Fabrication of Ag Nanorods-Embedded P3HT/PCBM ...

ECS Solid State Letters, 4 (4) Q5-Q9 (2015)

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Fabrication of Ag Nanorods-Embedded P3HT/PCBM Films for the Enhancement of Light Absorption Jong Bae Park,a,b Tae Sung Bae,a Jung Inn Sohn,b SeugNam Cha,b Jouhahn Lee,a Seong Kyu Kim,c and Woong-Ki Honga,z a Jeonju Center, Korea Basic Science Institute, Jeonju-si, Jeollabuk-do 561-712, Korea b Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, United Kingdom c Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea

We report a facile method for preparing hybrid nanostructured films composed of poly(3-hexlythiophene) (P3HT) and [6,6]-phenylC61 -butyric acid methyl ester (PCBM) with silver (Ag) nanorods (AgNRs) array. The AgNRs were synthesized by an electrochemical deposition method using an anodic aluminum oxide template with 50-nm-pore-diameter and 10-μm-thickness. The nanostructured P3HT/PCBM film was formed by the intercalation of AgNRs into the P3HT/PCBM film. The nanostructured P3HT/PCBM film with AgNRs showed enhanced optical absorption in the spectral range of 300–650 nm due to localized surface plasmon resonance and scattering effects around the AgNRs compared with spin-coated and nanopatterned P3HT/PCBM films without AgNRs. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/2.0021504ssl] All rights reserved. Manuscript submitted November 25, 2014; revised manuscript received January 7, 2015. Published January 23, 2015.

The design and manipulation of conjugated polymers have been extensively studied in basic science and for a wide range of applications.1 Conjugated polymers have many advantages, such as low production cost, high flexibility, and controllable conductivity, over inorganic materials.2 In addition, the chemical and physical properties of conjugated polymers can be improved by controlling their molecular weight and structure.3,4 Due to these advantages and the applicability of conjugated polymers, many efforts have been dedicated to the design of chemical structures with specific functionalities,5 efficient methodologies for synthesis processes,6 and a suitable structure for optoelectronic applications.4,7,8 Importantly, one key method for improving the efficiency of photovoltaic devices is to enhance the optical absorption in the organic active layers of these devices. Recent extensive studies on optoelectronic devices have shown that hybrid, nanostructured materials can give rise to remarkably improved energy conversion efficiency via various light-trapping mechanisms and optical absorption enhancement schemes. Moreover, the efficiency of optoelectronic devices largely depends on the structural design of these devices because the incorporation of nanostructures into polymer films offer advantages, such as adjustable material thickness,1 controllable interfaces,8 and large surface area,9 compared to the bulk structure in polymer films. Therefore, many types of nanostructures, including nanoparticles, nanowires, nanotubes, and tetrapods, have been fabricated, and their effectiveness in improving photovoltaic performance has been examined using hybrid material systems.10–12 In particular, the use of a porous template to prepare nanostructures is a promising method that has the advantages of highly regular pore arrays and the ability to control aspects of template geometry, such as pore diameter, pore depth, and inter-pore distance.13 Thus, the design and modification of the surface morphology of polymers using a highly ordered porous template can play an important role in improving efficiency and reducing the production cost of optoelectronic devices, such as organic light-emitting diodes (OLEDs) and organic solar cells.14,15 Recently, metallic nanostructures have been explored to improve the light absorption and efficiency of organic photovoltaic cells.16–18 In particular, Ag nanostructures are attractive because Ag exhibits the highest electrical and thermal conductivities among all metals. In addition, the incorporation of Ag nanostructures into polymers provides an enhancement in light absorption due to the generation of localized surface plasmon resonance at the interface between the Ag nanostructures and the surrounding matrix, namely, the active layer, which

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E-mail: [email protected]

can depend on the shape of the nanostructures, neighboring nanostructures, and the distribution of the nanostructures.19 Accordingly, to date, many efforts have been made to study on the incorporation of metallic nanostructures into polymer matrix.20–29 However, a strategy for an enhancement in optical absorption based on a precisely controlled fabrication method involving Ag nanostructures and highly ordered porous template remains unreported till now. In this study, we report a fabrication of a hybrid nanostructured P3HT/PCBM film with Ag nanorod (AgNR) arrays, resulting in an enhancement in light absorption. Specifically, the AgNR arrays were prepared by an electrochemical deposition technique using a nanoporous anodic aluminum oxide (AAO) template and were intercalated into P3HT/PCBM films. The nanostructured P3HT/PCBM films containing AgNR arrays showed enhanced optical absorption in the spectral range of 300–650 nm due to surface plasmon resonance and scattering effects around the AgNRs compared to the optical absorption of spin-coated P3HT/PCBM and nanopatterned P3HT/PCBM films. Experimental An aluminum sheet (99.999%, 30 mm × 40 mm × 0.5 mm) was purchased from Goodfellow. ITO (10 cm) glass was purchased from Samsung Corning Co. in Korea. P3HT (regioregularity: 95.4%) and PCBM (99.5%) were purchased from Rieke Metals Inc. and Nano-c, Inc., respectively. All chemical agents used to create the anodizing solution were purchased from Sigma-Aldrich. For the fabrication of a poly(dimethyl siloxane)-filled AAO sample, Sylgard-184 PDMS elastomer, which is supplied as a two-part liquid component kit, a pre-polymer base (part A) and a crosslinking curing agent (part B), was purchased from Dow Corning. A nanoporous AAO template was prepared electrochemically using a two-step anodization process to create an oxide film with a regularly ordered, porous structure, as shown in Figure 1a. First, an aluminum sheet was electro-polished at 15 V between Al and graphite electrodes in a mixture of ethanol and perchloric acid (2:1 v/v%) at 0◦ C. The first anodization was carried out using a 0.3 M oxalic acid solution at 0◦ C and 40 V for 8 to 12 hours, and then, the oxide layer was removed by immersing the samples in a mixture of chromic acid (1.8 wt%) and phosphoric acid (6 wt%) at 60◦ C. The desired thickness of the AAO film was obtained by a second anodization. In the second anodization step, the etched aluminum sheet was oxidized in 0.3 M oxalic acid at 0◦ C and 40 V. The thickness of the AAO film was controlled by the adjusting the duration of the second anodization; a film measuring 10 μm thick was produced after 5 h of anodization. After the second anodization, the AAO pores could be widened by increasing the anodization time and

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Figure 1. Schematic illustrations of (a) the formation process of an AAO template, (b) the fabrication of AgNR arrays and nanostructured PDMS, and (c) the fabrication of a nanostructured P3HT/PCBM film.

the concentration of the acid solution. To smooth the alumina surface and widen the pores of the template, the AAO was immersed in a 0.3 M phosphoric acid solution for 30 min. Then, the AAO was separated from the unoxidized aluminum substrate by immersion in a saturated HgCl2 solution. To create a through-type membrane, the barrier layer at the bottom of the AAO was removed by immersion in a 0.1 M NaOH solution for 10 min. We fabricated AgNR arrays and PDMS nanostructures using an AAO template. First, to grow AgNRs inside the AAO pores, one side of the AAO was deposited with a ∼300-nm-thick, thermally evaporated Ag film, which served as the working electrode in a threeelectrode electrochemical cell controlled by potentiostat (PGSTAT12, Autolab), as shown in Figure 1b (upper schematic illustrations). A Pt (99.99%) wire and an Ag/AgCl electrode were used as counter and reference electrodes, respectively. A −0.95 V potential was applied to reduce Ag+ in a commercially available Ag electroplating solution (1025 RTU, Technic co.). The length of the grown AgNRs was controlled by adjusting the amount of electric charge applied during the reduction process. Next, a PDMS nanopillar pattern was fabricated by using the highly ordered and dense nanopore arrays of the AAO film as a template. A PDMS-filled AAO sample was fabricated by filling the pores of the AAO with PDMS via capillary forces and subsequently curing the sample at 80◦ C for 6 h in a plastic petri dish. Then, the AAO was removed using a NaOH solution. The fabrication process of the PDMS nanopillar pattern is shown in Figure 1b (bottom row). To fabricated nanostructured P3HT/PCBM films, a mixture (2 w/v%) of P3HT and PCBM in chlorobenzene was dropped onto the PEDOT:PSS/ITO substrate. Then, the prepared AgNR arrays and PDMS nanopillars pattern were placed on top of the P3HT/PCBM films and pressed onto the P3HT/PCBM polymer layer, as shown in Figure 1c. The morphologies of the AAO template, Ag electrode with NR arrays, and PDMS nanopillar pattern were examined by field-emission scanning electron microscopy (FE-SEM, S-3600, Hitachi) and atomic force microscopy (Nanoscope IV mulimode, Veeco). The as-prepared arrays of AgNRs were characterized by X-ray photoelectron spec-

troscopy (XPS, Axis-Nova, Kratos Inc.) to confirm the existence of Ag2 O and Ag2 S thin layers. The light absorption of the structures was characterized using an UV-vis spectrophotometer (UV-3600, Shimadzu). Results and Discussion The as-fabricated AAO template yielded by the above-described synthesis procedure was examined by AFM and FE-SEM. Figure 2 shows top-view (Figures 2a and 2b) and cross-sectional view (Figure 2c) AFM and SEM images of the as-fabricated AAO template. The average pore diameter of the AAO was approximately 50 nm. To grow AgNRs inside the AAO pores, a 300-nm-thick Ag film used as a working electrode was deposited on the backsides of the AAO membrane using a thermal evaporator. We then deposited Ag metal electrochemically in the pores of the AAO membrane. The lengths of the AgNRs could be easily tuned by controlling the duration of electrodeposition. Only the AAO membrane was removed after electrodeposition from the sample to prepare a freestanding Ag film with AgNR arrays. In addition, a PDMS-filled AAO sample was fabricated by filling the pores of the AAO via capillary forces. The AgNRs exhibited a diameter of approximately 80 nm and an inter-rod distance of approximately 30 nm. The diameter of the AgNRs was larger than that of the pores of the AAO template, which is ascribed to the etching of the inner walls of the AAO pores by the Ag precursor solution during the electrochemical deposition of the AgNRs in the AAO template. Figures 3a and 3b show two-dimensional and three-dimensional AFM images of the surfaces of the as-synthesized AgNR arrays, respectively, after removing the top alumina layer with 1 M NaOH. Figures 3c and 3d also show two-dimensional and threedimensional AFM images of the surfaces of the PDMS nanopillar pattern after the elimination of the AAO template. To clarify the composition of the arrays of as-prepared AgNRs and to confirm whether the AgNR arrays were oxidized during the elimination of the AAO membrane by a NaOH solution, the NR arrays were examined by XPS. Figure 4a shows the wide-scan spectra



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Figure 3. Two-dimensional and three-dimensional AFM images of (a, b) AgNR arrays on 2 μm × 2 μm areas and (c, d) PDMS nanostructures on 3 μm × 3 μm areas.

Figure 2. (a) An AFM image of an AAO template on 3 μm × 3 μm areas. (b) Top-view and (c) cross-sectional SEM images of an AAO template.

obtained for the front and back sides of the Ag film with NR arrays. Figures 4b–4d show the high-resolution spectra of the Ag 3d, O 1s, and S 2p core level, respectively, indicating the existence of silver oxide (Ag2 O or AgO) and Ag2 S thin layers. In Figure 4b, the XPS

Figure 4. (a) Wide scan (survey) XPS spectra and (b-d) core level XPS spectra of (b) Ag 3d, (c) O 1s, and (d) S 2p for the front side (black line) and back side (red line) of the Ag electrode with and without NR arrays, respectively. The inset in (a) shows an SEM image of the Ag electrode with NR arrays, which were synthesized by an electrodeposition method.


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Figure 6. (a) UV-vis absorption spectra of the AgNR arrays of 600 nm length (i, green line), an as-coated P3HT/PCBM film (ii, black line), a P3HT/PCBM film with AgNRs (iii, red line), and a nanopatterned P3HT/PCBM film (iv, blue line) and (b) their corresponding schematic illustrations.

Figure 5. (a) A representative cross-sectional SEM image of ITO glass/ P3HT/PCBM structure with AgNR arrays. (b-d) Cross-sectional SEM images of arrays of AgNRs with lengths of (b) 600 nm, (c) 950 nm, and (c) 1,800 nm.

spectra of the Ag 3d show doublets with an area ratio of 2:3 and 6.0 eV splitting. The XPS spectra for O 1s peak with the binding energies around 529.5–530 eV show the existence of Ag2 O or AgO, which can be ascribed to the atmospheric oxygen adsorbed on the surface of AgNRs.30,31 In particular, the S 2p spectrum for the front side of the Ag electrode with AgNRs shows markedly the presence of a peak at around 161.2 eV, which is the characteristic binding energy for Ag2 S,32,33 compared with that for the back side of the Ag electrode without AgNRs. According to previous reports,16,18 metal nanomaterials such as Ag nanoparticles and Au nanodots exhibit localized surface plasmon resonance that gives rise to enhancements in light absorption in the visible region. Prior to the characterization of the light absorption of the nanostructured polymer films, we fabricated arrays of AgNRs with lengths of 600, 950, and 1,800 nm to verify the conformity of the NRs during their intercalation into polymer films. The arrays of AgNRs with different lengths were intercalated into P3HT/PCBM films on ITO substrates to create a hybrid nanostructure of P3HT/PCBM

containing AgNRs. Figure 5 shows cross-sectional SEM images of the arrays of AgNRs with different lengths after removing the ITO glass substrates. The polymer easily penetrated all the arrays of AgNRs of various lengths. The arrays of AgNRs measuring 600 nm and 950 nm in length remained in freestanding form without aggregations, as shown in Figures 5a and 5b. On the other hand, aggregation was observed in the arrays of 1800-nm-long AgNRs, as indicated by bundles of AgNRs shown in Figure 5c. The optical properties and the crystallinity of the nanostructured P3HT/PCBM films with AgNR arrays were characterized by UV-vis absorption spectroscopy. Figures 6a and 6b show UV-vis absorption spectra of the AgNR arrays (i, green line), an as-coated P3HT/PCBM film (ii, black line), a P3HT/PCBM film with AgNRs (iii, red line), and a nanopatterned P3HT/PCBM film (iv, blue line) and their corresponding schematic illustrations, respectively. The as-coated P3HT/PCBM film was prepared by the spin-coating technique. In Figure 6a, to clearly compare and emphasize differences, optical absorption curves were normalized with respect to the local maximum corresponding to the green light. The optical absorption peak of AgNRs was shown near around 370 nm (Figure 6a, (i)), which is similar to the reported absorption spectra by Zong et al.20 According to Gan’s theory, the optical absorption of AgNRs is associated with geometric factors, especially aspect ratio in which the geometric factors (p) are given by p=

1 − e2 e2

e=

1+e 1 ln −1 2e 1−e L2 − d2 L2

[1]

1/2 [2]

where the aspect ratio is L/d.20 The plasmon resonance wavelength is dependent on the aspect ratio. Zong et al.20 showed that the longitudinal resonance wavelength shifted to a longer wavelength with increasing aspect ratio, whereas the transverse resonance wavelength slightly shifted to shorter wavelength with increasing aspect ratio. Thus, AgNRs plasmonic response can depend on their size, shape, dielectric environment. It is reported that the behavior of a planar (continuous) Ag thin film, which does not exhibit any localized surface plasmon resonances.26 Additionally, AgNRs exhibit a strong


ECS Solid State Letters, 4 (4) Q5-Q9 (2015) absorption peak at approximately 428 nm, which is well known to be due to the surface plasmon resonance absorption of the NRs.11,34 The intercalation of AgNR arrays into the P3HT/PCBM polymer films improved the optical absorption of the films compared with that of the as-fabricated P3HT/PCBM and nanopatterned P3HT/PCBM films without AgNRs. Both films exhibited the characteristic maximum wavelength for P3HT in the 400–650 nm region. In addition, the absorption peak with a maximum wavelength of 330 nm can be assigned to PCBM. At wavelengths of 330–650 nm, where the P3HT/PCBM blend film is absorbing, an enhancement in optical absorption was observed. Therefore, although the existence of silver oxide and Ag2 S thin layers covered on the surface of AgNRs can affect partially the light absorption based on the XPS results, the increase in total optical absorption can be mainly due to localized surface plasmon resonance and scattering effects around the AgNR arrays through the conformal intercalation of the NRs into P3HT/PCBM films.35–37 Conclusions We report a facile method for preparing a nanostrucctured P3HT/PCBM film with AgNR arrays. AgNR arrays were prepared by an electrochemical deposition technique using a nanoporous AAO template and were intercalated into P3HT/PCBM films. The nanostructured P3HT/PCBM film with AgNR arrays showed the enhancement of optical absorption in the spectral range of 300–650 nm due to surface plasmon resonance and scattering effects around the AgNRs compared to the optical absorption of spin-coated P3HT/PCBM and nanopatterned P3HT/PCBM films. Our work will have implications for the facile surface modification of polymer films for the enhancement of light aborption in bulk heterojunction solar cells. Acknowledgment W.-K.H and J.B.P acknowledge the financial support from the Korea Basic Science Institute (KBSI) (T34516) and the NRF-BC Researcher Links through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (no. NRF-2013K2A1C1076556). References 1. J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante, and A. J. Heeger, Science, 317, 222 (2007). 2. C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, Adv. Funct. Mater., 11, 15 (2001). 3. I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. MacDonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang, M. L. Chabinyc, R. J. Kline, M. D. McGehee, and M. F. Toney, Nat. Mater., 5, 328 (2006). 4. B. C. Thompson and J. M. J. Fréchet, Angew. Chem. Int. Ed., 47, 58 (2008).

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