Article pubs.acs.org/JPCC
Gold Nanoparticle Assisted Self-Assembly and Enhancement of Charge Carrier Mobilities of a Conjugated Polymer Chakkooth Vijayakumar,† Bijitha Balan,† Akinori Saeki,*,†,‡ Tetsuya Tsuda,† Susumu Kuwabata,† and Shu Seki*,† †
Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama, 332-0012, Japan
‡
S Supporting Information *
ABSTRACT: A composite of bithiazole−benzothiadiazolebased semiconducting conjugated copolymer and gold nanoparticles (AuNPs) was prepared in situ and characterized by transmission electron microscopy, thermogravimetry, and UV−vis absorption spectroscopy. The polymer interacts with the nanoparticle surface through the nonbonding electrons of the nitrogen and sulfur atoms, which provides stability to the nanoparticles as well as planarity and rigidity to the polymer backbone. As a result, the effective conjugation length and delocalization of π-electrons of the polymer improved as evident from 130 nm red-shift in the UV−vis absorption spectrum. The nanoparticle along with the chemisorbed layer of polymer acts as a template for the self-assembly of the remaining polymer which is dispersed in the solution through π−π-stacking and van der Waals interactions. The self-assembly process enhances the polymer packing as well as ordering as seen from the shorter d spacing and from the more than threefold increase in the intensity of X-ray diffraction of the composite film. The charge carrier mobilities in the short and long ranges were measured by flash-photolysis time-resolved microwave conductivity and space-charge-limited current methods, respectively, which showed enhancement for the composite material compared to the pristine polymer. A more significant increase was observed in the hole mobilities (more than 12-fold), and hence the p-type nature of the composite was further studied by preparing blend films with typical acceptors such as phenyl-C61-butyric acid methyl ester (PCBM) and N,N′-bis(1-hexylheptyl)perylene-3,4−9,10tetracarboxylbisimide (PBI). Due to its spherical geometry, PCBM was found to disturb the ordering of polymer chains in the composite, resulting in the lowering of photoconductivity signals. On the other hand, planar PBI molecules coassemble with the composite leading to significant enhancement of photoconductivity. Thus, we demonstrated a versatile approach of controlling planarization, π-stacking, and ordering of a conjugated polymer leading to the improvement of optoelectronic properties using AuNPs as a template. selection of solvents,5−7 use of solvent additives,8−10 and/or annealing techniques11−14 were found to be effective. Metal nanoparticles constitute a unique class of functional materials exhibiting characteristic optical and electronic properties, which differ significantly from those of bulk metals.15,16 Because of the ease of synthesis and surface plasmon properties, gold nanoparticles (AuNPs) have been one of the most studied materials to date, and they are useful in a large variety of applications ranging from biology to material science and optoelectronics to catalysis.16−28 The incorporation of AuNPs into a conjugated polymer was found to be particularly interesting because it results in the formation of multifunctional materials with properties beyond those achievable with just the polymer or the nanoparticles alone.29−31 It has been reported that the AuNPs can assist the exciton dissociation process in a
1. INTRODUCTION Semiconducting polymers attract immense scientific interest because of their applications in low-cost, lightweight, and flexible optoelectronic devices such as photovoltaic cells, lightemitting diodes, field-effect transistors, and photodiodes.1−4 The most important property of a semiconducting polymer is the ability to transport electronic charges, which is measured in terms of charge carrier mobility (μ; μh for hole mobility and μe for electron mobility). The intrinsic carrier mobility of polymers depends on factors such as chemical nature of the individual building units and planarity of the conjugated backbone. Through the advances in molecular engineering and organic synthesis, design and synthesis of conjugated polymers with good conductive properties became possible. However, in many cases the charge carrier mobilities need to be further improved for obtaining optimum device performances. It has been shown that the electronic properties of conjugated polymers could be enhanced by tailoring the stacking and ordering in the film state. Toward this end, the rational © 2012 American Chemical Society
Received: April 24, 2012 Revised: July 24, 2012 Published: July 25, 2012 17343
dx.doi.org/10.1021/jp3039253 | J. Phys. Chem. C 2012, 116, 17343−17350
The Journal of Physical Chemistry C
Article
performed in an argon atmosphere using standard Schlenk techniques. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (BaytronP VP AI 4083) and indium tin oxide (ITO)-coated glass substrates (
