A Photoconductive, Thiophene–Fullerene Double-Cable Polymer

Letter pubs.acs.org/JPCL

A Photoconductive, Thiophene−Fullerene Double-Cable Polymer, Nanorod Device Hiroshi Imahori,*,†,‡ Shinji Kitaura,‡ Aiko Kira,‡ Hironobu Hayashi,‡ Masayuki Nishi,§ Kazuyuki Hirao,§ Seiji Isoda,† Masahiko Tsujimoto,† Mikio Takano,† Zhang Zhe,∥ Yuji Miyato,∥ Kei Noda,∥ Kazumi Matsushige,*,∥ Kati Stranius,⊥ Nikolai V. Tkachenko,⊥ Helge Lemmetyinen,⊥ Lidong Qin,# Sarah J. Hurst,▽ and Chad A. Mirkin▽ †

Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan § Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Department of Electronic Science and Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ⊥ Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland # Department of Nanomedicine, The Methodist Hospital Research, Weill Cornell Medical College of Cornell University, 6670 Bertner St., Office R7-121, Houston, Texas 77030, United States ▽ Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡

S Supporting Information *

ABSTRACT: Gold/double-cable copolymer/gold multisegmented nanorods were prepared electrochemically via a template-based method. These “bulk heterojunction” nanorods showed photoconductivity providing us with a platform to study photoinduced charge separation/transport at the nanointerface and begin to think about the rational design of nanoscale solar cells based on such structures.

SECTION: Electron Transport, Optical andElectronic Devices, Hard Matter achieve the formation of a “bulk heterojunction” (BHJ) nanorod composed of 3HT and C60, thiophene tethered to C60 (TC60) was prepared and copolymerized with 3HT to yield the corresponding copolymer exhibiting the covalent incorporation of C60 into the thiophene polymer (denoted P(3HT +TC60), Scheme 1). In a typical experiment, segmented metal− polymer−metal nanorods were synthesized by electrochemical deposition of gold into an AAO template, followed by electrochemical copolymerization of 3HT and TC60 with a molar ratio of 4:1 (weight ratio of thiophene:C60 unit ≈ 1:1) and final electrochemical deposition of gold (denoted Au/ P(3HT+TC60)/Au). The Au segments were introduced to create electrical connections between the P(3HT+TC60) segments and Au microelectrode. We expected that illumination of the Au/P(3HT+TC60)/Au nanorod would lead to

O

rganic/inorganic hybrid nanostructures are a new class of nanomaterials in which organic and inorganic components are integrated and exhibit unique properties that cannot be derived from the individual components or their bulk composites.1,2 In particular, research in one-dimensional (1D) organic/inorganic hybrid nanomaterials has focused on controlling the morphology of nanomaterials to generate structures with novel function and properties. However, although there are some reports of nanoscale organic/inorganic or organic/organic p−n junctions possessing unique electric and photoelectric properties,3−7 there are still challenges in connecting 1D organic/inorganic nanomaterials in a controllable and functional manner.8−12 We report herein the first example of p−n double-cable polymer nanorods that exhibit photoconductivity together with organic field effect transistor (FET) properties. We used ordered anodic aluminum oxide (AAO) templates (Scheme S1)8−13 and chose 3-hexylthiophene (3HT) and C60, as our p-type and n-type organic components, respectively. To © 2012 American Chemical Society

Received: January 5, 2012 Accepted: January 26, 2012 Published: January 26, 2012 478

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Letter

Electrochemical copolymerization of 3HT and TC60 was also performed using a planar ITO electrode to assess the photophysical and electrochemical properties of the P(3HT +TC60) polymer, as it was difficult to obtain a sufficient amount of nanorods to perform these measurements. An XPS spectrum of the P(3HT+TC60) polymer film exhibits characteristic peaks arising from the C, N, and S atoms (Figure S5). Given the relative intensity (C: 93.8%; N: 0.6%; S: 5.6%), the n:m ratio in the copolymer was determined to be 8:1 3HT:TC60 (weight ratio of thiophene:C60 units = ∼5:3). This ratio (8:1) is higher than the initial one (4:1), suggesting the low polymerization reactivity of TC60 compared to 3HT under these conditions. It was not possible to increase the initial amount of TC60 in the electrolyte solution due to its low solubility. The UV−visible absorption spectrum of the P(3HT+TC60) polymer film reveals a broad absorption ranging from 350 to 600 nm with the peak arising from the π−π transition seen at 400 nm (Figure S6A). This value is considerably blue-shifted compared to the reference polymer P3HT, where the corresponding band with a broad maximum is seen around 480 nm, which is typical of polythiophenes. Such a blue-shift, also observed in “doublecables,” is proposed to originate from the shortening of the effective conjugation length in the P(3HT+TC60) polymer.16 This effect may be explained by steric hindrance resulting from the bulkiness of the fullerene and hexyl substituents. The fluorescence spectrum of the P(3HT+TC60) polymer film also exhibits intense quenching of the polymer emission at 600−700 nm relative to the P3HT reference, consistent with the emission behavior of the nanorods (Figure S6B). Cyclic voltammetry measurement of the P(3HT+TC60) polymer film shows irreversible P3HT oxidation (ca. 1.2 V vs Fc/Fc+) and TC60 reduction (ca. −1.2 V vs Fc/Fc+) (Figure S7), as reported in similar copolymer films.16−18 We further investigated the nature of the photoexcitation in the P(3HT+TC60) and P3HT polymer films using transient absorption measurements (Figures S8 and S9). Upon excitation of the P3HT polymer film at 410 nm, a small absorption shoulder and broad absorption band arising from the singlet exciton are observed around 600−700 and at 1000 nm (Figure S8A).19−21 On the picosecond time scale, the absorption at 1000 nm decays rapidly with time constants of 0.28 ps (59%), 1.9 ps (32%), and 286 ps (9%) for a three-exponential fit of the data (Figure S8B). On the other hand, the P(3HT+TC60) polymer film exhibits much smaller absorption bands around 600−700 and 1000 nm immediately after laser excitation, suggesting that a rapid quenching pathway, such as CS, exists (even on a time scale of 2 ns (10%) for a three-exponential fit of the data (Figure S9B). The absorption can be assigned to the polymer polaron (radical cation),19−21 whereas the characteristic band at 1000 nm arising from the C60 radical anion16−22 could not be detected due to the small molar absorption coefficient and the poor signal-to-noise ratio at 1000 nm. These data demonstrate that photoinduced ET occurs from the polymer singlet exciton to the C60 to generate the charge separated state in the film. The photoelectric properties of the Au/P(3HT+TC60)/Au and Au/P3HT/Au nanorods were probed by current (I) versus bias voltage (V) measurements (Figure 2). Single nanorod devices were prepared by depositing multicomponent nanorods on top of a microelectrode array (Figure S10). The I−V curve

Scheme 1. Electrochemical Copolymerization of 3HT and TC60 for the Preparation of the P(3HT+TC60) Segments of Au/P(3HT+TC60)/Au Nanorods

charge separation (CS) within the P(3HT+TC60) segment and subsequent hole and electron transport to the respective Au segments of the rod to which external bias voltage was applied. An analogous three-segment nanorod without TC60 (denoted Au/P3HT/Au) was also prepared as a control using similar conditions. The morphology and size of the Au/P(3HT+TC60)/Au and Au/P3HT/Au nanorods were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy, and optical microscopy (Figure 1, and Supporting

Figure 1. (A) FE-SEM image of Au/P(3HT+TC60)/Au nanorod. (B) A magnified image of the rod shown in (A), highlighting the interface between the Au and P(3HT+TC60) segments.

Information Figures S1 and S2). These nanorods possess distinct interfaces between the Au and P(3HT+TC60) segments (Figure 1A,B). The Au/P(3HT+TC60)/Au nanorods have an average diameter of 330 nm (±10 nm), each with a total length of 2.7 μm (±0.4 μm) and a P(3HT+TC60) segment length of 400 nm (±200 nm). The Au/P3HT/Au nanorods also exhibit a similar diameter, 340 nm (±10 nm), each with a total length of 4.1 μm (±0.2 mm) and a P3HT segment length of 500 nm (±200 nm) (Figure S1). Elemental mapping by energy dispersive X-ray spectroscopy (EDS) was used to study the distribution of C, N, and Au elements within the rod architecture. The EDS results obtained from the three different segments of the hybrid nanorods unambiguously corroborate the chemical composition, showing that both the Au/P(3HT+TC60)/Au (Figure S3) and Au/ P3HT/Au nanorods (data not shown) are composed of an organic segment flanked by Au segments. The segmented structures of the Au/P(3HT+TC60)/Au and Au/P3HT/Au nanorods were also confirmed by fluorescence microscopy. Emission at 650 nm is observed for the P3HT segment of the Au/P3HT/Au nanorod (excitation at 443 nm, Figure S4A), but the corresponding emission is not visible for the P(3HT+TC60) segment of the Au/P(3HT+TC60)/Au nanorod (excitation at 443 nm, Figure S4B). This suggests that photoinduced electron transfer (ET) occurs from the excited state of the P3HT moiety to the C60 to generate the charge separated state.14,15 479

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Figure 3. Room temperature Ids−Vds characteristics for the Au/P3HT/ Au nanorod FET.

transport. The device characteristics of the Au/P3HT/Au nanorods are similar to those observed for metal electrodes with a 30 nm gap,23 Au/Ag/Au electrodes in which the Ag was replaced with spin-coated P3HT,24 and single-walled carbon nanotube electrodes with 5−6 nm gaps.25 As far as we know, this is the first observation of organic FET characteristic for well-defined organic/inorganic nanorods. In conclusion, we have successfully synthesized for the first time gold/double-cable copolymer/gold nanorods via a template-based method. More importantly, the nanorod shows photoconductivity, and the gold/polythiophene/gold nanorod exhibits unique FET characteristics. Such systems will be useful for probing photoinduced CS and transport at nanointerfaces and in exploring the rational design of nanoscale organic electronics including solar cells and FETs.

Figure 2. Typical current−voltage (I−V) curves for (A) a single Au/ P(3HT+TC60)/Au nanorod and (B) a single Au/P3HT/Au nanorod. White light with a power of 5 mW cm−2 was used for illumination at room temperature.

of an individual Au/P(3HT+TC60)/Au nanorod shows that the nanorods are highly insulating in the dark with resistivity of 1.2 × 105 Ω·cm. However when illuminated, a photoresponse is detected along with an increase in bias voltage. In this case, a typical I−V curve shows that the Au/P(3HT+TC60)/Au nanorod serves as a semiconductor with a resistivity of 8.4 × 103 Ω·cm. On the other hand, the Au/P3HT/Au nanorods are highly insulating both in the dark (1.6 × 105 Ω·cm) and under illumination (1.2 × 105 Ω·cm). These results therefore show that the p−n double-cable structure contributes to the enhanced photoconductivity of the hybrid nanostructure. One C60 moiety is estimated to appear for every nine thiophene units in the copolymer. Moreover, the strong intrachain and interchain π−π interactions between the C60 moieties would make the C60 disposition sufficiently close in the nanorod. Thus, inter- and intramolecular electron hopping may occur through the neighboring C60 moieties in the BHJ nanorod, leading to the photoconductivity. We also prepared organic FETs using Au/P3HT/Au nanorods (Figure S10). We observed a clear field effect in which the drain-source current (Ids) scales up with increasing gate voltage (Figure 3). With increasing Vds, the nanorod does not exhibit saturation behavior of Ids. The mobility could not be determined as the nanorods break at Vds < −5 V. By contrast, the Au/P(3HT+TC60)/Au nanorods do not show FET properties. The lower conjugation of P(3HT+TC60) than P3HT as a result of the bulkiness of TC60 may hinder p-channel (hole) transport, whereas a large barrier for electron injection from Au with work function of 5.1 eV to C60 with LUMO of 4.0 eV as well as a large number of electron traps (OH, etc.) on the SiO2 surface be responsible for no n-channel (electron)

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, and additional figures (Figures S1−S10). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.I.); matusige@kuee. kyoto-u.ac.jp (K.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grand-in-Aids (Scientific Research on Priority Area “Strong Photon-Molecule Coupling Fields (No. 20043017 to H.I.)), MEXT, Japan. A.K. and H.H. are grateful for a JSPS Fellowship for Young Scientists. N.T. and H.L. thank the Academy of Finland. H.I., N.T., and H.L. also thank the Strategic Japanese-Finish Cooperative Program (JST, Tekes, and AF). C.A.M. acknowledges support from the DOE through the Non-Equilibrium Research Center, the AFOSR, the DoD through a NSSEF Fellowship, and the DOE under award number DE-SC0005462.

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