Materials Science in Semiconductor Processing

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Improvement of photoelectrochemical and optical characteristics of MEH-PPV using titanium

dioxide nanoparticles

Farid Habelhames

ⁿ

, Leila Lamiri, Wided Zerguine, Belkacem Nessark

Laboratoire d’Electrochimie et Mate´riaux (LEM), Faculte´ de Technologie, Universite´ Ferhat Abbas de Se´tif, Se´tif 19000, Algeria

a r t i c l e i n f o

Available online 23 January 2013 Keywords:

Organic–inorganic compounds MEH-PPV

Morphology

Photoelectrochemical properties

a b s t r a c t

The use of bulk heterojunctions can increase the efficiency of exciton dissociation in polymer-based photovoltaics. We prepared and characterized bulk heterojunctions of poly[2-methoxy-5-(2⁰-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and titanium dioxide nanoparticles deposited by spin coating on indium tin oxide substrates. The surface morphology of the MEH-PPVþTiO2composite films revealed that addition of TiO2nanoparticles increased the film roughness. The effect of TiO2nanoparticles on the photoelectrochemical and optical characteristics of MEH-PPV polymer heterojunctions was studied. Addition of TiO2 nanoparticles improved the absorbance of MEH-PPV composite films. Moreover, the photocurrent of the composite devices increased with the TiO2 nanoparticle concentration. These observations provide an insight into new approaches to improve the light collection efficiency in photoconductive polymers.

1. Introduction

Photovoltaic cells based on organic–inorganic bulk heterojunction (BHJ) semiconductors have attracted much interest in the last two decades[1–4]. Based on intimate nanoscale mixing of an electron donor–usually a p-type conjugated polymer–and an electron acceptor–usually an n-type organic semiconductor–BHJs have shown regular improvements due to better control of the nanophase separation [5]. Research trends on this area are now focused on the development of new materials such as low-bandgap polymers[6,7] to absorb a greater fraction of the solar radiation spectrum.

Nanoparticles of a variety of inorganic materials such as CdSe [8–10], CdTe [11,12], silicon [13,14], InP [15], GaAs[16]and PbS[17,18] have been studied intensively for use in hybrid BHJ solar and photoelectrochemical cells. In particular, hybrid BHJs comprising transition metal oxide compounds and organic semiconductors have

shown promising results. Among the metal oxides, titanium dioxide (TiO2) [19,20] and zinc oxide (ZnO) [21–23] are of particular interest because of their easy fabrication, non-toxicity and relatively low production costs.

Poly[2-methoxy-5-(20-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is a photoactive polymer that forms excitons on exposure to light[24]. Despite its low electron mobility, MEH-PPV has been utilized in solar cell research because of its high absorbance in the visible spectral range [25]. However, since MEH-PPV has high hole mobility, it is often paired with an electron carrier for the preparation of photovoltaic devices[26].

BHJ materials can be deposited via solution-based techniques or electrochemical polymerization onto prin- table, flexible and large-scale films for various industrial applications[27]. For solar cell applications, nanocrystal- line TiO2/indium tin oxide (ITO) electrodes are generally combined with a conjugated polymer such as MEH-PPV [28,29]. Hybrid polymer solar cells that use metal oxides as acceptors have been reviewed[30–32]. Here we report on the morphology of MEH-PPV films modified by addition of TiO2nanoparticles and on the enhanced device performance resulting from the increase in the active surface area.

Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/mssp

Materials Science in Semiconductor Processing

http://dx.doi.org/10.1016/j.mssp.2012.12.015

nCorresponding author. Tel.:þ213 36 92 51 21; fax:þ213 36 92 51 33.

E-mail address:habelhamesfarid@yahoo.fr (F. Habelhames).

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2. Experimental

2.1. Materials and ﬁlm preparation

Commercial MEH-PPV (99%, Sigma-Aldrich) and TiO2

nanoparticles (40–80 nm, 99%, Aldrich) were dissolved in chlorobenzene (99.8%, Sigma-Aldrich) and stirred for 24 h.

ITO substrates (Merck; sheet resistance 10O/

m

) were successively cleaned with water and acetone for 20 min in an ultrasonic bath and ﬁnally dried at 1201C. Once dissolved, the MEH-PPVþTiO2 solution was deposited by spin coating on the ITO substrate at a spinner speed of 900 rpm for 40 s. Following deposition, ﬁlms were dried at 801C for 15 min to remove the solvent.

2.2. Characterization

The morphology of MEH-PPVþTiO2 films was exam- ined by atomic force microscopy (AFM) using a Thermo- microscope AutoProbe LP Research instrument. The film thickness, measured with a Dektak profilometer, varied from 100 to 120 nm.

Electrochemical analysis, including cyclic voltammetry, of MEH-PPV and MEH-PPVþTiO2ﬁlms deposited on ITO

(working electrode) was carried out in a one-compart- ment cell connected to a potentiostat/galvanostat (PGZ- 301 Voltalab) connected to a computer equipped with Voltamaster 4 software to select the electrochemical technique and set suitable parameters. The reference electrode was a saturated calomel electrode (SCE) with KCl and the auxiliary electrode was a platinum plate. The supporting electrolyte was 10¹mol l¹lithium perchlo- rate dissolved in acetonitrile.

Cyclic voltammograms were recorded at a scan rate of 50 mV s¹in the potential range from 0 toþ1500 mV vs.

SCE. The same cell used in current density measurements was used for photoelectrochemical experiments. A 500-W halogen lamp was used as a polychromatic light source and the illumination intensity was 100 mW cm². All measurements were made at room temperature.

3. Results and discussion 3.1. Atomic force microscopy

In general, AFM images provide information about height differences and the constituency of composite thin ﬁlms at the surface because hard nanoparticles can easily

Fig. 1.AFM images of (a,b) MEH-PPV and (c,d) MEH-PPVþTiO2(8%) ﬁlms.

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be distinguished from the soft polymer. The surface topography of MEH-PPVþTiO2composite films (thickness 120 nm) provides information on the distribution of phases. Fig. 1demonstrates that addition of TiO2 nanoparticles increased the roughness of the polymer film (Table 1)[33]. The AFM images show an interpenetrating network. The small size of TiO2 nanoparticles in MEH- PPVþTiO2films leads to more efficient exciton dissociation and provides more conducting channels for charge transfer and thus enhances device performance.

3.2. Cyclic voltammetry

Cyclic voltammograms for MEH-PPVþTiO2 deposited on ITO in CH3CN/LiClO4 shows two oxidation peaks at 0 andþ1500 mV vs. SCE (Fig. 2) corresponding to the ﬁrst and second oxidation events for MEH-PPV. The absence of any other peak for TiO2can be explained by the fact that this inorganic semiconductor is not electrochemically active because of its insolubility in the medium under our experimental conditions. The electrochemical stabi- lity observed for MEH-PPVþTiO2 is in good agreement with cyclic voltammograms for other organic–inorganic composites[34,35].

3.3. UV-Vis spectra

Fig. 3 shows UV-Vis absorbance spectra of MEH-PPV and MEH-PPVþTiO2 ﬁlms. For all the ﬁlms, the peak at 500 nm is attributed to a

p

–

p

ⁿ transition of the conjugated polymer[35,36]. The results for MEH-PPV composite ﬁlms containing different TiO2concentrations shows that addition of TiO2 nanoparticles improves the absorbance. The increase in absorbance can be attributed to an increase in the active surface layer by introduction of TiO2

nanoparticles. As a result, the MEH-PPVþTiO₂ﬁlms have slightly broader absorbance peaks. This is in agreement with previous results reported by other researchers for composites based on MEH-PPV[37].

3.4. Photoelectrochemistry

MEH-PPV and MEH-PPVþTiO2ﬁlms deposited on ITO substrate (surface of 1 cm²) were used as working electrodes in a photoelectrochemical cell containing CH3CN/

LiClO₄(10^–1mol l^–1) as electrolyte (Fig. 4).

The polymer and composite films were polarized at – 400 mV vs. SCE. After stabilization of the current, the working electrode was irradiated with polychromatic light at an intensity of 100 mW cm^–2(Fig. 5). The plots show that the polymer film presented a cathodic photocurrent peak immediately after irradiation. This response indicates that recombination processes are occurring in the film, probably due to the presence of charge carriers in the polymer bulk, mainly due to structural disorder. Our results are in agreement with the conducting behavior of p-type polymers [16,38,39]. This response indicates that the presence of charge carriers in the polymer film, Table 1

Root mean square (RMS) and average roughness (RA) of (a,b) MEH-PPV and (c,d) MEH-PPVþTiO2(8%) determined from AFM characterization of the ﬁlms shown inFig. 1.

Sample RMS (nm) RA (nm)

a 11.4 6.6

b 5.7 3.6

c 17.8 10.7

d 13.2 8.4

0 500 1000 1500

-50 0 50 100 150 200 250

Current density(μA/cm)2

Potential (mVvs SCE) MEH-PPV+TiO₂ 0%

MEH-PPV+TiO₂ 2%

Fig. 2.Cyclic voltammograms of MEH-PPV and MEH-PPVþTiO2ﬁlms in CH3CN/LiClO4electrolyte solution.

300 400 500 600 700 800 900

0.00 0.05 0.10 0.15 0.20

Absorbance

λ(nm)

MEH-PPV + TiO₂ 0 % MEH-PPV + TiO₂ 2 % MEH-PPV + TiO₂ 6 % MEH-PPV + TiO₂ 8 %

Fig. 3.UV-Vis absorbance spectra of MEH-PPVþTiO2ﬁlms containing different TiO2concentrations.

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mainly due to structural disorder. The presence of the space-charge region suggests that these polymers may produce photocurrents when illuminated.

As expected, addition of TiO2 to the polymer ﬁlms increased the photocurrent generated. We suggest that semiconducting TiO₂ nanoparticles act as dissociation centers for the polymer excitons, increasing the number of charge carriers that reach the TiO2–MEH-PPV interface.

Thus, when assembling photovoltaic devices based on a conducting polymer and an inorganic semiconductor, it is important to remember that the device performance is closely related to the concentration of the latter.

Fig. 6 shows photocurrent density as a function of applied potential (800 to þ800 mV vs. SCE) for MEH-PPVþTiO2/ITO electrodes with different TiO2concen- trations for a light intensity of 100 mW/cm². The photocurrent for MEH-PPVþTiO2was negative at all potential values and was higher than that for the non-modiﬁed polymer[40].

4. Conclusion

Composite ﬁlms of a conducting polymer and semiconductor nanoparticles were obtained by dissolving MEH-PPV with various TiO2 concentrations and were deposited by spin coating on ITO substrates. Addition of TiO2 nanoparticles improved the absorbance of the

Working electrode (Conductive layer/ITO)

Polychromatic lamp Electrochemical cell

Reference electrode (SCE)

Auxiliary electrode (Pt)

Electrolyte solution

Fig. 4.Schematic of the experimental photoelectrochemical technique.

140 160 180 200

-500 -400 -300 -200 -100 0

Current density nA/cm2

Time (s)

MEH-PPV MEH-PPV+TiO₂ 2%

MEH-PPV+TiO₂ 4% MEH-PPV+TiO₂ 8%

Dark Dark Dark

illumination illumination

Fig. 5.Current density–time plots for MEH-PPV/ITO and MEH-PPVþTiO2/ ITO electrodes in CH3CN/LiClO4(10¹mol l¹) at a potential of400 mV observed on switching the light on and off for light intensity of 100 mW cm².

-800 -600 -400 -200 0 200 400 600 800

-300 -200 -100 0

Photocurrent density (nA/cm2) Potential (mV vs SCE)

MEH-PPV + TiO₂ 0 % MEH-PPV + TiO₂ 2 % MEH-PPV + TiO₂ 6 % MEH-PPV + TiO₂ 8 %

Fig. 6.Photocurrent density as function of the applied potential for MEH-PPVþTiO2/ITO electrodes with different TiO2 concentrations in CH3CN/LiClO4solution at a light intensity of 100 mW cm².

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polymer ﬁlm and the photocurrent of composite MEH- PPVþTiO2was higher than that of MEH-PPV ﬁlms with- out TiO2. Moreover, the photocurrent increased with the TiO2 concentration. Our results demonstrate that TiO2

nanoparticles improved the optical and photoelectrochemical properties of MEH-PPV composite ﬁlms.

Acknowledgments

We are grateful to Mihaela Girtan for useful discus- sions and to Romain Mallet and Guillaume Mabilleau from the SCIAM (Angers, France) Microscopy Service for AFM micrographs.

References

[1] A. Hagfeldt, M. Gr ¨atzel, Acc. Chem. Res. 33 (2000) 269–277.

[2] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater.

11 (2001) 15–26.

[3] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693–3723.

[4] H. Hoppe, N.S. Sariciftci, J. Mater. Res. 19 (2004) 1924–1945.

[5] S. G ¨unes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324–1338.

[6] E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954–985.

[7] C. Winder, N.S. Sariciftci, J. Mater. Chem. 14 (2004) 1077–1086.

[8] B. Sun, H.J. Snaith, A.S. Dhoot, S. Westenhoff, N.C. Greenham, J. Appl.

Phys. 97 (2005) 014914.

[9] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425–2427.

[10] I. Gur, N.A. Fromer, C.P. Chen, A.G. Kanaras, A.P. Alivisatos, Nano Lett. 7 (2007) 409–414.

[11] Y. Kang, N.G. Park, D. Kim, Appl. Phys. Lett. 86 (2005) 1–3.

[12] I. Gur, N.A. Fromer, A.P. Alivisatos, J. Phys. Chem. B 110 (2006) 25543–25546.

[13] C.-Y. Liu, Z.C. Holman, U.R. Kortshagen, Nano Lett. 9 (2008) 449–452.

[14] F. Habelhames, L. Lamiri, Z. Wided, B. Nessark, Adv. Mater. Res 428 (2012) 78–83.

[15] F. Habelhames, B. Nessark, D. Bouhafs, A. Cheriet, H. Derbal, Ionics 16 (2010) 177–184.

[16] F. Hab Elhames, B. Nessark, N. Boumaza, A. Bahloul, D. Bouhafs, A. Cheriet, Synth. Metals 159 (2009) 1349–1352.

[17] K.P. Fritz, S. Guenes, J. Luther, S. Kumar, N.S. Sariciftci, G.D. Scholes, J. Photochem. Photobiol. A 195 (2008) 39–46.

[18] S.A. McDonald, G. Konstantatos, S. Zhang, P.W. Cyr, E.J.D. Klem, L. Levina, E.H. Sargent, Nat. Mater. 4 (2005) 138–142.

[19] I. Vaiciulis, M. Girtan, A. Stanculescu, L. Leontie, F. Habelhames, S. Antohe, Proc. Rom., Acad. Ser. A 13 (2012) 335–342.

[20] K.R. Millington, Photoelectrochemical cells: Dye-sensitized cells, in:

J. Garche (Ed.), Encyclopedia of Electrochemical Power Sources, Elsevier, 2009, pp. 10–21.

[21] M. Girtan, Sol. Energy Mater. Sol. Cells 100 (2012) 153–161.

[22] M. Soylu, M. Girtan, F. Yakuphanoglu, Mater. Sci. Eng. B 177 (2012) 785–790.

[23] G.G. Rusu, M. Girtan, M. Rusu, Superlattices Microstruct 42 (2007) 116–122.

[24] A.G. Manoj, A.A. Alagiriswamy, K.S. Narayan, J. Appl. Phys. 94 (2003) 4088–4095.

[25] J. Yang, I. Shalish, Y. Shapira, Phys. Rev. B 64 (2001) 035325.

[26] M.M. Alam, S.A. Jenekhe, Chem. Mater. 16 (2004) 4647–4656.

[27] K.M. Coakley, M.D. McGehee, Chem. Mater. 16 (2004) 4533–4542.

[28] A.J. Breeze, Z. Schlesinger, S.A. Carter, Phys. Rev. B 64 (2001) 125205.

[29] M.Y. Song, J.K. Kim, K.-J. Kim, D.Y. Kim, Synth. Metals 137 (2003) 1389–1390.

[30] B. Kannan, K. Castelino, A. Majumdar, Nano Lett. 3 (2003) 1729–1733.

[31] J. Boucle´, P. Ravirajan, J. Nelson, J. Mater. Chem. 17 (2007) 3141–3153.

[32] C. Brabec, V. Dyakonov, U. Scherf (Eds.), Wiley-VCH, Weinheim, 2008.

[33] P. Singh, O.P. Sinha, R. Srivastava, A.K. Srivastava, J. Kaur Bindra, R.P. Singh, M.N. Kamalasanan, Mater. Chem. Phys. 133 (2012) 317–323.

[34] Q.-T. Vu, M. Pavlik, N. Hebestreit, J. Pﬂeger, U. Rammelt, W. Plieth, Electrochim. Acta 51 (2005) 1117–1124.

[35] D. Verma, A. RangaRao, V. Dutta, Sol. Energy Mater. Sol. Cells 93 (2009) 1482–1487.

[36] S.S. Kim, J. Jo, C. Chun, J.C. Hong, D. Kim Yu, J. Photochem. Photobiol.

A Chem 188 (2007) 364–370.

[37] H. Derbal-Habak, C. Bergeret, J. Cousseau, J.M. Nunzi, Sol. Energy Mater. Sol. Cells 95 (2011) S53–S56.

[38] H. Kim, W. Chang, Synth. Metals 101 (1999) 150–151.

[39] F.L.C. Miquelino, M.A. De Paoli, E.M. Genies, Synth. Metals 68 (1994) 91–96.

[40] A. Petrella, M. Tamborra, P.D. Cozzoli, M.L. Curri, M. Striccoli, P. Cosma, G.M. Farinola, F. Babudri, F. Naso, A. Agostiano, Thin Solid Films 451–452 (2004) 64–68.