4620 Phys. Chem. Chem. Phys., 2012, 14, 4620–4625 This journal is c the Owner Societies 2012

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 4620–4625

Sea urchin TiO2–nanoparticle hybrid composite photoelectrodes for

CdS/CdSe/ZnS quantum-dot-sensitized solar cellsw

Eui-Hyun Kong,aYong-June Chang,

aYoon-Cheol Park,

aYeon-Hee Yoon,

a

Hyun-Jin Parkband Hyun Myung Jang*

a

Received 23rd December 2011, Accepted 7th February 2012

DOI: 10.1039/c2cp24106d

The sea urchin TiO2 (SU TiO2) particles composed of radially aligned rutile TiO2 nanowires are

successfully synthesized through the simple solvothermal process. SU TiO2 was incorporated into

the TiO2 nanoparticle (NP) network to construct the SU–NP composite film, and applied to the

CdS/CdSe/ZnS quantum-dot-sensitized solar cells (QDSSCs). A conversion efficiency of 4.2% was

achieved with a short-circuit photocurrent density of 18.2 mA cm�2 and an open-circuit voltage

of 531 mV, which corresponds to B20% improvement as compared with the values obtained

from the reference cell made of the NP film. We attribute this extraordinary result to the light

scattering effect and efficient charge collection.

Introduction

Dye-sensitized solar cells (DSSCs) consisting of a wide band-

gap semiconductor film, a dye and an electrolyte have been

regarded as a promising alternative to silicon-based photo-

voltaic cells.1 Instead of using a dye, the sensitization of a

photoanode can be achieved through modification of the oxide

surface with a narrow band-gap semiconductor quantum-dot

(QD). Quantum-dot-sensitized solar cells (QDSSCs) recently

have attracted a great deal of attention owing to their advan-

tages over DSSCs. The advantages include (i) higher molar

extinction coefficient2 of QDs than ruthenium complexes,3,4

(ii) tunable energy gaps,5 and (iii) multiple exciton generation6

which may potentially lead to a theoretical maximum efficiency7

higher than that of DSSCs.

Research on high-performance QDSSCs has been motivated by

the following achievements: (i) various semiconducting metal

chalcogenides (CdS,8,9 (B1.35%) CdSe,10–12 (B3.21%) PbS,13

etc.), (ii) surface modification of QDs with ZnS passivation14,15

(B2.02%) and the QD-size effect,11 (iii) improvement in linking

between the QDs and the TiO2 matrix via chemical bath deposition

(CBD) and self-assembly binding (SAB),14,15 (iv) non-corrosive

Co2+/Co3+ and polysulfide (S2�/Sx2�) complexes16,17 (B1.15%)

used as redox mediators, (V) Pt-free counter electrodes made of Au,

Cu2S, and carbon18–20 (B4.22%) and (Vi) structural optimization

of a TiO2 photoelectrode (B4.79%).21 In spite of extensive studies,

however, efficiencies of most QDSSCs are still below 4%.

Recently, hierarchically structured oxide materials such as

ZnO aggregates,22 nano-embossed hollow spherical TiO2,23

mesoporous TiO2 beads24–26 have been reported as functional

photoelectrode materials in the sensitized photovoltaic

devices. These porous spherical systems consist of nano-sized

crystallites that are aggregated to create micrometre- or

submicrometre-sized secondary particles and thereby can

function as light scattering materials maintaining a large

internal surface area for sufficient sensitizer-uptake. Thus,

these bi-functional materials having 0–3 hierarchy seem to

be more appealing than traditional nanocrystalline TiO2.

However, slow-trap-limited electron transport remains a

problem in the charge collection kinetics.

In this work, we have synthesized a different type of

hierarchical nanostructure: sea urchin TiO2 (SU TiO2) formed

by clustering nanoneedles that has a mean diameter of about

50 nm and a length of a few micrometres to construct a radially

aligned particle that is 4 to 7 mm in diameter. To our knowledge,

this is the first demonstration of 1–3 hierarchical structures for

the QDSSCs. It has been widely reported that 1D nanostructures

have superior electron lifetime and recombination time than

traditional nanoparticle films. In addition to the advantage of

charge collection efficiency, 1D structures can effectively scatter

visible light, thereby enhancing the overall light harvesting.

We present a new approach of implementing the benefits of

nanostructures by suitably combining the SU TiO2 and tradi-

tional TiO2 nanoparticles (NP) to construct a hybrid composite

photoelectrode (SU–NP) for the QDSSCs. In particular, the

a Laboratory of Multiferroic and Photovoltaic Nanostructures,Department of Materials Science and Engineering, and Division ofAdvanced Materials Science, Pohang University of Science andTechnology (POSTECH), Pohang 790-784, South Korea.E-mail: hmjang@postech.ac.kr; Fax: +82-54-279-2399;Tel: +82-54-279-2138

bDepartment of R&D, National Center for Nanomaterials Technology(NCNT), Pohang University of Science and Technology(POSTECH), Pohang 790-784, Republic of Korea

w Electronic supplementary information (ESI) available: Effects ofseveral parameters in the synthetic procedure, diffuse reflectance ofthe SU TiO2, Dn and Ln, EIS analysis are shown. See DOI: 10.1039/c2cp24106d

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nanoparticles offer a high surface area (73.29 m2 g�1) for

sufficient QD loading, whereas SU TiO2 particles provide a

highway for fast charge collection and multiple scattering

centers within the photoelectrode (Scheme 1). CdS/CdSe/ZnS

QDSSCs made of the SU–NP composite film exhibited remark-

able improvement in power conversion efficiency: 4.2% versus

3.5% for the reference cell made with the NP film.

Experimental

Preparation of the photoelectrode with the SU TiO2 (SU) and

the TiO2 nanoparticles (NP)

The SU TiO2 particles were synthesized by a simple solvothermal

process through the modification of the reported procedure for

TiO2 nanowire growth.27 For this, a precursor solution was

prepared: 15 ml titanium butoxide, 24 ml titanium tetrachloride

(1 M in toluene), and 8 ml hydrochloric acid (37 wt%) were

dissolved in 80 ml toluene. After centrifugation (3700 rpm), an

80 ml transparent solution was collected for solvothermal treat-

ment. Then, 16 ml absolute ethanol was added as a dispersing

agent to the above solution (Fig. S1, ESIw). Finally, the mixture

underwent autoclaving at 180 1C for 4 h. The autoclaved

precipitates were washed with absolute ethanol.

The known synthetic procedure was used to prepare 20 nm-

sized TiO2 nanoparticles (NP).28 As-prepared nanoparticles

were used for the SU TiO2–NP composite.

To make a viscous paste, SU TiO2 and NP were mixed with

a polymer binder and a-terpineol. The weight% in the SU–NP

paste was 12.5% SU TiO2, 12.5% NP, 10% ethyl cellulose,

and 65% a-terpineol. The mixture was then alternatively

stirred and sonicated three times. Since the TiO2 sol contained

a significant portion of ethanol, the mixture underwent rotary

evaporation to remove the residual ethanol. The viscous paste

was collected and kept in an opaque glass bottle. With the

same procedure, a NP paste was prepared for reference: 18%

NP, 9% ethyl cellulose, and 73% a-terpineol.

The viscous paste was printed on the FTO glass substrate.

After calcination at 500 1C for 30 min, the films were immersed

in a 40 mM aqueous TiCl4 solution at 70 1C for 30 min,

and washed with water and ethanol. The treated films were

heated at 500 1C for 30 min. The cross-sectional SEM images

(Fig. S2, ESIw) clearly show that the thicknesses of two

samples are almost identical (NP: 14.5 mm, SU–NP: 14.6 mm).

Deposition of CdS/CdSe/ZnS QDs

The successive ionic layer adsorption and reaction (SILAR)

technique,29 known as a modified version of chemical bath

deposition, was employed to assemble the CdS QDs on the

TiO2 anode. For the deposition of the CdS QDs from the

precursor solutions, two separate solutions were prepared: 0.1 M

Cd(NO3)2 in ethanol and 0.1 M Na2S in methanol. Two distinct

working electrodes (SU–NP composite, NP) were immersed into

the Cd2+ solution and the S2� solution successively for 2 minutes

each. After dipping into one solution, the electrodes were rinsed

with ethanol andmethanol to remove the excess of each precursor.

The number of the SILAR cycles was optimized to be 6 in our

experimental conditions. The CdS-deposited photoanodes were

dried on a hot plate at 60 1C and washed with deionized water.

Then, the CdSe QDs were deposited on the TiO2 films using

chemical bath deposition (CBD). For the nucleation and growth

of the CdSe QDs, TiO2/CdS films were immersed in an aqueous

solution containing Cd(SO4) :Na2SeSO3 :N(CH2COONa)3 =

80 mM:80 mM:160 mM at 10 1C for 12 h. The Na2SeSO3

aqueous solution was prepared by refluxing Se (0.5 M) in an

aqueous solution of Na2SO3 (0.6 M) at 120 1C for 7 h.

After the deposition of the CdSe QDs during the CBD process,

the sensitized films were rinsed with deionized water. Then, the

films were dipped again in 0.5 M aqueous Na2SeSO3 solution at

70 1C for 30 min to remove residual Cd2+ ions, which otherwise

could cause the formation of the CdS QDs on the deposited CdSe

QDs (Fig. S3, ESIw) by reacting with S2� ions during the ZnS

passivation.30 ZnS was finally deposited on top of the sensitized

films through the SILAR process. For the ZnS passivation, two

separate solutions were used: 0.1 M aqueous Zn(NO3)2, and

0.1 M aqueous Na2S. Two SILAR cycles were used.

Cell fabrication

The counter electrode was prepared by dripping a Pt solution

(Solaronix, Platisol T) on the FTO glass substrate, which was

followed by thermal treatment at 500 1C for 30 minutes. For

the cell assembly, a hot-melt 60 mm-thick Surlyn (Solaronix,

Meltonix 1170-60) was used as a spacer between the working

electrode and the Pt electrode. The polysulfide electrolyte was

composed of 1 M S and 1 M Na2S in deionized water.

Characterization

Morphology and crystallography of the SU TiO2 particle were

studied by employing a field-emission scanning electron

microscope (FE-SEM; JEOL, JSM-7401F) and an X-ray

diffractometer (Rigaku, RINT 2000). For the sensitized TiO2

films, higher resolution TEM images and EELS analysis data

were obtained by a TEM (JEOL JEM-2200FS) in NCNT

(National Center for Nanomaterials Technology in Pohang).

Nitrogen sorption isotherms were obtained by utilizing a

Scheme 1 A schematic diagram of a sea urchin TiO2–nanoparticle

composite (SU–NP) film that has fast charge collection and a light

scattering effect.

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4622 Phys. Chem. Chem. Phys., 2012, 14, 4620–4625 This journal is c the Owner Societies 2012

Micrometrics Tristar 3000 system. The photovoltaic performance

wasmeasured under an illumination of a solar simulator (Newport,

Oriel class A, 92251A) at one sun (AM 1.5, 100 mW cm�2)

with an active area of 0.16 cm2. The incident-photon-to-

current conversion efficiency (IPCE) values were recorded as

a function of the wavelength from 350 nm to 850 nm (PV

Measurements, Inc). A 75 W Xenon lamp was used as a light

source with a monochromator. Calibration was performed with

a NIST-calibrated photodiode G425 as a reference (chopping

frequency at 4 Hz). The monochromatic power density was

calibrated using a reference Si photodiode as a standard from

the NIST. UV-visible reflectance spectra were recorded with a

Perkin Elmer UV-Vis spectrometer (Lambda 750S). Electrical

impedance spectra were measured using an impedance analyzer

(BioLogic, SP-300), with a frequency ranging from 10�1 to

106 Hz, and analyzed using Z-view software. Intensity-modulated

photovoltage spectroscopy (IMVS) and intensity-modulated

photocurrent spectroscopy (IMPS) measurements were carried

out on a electrochemical workstation (Zahner, Zennium) with a

frequency response analyzer under a modulated red light emit-

ting diode (625 nm) driven by a source supply (Zahner, RTL01),

which can provide both DC and AC components of illumination.

The modulated light intensity was 10% or less than the base light

intensity. The frequency range was set from 10�2 Hz to 102 Hz.

Results and discussion

Fig. 1(a–c) show morphological features of the calcined SU

TiO2 particles. The high-magnification FE-SEM image

indicates that the SU TiO2 particles are composed of nano-

needles that have a mean diameter of about 50 nm and a length

of a few micrometres to construct a radially aligned particle that

is 4 to 7 mm in diameter. In the inset of Fig. 1b, broken particles

reveal the internal nanostructure of the SU TiO2. It seems that

many nanoneedles are densely packed within the spherical

particle. The high-resolution image of a single nanoneedle con-

stituting the SU TiO2 (Fig. 1d) shows that the material is fully

crystalline with the lattice spacing of 0.324 nm, which corre-

sponds to the rutile (110) plane.

Time-dependent analysis provides more detailed informa-

tion on the formation process of the SU TiO2. Fig. S4a (ESIw)shows that nanoparticles were initially created after 30 min

solvothermal reaction. With the prolongation of reaction time

(Fig. S4b, 1 h, ESIw), the aggregates were formed which finally

led to the 1–3 hierarchically-structured SU TiO2 (Fig. S4c, 4 h,

ESIw). The anisotropic growth of nanoneedles on the spherical

particle surface can be understood in terms of shape-

controlled chemistry.31–33 It is thought that Cl� ions adsorb

selectively onto the (110) crystal plane,34 which limit further

growth of this plane, resulting in anisotropic growth and hence

radially aligned TiO2 nanoneedle formation.

The XRD patterns indicate that two distinct samples are

in different crystalline phases (Fig. 2): SU TiO2 in rutile,

and NP in anatase, respectively (JCPDS card No. 21-1276

and 21-1272).

Fig. 3 shows the STEM images of the CdS/CdSe/ZnS QD-

sensitized NP (a) and SU films (c). Most of the QDs seem to be

individual on the NP film with less agglomeration of the QDs,

and a partially uncovered surface is also found. In contrast,

the SU TiO2 seemingly forms larger QD agglomerates with

better coverage on the surface of TiO2 nanoneedles. This

implies that the surface coverage of the QDs is superior at

the SU TiO2. Electron energy loss spectroscopy (EELS) shows

spatial distribution of the QDs deposited on the NP and SU

TiO2 samples at a high resolution.26 Magnified TEM images of

NP and SU TiO2 and the corresponding EELS data are shown

in Fig. 3b and d.

The current–voltage characteristics of the QDSSCs were

studied under one sun illumination (Fig. 4). The photovoltaic

properties of the SU–NP and NP cells are summarized in

Table 1. The SU–NP cell shows Voc, Jsc, fill factor (FF), and

power conversion efficiency (Z) of 531 mV, 18.2 mA cm�2, 0.43

and 4.2%, respectively; the reference NP cell shows 510 mV,

15.2 mA cm�2, 0.45, and 3.5%. Both Jsc and Z of the SU–NP

device are improved by around 20%, compared to those of the

NP cell. The quantum efficiency (IPCE) provides detailed

information on the light harvesting mechanism (inset of

Fig. 4). It is observed that the overall IPCE values of the

SU–NP cell are higher than the NP cell over the whole spectral

range. This implies that efficiency improvement in the SU–NP

composite QDSSC is mainly due to the enhanced current

density.

Fig. 1 SEM images of the SU TiO2 particles at different magnifica-

tions (a, b, c). The inset of (b) shows a cross-sectional view of broken

particles. A TEM image of a 1D nanocrystal in the SU TiO2 particle

(d). The inset of (d) shows a selected-area electron diffraction pattern.

Fig. 2 XRD patterns of the SU TiO2 particles and the TiO2 nano-

particles (NP).

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It is well known that the photocurrent density (Jsc) can be

calculated by the following expression:35

Jsc = qZlhZinjZccIo (1)

where q is the elementary charge, Zlh is the light harvesting

efficiency of a cell, Zinj is the charge-injection efficiency, Zcc is thecharge-collection efficiency, and I0 is the light flux. Here, Zlh is

commonly determined by the amount of adsorbed QDs and

light scattering of the films; Zcc is largely determined by the

competition between the charge collection and recombination.36

Since identical QDs (CdS/CdSe/ZnS) are applied both to the

SU–NP and NP films, Zinj values of two QDSSCs are assumed

to be the same. Accordingly, it can be concluded that the

photocurrent density under our experimental conditions is

determined by two parameters: the light harvesting efficiency

(Zlh) and the charge-collection efficiency (Zcc).The Zlh value is explained by the reflectance spectra. Since the

micrometre-sized SU TiO2 particles can function as scattering

centers (Fig. S5, ESIw), the SU–NP composite film can effec-

tively confine visible light (450 to 750 nm). We concluded that

improved red response of the NP–SU film contributes to the

enhancement in the Zlh, which is reflected to the increased IPCE

values at the long wavelength region. After the sensitization

with the CdS/CdSe/ZnS QDs, the reflectance spectra take on a

new aspect (Fig. 5b). The reflectance of each sample is reduced

at the short wavelength range (under 650 nm) due to the

absorption of the incident light by the QDs.

Even though the SU TiO2 particles possess an inferior QD

loading capacity owing to a lower surface area of 10.16 m2 g�1

(NP: 73.29 m2 g�1), the SU–NP device yielded remarkably

larger IPCE values at the short wavelength region. This

contradictory result comes from the difference between Zcc intwo distinct QDSSCs (SU–NP and NP cells). Note that Zcc isgenerally determined by the intensity-modulated photocurrent/

photovoltage spectra (IMPS/IMVS). In brief, IMPS measures

the periodic current response to the intensity-modulated

light, providing kinetic information on the charge transport

under short-circuit conditions. On the other hand, IMVS is

conducted with the same perturbed light but under open-

circuit conditions, which offers the recombination lifetime.

The electron transport time (tt) can be estimated from the

equation tt = 1/(2pft), where ft is the characteristic frequencyminimum of the IMPS imaginary component. Similarly,

the recombination lifetime (tr) can be determined from the

relation tr = 1/(2pfr), where fr is the characteristic frequency

Fig. 3 STEM images of the CdS/CdSe/ZnS QD-sensitized TiO2 nanocrystals. (a, b) The sensitized NP film, and (c, d) SU film. Each inset

compares EELS data of these two sensitized films.

Fig. 4 I–V characteristics of the CdS/CdSe/ZnS QDSSCs with the

SU–NP composite film and the reference NP film under illumination

(AM 1.5, 100 mW cm�2). The inset shows the corresponding IPCE

spectra.

Table 1 Photovoltaic parameters of the CdS/CdSe/ZnS QDSSCsmade of the SU–NP hybrid composite and the NP films

Sample Voc/mV Jsc/mA cm�2 FF Z (%)

SU–NP 531 18.2 0.43 4.2NP 510 15.2 0.45 3.5

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4624 Phys. Chem. Chem. Phys., 2012, 14, 4620–4625 This journal is c the Owner Societies 2012

minimum of the IMVS imaginary component. Fig. 6a presents

the plot of the electron transport time (tt). It is noticeable

that tt of the SU–NP hybrid composite-based QDSSC is

larger than that of the reference NP cell. The relatively faster

electron transport seems to be due to fewer interparticle

junctions within the SU–NP photoelectrode.37,38 The effective

electron diffusion coefficient (Dn) was also estimated from the

expression: Dn = L2/(2.35tt),39 where L is the film thickness.

The Dn of the SU–NP cell was 2.3 times larger than that of the

NP cell under 100 W m�2 illumination (Fig. S6, ESIw).Fig. 6b shows that tr of the SU–NP cell is longer than that

of the NP device over the whole light-intensity range. The

increased tr is attributed to the presence of the relatively

defect-free single crystalline TiO2 nanoneedles that have far

fewer grain boundaries, which may result in the reduced

electron trapping phenomena.40,41 Additionally, better surface

coverage of the QDs at the SU–NP film can promote longer

tr.38 This assumption is in agreement with the previous

analysis obtained from the TEM images. This result is also

consistent with Nyquist plots obtained by the electrochemical

impedance spectroscopy (EIS) employed to examine the inter-

facial reactions in the QDSSCs (Fig. S7, ESIw). The effective

electron diffusion length (Ln) was obtained from Dn and trusing the relationship Ln = (Dn � tr)

1/2. As shown in Fig. S6

(ESIw), the Ln (35.69 mm) of the SU–NP cell is around twice as

thick as that of the NP cell (17.97 mm) under the light intensity

of 100 W m�2. On the basis of the increased tr, improved

photoelectron densities n (n p Jsctr)42 yield a higher Voc

43 in

the SU–NP cell.

The charge-collection efficiency (Zcc) can be calculated by

the two values obtained from the above IMPS and IMVS

analysis via the following equation:44

Zcc = 1 – (tt/tr) (2)

Consequently, Zcc of the SU–NP cell (92.9%) was 20.6%

larger than that of the NP device. Therefore, it is reasonable

to conclude that higher Jsc is mainly due to the higher Zcc, andpartially due to the higher Zlh of the SU–NP hybrid composite

QDSSCs.

Conclusions

In summary, sea urchin TiO2 (SU TiO2) particles composed of

rutile TiO2 nanoneedles and TiO2 nanoparticles (NP) have

been synthesized through a solvothermal process. For the cell

fabrication, the SU–NP hybrid composite film was prepared,

and compared with the NP film. CdS/CdSe/ZnS QDSSCs

made of the SU–NP film achieved a noticeable improvement

in power conversion efficiency: 4.2% versus 3.5% for the

reference cell made of the NP film. We attribute this extra-

ordinary result to the higher photocurrent density (Jsc) of the

SU–NP cell (18.2 mA cm�2), corresponding to about 20%

improvement in comparison with the NP cell. The origin of the

enhanced Jsc was investigated in terms of two parameters: the

Fig. 5 Diffuse reflectance spectra of the SU–NP composite film and the NP film before (a) and after (b) QDs deposition.

Fig. 6 Incident light intensity dependent transport time constant (a) and the recombination time constant (b) for the QDSSCs based on the

SU–NP composite film and the NP film measured under 625 nm LED illumination.

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light harvesting efficiency (Zlh) and the charge-collection efficiency

(Zcc). The diffuse reflectance spectra and the intensity-modulated

photocurrent/photovoltage spectra confirmed that both Zlh and

Zcc were enhanced due to the internal light scattering and more

efficient charge collection. Therefore, we concluded that the

SU–NP hybrid composite can be exploited as a versatile building

block for the QDSSCs.

Acknowledgements

This work is financially supported by Pohang Steel Corporation

(POSCO) through Steel Nano-Fusion Program (Project No.

2010Y110) and partly by the World Class University (WCU)

program through the National Research Foundation of Korea

(Grant No. R31-2008-000-10059-0).

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