Ultrasound-Induced Organogel Formation Followed by Thin Film

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Ultrasound-induced Organogel Formation Followed by Thin Film Fabrication via Simple Doctor Blading Technique for Field-Effect Transistor Applications Jiaju Xu, Yulong Wang, Haiquan Shan, Yiwei Lin, Qian Chen, Vellaisamy.A.L. Roy, and Zongxiang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04817 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016

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ACS Applied Materials & Interfaces

Ultrasound-induced

Organogel

Formation

Followed by Thin Film Fabrication via Simple Doctor

Blading

Technique

for

Field-Effect

Transistor Applications Jiaju Xu,†,⊥ Yulong Wang, †,⊥ Haiquan Shan,† Yiwei Lin,† Qian Chen,† V. A. L. Roy,*,‡ and Zongxiang Xu*,† †

Department of Chemistry, South University of Science and Technology of China, Shenzhen, P.

R. China. ‡

Department of Physics and Materials Science, City University of Hong Kong, Hong Kong

SAR.

KEYWORDS: metal phthalocyanine, ultrasound; organogel, doctor blading, organic field-effect transistor

ABSTRACT: We demonstrate doctor blading technique to fabricate high performance transistors made up of printed small molecular materials. On this regard, we synthesize a new

ACS Paragon Plus Environment

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soluble phthalocyanine, tetra-n-butyl peripheral substituted copper (II) phthalocaynine (CuBuPc) that can easily undergo gel formation upon ultrasonic irradiation, leading to the formation of three-dimensional (3D) network composed of one-dimensional (1D) nanofibers structure. Finally, taking the advantage of thixotropic nature of the CuBuPc organogel, we use the doctor blade processing technique that limits the material wastage for the fabrication of transistor devices. Due to the ultrasound induced stronger π-π interaction, the transistor fabricated by doctor blading based on CuBuPc organogel exhibits significant increase in charge carrier mobility in comparison with other solution process techniques, thus paving a way for a simple and economically viable preparation of electronic circuits.

INTRODUCTION Small molecules and oligomers with delocalized π-electron conjugated systems have been the subject of intense attention because of their promising photophysical and transport properties.1,2 As one class of these materials, phthalocyanines (Pcs) are receiving a surge of interest in the field of organic semiconductors because of their high chemical/thermal stability, unique electronic structures, and strong intermolecular interactions in the condensed phase.3-6 Many Pcbased electronic devices, such as organic light-emitting diodes,7 organic field-effect transistors (OFETs),8,9 and organic photovoltaics3 have been studied and found to exhibit promising device performance. In most cases, the Pc thin films were deposited via vacuum sublimation during device fabrication because of the poor solubility of Pcs in common organic solvents.4,10,11 Unlike vacuum processes, solution processing methods have the advantages of low cost and the potential for mass production of electronic devices as well as compatibility with flexible


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substrates.4,9 During the past several decades, numerous studies have been conducted to develop new soluble Pcs. In particular, Pcs functionalized with solubilizing alkoxy or alkyl substituents on the peripheral benzene rings are of immense interest.12-16 Drop-casting and spin-coating are two main solution processes that are well suited to prepare devices with high reliability and performance on the laboratory scale.17-19 However, such experimental techniques are found to be inefficient due to the wastage of high amount of solution, and are not compatible for large scale industrial processing. Various alternatives such as screen printing, ink-jet printing, spray coating, and doctor blading have been developed for thin film preparation to fulfill the production demands for cost-effective and large-scale electronics manufacture.17,20-22 In particular, the doctor blading method is regarded as a material-saving deposition technique for both organic and inorganic thin films because it facilitates high throughputs.23 In addition, doctor blading is also compatible with roll-to-roll printing, allowing easy implementation in an established roll-to-roll coating environment. Therefore, doctor blading is a preferable technique to other solution-based processes to allow future use in mass production. Due to the low viscosity of small molecule solution which leads to rough surface property and device failures, it is difficult to carry out doctor blading process using small molecular materials in solution for device fabrication. Although doctor blading is widely used to produce thin films on large surfaces, this method is less frequently used to prepare organic thin films from small molecules for use in electronic devices. In this paper, we introduce a new soluble Pc, tetra-n-butyl peripheral-substituted copper(II) phthalocyanine (CuBuPc) (Figure 1a). We found that this material can easily form a gel upon ultrasonic irradiation, leading to the formation of nanofibers. To the best of our knowledge, this is the first example of ultrasound-induced gelation from alkyl-substituted Pc molecules.


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Supramolecular gels24,25 formed from low-molecular-weight gelators (LMWGs) are an important class of soft matter because of their various applications including drug delivery,26 wound healing,27 cell culture media,28 waste water treatment,29 gear and rolling bearing.30 Gelation of solutions containing low-molecular-weight organic compounds via ultrasonic treatment was not considered an appealing approach. However, in the year 2005, for the first time Naota and Koori reported the formation of ultrasound-induced gelation for a coordination compound which is a dinuclear palladium complex, and has been stabilized by the interaction due to intramolecular π–π stacking.31,32 Other LMWG examples include peptides,17,33 amino acid derivatives,33,34 ureas,35 and pyridines,36 most of which exhibit potential biological and biomedical applications. Recently, Prof. Zhu of City University of Hong Kong and Prof. Cho in Pohang University of Science and Technology reported that imbedding nanowires of diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene

(I)

or

Poly(3-hexylthiophene)

in

polystyrene led to enhanced transistors mobilities fabricated by inkjet printing. Comparing with that of thin film phase based devices, such higher device performance was induced by stronger molecular π-π interactions of the nanostructrures. Furthermore, such nanowires network led to good flexibility of as fabricated devices, exhibiting potential application in integrated organic transistor array on flexible substrate.37,38 Although thin film of nanowires semiconductor network with high device performance has been obtained, to date, no application of electronic device from nanowire network based on LMWGs fabricated by doctor blading has been reported. Herein, we report that CuBuPc assembles into a one-dimensional fibrous architecture upon sonication, resulting in organogel formation. Furthermore, we use doctor blade processing to fabricate an OFET using the ultrasound-induced organogel. The organogel transistor has a hole mobility of up to 0.030 cm2/Vs. This mobility is approximately 50 times of magnitude higher


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than that of devices formed by solution casting CuBuPc. We attribute the increased mobility to stronger π–π interactions of organogel induced by ultrasonic irradiation.

Figure 1. (a) Molecular structure of CuBuPc; (b) CuBuPc solution (40 mg/mL) in 1,2dichlorobenzene; (c) Ultrasound-induced CuBuPc gel; (d) Sol states after vigorous shaking; and (e) Stable gel states after standing for 20 min.

RESULTS AND DISCUSSION Material. The synthesis of CuBuPc is described in the Supporting Information and involves a five-step synthesis from 4-methylphthalonitrile (Scheme S1 and S2, Supporting Information). CuBuPc exhibited good thermal stability when tested by thermal gravimetric analysis, with a 5% weight loss occurring at temperatures higher than 400 ℃ (Figure S1, Supporting Information). The final product was purified by vacuum sublimation to ensure adequate purity for electronic applications. Purity was determined by elemental analysis and high-resolution mass spectrometry (Supporting Information). The presence of n-butyl substituents resulted in CuBuPc exhibiting –1

good solubility in organic solvents such as chloroform and dichlorobenzene (>50 mg mL

at

room temperature). UV-Vis Spectra. The absorption spectrum of CuBuPc in 1,2-dichlorobenzene solution displayed two sets of strong absorption bands (Figure S2a, Supporting Information). One band is


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an intense single absorption at ~678 nm, and the other is a shoulder at ~610 nm (Q band). Both of these absorptions are attributed to π→π* transitions. Another absorption at ~338 nm is denoted as the B band and arises from the deeper π levels→LUMO transitions.39 Thixotropic Nature. The so-called “inverted test tube” method was used as a visual technique to test gelation of the CuBuPc solution in 1,2-dichlorobenzene after ultrasonic irradiation.32,40 Glass vials containing 1 mL of CuBuPc in 1,2-dichlorobenzene (10, 20, or 40 mg/mL) were placed in an ultrasonic bath and sonicated (200 W) for 30 min at temperature of 50 ℃ to promote complete sol-to-gel conversion. Complete gel formation was considered successful at 40 mg/mL because no sample flow occurred upon inverting the vial at room temperature (Figure 1b, c). It should be noted that the gelation only occurs upon sonication, with other external stimuli such as vigorous shaking, rapid heating/cooling, and microwave irradiation unable to cause aggregation. The thixotropic property of the resulting gel was studied. To obtain a solution phase (Figure 1d), the gel has to be shaken vigorously. A self-supporting gel was reformed after allowing the vial to stand for ~20 min (Figure 1e). The observed thixotropic nature of the CuBuPc organogel was analyzed by a rheological behavior study using a reported method.25 A frequency sweep analysis of the CuBuPc gel recorded the dynamic mechanical properties (e.g., storage modulus Gʹ and loss modulus Gʺ) as a function of angular frequency at a constant strain of 0.1% at 25 ℃. The value of Gʹ is higher than that of Gʺ (Figure 2a). No crossover point is observed over the tested frequency range, indicating the formation of a soft “solid-like” gel.41 Both Gʹ and Gʺ increase slightly with increasing angular frequency, showing a weak dependency on the angular frequency in the low frequency region (0.1–100 rad/s). These observations indicate that the gel matrix is stable and has good


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tolerance to external stimuli,26 which may be because of strong intermolecular π–π stacking interaction between Pc molecules after ultrasonic irradiation. A continuous time dependent step-strain analysis of the gel at a constant temperature of 25 ℃ was undertaken to further confirm the thixotropic behavior observed by the naked eye (Figure 2b). The gel was subjected to a continuous strain of 0.1% for 100 s followed by a sudden strain increase to 40% to destroy the gel matrix. During the application of the 40% strain, the Gʺ values were greater than those of the corresponding Gʹ values, indicating a gel-to-sol transition had occurred.25,41 Reverting the strain to a constant value of 0.1% from 200 s resulted in Gʹ values greater than the Gʺ ones, indicating the system again exhibits gel-like character.41 The gel takes ca.720 s to recover almost all of its original strength after the withdrawal of the large strain; i.e., for Gʹ to recover the original value recorded at a low constant strain of 0.1% (Figure 2b).

Figure 2. (a) Frequency sweep rheological study of the CuBuPc organogel at a constant strain of 0.1% at 25 ℃; (b) Time dependent step-strain rheological study of the CuBuPc organogel at a fixed angular frequency of 1 rad/s at 25 ℃.

Rheoreversible organogels are potentially good candidates for doctor blading because the gelto-sol transition occurs when mechanical stimuli is applied. Ideally, the gel state returns after


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complete withdrawal of external physical stress, causing the gel to remain immobilized in the target area after coating to allow further processing.25 Drop casting a solution of CuBuPc in 1,2dichlorobenzene on indium tin oxide (ITO)-coated glass resulted in CuBuPc aggregating to form large solid grains with poor film formation and prominent coffee ring effect (Figure S3a, Supporting Information). However, the CuBuPc organogel formed a smooth transparent film on ITO glass by doctor blading (Figure S3b, Supporting Information), indicating the potential use of this process for large-area electronic device fabrication. Morphological Analysis. Morphological analysis was carried out with transmission electron microscopy (TEM) and atomic force microscopy (AFM) to investigate the effect of ultrasonic irradiation on microstructural changes of molecular aggregation of CuBuPc. Samples were prepared by drop casting CuBuPc solution (40 mg/mL) or solution obtained from the ultrasoundinduced CuBuPc organogel onto the TEM grids or SiO2/Si substrates and allowing them to dry under ambient conditions prior to testing. The morphological difference between the CuBuPc xerogel and drop-casting sample is manifested in the TEM and AFM images. The CuBuPc xerogel shows a well-defined 3D network composed of 1D nanofibrillars, while the casting sample from solution exhibits a relatively featureless and amorphous structure (Figure 3). These differences are similar to those reported for other xerogels.1,25,26 The AFM images (Figure 3a, b) reveal that the diameter of fibers varies from 35 to 60 nm, while the TEM image (Figure 3c) shows a fiber diameter of 90 to 180 nm. As seen from the images of two microscopic techniques, the two approaches for sample preparation show differences in the width of the nanofibers.25 The self-assembled CuBuPc fibers are several micrometers in length. The cross-linked fibrous network of the xerogel provides a three-dimensional matrix that may trap many small solvent


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molecules to afford a self-supportive gel.25 In contrast, the drop-casting CuBuPc sample exhibits an amorphous film structure (Figure 3d–f).

Figure 3. (a) Height, (b) phase AFM images and (c) TEM image of CuBuPc xerogel obtained from organogel; (d) Height, (e) phase AFM images and (f) TEM image of CuBuPc cast from solution.

X-ray Diffraction Analysis. The quality of the fabricated thin film samples was assessed using grazing incidence X-ray diffraction (Figure S4, Supporting Information). The organogel sample has a strong diffraction peak with a 2θ value of 6.4°, indicating a single molecular orientation with a d spacing of 14.0 Å and high crystallinity (Figure S4a, Supporting Information). However, a much weaker diffraction peak at 2θ value of 6.3° was present in the drop-casting sample (Figure S4b, Supporting Information), suggesting a thin film structure with significantly lower crystallinity. The substitution location on the peripheral ring of the Pc was not controlled, leading to the formation of multiple isomers, and a single crystal suitable for X-ray crystallographic structural determination could not be obtained. The molecular packing of the CuBuPc xerogel was examined by X-ray powder diffraction (XRPD) and selected-area electron diffraction (SAED)


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(Figure 4). The XRPD pattern of the CuBuPc xerogel shows four intense diffraction peaks at 2θ values of approximately 5.8°, 6.3°, 14.7°, and 26.4° (Figure 4a), which correspond to d spacing values of 15.2, 14.0, 6.0, and 3.37 Å, respectively. The spacing of 3.37 Å is attributed to strong π–π interactions. The XRPD d spacings of 14.0 and 3.37 Å are consistent with those obtained from SAED measurements (13.96 and 3.35 Å, Figure 4b). For comparison, XPRD measurement of CuBuPc solution drop-casting sample was performed (Figure S4c, Supporting Information). Consistent with GIXRD measurement, the XRPD pattern of the CuBuPc solution drop-casting sample exhibited similar diffraction signals at 2θ values of 6.6° and 14.8° with much weaker diffraction intensity. Moreover, no prominent diffraction peak at 2θ values of ~26° corresponding to π–π interactions was observed, indicating the poor crystallinity of as prepared sample from CuBuPc solution by drop-casting. Furthermore, due to its amorphous structure no diffraction pattern was observed from SAED measurement of solution drop casting sample.


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Figure 4. (a) XRPD pattern of CuBuPc xerogel; (b) SAED pattern of CuBuPc xerogel; (c) Proposed molecular packing diagram of CuBuPc xerogel.

Based on the assumptions that the n-butyl chain has a length of 1.2 Å per ‒CH2 unit and adopts an all-trans configuration, the theoretical molecular length of CuBuPc was calculated to be 20.2 Å by Gaussview and 6-311g+/B3LYP simulation. This length is larger than the d spacing values obtained from XRPD and SAED measurements, suggesting a π-stacked arrangement of CuBuPc molecules in the xerogel (Figure 4c). We propose that the CuBuPc molecules selfassemble along the π–π stacking axis during ultrasonic irradiation to form nanofiber structures that accept mutual interdigitation of n-butyl chains of neighboring Pc molecular stacks oriented at an angle of approximately 67° from the plane of the nanofiber growth axis. The strong π–π


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interaction along the one-dimensional nanostructure axis could be advantageous for charge transport in a planar structure transistor. Device Performance. We utilized the thixotropic property of the CuBuPc organogel to prepare thin films for use in OFETs using a doctor blade coating method (Figure 5). A tape mask was applied on the surface of a lithographically patterned bottom-contact FET substrate. A CuBuPc organogel film was formed by scraping the gel onto the uncovered FET area by a doctor blade, connecting the source and drain electrodes. After removal of the mask, the film was allowed to dry at 50 ℃ to afford an OFET device and measured in N2 glove box. A drop-casting OFET was also fabricated from CuBuPc solution and characterized for comparison. OFET fabrication and measurement are described in more detail in the Supporting Information. On each patterned substrate, more than 20 FET devices were fabricated and measured. The output and transfer characteristics of the fabricated transistors are illustrated in Figure 6 and Figure S5 (Supporting Information). Both types of devices were found to exhibit p-type FET behavior. The Ids versus Vg relation was used to extract the charge-carrier mobility in the saturation region.42

Figure 5. Doctor blade fabrication process of an OFET using the CuBuPc organogel.


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The CuBuPc xerogel-based OFET exhibited an on/off ratio of 103 and threshold voltage of 8.5 V, which were 102 and -6.3 V for CuBuPc drop-casting device, respectively. The highest charge carrier mobility of the CuBuPc xerogel-based OFET (3.0×10−2 cm2/Vs, Figure 6) is almost 50 times of magnitude higher than that of the drop-casting OFET (6.5×10−4 cm2/Vs, Figure S5, Supporting Information). Only five of 20 devices fabricated by drop-casting successfully exhibited field-effect performance. In contrast, 17 out of 20 devices fabricated by doctor blading displayed field-effect performance. This demonstrates the higher reliability and throughput of the doctor blading process using organogel than drop-casting using the solution. Although metal Pc and related macrocycles have been extensively studied over the past decades, the formation of a Pc organogel and utilization of this gel to fabricate OFETs by doctor blading has received little attention. Soluble Pc-based transistors with high carrier mobility (higher than 0.1 cm2/Vs) have been reported.9,16 However, most of these devices were fabricated by spin-coating. In this work, an organogel was easily fabricated by ultrasonic treatment of CuBuPc and used for organic electronic device fabrication by simple doctor blading. We believe this to be the first report of OFETs fabricated from a Pc-based organogel by doctor blading. The organogel fabricated transistor had a charge mobility of 0.030 cm2/Vs, and this value is considerably higher that of the OFETs constructed from a CuBuPc solution by drop-casting (6.5×10−4 cm2/Vs). This discrepancy in charge-carrier mobility can be attributed to better overlap of the CuBuPc molecules and higher crystallinity in the organogel43 caused by enhanced π–π stacking as a consequence of ultrasonic irradiation treatment.


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Figure 6. (a) Output and (b) Transfer characteristics of bottom-contact (channel length 100 µm, channel width 3000 µm) OFETs fabricated by doctor blading from CuBuPc organogel

Gel Formation Mechanism. The proposal of stronger π–π interactions was supported by a UV-Vis absorption study. The UV-vis spectrum of a CuBuPc xerogel film exhibits two absorption maxima at 622 and 696 nm in the Q-band range (Figure S2b, Supporting Information), which is a considerable red shift (19 and 12 nm, respectively) relative to those of the thin film sample prepared from CuBuPc solution. These results suggest enhanced π–π stacking between molecules of the ultrasound-induced organogel.44 Treatment of the CuBuPc solution with ultrasonic irradiation transformed the solution into an organogel with nanofibrous architecture. This transformation indicates that CuBuPc molecules underwent a reorganization via self-assembly, involving the breaking of intermolecular interactions and subsequent formation of new interactions. The shear forces induced by wave shocks generated at both the interfaces of microbubbles and in the bulk media during the sonication process is likely responsible for this molecular aggregation.32 Intermolecular π–π stacking interactions and attraction between alkyl chains play important roles in the self-assembly of the Pc molecules.3 However, these attractive forces can easily be


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destroyed by external physical stress.45 Sonication of a solution of CuBuPc causes shear force that disrupts the weak noncovalent interactions. The disassociated Pc molecules then undergo geometrical rearrangement and adopt an orientation that facilitates stronger intermolecular π–π stacking that is not disrupted by sonication. Only stable reconstructed interactions between molecules survive sonication, causing the Pc molecules to cluster into a noncovalently bonded matrix that captures the solvent. This process has been reported for other chemicals.31 The growing molecular network exhibits gel-like properties and can be used for OFET fabrication.46 CONCLUSION In summary, we introduce a new soluble Pc molecule, and investigate ultrasonic irradiation of a solution of the Pc as a simple approach to enhance intermolecular π–π stacking, resulting in the formation of a thixotropic organogel. Doctor blade processing was used to fabricate OFETs from this gel. The OFETs produced from the gel had a charge-carrier mobility 50 times of magnitude higher than that of OFETs produced by drop-casting. This research paves the way for the simple and economic preparation of OFET active layers. Future work is necessary to optimize device fabrication techniques and develop other Pc derivatives with ultrasound-induced gelling properties.

EXPERIMENTAL SECTION Synthesis of CuBuPc. The product was synthesized by 4-butylphthalonitrile and copper(II) chloride in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene in 1-pentanol. In detail please see the Supporting Information. Elemental analysis calcd (%) for C48H48CuN8: C, 72.02; H, 6.04; N, 14.00. Found: C, 71.89; H, 6.10; N, 13.89. ESI-MS: m/z (M+) 799.33. UV-Vis (CHCl3): λmax =


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678 nm (195625 L·mol⁻¹·cm⁻¹), 611 nm (43360 L·mol⁻¹·cm⁻¹). IR (KBr): ν = 2950, 2924, 2855, 1611, 1504, 1456, 1405, 1337, 1159, 1095, 1064, 819, 737, 720 cm-1. Gel preparation. A 1 mL solution of CuBuPc in 1,2-dichlorobenzene (at a concentration of 10, 20, or 40 mg/mL) was added to a glass vial and enclosed with a cap. The solution was sonicated (40 kHz, 200 W) for 30 min at 50 ℃ to form an organogel. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Material synthesis procedures, TGA data, UV-Vis spectra, Rheological measurement, Electron microscopy, GIXRD patterns, and FET fabrication and characterization (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ⊥

These authors contributed equally. The paper was written through contributions of all authors.

All authors have given approval to the final version of the paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation Youth Project (Project No. 21303081), Shenzhen overseas high level talents innovation plan of technical innovation project


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Ultrasound-Induced Organogel Formation Followed by Thin Film

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