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Thin Solid Films 516 (2

Evaluation of molecular orientation and alignment of poly(3-hexylthiophene)on Au (111) and on poly(4-vinylphenol) surfaces

Koji Yamamoto, Shizuyasu Ochiai ⁎, Xin Wang, Yoshiyuki Uchida,Kenzo Kojima, Asao Ohashi, Teruyoshi Mizutani

Department of Electrical Engineering, Aichi Institute of Technology, 1247 Yakusa, Toyota 470-0392, Japan

Available online 3 May 2007

Abstract

The orientation and alignment of regioregular poly(3-hexylthiophene) (P3HT) molecules on Au (111) surface and on poly(4-vinylphenol)(PVP) thin film were investigated. The P3HT molecules on the smooth Au (111) are oriented with both the backbones and the side chains parallelto the substrate (plane-on orientation) as revealed by the scanning tunneling microscope (STM) images. However, the P3HT molecules on the PVPthin films are preferably oriented with side chains perpendicular to the surface (edge-on orientation). Surface modification of the PVP byhexamethyldisilazane (HMDS) can increase the crystalline size in the P3HT semicrystalline films. The performance of an all-polymer organicfield-effect transistor (OFET) with the drop-cast P3HT semiconductor layer and the crosslinked PVP gate insulator on poly(ethylene naphthalate)(PEN) substrate was evaluated.© 2007 Elsevier B.V. All rights reserved.

Keywords: P3HT; PVP; Molecular orientation; Surface modification; OFET

1. Introduction

Over the past two decades, there has been great interest inorganic semiconductors, driven by their potential use in light-emitting diodes, photovoltaic devices, organic integrated cir-cuits, physical–chemical sensors, and low-cost disposable elec-tronics applications [1–4]. In particular, there has been specialinterest in solution processable π-conjugated organic moleculesdue to their flexible chemical tunability, low-temperature pro-cessing, large area coverage, and light-weight low-cost applica-tions. They are suitable to use wet coating techniques andeliminating the need for many of the major semiconductor-manufacturing cost points, including physical vapor deposition(PVD) and chemical vapor deposition (CVD), plasma etching,and the high waste management costs.

Comparing with small organic molecules, polymers showunique charge transfer characteristics in chains and usuallysuperior mechanical and thermal stabilities. Regioregular poly(3-hexylthiophene) (RR-P3HT) has been adopted as an active

⁎ Corresponding author. Tel.: +81 565 48 8121; fax: +81 565 48 0010.E-mail address: ochiai@aitech.ac.jp (S. Ochiai).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.04.145

material for organic solar cells and organic field-effect transistors(OFETs). The performances of P3HT based OFETs on SiO2 gateinsulators (in most cases modified with self-assembled mono-layers [SAM]) were reported [5–9]. The field-effect mobility ofRR-P3HT usually ranges from 10−4 to 10−2 cm2/Vs dependingon the castingmethod (spin or drop) and the highest value reachesaround 0.1 cm2/Vs. It was shown that molecular orientation andalignment of P3HT was affected by the property of the SAM onSiO2 substrate, and resulted in different carrier mobility [7]. At thesame time, the mobility of P3HT also depends on the solventbeing used; enhanced mobility was found from high-boiling-point solvents (for example, 1,2,4-trichlorobenzene) for spin-coated RR-P3HT films [8]. It is due to the higher crystallinity,stronger interchain interactions (π–π stacking), and bettermolecular alignment (with the (100)-axis of the π–π stackedcrystalline planes preferentially normal to the film) in RR-P3HTfilms spin-coated from the solvents with higher boiling points(slower evaporation rates). Similar results were found for RR-P3HT thin films drop-cast from different solvents with differentboiling points and solubilities [9].

In this work, an all-polymer OFET with an RR-P3HTsemiconductor layer and a crosslinked poly(4-vinylphenol)

Fig. 1. Structure of the all-polymer OFETwith the “bottom-gate” and “bottom-contact” configuration.

Fig. 2. STM image of the Au (111) surface deposited on a smooth mica substratevia PVD.

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(PVP) dielectric layer on a poly(ethylene naphthalate) (PEN)substrate was fabricated and the performance was evaluated.Moreover, the orientations and alignment of RR-P3HT mole-cules drop-cast on an Au (111) surface, on a crosslinked PVPdielectric layer surface, and on PVP modified with 1,1,1,3,3,3-hexamethyldisilazane (HMDS), were studied by using the high-resolution scanning tunneling microscopy (STM), atomic forcemicroscopy (AFM) and out-of-plane X-ray diffraction (XRD).

2. Experiment

High quality head-to-tail coupled regioregular RR-P3HT(regioregularityN98.5%), PVP, HMDS, and other chemicalswere purchased from Aldrich Chemical Co. without furtherpurifications.

For the measurements of the orientation and morphology inP3HT thin films, two kinds of substrates were prepared. Onewas the Au (111) surface which was physical vapor depositedon the mica substrate, the other was crosslinked PVP thin filmwhich was spin-coated (3000 rpm) on the PEN substrate.Crosslinking agent [methylated poly(melamine-co-formalde-hyde)] of 4 wt.% was mixed in the 11 wt.% PVP solution ofpropylene glycol monomethyl ether acetate; and heat-cross-linking of PVP was performed with prebaking at 100 °C for10 min and then baking at 200 °C for one hour under theprotection of nitrogen. Surface modification of the PVP thinfilm was done by immersion into the HMDS solution for 24 hand then baked at 80 °C for 2 h. P3HT molecules were drop-castonto the Au (111) surface from a chloroform solution withthe concentration of 10−4 wt.%. On the other hand, P3HTmolecules on the PVP substrate and on PVP modified byHMDS were drop-cast at a higher concentration of 0.1 wt.%.Drop-cast films were enclosed in a Petri dish to increase theevaporation time. After being dried, they were transferred intothe ultra high vacuum (UHV) chamber (10−9 Pa) of an UHVSPM system (SPM Probe VT STM, Omicron MicroscopyGmbH). P3HT molecules on Au (111) and on PVP surfaceswere observed by the STM mode and the AFM moderespectively.

Fabrication of the all-polymer OFET is as the following.Firstly, a gold gate electrode was deposited onto a PEN substratevia PVD technique. Secondly, a PVP layer was spin-coated(3000 rpm) and then heat-crosslinked in an oven. Thirdly, goldsource and drain electrodes were evaporated onto the PVP layervia PVD under a slow evaporation speed (≤0.5 Å/s) by using amask (with the channel wide W of 1000 μm and length L of

50 μm). Lastly, a P3HT thin film was drop-cast onto the channelat the concentration of 0.1 wt.%. This structure (Fig. 1) is calledthe “bottom-gate” and “bottom-contact” configuration. Forcomparison, another OFET was made with the same structurebut the P3HT film was spin-coated (2000 rpm) onto the channelat a concentration of 0.4 wt.%. The output and transfer char-acteristics of the OFETs were measured by equipments com-prising a picoammeter and two DC sources. Mobilities μ forOFETs were calculated in the saturation regime from the transfercurves. The capacitance per unit area Ci for crosslinked PVPthin film was calculated using a dielectric constant ε≈3 and athickness of about 800 nm.

3. Results and discussion

Fig. 2 shows the STM image of the Au (111) surface whichwas deposited via PVD on a smooth mica substrate. Thetriangular terraces, representing the characteristic structure ofthe Au (111) surface, are observed. Fig. 3(a) shows a STMimage of the RR-P3HT film drop-cast on the Au (111) surface.We see that both monolayer and multilayer (aggregates) ofP3HT can form on the Au (111) surface and there are some self-organized ordered structures as indicated by the circles ofdashed lines. More aggregation of the P3HTchains occurs at theboundary between the terraces (steps). Fig. 3(b) shows the lineprofile measured along the arrow in Fig. 3(a). The separationbetween the polymer chains is about 1.6 nm and the height ofthe chain is only about 0.4–0.7Å, so it is clearly a monolayer ofP3HT polymer. According to the crystal structure of RR-P3HT[5,6], Fig. 3 indicates that the first layer of P3HT polymers onthe smooth Au (111) are oriented with both the backbones andthe side chains parallel to the substrate, i.e. with the plane-onorientation. This orientation is illustrated as the inset in Fig. 3(a)with the backbone chains lying in the plane.

XRD profiles of the P3HT thin films prepared by drop-casting and spin-coating (2000 rpm) on crosslinked PVP areshown in Fig. 4. The (100) diffraction peak of drop-cast film islocated at 2θ=5.25°, which corresponds to a lattice parameterof 16.8 Å. So in the drop-cast P3HT film on PVP, the polymers

Fig. 4. XRD profiles of the P3HT thin films prepared by drop-casting and spin-coating (2000 rpm) on crosslinked PVP film. The inset shows the model of edge-on orientation in drop-cast P3HT film on PVP.

Fig. 3. (a) STM image of the RR-P3HT film drop-cast on the Au (111) surface.Some self-organized ordered structures are indicated by the circles of dashedlines. The inset illustrates the plane-on orientation of the P3HT monolayer withthe backbone chains lying in the plane. (b) The line profile measured along thearrow in (a).

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are preferably oriented with backbones parallel but side chainsperpendicular to the surface, i.e. with the edge-on orientation.The inset in Fig. 4. shows the model of polymer orientation indrop-cast P3HT film on PVP. However, spin-coated P3HT filmis almost featureless in XRD so it is considered to be amorphouswith almost non-crystalline structure. It can be understoodby the fast solvent evaporation rate and large spin-inducedcentrifugal force during the high-speed spinning which impedesthe growth of P3HT microcrystallites. The results in Figs. 3 and4 reveal that the polymer orientations and alignments can becontrolled by both the surface properties of the substrate and themethod of deposition.

For the better control of the P3HT crystalline growth onpolymer substrates, we tried the surface modification of PVP.Fig. 5 shows the AFM images of drop-cast P3HT films on thesurfaces of untreated crosslinked PVP and crosslinked PVPmodified by HMDS. It shows that P3HT polymers aggregate toform worm-like grains with only short-range ordering. Thegrain size of P3HT aggregates on HMDS treated crosslinked

PVP [Fig. 5(b)] is obviously larger than that on untreatedcrosslinked PVP [Fig. 5(a)]. The typical worm width a andlength b in Fig. 5(a) are 10–15 nm and 40–80 nm respectively;and the typical a and b in Fig. 5(6) are 20–40 nm and 60–150 nm respectively. It is known that polymeric materials areusually not subject to silanization by HMDS because of the lackof reaction sites [10]. However, the –OH group of the phenol inPVP can react with HMDS and result in a hydrophobic surfacewith short chains of silicon methyl. These hydrophobic chainscan favor the interaction with the side-chains of P3HTand resultin a more ordered edge-on orientation, and hence the largercrystalline size. XRD profile proved that the microcrystallitesin drop-cast P3HT film on HMDS treated PVP are oriented withthe edge-on orientation similar to drop-cast P3HT film onuntreated PVP (Fig. 4.). Fig. 6 shows the suggested self-assembled lamellar structure of RR-P3HT polymers with theordered edge-on orientation on HMDS treated PVP film. It alsodisplays that the π–π interchain interactions (π–π stacking)play an important role in the formation of RR-P3HT crystallites.With the edge-on orientation, the π–π stacking in the worm-likecrystals can stack along the major axis (b-axis) or minor axis (a-axis). However, the stacking along the a-axis seems to bedifficult, because the backbone of the P3HT polymer is easy toform folding or bending structure as shown in Fig. 3, forexample, it is expected to form “U”-type or “S”-type bendingstructures due to the intra-and inter-polymer side-chain in-teractions. However, we can hardly find any “U”-type or “S”-type worms in the STM images (Fig. 5) even when the wormlength b reaches about the same as the polymer contour length(about 150 nm for our P3HT sample). Some interesting AFMimages were also found by Kline et al for RR-P3HTwith lowermolecular weight [11]. It formed straight fibers much longerthan the polymer contour length, and a π–π stacking modelalong the major axis of the fiber was proposed for the lowermolecular weight P3HT. However, for high molecular weightP3HT (as we used here), Kline et al proposed a loosely π-πstacking model and the stacking direction was along the minoraxis. According to our results here, and the similarity betweenthe low and the high molecular weight P3HT films (forexample, similarities between XRD profiles and UV–vis

Fig. 6. The suggested self-assembled lamellar structure of RR-P3HT polymerswith the ordered edge-on orientation on HMDS-treated PVP film.

Fig. 7. The output and transfer curves of an all-polymer OFET with drop-castRR-P3HT active layer and crosslinked PVP gate insulator on a PEN substrate.

Fig. 5. AFM images of drop-cast P3HT films on the surfaces of: (a) untreatedcrosslinked PVP, and (b) crosslinked PVP modified by HMDS. The proposedπ–π stacking model in the worm-like microcrystallites is illustrated in theenlarged circle in (a).

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absorption spectra), we propose that the π–π stacking directionis still along the major axis of the microcrystallite in long-chainP3HT films as in short-chain P3HT films. This model isillustrated as in the enlarged circle in Fig. 5(a). The short solidlines in these worm-like microcrystallites only represent part ofone backbone; and it means that the P3HT backbone chains are

folded in these microcrystallines. More experiments are stillneeded to prove and support this model.

The morphology and crystalline structure will affect themobility and performance of the OFETs. Fig. 7 shows theoutput and transfer curves of an all-polymer OFET with drop-cast RR-P3HT active layer and crosslinked PVP gate insulatoron a PEN substrate. It displayed an excellent saturation propertyin the output curve. From the saturation region in transfer curve,the field-effect mobility was calculated to be 2.0×10−2 cm2/Vsand the threshold voltage was only about-2V. The on/off ratiowas over 103. This field-effect mobility is about 2 times of therecently reported one with a similar OFETstructure on the SiO2/

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Si substrate [9]. For comparison, the performance of anotherOFET with the same structure but a spin-coated P3HT filmwas also investigated. Its field-effect mobility was only about4.0×10−4 cm2/Vs. These results reveal again the difference ofpolymer alignment and crystalline structure between the drop-cast and the spin-coated P3HT films, and support again thefindings that π–π stacking plays an important role in the carriertransport in the two-dimensional active channel [6]. However,the performance of the OFET with P3HT on a HMDS-treatedcrosslinked PVP gate insulator was not investigated at this time.We will measure and evaluate the performance of the OFETwith HMDS-treated PVP film in the near future.

4. Conclusions

The orientation and alignment of RR-P3HT molecules onAu (111) surface and on crosslinked PVP thin film wereinvestigated. The P3HT monolayers on the smooth Au (111) areoriented with both the backbones and the side chains parallel tothe substrate (plane-on orientation) as revealed by the STMimages. However, the P3HT molecules on the crosslinked PVPthin films are preferably oriented with side chains perpendicularto the surface (edge-on orientation) with worm-like microcrys-talline structures as revealed by the AFM images and XRDprofiles. Surface modification of the PVP by HMDS canincrease the crystalline size in the P3HT semicrystalline films.The hydrophobic chains of HMDS on PVP surface can favor theinteraction with the side-chains of P3HT and result in a moreordered edge-on orientation, and hence the larger crystallinesize. For drop-cast P3HT thin films, the π-π stacking directionis supposed to be along the major axis of the worm-likemicrocrystallines. The performance of an all-polymer OFETwith the drop-cast P3HT semiconductor layer and the cross-linked PVP gate insulator on PEN substrate was evaluated. Thefield-effect mobility was calculated to be 2.0×10−2 cm2/Vs andthe threshold voltage was only about-2V. These values are

excellent for an all-polymer OFET. In conclusion, the polymerorientations and alignments can be controlled by both thesurface properties of the substrate and the method of deposition.Higher performance can be achieved by optimize the substratesurface properties, the deposition method, and other fabricationparameters.

Acknowledgments

The present work was carried out as part of the researchproject “Materials for the 21st Century—Materials Develop-ment for Environment, Energy, and Information” (for fiscalyears 2002–2006) supported by the Ministry of Education,Culture, Sports, Science, and Technology of Japan.

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