High-performance dye-sensitized solar cells containing double-layer organizedmesoporous TiO2 films sensitized by a dye with a high molar extinction coefficientJuangang Wang and Yunli Shang Citation: Applied Physics Letters 102, 143113 (2013); doi: 10.1063/1.4801755 View online: http://dx.doi.org/10.1063/1.4801755 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High open circuit voltages of solar cells based on quantum dot and dye hybrid-sensitization Appl. Phys. Lett. 104, 013901 (2014); 10.1063/1.4861163 Unexpected effect of dye's molar extinction coefficient on performance of back contact dye-sensitized solar cells Appl. Phys. Lett. 101, 233905 (2012); 10.1063/1.4769897 High-performance dye-sensitized solar cell with a multiple dye system Appl. Phys. Lett. 94, 073308 (2009); 10.1063/1.3086891 Double-layer porous Ti O 2 electrodes for solid-state dye-sensitized solar cells Appl. Phys. Lett. 92, 193108 (2008); 10.1063/1.2924277 The influence of the time-of-flight mobility on the efficiency of solid-state dye-sensitized Ti O 2 solar cells Appl. Phys. Lett. 85, 6185 (2004); 10.1063/1.1834717

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High-performance dye-sensitized solar cells containing double-layerorganized mesoporous TiO2 films sensitized by a dye with a highmolar extinction coefficient

Juangang Wang and Yunli ShangCollege of Chemistry and Material Science, Huaibei Normal University, Huaibei 235000, Anhui, China

(Received 27 December 2012; accepted 27 March 2013; published online 10 April 2013)

In the present work, we describe a practical technique to construct double-layer organized

mesoporous TiO2 films with a combined thickness of 0.85 lm. Large mesopores (25.74 nm) formed

in the film by using ovalbumin as the main template facilitate entry and adsorption of dye

molecules. The films were sensitized by a dye that exhibits a high molar extinction coefficient

because it contains a ligand with a fused-ring system. The double-layer films exhibited a solar

conversion efficiency of 7.37%, which was about 35% higher than that of monolayer films. VC 2013AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4801755]

Previous papers have shown that the morphology of

nanocrystalline TiO2 can strongly influence the efficiency of

collecting injected electrons, photovoltage, photon-to-current

conversion efficiency (IPCE), quantum yield of electron

injection, and fill factor (FF) of dye-sensitized solar cells

(DSSCs).1–4 It has been argued that a porous framework is of

great importance to increase surface area compared with that

of a compact framework.5–10 Porous TiO2 thin film electro-

des can be prepared through various methods such as spray

pyrolysis, screen printing, thermal evaporation, sputter depo-

sition, nanoparticle paste, sol-gel methods, electrochemical

deposition, and chemical vapor deposition.5–16 Interestingly,

a double-layer porous TiO2 electrode constructed using a

one-step process has been reported.17 The electrode con-

sisted of a thin transparent layer on conductive glass and a

rough opaque top layer, which is the reverse order compared

with that in other studies. The cell displayed a high open cir-

cuit voltage of 1.18 V, but its short circuit current was low

because of the low mobility of carriers in the hybrid

structure.

In the past, the overlying nanocrystalline TiO2 film in

double-layer TiO2 electrodes covered the underlying TiO2

film, resulting in a small surface area.17 Importantly, even

though there were pores on the surface of the structure, the

small size of these pores (4–10 nm) restricted the entry of

dye molecules, which inevitably affected the conversion effi-

ciency of DSSCs.14–17 In the current work, we present a

practical technique to prepare double-layer organized meso-

porous TiO2 films with a total thickness of 0.85 lm by layer-

by-layer deposition. Ovalbumin was used as the main

template to form large mesopores. A thin layer of grease was

coated on the surface of the underlying TiO2 film to prevent

the overlying TiO2 paste from packing into the interspace of

the underlying TiO2 film. The solvothermal method is often

used to prepare porous TiO2 electrodes.18 The reaction is

usually performed in the solution phase; use of the gas phase

in such reactions as a practical medium has seldom been

studied. However, here solvothermal crystallization is

achieved at a solid/gas interface when the gas phase is satu-

rated with water/ethanol solution. Both mono- and double-

layer TiO2 films were prepared by solvothermal treatment at

the solid/gas interface before calcination. This reaction

allowed direct conversion of the solid precursor into the

desired sample.

All chemicals in this work were used as purchased with-

out further modification. Ti(OC2H5)4 (13.6 g) was added

slowly to 1-butanol (40 g) under vigorous stirring to form a

solution. At the same time, ovalbumin (3.0 g), hexadecyl

trimethyl ammonium bromide (3.2 g), and block copolymer

Pluronic P123 [OH(CH2CH2O)20 (CH2CH(CH3)O)70

(CH2CH2O)20 H] (2.5 g) were dissolved in 1-butanol (90 g)

and then added to the Ti(OC2H5)4 solution. This resulting so-

lution was aged by vigorous stirring at ambient temperature

for at least 7 days. The films were constructed by dip coating

SnO2-coated conductive glass (Asahi Glass, 10 X/square,

0.4� 0.8 cm2) with the aged solution. After drying under

vacuum at room temperature, the coated substrates were set

on a glass bottle in a Teflon-lined autoclave. A small amount

of water/ethanol solution was added to the bottom of the

Teflon-lined autoclave so that the samples were in direct

contact with gas rather than ethanol and water solution dur-

ing the reaction. The Teflon-lined autoclave was placed in an

oven at 378 K for 15 days. After solvothermal treatment, the

films were removed from the autoclave and washed in water/

ethanol solution. The films were calcined in air at 723 K for

48 h (heating rate: 0.5 K/min). To construct thicker films

consisting of two layers, the procedure described above was

repeated once after a thin layer of grease was evenly coated

onto the surface of the underlying TiO2 film. Finally, the

film was calcined at 723 K for 120 h (heating rate:

0.5 K/min). The double-layer film had a combined thickness

of 0.85 lm, which was measured using a Tencor Alpha step

profiler.

We used the adsorption of n-pentane, which has a satu-

ration vapor pressure of 118 Pa at 195 K and a melting point

of 143.5 K,19 to measure the surface areas of the mesoporous

films. Isotherms were measured directly on the actual glass-

supported films. The adsorption of n-pentane was calibrated

using anatase powder (Aladdin) with a BET (Brunauer,

Emmett, and Teller)-nitrogen surface area of 189 m2/g. The

pore size distribution was calculated from the desorption

branch of the n-pentane isotherm.19 The mesopore volume

0003-6951/2013/102(14)/143113/4/$30.00 VC 2013 AIP Publishing LLC102, 143113-1

APPLIED PHYSICS LETTERS 102, 143113 (2013)

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(VP) is expressed analogously in cm3/cm2 of the geometric

area of the film, and mesopore diameter (DP) is expressed in

nm (Figure 1). The monolayer mesoporous nanocrystalline

TiO2 film has an almost uniform pore size centered at ca.

23.86 nm, although the mesopores are not uniformly perpen-

dicular to the substrate. The mesopores formed using ovalbu-

min as the main template are much larger than those of

porous films (DP¼ 4–10 nm) for DSSC applications reported

previously.17–19 The size of the pores in the double-layer

film increases to 25.74 nm. The mesoporous framework

changes only slightly during the repeated dip-coating proce-

dure, when the inorganic framework expands during calcina-

tion. Therefore, the underlying film remains almost intact

during deposition of the subsequent layer. Scanning electron

microscopy (SEM) images (Figure 2) further indicate that the

surface morphologies of the mono- and double-layer films

are similar. All calcined samples appear organized, mesopo-

rous, smooth, and semitransparent. The overlying organized

mesoporous TiO2 film in the double-layer TiO2 electrodes

covered most of the underlying TiO2 film. The interface

between the under- and overlying TiO2 films can be clearly

seen because of the grease layer used to separate them.

The dry, warm TiO2 electrodes were placed in a solution

of [(DIBY)Ru(PEYQ)](SCN)2 (hereafter called N313

(Figure 3), DIBY: 4,40-dicarboxy-2,20-bipyridine; PEYQ:

2-Perylen-3-yl-thiazolo[5,4-h]quinoline, 0.5 mM) in N,N0-

dimethylformamide overnight at room temperature to sensi-

tize them. The N313 sensitizer was provided by Epworth

Chemical Co., Ltd. N313 exhibits a metal-to-ligand charge

transfer transition at kmax¼ 542 m with a molar extinction

coefficient e of 21 900 M�1 cm�1. The corresponding values

for N719¼ bis(tetrabutylammonium)cis-dithiocyanatobis-

(2,20bipyridine-4-COOH, 40-COO�) ruthenium(II)) are

kmax¼ 530 nm and e¼ 13 500 M�1 cm�1.20 The high e of

N313 can be attributed to electronic transitions from the

RuII-based t2g orbital to a ligand-based p* orbital. This

extended p conjugated system of the ancillary ligands with

fused-ring systems in N313 improves electron transport.

3-Methoxypropionitrile containing 0.6 M N-methyl-N-butyl

imidazolium iodide, 0.05 M I2, 0.5 M tert-butylpyridine, and

0.1 M guanidine thiocyanate was used as an electrolyte.

Photocurrent-voltage measurements under illumination with

AM 1.5 (100 mW cm�2) simulated sunlight and photocurrent

action spectra under monochromatic light illumination with

a constant photon number (1016 cm�2 s�1) were obtained

using a Bunko–Keiki CEP-2000 system. The electrode active

area, determined by the aperture of a black mask, was

0.16 cm2.

Incident monochromatic IPCE spectra show a maximum

of 81.6% for N313 on the double-layer organized mesopo-

rous TiO2 electrode (Figure 4). The higher IPCE obtained

for the double-layer film compared with that of the mono-

layer film (52.5%) is a result of increased light harvesting in

the double-layer film because it can adsorb two layers of dye

molecules. As a result, the quantum yield of charge injection

FIG. 1. Pore size distributions of monolayer (gray) and double-layer (black)

mesoporous TiO2 films.

FIG. 2. SEM images of TiO2 films: (a) mono-

layer film and (b) double-layer film.

FIG. 3. Molecular structure of the sensitizer N313.

143113-2 J. Wang and Y. Shang Appl. Phys. Lett. 102, 143113 (2013)

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and collection efficiency of injected electrons at the back

contact are both enhanced in the double-layer film.

Photocurrent-voltage measurements were performed in a

two-electrode sandwich configuration. The cell containing

the double-layer TiO2 electrode exhibited an open circuit

potential (UOCP) of 0.802 V, FF of 0.78, and a short circuit

current (Isc) of 11.78 mA/cm2, yielding a conversion effi-

ciency of 7.37% (Figure 5). This solar conversion efficiency

is larger than that of three-layer mesoporous film (4.04%)

containing mesopores with a size of 7 nm reported by

Gr€atzel.19 This is because the large mesopores in the double-

layer film increase its internal surface area, facilitating dye

anchoring and therefore light adsorption. The large pores

should also enhance the movement of the electrolyte into the

mesoporous TiO2 electrodes, increasing the photoelectrical

conversion efficiency of the DSSC. For comparison, a TiO2

film composed of a double-layer electrode sensitized by

N719 was also measured under similar conditions. The

N719-sensitized solar cell exhibited IPCE¼ 80.3%,

FF¼ 0.76, Isc¼ 10.24 mA/cm2, and UOCP¼ 0.792 V, yield-

ing a conversion efficiency of 6.16%. The higher conversion

efficiency of the N313 DSSC is caused by stronger light ab-

sorbance of N313 molecules across the visible spectrum

because of its fused-ring system. A cell sensitized by N313

containing the monolayer TiO2 electrode achieved

Isc¼ 8.75 mA/cm2, FF¼ 0.73, and UOCP¼ 0.751 V, yielding

a conversion efficiency of 4.78%. The conversion efficiency

of DSSCs with an organized double-layer mesoporous film

is about 35% higher than that with a monolayer film. This

improvement results from a marked enhancement of the

short circuit photocurrent because the double-layer electrode

possesses a large physical surface area per unit of projected

area, which increases light harvesting, as well as improving

open circuit potential because of the continuity and homoge-

neity of the double-layer organized mesoporous skeleton. In

addition, electrons possess a longer residence time because

of a charge trapping effect, raising the Fermi level of the

semiconductor. The improved FF may be related to increased

adsorption of the dye as the thickness of the porous TiO2

film increases on going from mono to double layer.

In summary, we have developed a method to fabricate a

double-layer organized mesoporous nanocrystalline TiO2

electrode. The large organized mesopores formed using oval-

bumin as the main template facilitated entry and adsorption

of dye molecules. Moreover, the large surface area of the

double-layer film improved light harvesting because of the

possibility of two layers of adsorbed dye molecules, evi-

denced by the notable photocurrent density and IPCE

obtained using the sensitizer N313, which possesses a high

molar extinction coefficient because it contains a fused-ring

system. The performance of these thin double-layer organ-

ized mesoporous films (0.85 lm thick) is markedly improved

compared with that of other thicker electrodes (12–18 lm

thick) in dye-sensitized photovoltaic devices,5–10 revealing a

way to increase the performance of photovoltaic cells.

This work was supported by the Science and

Technology Research Projects of the Education Office of

AnHui Province (No. KJ2012Z348). We thank Epworth

Chemical Co., Ltd. for supplying N313 sensitizer.

1M. K. Nazeeruddin, R. Humphry-Baker, P. Liska, and M. Gr€atzel, J. Phys.

Chem. B 107(34), 8981 (2003).2L. Mattt, G. Lorie, C. J. Justin, S. Richard, and Y. Peidong, Nat. Mater. 4,

455 (2005).3D. Zhang, J. A. Downing, F. J. Knorr, and J. L. McHale, J. Phys. Chem. B

110, 21890 (2006).4B. O’Regan and M. Gr€atzel, Nature (London) 353, 737 (1991).5K. Liu, H. Fu, K. Shi, F. Xiao, L. Jing, and B. Xin, J. Phys. Chem. B 109,

18719 (2005).6S. Ito, T. Kitamura, Y. Wada, and S. Yanagida, Sol. Energy Mater. Sol.

Cells 76, 3 (2003).7M. Aoyagi and M. Funaoka, J. Photochem. Photobiol. A: Chem. 164, 53

(2004).8S. Hore, E. Palomares, H. Smit, N. J. Bakker, P. Comte, P. Liska, K. R.

Thampi, J. M. Kroon, A. Hinsch, and J. R. Durrant, J. Mater. Chem. 15,

412 (2005).9D. Guti�errez-Tauste, I. Zumeta, E. Vigil, M. A. Hern�andez-Fenollosa, X.

Domenech, and A. Jos�e, J. Photochem. Photobiol. A: Chem. 175, 165

(2005).10T. Ma, T. Kida, M. Akiyama, K. Inoue, S. Tsunematsu, K. Yao, H. Noma,

and E. Abe, Electrochem. Commun. 5, 369 (2003).11S. Pavasupree, Y. Suzuki, S. Pivsa-Art, and S. Yoshikawa, Sci. Technol.

Adv. Mater. 6, 224 (2005).12G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, and C. A. Grimes,

Nano Lett. 6, 215 (2006).13M. Adachi, Y. Murata, J. Takao, J. Jiu, M. Sakamoto, and F. Wang, J. Am.

Chem. Soc. 126, 14943 (2004).

FIG. 4. IPCE of solar cells based on TiO2 films sensitized by N313.

Monolayer film (gray), double-layer film (black).

FIG. 5. Photocurrent-voltage characteristics of solar cells based on TiO2

films sensitized by N313. Monolayer film (gray), double-layer film (black).

143113-3 J. Wang and Y. Shang Appl. Phys. Lett. 102, 143113 (2013)

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160.36.178.25 On: Fri, 19 Dec 2014 12:40:46

14B. C. O’Regan, S. Scully, and A. C. Mayer, J. Phys. Chem. B 109, 4616

(2005).15I. Palomares, J. N. Clifford, S. A. Haque, T. Lutz, and J. R. Durrant,

J. Am. Chem. Soc. 125, 475 (2003).16X.-t. Zhang, I. Sutanto, T. Taguchi, K. Tokuhiro, Q.-b. Meng, T. N. Rao,

A. Fujishima, H. Watanabe, T. Nakamori, and M. Uragami, Sol. Energy

Mater. Sol. Cells 80, 315 (2003).

17F. Xiaojuan, P. D. David, M. S. J. Jessica, T. Akhilesh, and R. J. O. Scott,

Appl. Phys. Lett. 92, 193108 (2008).18D. Zhang, T. Yoshida, K. Furuta, and H. Minoura, J. Photochem.

Photobiol. A: Chem. 164, 159 (2004).19M. Zukalov�a, A. Zukal, L. Kavan, M. K. Nazeeruddin, P. Liska, and M.

Gr€atzel, Nano Lett. 5, 1789 (2005).20A. O. Adeloye, Molecules 16, 8353 (2011).

143113-4 J. Wang and Y. Shang Appl. Phys. Lett. 102, 143113 (2013)

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