNb was successfully doped into anatase TiO2 lattice to form Nb5+-Obonds by this synthesis. The uniform NPs distribution can be

nts and (b) the magnified spectra in the range of 450�600 cm�1 and (c) the range of

Fig. 4. The XPS survey spectra of Nb-doped TiO2 nanoparticles with different Nb contents: (a) 0 mol%, (b) 2 mol%, (c) 5 mol%.

Fig. 5. (a) The Ti 2p, (b) Nb 3d and (c) O 1s high resolution XPS spectra of Nb-doped TiO2 nanoparticles with different Nb contents. (d) is the fitting result of the 2 mol% Nb-doped TiO2 sample for the O 1s spectra.

H. Su et al. / Electrochimica Acta 182 (2015) 230–237 233

observed by field-emission scanning electron microscopy (FE-SEM) for the standard TiO2 and Nb-doped TiO2 NPs films (Fig. S5(a)–(i)). Fig. S5(b)–(j) shows the cross-section image of thecompact Nb-doped TiO2 film with the thickness about 10 mm.

3.2. Optical properties of Nb-doped TiO2 NPs

Fig. 6(a) shows the transmission spectra of the standard TiO2

and Nb-doped TiO2 NPs films measured by UV–vis in thewavelength range of 300–800 nm. The corresponding absorbancespectra are shown in Fig. 6(b). As seen, all films exhibit 10–35%transmission in visible region and strong absorption below400 nm. When increasing the Nb dopants, the absorbance intensityof visible light increases and the absorption edge shifts toward thelonger wavelength, as shown in Fig. 6(b)-(c). As known, the optical

band gap (Eg) can be calculated according to the Eq. (1) below:

ahn ¼ Aðhn�EgÞn ð1Þwhere a is the optical absorption coefficient, hv is the photoenergy, Eg is the absorption band gap, A and n are constants. For theindirect semiconductor of anatase TiO2, n is equal to 2 [7,19].Therefore, the tangent intercept in the plot of (ahv)1/2 versus hvrepresents Eg, as shown in Fig. 6(d). The inset of Fig. 6(d) shows thatEg decreases with increasing Nb dopant. By Nb doping into TiO2,Nb5+ substitutes Ti4+ so that the Ti3+ 3d1 and Nb5+ 4d0 levels exist inthe nanocrystals to form the intraband states making a positiveshift of CBM [25]. Thus, the higher absorbance can be seen in theNb-doped TiO2 film due to the smaller band gap. Moreover, oneexcess electron in the Ti 3d orbital for each Nb5+ substitution wouldraise the electron concentration, which improves the electron

Fig. 6. (a) The Uv–vis transmission and (b) absorbance spectra of DSSCs based on the Nb-doped TiO2 NPs electrodes with different Nb contents. (c) is the magnifiedabsorbance spectra in the wavelength range of 400�500 nm. (d) is the (ahv)1/2 versus hv plots for the Nb-doped TiO2 electrodes. The inset in (d) shows the variation of opticalband gap with the increase of Nb dopant content.

234 H. Su et al. / Electrochimica Acta 182 (2015) 230–237

conductivity. The similar tendency of Eg shift was also reported byLü et al. [8] and Kim et al. [25] As seen, the band gap of 5 mol%sample is larger than that of 2 mol% sample. The reason could bethe increase of structural defect when incorporating a relativelarge amount of Nb dopant in the lattice. Ti3+ 3d1 can also trapelectrons when the TiO2 has poor crystallinity. Fig. 7 showsincident photon-to-current conversion efficiency (IPCE) spectra forthe undoped and doped TiO2 photoanodes used in DSSCs. Asignificant enhancement of IPCE can be seen in the DSSCs with Nb-doped TiO2 electrode compared with the undoped one, which is

Fig. 7. The incident photon-to-current conversion efficiency spectra of DSSCs basedon the undoped and Nb-doped TiO2 NPs electrodes.

ascribed to the enhancement of electron injection and transport.The increased driving force between the dye’s LUMO and the TiO2’sdownshifted CB enhances the electron injection, and the improvedelectron conductivity facilitates the electron transport [26]. Asnoted, the lowest IPCE was also found in the DSSC with 5 mol% Nb-doped TiO2 electrode. The recombination effect will be explainedin the following EIS measurement. As seen in Fig. 7, the use of theNb-doped TiO2 photoanode in DSSCs leads to the absorption edgeshift in 700�730 nm. It was reported that the incorporation oftransition metals, such as V, Nb, Fe, creates the oxygen vacanciesas active sites in TiO2, giving rise to a red-shift of photo-response[27]. This broad absorption would benefit the photoconversionefficiency.

3.3. Photovoltaic performance of DSSCs

Fig. 8(a) shows the photovoltaic performance of DSSCs based onthe undoped and Nb-doped TiO2 electrodes with different Nbcontents under one sun illumination (AM 1.5 G, 100 mW/cm2). Theobtained photovoltaic parameters are listed in Table 1. Thephotovoltaic performance of DSSCs based on the Nb-doped TiO2

electrodes exhibits a pronounced improvement when increasingthe Nb contents from 0.5 to 2.0 mol%. The DSSCs with 2.0 mol% Nb-doped TiO2 electrode achieve a high photoconversion efficiency(h) of 8.44%, which is 18.9% improvement as compared with thestandard DSSCs. The main contribution is the high short-circuitphotocurrent density (Jsc) of 16.17 mA cm�2 due to the enhance-ment of electron injection and transport based on the shifted CBMand improved electron conductivity. As noted in Table 1, thehighest Voc is found in the DSSCs with 1 mol% Nb-doped TiO2. The

Fig. 8. (a) The photocurrent density-photovoltage (J–V) curves and (b) dark current density-voltage curves for DSSCs based on the undoped and Nb-doped TiO2NPs electrodeswith different Nb contents. (c) The Nyquist plots and (d) Bode plots of the EIS for DSSCs based on the Nb-doped TiO2 NPs electrodes with different Nb contents. The inset in (c)is the equivalent circuit.

Table 1Performance characteristics of DSSCs based on the undoped and Nb-doped TiO2 NPselectrodes.

DSSCs Jsc (mA/cm2) Voc

(Volt)FF h

(%)Adsorbed dye (x10�7mole)

Nb 0 mol% 13.358 0.732 0.724 7.062 0.256Nb 0.5 mol% 14.404 0.725 0.708 7.375 0.263Nb 1.0 mol% 14.674 0.741 0.722 7.841 0.260Nb 2.0 mol% 15.907 0.742 0.717 8.459 0.259Nb 3.0 mol% 15.437 0.700 0.726 7.841 0.243Nb 4.0 mol% 14.826 0.706 0.727 7.604 0.279Nb 5.0 mol% 12.862 0.733 0.728 6.831 0.251

Table 2The electrochemical impedance parameters of DSSCs based on the undoped andNb-doped TiO2 NPs electrodes.

DSSCs Rs (V) Cpt (F) Rpt (V) vk (s�1) vd (s�1) Rt (V) Rk (V)

Nb 0 mol% 16.3 1.10E-05 3.81 97.42 545.9 3.21 17.96Nb 0.5 mol% 16.3 1.10E-05 3.92 91.17 568.5 2.66 16.57Nb 1.0 mol% 16.3 1.10E-05 4.76 89.62 575.1 2.49 15.94Nb 2.0 mol% 16.3 1.10E-05 3.57 94.30 607.7 2.37 15.29Nb 5.0 mol% 16.3 1.10E-05 3.61 111.3 534.6 4.56 21.73

H. Su et al. / Electrochimica Acta 182 (2015) 230–237 235

value decreases with further increase in the Nb content. It isbelieved that the influence of Nb doping in Voc combines twoeffects involving the shift of EF and the suppression of surfacerecombination at the TiO2-electrolyte interface. First, the de-creased difference between the positive shift EF of Nb-doped TiO2

and the redox potential of the I�/I3� couple leads to the decrease ofVoc [1]. Second, Nb-doping would decrease the concentration ofthe oxygen vacancies at the TiO2 surface so that widen the spacecharge region to suppress the recombination at the TiO2-electrolyte interface, leading to the increase of Voc [7]. In the caseof low Nb-doping (0.5�1.5 mol%), the shift of EF is small so that theimproved surface recombination results in the increased Voc.Fig. 8(b) is the dark current density-voltage (J–V) curves, providingevidence that the recombination was suppressed by Nb-doping. Onthe other hand, the high Nb-doping has a large positive shift of EF todominate the decreased Voc. Therefore, considering Voc and Jsc inorder to achieve the maximum h, there should be an optimum

concentration for Nb-doping. In our case, it is 2 mol%. In addition,Table 1 shows that the adsorbed dye is relative the same, which isnot the dominate factor for increase of Jsc.

To advance our understanding of the electron transport andrecombination in Nb-doped TiO2 film, EIS was used to measureDSSCs under 1 sun illumination at open-circuit potential. Fig. 8(c)shows the Nyquist plots for DSSCs based on undoped and Nb-doped TiO2 photoanodes. The equivalent circuit is depicted in theinset of Fig. 8(c). The impedance Z(v) function for the photoanodecan be expressed in the Eq. (2) [28]:

ZTiO2ðvÞ ¼ RtRr

1 þ iv=vk

� �1=2

cothððvk þ ivvd

Þ1=2Þ ð2Þ

where Rt, Rr represent the electron transport resistance in TiO2, thecharge-transfer resistance related to electron recombination at theTiO2/dye/electrolyte interface, respectively. vd and vk stand for thecharacteristic frequency of electron diffusion in a finite layer andthe rate constant for recombination, respectively, which inverselyrelated to the transit time and lifetime. Therefore, the larger vd is,

236 H. Su et al. / Electrochimica Acta 182 (2015) 230–237

the faster the electron transports, resulting in a higher Jsc. Incontrast, the smaller vk is, the longer electron lifetime is, leading toa larger Voc. The Rs, Cpt, and Rpt represent the series resistance ofFTO, the interfacial capacitance and the charge-transfer resistanceat the Pt/electrolyte interface, respectively. The fitting results aresummarized in Table 2. As seen in Fig. 8(c), the second semicirclesin the medium frequency (10–1000 Hz) correspond to thecompetition of the electron transport within TiO2 film andrecombination at the TiO2/dye/electrolyte interface. As seen, thesemicircles at the intermediate frequency regions decrease withincreasing Nb dopant content from 0 to 2 mol%, indicating that theelectron transport becomes faster within the TiO2 film, implyingthe film conductivity is improved. The trend of vd values in Table 2are in good agreement with the trend of Jsc in Table 1. The largestand the smallest vd occurring on the 2 mol% and 5 mol% Nb-dopingDSSCs lends support to the highest and lowest Jsc. The trend of vk inTable 2 is also well consistent with the trend of Voc in Table 1. Forexample, the Nb 1 mol% DSSC has the smallest vk, which supportsthe highest Voc. In addition, the peak frequency (vmax) in themedium frequency region in the Bode plots shown in Fig. 8(d)provides the direct information of electron recombination rate,whose trend is in good agreement with fitting vk values. Inaccordance with the previously noted mechanism, the improved Jscand Voc of DSSCs can be achieved by using the Nb-doped TiO2

photoanodes due to the enhancement of electron injection,electron conductivity, and the suppression of surface recombina-tion at the TiO2/dye/electrolyte interface.

The long-term stability of undoped and doped-Nb DSSCs withMPN-based electrolyte was investigated. A 1200 hr-aging test onwell-sealed cell was carried under dark condition and stored at twodifferent specifications, (a) 25 �C and 50% relative humidity (similarto the indoor environment) and (b) 60 �C and 80% relative humidityto simulate a damp condition. Fig. S6 and Fig. S7 show the change innormalized photovoltaic parameters (Jsc, Voc, FF and h) as a functionof storage time. Overall, h and Jsc decrease with time, and FF and Voc

increasewith time. Inthe long-term agingtest,G.Xueetal. suggestedthat the occurrence of chemical reaction in dye would widen the dyeband gap to degrade h [29]. Compared with undoped-Nb cells, Nb-dopedcellshave a 10–20% improvement forh and Jscwhile FFandVoc

have a similar increasing trend. The improved degradation by Nb-dopingpossiblyresults fromtheenhancedbindingofdyeadsorption,which is mentioned in XPS results.

4. Conclusion

This paper presents a cost-effective way to prepare Nb-dopedTiO2 nanoparticles (NPs) as the photoanode of DSSCs by simplymixing TiO2 paste with Nb2O5 sol gel. This synthesis method issuitable for mass production. After screen printing and annealing,Nb was successfully doped into anatase TiO2 lattice to form Nb5+-Obonds based on XRD, Raman and XPS investigations. By Nb dopinginto TiO2, the Ti3+ 3d1 and Nb5+ 4d0 levels existing in thenanocrystals make a positive shift of CBM to enhance the drivingforce of electron injection, and the increase of electron concentra-tion improves the electron conductivity. The enhancement ofelectron injection and transport leads to the improvement of Jsc. Inaddition, the concentration of the oxygen vacancies at the TiO2

surface would be decreased by Nb-doping so that the space chargeregion is widened to suppress the recombination at the TiO2-electrolyte interface, leading to the increase of Voc. EIS measure-ment supports the mechanism of Nb-doping effect for theimproved electron injection, transport, and recombination. TheDSSCs with 2.0 mol% Nb-doped TiO2 electrode improve both Jsc andVoc to achieve a high photoconversion efficiency of 8.44%, which is18.9% improvement as compared with the standard DSSCs. Thelong-term stability test shows an improved performance on the

DSSCs with Nb-doped TiO2 photoanode, which possibly resultsfrom the enhanced dye anchoring.

Acknowledgments

Miss Chih-Fan Lin assisted in making DSSCs devices and datacollection of photovoltaic performance. Miss WANG Qian assistedICP-OES analysis. This work was supported by Hong KongInnovation and Technology Fund under Award: ITP/013/12NPand General Research Fund from Research Grants Council of HongKong Special Administrative Region, China, under Award Number:HKU 712213.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.electacta.2015.09.072.

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