Toward Precision Control of Nanofiber Orientation in Conjugated

Article

Toward Precision Control of Nanofiber Orientation in Conjugated Polymer Thin Films: Impact on Charge Transport Ping-Hsun Chu, Nabil Kleinhenz, Nils Persson, Michael McBride, Jeff L. Hernandez, Boyi Fu, Guoyan Zhang, and Elsa Reichmanis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04202 • Publication Date (Web): 27 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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Chemistry of Materials

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Toward Precision Control of Nanofiber Orientation

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in Conjugated Polymer Thin Films: Impact on

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Charge Transport

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Ping-Hsun Chu, † Nabil Kleinhenz, ‡ Nils Persson, † Michael McBride, † Jeff L. Hernandez, ‡ Boyi

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Fu, † Guoyan Zhang, † and Elsa Reichmanis*,†,‡,§

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†

School of Chemical and Biomolecular Engineering, ‡School of Chemistry and Biochemistry,

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and §School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta,

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Georgia 30332, United States.

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KEYWORDS: conjugated polymers, alignment, nanofibrillar structure, tie-chains, field-effect

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transistors

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ABSTRACT

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The deposition of conjugated polymers is typically subject to chain entanglement effects,

13

which can severely hinder chain unfolding, alignment and π-π stacking during rapid solution

14

coating processes. Here, long-range ordering and highly aligned poly(3-hexylthiophene) (P3HT)

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thin films were demonstrated by preprocessing the polymer solution with ultraviolet (UV)

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irradiation/solution aging and then depositing via blade coating method, which is compatible

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with roll-to-roll printing processes. The surface morphologies and optical anisotropy of deposited 1

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films revealed that the degree of chain alignment was greatly improved with increased levels of

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polymer assembly that can be precisely controlled by solution aging time. The correlations

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between oriented nanofibrillar structures and their charge transport anisotropy were further

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systematically investigated by blade coating pretreated solutions parallel and perpendicular to the

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direction of the source and drain electrodes. Interestingly, charge transport across the well-

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aligned P3HT nanofibers was more efficient than along the long-axis of nanofibrillar structures

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owing to enhanced intramolecular charge transport and tie-chains. The facile and scalable

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solution coating method investigated here suggests an effective approach to induce anisotropic

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crystalline structures, which are readily obtained by directly controlling their intrinsic solution

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properties without the need for extrinsic techniques such as surface templating or shearing blade

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patterning.

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INTRODUCTION

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Conjugated polymers have emerged as key functional materials for a variety of electronic

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device applications such as solar cells,1, 2 biological sensors3, 4 and organic field-effect transistors

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(OFETs).5-7 These organic semiconductors typically exhibit a semicrystalline thin film structure,

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with macroscopic charge transport properties that represent some combination of transport

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through highly conductive crystalline regions and poorly conductive amorphous domains.8-10

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Therefore, controlling the structure and morphology of the semiconducting layer is essential to

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improve the charge carrier mobility and overall device performance. Moreover, it has been

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shown that charge transport within organic semiconductors is highly anisotropic.11, 12 In the case

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of poly(3-hexylthiophene) (P3HT), the charge carrier mobility along the conjugated polymer

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backbone is significantly higher than in the direction of insulating alkyl side chains.13

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Accordingly, the long-range alignment of organic semiconductors is beneficial for efficient

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charge transport when the crystallographic direction of the thin films is oriented along the active

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channel of electronic devices. For example, out-of-plane alignment of polymer chains is desired

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for fast vertical charge transport in organic photovoltaics (OPVs)14 and organic light-emitting

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diodes (OLEDs),15 on the other hand, the in-plane direction is the major charge transport

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pathway for thin film transistors.11

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To obtain long-range aligned thin film structures, several methods have been proposed,

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including evaporation-induced self-assembly,16,

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epitaxial crystallization,18,

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lithography,14, 20, 21 electrospinning,22 mechanical stretching23, 24 and high temperature rubbing.25-

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28

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structures, leading to charge transport and optical anisotropy, some serious problems still exist.

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For instance, the improvement of device performance obtained from strained or rubbed thin films

nanoimprint

Although these approaches are able to precisely control the orientation of crystalline

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is very limited since some proportion of crystal planes still adopt a face-on orientation, which is

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not favorable for effective in-plane charge transport.23,

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transport limitations during the solvent evaporation process may lead to sharp grain boundaries

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and void formation which will become charge carrier traps and thus hinder transport.29 Most of

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all, the above mentioned methods are difficult and costly to implement on an industrial-scale and

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with continuous device production.

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For directional solidification, mass

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Meniscus-guided coating techniques such as zone casting, blade coating and solution shearing

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are alternative, simple and scalable strategies to induce long-range ordering and well-aligned thin

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film structures.30 For example, Park et al.31 suggested that the combination of differential

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substrate surface energy and solution shearing can generate highly oriented and precisely self-

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registered crystalline structures within the active channel, affording low device-to-device

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variability. Alternatively, Diao et al.32 showed that shear coating with a micropillar-pattered

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printing blade can facilitate the formation of well-aligned small molecule crystals.

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reported solution coating methods, however, still relied on complex equipment and/or

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experimental procedures such as pre-patterned substrates or microstructured coating blades.33-36

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In addition, most investigations focused on the fabrication of anisotropic crystalline conjugated

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small molecule assemblies.31-33, 37-39 To date, only a few studies have discussed the alignment of

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semiconducting polymers via unidirectional solution coating techniques, which would be

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compatible with roll-to-roll printing processes. For example, the McCulloch research group

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demonstrated a flow-coating process that is able to control the crystal orientation of poly(2,5-

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bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT). However, this approach required

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relatively high processing temperatures to induce the phase transition, and thereby form a well-

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oriented terraced ribbon structure.40 Recently, Bao and coworkers introduced a flow-enhanced 4


These

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solution printing method to increase polymer crystallinity, which is facilitated by pre-stretched

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polymer chains through extensional flow. However, a micropillar-patterned printing blade was

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required.34

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The difficulties associated with chain alignment are especially true for high molecular weight

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conjugated polymers, which generally exhibit greater charge carrier mobility than their low

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molecular weight counterparts.41,

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nucleation and growth of self-assembled conjugated polymers due to chain entanglement effects

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that can severely hinder the alignment and π-π stacking of polymer chains during rapid solvent

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evaporation associated with solution coating methods.43-45

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One reason for this is the difficulty associated with

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Here, we demonstrate a blade coating approach to induce long-range alignment of conjugated

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polymer nanocrystalline structures by preprocessing the polymer solution with low-dose

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ultraviolet (UV) irradiation and solution aging. The demonstration vehicle was P3HT. It has been

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reported that the low-dose UV-irradiation is able to induce solution phase P3HT aggregation into

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nanofibrillar structures via a shift in the overall balance between ‘aromatic-’ and ‘quinoidal-like’

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configurations. The latter polymer conformation exhibits a higher degree of backbone

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coplanarity, and thus facilitates the π-π interchain interactions necessary for nanofiber

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formation.46 However, the development of UV-irradiated aggregation becomes saturated after ~8

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min UV-irradiation time. Fortunately, further P3HT aggregation can be driven by time-

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dependent self-assembly.47 As a result, the population of P3HT nanofibrils can be controlled and

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enhanced by simply storing the UV-irradiated P3HT solutions under ambient conditions for the

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designated period of time in a so called solution aging approach.

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This combined approach takes advantage of the facile and straightforward solution

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preprocessing steps that were found to effectively facilitate the controlled nucleation and crystal 5



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growth of conjugated polymers prior to the rapid thin film formation process.48, 49 Interrogation

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of the resultant thin films revealed that the degree of crystallinity and chain alignment were both

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significantly improved with increased solution aging time, and the optical and charge transport

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properties were found to be anisotropic. Analysis of the surface morphology coupled with

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mechanistic interpretation supports the supposition that polymer chain entanglement effects,

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which negatively impact polymer self-assembly and chain alignment, were greatly reduced when

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the semiconducting films were fabricated by blade coating preaggregated polymer solutions. The

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anisotropic charge carrier mobility of uniaxially aligned nanofiber arrays further suggests that

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interfiber tie chains facilitate the alignment of nanoaggregates and provide for efficient charge

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transport pathways between nanofibrillar structures.

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RESULTS & DISCUSSION

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Photophysical Properties

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Figure 1a depicts the experimental polymer solution preprocessing and thin film formation

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sequence. The P3HT solutions were prepared in CHCl3 at a concentration of 5 mg mL-1, and

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treated with low dose UV-irradiation.46 Next, the UV-irradiated P3HT (UV-P3HT) solutions

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were aged for a designated period of time (0, 3, 8, 15, 24 h) under ambient conditions in the dark.

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As shown in Figure 1b, the UV-vis spectrum obtained from pristine P3HT solutions exhibited a

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single absorption maximum at ca. 453 nm, and no distinct spectral features were observed. Upon

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UV-irradiation, the orange polymer solutions turned to dark purple/brown, and absorption bands

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at ca. 550 nm and ca. 605 nm associated with intrachain (0‒1) and interchain (0‒0) vibronic

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transitions, respectively,50 began to develop. These spectral features are indicative of P3HT

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aggregate formation.50 With aging, the intensity of the low energy absorption bands notably

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increased in a time dependent manner, suggesting that the polymer aggregates further evolved

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into longer nanofibrillar structures. The degree of polymer aggregation in solution as a function

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of aging time was estimated from Franck-Condon fits to the absorption spectra as shown in

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Figures 1c and S1.44, 51 The percentage of P3HT aggregates increased gradually from 9.0 to 20.4

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% through 24 h of aging. The spectroscopic results are indicative of a time-dependent polymer

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self-assembly process that is driven by interchain interactions.

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Figure 1d and 1e present the absorption spectra of pristine and processed P3HT thin films

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deposited by spin coating and blade coating, respectively. A red-shift of the π‒π* intraband

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transition and increased intensity of the (0‒0) absorption peak at ca. 605 nm were observed from

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both spin coated and blade coated thin films, and the evolution of polymer aggregation appeared

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consistent with the solution phase UV-vis spectral results. Given that the band intensities 7



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increased irrespective of coating method, it is likely that the nanofibers formed in solution and

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survived the thin-film fabrication process.52, 53 According to Spano’s model, the intensity ratio

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of the (0‒0) and (0‒1) vibronic bands can be related to the free exciton bandwidth (W), which

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enables estimation of the magnitude of interchain coupling within aggregates.54, 55 A decrease in

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W is associated with increased conjugation length and/or improved chain ordering. The values of

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W were calculated by the following equation 1.56

1 − 0.24 / = 1 + 0.073 /

(1)

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I0-0 and I0-1 represent the intensities of the 0‒0 and 0‒1 vibronic bands, respectively, obtained

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from Gaussian fits to the experimental spectra (Figure S2 and S3). Ep is the vibrational energy of

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the symmetric vinyl stretch (taken as 0.18 eV).54 Figure 1f provides the calculated free exciton

10

bandwidths for corresponding thin films. The value of W decreased approximately linearly with

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longer aging time, regardless of the film formation process. Notably, the blade coated samples

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exhibited lower exciton bandwidth values compared with spin coated analogs, suggesting that

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the blade coating approach more effectively extended and coplanarized the polymer chains

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thereby promoting intermolecular ordering.

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Figure 1. (a) Schematic illustration of polymer solution preprocessing and thin film formation

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experimental sequence. (b) Normalized UV-vis absorption spectra of pristine and UV-P3HT in

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solution state with varying solution aging time. (c) Percentage of UV-P3HT aggregates as a

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function of solution aging time in solution. Normalized UV-vis absorption spectra of pristine and

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aged UV-P3HT thin films deposited by (d) spin coating and (e) blade coating. (f) Evolution of

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exciton bandwidth (W) as a function of solution aging time.

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Surface Morphology

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Based on examination of the photophysical properties, the level of solution state polymer

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aggregation was successfully manipulated by the combined UV-irradiation/aging method. The

4

impact of solution preprocessing on the surface morphologies of blade coated thin films as a

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function of aging time was evaluated using Atomic Force Microscopy (AFM) imaging. As

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shown in Figure 2a, pre-deposition processing with just UV-irradiation led to the development of

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randomly oriented, nanoscale fibrillar structures that were isolated by disordered regions. With

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increased solution aging time, the average nanofiber length and density increased significantly.

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In addition, P3HT nanofiber orientation became more unidirectional. (Figure 2b-e).

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As shown in Figure 2f-j, the AFM phase images (Figure S4) were analyzed for their nanofiber

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orientation distribution and degree of nanofiber alignment using an open-source image

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processing algorithm.57 According to the polar figure (Figure 2k), which is generated by

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counting the number of fiber backbone pixels of each orientation, the fiber orientation

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distribution of aged UV-P3HT thin films was mainly oriented along the horizontal axis, which

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corresponded to the blade coating direction. The nanofiber alignment was further quantified by

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the orientational order parameter (S2D),58, 59 which is defined for a population of oriented fiber

17

segments (pixels) as follows in equation 2. S = 2〈 〉 − 1

(2)

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where is the angle between a fiber segment and the population’s director, !, which is

19

chosen as the average orientation of the populations; for completely aligned nanofibrillar

20

structures S = 1, whereas for totally randomly oriented thin films S = 0. S , which can be

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correlated to the degree of nanofiber alignment as plotted against aging time in Figure 2l. The

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films blade coated with fresh (0 h) UV-P3HT solutions provide an S2D value of 0.53, showing a 10


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weakly oriented microstructure. However, this value gradually increased with longer solution

2

aging times, and reached a highest value of S2D at 0.89 for an aging time of 24 h, suggesting a

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highly aligned nanofibrillar structure.

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The UV-P3HT solution with 24 h aging time was also deposited onto a substrate via spin

5

coating. The spin coated thin films showed a distinct surface morphology compared with blade

6

coated samples as shown in Figure S5. In contrast to the long-range aligned nanocrystalline

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structure observed from blade coated samples, films deposited by spin coating the 24 h aged UV-

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P3HT solution exhibited randomly oriented P3HT nanofibrillar surface morphology, indicating

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that a unidirectional coating process is a prerequisite for aligning the conjugated polymer

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nanofibers along the coating direction.

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Figure 2. AFM phase images (2 x 2 µm) of P3HT thin films deposited by blade coating UV-

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P3HT solutions with (a) 0 h (b) 3 h (c) 8 h (d) 15 h (e) 24 h solution aging time. Image Analysis

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(5 x 5 µm) of P3HT thin films deposited by blade coating UV-P3HT solutions with (f) 0 h (g) 3 h

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(h) 8 h (i) 15 h (h) 24 h solution aging time. (k) Polar Plots and (l) orientational order parameter

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(S2D) of blade coated films as function of solution aging time.

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Optical Anisotropy

2

Cross-polarized optical microscopy (POM) was used to investigate the optical anisotropy of

3

aged UV-P3HT thin films. As evidenced by POM (Figure 3), the fresh (aged 0 h) UV-P3HT

4

blade coated thin film appeared completely dark when viewed through crossed polarizers

5

regardless of angle, demonstrating isotropic characteristics. Alternatively, films obtained by

6

blade coating aged UV-P3HT solutions exhibited clear birefringence, and the brightness

7

concomitantly appeared to be enhanced with longer solution aging time. The almost complete

8

extinction and re-emergence of brightness upon rotation of the thin film long axis from 0 to 45o

9

with respect to the crossed polarizers suggests a highly anisotropic crystalline structure.60, 61 The

10

POM results are well correlated with AFM images that showed well-aligned nanofibrillar surface

11

morphologies. It is worth noting that the POM image area is 1000 times larger than that of the

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AFM images (2 x 2 µm), pointing to macroscopically aligned nanofibrillar structures. The spin

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coated thin films were also investigated with POM. As shown in Figure S6, the randomly

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oriented nanofibrillar thin film structure leads to no significant difference in light intensity upon

15

rotation.

16

The extent of polymer chain alignment can be further evaluated through polarized UV-vis

17

spectroscopy. For spin coated samples, 24 h aged UV-P3HT thin films exhibited no differences

18

with respect to polarization direction owing to their isotropic nature (Figure 4a). The 24 h aged

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UV-P3HT films fabricated by blade coating showed distinct polarized UV-vis spectra as shown

20

in Figure 4b. As anticipated, the anisotropic thin films exhibited a stronger absorbance when the

21

polarized light beam was parallel to the aligned polymer chains; note that the transition dipole

22

moment is typically along the long axis of conjugated polymer backbones.62, 63 As a result, the

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low energy absorption peak (0‒0) at ca. 605 nm was concomitantly enhanced when the UV-vis 13



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polarization was vertical to the coating direction. In contrast, the overall polarized optical

2

absorbance, which is parallel to the blade coating direction was significantly reduced. The above

3

features demonstrated that the polymer backbones associated with the 24 h aged UV-P3HT thin

4

films fabricated via blade coating are closely packed and perpendicular to the long axis of the

5

nanofibers, which in turn are well-aligned along the blade coating direction as shown in Figure

6

4c.

7

To quantitatively analyze the structural anisotropy as a function of solution aging time, the

8

dichroic ratio (R = A⊥/A∥),23 defined as the absorption of (0‒0) peak obtained from polarized

9

UV-vis spectra (Figure S7 and S8) in the direction perpendicular (⊥) to blade coating over the

10

absorption in the parallel direction (∥), was calculated. As shown in Figure 4d, the dichroic ratios

11

of spin coated thin films were ca. 1.0, and remained constant even with extended solution aging

12

time, providing additional support for the premise that spin coated thin films were randomly

13

oriented. Alternatively, the dichroic ratio of blade coated thin films increased linearly by four

14

orders of magnitude as a function of solution aging time, demonstrating that polymer chain

15

alignment was improved and was easily tunable by varying the solution aging time. These

16

factors are further correlated with the degree of polymer aggregation in solution.

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Figure 3. POM micrographs of blade coated P3HT films with varying solution aging time,

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rotated in increments of 15o between crossed polarizers.

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Figure 4. Polarized UV-vis spectra of 24 h aged UV-P3HT thin films deposited by (a) spin

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coating and (b) blade coating. (c) Schematic diagram of P3HT nanofibrillar structures in spin

4

coated (SC) and blade coated (BC) thin films. The UV-vis polarization direction (red arrows)

5

vertical (V) and parallel (P) to the blade coating direction (gray arrow) are provided in the figure.

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The short blue lines schematically indicate the individual polymer chains, which are vertical to

7

the long axis of nanofibers. (d) Evolution of the dichroic ratio as a function of solution aging

8

time.

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Thin Film Crystallinity

2

To probe the thin film crystallinity and microstructure, the 24 h aged UV-P3HT thin films

3

deposited by spin coating were investigated using two-dimensional grazing-incidence wide-angle

4

X-ray scattering (2D-GIWAXS) as shown in Figure 5a. The blade coated thin films, which

5

exhibited highly anisotropic microstructures, were measured in the two orientations where the

6

blade coating direction was parallel or vertical to the incident X-ray beam (Figure 5b and 5c).

7

The 2D-GIWAXS profiles of these films present a series of intense characteristic (h00) (h = 1, 2,

8

3) reflection spots along the out-of-plane (qz) direction, corresponding to lamellar stacking of

9

P3HT oriented normal to the substrate.64 The appearance of these high order reflections is a

10

strong indicator of enhanced thin film crystallinity.65 For the high q regime, the halo-like patterns

11

that appeared along the azimuthal angle are attributed to the random oriented microstructures

12

within the film.66

13

Figure 5d depicts the GIWAXS in-plane profiles for spin coated and blade coated thin films

14

with the incident beam oriented parallel and perpendicular to the coating direction. A reflection

15

peak at q = 0.379 (Å-1) is evident in the in-plane scattering of the spin coated sample, indicating

16

that a certain portion of the polymer crystal planes lie parallel to the substrate.64 Notably, this

17

(100) in-plane diffraction peak, which is correlated with face-on crystal orientation was absent

18

for bladed coated thin films. Additionally, a sharp (010) peak was recorded at q = 1.649 (Å-1) for

19

the blade coated sample when the incident X-ray beam was oriented vertical to the blade coating

20

direction. These results suggest strong π-π interchain stacking of polymer chains comprising the

21

blade coated sample and the crystalline microstructure of these films exhibited a primarily edge-

22

on orientation, expected to facilitate efficient charge transport in the plane of the substrate.11 It

23

has been reported that the characteristic π-π stacking peak would exhibit a maximum intensity 17



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when the X-ray incident beam is aligned along the polymer backbone as shown in Figure 5e.67

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Thus, the distinct intensity of (010) reflection peaks of blade coated thin films between the

3

parallel and vertical directions provides further support that the polymer backbones are well-

4

aligned and oriented perpendicular to the blade coating direction.

5 6

Figure 5. 2D-GIWAXS patterns of (a) spin coated and bladed coated thin films with the incident

7

beam oriented (b) parallel and (c) vertical to the coating direction. (d) The GIWAXS in-plane

8

profiles for spin coated and blade coated thin films with the incident beam oriented parallel and

9

perpendicular to the coating direction. (e) Schematic illustration of in-plane (010) reflections

10

when the incident beam is parallel to aligned polymer backbones. 18


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1


Mechanistic Interpretation

2

In the blade coating process, the blade is used to drag the solution moving along one direction

3

relative to the substrate at a controlled speed. The concentration gradient of the polymer solution

4

that arises from solvent evaporation will force the migration of polymer chains toward the

5

solution meniscus, leaving behind a uniform solid film. During thin film formation, polymer

6

crystallization depends upon nucleation and crystal growth. Nucleation is the first step in the

7

formation of polymer aggregates, which requires overcoming of a free energy barrier and is

8

intrinsically stochastic.30 Subsequent crystal growth will be initiated from the available

9

nucleation sites, leading to development of nanofibrillar structures.68,

69

However, these self-

10

assembly processes are time-dependent and typically require long periods of time to evolve and

11

form an ordered thin film structure.70 As shown in Figure 6a, in pristine solutions, P3HT polymer

12

chains behave as random coils, preventing the chains from stretching and packing into a well-

13

ordered microstructure during a rapid solvent evaporation process, resulting in an amorphous

14

thin film morphology.48

15

Low-dose UV-irradiation (Figure S9) was suggested to induce a higher degree of polymer

16

backbone planarity thereby facilitating π-π stacking necessary for nanofiber formation in the

17

solution phase.71 Although crystalline regions were obtained from blade coated fresh (aged 0 h)

18

UV-P3HT solutions (Figure 6b), the population of aggregates was too low to form long-range

19

ordered films due to the presence of significant numbers of solubilized polymer chains that still

20

rely on the slow diffusion of polymer chains to the crystal growth front by π-π interchain

21

interactions. Additionally, a certain degree of flexibility and entanglement of unstacked polymer

22

chains may inhibit chain alignment, especially for high molecular weight P3HT.

19



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Page 20 of 42

1

The additional solution aging step allowed for a further increase in the proportion of π-π

2

stacked polymer chains in the solution state via time-dependent polymer self-assembly. Note that

3

the level of aggregation is positively correlated with solution aging time, confirmed by UV-vis

4

analysis. As shown in Figure 6c, the high degree of polymer aggregation obtained by the

5

synergistic combination of UV-irradiation and solution aging effectively reduced the number of

6

unstacked and tortuous polymer chains, which may cause severe chain entanglement effects.43, 44

7

Moreover, interfiber connections, such as tie-chains, may further limit the freedom of nanofiber

8

motions to a single direction upon blade coating the preaggregated polymer solutions,45, 72, 73

9

thereby affording a long-range and highly aligned nanocrystalline structure.

10 11

Figure 6. Schematic illustration of blade coating process with (a) pristine P3HT (b) fresh UV-

12

P3HT (c) Aged UV-P3HT Solutions.

13 20


Page 21 of 42

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1


Charge Transport Anisotropy

2

While previous studies focused on the alignment of individual polymer backbones along a

3

certain direction,22-28 the approach presented here effects long range ordering of conjugated

4

polymers into nanofibers, and induces macroscopic alignment. Given the alignment of numerous

5

well-packed and coplanarized conjugated polymer chains, the system presents an opportunity to

6

study charge transport anisotropy in a highly aligned nanofibrillar structure.

7

OFETs were fabricated to evaluate the impact of nanofiber orientation on the charge carrier

8

transport properties of the semiconducting polymer, which was deposited by blade coating and

9

spin coating the respective UV-P3HT solutions with different aging times onto device substrates.

10

For the blade coated thin films, the preprocessed polymer solutions were coated in vertical and

11

parallel directions with respect to the source/drain electrodes as shown in the Figure S10.

12

Representative transfer and output curves acquired from OFETs with semiconducting layers

13

deposited by spin coating and blade coating in parallel and vertical directions are presented in

14

Figure S11.

15

The calculated charge carrier mobilities extracted from the transfer characteristics were plotted

16

against the solution aging time as shown in Figure 7a. The charge transport performance of spin

17

coated and blade coated OFETs all gradually increased with extended solution aging time, as

18

anticipated due to the improvement in thin film crystallinity. However, the blade coated samples

19

exhibited anisotropic charge transport properties when the films were fabricated by blade coating

20

in directions parallel and perpendicular to the source/drain electrodes. For well-aligned

21

nanofibrillar thin films, such as those obtained from blade coated 24 h aged UV-P3HT, the

22

charge carrier mobility was an average of 0.198 ± 0.003 cm2V-1s-1 and 0.122 ± 0.004 cm2V-1s-1

23

for films fabricated in the vertical and parallel directions, respectively. These results can be 21



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Page 22 of 42

1

ascribed to the fact that the intramolecular charge transport of well-aligned nanofibrillar

2

structures in the vertical direction with respect to source/drain electrodes is more effective than

3

in the parallel direction (Figure 7b). Note that intramolecular charge transport along the

4

conjugated polymer backbone is reported to be much faster than intermolecular charge transport

5

parallel to π-π stacking direction (Figure 7c).11 As a result, charge transport anisotropy was

6

enhanced with increased solution aging time, and can be correlated with the degree of chain

7

alignment.

8

Furthermore, the remarkable charge transport mobility observed from the films deposited by

9

blade coating in the vertical direction could further support the premise that the nanofibrillar

10

structures are interconnected by tie-chains, which serve as bridges between well-aligned

11

nanofibers to afford efficient intramolecular charge transport.9,

12

reproducibility, the mobility distribution of over 20 devices is illustrated in Figure S12. For spin

13

coated thin films, the randomly oriented nanofibrillar structure may exhibit binary charge

14

transport behavior, resulting in an intermediate charge carrier mobility.

22


26,

73

To demonstrate

Page 23 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 7. (a) Average charge carrier mobilities of OFETs with active layers deposited by spin

3

coating and blade coating in parallel and vertical directions. (b) Schematic diagram of blade

4

coated films with perfect aligned nanofibrillar structures parallel and vertical to source/drain

5

electrodes. The blade coating direction (gray arrows) are provided in the figure. The short blue

6

lines schematically indicate the individual polymer chains. Long blue and red line represent the

7

intermolecular and intramolecular charge transport, respectively (c) Charge transport direction in

8

P3HT crystalline structure.

9 10 11

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Page 24 of 42

CONCLUSIONS

2

The combination of UV-irradiation and solution aging was shown to facilitate the control of

3

polymer self-assembly, which exhibits a strong correlation with thin film crystallinity and charge

4

transport anisotropy. Rather than effecting alignment of the individual polymer backbones

5

without long-range positional ordering, we demonstrated a facile strategy to induce macroscopic

6

alignment of P3HT by pre-stacking a significant fraction of polymer chains into orthorhombic

7

nanofibers prior to the rapid thin film formation process. The preaggregated polymer chains were

8

bonded by cofacial π-π interactions, and therefore the tortuosity and entanglement effects of long

9

polymer chains were significantly reduced during the blade coating process. According to

10

surface morphology analysis, the orientation distribution of P3HT nanofibers were aligned

11

parallel to the blade coating direction. The degree of chain alignment is greatly improved with

12

increased polymer aggregation which can be easily tuned by solution aging time. Furthermore,

13

the crystal planes are mainly oriented edge-on, which benefits in-plane charge transport. The

14

strategy afforded well-aligned and long-range, macroscopically ordered nanofibrillar structures.

15

It is noteworthy that the approach obviates the need for additional blade patterning or surface

16

functionalization steps, suggesting a low-cost and scalable process for high-throughput flexible

17

polymer semiconductor device manufacturing.

18

To the best of our knowledge, for the first time, we have demonstrated an evolution of P3HT

19

nanofibers from totally randomly oriented to long-range aligned nanocrystalline architectures.

20

The highly aligned nanofibrillar structure provides a platform to mechanistically elucidate the

21

anisotropic charge transport behavior. The results indicate that the charge carrier mobilities were

22

determined by P3HT nanofiber orientation with respect to the direction of source and drain

23

electrodes. The charge carrier mobility reached a maximum value as high as 0.241 cm2V-1s-1 24


Page 25 of 42

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1

when the nanofibers were well-aligned perpendicular to the direction of source and drain

2

electrodes, presumably due to enhanced intramolecular charge transport within the film. These

3

results also support the concept that semicrystalline, semiconducting polymer nanofibers are

4

often interconnected by tie-chains which allow for efficient charge transport between

5

nanoaggregated structures. The significant insights that correlate thin film morphology/structure

6

and charge transport anisotropy for well-aligned conjugated polymers are expected to impact the

7

design and development of future high performance organic electronic devices.

8 9 10 11 12 13 14

25



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1

Page 26 of 42

MATERIALS & METHODS

2

Materials. Regioregular P3HT (Mw: 90 kDa, Regioregularity: 96 %) and Chloroform

3

(anhydrous) were purchased from Rieke Metals Inc. and Sigma-Aldrich, respectively, and used

4

without further purification.

5

Solution Processing of P3HT. A combination of UV-irradiation and solution aging was used

6

to control the aggregation of P3HT in solution state. First, 10 mg of P3HT was dissolved in 2 mL

7

of Chloroform at 55 oC under stirring for 30 min. After complete dissolution of P3HT, the

8

solution was left to cool to room temperature. Then, the vial containing the solution was placed

9

on a hand-held UV lamp (Entela UVGL-15, 5 mW cm-2, 254 nm), which had been placed on a

10

magnetic stirrer (Corning Inc.) and the solution was then stirred at 300 rpm. The UV -irradiation

11

time was fixed at 8 min and the solution exhibited a notable color change from bright orange to

12

dark purple/brown, indicating that the P3HT began to aggregate and form nanofibrillar structures

13

in the solution. The UV-P3HT solutions were then stored for a designated period of time (0, 3, 8,

14

15, 24 h). The vials containing the solution were covered with aluminum foil to avoid additional

15

exposure to light. All solution processing was done in ambient condition.

16

OFET Fabrication and Characterization. The OFETs with a bottom-gate bottom-contact

17

configuration were fabricated using a heavily n-doped silicon wafer as the gate electrode. A 300

18

nm thick layer of thermally grown SiO2 served as the gate dielectric. The 50 nm Au source and

19

drain electrodes with 3 nm Cr as the adhesion layer were both deposited via standard

20

photolithography based lift-off processing, followed by E-beam evaporation onto the SiO2 layer.

21

Before the semiconducting layer deposition, the device substrates were cleaned in an ultrasound

22

cleaning bath and sequentially rinsed with acetone, methanol and isopropanol. At the final step

23

of the cleaning process, the devices were exposed to UV-ozone (Novascan PSD-UV) to ensure 26


Page 27 of 42

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1

complete removal of residual photoresist and other organic contaminants. OFET fabrication was

2

completed by depositing the preprocessed P3HT solutions onto the precleaned substrates via

3

either spin coating (WS-650MZ-23NPP, Laurell) at a spin speed of 1500 rpm for 60s or blade

4

coating at a speed of 3.0 mm s-1. For the blade coating system, the coating blade was vertical to

5

the substrate at a fixed gap of 10 µm. The device substrate was placed onto a homemade vacuum

6

chuck equipped with a motorized linear stage (A-LSQ150A-E01, Zaber).

7

OFET electrical properties were investigated in nitrogen glovebox using an Agilent 4155c

8

semiconductor parameter analyzer. The charge carrier mobilities were calculated in the

9

saturation regime from the transfer plots of drain current (IDS) versus gate voltage (VG) using the

10

following equation 3: $ =

%&' )*+,$ − +-. / 2(

(3)

11

where W and L refer to channel width (50 µm) and length (2000 µm), respectively. µ presents

12

the hole mobility, Vth is the threshold voltage and Cox is the capacitance per unit area of the SiO2

13

gate dielectric (1.15 x 10-8 F cm-2).

14

UV-vis Spectroscopy. The solution and solid state UV-vis spectra were determined using an

15

Agilent HP 8510 UV-vis spectrophotometer. The corresponding P3HT films were deposited onto

16

precleaned glass slides, which had undergone the same clean procedure as indicated above for

17

OFET devices. This spectrophotometer was also used for Polarized UV-vis absorption

18

measurements by polarizing the incident light vertical or perpendicular to the blade coating

19

direction.

20

Atomic Force Microscopy (AFM). The surface morphologies of the thin films were

21

characterized by AFM using a Bruker Dimension Icon atomic force microscopy system,

22

operating in tapping mode with a n-type silicon tips (HQ:NSC14, MikroMasch). 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 42

1

Polarized Optical Microscopy (POM). POM images were obtained using a Leica DMRX

2

optical microscope equipped with a rotatable polarizer, analyzer and Nikon D300 digital SLR

3

camera.

4

Grazing-Incidence

Wide-Angle

X-Ray

Scattering

(GIWAXS).

The

GIWAXS

5

measurements were carried out on beamline 11-3 at the Stanford synchrotron radiation light

6

source (SSRL) with the help of Dr. Chris Tassone. The beam was kept at an energy of 12.7 KeV

7

and the critical angle of measurement was 0.13o. A LaB6 standard sample was used to calibrate

8

the instrument and the software WxDiff was used to reduce the 2D scattering data into the

9

corrected 1D integration plots.

10 11 12 13

28


Page 29 of 42

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1

ASSOCIATED CONTENT

2

Supporting Information.

3

The results of fits to UV-vis spectra, AFM phase images, POM micrographs, polarized UV-

4

vis spectra, AFM image analysis, transfer and output characteristics and mobility distribution of

5

OFETs. This material is available free of charge via the Internet at http://pubs.acs.org.

6

AUTHOR INFORMATION

7

Corresponding Author

8

*E-mail: [email protected]

9

Present Addresses

10

Boyi Fu: Applied Materials, Inc., 3050 Bowers Ave, Santa Clara, CA 95054, United States.

11 12

ACKNOWLEDGEMENTS

13

The authors would like to acknowledge support from NSF Grant No. 1264555: Morphology

14

and Mobility Control for Functional Robust Flexible Electronics and Photovoltaics, the Georgia

15

Institute of Technology and the Brook Byers Institute for Sustainable Systems for support. Nils

16

Persson also gratefully acknowledges financial support from the NSF FLAMEL IGERT

17

Traineeship program, NSF Grant No. 1258425, IGERT-CIF21: Computation-Enabled Design

18

and Manufacturing of High Performance Materials. Jeff Hernandez, Nabil Kleinhenz and

19

Michael McBride acknowledge NSF IGERT NESAC Traineeship support: NSF Grant No.

20

1069138: Nanostructured Materials for Energy Storage and Conversion.

21 22

29



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Toward Precision Control of Nanofiber Orientation in Conjugated

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