Dye-sensitized solid-state solar cells fabricated

by screen-printed TiO2 thin film with

addition of polystyrene balls

Jing Han a, Jin Mao Chen b, Xiao Wen Zhou b, Yuan Lin b,Jing Bo Zhang b,*, Jian Guang Jia a,*

a Department of Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, Chinab Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry,

Chinese Academy of Sciences, Beijing 100190, China

Received 25 January 2008

Abstract

The screen-printed nanoporous TiO2 thin film was employed to fabricate dye-sensitized solid-state solar cells using CuI as

hole-transport materials. The solar cell based on nanoporous TiO2 thin film with large pores formed by the addition of

polystyrene balls with diameter of 200 nm to the TiO2 paste exhibits photovoltaic performance enhancement, which is

attributed to the good contact of CuI with surface of dye-sensitized thin film due to easy penetration of CuI in the film with

large pores.

# 2008 Jing Bo Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Dye-sensitized solar cells; Screen-printed TiO2 film; Cuprous iodide; Polystyrene balls

Dye-sensitized solar cell has attracted much research attention due to its high conversion efficiency and low

production cost. Since it was first reported by O’Regan and Gratzel , the solar cell based on liquid electrolyte has been

developed rapidly [1]. However, liquid electrolyte shows several technological disadvantages such as easy leakage and

evaporation of solvent and so on. For escaping these problems, some studies on the properties and performances of

dye-sensitized solid-state solar cells (DSSSCs) have been carried out. DSSSCs using CuI as hole conductors to replace

liquid electrolyte were paid much attention. Most of researches focused on discussing the filling method of CuI into

porous film and addition of ionic liquid to inhibit CuI crystal growth, as well as inhomogeneous photocurrent

generation, etc. [2–5]. Preparation of TiO2 porous thin film for DSSSCs usually followed the method reported by

Tennakone et al., which is costly in time, energy and material consumption [6]. In this letter, the screen-printed

method, which favors industrialized production for DSSSCs, was employed to prepare nanoporous TiO2 thin film with

addition of polystyrene (PS) balls, which was used as photo-electrode to fabricate DSSSCs based on CuI as hole-

transport materials.

www.elsevier.com/locate/cclet

Available online at www.sciencedirect.com

Chinese Chemical Letters 19 (2008) 1004–1007

* Corresponding authors.

E-mail address: jbzhang@iccas.ac.cn (J.B. Zhang).

1001-8417/$ – see front matter # 2008 Jing Bo Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2008.05.013

1. Experimental

The fluorine-doped tin oxide conductive glass (FTO, 20 V/&) was used as substrate to prepare nanocrystalline

TiO2 thin film. To avoid the short contact between CuI and the conducting substrate, prior to screen-printing, n-butanol

solution containing 0.1 mol/L titanium isopropoxide was spread on FTO to hydrolyze a compact TiO2 layer followed

by sintering at 450 8C for 30 min. To prepare TiO2 paste for screen-print, 12 g n-butanol solution containing 3 g

commercial nanocrystalline TiO2 powder P25 (Degussa, average diameter 25 nm) was stirred overnight, to which

ethocel and terpineol were added, and the mix suspension was grinded in an agate mortar for 40 min. Then, different

amount (mass percent of 5%, 10% and 20%) of PS balls with diameter of 200 nm were added to the suspension to

screen-print TiO2 porous films on a compact layer TiO2-coated FTO. After sintered at 450 8C for 30 min, the porous

film was sensitized in 10�4 mol/L Ru(2,20-bipyridine-4,40-dicarboxylic acid)2(NCS)2 ethanol solution at room

temperature for 12 h. CuI of 2.5 g were dissolved in 6 mL acetonitrile, in which a small amount of triethylamine

hydrothiocyanate [(C2H5)3NHSCN] were added as CuI crystal growth inhibiting agent. 0.1 mL CuI acetonitrile

solution was spread on the 0.2-cm2 dye-sensitized nanoporous TiO2 thin film. After evaporation of acetonitrile,

TiO2jdyejCuI thin film was pressed by a platinum-coated FTO glass as the back contact to fabricate TiO2jdyejCuIjPt

solid-state solar cell. Carbon powders were put between TiO2jdyejCuI thin film and the platinized FTO to improve

their contact.

I–V curves of the DSSSCs were measured with Potentiostat/Galvanostat Model 273 (EG&G) under the light

intensity at AM1.5 (Oriel 961160-1000), and the active area was 0.2 cm2. The morphology of TiO2 thin film was

observed by using scanning electron microscope (SEM, S-4200F, HITCHI, 15 kV). X-ray diffraction pattern (XRD) of

CuI covered on the porous film was measured with Riguku D/max 2500 using Cu Ka irradiation. The dye loading

amount was determined by dissolving a known area of the film into a known volume of NaOH solution (0.1 mol/L) and

measuring its absorption spectrum with UV (Labtech, UV 2000) spectrometer.

2. Results and discussion

The thickness of the screen-printed TiO2 thin film with P25 paste containing 10% PS balls determined from the

cross-section image shown in Fig. 1(a) is about 3 mm, and this film shows some large size pores (Fig. 1(b)). While the

TiO2 thin film without addition of PS balls has homogeneous porous structure (Fig. 1(c)). In porous film with some

large size pores, penetration of CuI should be much easier.

Crystalline structure of CuI spread on the porous TiO2 thin film was characterized by XRD and assigned to g-

CuI phase (not shown). There are three kinds of crystalline structures for CuI. a-CuI and b-CuI are ionic conductors

and g-CuI is p-type semiconductor. Therefore, g-CuI is requirement as hole-transport materials to fabricate

DSSSCs [7].

Fig. 2 shows the photovoltaic behavior of the DSSSCs without and with PS balls of different addition amounts in

the TiO2 thin films. Significantly enhanced light-to-electricity conversion efficiency was observed for the

TiO2jdyejCuIjPt cell based on the TiO2 thin film with addition of PS balls. The added amount of PS balls was

optimized as 10% according to performance of DSSSCs fabricated by TiO2 porous films with different percentage of

PS balls. The detailed performance parameters such as conversion efficiency (h), fill factor (ff), short circuit photo-

current density (JSC) and open circuit photo-voltage (VOC), as well as dye loading amount for DSSSCs were collected

in Table 1. And the enhancement percentage of parameters for DSSSC with 10% PS balls was calculated and listed in

Table 1.

J. Han et al. / Chinese Chemical Letters 19 (2008) 1004–1007 1005

Table 1

The performance and dye loading amount of the DSSSCs

TiO2 thin film JSC (mA/cm2) VOC (V) ff h (%) Dye loading amount (mol cm�2)

5% PS 2.23 0.62 0.60 0.83 3.4 � 10�8

10% PS 3.56 0.67 0.68 1.63 2.2 � 10�8

20% PS 2.17 0.62 0.55 0.74 2.0 � 10�8

Non-PS 1.57 0.68 0.45 0.48 3.0 � 10�8

Enhancement (%)a 127 �1.5 51.1 240 �27.1

a The enhancement percentage was calculated by comparing data of 10% PS with non-PS.

Compared with the cell fabricated by the TiO2 nanoporous thin film without PS balls addition, there is significant

increase in JSC and ff for the cell with 10% PS balls addition as shown in Table 1. The conversion efficiency

enhancement is mainly attributed to the increase of JSC of DSSSCs due to large size pores present in porous thin film

formed by the addition of PS balls. Dye loading amount for the cell with PS balls is lower than that without PS balls

J. Han et al. / Chinese Chemical Letters 19 (2008) 1004–10071006

Fig. 1. The cross-section (a) and surface (b) images of nanoporous TiO2 thin film with 10% PS balls and surface image (c) of the film without

addition of PS balls.

revealing the improvement of JSC is not due to increasing of dye loading amount. ff representing the inner resistance of

DSSSCs was increased by 51.1%. Summarizing the above analysis, we can conclude that the addition of PS balls

forms some large size pores in porous TiO2 thin film after burning PS balls during sintering process, which reduces the

porous surface of the film, thus decreases dye loading amount by 27.1%. Therefore, the reason for enhancement of JSC,

ff and h is attributed to the good contact between the dyed TiO2 and CuI due to good penetrability of CuI in porous film

with some large pores.

3. Conclusions

We applied screen-printed nanoporous TiO2 thin films with large size pores due to the addition of PS balls in

fabricating DSSSCs using CuI as hole collector. CuI could penetrate in film with some large pores more facilely to

form good contact between CuI and surface of dye-sensitized thin film, which significantly enhanced performance of

DSSSCs. The method of screen-printing being suitable for industrial production of large area DSSSCs will facilitate

its application in future.

Acknowledgments

The authors appreciate the financial support of this work by the Major State Basic Research Development Program

(No. 2006CB202605), High-Tech Research and Development of China Program (No. 2007AA05Z439), the National

Nature Science Foundation of China (No. 50221201), Innovative Foundation of the Center for Molecular Science,

Chinese Academy of Sciences (No. CMS-CX200718).

References

[1] B. O’Regan, M. Gratzel, Nature 353 (1991) 737.

[2] G.R.A. Kumara, S. Kaneko, M. Okuya, K. Tennakone, Langmuir 18 (2002) 10493.

[3] P.M. Sirimanne, H. Tributsch, Solar Energy 81 (2007) 535.

[4] K. Tennakone, G. Kumara, I.R.M. Kottegoda, K.G.U. Wijayantha, V.P.S. Perera, Journal of Physics D-Applied Physics 31 (1998) 1492.

[5] Q.-B. Meng, K. Takahashi, X.-T. Zhang, et al. Langmuir 19 (2003) 3572.

[6] K. Tennakone, G. Kumara, I.R.M. Kottegoda, et al. Semiconductor Science and Technology 12 (1997) 128.

[7] K. Tennakonr, G. Kumara, I.R.M. Kottegoda, et al. Solar Energy Materials and Solar cells 55 (1998) 283.

J. Han et al. / Chinese Chemical Letters 19 (2008) 1004–1007 1007

Fig. 2. I–V characteristics of TiO2jdyejCuIjPt solid-state solar cells fabricated by the nanoporous TiO2 thin films without and with different addition

amount of PS balls.