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Research ArticleReceived: 2 April 2008 Revised: 10 June 2008 Accepted: 12 June 2008 Published online in Wiley Interscience: 2 September 2008

(www.interscience.com) DOI 10.1002/sia.2913

Efficiency enhancement of solid-state dyesensitized solar cell by in situ deposition of CuIJing Han,a Jin Mao Chen,b Xiao Wen Zhou,b Yuan Lin,b Jing Bo Zhang,b∗Jian Guang Jiaa∗ and Babasaheb Raghunath Sankapalc

Solid-state dye-sensitized solar cells based on nanocrystalline TiO2 thin film with the structures TiO2/dye/CuI/Pt andTiO2/dye/In situ CuI/CuI/Pt were developed in order to compare the use of In situ deposited CuI (In situ CuI). Porosity ofthe screen-printed nanoporous TiO2 thin film was enhanced by the addition of polystyrene balls. Evidence of decrease ininterfacial resistance was observed by electrochemical impedance measurement for the device with In situ deposited CuI ascompared to that without In situ deposited CuI as hole conductor. This was attributed to good interfacial contacts and bettercharge transfer between CuI and dye-sensitized TiO2 nanoparticles, which resulted in the enhancement of power conversionefficiency from 0.058 to 1.01%. Copyright c© 2008 John Wiley & Sons, Ltd.

Keywords: in situ deposition of CuI; screen-printed TiO2 thin film; solid-state dye sensitized solar cell; interfacial improvement; efficiencyenhancement

Introduction

The first report on highly efficient dye-sensitized solar cells (DSSCs)appeared in 1991, which made a breakthrough in the solar cellresearch activity in making low cost solar cells[1] and henceforthmany groups have been working on this type of solar cells toimprove their efficiency as well as longterm stability. Currently,the highest certified efficiency value is 11.0% for small, liquidjunction type laboratory cell. The use of liquid electrolyte leadsto several technological problems such as solvent evaporation,degradation, and seal imperfection.[2] This can be overcome byusing solid or quasi-solid type hole transport material in place ofliquid electrolyte.[3 – 5]

Cuprous iodide (CuI) has received great attention as a catalystfor many organic coupling reactions and hole transportingmaterial in solid-state DSSCs.[6 – 9] γ -CuI is a convenient p-typesemiconducting material having a wide band gap of 3.1 eV.Because of its high optical transparency and good electricalconductivity, γ -CuI was extensively used to fabricate solid-state solar cell.[8 – 9] It can be cast from solution or by vacuumdeposition with conductivity above 10−2 S cm−1, which facilitatesits hole conducting ability.[10] γ -CuI was first used successfully byTennakone et al. to fabricate complete solid-state DSSC havingthe structure of TiO2/dye/CuI/Pt.[11] Typically, CuI from acetonitrilesolution was used to deposit inside the porous structure ofnanocrystalline thin film. This causes limited efficiency in sucha solar cell due to the poor interfacial contact between CuI anddye-coated TiO2 nanoparticles inside the porous structure. Thisis mainly attributed to the formation of large crystallite size ofCuI deposited by this method, which can result in improperfilling inside the pores of nanocrystalline matrix. To overcome thisdifficulty, a crystal growth inhibitor was used to control the size ofCuI for better filling of the pores. Controlling the amounts of thecrystal growth inhibitor added is most important and tedious aswell; otherwise a reverse effect can be seen.[12 – 13]

In the present investigation, we report in situ deposition of CuIprior to usual CuI deposition in order to make better contactbetween CuI and dye-coated TiO2 particles and for better filling ofthe pores inside the porous structure. We evaluate the efficiencychange due to in situ deposition of CuI and results are reportedtherein.

Experimental

Fluorine-doped tin oxide (FTO) coated glasses (20 �/square) wereused as conductive substrates to deposit nanocrystalline TiO2 thinfilm by screen-printing method. Prior to the printing, a compactTiO2 layer was formed by spreading n-butanol solution containing0.1 M titanium isopropoxide on FTO followed by sintering at450 ◦C for 30 min. For the formation of a nanoporous layer ofscreen-printed TiO2, 12 g n-butanol solution containing 3 g TiO2

powder (Degussa P25, 30% rutile and 70% anatase; particle size,25 nm; BET surface area, 55 m2 g−1) was stirred overnight. Inorder to enhance the porosity of TiO2, polystyrene (PS) balls(10%, mass percentage) with a mean diameter of 200 nm alongwith a little terpineol were added to the above suspension toget P25 TiO2 paste by further grinding in an agate mortar. Thispaste was screen-printed on the compact TiO2 layer followed

∗ Correspondence to: Jing Bo Zhang Jian Guang Jia, Key Laboratory of Pho-tochemistry, Center for Molecular Science, Institute of Chemistry, ChineseAcademy of Sciences, Beijing 100190, China.E-mail: jbzhang@iccas.ac.cn, jgjia@hotmail.com

a Department of Chemistry, School of Science, Beijing University of ChemicalTechnology, Beijing 100029, China

b Key Laboratory of Photochemistry, Center for Molecular Science, Institute ofChemistry, Chinese Academy of Sciences, Beijing 100190, China

c Department of Physics, North Maharashtra University, Umavinagar Jalgaon425001, India

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(a)

(b)

Porous TiO2 structure

Pores filled by CuI

FTO layer

Figure 1. SEM cross-sectional views of the screen-printed P25 TiO2 thinfilm (a) and the TiO2 thin film covered with CuI by spreading deposition(b).

by sintering at 450 ◦C for 30 min. These films were sensitized bydipping in 5 × 10−4 M Ru(dcbpy)2(NCS)2 (dcbpy: 2,2′-bipyridine-4,4′-dicarboxylic acid) (N3, Solaronix) overnight.

In situ deposition of CuI was carried as follows: Ethanolic solutionof anhydrous CuSO4 (0.8 mM) was dropped into 8 mM NaI (ethanolicsolution) in a sealed container with proper magnetic stirring andN2 bubbling, in which dye-sensitized thin films were suspended.After 4 h, the films with in situ deposited CuI were taken out andwashed with ethanol. Some of these films were further used todeposit CuI by the usual procedure. For this, a 6-ml solution ofacetonitrile containing 2.5 g of CuI and 0.026 g of triethylaminehydrothiocyanate was spread on dye-sensitized TiO2 thin filmswith and without in situ deposited CuI. After the evaporation ofacetonitrile, a platinized FTO counter electrode was pressed onthese films to complete the devices.

I –V curves of the cells were measured with a Potentio-stat/Galvanostat Model 273 (EG&G) under a light intensity of100 mW cm−2 at AM1.5 (Newport solar simulator), and the cellactive area was 0.2 cm2. Cross-section of the film was observedby an SEM (S-4500, Hitachi, 15 kV). The XPS data were obtainedwith an ESCALab220i-XL electron spectrometer from VG Scien-tific using 300 W Mg Kα radiations. The X-ray diffraction pattern(XRD) of CuI-covered porous TiO2 thin film was studied fromRiguku D/max 2500 using Cu Kα irradiation. The electrochemicalimpedance spectra of the thin layer cells were measured using So-lartron 1255B frequency response analyzer and Solartron SI 1287electrochemical interface system at a bias of −0.6 V in darkness.The scan frequency range was from 1 MHz to 1 Hz.

0 200 400 6000

1

2

3

Pho

tocu

rren

t den

sity

(m

A/c

m2 )

Voltage (mV)

a

b

c

Figure 2. Current-voltage characteristics of DSSCs with the structuresof TiO2/dye/in situ CuI/Pt (a), TiO2/dye/CuI/Pt (b), and TiO2/dye/in situCuI/CuI/Pt (c).

1000 800 600 400 200 0

κ (a

.u.)

Binding Energy (eV)

b

a

Cu(2p3/2)

I(3d5/2)

C(1s)

Figure 3. Wide X-ray photoelectron spectra of the TiO2 films filled by CuI.(a) TiO2 film with in situ growth of CuI and (b) TiO2 film with in situ growthand spreading deposition of CuI.

Results and Discussion

Screen-printing technology is suitable and versatile for a large-scale industrial production process. This process was recently usedto deposit nanocrystalline semiconductor thin film electrode forliquid-state DSSCs. To the best of our knowledge, no attempt hasbeen made to use this process to fabricate solid-state DSSC usingCuI as hole transporting material. We screen-printed TiO2 thin filmby the addition of PS balls that facilitates the formation of morepores on burning the PS balls during sintering at 450 ◦C favoringbetter penetration of CuI inside the porous thin film (as shown inFig. 1(a)). The entire film thickness was about 6 µm. Although TiO2

thin film with some big pores was employed, the usual methodof CuI deposition (spreading from acetonitrile solution of CuI)cannot make CuI to effectively penetrate into the porous innerfilm and establish good contact with the porous surface as shownin Fig. 1(b).

Figure 2 shows current-voltage characteristics of the deviceswith only in situ deposited CuI (a), spreading of CuI from acetonitrile(b), and in situ deposited CuI followed by spreading of CuI (c). Dueto the solubility limit of CuSO4 in ethanol, the increments ofCuI are not sufficient to fill up the pores and to expedite hole-transporting process between TiO2 and a platinum electrode. Theperformance of solid-state DSSC based on in situ deposited CuI is

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920 930 940 950 960 970

κ (a

.u.)

κ (a

.u.)

Binding Energy (eV)

Cu (2p3/2)

(a) (b)

610 615 620 625 630 635 640 645

Binding Energy (eV)

I (3d5/2)

Figure 4. High-resolution XPS spectra taken for Cu(2p) (a) and I(3d) (b) region of the in situ grown CuI layer on TiO2 thin film.

10 20 30 40 50 60 70 80

Inte

nsity

(a.

u.)

2θ (°)

Figure 5. X-ray diffraction pattern of CuI layer deposited on the porousTiO2 thin film. Peaks marked as ‘o’ correspond to γ -CuI phase.

far from satisfactory (conversion efficiency 0.016%). Furthermore,for the spreading method of CuI, due to the improper poresfilling and lack of good contact between CuI and TiO2 particlesas shown in Fig. 1(b), the efficiency of such a device is limitedto 0.058% (b curve in Fig. 2). On combining both methods bymeans of prior in situ deposition of CuI followed by spreading ofCuI from acetonitrile, the device showed enhancements in bothphotocurrent and photovoltage, as well as in conversion efficiencyup to 1.01%. This indicates better pores filling inside the porousstructure of TiO2 and forming of good internal contact betweenTiO2 and CuI by in situ deposition and good coverage of CuI onTiO2 by the spreading method.

The wide XPS picture of porous TiO2 thin films with in situgrown CuI or spreading of CuI were shown in Fig. 3. Further,the high-resolution XPS of Cu(2p) and I(3d) for in situ grown CuIwas shown in Fig. 4. Adventitious carbon (C1s) as reference wasobserved at the binding energy of 284.8 eV (not shown).[14] The3d5/2 photoelectron peak of iodine (I−) appeared at a bindingenergy of 618.8 eV for CuI. This is close to 619.0 eV, the reportedvalue for CuI in the literature.[15] The peak could be fitted very wellwith a single peak indicating that only one type of iodine is presentin the sample. For the Cu2p3/2, the signal appeared at 931.8 eV forCuI. For comparison, the binding energies reported for Cu+ andCu2+ are 932.5 and 933.8 eV, respectively.[14,15] According to thepeak area of XPS of in situ grown CuI, the molar ratio of Cu+ andI− was calculated as 1 : 1, certifying that CuI can be deposited onTiO2 porous thin film by in situ deposition method.

There are three kinds of crystalline structures for CuI. α-CuIand β-CuI are ionic conductors and γ -CuI is a hole transport

0 100 200 300 400 500 6000

50

100

150

200

250

300

Z"(

ohm

)

Z' (ohm)

Figure 6. Electrochemical impedance spectra of cells with the structuresof TiO2/dye/CuI/Pt (solid square) and TiO2/dye/in situ CuI/CuI/Pt (opensquare).

semiconductor. Therefore, γ -CuI is required as a hole transportingmaterial to fabricate solid-state DSSCs. The crystal structure of CuIdeposited on dye-sensitized TiO2 thin film by the two methodswas measured by XRD and shown in Fig. 5. The diffraction peaksof CuI marked as o are in good agreement with the standard peakof γ -phase.

Electrochemical impedance measurements were carried out toanalyze the interfacial contact improvement of CuI and poroussurface of dye-sensitized TiO2 thin film for in situ deposited CuI.Figure 6 shows impedance spectra of solid-state DSSCs with thestructures of TiO2/dye/CuI/Pt and TiO2/dye/in situ CuI/CuI/Pt. Twodistinct arcs are observed for both cells. In general, the impedancespectrum of DSSC based on liquid electrolyte shows three arcs fromhigh to low frequency corresponding to resistances at FTO/TiO2,Pt/electrolyte, TiO2/dye/electrolyte, and of redox diffusion in theelectrolyte with the first two resistances overlapping to showone arc in the impedance spectrum. In the case of solid-stateDSSC based on hole transporting materials, there is no redoxdiffusion process in the electrolyte. Therefore, the second arcin Fig. 6 is assigned to the interfacial resistance of TiO2/dye/CuI.The first arc with almost the same radius for the two samplesis reasonable for them to be with the same TiO2 thin film onFTO and the same contact between platinized FTO and CuI.Their difference is located at the second arc, where the cellwithout in situ deposited CuI shows a bigger radius than thecell with in situ deposited CuI revealing that in situ depositionimproves interfacial contact of CuI with dye-sensitized TiO2 thinfilm. Therefore, significant enhancement of conversion efficiency

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has been achieved because of better improvement of interfacialcontacts. In situ deposition of CuI is a new point of view to fabricatesolid-state solar cells with better interfacial contacts leading tobetter charge transport. The conversion efficiency of the deviceis still lower than the earlier reported highest value.[16] By usingthis new approach and optimizing the deposition parameters,there is much more scope to improve the conversion efficiency ofsolid-state DSSCs.

Conclusion

The screen-printed TiO2 nanocrystalline thin film with porousstructure was applied for the fabrication of solid-state DSSC. In situdeposited CuI along with the usual CuI deposition from acetonitrileshowed improved interfacial contact of CuI with the dye-coatedTiO2 porous thin film, which further led to the overall enhancementin power conversion efficiency.

Acknowledgements

The authors appreciate the financial support for this workby the Major State Basic Research Development Program(2006CB202605), High-Tech Research and Development of ChinaProgram (2007AA05Z439), the National Nature Science Foun-dation of China (50221201), and Innovative Foundation of theCenter for Molecular Science, Chinese Academy of Science (CMS-CX200718).

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