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Visible-light-response iodine-doped titanium dioxide nanocrystals fordye-sensitized solar cells†

Qian Hou,ab Yanzhen Zheng,ab Jian-Feng Chen,*b Weilie Zhou,c Jie Denga and Xia Tao*a

Received 3rd October 2010, Accepted 24th November 2010

DOI: 10.1039/c0jm03327h

We report on the synthesis of visible-light-response iodine-doped TiO2 nanocrystals (I-TNCs), and

their implementation as photoanodes of dye-sensitized solar cells (DSSCs). The preparation of I-TNCs

starts with hydrolysis of titanium isopropoxide in iodine-based aqueous system, followed by

hydrothermal treatment and annealing process to achieve anatase structure. The I-TNCs were

characterized by TEM, SEM, XRD, EDS and XPS. Under an optimized I/Ti doping ratio of 20 mol%,

the photovoltaic performance of the cell based on I-TNCs is significantly better, with an energy-

conversion efficiency of 7.0% under 100 mW cm�2 illumination, which is 42.9% higher than that of the

cell based on undoped TNCs. Interestingly, the I-TNCs-based cell shows a higher conversion efficiency

of 10.0% under 30 mW cm�2 illumination. The improved performance is explained by the expanded

visible-light harvesting (characterized by incident photon to current conversion efficiency and UV-vis

absorption spectra), lowered recombination resistance (characterized by electrochemical impedance

spectroscopy, Rrec) together with prolonged electron lifetime se. These results suggest substantial

potential of TiO2 nanocrystals with controlled doping in DSSC application.

Introduction

Dye-sensitized solar cells (DSSCs) have attracted particular

attention as low-cost alternatives to conventional silicon-based

photovoltaic devices.1–3 One of the key elements in DSSCs is the

semiconductor photoanode, which transfers the electrons from

the dye molecules to the transparent conducting substrate and

concurrently allows the electrolytes to diffuse to the anchored

dyes.4,5 For an efficient DSSC, the porous electrode composed of

anatase TiO2 nanocrystals (10–20 nm in diameter) is essential due

to the high internal surface area which maximizes the uptake of

dye molecules, thereby increasing the cell performance.6,7

However, such TiO2 nanocrystalline films usually show poor light

harvesting due to the shortage of optical elements in the electrode

films. One way to successfully enhance photocapture efficiency

and optical absorption of the photoelectrode films is the intro-

duction of highly scattering layers.8–12 Many research groups have

succeeded in improving the photon-to-current conversion

aKey Laboratory for Nanomaterials of the Ministry of Education, BeijingUniversity of Chemical Technology, Beijing, 100029, China. E-mail:taoxia@yahoo.com; Fax: +86-10-64434784; Tel: +86-10-64453680bResearch Center of the Ministry of Education for High GravityEngineering & Technology, Beijing University of Chemical Technology,Beijing, 100029, China. E-mail: chenjf@mail.buct.edu.cn; Fax: +86-10-64434784; Tel: +86-10-64446466cAdvanced Materials Research Institute, University of New Orleans, NewOrleans, LA, 70148, USA

† Electronic supplementary information (ESI) available: SEM images ofundoped TNCs, 10 mol% I-TNCs and 30 mol% I-TNCs films (Fig. S1).See DOI: 10.1039/c0jm03327h

This journal is ª The Royal Society of Chemistry 2011

efficiency of TiO2-based DSSCs by light-scatterer.10–12 As exam-

ples, Qiu et al.13 reported double-layered photoanodes from

variable-size anatase TiO2 nanospindles that not only provide

high specific surface area, but also exhibit stronger aggregation-

induced light scattering in the visible wavelength of the solar

spectrum. Recently, our group also developed a new bilayer-

structured film with TiO2 nanocrystals as the underlayer and TiO2

nanotubes as the overlayer exhibiting an overall energy-conver-

sion efficiency of 44.7% higher than that formed by pure nano-

crystalline TiO2.14 Additionally, the enhanced visible absorption

and photocurrent generation can also be achieved by doping

nonmetal elements in TiO2 nanostructures.15–24 For instance, Ma

and coworkers20 reported that a nitrogen-doped nanocrystalline

TiO2 film as a favorable electron-transfer mediator in photovol-

taic devices exhibited improved incident photon to current

conversion efficiency (IPCE) and great stability due to the

replacement of oxygen-deficient TiO2 by visible-light-active

nitrogen-doped TiO2, which was further testified by other

research groups.21–24 Indeed, iodine has been demonstrated to be

promising for n-type doping and iodine-doped (I-doped) TiO2 has

been shown to be effective in narrowing TiO2 band gap subjected

to absorbing visible light of the solar spectrum in recent years.25–28

More importantly, the recombination of electron-hole pairs could

be sufficiently inhibited because the doping iodine sites can act as

trapping sites to capture the photo-generated electrons. Further-

more, the trapped holes generated in I-doped TiO2 were evidenced

to have no significant oxidation reactivity toward substrates

adsorbed on the TiO2 surface.29–31 All the features above of I-

doped TiO2 can be considered as favorable influencing factors for

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improving overall energy-conversion efficiency of TiO2-based

DSSCs. However, to date, I-doped TiO2 nanomaterials for

DSSCs have yet to be reported.

In this paper, a new visible-light-response I-doped TiO2 pho-

toanode was fabricated through the packing of nanocrystal

particles that were prepared via a combination of sol–gel process

and hydrothermal treatment using iodic acid as doping source. It

was observed that as-prepared I-doped TiO2 nanocrystals (I-

TNCs) exhibited strong absorption in the 400–550 nm range with

a red shift in the band gap transition. In the I-doped TiO2 film,

nanocrystal particles offered a large internal surface area for

sufficient dye-adsorption; on the other hand, the introduction of

iodine in the TiO2 matrix was able to promote the light-har-

vesting efficiency as well as the overall energy-conversion effi-

ciency of the cells. By fabricating the I-doped TiO2 photoanode

with a low volatile 3-methoxypropionitrile-based electrolyte in

the DSSC, we achieved conversion efficiencies of 7.0% and 10.0%

under 100 and 30 mW cm�2 illumination, respectively. The

mechanism behind the improvement caused by I-doping was also

discussed.

Experimental

Materials

Titanium(IV) isopropoxide (TTIP), anhydrous lithium iodide

(LiI), iodide (I2), 4-tert-butylpyridine (TBP), 1,2-dimethyl-3-

propylimidazolium iodide (DMPII), 3-methoxypropionitrile,

polyethylene glycol M-20000 (PEG20000) and chloroplatinic

acid (H2PtCl6) were received from Sigma. cis-Di(thiocyanate)-

bis(2,20-bipyridyl-4,40-dicarboxylate)-ruthenium(II) (N3) was

obtained from Solaronix (Aubonne, Switzerland). Iodic acid

(HIO3), acetic acid (HAc), nitric acid (HNO3), ethanol

(C2H5OH), and all other chemicals used in this study were

purchased from commercial sources of analytical grade and used

without further purification. Milli-pore water with a resistivity of

18.2 MU cm was used throughout the study.

Preparation and characterization of I-TNCs

As an example, 20 mol% I-TNCs (I/Ti molar ratio of 20%) were

obtained as follows: 70 mmol HAc was dropwise added into 75

mmol TTIP under stirring for 15 min. Then the modified titanic

precursor was poured into distilled water containing HIO3 (15

mmol) with vigorous stirring. The obtained suspension was

stirred for 1 h, followed by heating from room temperature to 75�C at a rate of 1 �C min�1 and peptizing for 75 min. After that, the

suspension was transferred into Teflon-lined autoclave for

thermal treatment at 180 �C for 12 h. The resultant I-TNCs

colloids were dispersed with an ultrasonic titanium probe at

a frequency of 15 pulses per s, and then centrifuged and washed

with ethanol for several times. Finally, the obtained products

were calcined at 500 �C for further characterizations. In parallel,

other samples with different I-doping ratios (I/Ti molar ratio of

0, 10%, 30%) were also synthesized by following the same

procedure as mentioned above.

The morphology of the synthesized I-TNCs was observed by

transmission electron microscopy (TEM) and high resolution

transmission electron microscopy (HR-TEM) images character-

ized on a JEOL JEM-3010 microscope. The surface area was

3878 | J. Mater. Chem., 2011, 21, 3877–3883

calculated by the Brunauer-Emmett-Teller (BET) method

recorded on an ASAP 2010 surface area analyzer. The crystalline

structures of the sintered samples were identified using an X-ray

diffractometer (XRD) (X0Pert PRO MPD, Panalytical). The

optical properties of I-TNCs were studied by the adsorption

spectra detected using a UV-vis spectrophotometer (UV 2501

spectrometer, Shimadzu). The chemical composition of doped

TNCs samples was determined by X-ray photoelectron spec-

troscopy (XPS) performed on a Thermo ESCALAB250 XPS

system using Al Ka as X-ray source.

Preparation and characterization of I-TNCs electrode films

The I-TNCs paste was obtained by dispersing 0.8 g PEG 20000 in

2 ml I-TNCs colloids by means of sufficient grinding. Fluorine-

doped tin oxide (FTO) conducting glass (Hartford, 14 U/sq, 80%

transmittance) used as the substrates were ultrasonically cleaned

sequentially in acetone, ethanol, water, and pretreated with 40

mM TiCl4 aqueous solution at 70 �C for 30 min. The paste was

then coated on the FTO glass by a typical doctor-blade technique

and dried at room temperature. Finally, the I-TNCs electrode

films were gradually heated under a programmed annealing

process, i.e. at 325 �C for 5 min, at 375 �C for 5 min, at 450 �C for

15 min, and at 500 �C for 15 min. The surface morphology of I-

TNCs films were observed by scanning electron microscope

(SEM) images characterized on a Hitachi S-4700 microscope.

Dye adsorption of I-TNCs electrode films

After thermal treatment with the programmed annealing

process, the I-TNCs electrode films were cooled to 80 �C and

immersed in a 0.5 mM ethanolic solution of the ruthenium

complex cis-[RuL2(NCS)2] (commercially called N3 dye) for 24 h

at room temperature. Then the sensitized films were rinsed with

ethanol and dried at room temperature. In order to examine the

loading amount of the dye in I-TNCs electrode, the dye was

desorbed from I-TNCs electrode into a 1.0 M NaOH solution in

water–ethanol (50 : 50, v/v). The absorption spectrum of the

desorbed-dye solution was detected using a UV-vis spectropho-

tometer (UV 2501 spectrometer, Shimadzu).32,33

Cell assembly and photoelectrochemical measurements

The counter electrodes were prepared by depositing a platinum

thin film on the FTO glass using an H2PtCl6 solution (2 mg Pt in

1 ml isopropanol), followed by annealing at 400 �C for 15 min.

The dye-adsorbed I-TNCs electrode and the Pt electrode were

assembled into a sandwich cell by heating at 80 �C using

a thermal adhesive film (Surlyn, 25 mm) as a spacer between the

electrodes. A drop of an electrolyte solution (0.1 M LiI, 0.12

M I2, 1.0 M DMPII and 0.5 M TBP in 3-methoxypropionitrile)

was infiltrated into the cell. The effective area of the cell exposed

in light was 0.25 cm2 (�0.5 cm � 0.5 cm).

The photocurrent-voltage characteristics and the electro-

chemical impedance spectroscopy (EIS) measurements of the

DSSCs were recorded by a electrochemical workstation

(CHI660C, ShangHai) under one sun condition using a solar

light simulator (Oriel 69911 1000 W xenon lamp, AM 1.5) with

the intensity of 100 mW cm�2. The frequency range was explored

from 0.1 Hz to 10 kHz. The applied bias voltage and

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ac amplitude were set at open-circuit voltage of the DSSCs and

10 mV between the FTO-Pt counter electrode and the FTO-

TiO2-dye working electrode, respectively. The impedance spectra

were analyzed by an equivalent circuit model for interpreting the

characteristics of the DSSCs. The incident photo to current

conversion efficiency (IPCE) was measured as a function of

wavelength from 400 to 800 nm on a Keithley model 2000

SourceMeter under short circuit conditions using a tungsten

source.

Fig. 2 (a) XRD patterns of as-prepared samples with different iodine

contents. (b) The fine-scanned (101) peak of the XRD patterns around

25�.

Results and discussion

The morphology and structure of I-TNCs

Fig. 1 shows TEM and HR-TEM bright field images of 20 mol%

I-TNCs. It can be seen that as-synthesized I-TNCs have

a uniform spherical structure with an average diameter of �15

nm (Fig. 1a). HRTEM analysis (Fig. 1b) reveals that a tiny

nanocrystal particle is single-crystal structure. The lattice fringe

of d¼ 0.35 nm, corresponding to the (101) plane, can be assigned

to the most stable anatase phase with a lower surface energy

according to the calculation by Oliver et al.34 For comparison,

undoped TNCs as well as 10 mol% I-TNCs and 30 mol% I-TNCs

were also synthesized by following the same procedure as 20

mol% I-TNCs, and it was found that all these particles main-

tained analogous structure and morphology (not shown here).

Structural characterization of the TiO2 powders was per-

formed by XRD measurements. Fig. 2 shows the XRD patterns

of the undoped and I-doped TNCs with various doping contents.

All diffraction peaks of as-prepared powders after annealing at

500 �C exhibit good agreement with anatase reference data

(JCPDS 21-1272) (Fig. 2a), indicating that the anatase nano-

crystalline structure is retained after doping. The average crys-

tallite size calculated from the full width at half-maximum

(FWHM) of the (101) peak based on the Scherrer’s formula (d ¼0.9l/b1/2cosq) is within a narrow range of 13–16 nm, and it is

shown to be well consistent with the TEM image shown in

Fig. 1a. Note that shift of the characteristic (101) peaks is

observed (Fig. 2b). With increasing the contents of iodine doped

in TiO2 nanocrystals the peak with the index of (101) gradually

shifted negatively from 25.45� to 25.22�, indicating that the

interplanar spacing between parallel crystallographic planes in

the anatase structure expanded after I-doping.35 By knowing that

the ionic radii of I5+ (0.095 nm) is larger than that of Ti4+ (0.060

nm), it is reasonably deduced that iodine should be incorporated

into the crystal structure.36,37 Such a diffraction peak shift has

Fig. 1 TEM (a) and HR-TEM (b) images of 20 mol% I-TNCs.

This journal is ª The Royal Society of Chemistry 2011

also been observed in other doping systems such as Nd-doped

TiO2 nanoparticles or Cl-doped ZnO nanowire arrays.38,39 The

physical parameters of all prepared samples including crystal

structure, crystallite size and BET surface areas are summarized

in Table 1.

To further investigate the chemical states of iodine and the

possible changes on the binding energies of Ti and O after I-

doping, we conducted the measurements of I 3d, Ti 2p and O 1s

core levels using the XPS technique. Fig. 3a shows a typical XPS

Table 1 Physical parameters of undoped and I-doped TNCs

SamplesCrystalstructure

Crystallitesize/nm

SBET/m2

g�1

Eg/eV

Pure TNCs Anatase 15.3 145.7 3.210 mol% I-

TNCsAnatase 14.4 153.4 2.9

20 mol% I-TNCs

Anatase 13.2 161.8 2.4

30 mol% I-TNCs

Anatase 15.7 146.4 2.2

J. Mater. Chem., 2011, 21, 3877–3883 | 3879

Fig. 3 XPS survey spectrum (a) and high resolution scan over I 3d (b),

Ti 2p (c) and O 1s (d) spectral regions of 20 mol% I-TNCs. Note: The

actual percentage of iodine in the doped TiO2 samples obtained by XPS is

estimated to be ca. 6 mol% for I/Ti doping ratio corresponding to

experimentally controlled ratio of 20 mol%.

Fig. 4 (a) Cross-sectional SEM image of I-TNCs film, showing

approximately 7 mm in thickness; and (b) surface SEM image for 20 mol%

I-TNCs film.

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survey spectrum of 20 mol% I-TNCs containing Ti, O and I

elements. There are doublet peaks of I 3d observed at the binding

energy around 619.8 and 624.0 eV (Fig. 3b). According to recent

published papers,29,40–42 the peak of I 3d5/2 at 624.0 eV, close to

HIO3 (623 eV), can be attributed to I5+ allowing replacing Ti4+ in

TiO2 matrix. The other peak detected at 619.8 eV may be origi-

nated from I� species, in agreement with the results reported by

Su et al.26 Furthermore, one can see that the intensity of I 3d5/2

peak at 624 eV is much weaker than the peak at 619.8 eV, which

could be understood by the reduced concentration of I5+ on the

surface of TiO2 due to substitutional I5+ in TiO2 lattice.25,27,40

Besides, that the I 3d5/2 peak of I5+ decreased with a significant

increase in the peaks of I� during the whole XPS measurement

was also observed by Tojo et al.,29 in which the doped I5+ was

proposed to be as an electron acceptor to produce measurable I�

species. Additionally, the high resolution XPS peaks for Ti 2p

(Fig. 3c) and for O 1s (Fig. 3d) also exhibit obvious changes after

I-doping. Compared with the binding energies of undoped TNCs

(458.9 eV for Ti 2p and 530.3 eV for O 1s), the binding energies of

I-TNCs shifted toward lower values (458.5 eV for Ti 2p and 529.8

eV for O 1s), corresponding to the reduction of 0.4 eV and 0.5 eV,

respectively. The low energy shifts of Ti and O binding energies

after I-doping, together with the low angle shift of (101) peak in

XRD pattern, synergistically provide favorable evidence for the

successful incorporation of iodine into the TiO2 matrix.23,24,29

Fig. 5 (a) Optical absorption spectra of undoped and I-doped TNCs

with different doping ratios. The inset shows the color change after iodine

doping. (b) Plots of the square of absorbance coefficient (a2) versus

photon energy (hn). Band gap values are determined from the extrapo-

lation of the linear portion of the absorption band to the photon energy

abscissa.

The morphology and optical properties of I-TNCs films

The monolayer I-TNCs films were formed using the doctor-blade

method onto FTO a substrate and subsequently followed by

thermal treatment. A cross-sectional SEM image provides

a direct observation on the fabricated film thickness (see Fig. 4a).

In this work, the thickness of all films was experimentally

controlled to be identical to �7 mm. A typical electrode film

fabricated by 20 mol% I-TNCs as shown in Fig. 4b exhibits

3880 | J. Mater. Chem., 2011, 21, 3877–3883

smooth and homogeneous surface morphology without cracks

and big agglomerates, facilitating electron transport between

neighbouring nanoparticles.43 Accordingly, we also obtained

undoped TNCs film as well as 10 mol% and 30 mol% I-TNCs

films with similar surface morphology and texture framework

compared with 20 mol% I-TNCs film. (see Fig. S1 of the ESI†).

The optical properties of undoped and doped TNCs films were

investigated by UV-vis absorption spectra. As can be seen from

This journal is ª The Royal Society of Chemistry 2011

Table 2 Comparison of Jsc, Voc, ff, and h together with the amount ofN3 dye for the films consisting of undoped and I-doped TNCs

Samples

Adsorbeddye/�10�7

mol cm�2 Jsc/mA cm�2

Voc/mV ff h (%)

Pure TNCs 1.99 10.9 672 0.67 4.910 mol% I-TNCs 2.18 12.5 702 0.70 6.220 mol% I-TNCs 2.37 14.1 715 0.67 7.030 mol% I-TNCs 2.01 10.3 681 0.62 4.4

Fig. 7 IPCE spectra of DSSCs based on undoped and I-doped (10

mol%, 20 mol% and 30 mol%) TNCs.

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Fig. 5a, pure TNCs sample shows the characteristic spectrum

with the fundamental absorbance stopping edge at �380 nm.

With increasing the iodine contents, I-doped samples exhibit

a noticeable expanded response in the visible light region from

400 to 550 nm; concomitantly, the color of pure and I-doped

TNCs films changes from colorless (transparent) to yellow

brown (see the inset of Fig. 5a). The visible light response is more

likely to be due to I-doping in the TiO2 crystal structure inducing

a new state lying close to the valence band edge.26,44,45 To further

gain insight into the absorption red-shift of materials originated

from I-doping, the band gap energies (Eg) of samples were esti-

mated by plotting square of absorbance coefficient (a2) versus

photon energy (hn) (see Fig. 5b).46 The photon energy can be

calculated from the transformation of absorption wavelength

using the following equation:

hnðeVÞ ¼ 1240

l0ðnmÞ

Extrapolation of the linear portion of the absorption band to

hn axis at zero absorption value yields the band gap energies

associated with light harvesting efficiency, and the results of Eg

are listed in Table 1. As expected, for pure anatase TNCs its band

gap is�3.2 eV, which is consistent with the very poor visible light

absorbance ability (see Fig. 5a). I-TNCs samples show an

obvious band gap narrowing ranging from �2.9 to 2.2 eV

accompanied with the increase of iodine ratios from 10 mol% to

30 mol%, which explains the excellent visible light absorbance

ability they demonstrate.

The photovoltaic performance of I-TNCs-based DSSCs

Solar cells consisting of undoped TNCs as well as a series of I-

TNCs (10 mol%, 20 mol%, 30 mol%) films were tested under the

AM 1.5 simulated sunlight with a power density of 100 mW

cm�2. Fig. 6 displays the photocurrent density-voltage (I–V)

curves of the cells and the performance characteristics including

short-circuit current density (JSC), open-circuit voltage (VOC), fill

factor (ff) and efficiency (h) are summarized in Table 2. For

Fig. 6 Photocurrent density-voltage curves of DSSCs based on undoped

and I-doped (10 mol%, 20 mol% and 30 mol%) TNCs, measured under

100 mW cm�2 illumination.

This journal is ª The Royal Society of Chemistry 2011

undoped TNCs cell, the JSC of 10.9 mA cm�2, VOC of 672 mV, ff of

0.67 and h of 4.9% were achieved. After introducing iodine into

TNCs, the JSC of I-TNCs-based cells increases markedly, with

12.5 mA cm�2 for 10 mol% I-TNCs-based cell and 14.1 mA cm�2

for 20 mol% I-TNCs-based cell, respectively. Afterwards, upon

continuing increasing the doping ratio up to 30 mol%, the JSC

value shows an obvious decrease to 10.3 mA cm�2, in which the

excess amounts of dopants in electrode films could be considered

as recombination centers to inhibit the efficient separation of

photo-induced electron-hole pairs.38,47 Besides, the Voc of all the I-

TNCs-based cells is slightly higher than that of undoped one,

reaching up maximum to 715 mV at the doping ratio of 20 mol%.

In a typical n-type doping system based on identical electrolyte,

the Voc enhancement is assumed to be arisen from the increase of

the electron concentration and upward shift of the Fermi level.48,49

Hence, under our current experimental conditions the highest h of

7.0% was achieved for a 20 mol% I-TNCs-based DSSC, corre-

sponding to a 42.9% increment of the undoped one.

The photoactive wavelength regime for the undoped and I-

doped TNCs cells was studied by incident photon-to-current

conversion efficiency (IPCE) (Fig. 7). The IPCE spectra were

observed to follow a similar trend to I–V curves described above

(see Fig. 6). Specifically, with increasing I-doping ratio, the

maximum efficiency contributed by the N3 dye absorption at the

approximately 520 nm first increases gradually from 45% for

undoped cell to 54% for 10 mol% I-TNCs-based cell and 60% for

20 mol% I-TNCs-based cell, and then decreases to 43% for 30

mol% I-TNCs-based cell. Note that 10 mol% and 20 mol%

I-TNCs-based cells both exhibit better photoelectrical response

J. Mater. Chem., 2011, 21, 3877–3883 | 3881

Fig. 8 Nyquist plots (a) and Bode phase plots (b) of DSSCs based on

undoped and I-doped TNCs with different molor ratios of I to Ti

including 10 mol%, 20 mol% and 30 mol%. The inset of (b) is a magnified

profile of selected rectangle area in low-frequency peak region.

Fig. 9 I–V curves of the 20 mol% I-TNCs-based cell measured repeated

under various light intensity including 100 mW cm�2, 30 mW cm�2 and 10

mW cm�2.

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over the entire wavelength region of 400–800 nm, matched well

with their enhanced JSC and h. It should also be pointed out that

the enhance IPCE in the range of 400–550 nm could be consid-

erably ascribed to the photoresponse of I-doped TiO2 in the

visible light region.20 To clarify the contribution to the increase in

IPCE, the amounts of dye absorbed on all four electrode films

were measured and the data are presented in Table 2. One can see

that the adsorbed amount of dye on all films maintains almost

unchanged before and after iodine doping, which is also

coincided very well with their analogous surface area data (see

Table 1). In this case, the influence of dye uptake on IPCE could

be neglected and the increases in JSC and IPCE can be reasonably

inferred to be arising from the intrinsic component of the films as

well as the enhanced light-harvesting efficiency and the electron

transfer process of the cells.15–24,50,51 Detailed explanation for the

enhancement of photovoltaic performance of I-TNCs-based cells

will be further discussed below.

Electrochemical impedance spectroscopy (EIS) technique was

used to investigate the recombination resistance and the electron

lifetime of DSSCs. Fig. 8 shows the Nyquist and Bode phase

plots of the DSSCs based on undoped and I-doped TNCs. In

general, two semicircles were observed in the Nyquist plots (see

Fig. 8a). The small semicircle in the high-frequency region (>1

kHz) represents the redox reaction of I�/I3� at the Pt/electrolyte

interface, and the other large semicircle in the low-frequency

region (100–1 Hz) is assigned to the accumulation/transport of

the injected electrons within TiO2 film and the electron transfer

across the TiO2/dye/electrolyte interface.52,53 The large one

dominates the impedance of the DSSC and is recognized as the

characteristic shape of recombination through the TiO2 elec-

trode.54,55 It is worth noting that the charge transport resistance

at the TiO2/dye/electrolyte interface just shows an opposite trend

to I–V curves (Fig. 6) and IPCE spectra (Fig. 7a). This implies

that among our four types of cells, 20 mol% I-TNCs-based cell

possesses the lowest recombination resistance and hence more

effective electrons could be captured. Correspondingly, the Bode

phase plots of EIS spectra, as shown in Fig. 8b, display the

characteristic frequency peaks of the charge transfer process for

all cells. The lifetime of electrons (se) in TiO2 film can be related

to as the inverse of the characteristic frequency and estimated by

the equation: se¼ 1/umax¼ 1/2pfmax, where fmax is the maximum

frequency of the low-frequency peak.52,53,56,57 The calculated se

values show an increased trend in a sequence of 20 mol% I-

TNCs-based cell (14.2 ms) > 10 mol% I-TNCs-based cell (12.5

ms) > pure TNCs-based cell (9.1 ms) > 30 mol% I-TNCs-based

cell (7.1 ms). Obviously, the 20 mol% I-TNCs-based cell has the

lowest recombination resistance and the longest electron lifetime,

which could favor the electron transport through a longer

distance with less diffusive hindrance, and finally leading to

enhanced photoconversion efficiency.

As a supplement, the dependence of the performance charac-

teristics on light intensity for the optimized cell, viz. 20 mol% I-

TNCs-based cell, was also studied (see Fig. 9). Under standard

global AM 1.5 solar condition, the cell shows an overall effi-

ciency of 7.0%. Interestingly, under lower light intensity, 30 mW

cm�2 and 10 mW cm�2, the cell exhibits better photovoltaic

performance, with efficiencies of 10.0% and 8.2%, respectively.

This implies that the fabricated DSSCs could work well even

under weak light intensity.

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Conclusions

Visible-light-response I-doped TiO2 nanocrystals with various

molar ratios of I/Ti synthesized via a combination of sol–gel

process and hydrothermal treatment have successfully been

applied as photoanodes in DSSCs. The doped materials were

confirmed to possess huge internal surface area for adequate dye-

adsorption as well as improved light absorption and light

harness. The photovoltaic measurement results showed that the

cell with an optimizing I-doping ratio of 20 mol% gave conver-

sion efficiencies of 7.0% and 10.0%, under 100 mW cm�2 (AM

1.5) and 30 mW cm�2 illumination, respectively. The improved

performance could be explained by effective harvesting

of sunlight, reduced electron-hole recombination process and

pro-longed electron lifetime. It is anticipated that the visible-

light-response doping material can be extended to other

nonmetal-doped semiconductors for high efficient DSSCs.

Acknowledgements

This work was supported financially by NSFC (Nos. 20776014,

20906004, 20977005, 20821004), CPSF (No. 20080440303), 863

project (2007AA03Z343).

Notes and references

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