Electrochemistry Communications 11 (2009) 75–79

Contents lists available at ScienceDirect

Electrochemistry Communications

journal homepage: www.elsevier .com/locate /e lecom

High efficiency solar cell based on dye sensitized plasma treatednano-structured TiO2 films

Kyung-Hee Park a, Marshal Dhayal b,*

a Department of Electrical Engineering, Chonnam National University, Republic of Koreab Department of Bioengineering, University of Washington, Seattle, USA

a r t i c l e i n f o

Article history:Received 25 August 2008Received in revised form 30 September2008Accepted 3 October 2008Available online 18 October 2008

Keywords:Plasma treatmentDye-sensitized solar cellSurface structure

1388-2481/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.elecom.2008.10.020

* Corresponding author. Tel.: +1 206 543 7331; faxE-mail address: marshaldhayal@yahoo.com (M. Dh

a b s t r a c t

Radio frequency (RF) plasma treatment of nano-structured TiO2 was carried out to enhance the efficiencyof dye-sensitized solar cell. Relative change in surface chemistry and hydrophilic characteristics wasinvestigated by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Increase insurface hydrophilic and carboxyl functionality had enhanced the dye molecule adsorption. Plasma treat-ment has also increased proportion of Ti3+ surface site which supported transport of electron and holebetween dye molecules and photoelectrode. Plasma treated TiO2 at optimum condition had increasedefficiency by 40% relative to untreated TiO2.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Dye-sensitized nano-structured TiO2 solar cells (DSSCs) havepotential to develop low cost alternative energy to traditional pho-tovoltaic device [1–4]. The electron transport mechanism in nano-particle thin films is fairly well understood and nano-structureshas increased surface area by three orders and achieved longeroptical path in DSSCs [4]. Since the discovery of TiO2 base DSSCs,several other types of composite nano-structured oxides suchZrO2 [5], GeO2 [6], Al [7], Si [8], La [9] etc. with titania had beenuse to improve sensitivity, thermal stability and performance ofDSSC [10].

DSSC working principle is very simple in which photons inter-action with dye molecules creates excitons and at nanoparticlesurface these are rapidly split with electrons and holes. Electronsattracted toward photoelectrode and holes moved by means of re-dox species in electrolytes used in DSSC [11–14]. The conversionyield of incident photons into current density (J1 = gabs �Uinj

� gcoll). Where gabs is dye absorption coefficient of incident pho-tons, Uinj is quantum yield of charge injection and gcoll is efficiencyof charge collection at photoelectrode [4]. Therefore efficiency ofDSSC depends on (i) electrolyte, (ii) electrode surface areas, (iii)characteristics of dye molecules, (iv) recombination at electrodesurfaces, (v) regeneration of electrolyte and dye, and (vi) surfaceactive sites of photoelectrodes [15].

ll rights reserved.

: +1 206 685 3300.ayal).

The efficiency (g) of DSSC is proportional to short circuit current(Isc), open circuit voltage (Voc) and filed factor (FF). Where, Isc de-pends on dye and photon interaction efficiency and diffusionlength of exciton whereas Voc depends on Fermi energy level ofTiO2 and closest potential of the redox couple in the electrolyte.There are several excellent reports on development of new materi-als for electrolyte and stability [16], photoelectrodes [17,18] anddyes [19]. Recently hybrid electrolyte containing both liquid andsolid electrolytes [20], plasma treatment [21], and natural bindersin TiO2 photoelectrode [22] had also showed enhancement in DSSCefficiency.

To have higher sensitivity in DSSC it is necessary to ensure elec-trons should travel to the conducting electrode before chargerecombination occurs and diffusion length > thickness of the TiO2

film. Hence, controlling dynamics of electron transport and reduc-ing interfacial recombination will allow us to develop next gener-ation high-sensitivity DSSC. In this communication plasma surfacemodification of TiO2 had been used to increase the efficiency ofDSSC. Effects of plasma treatment on TiO2 surface charge statesand surface functionalities were investigated. Enhanced electro-chemical response of plasma treated photo electrode was corre-lated with increase in oxygen vacancies and enhanced electrontransport.

2. Experimental

Using squeeze printing method 10 lm thick with 0.25 cm2 ac-tive area of the TiO2 photoelectrode films were prepared ontopre-cleaned fluorine-doped tin dioxide (FTO, Pilkington TEC glass,

76 K.-H. Park, M. Dhayal / Electrochemistry Communications 11 (2009) 75–79

8 X cm�2) substrate as described previously [18]. Capacitive cou-ple radio frequency (RF) driven plasma reactor made of a glass cyl-inder was used for plasma treatment of TiO2 thin film. Thedischarge was sustained by applying 13.56 MHz RF excitationthrough a matching network. Different RF power and treatmenttime were used at constant 100 mTorr operating pressure.

Plasma treated and untreated TiO2 photoelectrodes were im-mersed into the ethanol solution of bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylato)-ruthenium(II)-bis-tetrabutyl ammo-nium (N719 dye, Ruthenium 535-bis TBA, Solaronix,0.5 m mole) for 24 h. The sandwich-type solar cell was assem-bled by placing a platinum-coated conducting glass on thephotoelectrode and was sealed after adding the electrolytesolution consisted of 0.3 M 1,2-dimethyl-3-propylimidazoliumiodide, 0.5 M LiI, 0.05 M I2 and 0.5 M 4-t-butylpyridine in 3-methoxypropionitile.

X-ray photoelectron spectroscopy (XPS) was performed on aMultiLab200 with standard Mg Ka radiation. The binding en-ergy was calibrated by using the C1s line (284.6 eV) fromadventitious carbon. Field emission scanning electron micros-copy (FE-SEM, Hitachi, S-4700) was use to measure film surfacemorphology. An AM1.5 solar simulator (Thermo–Oreal with a1000 W Xe lamp and an AM 1.5 filter) was used and incidentlight intensity was calibrated with a standard Si solar cell pro-duced by Japan Quality Assurance Organization (JQA). Four par-allel samples for each kind of layer structure were made for I–Vmeasurement, and the reproducibility of TiO2 photoelectrodewas quite good.

0.1

0.3

0.5

0.7

0.9

1.1

454456458460462464466

CP

S(1

04 )

Ti3+2p3/2

Ti4+2p3/2

Ti4+2p1/2

Ti3+2p1/2

a

Environment Position / eV FWHM / eV Ti / at %

Ti 4+ 2p3/2 456.6 1.1 57.1

Ti 4+ 2p1/2 462.2 2.3 36.6

Ti 3+ 2p3/2 455.4 1.2 4.1

Ti 3+ 2p1/2 458.1 1.2 2.2

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

454456458460462464466

CP

S(1

04 )

Ti3+2p3/2

Ti4+2p3/2

Ti4+2p1/2

Ti3+2p1/2

c

Environment Position / eV FWHM / eV Ti / at %

Ti 4+ 2p3/2 456.6 1.19 52.8

Ti 4+ 2p1/2 462.3 2.45 34.8

Ti 3+ 2p3/2 455.6 1.5 7.5

Ti 3+ 2p1/2 458.1 1.8 4.9

Untreated

20 min and 30 W

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

2288290292

CP

S(1

04 )X

X

C-OX

C=OC(O)OX

a

Name Position / eVC-H/C-C 284.C-C(=O)OX 28C -OX 286.C =O 287.C (=O)OX 288.

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2288290292

CP

S(1

04 )X

X

C-OX

C=OC(O)OX

c

Untr

20 min a

Ti2p

Ti2p

Name Position / eVC-H/C-C 284.6C-C(=O)OX 285C -OX 286.1C =O 287.6C (=O)OX 288.6

C1s

C1s

Binding EnBinding Energy (eV)

Binding Energy (eV) Binding E

Fig. 1. Ti2p, C1s and O1s high resolution XPS spectra of untreated and plasm

3. Results and discussion

XPS analysis was carried out to determine changes in surfacecharge states of TiO2 thin film and functionalities at surface. Highresolution Ti2p and C1s XPS spectra of untreated and plasma trea-ted at 30 W RF power for 20 min were shown in Fig. 1. A Ti2p XPSspectrum of untreated film was fitted with four peaks and chemi-cal shift relative to the Ti4+ 2p3/2 peak was shown in figure. About94% of titanium surface state of Ti2p in untreated TiO2 film was asTi4+. Plasma treated TiO2 film spectra was also fitted with samefour peaks and details of chemical shift relative to the Ti4+ 2p3/2

peak was shown in figure. Ti3+ surface state in Ti2p was increasedfrom 6.3% to about 12.4% after plasma treatment. Similarly an in-crease in Ti2O3 surface states in O1s XPS spectra was observed asshown in Fig. 1.

Plasma interaction with TiO2 film is a complex process due topresence of ions, energetic photons (ht) and radicals (mainly H�and OH�) in the discharge. In this type of discharges D Bartonet al. [23] had reported that at 10 W RF power and 100 mTorr pres-sure (similar conditions were used in this study) radiation flux(�6 mW cm�1) was about three times higher than ionic flux.Hence, it was expected that higher level of radiation flux in theplasma has increased TiO2 surface active states. Hence, it is ex-pected that during the plasma treatment at the surface of TiO2

(Ti4+) ? Ti2O3 (Ti3+) reaction accurse. This process can generateoxygen vacancy at the surface (seen in O1s XPS spectra) and excesselectrons in Ti which can change the electronic structure of TiO2

[24].

28028228486

C-C/C-H

C-C(O)OX

FWHM / eV Carbon / at %6 1.8 60.75 1.6 12.83 1.5 126 1.2 1.78 1.6 12.8

28028228486

C-C/C-H

C-C(O)OX

eated

nd 30 WFWHM / eV Carbon / at %

1.56 57.11.6 13.2

1.54 151.2 1.51.6 13.2

0.2

0.4

0.6

0.8

1.0

1.2

1.4

524526528530532534

CP

S(1

04 )

Binding Energy (eV)ergy (eV)

nergy (eV) Binding Energy (eV)

Ti2O3

TiO2

Untreated

O1s

Environment Position / eV FWHM / eV O1s / at %TiO2 528.9 1.4 87.6

Ti2O3 530.7 1.7 12.4

0.5

1.0

1.5

2.0

2.5

3.0

524526528530532534

CP

S(1

04 )

Ti2O3TiO2

20 min and 30 W

O1s

Environment Position / eV FWHM / eV O1s / at %TiO2 528.8 1.4 72.4

Ti2O3 530.5 2.1 27.6

a treated TiO2 (at 100 mTorr pressure and 30 W RF power for 20 min).

K.-H. Park, M. Dhayal / Electrochemistry Communications 11 (2009) 75–79 77

Details of different carbon functionalities at the surface were alsodetermined from C1s high resolution XPS spectrum. The C1s spec-trums for untreated TiO2 film was fitted with five peaks as; hydrocar-bon (C–H/C–C) at 284.6 eV, C–C(@O)OX at 285 eV, alcohol/ether (C–OX) at 286.3 eV, carbonyl (C@O) at 287.6 eV, and carboxylic/ester

11

12

13

14

15

16

0 5 10 15 20 25Time (min.)

% o

f CO

X in

C1s

4

6

8

10

12

14

0 5 10 15 20 25Time (min.)

% o

f Ti3+

in

Ti2p

Fig. 2. Relative proportion of carboxyl and Ti3+ surface states

Untreated

EV

EC

EF

D

EΔVU

Load

Ti4+

Untreated ΔVU <

extra energleve

TiO2

Fig. 3. Untreated and plasma treated (at 100 mTorr pressure and 30 W RF power for 20 mtreated TiO2 DSSC. Valance band energy (EV), conduction band energy (Ec), fermi energy

(C(@O)OX) at 288.8. Increase in each oxygen atoms with carbonhad showed increase of 1.6 ± 0.2 eV in binding energy. C1s spectrumof plasma treated TiO2 was also fitted with similar five peaks.

The quantitative analysis of Ti3+ surface states in Ti2p withdifferent plasma treatment conditions was shown in Fig. 2. A rapid

11

12

13

14

15

16

0 10 30RF Power (W)

% o

f CO

X in

C1s

4

6

8

10

12

14

0 10 30RF Power (W)

% o

f Ti3+

in

Ti2p

in C1s and Ti2p for untreated and plasma treated TiO2.

Plasma Treated

EV

ECEF

D

EΔVP

Ti3+

Load

Plasma TreatedΔVP

e-

y l

TiO2

in) TiO2 FE-SEM pictures. A schematic diagram to explain the principle of the plasmalevel (Ef), dye (D) and electrolyte (E).

0

2

4

6

8

10

12

14

16

0 0.2 0.4 0.6 0.8Voltage (V)

TiO25min15min20 min

0

2

4

6

8

10

12

14

16

0 0.2 0.4 0.6 0.8

Voltage (V)

J (m

A/c

m2 )

J (m

A/c

m2 )

TiO2

10W

30W

Sample Voc (V) Isc (mAcm- 2) FF η (%)

10W 0.70 13.8 0.66 6.3

30W 0.68 14.6 0.64 6.4

Sample Voc (V) Isc (mAcm- 2) FF η (%)

TiO2 0.64 12.3 0.58 4.6

5min 0.73 12.5 0.66 6.4

15min 0.68 14.5 0.65 6.4

20min 0.68 14.5 0.65 6.4

Fig. 4. Four characteristics of untreated and plasma treated TiO2 in dye-sensitized solar cell including details of all the parameters.

78 K.-H. Park, M. Dhayal / Electrochemistry Communications 11 (2009) 75–79

increase in Ti3+ was observed even after 5 min plasma treatment at30 RF power. There was no significant difference between low andhigher power plasma treatment for 10 min at 100 mTorr pressure.A quantitative analysis of the variation of C–OX functionality withdifferent plasma treatment conditions was also carried out andshown in Fig. 2. The result showed that the proportion of carbonatoms as C–OX functionality gradually increased with increasingthe plasma treatment time. Similarly increase in RF power had alsoshowed increase in carboxyl functionality at surface.

Fig. 3 shows FE-SEM images of TiO2 film on FTO glass before andafter plasma treatment. The observations had showed very goodfilm surface uniformity with about 15 nm TiO2 nanoparticles. Thistype of structure has advantages to have higher adsorption of dyemolecules and also supports penetration of the I�/I3

� redox coupleinto the TiO2 film. There was no significant difference in surfacemorphology after plasma treatment however surface oxidationcontamination was decreased drastically. This can be seen fromthe brightness of film surface which is an indication of very cleansurface and high electron conduction. The water contact angle ofuntreated film was 45 ± 3� and it was decreased to 14 ± 4�. Plasmatreatment had increased surface energy which sports higher levelof dye molecule adsorption at the surface. A schematic diagramis also shown in Fig. 3 to understand the effects of plasma treat-ment on DSSC.

Fig. 4 shows photocurrent–voltage characteristics of the dye-sensitized solar cells based on untreated and plasma treatedTiO2. At low RF power treatment open circuit voltage was higherand increasing the plasma treatment time an increase in short cir-cuit current density was observed. This increase in Voc can be ex-plained as: in the discharge following reaction at the surface wasexpected: TiO2 + ht (in plasma) ? TiO2

* (h+vb e�cb). Hence, increase

in number of electrons in conduction band of TiO2 was expected.Therefore excess electrons in Ti can increase the fermi energy levelof TiO2 and potential of the redox couple in the electrolyte was in-creased (schematic is shown in Fig. 3). Hence, an increase in Voc

was expected. Enhanced value of Isc was explained with increasein surface hydrophilic nature and hydroxyl groups. Lower surfacecontract angle can increase the adsorption of dye molecules at thesurface. Therefore, higher current density was observed. Plasma

treatment has increased the conversion efficiency by 40% which isvery significant contribution toward the development of high-sensi-tivity DSSCs. Overall efficiency of this cell was 6.4% and this can befurther increased by using optimum photoelectrode structure andelectrolyte combination.

4. Conclusion

Plasma treatment had significantly increased surface hydro-philic characteristics and carboxyl functionality. The increase inhydrophilic nature with carboxyl functionality at surface had en-hanced the dye molecule adsorption. Plasma treatment has also in-creased proportion of Ti3+ surface site which supported transportof electron and hole between dye molecules and photoelectrode.Plasma treated TiO2 at optimum condition had increased efficiencyby 40% relative to untreated TiO2.

Acknowledgements

Authors are thankful to Dr. M.Y. Kim for helping in obtainingXPS data and for partial funding support from second-stage BrainKorea 21.

References

[1] M.S. Akhtar, J.M. Chen, O.B. Yang, Electrochem. Commun. 9 (2007) 2833.[2] M.K. Nazeeruddin, P. Péchy, M. Grätzel, J. Am. Chem. Soc. 123 (2001) 1613.[3] M.K. Nazeeruddin, A. Kay, I. Rodicio, M. Grätzel, J. Am. Chem. Soc. 115 (1993)

6382.[4] B. O’Regan, M. Grätzel, Nature 353 (1991) 737.[5] A. Kitiyanan, S. Ngamsinlapasathian, S. Pavasupree, S. Yoshikawa, J. Solid State

Chem. 178 (2005) 1044.[6] A. Kitiyanan, T. Kato, Y. Suzuki, S. Yoshikawa, J. Photochem. Photobio. A. Chem.

179 (2006) 130.[7] K.P. Kumar, Appl. Catal. A: General 119 (1994) 163.[8] B.M. Reddy, S. Mehdi, E.P. Reddy, Catal. Lett. 20 (1993) 317.[9] C.A. Leduc, J.M. Campbell, J.A. Rossin, Ind. Eng. Chem. Res. 35 (1996) 2473.

[10] Y. Diamant, S.G. Chen, O. Melamed, A. Zaban, J. Phys. Chem. B 107 (2003) 1977.[11] P. Wang, S.M. Zakeeruddin, I. Exnar, M. Grätzel, Chem. Commun. (2002) 2972.[12] G.R.A. Kumara, S. Kaneko, M. Okuya, K. Tennekone, Langmuir 18 (2003) 10493.[13] J. Bandara, H. Weerasinghe, Sol. Energy Mater. Sol. Cells 85 (2005) 385.[14] J. Kang, W. Li, X. Wang, Y. Lin, X. Li, X. Xiao, S. Fang, J. Appl. Electrochem. 34

(2004) 301.

K.-H. Park, M. Dhayal / Electrochemistry Communications 11 (2009) 75–79 79

[15] E. Stathatos, P. Lianos, S.M. Zakeeruddin, P. Liska, M. Grätzel, Chem. Mater. 15(2003) 1825.

[16] Y. Bai, Y. Cao, J. Zhang, M. Wang, R. Li, P. Wang, S.M. Zakeeruddin, M. Gratzel,Nature Mater. 7 (2008) 626.

[17] S. Ito, S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.K.Nazeeruddin, P. Péchy, M. Takata, H. Miura, S. Uchida, M. Grätzel, Adv. Mater.18 (2006) 1202.

[18] K.H. Park, H. Li, M. Dhayal, J.W. Lee, H.B. Gu, Journal of Nanoscience andNanotechnology 8 (2008) 5252.

[19] Z.S. Wang, Y. Cui, K. Hara, Y. Dan-ohh, C. Kasada, A. Shinpo, Adv. Mater. 19(2007) 1138.

[20] K.H. Park, D.W. Park, M. Dhayal, H.B. Gu, Electrochem. Commun. 10 (2008)1098.

[21] Y.M. Sung, H.J. Kim, Thin Solid Films 515 (2007) 4996.[22] K.H. Park, C.K. Hong, Electrochem. Commun. 10 (2008) 1197.[23] D. Barton, J.W. Bradley, K.J. Gibson, D.A. Steele, R.D. Short, J. Phys. Chem. B 104

(2000) 7150.[24] K. Sakata, Physica Status Solidi B 116 (1) (1983) 145.