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Materials Chemistry and Physics 143 (2014) 1440e1445

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Hybrid photoelectrode by using vertically aligned rutile TiO2nanowires inlaid with anatase TiO2 nanoparticles for dye-sensitizedsolar cells

Eui-Hyun Kong, Yeon-Hee Yoon, Yong-June Chang*, Hyun Myung Jang*

Department of Materials Science and Engineering, and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH),Pohang 790-784, Republic of Korea

h i g h l i g h t s

* Corresponding authors. Tel.: þ82 54 279 2138; faE-mail address: hmjang@postech.ac.kr (H.M. Jang)

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.11.060

g r a p h i c a l a b s t r a c t

� The hybrid system was designed bycombining TiO2 nanowires with TiO2

nanoparticles.� Vertically aligned nanowires showrapid charge transfer and light scat-tering effects.

� The inlaid nanoparticles provide anadditional surface area for dyeloading.

� The hybrid structure exhibits anoticeable enhancement in the con-version efficiency.

a r t i c l e i n f o

Article history:Received 27 July 2013Received in revised form6 November 2013Accepted 30 November 2013

Keywords:SemiconductorsNanostructuresElectron microscopyUltrasonic techniquesLight scattering

a b s t r a c t

We report a hybrid photoelectrode fabricated by using single crystalline rutile TiO2 nanowires (NWs)inlaid with anatase TiO2 nanoparticles (NPs) for efficient dye-sensitized solar cells. For this purpose, w4-mm-thick vertically aligned NWs were synthesized on the FTO glass substrate through a solvothermaltreatment. Then, as-prepared NW film was treated with the NP colloidal dispersion to construct the NWeNP film. In particular, the NWs offer a fast pathway for electron transport as well as light scatteringeffect. On the other hand, the inlaid NPs give an extra amount of space for the dye-uptake. Accordingly,the present NWeNP electrode exhibited 6.2% of the conversion efficiency, which corresponds to w48%improvement over the efficiency of the NP-DSC. We attribute this notable result to the synergetic effectsof the enhanced light confinement, charge collection, and dye-loading.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The rising demand for global energy use has promoted furtherutilization of sustainable and renewable energies. Solar power, oneof the most abundant sources, is expected to contribute to theworld energy production in the near future [1]. During the past twodecades, dye-sensitized solar cells (DSCs) have attracted great

x: þ82 54 279 2399..

All rights reserved.

attention as a promising alternative to the conventional solid-statepen junction device by virtue of their low-cost of fabrication andtheoretical high efficiency [2,3].

Till now, extensive researches have been conducted to improvethe DSC efficiency in the field of materials engineering. Althoughvarious nanostructures have been suggested as the working elec-trodes, anatase TiO2 nanoparticles (NPs) are still the most widelyused material for the advantage of the optimal dye-loading [4,5].However, high transparency of the NP film usually results in a hugeoptical loss to the incident light, thereby degrading the photocur-rent generation. Thus, the light management becomes one of the

E.-H. Kong et al. / Materials Chemistry and Physics 143 (2014) 1440e1445 1441

key issues to be resolved in the DSCs [6e8]. For this, various ar-chitectures including photonic crystals [9e12], scattering over-layers [13e16], or graded-series of multi-layers [17,18] were pro-posed in order to achieve strong light confinement over a broadwavelength range. Nonetheless, their structural complexity maylimit wider applications. Another feasible strategy is to improve thecharge collection efficiency by employing one-dimensional (1-D)nanostructures: nanowires (NWs) [19,20], nanotubes (NTs) [21e23], nanoforests [24,25], etc. It is reported that the photo-injected-electrons in a typical NP film cannot but travel throughnumerous grain boundaries where the electrons are easily trapped[26]. On the contrary, 1-D nanostructures can provide a directpathway for the electrons with minimized interfacial recombina-tion since they consist of a nearly defect-free single crystallinephase [27e29]. However, these architectures have a very low sur-face area as compared with the NP film. This fatal drawback limitsthe dye-loading capacity and reduces the photocurrent density andthe overall conversion efficiency. Consequently, a moderate con-version efficiency of 4e5% was demonstrated in the DSCs madewith thin pristine TiO2 NW or NT films [20,30]. To overcome theselimitations, the hybridization of 1-D nanomaterials and nano-particles has been extensively conducted in the ZnO-based DSCs.However, the overall efficiency remains 2e4% [31e33].

Herein, we present a new approach of implementing the ben-efits of hybrid nanostructures by combining vertically aligned rutileTiO2 nanowires (NWs) with anatase TiO2 nanoparticles (NPs). Inparticular, the single crystalline NWs offer a highway for rapidcharge transport and multiple scattering centers, whereas theinlaid NPs provide an additional surface area (72.5 m2 g�1) for dyeloading (Fig. 1). As a result, w4-mm-thick NWeNP hybrid photo-electrode exhibited a noticeable enhancement in the power con-version efficiency: 6.2% versus 4.2% for the reference cell madesolely with the NPs, and 4.5% for the NWs at the same filmthickness.

2. Experimental

2.1. Preparation of anatase TiO2 nanoparticle (NP TiO2) film

20-nm-sized commercial TiO2 nanoparticle (NP TiO2) paste waspurchased from ENB Korea. Then, the NP TiO2 paste was screen-

Fig. 1. A schematic of the hybrid photoelectrode consisting of the vertically-alignedNWs array infiltrated with the NPs.

printed on the transparent fluorine-doped tin oxide (FTO) glasssubstrate to construct a w4-mm-thick working electrode. As-prepared photoelectrode was heat-treated under air environ-ment with the following temperature profile: (i) 325 �C for 5 min,(ii) 375 �C for 5 min, (iii) 450 �C for 15 min, and (iv) 500 �C for15 min.

2.2. Preparation of rutile TiO2 nanowire (NW TiO2) film

The NW TiO2 was synthesized on the FTO substrate by adopt-ing a simple solvothermal process. Firstly, the FTO substrate wascleaned with isopropyl alcohol, deionized (DI) water, and acetone.Then, the FTO substrate was soaked in 0.2 M TiCl4 aqueous solu-tion at room temperature for 15 h to form a seed layer for theNWs. After this step, white precipitates were cleaned from the FTOsurface, and the substrate underwent thermal treatment at 500 �Cfor 30 min to completely remove the residue. It was found that theresidual precipitates may influence the porosity and individualityof the NWs, thereby degrading the film adhesion properties (FigS1a and b). As a next step, the surface-treated FTO substrate wasplaced in a Teflon-sealed reactor at 180 �C for 0.5 h (heating rate of0.5 �C min�1) with the following precursor solution having opti-mized compositions: 2 mL titanium butoxide, 2 mL titanium tet-rachloride (1 M in toluene), and 0.8 mL hydrochloric acid (37 wt %)dissolved in 20 mL toluene. In the synthetic procedure, a Cl� ion isthought to selectively adsorb onto the (110) plane of TiO2, whichsuppresses growth of the plane. This underlying mechanism re-sults in anisotropic growth of nanocrystallites. Accordingly, rutileTiO2 nanowires are formed [20]. It is observed that hydrochloricacid played a key role in the porosity, and 0.8 mL of hydrochloricacid seemed to be most ideal considering the individuality of theNWs (Fig. S2). Indeed, the aforementioned chemical compositionresulted in the highest efficiency of 4.5% at the film thicknessof w4 mm.

2.3. Preparation of the hybrid NWeNP film

To construct a hybrid photoelectrode (NWeNP), the anataseTiO2 nanoparticles (NPs) were infiltrated into as-prepared NW film.For this, the NP powder (w20 nm-sized anatase TiO2 particlespurchased from ENB Korea) was prepared in the form of colloidalsuspension. 10 M NP TiO2 was dispersed in absolute ethanol using atip-type ultrasonic generator (VCX130, Sonics, USA; frequency,20 kHz; generating power, 130 W) for 5 min (2 s ON, 2 s OFF, 20%amplitude). Finally, the NW film was dipped in the NP TiO2dispersion to make the NWeNP film. In order to insert the NPs intothe NW array, the NW photoelectrode was dipped in the well-dispersed NP colloidal solution for 30 min. Then, the photo-electrode was rinsed and sonicated in absolute ethanol for 1 min.After that, the photoelectrode was dried at 70 �C for 30 min, andfinally kept in vacuum oven at room temperature.

2.4. Cell fabrication

The devices were fabricated according to the previous literaturewith several modifications [20]. Three distinct films (the NP, NW,and NWeNP films) were processed with 40 mM of TiCl4 solution at70 �C for 30 min, and rinsed with deionized water. Then, the TiCl4-treated films were heated at 500 �C for 30 min. For sensitization,they were immersed in 0.3 mMN719 solution (in absolute ethanol)for 12 h at room temperature. The counter electrode was preparedby dripping a Pt solution (Solaronix, Platisol T) on the FTO substrate,which was followed by thermal treatment at 500 �C for 30 min. Forthe cell assembly, a hot-melt 60-mm-thick Surlyn (Solaronix, Mel-tonix 1170-60) was used as a spacer between the working electrode

Fig. 2. XRD patterns of the NW film grown on the FTO substrate.

E.-H. Kong et al. / Materials Chemistry and Physics 143 (2014) 1440e14451442

and the counter electrode. Since the NWs have been grown on thetransparent FTO substrate, the front-illuminated DSCs can befabricated. A commercially available electrolyte (Solaronix, AN50)was used as a redox mediator.

Fig. 3. Top-view (a, b) and cross-section (c) SEM images of the NW film. T

2.5. Characterization

The crystallography of the NW TiO2 was studied by employingX-ray diffraction method (Rigaku RINT 2000 with Cu Ka ray). Themorphology of the NWandNWeNP filmswere investigated using afield-emission scanning electronic microscope (FE-SEM; JEOL JSM-7401). To observe detailed nanostructures, a high-resolutiontransmission electronic microscope (HR-TEM; JEOL JEM-2200FS)located at NCNT (National Center for Nanomaterials and Technol-ogy in Pohang) was used. A surface profiler (Tencor, Alpha-Step500) was used in order to measure the film thickness. The trans-mittance and reflectance spectra were recorded with a PerkinElmer UVeVis spectrometer (Lambda 750S). The photovoltaicperformance was measured under an illumination of a solarsimulator (Newport, Oriel class A, 92251A) at 1 Sun (AM 1.5,100 mW cm�2) without using a mask. The cell active area was0.25 cm2. The incident-photon-to-current conversion efficiency(IPCE) values were recorded as a function of the wavelength from400 nm to 800 nm (PV Measurements, Inc.). A 75 W Xenon lampwas used as a light source with a monochromator. Calibration wasperformed with a NIST-calibrated photodiode G425 as a reference(chopping frequency: 10 Hz). The monochromatic power density

op-view (d, e) and cross-section (f) SEM images of the NWeNP film.

E.-H. Kong et al. / Materials Chemistry and Physics 143 (2014) 1440e1445 1443

was calibrated using a reference Si photodiode as a standard fromthe NIST. Electrical impedance spectra were measured using animpedance analyzer (BioLogic, SP-300), with a frequency rangingfrom 10�1 to 106 Hz, then analyzed using Z-view software.

3. Results and discussion

Prior to prepare the NWeNP hybrid photoelectrode, singlecrystalline rutile TiO2 nanowires (NWs) were fabricated directly onthe FTO substrate through solvothermal process. As-grown NWTiO2 array was characterized by XRD spectroscopy (Fig. 2). Themarked (002) peak and diminished (110) peak indicate that theNWs are in rutile phase after calcination, and their crystal growth is(001)-oriented. Fig. 3aec presents the top-view and cross-sectionalSEM images of the NW film. It is observed that the NWs possesssquare top facets and they are almost individually positioned. Thecross-sectional view shows that the NWs with a length of w4 mmare vertically aligned on the FTO substrate. The microstructures ofthe NWs array, however, take on a different aspect after the infil-tration of anatase TiO2 nanoparticles (NPs) using the dip-coatingand ultrasonication method (Fig. 3def). As shown in the SEM im-ages of the NWeNP film, the NPs are densely inlaid on the NWsurface without increasing the film thickness of w4 mm. The cross-section SEM images also indicate that the NPs were successfullyinfiltrated all the way to the bottom of the NW array (Fig. S3). It isobvious that the NW and NP have the identical chemical states ofelemental Ti and O (Ti 2p3/2 binding energy 458.78 eV; O 1s bindingenergy 529.89 eV) as shown in XPS spectra (Fig. S4a and b).

The HR-TEM images provide more detailed structural informa-tion on the NW TiO2 (Fig. 4aec) and the NWeNP (Fig. 4def). Fig. 4aand b shows that the NWs are 5e10 nm in their diameter. Fig. 4cpresents a higher resolution image, indicating that the NWs arefully crystalline with the lattice spacing of 0.322 nm. This corre-sponds to the rutile (110) plane. On the other hand, Fig. 4def shows

Fig. 4. HR-TEM images of the NW (aec) and NW

the HR-TEM images of the NWeNP hybrid structure. It can be seenthat w20-nm-sized NPs are evenly distributed all the way throughthe single crystalline NW array to construct the anatase-rutilehetero-junction.

As a next step, approximately 4-mm-thick films (the NP-, NW-,and NWeNP films) were prepared on the transparent FTO glasssubstrates according to the cell-fabrication procedures (see“Experimental section”). Then, their optical properties were char-acterized by measuring the transmittance and reflectance spectra(Fig. 5a and b). Since the NPs are weak light scatterers in the visiblerange, a large portion of the incident photons inevitably penetratevia the NP film, resulting in a great optical loss at the long wave-length region. On the contrary, the NWs are strongly photonic withthe incident light. As a result, the NW films performed excellentlight confinement via reduced reflectance and transmittance in agiven spectral range [34,35]. For the NWeNP sample, the reflec-tivity is increased, whereas the transmittance is slightly decreased,as compared with the NW film. This can be attributed to the van-ishing porosity in the NW array [36]. Therefore, it is reasonable toexpect that the NW-based DSCs are more beneficial in terms of thephoton management.

Photovoltaic experiments were conducted in order to evaluatethe performance of the DSCs. IeV curves of the NP-, NW-, and NWe

NP-DSCs are shown in Fig. 6a. Corresponding photovoltaic prop-erties of three devices are summarized in Table 1. The NP-DSCshowed the overall efficiency (h) of 4.2% with a photocurrentdensity (Jsc) of 7.6 mA cm�2 and an open-circuit voltage (Voc) of796 mV under AM 1.5, 1 Sun illumination; the NW cell performed h

of 4.5% with Jsc of 7.8 mA cm�2 and Voc of 835 mV under sameirradiation. In spite of lower surface area, the NW device is slightlymore efficient as compared with the NP cell at the same filmthickness of w4 mm. This result is mainly due to the fast chargetransport and partially to the light scattering effect (The film-thickness-dependent-h is shown in Fig. S5.). By suitably

eNP film (dee) at various magnifications.

Fig. 5. Optical properties of the NP, NW, and NWeNP films: transmittance (a) and reflectance (b).

Fig. 6. IeV characteristics (a) and IPCE spectra (b) of the NP, NW, and NWeNP-DSCs.

E.-H. Kong et al. / Materials Chemistry and Physics 143 (2014) 1440e14451444

constructing the NWeNP hybrid photoelectrode, one can achieve aremarkable improvement in the photocurrent density, which leadsto the increase in the power conversion efficiency. The NWeNPdevice exhibited h of 6.2% with Jsc of 10.5 mA cm�2 and Voc of844 mV under AM 1.5, 1 Sun. Since the NPs possess much higherinternal surface area than the NWs (72.5 m2 g�1 for the NP),combination of these two nanomaterials can bring about an addi-tional effect on the photocurrent density. This is quantitativelyverified in the dye-loading capacity of each working electrode:2.947 � 10�8 mol cm�2 for the NW film, 5.937 � 10�8 mol cm�2 forthe NP film, and 4.133 � 10�8 mol cm�2 for the NWeNP film(Table 1).

The incident-photon-to-current conversion efficiency (IPCE)provides us with more detailed information on the origin of thelight harvest. It is observed in Fig. 6b that the NP-DSC exhibitshigher IPCE values than the NW cell in the shorter wavelengthregion. On the contrary, the NW-DSC performs markedly higherIPCE values than the NP device in the longer wavelength range. Thisopposing result comes from two factors: higher dye-loading abilityof the NP film and stronger light confinement of the NW film. Onthe other hand, the integration of NWs and NPs noticeably im-proves IPCE values over the whole spectral range. Therefore, it can

Table 1Photovoltaic parameters of devices with the dye-loading capacity of the NP, NW, andNWeNP films.

Sample Voc (mV) Jsc (mA cm�2) FF h (%) Adsorbed dye(10�8 mol�1

cm�2)

NP 796 � 2.3 7.8 � 0.11 0.69 � 0.02 4.2 � 0.10 5.937NW 835 � 0.9 7.6 � 0.11 0.70 � 0.01 4.5 � 0.11 2.947NWeNP 844 � 1.3 10.5 � 0.12 0.70 � 0.01 6.2 � 0.08 4.133

be inferred that the combined effects of high dye-loading, fastcharge transport, and light confinement lead to an enhancedphotocurrent generation in the NWeNP cell, which eventuallyexpedites the overall efficiency [29].

The electrochemical impedance spectroscopy (EIS) wasemployed to examine the interfacial reactions in the NP-, NW-,and NWeNP-DSCs (Fig. 7). Typical Nyquist plots of three deviceswere recorded at the Voc under dark using the equivalent circuitshown in the inset of Fig. 7a. Fig. 7b presents the Bode phase plotsof the EIS. It is widely known that the second semi arc (at the lowfrequency region) is related to the charge-recombination resis-tance between the photoelectrode and the electrolyte, which iscrucial to an overall light harvest. The recombination lifetime ofphotoelectrons (sr) is determined by sr ¼ 1=2pfr ; where fr is thecharacteristic peak frequency at the frequency region of 10�1e102 Hz. Accordingly, it can be estimated that sr is noticeablyincreased in the NW-based devices, which implies that the darkcurrent is effectively suppressed at the photoelectrodeeelectro-lyte interface, leading to an improved Voc. It is also worth pointingout that the recombination reaction is even more retarded afterconstructing the hetero-junction of rutile-anatase TiO2 (NWeNP).It is widely known that the conduction band energy level of rutilephase is lower than that of anatase. Since the vertically-alignedrutile TiO2 NWs are surrounded by the anatase TiO2 NPs in thepresent hybrid photoelectrode, the outer nanostructure (theinlaid NPs) naturally forms an energy barrier at the interface be-tween the core nanostructure (the NWs/dye/electrolyte). Thisparticular energy band structure can promote the electrontransport in the photoelectrodewhile the charge recombination iseffectively retarded [37e39]. Therefore, we can conclude that thesingle crystalline NWs provide us with a highway for the chargetransport, and this kinetic advantage seems to be more evident inthe NWeNP photoelectrode.

Fig. 7. Impedance spectra of the NP, NW, and NWeNP-DSCs under dark; Nyquist plots (a) and Bode phase plots (b).

E.-H. Kong et al. / Materials Chemistry and Physics 143 (2014) 1440e1445 1445

4. Conclusions

In summary, w4-mm-thick vertically-aligned rutile TiO2 nano-wires (NWs) were synthesized on the FTO glass substrate via asimple solvothermal process. Then, anatase TiO2 nanoparticles(NPs) were infiltrated into the single crystalline NWs in order toconstruct a hybrid photoelectrode (NWeNP) by using the ultra-sonication. In detail, the NWs function not only as a highway forthe electron transport but also as multiple light scatterers. On thecontrary, the inlaid NPs compensate an insufficient surface area ofthe NW film. Consequently, the NWeNP film creates synergeticeffects of the enhanced light confinement, charge collection, anddye-loading. As a result, maximum 6.2% efficiency was achieved ina very thin film by adopting the NWeNP photoelectrode (w4 mm).This corresponds to w48% improvement over the efficiency of theNP device at the same film thickness (w38% for the NW cell). Weexpect that the present hybridization approach can be extended tovarious photo-electrochemical applications, especially where thecompactness of the device is strictly required.

Acknowledgments

This work was financially supported by the Basic ScienceResearch Program (Grant No. 2012R1A1A2041628) through theNational Research Foundation (NRF) of Korea funded by the Min-istry of Education, Science and Technology.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.matchemphys.2013.11.060.

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