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Current Applied Physics 10 (2010) e189ee191

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Current Applied Physics

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

Fabrication of gold nanoparticle arrays with diblock copolymers for enhancedabsorption of P3HT

Taehee Kim, Seon Kyoung Son, Doh-Kwon Lee, Min Jae Ko, Kyungkon Kim*

Solar Cell Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

a r t i c l e i n f o

Article history:Received 26 December 2009Accepted 27 August 2010Available online 8 September 2010

Keywords:Surface plasmonNanoparticlesPhotovoltaicsBlock copolymerMicelle

* Corresponding author. Tel.: þ82 2 958 5362; fax:E-mail address: kimkk@kist.re.kr (K. Kim).

1567-1739/$ e see front matter � 2010 Elsevier B.V.doi:10.1016/j.cap.2010.08.035

a b s t r a c t

Enhanced optical absorption of Poly(3-hexylthiophene) films deposited on the gold nanoparticle arrayswas investigated. Highly dense arrays of gold nanoparticles were produced using the amphiphilic poly(styrene-block-4-vinylpyridine) diblock copolymer. The local electric field enhancement due to thelocalized surface plasmon resonance excitation of novel metal nanoparticles can increase the absorptionof light from nearby photoactive materials. This result can be applicable to improve the solar harvestingin organic photovoltaics.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Surface plasmons are created when incident light excitescoherent oscillation of the free electrons in surfaces of noble metals[1]. The study of the localized surface plasmon resonance (LSPR) ofnanostructuredmetal particles has beenmotivated by fundamentalresearch [2] and by possible applications such as sensors [3], andphotocatalysts [4], and solar cells [5e8]. Recent experimentalstudies reported the improved performance of organic solar cells byemploying the LSPR phenomena [5,6]. One of the limiting factor tothe efficiency is weak absorption of photoactive materials. In orderto increase the light absorption, the photoactive layer thicknesscould be also increased, whereas exciton diffusion and carriertransport cannot occur efficiently in the thicker film. In optimumthickness of photoactive layer, enhancement of light absorption canbe realized with the LSPR effect. However, there is no systematicinvestigation of the plasmon-induced enhancement of lightabsorption of photoactive layer with a uniform areal density ofmetal nanoparticles (NPs) by the controlled manner.

In this work, we report a enhancement of light absorption ofphotoactive materials in the vicinity of metallic nanoparticles. Thewell-known method of the fabrication of the Au NP arrays withamphiphilic poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) di-block copolymer was used [9e11]. HAuCl4 could be introducedselectively within P4VP nanodomains in PS-b-P4VP. Subsequent O2plasma treatment removed the organic matrix and produced the

þ82 2 958 6649.

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highly dense arrays of AuNP on the substrates. Blendfilms of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) deposited on the Au NP arrays show theenhanced optical absorption.

2. Experimental

A PS-b-P4VP block copolymer with the total molecular weightof 59,000 g mole1 and polydispersity of 1.07 was purchased fromPolymer Source, Inc. The molecular weights of PS and P4VP are41,500 g mole1 and 17,500 g mole1, respectively. PS-b-P4VP copoly-mers were dissolved in toluene at 70 �C for 2 h and cooled to roomtemperature to yield a 0.5 wt% polymer solution. PS-b-P4VP filmswere prepared from 0.5wt% solution by spin-coating at 2000 rpmonIn:SnO2 (ITO) conductive glass substrates (Delta Technologies,Rs ¼ 10 U/sq), which was sequentially cleaned with sonication inisopropanol, acetone, and isopropanol.

The PS-b-P4VP films were immersed in a 0.015 M of HAuCl4/ethanol solution. After immersion, the sample was thoroughlyrinsed with water and dried under a nitrogen stream. Oxygenplasma was used to remove the polymer and reduce the Au ions.Oxygen plasma treatments were performed at 50 mtorr and 30 Wof power for 2 min.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) (BaytronP, AI4083)was deposited on theAu-depositedITO substrates or bare ITO substrates by spin-coating at 4000 rpmand then dried at 80 �C for 10 min. For the preparation of thin andthick films of photoactive layer, 12 mg of P3HT (Rieke Metals, inc)

Fig. 1. Schematic representation of the fabrication process of Au NP arrays on a ITOsubstrate. A) PS-b-P4VP film is deposited from toluene solution onto a ITO substrate byspin-coating. B) PS-b-P4VP film is immersed in a HAuCl4/ethanol solution. C) Oxygenplasma treatments.

T. Kim et al. / Current Applied Physics 10 (2010) e189ee191e190

was dissolved in 2 and 1 ml of chlorobenzene, respectively, andblended with PCBM (Nano-C) in ratio of 1:0.8 polymer to PCBM byweight. The composite material was spin-coated onto thePEDOT:PSS layer at 1000 rpm.

The surface topography of the PS-b-P4VP films on ITO substrateswere imagedbyatomic forcemicroscopy (MFP-3D,AsylumResearch)

Fig. 2. AFM height image of the PS-b-P4VP micellar film as-prepared (a) and after immersiofilm containing HAuCl4 by oxygen plasma treatment.

Fig. 3. Optical absorption spectra of photoactive P3HT/PCBM films deposited on PEDOT:PSindicate the films without Au NPs and solid lines are the films with Au NPs. Inset in (b) is

in the tappingmode. The film thickness wasmeasured by alpha-step(Alpha step IQ surface profiler, KLA Tencor). Scanning electronmicroscopy (NOVA-SEM, FEI)was operated at an accelerating voltageof 10 kV. The absorption spectra of blend films of P3HT/PCBMdeposited on PEDOT:PSS layers was measured with UVeVisiblespectrometer (Lambda35, Perkin Elmer).

3. Results and discussion

The overall process for the fabrication of Au NP arrays on ITOsubstrates is schematically described in Fig 1. PS-b-P4VP blockcopolymers assemble into spherical micelles with a PS corona anda P4VP core in toluene, which is a strongly selective solvent for thePS block. Fig. 2a shows a typical atomic force microscopy (AFM)height image of a monolayer of PS-b-P4VP micellar film. Themicellar filmwas coated over the entire substrate (2.5 cm� 2.5 cm)except the edges. Fig. 2b shows a AFM height image of the PS-b-P4VP micellar film after immersion in HAuCl4/Ethanol solution for1 h. The size of micelle was increased and the lateral periodicity ofthe core changed from 50 nm to 60 nm due to the addition ofHAuCl4 complex into the core and the swelling of P4VP core inethanol. After the oxygen plasma treatment, polymer micelles wereremoved thoroughly and Au NPs were produced (Fig. 2c.). The

n in HAuCl4/Ethanol solution (b). (c) Array of Au NPs produced from PS-b-P4VP micellar

S layer before (a) and after (b) thermal annealing at 150 �C for 10 min. Dotted linesa schematic showing the structure of the samples.

T. Kim et al. / Current Applied Physics 10 (2010) e189ee191 e191

highly dense and regular array of Au NPs with a average diameter of20 nm was successfully fabricated from the monolayer of PS-b-P4VP micelles containing HAuCl4. The lateral periodicity of Au NPswas the same as that of PS-b-P4VP micelles containing HAuCl4 onthe substrate. With this method, we could reproducibly fabricatethe monolayer of Au NPs having a uniform areal density of about 25billion per square centimeter.

The P3HT/PCBM layer and the PEDOT:PSS layer are commonlyused in polymer solar cells as a photoactive material and a holeconducting buffer layer, respectively. To study the enhancementof light absorption of the photoactive layer, P3HT/PCBM blendfilms with two different film thicknesses (about 40 nm and80 nm) were deposited on the PEDOT:PSS layer supported by ITOsubstrates with dense Au NP arrays. Fig. 3 shows the opticalabsorption spectra of P3HT/PCBM films deposited on PEDOT:PSSlayer. The absorbances from the films with Au NPs are larger thanthat from the films without Au NPs. The enhancement ofabsorption can be regarded as the result from the LSPRphenomena. This result was confirmed by additional experi-ments using Pt NPs prepared with the same procedure. P3HT/PCBM film deposited on Pt NP arrays didn’t show the enhancedabsorption (not shown here). Because the plasmon-enhancedelectromagnetic field is an evanescent wave that decays with thedistance from the surface of the metal [12], the enhancement oflight absorption of P3HT/PCBM is more valid in the thinner film.The enhancement of absorption at a wavelength of 520 nm isreduced from 15% and 12% for thinner film to 12% and 8% forthicker film before and after thermal annealing, respectively.Demonstration of improved efficiency of polymer solar cells withAu NP arrays will be published at near future.

4. Conclusion

The highly dense and regular array of Au NPs was successfullyfabricated on the ITO substrate from the monolayer of PS-b-P4VPmicelles containing HAuCl4. Plasmon-enhanced absorption of pho-toactive P3HT/PCBM films deposited on the Au NP arrays wasinvestigated. The enhancement of light absorption is more valid inthe thinner film because the plasmon-enhancement of electro-magneticwave near themetal surface decays away from the surface.

Acknowledgments

This research was supported by the Pioneer Research CenterProgram through the National Research Foundation of Korea fundedby the Ministry of Education, Science and Technology (2009-0081500) andKIST internal research fund under the contract numberof 2E20980.

References

[1] S.K. Ghosh, T. Pal, Chem. Rev. 107 (2007) 4797.[2] S. Link, M.A. El-sayed, J. Phys. Chem. B 103 (1999) 8410.[3] A.G. Brolo, R. Gordon, B. Leathem, K.L. Kavanagh, Langmuir 20 (2004) 4813.[4] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc. 129 (2007) 4538.[5] A.J. Morfa, K.L. Rowlen, T.H. Reilly III, M.J. Romero, J. van de Lagemaat, Appl.

Phys. Lett. 92 (2008) 013504.[6] S.-S. Kim, S.-I. Na, J. Jo, D.-Y. Kim, Y.-C. Nah, Appl. Phys. Lett. 93 (2008) 073307.[7] K. Nakayama, K. Tanabe, H.A. Atwaterb, Appl. Phys. Lett. 93 (2008) 121904.[8] S.D. Standridge, G.C. Schatz, J.T. Hupp, J. Am. Chem. Soc. 131 (2009) B8407.[9] J.P. Spatz, S. Mössmer, C. Hartmann, M. Möller, Langmuir 16 (2000) 407.

[10] Y. Boontongkong, R.E. Cohen, Macromolecules 35 (2002) 3647.[11] S.-H. Yun, S.I. Yoo, J.C. Jung,W.-C. Zin, B.-H. Sohn, Chem. Mater. 18 (2006) 5646.[12] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824.