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Enhanced and anisotropic charge transport in polymerbased thin-film transistors by guiding polymer growth Fu-Chiao Wu, Cheng-Chang Lu, Jrjeng Ruan, Fu-Ching Tang, Horng-Long Cheng, and Wei-Yang Chou Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01466 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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Crystal Growth & Design

Cover Page Enhanced and anisotropic charge transport in polymer-based thin-film transistors by guiding polymer growth Fu-Chiao Wu,† Cheng-Chang Lu,† Jrjeng Ruan,‡ Fu-Ching Tang,§ Horng-Long Cheng,† and Wei-Yang Chou*,† †

Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan

701, Taiwan ‡

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan

§

Department of Physics, National Cheng Kung University, Tainan 701, Taiwan Crystalline P3HT

Amorphous P3HT

Ag

Ag

Ag

Ag P3HT SiO2 Heavy-doped Si

Ag

Ag

Ag


Spin-coated P3HT

Ag

Ag

Ag


HMB-processed P3HT

We prepared and processed hexamethylbenzene (HMB)/poly(3-hexylthiophene) (P3HT) mixtures using a thermal gradient system to fabricate P3HT-based organic thin-film transistors (OTFTs). In the thermal gradient system, the HMB separated from the HMB/P3HT mixtures and crystallized along the sample movement direction. The crystallized HMB affected and guided the growth behavior of P3HT at the molecular level. Observations from joint microscopic and spectroscopic analyses revealed that the HMBprocessed P3HT (H-P3HT) thin film possessed a directional stripe microstructure, where the crystalline P3HT was abundant and attained improved molecular features and microstructural properties that resulted in more extended π-conjugation, lower reorganization energy, and stronger π–π overlaps than those of the spin-coated P3HT (S-P3HT) thin film. The improvements also augmented the intramolecular and intermolecular charge transport. The electrical performance of the H-P3HT OTFT was augmented significantly with respect to that of the S-P3HT OTFT. In addition, the H-P3HT OTFT exhibited an anisotropic charge transport property, correlating with microstructure directionality and resulting from the difference in the directions of the π–π overlaps.

*Wei-Yang Chou. Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan. E-mail: [email protected]

ACS Paragon Plus Environment

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Enhanced and anisotropic charge transport in polymer-based thin-film transistors by guiding polymer growth Fu-Chiao Wu,† Cheng-Chang Lu,† Jrjeng Ruan,‡ Fu-Ching Tang,§ Horng-Long Cheng,† and Wei-Yang Chou*,† †

Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung

University, Tainan 701, Taiwan. E-mail: [email protected] ‡

Department of Materials Science and Engineering, National Cheng Kung University, Tainan

701, Taiwan §

Department of Physics, National Cheng Kung University, Tainan 701, Taiwan

ABSTRACT Ideal molecular features and microstructural properties of organic semiconducting thin films are being explored to achieve high-performance organic thin-film transistors (OTFTs). We prepared and processed hexamethylbenzene (HMB)/poly(3-hexylthiophene) (P3HT) mixtures using a thermal gradient system to fabricate P3HT-based OTFTs. In the thermal gradient system, the HMB separated from the HMB/P3HT mixtures and crystallized along the sample movement direction. The crystallized HMB affected and guided the growth behavior of P3HT at the molecular level. Observations from joint microscopic and spectroscopic analyses revealed that


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the HMB-processed P3HT (H-P3HT) thin film possessed anisotropic and improved microstructures, particularly in crystalline domains. The improved molecular features and microstructural properties of the H-P3HT thin film enhanced the intramolecular and intermolecular charge transport by extending the π-conjugation, decreasing the reorganization energy, and strengthening the π–π overlaps. The electrical performance of the H-P3HT OTFT was augmented significantly with respect to that of the spin-coated P3HT OTFT. In addition, the H-P3HT OTFT exhibited an anisotropic charge transport property, correlating with microstructure directionality and resulting from the difference in the directions of the π–π overlaps. This effective and simple technique can be applied to other device types and has the potential to achieve high-performance organic electronic/photonic devices.


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INTRODUCTION Organic semiconductors have attracted considerable attention because of their unique advantages, such as low-temperature processing, flexibility, and easy fabrication across a large area. Thus, organic electronic/photonic devices are considered the next generation of electronic/photonic technologies, particularly in the field of wearable devices. The performances of organic electronic/photonic devices, including organic thin-film transistors (OTFTs), organic solar cells, and organic light-emitting diodes, are determined by the charge behavior in the active layers.1,2 Considering OTFTs, efficient charge transport is essential for high-performance devices. Charge transport is correlated with the microstructural properties of an active layer.1,3–6 For small-molecule active layers, charge transport mainly stems from adjacent molecules, socalled intermolecular charge transport.3,4,7,8 The order of molecules determines the distance that the charges can travel. For polymer active layers, charges can travel within a molecule and between adjacent molecules, which correspond to intramolecular and intermolecular charge transport, respectively.3,8–10 Depending on molecular conformation, a polymer thin film contains amorphous and crystalline regions. In crystalline regions, molecules exhibit extended conjugation lengths and form ordered structures, which result in efficient intramolecular and intermolecular charge transport. However, amorphous regions involve molecules with reduced conjugation lengths and form disordered structures that can interrupt the charge transport between crystalline regions. Thus, the interconnections of crystalline regions are important for efficient charge transport in a polymer thin film.8,9,11 In general, an amorphous/disordered microstructure is unfavorable for charge transport, whereas a crystalline/ordered microstructure is beneficial for charge transport.1,3–7 Consequently, the amelioration of the microstructural qualities of active layers is essential to the performance optimization of OTFTs.


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Various methods have been proposed to improve the microstructural properties of active layers and enhance OTFT performance. Several kinds of commonly used techniques exist, as follows: (1) Material design: New organic semiconducting molecules are developed to attain features of planar structures, extended π-conjugation, strong intermolecular π–π interactions, and easy formation of large crystalline domains.1,4,10,12 (2) Solvent selection: High-boiling-point solvents, solvent additives, and poor solvents are used to drive molecules to self-assemble and become crystalline domains.3,13–16 (3) Thermal annealing: Thermal energy is applied to organic thin films to supply molecules with kinetic energy and induce these molecules to evolve into ordered microstructures.3,17–19 (4) Binary blend: Adding secondary materials, such as semiconductors and insulators, to organic thin films facilitates the growth of homogeneous and ordered microstructures.20–22 (5) Meniscus-guided coating: This type of thin film process, which includes slot die coating, blade coating, capillary action, and solution shearing, provides an external force that induces molecules to form ordered and oriented microstructures.23–27 However, the design and synthesis of a new material with these advantages is difficult. The methods of solvent selection and thermal annealing are difficult to use for the effective manipulation of the evolution of crystalline/ordered domains, such as domain qualities and orientation, in organic thin films. Such approaches would be limited by the nature of materials, such as through the sensitivity of materials to solvent or heat. The addition of secondary materials complicates the microstructures of the organic thin films because of the coexistence of different materials. As a result, the microstructural properties become more difficult to manipulate than in other ways. Although meniscus-guided coating is effective in facilitating the formation of highly ordered and oriented domains of organic thin films, the molecular features inside such domains (domain qualities) are difficult to control directly. Moreover, the meniscus-guided coating systems are often


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sophisticated. Therefore, developing simple techniques capable of effectively manipulating the molecular features and microstructural properties of organic semiconducting thin films is necessary for achieving and commercializing high-performance OTFTs. Hexamethylbenzene (HMB), an easily crystallized organic material, was used to modify the microstructures of insulating polymer films by simple methods.28–30 Insulating polymer was dissolved in molten HMB to form a eutectic mixture. Through a cooling procedure, HMB crystallization effectively influenced the behavior of growth and/or crystallization of the insulating polymer. After HMB was removed, modified microstructures of the insulating polymer films were obtained. Recently, we successfully adopted HMB to manipulate the molecular features and microstructural properties of semiconducting polymer films, including the changes in the crystalline forms of poly(9,9-di-n-octyl-2,7-fluorene) and in the molecular organizations of poly(3-hexylthiophene) (P3HT).31,32 HMB is a suitable material for the microstructural modulation of organic semiconducting thin films at the molecular level. In this study, we used a technique that incorporates HMB into the thermal gradient system to fabricate P3HT-based OTFTs. The thermal gradient system successfully guided the direction of HMB crystallization. Then, the crystallized HMB steered the growth direction and modified the microstructures of P3HT. By joint microscopy and spectroscopy methods, we observed the improved and directional microstructures of the HMB-processed P3HT (H-P3HT) thin film. These microstructural properties enhanced the intramolecular and intermolecular charge transport. The H-P3HT-based OTFTs showed enhanced electrical performance and anisotropic charge transport property. The mechanism of microstructure-correlated charge transport in the HP3HT-based OTFTs is discussed subsequently.


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EXPERIMENTAL SECTION Sample preparation. Heavily doped silicon wafers covered with a 300 nm silicon dioxide layer were used as substrates. A semiconducting polymer, P3HT (Rieke Metals, 4002-E, regioregularity was 91-94%, molecular weight was 50-70K g/mol and polydispersity index was 2.2), was chosen as the active material and used without further purification. P3HT was dissolved in chloroform at a concentration of 0.05 wt%. For the control samples, the P3HT solution was spin-coated on the substrates (spin-coated P3HT; S-P3HT) at 2,000 rpm and baked at 90 °C for 60 min in a nitrogen-filled glove box. For the experimental samples, HMB (99%; Tokyo Chemical Industry) with a weight ratio of 95:5 (HMB/P3HT) was added to the P3HT solution. The 0.4 mL HMB/P3HT solution was dropped onto the substrates. After evaporating chloroform, the samples (HMB/P3HT mixtures) were covered with slides and transferred onto the thermal gradient system in the atmosphere. The system consisted of a longboard aluminum plate with a water circulation instituted for cooling. The eutectic temperature of the HMB/P3HT mixtures is approximately 180 °C, and the melting point of HMB is approximately 165 °C.32 Therefore, the plate was heated to 180 °C and controlled in the range of 150 °C to 180 °C by the water cooling system (Figure 1a). The samples were moved under a constant speed from the 180 °C point to the 150 °C point (Figure 1a). As the samples reached the point below the 165 °C spot, the HMB began to separate from the HMB/P3HT mixtures and crystallize.32 After this process, the samples were placed in vacuum for one day to completely remove HMB and form the H-P3HT samples. A 100 nm-thick silver film was thermally evaporated on the S-P3HT and H-P3HT samples through shadow masks to serve as source and drain electrodes to complete the top-contact OTFTs (Figure 1b). The channel width and length of the OTFT devices were 2000 and 100 µm, respectively.


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(a)

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(b) Sample

Ag

Ag P3HT SiO2

Heavy-doped Si

Figure 1. (a) Schematic of the thermal gradient system guiding the growth direction of crystalline P3HT. (b) Device architecture of a P3HT-based OTFT.

Characterization. The surface morphologies and optical properties of thin films were detected by atomic force microscopy (AFM; Park XE-100) and a polarizing microscope (Zeiss Axioskop 40), respectively. The absorption, Raman, and photoluminescence (PL) spectra of thin films were measured using a GBC Cintra 202 UV–Vis spectrometer with a resolution of less than 0.9 nm, a Jobin Yvon LabRAM HR spectrometer with a resolution of less than 0.4 cm−1 for the 532 nm laser and less than 0.2 cm−1 for the 633 nm laser, and a HORIBA iHR320 imaging spectrometer with a 532 nm excitation light source and resolution of 1 nm. The X-ray diffraction (XRD) patterns of thin films were recorded through a Rigaku RINT 2000 diffractometer with 1.5406 Å X-ray and scan step size of 0.01°. The electrical characteristics of the OTFTs were analyzed using a semiconductor parameter analyzer (Keithley 4200SCS) in a nitrogen-filled glove box.

RESULTS AND DISCUSSION Microstructural features of thin films. Figure 2 shows the surface morphologies of P3HT thin films from different processes. The S-P3HT thin film presents a surface morphology of randomly


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distributed small domains (Figure 2a). This topography is commonly observed in a spin-coated polymer thin film. However, in contrast to the surface morphology of the S-P3HT specimen, that of the H-P3HT thin film shows directional stripe patterns. During the movement of the H-P3HT specimen from the 180 °C point to the 150 °C point, HMB molecules began to separate from the P3HT/HMB mixture and crystallize along the movement direction of the specimen. Meanwhile, HMB crystallization helped guide the growth of the P3HT molecules along the movement direction of the specimen. Therefore, the morphology of the H-P3HT thin film evolved into directional stripe patterns (Figure 2b).

(a)

(b)

Figure 2. AFM topographies of the (a) spin-coated and (b) H-P3HT thin films.

The growth of P3HT molecules in a thin film includes amorphous and crystalline portions. We aim to improve the order and orientation of the crystalline P3HT in a P3HT thin film. The optical properties of the S-P3HT and H-P3HT thin films were examined through a polarizing microscope. Figure 3 shows the polarizing microscopic images of the S-P3HT and H-P3HT thin films. All of the images of the S-P3HT specimen at the different rotation angles of the polarizer are in complete darkness (Figure 3a). The crystalline P3HT in the S-P3HT thin film is randomly


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distributed, thus rendering the overall crystalline portion isotropic. The amorphous portion of P3HT essentially exhibits no directionality, again showing an isotropic pattern. As a result, all of the images of the S-P3HT specimen are dark. By contrast, the H-P3HT specimen presents images of directional stripe patterns with a dark pink color under various rotation angles of the polarizer (Figure 3b). This result is consistent with the AFM image. With the aid of HMB, the crystalline P3HT in the H-P3HT thin film grew along an identical direction. Thus, the overall crystalline portion appears anisotropic, resulting in directional stripe patterns on the images of the H-P3HT specimen. Some dark parts are also visible in the images. These dark parts may have resulted from the amorphous P3HT in the H-P3HT specimen.

(a)

0°

0°

45°

45°

90°

90°

(b)

Figure 3. Polarizing microscopic images of the (a) spin-coated and (b) H-P3HT thin films under the various rotation angles of the polarizer.


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Figure 4a shows the normalized absorption spectra of S-P3HT and H-P3HT thin films. In the absorption spectrum of a P3HT thin film, the absorption in the range greater than 2.3 eV mostly resulted from the amorphous P3HT and that less than 2.3 eV mainly stemmed from the crystalline P3HT.33–35 Thus, the absorption spectra of the two specimens were normalized by the absorbance at 2.3 eV (Figure 4a). The absorbance of the H-P3HT thin film is slightly higher than that of the S-P3HT thin film in the absorption range greater than 2.3 eV, indicating the existence of a slightly more amorphous P3HT in the H-P3HT specimen. Compared with the absorption spectrum of the S-P3HT thin film, that of the H-P3HT thin film presents a tail state in the lowenergy portion. This finding reflects the entangling of P3HT in the H-P3HT specimen.36 At approximately 2 eV (shoulder position), the absorbance of the H-P3HT specimen is higher than that of the S-P3HT specimen. This finding indicates that the amount of the crystalline P3HT in the H-P3HT specimen is larger than in the S-P3HT specimen and their features in the two specimens are dissimilar.33,34,36 The absorption from the crystalline P3HT can be identified by fitting the absorption spectrum of a P3HT thin film using the modified Franck–Condon equation, as follows:33,34,36 2

  e − S S m   We− S A ∝ ∑ Gm  Γ ( hω − E0−0 − mEP ) ,  1 − m!   2 EP m=0  

(1)

where A is the relative absorbance, m is the vibrational level, S is the Huang–Rhys factor assumed to be 1 in this study,33,34 W is the exciton bandwidth, EP is the energy of the main vibrational mode of P3HT coupled with the electronic transition, Gm is a constant equal to Σn(≠m)Sn/n!(n − m) (n is the vibrational quantum number), Γ is a Gaussian function, ω is the


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vibrational frequency, and E0–0 is the energy of the 0–0 electronic transition. Therefore, the absorption spectra of S-P3HT and H-P3HT thin films were fitted using eq (1) to analyze the features of the crystalline P3HT (Figures 4b and 4c). In the fitting procedure, the variable factor of eq (1) was W. A specific value of W for each specimen was selected to achieve the best fitting result. As shown in Figures 4b and 4c, the W of the S-P3HT specimen is larger than that of the H-P3HT specimen. Considering the free excitons, the exciton bandwidth was correlated to excitonic coupling (J), as indicated by the formula W = 4J.33 For P3HT, the excitonic coupling and the effective conjugation length (Leff, defined as the number of repeat units) of the molecules followed the relationship J ~ Leff−1.81.37 Thus, Leff of the S-P3HT and H-P3HT specimens can be estimated to be 32 and 44, respectively. Leff of the crystalline P3HT in the H-P3HT thin film is longer than that in the S-P3HT thin film.

Normalized absorbance (a.u.)

1.2

crystalline

amorphous

0.9

(a) 0.6 0.3 Spin-coated HMB-processed

0.0 1.6

2.0

2.4 Energy (eV)

2.8

3.2

(c) Absorbance (a.u.)

(b) Absorbance (a.u.)

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

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W = 51 meV

W = 90 meV

1.8

2.1 2.4 2.7 Energy (eV)

3.0

1.5

1.8

2.1 2.4 2.7 Energy (eV)

3.0


12

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Figure 4. (a) Normalized absorption spectra of the different P3HT thin films. The dashed line indicates the position of 2.3 eV. Experimental (open circles) and theoretical (from the crystalline portion, red lines) absorption spectra of the (b) spin-coated and (c) H-P3HT thin films. The values of the exciton bandwidth (W) of the specimens are also shown.

The two kinds of P3HT thin films were investigated through their out-of-plane XRD patterns to discuss the properties of the crystalline P3HT further (Figure 5a). The diffraction peak at approximately 5.38° (2θ) reflects the (100) lattice plane of stacked edge-on P3HT molecules along the a-axis, which forms the lamellar supramolecular structures (Figure 5b).38,39 The twodimensional grazing incidence XRD (2D GIXRD) patterns of P3HT thin films present the (100) and (010) diffraction rings (Figure S1), which respectively indicates the existence of the edge-on and face-on P3HT stacking along the a-axis in P3HT thin films.38,39 The intensity of the (100) diffraction ring is much higher than the (010) diffraction ring. Thus, the dominant molecular configuration of P3HT in P3HT thin films is the edge-on type. The intensity of the (100) peak of the H-P3HT specimen is stronger than that of the S-P3HT specimen (Figure 5a). Moreover, the intensity of the diffraction rings in the 2D GIXRD pattern of the H-P3HT specimen is also higher than that of S-P3HT specimen (Figure S1). These results indicate the existence of a greater amount of the crystalline portion in the H-P3HT thin film than in the S-P3HT thin film. The HP3HT specimen exhibits (100), (200), and (300) peaks, in contrast to the S-P3HT specimen, which presents only a single (100) peak. The 2D GIXRD pattern of the H-P3HT specimen also shows more obvious diffraction rings than that of the S-P3HT specimen (Figure S1). The halfwidth of the (100) peak (β) of the H-P3HT specimen is smaller than that of the S-P3HT specimen (Figure 5a). These observations signify that the order and orientation of the crystalline


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portion was better in the H-P3HT thin film than in the S-P3HT thin film. β was correlated with the Debye–Scherrer dimension of coherently scattering crystalline domains. The size of the crystalline P3HT along the a-axis (La) in thin films (Figure 5b) can be estimated by adopting the Scherrer equation, as follows:36,40

La =

Kλ , β × cos θ

(2)

where K is the Scherrer constant and a value of 0.9 was used herein,40 λ is the X-ray wavelength, and θ is the diffraction angle. La of the H-P3HT specimen (11.05 nm) is longer than that of the SP3HT specimen (9.47 nm). Furthermore, the size of the crystalline P3HT along the c-axis (Lc) in thin films (Figure 5b) is relevant to Leff. From the theoretical calculation, the length of one repeat unit of a P3HT molecule is approximately 3.94 Å. Given the Leff value, Lc of the H-P3HT (17.32 nm) and S-P3HT (12.6 nm) specimens were obtained. By combining La with Lc, the dimensions of the crystalline P3HT in the thin films were determined (Figure 5b). The H-P3HT specimen shows larger dimensions of the crystalline P3HT than those of the S-P3HT specimen.

Crystalline domain

(b)

(a)

a-axis a-axis

(100)

S

S S

S

S S

S

S S

S

S

S S

S

S S

S S

S

S S

S S

S

S S

S S

S

S S

S

S

S S

S S

S S

S

S S

S

S S

S

b-axis b-axis c-axis // substrate

β

c-axis // polymer main chain

Lc (300)

Spin-coated

0.84 10 15 2θ (degree)

20

HMB-processed

11.05

(200)

9.47

0.72

5

S S

La

Spin-coated HMB-processed

Intensity (a.u.)

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

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12.60

17.32 (Unit: nm)


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Figure 5. (a) XRD patterns of P3HT thin films from different processes. The dashed line indicates the position of the (100) peak. The half-width of the (100) peaks (β) is also shown. (b) Illustration of the dimensions of the crystalline domain in different P3HT thin films. La and Lc are the crystallite sizes along the a- and c-axes, respectively.

The microstructural properties of the S-P3HT and H-P3HT thin films were analyzed by Raman spectroscopy. From the absorption spectrum of a P3HT thin film, 532 and 633 nm lasers were adopted to examine the amorphous and crystalline P3HT, respectively. The Raman spectra of different specimens are shown in Figure 6. For the amorphous P3HT (Figure 6a), the selected band at approximately 1,448 cm−1, denoted as the v band, reflects a vibrational mode that originates from the symmetric C=C stretching deformation in the aromatic thiophene rings of a P3HT molecule.41 However, with respect to the crystalline P3HT (Figure 6b), the v band shifts to approximately 1,444 cm−1. The v band follows the exponential decay law to shift to a lower wavenumber with the increasing Leff of a P3HT molecule.35 Given the longer Leff of the P3HT molecules in the crystalline region than in the amorphous region, a downshift of the v band of the crystalline P3HT is observed. As shown in Figure 6, the half-width of the v band of the H-P3HT specimen is narrower than that of the S-P3HT specimen, either for the amorphous or crystalline region. This observation signifies that the H-P3HT thin film achieved a more homogeneous distribution of molecular conformations, that is, better microstructures, than those of the S-P3HT thin film. Figure 7 shows the PL spectra of the S-P3HT and H-P3HT thin films. Compared with the S-P3HT specimen, the position of the maximum PL intensity of the H-P3HT specimen shifts from 644 nm to 704 nm. For P3HT, the stronger PL intensity at the low-energy (longwavelength) position than at the high-energy (short-wavelength) position reflects the existence of


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a more ordered microstructure in a thin film.5,42 Consequently, the microstructures of the HP3HT thin film are superior to that of the S-P3HT thin film, coinciding with the observations from the Raman spectra.

v βv

Intensity (a.u.)

v

(b) 37.5 31.9

βv

Intensity (a.u.)

(a)

27.3

24.5


1410

1440 1470 -1 Raman shift (cm )


1500

1410

1440 1470 -1 Raman shift (cm )

1500

Figure 6. Raman spectra of different P3HT thin films excited by (a) 532 nm and (b) 633 nm lasers. The dashed line denotes the position of the v band. The half-width of the v band (βv) is also displayed.


Intensity (a.u.)

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600

650

700 750 800 Wavelength (nm)

850

900

Figure 7. PL spectra of the P3HT thin films from different processes. The wavelength of the excitation light source was 532 nm.


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Electrical performances of the OTFTs. The S-P3HT and H-P3HT thin films were used as the active layers of bottom-gate and top-contact thin-film transistors (TFTs) (Figure 1b). The morphology of the H-P3HT thin film exhibits directional stripe patterns. In a TFT structure, these directional stripes can be arranged to be parallel or perpendicular to the source and drain electrodes (Figure 8). The H-P3HT devices with stripes parallel and perpendicular to the electrodes are defined as the PA-H-P3HT and PE-H-P3HT devices, respectively. The electrical characteristics of various P3HT-based OTFTs are presented in Figure 9. The S-P3HT device has a hole mobility (µh) of 1.7 × 10−3 cm2/Vs, on/off current ratio (Ion/Ioff) of 2.1 × 102, and subthreshold swing (SS) of 12.7 V/dec. In the transfer curve of the S-P3HT device, the gate current (IG) varies with the change in the drain current (ID) (Figure 9d), indicating the existence of current leakage pathways in the device. Compared with that of the S-P3HT device, the PA-HP3HT device attained an approximately two-order increase in ID (Figures 9b and 9e). Moreover, Ioff and IG of the PA-H-P3HT device were at the order of 10−11 to 10−10 A, and IG was independent of ID (Figure 9e). Thus, the PA-H-P3HT device enhanced µh to 8 × 10−2 cm2/Vs, boosted Ion/Ioff to 2.5 × 105, and achieved an excellent SS at 1 V/dec. However, in contrast to the PA-H-P3HT device, the PE-H-P3HT device achieved a smaller ID and higher Ioff and IG at the order 10−9 A (Figures 9c and 9f). In particular, the output and transfer curves (other than the IG variation) of the PE-H-P3HT device are similar to those of the S-P3HT device. Thus, the PE-HP3HT device performed with electrical parameters close to those of the S-P3HT device, that is,

µh of 7.3 × 10−4 cm2/Vs, Ion/Ioff of 1.5 × 102, and SS of 18.5 V/dec. The statistical electrical parameters of different P3HT-based TFT devices are summarized in Table S1.


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Ag

Ag

Ag

Ag

P3HT SiO2 Heavy-doped Si


(a)

(b)

Figure 8. Device architectures of the H-P3HT OTFTs with the stripe morphology (a) parallel and (b) perpendicular to the Ag electrodes.

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Figure 9. Output (left panels) and transfer (right panels) curves of the (a, d) spin-coated P3HT OTFT and H-P3HT OTFT with stripe morphology (b, e) parallel and (c, f) perpendicular to the source and drain electrodes.

Microstructure-correlated charge transport. In the S-P3HT thin film, the crystalline P3HT attained a short Leff and small crystallite size. The amount of the crystalline P3HT was small. The crystalline P3HT was randomly distributed in the thin film and lacked directionality. The order and orientation of the crystalline P3HT were poor. An inferior microstructure is presented by the S-P3HT thin film in Figure 10a. The short Leff of the P3HT molecules produced a high reorganization energy detrimental to intermolecular charge transport37,43,44 and a reduced πconjugation unfavorable for intramolecular charge transport. From the XRD spectra, the most molecular configuration of P3HT on the substrates was of the edge-on type. Therefore, in the SP3HT OTFT, the randomly distributed crystalline P3HT without directionality and high order caused a weak π–π overlapping between adjacent crystalline domains along the source-to-drain direction and impeded the connections between domains (intermolecular charge transport). As such, the S-P3HT decreased the charge mobility (Figure 10a). The randomly distributed crystalline domains also formed strong π–π overlaps in the directions other than that of the source-to-drain direction and produced current leakage pathways. Moreover, the occurrence of the unintentional doping in the S-P3HT thin film could serve as current leakage pathways.45,46 These unfavorable microstructural properties resulted in the poor electrical performance of the SP3HT OTFT. Opposite to that in the S-P3HT thin film, a large amount of crystalline P3HT existed in the H-P3HT thin film and presented long Leff, large crystallite size, directionality, and


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improved order and orientation. In Figures 10b and 10c, the H-P3HT thin film shows superior microstructures. The long Leff of the P3HT molecules facilitated intermolecular charge transport because of low reorganization energy37,43,44 and intramolecular charge transport due to extended π-conjugation. In addition, HMB could help remove some of the unintentional doping (current leakage pathways) in P3HT during the formation of the H-P3HT thin film. However, with regard to the TFT device, the directionality of the crystalline P3HT was correlated with the electrical performance of the H-P3HT OTFT (Figure 9). For the PA-H-P3HT OTFT, the π–π stacking direction in the crystalline P3HT was identical to the source-to-drain direction, and the molecular configurations between adjacent crystalline domains tended to assume a face-to-face type of configuration, which generated strong π–π overlaps and domain interconnections (Figure 10b).44,47 As a result, the intermolecular charge transport along the source-to-drain direction was enhanced. Furthermore, these directional crystalline domains diminished the π–π overlap in the directions aside from that of the source-to-drain direction, suppressing the formation of current leakage pathways. These favorable microstructural features augmented the electrical performance of the PA-H-P3HT OTFT. For the PE-H-P3HT OTFT, the π–π stacking direction in the crystalline P3HT was perpendicular to the source-to-drain direction (Figure 10c) and provided current leakage pathways. In the source-to-drain direction, although charges traveled within P3HT molecules (intramolecular charge transport), the charge transport between crystalline domains (intermolecular charge transport) became difficult. This occurrence was due to the molecular configurations between crystalline P3HT, which assumed an edge-to-edge type. This type of configuration lacks π–π overlaps and obstructs intermolecular charge transport (Figure 10c).47 Thus, similar to that in the S-P3HT OTFT, such inferior microstructural properties worsened the electrical performance of the PE-H-P3HT OTFT relative to that of the


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PA-H-P3HT OTFT (Figure 9). Although the PA-H-P3HT OTFT exhibited better electrical characteristics, excess amorphous and entangled P3HT were retained in the H-P3HT thin film (Figure 4a), which can hinder charge transport. If such undesirable microstructures can be diminished, the electrical performance of the PA-H-P3HT OTFT would be further improved.

Crystalline P3HT

Amorphous P3HT

Ag

Ag

Ag

Ag

Ag

Ag

Ag

Ag

Ag

Ag

Ag

Ag




(a)

(b)

(c)

Figure 10. Schematic of the microstructures within active layers of (a) spin-coated P3HT OTFT and H-P3HT OTFT with stripe morphology (b) parallel and (c) perpendicular to the Ag electrodes. The green dotted lines indicate the paths of hole transport.

CONCLUSIONS The microstructures of a P3HT thin film were significantly improved by incorporating HMB into a thermal gradient system. Through the HMB-guided growth of P3HT molecules, the H-P3HT thin film achieved a directional stripe microstructure, where the crystalline P3HT was abundant and attained enhanced features that resulted in more extended π-conjugation, lower


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reorganization energy, and stronger π–π overlaps than those of the S-P3HT thin film. The improvements also augmented the intramolecular and intermolecular charge transport. The direction of the stripe patterns of the H-P3HT thin film was relevant to the electrical performance of the P3HT-based OTFT. In the PA-H-P3HT OTFT, the direction of most of the π–π overlaps was consistent with that of the charge transport from source to drain. This behavior improved the electrical performance of the device. By contrast, the PE-H-P3HT OTFT exhibited poor electrical characteristics similar to those of S-P3HT OTFT because of the perpendicular direction of most of the π–π overlaps with respect to the source-to-drain direction. We proposed an effective and simple technique for enhancing the electrical performance of solution-processed polymer-based OTFTs. This technique can be easily applied to other solution-processable organic semiconductors and has the potential to achieve and commercialize high-performance solution-processable organic electronic/photonic devices.

ASSOCIATED CONTENT Supporting Information. Two-dimensional grazing incidence XRD patterns of thin films. Electrical parameters of various P3HT-based OTFT devices. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology, Taiwan, through Grant MOST 105-2112-M-006-003- MY3, and prof. Hwo-Shuenn Sheu in National Synchrotron Radiation Research Center, Taiwan, for his equipment support and cooperation.

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For Table of Contents Use Only Enhanced and anisotropic charge transport in polymer-based thin-film transistors by guiding polymer growth Fu-Chiao Wu, Cheng-Chang Lu, Jrjeng Ruan, Fu-Ching Tang, Horng-Long Cheng, and Wei-Yang Chou* -5

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Spin-coated P3HT -11

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The HMB-processed P3HT thin film presented improved molecular features and microstructures with directionality that extended π-conjugation, decreased reorganization energy and strengthened π–π overlaps. The improvements facilitated the intramolecular and intermolecular charge transport. The improved and directional microstructures of the HMB-processed P3HT thin-film transistor resulted in efficient anisotropic charge transport and enhanced electrical performance.


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Enhanced and Anisotropic Charge Transport in ... - ACS Publications

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