Nano Res

1

Preparation of Dye Sensitized Solar Cells with high

photocurrent and photovoltage by using mesoporous

TiO2 particles as photoanode material

Yi Zhang1,2

, Bao Zhang1,†

(), Xiao Peng1, Lin Liu

1, Shuo Dong

1, Liping Lin

1, Si Chen

1, Shuxian Meng

1,

and Yaqing Feng1,2,†

()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0883-y

http://www.thenanoresearch.com on August 20, 2015

© Tsinghua University Press 2015

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TABLE OF CONTENTS (TOC)

Preparation of Dye Sensitized Solar Cells with high

photocurrent and photovoltage by using mesoporous

TiO2 particles as photoanode material

Yi Zhang1, 2, Bao Zhang1, †, Xiao Peng1, Lin Liu1, Shuo

Dong1, Liping Lin1, Si Chen1, Shuxian Meng1, and Yaqing

Feng1, 2, †

1School of Chemical Engineering and Technology, China

2Collaborative Innovation Center of Chemical Science and

Engineering, China

The DSSCs with mesoporous TiO2 particles as photoanode

could show high photocurrent and photovoltage by the unique

structure of these materials.

Provide the authors’ webside if possible.

Author 1, webside 1

Author 2, webside 2

Preparation of Dye Sensitized Solar Cells with high

photocurrent and photovoltage by using mesoporous

TiO2 particles as photoanode material

Yi Zhang1, 2

, Bao Zhang1, †

(), Xiao Peng1, Lin Liu

1, Shuo Dong

1, Liping Lin

1, Si Chen

1, Shuxian Meng

1,

and Yaqing Feng1, 2,†

()

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Dye Sensitized Solar

Cells, Mesoporous TiO2,

high photocurrent and

photovoltage.

ABSTRACT

Several mesoporous TiO2 (MT) materials were synthesized under different

conditions following a hydrothermal procedure using Poly(ethylene-glycol)

-block-poly(propylene-glycol)-block-poly(ethylene-glycol) (P123) as template

and titanium isopropoxide as titanium source. The molar ratios of Ti/P123 and

the pH values of reaction solution in autoclave were investigated. Various

techniques such as Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD),

X-ray photoelectron spectroscopy (XPS), laser Raman spectrometry (LRS),

scanning electron microscope (SEM) and high resolution transmission electron

microscopy (HRTEM) were used to characterize the products. Then, these

materials were assembled into dye sensitized solar cells (DSSCs). The analysis

of J-V curves and electrochemical impedance spectroscopy (EIS) were applied to

characterize the cells. The result indicated the specific surface area and

crystalline structure of materials provided the possibility of high photocurrent

for cells, and the structural characteristics of specimen led to the increased

electron transfer resistance of cells which was beneficial for the improvement of

the photovoltage of DSSCs. The highest photoelectric conversion efficiency of

the cells involving MT materials reached 8.33% which, compared to that of

P25-based solar cell (5.88%), increased by 41.7%.

1 Introduction

Sunlight is arguably the most abundant clean

source of energy that is capable of enabling

“indefinite” and sustainable economic growth, with

minimum detrimental impact on the environment.

Crystalline silicon solar cells shave been

continuously advancing in efficiency and reducing in

cost of fabrication over the last 40 years [1-2], but in

most places of the world, they are still expensive

compared to the price of electricity generated from

fossil fuels [3]. In order to cut the cost of photovoltaic

energy, there are many other new solar cell

technologies that have attracted considerable

attentions, such as dye-sensitized solar cells (DSSCs).

DSSC represents an attractive alternative with the

advantages of low material costs, simple

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2 Nano Res.

manufacture process, desired durability and

compatibility with flexible substrates [4-5].

The photoanode material of DSSCs is one of the

key factors affecting the photoelectric conversion

efficiency. Nanoparticles were conventional and most

widely studied for use in DSSCs to form photoanode

film [6-7]. However, due to the restriction of

characteristics itself, the small size of individual

nanoparticles has been regarded as an unfavorable

factor that may increase interfacial charge

recombination occurring between the

photogenerated electrons and the redox species in

electrolyte [8-9]. In order to improve the efficiency of

solar cells, several semiconductor materials with

special morphology were prepared for use in

photoanodes. For example, Chen et al. [9] and

Diamant et al. [10-11] prepared a series of core-shell

nanostructures to decrease the charge recombination

based on hypothesis that a coating layer could build

up an energy barrier at the semiconductor/electrolyte

interface. However, this nanostructure has been

proved to be less effective and lack of consistency

and reproducibility. One-dimensional nanostructures,

such as nanowires [12-13], nanorods [14] and

nanotubes [15-16], were synthesized to solve this

problem based on a consideration that these

materials can provide direct pathways for electron

transport from the site occurring electron injection to

the conducting film of collector electrode. However,

these materials face drawback of insufficient internal

surface area, leading to relatively low conversion

efficiency. The oxide aggregates, so called

three-dimensional nanostructures, are addressed to

be a potentially promising structure since it is

believed that the aggregates may simultaneously

generate light scattering and provide a ‘‘highway’’

for electrolyte diffusion in view of the existence of

pores [17-18]. But the surface area of usual

aggregates was not obviously higher than that of

nanoparticles.

Since 1991, mesoporous materials with high

surface area, large pore size and narrow pore size

distribution have caused the extensive interest of

researchers [19-20]. From the view-points of

adsorption, the mesoporous materials can increase

greatly the adsorption capacity of dye. Hence, DSSCs

assembled using mesoporous TiO2 materials

(denoted as “MT” hereinafter) as photoanode films

should show immense potential to improve

photocurrent and photoelectric conversion efficiency.

And the mesoporous TiO2 could provide extra

channels to transmit light ray besides the stacking

mesoporous of traditional scaffold particle. In our

studies, a series of MT materials were prepared by a

hydrothermal soft-templating method employing

different Ti:P123 molar ratios and pH values. The

properties of these materials were characterized

using XRD, XPS, LRS, BET and HRTEM techniques.

For a comparison, DSSCs with newly prepared MT

materials and with the commercial P25 as

photoanode materials were fabricated via a doctor

coating method. The cells were analysed by J-V

curves and EIS. The photoelectric conversion

efficiencies of these cells were examined and it was

shown that the cell performance could be influenced

by the characteristics of MT materials. The highest

photoelectric conversion efficiency of the cells

involving MT materials reached 8.33%. And the

photovoltaic results were well explained based on

the properties of MT materials.

2 Experimental

2.1. Synthesis of MT materials

The MT materials were synthesized under

various conditions (Table 1), since the synthesis

conditions of materials were closely related with

their morphology [21]. Firstly, to decrease the

hydrolysis rate of titanates, 0.05 mol of titanium

isopropoxide and an equivalent amount of

acetylacetone were transferred into a beaker and

stirred at room temperature (RT). Then, an

appropriate amount of P123 was dissolved with 100

mL of distilled water (DW) and the concentration of

P123 was represented as x wt% in Table 1. After

stirring at 40 oC for 12 h, the treated mixed solution

was added dropwise to the aforementioned water

solution. In the meantime, 1 mol/L of H2SO4 solution

was used to adjust pH with stirring continually for

another 2 h. Finally, the obtained sol was transferred

into a Teflon-lined autoclave and hydrothermal

synthesis was conducted at 90 oC for 12 h. The

products were filtered, washed to neutral and

calcined at 550 oC for 3 h. The obtained samples were

then denoted as “MT-x-pH” hereinafter.

2.2. Fabrication of DSSC

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3 Nano Res.

According to the reported procedure [22-24], the

pastes used for fabricating porous films were

prepared by mixing 1 g TiO2, 0.5 g ethyl cellulose

(EC), 3.5 g terpineol and 25 g ethanol together. After

ball milling for 3 h, the mixture was concentrated

using a rotary evaporator to obtain viscous pastes.

FTO was ultrasonically cleaned with soapsuds,

distilled water and ethanol, respectively, and then

treated with 50 mmol TiCl4 at 70 oC for 0.5 h. A thin

coating was assembled by the pastes on an FTO glass

using a doctor blade method, and the film thickness

of all samples was 12±2 μm. After dried at 80 oC for

30 min, these samples were annealed at 500 oC for 1 h

and then treated with 50 mmol TiCl4 at 70 oC for 30

min. Another heat treatment proceeded at 500 oC for

30 min. After they were cooled to 110 oC, the films

were immersed in an ethanol solution of N719 (3 ×

10−4 M) for 20 h. The final TiO2 film electrodes were

obtained by rinsing these films with anhydrous

ethanol and drying them at 100 oC in oven. The

photoelectrode was assembled with Pt counter

electrode and electrolyte to form a sandwich-type cell.

And the injected electrolyte consisted of 0.6 M DMPII,

0.03 M I2, 0.5 M 4-TBP and 0.1 M GuSCN in

acetonitrile and valeronitrile (the volume ratio of

85/15).

2.3. Characterization of synthesized materials

The nitrogen adsorption isotherms of MT

materials were measured on a home-made N2

adsorption apparatus at 77 K [25-26]. Prior to

analysis, the samples were treated in vacuum at 200 oC for 2 h. The BET surface area, pore volume and

pore size of the samples were calculated by

Barrett-Joyner-Halenda (BJH) method via correlative

isotherms. Wide-angle powder X-ray diffraction

(XRD) investigation was conducted over a

PANalytical X’Pert PRO Diffractometer with Cu Kα

radiation (λ = 0.15406 nm) and Ni filter at settings of

“40 kV, 50 mA”. X-ray photoelectron spectroscopy

(XPS) experiments were carried out on a RBD

upgraded PHI-5000C ESCA system (Perkin Elmer)

with Mg Kα radiation (h =1253.6 eV). The X-ray

anode was run at 250 W and high voltage was kept at

14.0 kV with a detection angle at 54. The whole

spectra (0~1100 (or 1200) eV) and the narrow spectra

of all the elements with high resolution were

recorded using RBD 147 interface (RBD Enterprises,

USA) through the AugerScan 3.21 software. The

effect of surface charging was calibrated via the C 1s

binding energy (BE) (284.6 eV) of contaminant

carbon.

The laser Raman spectroscopy (LRS) study was

performed by single spectrum using a Thermo

Fischer DXR Raman Microscope. We select the

50×objective of the confocal microscope together with

a laser source of 532 nm at 10 mW in mode laser

power at 100%. Ten spectrum signals of 10 s exposure

were averaged to improve the signal to noise ratio.

Spectra were analyzed using the OMNIC for

Dispersive Raman Software. The scanning electron

microscope (SEM) images were performed on a

Hitachi S4800 electron microscopy operated at 15 kV.

The high resolution transmission electron

micrograph (HRTEM) images and selected area

electron diffraction (SAED) analysis were performed

on a Tecnai G2 F20 electron microscopy operated at

200 kV.

Table 1． Specific surface area (SBET), total pore volume (VT) and average pore size (Da) of fresh and calcined samples isotherms.

3 Results and discussion 3.1 BET characterization of MT materials

Synthesis conditions Physical properties of materials

Fresh sample

Calcined sample

MT-x-pH SBET

(m2/g)

VT

(mm3/g)

Da

(nm)

SBET

(m2/g)

VT

(mm3/g)

Da

(nm)

MT-12-0.8

MT-12-1.2

MT-12-1.6

MT-12-2.0

MT-8-1.2

MT-10-1.2

MT-14-1.2

218 242 4.26 121 224 7.40

247 203 3.92 142 266 7.50

209 207 3.96 133 248 7.47

196 131 2.68 123 176 5.73

232 247 3.08 137 219 6.39

217 248 4.59 130 246 7.57

252 211 3.36 125 149 6.44

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4 Nano Res.

Figure 1 (a) N2 adsorption isotherms and (b) pore

diameter distribution of (A) fresh and (B) calcined MT-12-0.8,

MT-12-1.2, MT-12-1.6, MT-12-2.0, MT-8-1.2, MT-10-1.2 and

MT-14-1.2.

The N2 adsorption isotherms and the

corresponding pore size distributions of fresh MT

materials were shown in Fig. 1A. The isotherms of

samples synthesized at different conditions could be

classified as type-IV isotherm (Fig. 1A), which

indicated that these samples were typical

mesoporous materials. The maximum of the pore

size distribution for MT materials (Fig. 1A(b))

slightly declined with increasing pH and parabolicly

changed with the rise of concentration of P123, the

peak at x = 10. When samples were calcined at 550 oC

for 2 h, the isotherms of samples still maintained as

type-IV (Fig. 2A). And the property difference among

different calcined MT materials was in consonance

with fresh materials, which denoted that these

samples were high-temperature stable. Table 1

summarized physical properties for both fresh and

calcined MT materials prepared at different

conditions. For fresh samples, the values of specific

surface area (SBET) remained more or less at 200 m2/g,

which were about 4 times larger than fresh P25

nanoparticle [27]. All the average pore sizes (Da)

were higher than 2 nm that was the minimum size

for mesoporous materials. As reported by Kresge et

al. [28], when mesoporous materials were calcined,

the SBET of materials decreased as well as the Da

increased evidently. Based on comprehensive

consideration of SBET and Da, it was indicated that the

physical properties of MT-10-1.2, MT-12-1.2 and

MT-12-1.6 were more suitable for support of dye

molecules.

3.2 XRD and LRS analysis of materials

Figure 2 Wide-angle XRD patterns of (A) fresh and (B, C)

calcined TiO2: (a) MT-12-0.8, (b) MT-12-1.2, (c) MT-12-1.6, (d)

MT-12-2.0, (e) MT-8-1.2, (f) MT-10-1.2, (g) MT-14-1.2 and (h)

P25.

The structure of MT materials synthesized under

different conditions was characterized by means of

wide-angle XRD technique, as shown in Fig. 2. For

fresh samples (Fig. 2A), the XRD patterns of TiO2

have only anatase crystal phase (2θ = 25.3, 37.8, 48.1,

53.9 and 55.1o) with tetragonal structure

[PDF#21-1272]. As shown in Fig. 2A, all samples have

similar full width at half maximum (FWHM) values

and the grain sizes were contiguous and calculated to

be equal to about 7 nm. The XRD curve of MT-10-1.2

(Fig. 2A(f)) showed the largest peak area of (1 0 1)

crystal planes, which meant that the surface of

MT-10-1.2 exposed the most (1 0 1) crystal planes. As

is reported [29-30], the anatase TiO2 with exposed (1

0 1) planes was a key factor for a good bonding

20 30 40 50 60 7020 30 40 50 60 70

g

f

e

d

c

b

a

B

Inte

nsity (

Arb

itra

ry u

nits)

Anatase

C

h

Rutile

2 Theta (degree)

g

f

e

d

c

b

2 Theta (degree)

AnataseA

a

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5 Nano Res.

interaction between the TiO2 and ruthenium-based

dye molecules, which, shows low surface energies,

enables the dye molecules to form high density

monolayer chemisorption on the particle surface and

the electrons in dye molecules to inject into

semiconductor highly efficiently. Moreover,

according to the analysis of BET, the SBET property of

MT-10-1.2 was also excellent relatively. After the

samples were calcined in air for 3 h at 550 oC, and

then calcined under the same conditions of

assembling photoanode, there were still the

diffraction peaks of anatase crystal phase and the

MT-10-1.2 peak area of (1 0 1) planes was still the

largest (Fig. 2B). And the grain sizes of calcined

samples increased by about 10 nm. For comparison,

the XRD pattern of commercial P25, calcined under

the same conditions of assembling photoanode, was

also shown in Fig. 2C. Obviously, the FWHM of MT

samples were much wider than that of P25, and the

grain sizes of MT samples were smaller compared to

those of P25.

Figure 3 Laser Raman spectroscopies (LRS) of calcined (a)

MT-10-1.2 and (b) P25.

The calcined samples of P25 and MT-10-1.2 were

characterized via RT laser Raman spectroscopy (LRS)

as a complementary structural characterization of

XRD, and the corresponding results were exhibited

in Fig. 3. As shown in Fig. 3a, several Raman bands

of MT material were observed at about 144, 196, 396,

516, and 638 cm−1, which could be assigned to the

Eg(1), Eg(2), B1g(1), A1g + B1g(2), and Eg(3) vibration

modes of anatase TiO2, respectively [31-32]. For

comparison, the Raman spectrum of P25 was also

shown in Fig. 3b. It was found that compared with

MT material, the LRS band of P25 occurred a slight

shift toward red, and the peak height of it was

significantly low. According to the report of Zuo et al.

[33-34], this phenomenon represented that the P25

particle was smaller and more highly dispersed.

Interestingly, the previous analysis of XRD denoted

that the grain size of P25 was much larger than MT

samples. This difference might originate from the

different analysis mechanism for two

characterization technique. XRD studied particles

with common crystal planes (grains) and LRS

analyzed particles by linking of the chemical bond.

Therefore, we could reasonably infer that the MT

samples were larger particles which formed via

smaller grains by bond-linking.

3.3 SEM and TEM characterization of materials

Figure 4 SEM images of calcined MT-10-1.2 taken with

the (a) low power and (b) high power and HRTEM images of

calcined MT-10-1.2 (c) electron beam parallel to pore axis and (d)

lattice fringes and SAED analysis.

In order to understand the type of morphology

clearly and directly, SEM and HRTEM images of

calcined MT-10-1.2 were obtained in Fig. 4. The low

power SEM image of MT-10-1.2 (Fig. 4a) showed this

sample was monodispersed spherical TiO2 particles.

The high power SEM image of MT-10-1.2 indicated

that the specimen surface was not smooth, as shown

100 200 300 400 500 600 700

195.61

Inte

nsity (

Arb

itra

ry u

nits)

Raman shift (cm-1)

637.41515.06395.10

638.57

516.51396.10

142.64

MT-10-1.2

P25

143.78

a

b

amplify

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6 Nano Res.

in Fig. 4b, and constructed by massive nano-particles,

which was in consistent with the analysis of XRD and

LRS. In order to assess the internal structure of

MT-10-1.2, HRTEM images of sample were measured

and revealed in Fig. 4c. It was shown that there were

circular holes in this sample and the diameter is

approximately 7 nm, which pointed out that the

mesoporous structure is present in this sample and

the pore size accorded with the analysis of BET. The

high power HRTEM images (Fig. 4d) showed the size

of the crystal lattices of the sample was found to be

approximately 0.353 nm which was similar to the

calculated value of SAED fringes (the inset of Fig. 4d)

via reciprocity reciprocal. The above results

suggested that these crystal lattices belonged to (1 0 1)

crystal plane of anatase [4].

3.4 XPS analysis of materials

Figure 5 Ti 2p and O 1s XPS spectra of (a) MT-10-1.2 and (b)

P25 after calcined.

The elemental states of calcined MT-10-1.2 and

P25 could be obtained accurately from the XPS

analysis (Fig. 5). As shown in Fig. 5A, the XPS spectra

at 458 and 464 eV should be assigned to Ti 2p3/2 and

Ti 2p1/2 and the splitting width of 5.8 eV between Ti

2p3/2 and Ti 2p1/2 suggested that the normal state of

Ti4+ (TiO2) is dominant in all these samples [35].

However, the Ti 2p binding energy in P25 was

slightly lower than those of the reported Ti4+ values

(458.8–459.3 eV/464.4–464.8 eV), which denoted that

a small amount of titanium suboxide species such as

Ti3+ (∼457 eV/463.1 eV) and Ti2+ (∼455.3 eV/461.7 eV)

might also exist in these samples [36-37]. It is clear

from Fig. 5B that the O 1s peak of MT-10-1.2 can

showed the peak appeared at 530.09 eV [38], and that

of P25 showed a small shift of 0.18 eV to lower

binding energy. As the analysis above, the Ti:O ratio

of P25 was greater than that of MT-10-1.2. That is to

say, in P25, the oxygen element was encompassed by

greater number of titanium element than that in

MT-10-1.2 samples, which could decrease the binding

energy of O 1s in P25 and result in slightly decreased

O 1s peak of it.

3.5 J–V curves of DSSC

0.0 0.2 0.4 0.6 0.8

0

20

MT-12-0.8

MT-12-1.2

MT-12-1.6

MT-12-2.0

MT-8-1.2

MT-10-1.2

MT-14-1.2

P25

Jsc (

mA

/cm

2)

Voc (V)

Figure 6 Photocurrent–voltage curves of DSSCs based on

different photoelectrode films.

The N-719 sensitized solar cells involving newly

prepared mesoporous TiO2 materials such a

MT-12-0.8, MT-12-1.2, MT-12-1.6, MT-12-2.0, MT-8-1.2,

MT-10-1.2, MT-14-1.2 and commercial p25 (all in the

similar thickness of 12 ± 2 μm and active area of 0.16

cm2) as the photoanode films were fabricated. And

then, as shown in Fig. 6, the photovoltaic properties

of these cells were characterized using sunlight

simulator under one-sun illumination, and the step

of scanning was 10 mV. The related photoelectric

performance parameters and the amount of dye

loading for each photoelectrode film are collected in

Table 2. It was shown that the dye adsorption of MT

films was obviously greater than that of P25 film.

Table 2. Parameters of photovoltaic performance of cells based on different photoelectrode films.

536 534 532 530 528

530.09B

Binding energy (eV)

ab

O 1s

52

9.9

1

465 460 455

458.46

464.58

46

4.2

5

A

Binding energy (eV)

a

b

Ti 2p458.84

According to the BET results, the SBET values of MT

materials, compared with P25, were much larger,

which could provide more sites for the adsorption of

dye molecules. And the larger area for the dye

adsorption could lead to more “active sites” for

photoelectric conversion, which would be helpful for

the generation of higher photocurrent for DSSCs.

This can be demonstrated by the results from J-V

curves that the photocurrents of the cells based on

MT films are all greater than that of P25-based cell

(Fig. 6). However, it was also shown in Fig. 6 that the

trend of photocurrent values of different MT

materials was not in consistent with that of their SBET

values or dye adsorption amounts, which can also be

observed in Table 2. For example, the

MT-10-1.2-based solar cell showed the highest

photocurrent but the corresponding dye adsorption

amount was not the greatest, which denoted that

there were other factors affecting the photocurrent of

MT based cells. According to the previous

discussions on XRD pattern of MT materials (Fig. 2A),

the surfaces of MT-10-1.2 exposed the most (1 0 1)

crystal planes among all MT materials. It is known

that for DSSCs, the electrons in dye molecules

injected into semiconductor more efficient in (1 0 1)

planes of TiO2, which could lead to greater

photocurrent of cells [29-30]. Therefore, the high

photocurrent property of MT films was the

common-effect of specific surface structure and

crystal structure. Furthermore, generally, the DSSC

which showed higher photocurrent would exhibit

lower photovoltage for similar photoanodes [6].

However, interestingly, the solar cells with MT

materials as photoanode, compared with P25

material, revealed higher photocurrent and

photovoltage, simultaneously. So some special

structural characteristics of MT materials must be

suitable for DSSC.

3.6 EIS analysis of DSSC

Figure 7 Nyquist and Bode plots of each DSSCs based on

different porous TiO2 films

To further understand the electrochemical and

photoelectron transfer processes in our DSSCs,

especially to reveal the charge transfer resistance at

the interface of conducting layer/TiO2, Pt/electrolyte,

dye-coated TiO2/electrolyte and diffusion of the I3-/I-

redox electrolyte, the EIS was performed [39–41]. Fig.

7a shows the typical Nyquist plots of DSSCs and the

equivalent circuit model of them was shown in the

inset. Two semicircles were observed for the

MT-based cells while three semicircles were observed

for the cell involving P25 under the measured

frequency range of 0.05 Hz-1 MHz. The three

semicircles of P25 cell showed the kinetics of

electrochemical process at the FTO/TiO2 interface and

the redox reaction occurring at the platinum counter

electrode (Z1), the electron transfer at the dye-coated

TiO2/electrolyte interface and the electron transport

at the inner of TiO2 photoanode (Z2), and the Nernst

diffusion impedance (Z3), respectively [42].

Samples

Jsc

(mA/cm2)

Voc

(V)

FF

(%)

η

(%)

Γ

(mol/cm2)

MT-12-0.8 14.45 0.762 66.39 7.31 1.685×10-7

MT-12-1.2 15.85 0.747 69.01 8.17 2.082×10-7

MT-12-1.6 14.38 0.746 69.82 7.49 1.784×10-7

MT-12-2.0 14.15 0.760 68.63 7.38 2.553×10-7

MT-8-1.2 16.06 0.740 67.15 7.98 2.181×10-7

MT-10-1.2 15.72 0.767 69.09 8.33 2.305×10-7

MT-14-1.2 13.99 0.745 67.64 7.05 1.691×10-7

P25 11.16 0.739 71.22 5.88 1.412×10-7

0 5 10 15 20 25 30 35 40 45 50 55 60 65

0

5

10

15

20

0.1 1 10 100 1000 10000100000

5

10

15

20

25

MT-12-0.8

MT-12-1.2

MT-12-1.6

MT-12-2.0

MT-8-1.2

MT-10-1.2

MT-14-1.2

P25

-Z''

(oh

m)

Z' (ohm)

A

z1

z2

z3

Ph

ase

(D

eg

ree

)

Frequency (Hz)

B

Table 3 The values of resistances of RS, R1, R2 and frequency at the first peak in Bode plots of each DSSC sample.

Table 3 presented the fitting results of the parameters

for the MT or P25-based cells in the light at a bias of

DSSCs’ open current voltage (Voc). The resistance

elements, Rs, R1, Y02, n2 and R2 are summarized in

Table 3. Rs, R1 and R2 represent the resistance of FTO,

Z1 and Z2, respectively. The Bode phase plots were

shown in Fig. 7b, and the electron lifetimes (τe) can be

obtained via the conversion of the first peak value in

these plots [43]. As shown in Nyquist plots, the

diameter of the middle frequency semicircle of the

DSSCs based on MT electrodes was much bigger

than that of P25-based solar cell. And for MT-based

cells, only two arcs in the EIS plot were observed,

which is because the middle frequency impedances

were great enough to cover low frequency

impedances.

According to the report of Liu et al., the middle

frequency impedance reflected the electron transport

resistance at the inner of TiO2 photoanode which is

inversely correlated to the photocurrent of DSSC, and

the electron transfer resistance at the dye-covered

TiO2/electrolyte interface which is inversely related to

electron recombination at this interface [42].

Obviously, as the analysis of J-V curves (Fig. 6), the

photocurrent of MT-based cells were much greater

than that of the P25-cell cells, which indicated that in

our systems the electron transport resistance cannot

remarkable affect the photoelectric properties of

DSSCs. According to the analysis of XRD and LRS,

the MT samples were larger particles which formed

via smaller grains by bond-linking, which could

effectively decrease the grain boundary of TiO2

particles and, thus, increase the electron transfer

resistance of cells. And, for DSSCs, the higher

electron transfer resistance implied that the degree of

electron recombination at the dye-covered

TiO2/electrolyte interface was lesser, which

maintained the lower concentration of I3- nearby

photoanode. It is well known that Voc depends

logarithmically on the inverse concentration of I3- and

increases with an incident photon flux I

( 0 1 3 0 2

ln( )[I ] n k [D ]

OC

RT AIV

F n k

) [44].Therefore, as

showed in Fig. 6., the MT-based could cells show

higher photovoltage. Y02 and n2 are the parameter of

constant phase element (CPE) of Z2 and Y02-1 could

represent the differential capacity of DSSCs in the

case where n2 is more than 0.7 [45]. Moreover, as

shown in Table 3, the electrical capacitance of MT

materials was smaller than the P25 one. Consistent

with previous XPS analysis, there is more oxygen

vacancies present in P25, which could accept more

electrons leading to higher capacity. However, these

oxygen vacancies could also work as a

“recombination centre” of dark current for DSSCs,

which increased the electron recombination and

decreased the photovoltage of DSSCs. That is to say,

the MT-based cells showed higher photovoltage were

the result by common-effecting of lesser grain

boundary and surface vacancies of MT materials. The

Bode phase plots in Fig. 7b showed that the middle

frequency peaks of the MT-based DSSCs shift to a

lower frequency relative to the P25 cell, which

indicated a longer electron lifetime for the MT-based

DSSCs. The electron lifetime of the DSSCs based on

MT materials was calculated to be in the range

61.2-74.1 ms, which are significantly greater than that

the P25 cell (28.2 ms). The structure of MT materials

could effectively reduce the grain boundaries of TiO2

particles, and the surface of them exposed less

oxygen vacancies, which all could result in the

decreased electron recombination and, therefore,

increased electron lifetime of cells. In conclusion,

compared with the traditional photoanode P25

Samples

Rs (Ω)

R1 (Ω) Y02 (S·secn/cm2) n2 R2 (Ω) τe (ms)

MT-12-0.8 8.671 7.86 0.004595 0.8777 27.76 61.2

MT-12-1.2 9.591 7.906 0.005482 0.8465 25.61 67.7

MT-12-1.6 9.195 10.08 0.005795 0.8381 25.05 70.6

MT-12-2.0 9.079 6.582 0.005404 0.8443 31.41 73.1

MT-8-1.2 9.873 7.893 0.005439 0.832 32.48 74.1

MT-10-1.2 8.496 6.739 0.005011 0.8534 35.34 67.8

MT-14-1.2 9.031 8.12 0.005627 0.853 28.41 73.9

P25 8.113 8.359 0.002636 0.9115 21.55 28.2

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Nano Res.

material, the structural characteristics of MT

materials could afford the DSSCs with greater

photocurrent and photovoltage, simultaneously.

4 Conclusions

The MT materials were synthesized by

hydrothermal procedure using P123 as template and

titanium isopropoxide as titanium source. A series of

DSSCs were assembled by using doctor coating

methods with these materials. The BET, XRD, LRS,

XPS, SEM and HRTEM techniques were employed to

characterize the fresh and calcined materials and the

J-V curves and EIS were used to analyze the DSSCs.

The results showed that the MT materials presented

excellent structural properties and the SBET values of

them were even four times higher than that of P25,

which could contribute to enhanced photocurrent of

cells. XRD analysis showed that the MT materials

were pure anatase phase and MT-10-1.2 exposed the

most (1 0 1) crystal planes, which was also propitious

to enhanced photocurrent of cells. Combined results

from XRD and LRS indicated that the MT materials

were a series of large particles composed of

numerous small nanocrystal grains via connection of

chemical bondings. SEM and HRTEM images of

samples further verified these studies. This structure

decreased the grain boundary among TiO2 particles

and, consistent with the analysis of EIS, indicated

that MT-based cells could show higher middle

frequency impedances than P25 cells, which leads to

increase the photovoltage of cells. XPS results

revealed that, for MT materials, the amount of

titanium suboxide species was less than that for P25.

These titanium suboxide species could increase the

amount of “recombination centre” of dark current for

DSSCs, which also decreased photovoltage of cells.

Overall, compared to P25, The MT-based DSSCs

presented higher photoelectric conversion efficiency

by increasing photocurrent and photovoltaic,

simultaneously. The highest photoelectric conversion

efficiency of the cells involving MT materials reached

8.33% which, compared to that of P25-based solar cell

(5.88%), increased by 41.7%.

Acknowledgements

This work is supported by Doctoral Program of Higher

Education of China (No. 20120032120011), National

Natural Science Foundation of China (No. 21076147,

No 21476162) and China International Science and

Technology Project (No. 2012DFG41980).

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