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Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films Wei Peng Goh, Evan Laurence Williams, Ren-Bin Yang, Wee-Shing Koh, Subodh G Mhaisalkar, and Zi-En Ooi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06724 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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ACS Applied Materials & Interfaces

Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films Wei-Peng Goh,† Evan L. Williams,† Ren-Bin Yang,† Wee-Shing Koh,‡ Subodh Mhaisalkar,§ Zi-En Ooi†* †

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology

and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634 ‡

Institute of High Performance Computing, A*STAR (Agency for Science, Technology and

Research), 1 Fusionopolis Way, #16-16 Connexis North, Singapore 138632 §

Division of Materials Technology, School of Materials Science and Engineering, Nanyang

Technological University, Block N4.1 Nanyang Avenue, Singapore 639798 KEYWORDS Metallic nanoparticles, insulating oxide, polaron generation, localized surface plasmons, localized field enhancements, plasmon enhanced organic photovoltaic cell, optical scattering, photoinduced absorption spectroscopy

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ABSTRACT

Embedding metal nanoparticles in the active layer of organic solar cells has been explored as a route to improving charge carrier generation, with localized field enhancement as a proposed mechanism. However, embedded metal nanoparticles can also act as charge recombination sites. To suppress such recombination, the metal nanoparticles are commonly coated with a thin insulating shell. At the same time, this insulating shell limits the extent that the localized enhanced electric field influences charge generation in the organic medium. It is presumed that there is an optimal thickness which maximizes field enhancement effects while suppressing recombination. Atomic Layer Deposition (ALD) was used to deposit Al2O3 layers of different thicknesses onto silver nanoparticles (Ag NPs), in a thin film of P3HT. Photoinduced absorption (PIA) spectroscopy was used to study the dependence of the photogenerated P3HT+ polaron population on the Al2O3 thickness. The optimal thickness was found to be 3-5 nm. This knowledge can be further applied in the design of metal nanoparticle-enhanced solar cells.


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INTRODUCTION Development in the field of OPV has pushed the power conversion efficiency (PCE) past 10%.1-2 Efforts have focused on improving optical absorption. One approach to improve absorption is to introduce plasmonic nanostructures into OPV devices such as zero-dimensional (metallic nanoparticles), one-dimensional (metallic gratings) and two-dimensional (metallic periodic array) structures.3 Organic solar cells incorporating silver,4-8 gold9-12 or aluminum nanoparticles13-14 have shown increased device photocurrent. The presence of the plasmonic materials is advantageous as they possess strong optical scattering and large localized field enhancement properties.15 These qualities are usually exploited for their ability to improve device performance, which is mainly derived from the increase in light absorption. Localized electric fields associated with surface plasmons on metallic nanoparticles can enhance light absorption in surrounding organic media. While enhancing light absorption, metal nanoparticles also act as centers for charge carrier recombination, so that the gains made in photogeneration can be offset by losses due to recombination. Coating the metal nanoparticles with an insulating layer (e.g. silica16-18) is a common strategy to reduce recombination losses. Unfortunately, this insulating layer also limits the extent to which the localized plasmonic electric field extends into the active organic medium. Therefore, we hypothesize that an optimal insulator thickness exists in such metal@insulator nanoparticle systems that maximizes photogeneration of charge carriers while suppressing carrier recombination. In this manuscript, we describe the use of photoinduced absorption (PIA) spectroscopy to study the hole polaron concentration in poly(3-hexylthiophene) (P3HT) films, as enhanced by Ag NPs. A series of samples were made in which Ag NPs were coated with different thicknesses



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of aluminum oxide (Al2O3) deposited by atomic layer deposition. This allowed for the study of the effect of insulator shell thickness on the concentration of photogenerated charge carriers. PIA is a pump-probe technique that is used to investigate photoexcited states in organic solar cells. The optical absorption of P3HT+ hole polarons shows a well-documented characteristic peak around 1000 nm (1.24 eV).19-23 The Ginger group has investigated the role of plasmonic materials in organic solar cells using PIA.24 Their experiments on silver nanoprisms in poly(3hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) films revealed an increase of photocharge generation by up to 3 times. In a follow-up experiment, the group also underlined the importance of an insulating shell on the silver nanoprisms to prevent metal-mediated losses in plasmon-enhanced solar cells.17 In a more recent development, polaron yield enhancements of ~7 were observed for edge-gold-coated silver nanoprisms.25


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Figure 1. Simulated plot of the electric field strength distribution of surface plasmon resonance in a quarter unit of Al2O3-encapsulated Ag NP embedded in P3HT. The wavelength used was the wavelength at which each structure showed peak resonance. The structure rests on top of a glass/ITO substrate. When there is no shell, the enhanced electric field extends well into the P3HT. This extension of the plasmonic field into the P3HT diminishes as the shell thickness increases. When the shell is 10 nm thick, there is almost no plasmonic field in the P3HT. The specific objective of this investigation is to find an optimal thickness (if it exists) of Al2O3 insulating layer for maximizing photogenerated polaron concentrations in P3HT films that 5 ACS Paragon Plus Environment


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incorporate Ag NPs. The rationale for an optimal thickness is as follows: The strength of the localized electric field decays rapidly with distance from the nanoparticle. Therefore, polaron generation may be expected to be higher for thin oxide layers compared to thick oxide layers. On the other hand, thicker oxide layers increase the separation between metal nanoparticle and polarons and thus, thick oxide layers are more effective at preventing charge carrier recombination. Figure 1 shows the simulated electric field distribution plot of an Al2O3encapsulated Ag NP at peak resonance, embedded within a P3HT layer. The structure rests on a glass/ITO substrate. This was calculated using COMSOL 4.4, for monochromatic, linearly polarized plane waves, normally incident on the ITO/glass face. In the absence of any Al2O3 shell, the plasmonic field extends ~10nm into the P3HT. This extension diminishes as the shell thickness increases, such that when the shell reaches 10 nm, the plasmonic field is nearly nonexistent in the P3HT.

EXPERIMENTAL METHODS Ag NPs capped with polyvinylpyrrolidone (PVP) were purchased from Nanocomposix in powder form and were prepared by dispersing them in ethanol at a concentration of 1 mg/ml. The average diameter of the nanoparticles is 50 nm. Regioregular (>98%) Sepiolid™ P200 P3HT was acquired from Rieke Metals and used as received. Indium tin oxide (ITO) substrates purchased from Xin Yan Technology Ltd were cleaned by sonicating in Hellmanex solution, DI water, acetone and isopropanol alcohol sequentially. After drying in a 60 °C oven, the substrates were treated with argon plasma before spin-coating the Ag NPs in an inert environment. Subsequently, they were annealed at 165 °C for 10 min to remove residual solvent. The samples were treated in a UV ozone cleaner for 1 min to remove the PVP


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from the Ag NPs. The PVP capping was removed so that the spacing between the metal and P3HT was determined solely by the thickness of the Al2O3 layer. Next, the Al2O3 layer was deposited using Atomic Layer Deposition (ALD), which consisted of 0.1 s pulse of water, 3 s soaking, 40 s purge, 0.1 s pulse of trimethylaluminum, 3 s soaking and 20 s purge per cycle. Different thicknesses of the insulator were investigated (0, 0.2, 0.8, 2, 3, 8, 15 and 30 nm). The oxide thickness of the thickest sample (30 ± ~0.5 nm) was measured from a Transmission Electron Microscope (TEM) image, and the thicknesses of the thinner samples were determined by linear interpolation from the cycle numbers. We have reported the thicknesses to either one significant figure (if

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