Synthesis, Morphology, and Field-Effect Transistor Characteristics of

Apr 3, 2013 - We report the synthesis, morphology, and the field effect transistor (FET) characteristics of the crystalline diblock copolymers of ...
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Synthesis, Morphology, and Field-Effect Transistor Characteristics of Crystalline Diblock Copolymers Consisted of Poly(3-hexylthiophene) and Syndiotactic Polypropylene Jing-Yu Lee,† Chih-Jung Lin,‡ Chen-Tsyr Lo,‡ Jing-Cherng Tsai,*,† and Wen-Chang Chen*,‡ †

Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 62142, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan

‡

S Supporting Information *

ABSTRACT: We report the synthesis, morphology, and the field effect transistor (FET) characteristics of the crystalline diblock copolymers of poly(3-hexylthiophene) and syndiotactic polypropylene (P3HT-b-sPP). Four diblock copolymers with various sPP block lengths, P3HT16K-b-sPP3K (P1), P3HT 1 6 K -b-sPP 6 K (P2), P3HT 1 6 K -b-sPP 9 K (P3), and P3HT16K-b-sPP14K (P4), were prepared by the click coupling of N3-capped sPP and ethynyl-capped P3HT. The stereoregular crystalline block sPP developed different types of molecular stacking structures and led the P3HT domains to pack lamellar edge-on structure with improved charge transporting characteristics, as evidenced by the grazing incidence wideangle X-ray scattering (GIWAXS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The FET hole mobilities of P1−P3 thin films were 4.15 × 10−3, 4.16 × 10−2, and 3.95 × 10−3 cm2 V−1 s−1, respectively, which were up to 1 order of magnitude higher than that of the parent P3HT thin film (1.43 × 10−3 cm2 V−1 s−1). The crystalline-stereoregular crystalline diblock P3HT-b-sPP demonstrates that using the lattice matching concept could well clarify the molecular stacking structure of conjugated polymer segments in order to further improve the performance of organic electron devices.

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poly(L-lactide).30,31 In contrast to rod−coil block copolymers with an amorphous insulated coil block, the diblock copolymers consisting of a P3HT rod and a crystalline insulated block have been explored recently. Müller and co-workers reported the FETs based on the block copolymers of P3HT and polyethylene (PE) with the saturated FET hole mobility of 0.05 cm2 V−1 s−1, but the semicrystalline PE required a relatively high processing temperature.21 We previously discovered that the FET hole mobility of P3HT-blockpoly(steryl acrylate) could be higher than that of the parent P3HT at a low P3HT composition. For the crystalline− crystalline diblock copolymers, the morphology and molecular structure would result from the interplay between the strength of the crystallization driving force and microphase-separated ability.32−36 The well-oriented crystalline structures are obtained from epitaxial crystallization of structural similar (lattice matching) crystalline block near the crystalline domains.32−34 However, stereoregular coil block has not been employed in conjugated rod−coil block copolymers yet, as far as we know. In this study, crystalline-stereoregular crystalline diblock copolymers, poly(3-hexylthiophene)-block-syndiotactic poly-

INTRODUCTION Organic semiconductors have been extensively studied for flexible electronic devices due to their electronic characteristics, light weight, low cost, flexibility, and solution processability.1−4 Poly(3-hexylthiophene) (P3HT) is considered as one of the most promising materials for field effect transistor (FETs) and organic photovoltaics (OPVs) due to the high carrier mobility and solution processability. While the device performance highly depends on both the molecular packing and the microstructure, P3HT-based devices typically show limited performance due to the difficulty in controlling its phase behavior based on its inherent chain rigidity. Conjugated rod−coil block copolymers containing a semiconducting polymer segment and an insulating flexible block are promising to stack highly ordered structures and improve the charge transporting characteristics and the resulting device performance.5−11 Numerous insulating polymer blocks have been incorporated with the P3HT block segment for tuning the molecular stacking structures and morphology, including P3HT-block-polystrene,12,13 P3HT-block-poly(fluorinated alkyl methacrylate),14 P3HT-block-poly(2-vinylpyridine),15 P3HTblock-poly(4-vinylpyridine),16 P3HT-block-poly(methyl acrylate),17,18 P3HT-block-poly(methyl methacrylate),19,20 P3HTblock-polyethylene,21−23 P3HT-block-poly(steryl acrylate),24 P3HT-block-poly(lactide),25−27 P3HT-block-poly(ethylene glycol),28 P3HT-block-poly(tetrahydrofuran),29 and P3HT-block© 2013 American Chemical Society

Received: February 21, 2013 Revised: March 26, 2013 Published: April 3, 2013 3005

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Scheme 1. Synthetic Scheme for the P3HT-b-sPP by Click Coupling of N3-Capped sPP and Ethynyl-Capped P3HT

Table 1. Polymerization Conditions, Molecular Weights, and Thermal Properties of Azide-Capped sPP, Ethynyl-Capped P3HT, and the Corresponding Diblock Copolymers, P3HT-b-sPP Tce (°C) a

entry

polymer

A1 A2 A3 A4 C1 C2 C3 C4 P1 P2 P3 P4

P3HT1 P3HT2 P3HT3 P3HT4 sPP3K sPP6K sPP9K sPP14K A1-b-C1 A2-b-C2 A3-b-C3 A4-b-C4

Mnb

(g/mol)

16 000 15 300 15 500 16 200 3 400 6 200 9 000 14 600 19 000 21 000 23 000 31 800

PDI

b

1.13 1.16 1.14 1.09 1.25 1.31 1.34 1.42 1.13 1.20 1.19 1.30

c

[rrrr] (%)

Tdd

87.5 87.8 88.1 88.7

(°C)

279.64 358.76 371.46 367.76 418.35 424.60 415.34 407.92

Tme (°C)

sPP

P3HT

sPP

P3HT

58.16 61.74 61.03 88.52

190.16 190.50 187.37 183.16

118.62 133.25 141.01 147.10

223.12 225.18 223.32 222.55

Propylene polymerization condition: toluene 50 mL; [Zr] = 2.5 × 10−6 mol; Al/Zr = 2000; propylene pressure = 1 bar; time = 30 min; temperature, Tp = 25 °C. bMolecular weight (Mn) and molecular weight distribution (Mw/Mn, PDI) were determined by high-temperature GPC at 110 °C with 1,2,4-chlorobenzene as solvent. cSyndiotactity (rrrr) was determined by 13C NMR analyses. dDecomposed temperature (Td) was obtained from TGA analyses. eCrystallization temperature (Tc) and melting point (Tm) were obtained from DSC analyses. a

successful preparation of stereoregular PP-based diblock copolymers was demonstrated through postpolymerization of end-functionalized sPP, which were generated via metallocenecatalyst-mediated selective chain transfer reactions.40,43−48 The end-functionalized sPP that can be used to generate azidecapped sPP and couple with the ethynyl-terminated P3HT to offer the P3HT-b-sPP, as illustrated in Scheme 1. The molecular structures and morphologies of the prepared P3HT-b-sPP thin films were investigated by grazing incidence wide-angle X-ray scattering (GIWAXS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The charge carrier mobilities were obtained on untreated SiO2/ Si substrate by FETs. It is found the stereoregular crystalline block sPP could develop different types of stacking structure, resulting in the effect on the P3HT domain packing and further the field-effect mobility.

propylene (P3HT-b-sPP), were prepared and used for investigating the relationships between the molecular packing structures, morphologies, and their field effect transistor characteristics. To date, P3HT-based diblock copolymers can be typically prepared via three synthetic routes: (i) sequential polymerization of different thiophene monomers for producing rod−rod all conjugated block copolymers via nickel- or palladium-catalyzed quasi-living chain growth polycondensation reactions,37,38 (ii) generation of the end-functionalized P3HT by Grignard metathesis for connecting onto other polymer blocks via controlled living polymerization reactions, and (iii) combining the separately synthesized alkynyl end-capped P3HT and azide-capped polymers, for the construction of diblock copolymers through “click” chemistry.28,39−42 Despite quasi-living polymerization, route i offers the direct syntheses of block copolymers; restrictions on the structures of monomers hamper the utilization of this synthetic route for preparing P3HT-based rod−coil diblock copolymers. Also, the sPP coil block of our targeting P3HT-b-sPP diblock copolymer is unable to be synthesized via living polymerization reactions. Recently,

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EXPERIMENTAL SECTION

Synthesis. General Procedure. All reactions and manipulations were conducted under a nitrogen atmosphere using the standard

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the resulting reaction solution was allowed to warm to room temperature and then kept at room temperature for an additional 1 h. After that, the reaction solution was charged with 0.8 mL of ethynylmagnesium bromide (0.5 M in THF). After stirring at room temperature for 20 min, the reaction solution was quenched with excess methanol, which led to the deposition of the ethynyl-capped P3HT as a purple precipitate. The polymer product was isolated by filtration and washed sequentially with excess methanol and then with heptane. The resulting purple product was then dried under vacuum for 24 h to provide 0.42 g of the ethynyl-capped P3HT as a purple solid (yield: 91%) [Mn = 15 300 g/mol, Mw/Mn = 1.16 by GPC (in 1,2,4-trichlorobenzene at 110 °C)]. Synthesis of P3HT-b-sPP. Representative experiment (for entry P2 of Table 1): A 250 mL Schlenk tube, equipped with a magnetic stirrer, was allowed to charged sequentially with 0.2 g of ethynyl-capped P3HT (Mn = 15 300 g/mol, Mw/Mn = 1.16), 0.13 g of azide-capped sPP (Mn = 6200 g/mol, Mw/Mn = 1.30), 1.5 mg of CuBr (10 μmol), 2.0 mg of PMDETA (11 μmol), and 40 mL of THF. The mixture was degassed by one freeze−pump−thaw cycle and was maintained at 50 °C. After stirring for 24 h, the reaction mixture was passed through a short column of neutral alumina to remove the residual catalyst. The purification of P3HT-b-sPP block copolymer was completed in two steps. First, the excess sPP (soluble in boiling hexane) was removed by Soxhlet extraction in boiling hexane. The resulting hexane insoluble fraction was extracted with THF/cyclohexane (1:1) to remove the insoluble P3HT homopolymer. The collected THF/cyclohexane solution was charged with excess methanol, which led to the deposition of the P3HT-b-sPP as a purple precipitate. The resulting precipitate was isolated by filtration and dried under vacuum overnight to provide 0.25 g of the pure P3HT-b-sPP block copolymer (yield: 89%). [Mn = 21 000 g/mol, Mw/Mn = 1.20 by GPC (in 1,2,4trichlorobenzene at 110 °C)]. Device Fabrication. The FET devices using the heavily doped (n+2) silicon (100) wafer as the substrate and a 300 nm SiO2 layer as the gate insulator were thermally grown onto the silicon wafer. Prior to deposit, the wafers are rinsed with toluene, acetone, and IPA in sequence. The P3HT-b-sPP solution (3 mg/mL) was dissolved in anhydrous CF. The polymer solution was then heated to 40 °C for 1 h to make sure the polymer is well-dissolved and then are cooled at room temperature. The thin film was spin-coated at 600 rpm for 60 s onto the bare SiO2/Si substrate in a N2-filled glovebox and vacuumed for 8 h. The top-contact source and drain electrode were defined by a 100 nm thick Au layer through a regular shadow mask, where the channel length (L) and width (W) were 50 and 1000 μm, respectively. Characterization. The molecular weight and molecular weight distribution (MWD) were determined through GPC (Waters 150CALAC/GPC) with a refractive index (RI) detector and a set of UStyragel HT columns with 106, 105, 104, and 103 pore sizes in series. The measurements were taken at 110 °C using 1,2,4-trichlorobenzene as the solvent. Polystyrene (PS) samples with narrow MWDs were used as the standards for calibration. The standards were in the range of absolute molecular weight from 980 to 2 110 000 g/mol; the R square of the ideal calibrated line was limited to up to 0.999. All 1H and 13C NMR spectra were recorded on a Bruker AV-500 NMR spectrometer. The P3HT-based samples were dissolved in CDCl3. The recorded temperature was 50 °C. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were measured under a nitrogen atmosphere at the heating rate of 10 and 5 °C/min using a TA Instruments (TGA-951 and DSC-910S, respectively), where the DSC measurement was heated with both 10 and 5 °C/min. UV−vis absorption spectra were measured with Hitachi U4100 spectrophotometer. Grazing incidence wide-angle X-ray scattering (GIWAXS) patterns were recorded from Rigaku Nano Viewer. The incident angle and scan range of GIWAXS measurement are 0.23° and 2−30°, respectively. Transmission electron microscope (TEM) images were obtained with JEOL JEM-1230 at 100 kV with a GATAN DualVision CCD. The thin film samples for TEM characterization were prepared by spin-coating at 600 rpm for 60 s on the copper grids with carbon, which were fixed on glass slides. Note that the surface

Schlenk line or drybox techniques. Solvents and common reagents were commercially obtained and used either as received or purified by distillation from sodium/benzophenone. Propylene (purity >99.9%, Matheson), triethylaluminum (TEA, 1 M in hexane, Aldrich), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, purity >98%, Aldrich) were used as received. Methylaluminoxane (MAO, 14% in toluene), purchased from Albemarle, was dried under vacuum to remove residual TMA.49 The resulting TMA-free MAO was diluted in toluene to the desired concentration before use. Me2C(Cp)(Flu)ZrCl2,50,51 2-bromo-3-hexylthiophene,52 and lithium diisopropylamide (LDA)52 were synthesized using the method described in the literature.50−52 Preparation of Bromide-Capped sPP (Br-Capped sPP). Representative experiment [for the synthesis of the precursor used in the preparation of azide-capped sPP (entry A2 of Table 1)]: A modified end-functionalization method,48,53,54 which offered the preparation of hydroxyl-capped PPs, was used for the synthesis of Br-capped sPP. A 250 mL stainless steel reactor, equipped with a magnetic stirrer, was allowed to dry at 80 °C under vacuum. After being refilled with nitrogen, the reactor was charged sequentially with 50 mL of toluene, 10.0 mmol of MAO, and 5.0 μmol of Me2C(Cp)(Flu)ZrCl2. After allowing the solution to stir at 25 °C for 5 min, the reactor was charged with 30.0 mmol of TEA and then with propylene (0.5 bar) to initiate the polymerization reaction. Polymerization was conducted at 25 °C for 1 h, after which propylene gas was discharged from the reactor. The polymer solution was then allowed to cool within an immersion cooler to −40 °C and was then charged with sequentially with bromine (8 mL, 150 mmol) and pyridine (10 mL) to undergo the end-functionalization reaction. After maintaining the solution at 0 °C for 30 min under stirring, the resulting solution was slowly warmed to room temperature and was then charged with excess methanol (ca. 40 mL), which led to the deposition of the Br-capped sPP as a white precipitate. The resulting polymer was isolated after filtration and dried under vacuum to provide 2.30 g of Br-capped sPP. The crude Brcapped sPP was then placed in a Soxhlet extractor and allowed to undergo Soxhlet extraction in a boiling MEK/THF mixture (1:1) for 24 h. The resulting MEK/THF soluble polymer was collected and allowed to concentrate under vacuum to 20 mL. The resulting solution was then charged with excess methanol (ca. 20 mL), which resulted in the deposition of the MEK/THF-soluble Br-capped sPP sample as a white precipitate. The resulting precipitate was isolated by filtration and dried under vacuum to provide 1.67 g of Br-capped sPP [Mn = 6200 g/mol, Mw/Mn = 1.30 by GPC (in 1,2,4-trichlorobenzene at 110 °C)]. Synthesis of Azide-Capped sPP. Representative experiment (for entry A2 of Table 1): In a 100 mL round-bottom flask, 0.40 g of Brcapped sPP (Mn = 6200 g/mol, Mw/Mn = 1.30) was dissolve in 40 mL of toluene. After that, 0.10 g (1.5 mmol) of NaN3 in 10 mL of DMF was slowly added into the flask. The resulting solution was kept under stirring at 110 °C for 24 h. After that, the reaction solution was cooled to room temperature and then charged with 20 mL of methanol/water (v/v = 4/1), which led to the deposition of the sPP-based polymer as a white precipitate. The resulting reaction product was collected after filtration, washed with methanol, and dried under vacuum to provide 0.38 g of azide-capped sPP (yield: 95%) [Mn = 6200 g/mol, Mw/Mn = 1.30 by GPC (in 1,2,4-trichlorobenzene at 110 °C), Mn = 3200 g/mol by 1H NMR (in d-chloroform at 50 °C)]. Synthesis of Ethynyl-Capped P3HT. Representative experiment (for entry A2 of Table 1): A modified nickel-catalyzed Grinard metathesis polycondensation reaction was used for the production of ethynyl-capped P3HT with high head-to-tail ratio.53−56 A 100 mL Schlenk tube, equipped with a magnetic stirrer, was charged with 3.1 mmol of diisopropylamine (in 40 mL of THF) and THF (40 mL). The resulting solution was allowed to cool to −78 °C and was then charged with 0.62 g of 2-bromo-3-hexylthiophene (2.5 mmol). The resulting solution was kept at −78 °C for 1 h. After that, the reaction solution was charged with 0.44 g of anhydrous ZnCl2 (3.2 mmol). The resulting solution was allowed to maintain at −78 °C under stirring for 0.5 h. Then, the reaction solution was slowly warmed to 0 °C and was then charged with 21 mg of [Ni(dppp)Cl2] (0.04 mmol). After that, 3007

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energies of carbon (62 mJ/m2) and SiO2 (54 mJ/m2) are similar, and thus carbon grid substrate is generally regarded as the suitable and reliable material to simulate the morphology on the SiO2/Si surface. The surface structure of the film surface was studied by AFM using a Nanoscope 3D Controller (AFM, Digital Instruments) operating in tapping mode at room temperature. For the measurements of optical absorption, GIWAXS, TEM, and AFM, the preparing conditions of polymer samples were the same as the device fabrication to simulate the polymer transistor structures. Output and transfer characteristics of the FETs were measured using Keithley 4200 semiconductor parametric analyzer. All the electronic measurements proceeded in the N2-filled glovebox.

respective azide-functionalized sPP in a high yield. The comparison of 1H NMR spectra between Br-capped sPP and N3-capped sPP is shown in Figure 1. As revealed by Figure 1a,b, the replacement of the Br functional end group of Br-capped sPP with azide (azide-capped sPP) shifts the methylene proton resonance from δ = 3.31 (−CH2−Br, for Br-capped sPP) to δ = 3.11 ppm (−CH2−N3, for azide-capped sPP). The detailed chemical assignments for azide-capped sPP are based on the structural information reveals in 13C (DEPT135) (Figure S3) and 2-D 1H and 13C HMBC NMR (Figure S4) spectra in the Supporting Information. Syntheses and End-Group Analyses of EthynylCapped P3HT. Ethynyl-capped P3HT with high head-to-tail ratios were generated from 2-bromo-3-hexylthiophene, lithium diisopropylamine, and zinc chloride via a Ni-catalyzed Grignard metathesis (GRIM) polymcondensation reaction.55−58 This method allows the synthesis of ethynyl-capped P3HT with different molecular weight, which can be regulated by adjusting the monomer to catalyst ratio in the feed. After precipitation by the addition of methanol, the ethynyl-capped P3HT can have the high yield of 89−91%. Structural analyses of the resulting polymer by GPC reveal that polymer products generated by our study has a characteristic narrow range of molecular weight distribution of 1.09−1.16, indicating the participation of a quasi-living chain growth polycondensation pathway for the production of ethynyl-capped P3HT. Figure 2 shows the 1H

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RESULTS AND DISCUSSION Syntheses and End-Group Analyses of BromideCapped sPP. Br-capped syndiotactic polypropylene with different chain lengths were synthesized via metallocene (Me2C(Cp)(Flu)ZrCl2)-mediated syndiospecific polymerization of propylene in the presence of different concentrations of triethylaluminum (see Scheme 1). The resulting alkylaluminum-capped sPP samples were in situ treated with bromine to provide the Br-capped sPP. Figure 1a shows the 1H NMR

Figure 1. 1H NMR spectra (500 MHz) of (a) Br-capped sPP (Mn = 6200 g/mol, PDI = 1.30) and (b) N3-capped sPP (Mn = 6200 g/mol, PDI = 1.30) in CDCl3 (temperature = 50 °C).

spectrum (with an inset of the expanded region and chemical shift assignments) of the Br-capped sPP (Mn = 6200 g/mol, Mw/Mn = 1.31) isolated after the treatment of Br2. As shown the in the 1H NMR spectrum, in addition to the three major upfield 1H resonances (δ = 0.84, 1.04, 1.59) corresponding to the −CH3, −CH2, and −CH proton resonances of the sPP backbone, respectively, a weak downfield resonance at 3.31 ppm is attributed to the chain end −CH(CH3)−CH2−Br proton resonance. The detailed chemical assignments are based on the structural information revealed in 13C (DEPT 135) and 2-D 1H−13C HMQC NMR spectra (Figures S1 and S2 in the Supporting Information) and by comparison with prior NMR analyses of OH-capped sPP. These NMR spectra clearly reveal that Br-capped sPP prepared by selective chain transfer to TEA/Br2 has a single bromide end group, making it suitable as an end-functionalized prepolymer for the construction of stereoregular block copolymers by subsequent reactions. Syntheses and End-Group Analyses of Azide-Capped sPP. Displacement of the bromide end group can be accomplished by treating the Br-capped sPP with NaN3 in toluene/DMF mixed solvent (110 °C), which affords the

Figure 2. 1H NMR spectra (500 MHz) of ethynyl-capped P3HT (Mn = 15 300 g/mol; PDI = 1.16) in CDCl3 (temperature = 50 °C).

NMR spectrum (with an inset of chemical structure and chemical shift assignments) of the ethynyl-capped P3HT (Mn = 15 300 g/mol, Mw/Mn = 1.16). As revealed by Figure 2, in addition to the five major upfield resonances (δ = 0.93, 1.37, 1.45, 1.72, and 2.82) corresponding to those proton resonances of the aliphatic hexane side chain attached to the thiophene backbone, a downfield resonance at 6.99 ppm is attributed to the proton resonance situated at the sp2 hybrid carbon within the five-membered thiophene ring and a weak resonance at 3.52 ppm corresponding to the proton resonance of the terminal ethynyl functional group (−CC−H). The detailed chemical assignments are based on the structural information revealed in 13 C (DEPT 135) (Figure S5) NMR spectra and by comparison with the prior NMR data reported in the literature.59 The headto-tail ratio of the ethynyl-capped P3HT determined by 1H NMR spectroscopy is 97% (in d-chloroform at 40 °C). These analytical results indicate that pure ethynyl-capped P3HT with 3008

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a high head-to-tail ratio is suitable as the prepolymer for the construction of block copolymer via the subsequent “click” reaction. Synthesis of P3HT-b-sPP via “Click” Chemistry. The combination of the synthesized N3-capped sPP prepolymer and the synthesized ethynyl-capped P3HT allows the construction of P3HT-b-sPP through the “click” chemistry. Accordingly, the synthesis of P3HT-b-sPP was carried out by the combination of ethynyl-capped P3HT and azide-capped sPP in THF using CuBr/PMDETA as the catalyst. The structural characteristics and thermal properties of the prepolymers (ethynyl-capped P3HT and azide-capped sPP) and block copolymer (P3HT-bsPP) are summarized in Table 1. Figure 3 and Figure S6

Figure 4. 1H NMR spectrum (500 MHz) of P3HT-b-sPP (Mn = 21 000 g/mol; PDI = 1.20) in CDCl3 (temperature = 50 °C).

corresponds to the 1H resonance situated at the sp2 hybrid carbon within the five-membered triazole ring, whereas the weak 4.24 ppm proton resonance corresponds to the 1H resonance situated at the sp3 hybrid carbon (sPP−CH2− triazole ring−P3HT) at the junction structure between the P3HT and sPP blocks. The four P3HT-b-sPP block copolymers have a low polydispersity index of 1.13−1.30. The successful identification of these junction structures provides the addition evidence on obtaining the target P3HT-b-sPP. Thermal and Optical Properties of P3HT-b-sPP Diblock Copolymers. The thermal properties of P3HT-b-sPP diblock copolymers are characterized from TGA (Figure 5) and

Figure 3. Azide-capped sPP (Mn = 6200 g/mol; PDI = 1.31), ethynylcapped P3HT (Mn = 15 300; PDI = 1.16), and P3HT-b-sPP (Mn = 21 000 g/mol; PDI = 1.20) (entry P3 of Table 1).

compare the GPC curves of ethynyl-capped P3HT, azidecapped sPP, and P3HT-b-sPP. The successful preparation of pure P3HT-b-sPP is further elucidated by NMR analyses. Figure 4 (with an inset of chemical structure and chemical shift assignments) shows the 1H spectrum of P3HT-b-sPP block polymers of entry P2 in Table 1 (the 1H spectra of P1, P3, and P4 are shown in Figures S7−S9). A comparison between Figures 1b, 2, and 4 clearly reveals the disappearance of the characteristic 3.52 ppm proton resonance (as in Figure 4), which corresponds to the proton resonance of the terminal ethynyl group (−CC−H) and the presence of two new proton resonances at δ = 7.51 and 4.24. As revealed by Figure 4, the weak downfield proton resonances at 7.51 ppm

Figure 5. TGA curves of P3HT-b-sPP block copolymers at the heating rate of 10 °C/min under a nitrogen atmosphere.

DSC (Figure 6) curves. The detailed data are summarized in Table 1. The thermal decomposition temperatures (Td, 95 wt % residue) of the P3HT-b-sPP diblock copolymers (P1−P4) are all above 400 °C, indicating their good thermal stability and high processing ability. Because of the relatively lower thermal stability of sPP compared to P3HT, Td is slightly decreased with the longer sPP block length. The DSC curve of P1 with a shorter sPP segment exhibits similar crystallization temperature (Tc) and melting temperature (Tm) to those of P3HT. Followed by the decreasing endo- and exothermal peaks of 3009

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processing solvent. For P3HT-b-sPP diblock copolymers, sPP segments do not absorb in the visible region; hence, the features of absorption spectra are only contributed to the P3HT block. Absorption spectra (Figure 7) of P1−P4 exhibit three absorption bands at λ = 520 nm (shoulder), 560 nm (λmax), and 605 nm (shoulder). The λmax of P3HT-b-sPP does not shift with the variation of sPP block length. It suggests that the sPP segments do not affect the effectively conjugating length of P3HT in solid state.63,64 The lower energy absorption bend (λπ−π, λ = 605 nm) is ascribed to the interchain exciton delocalization of the excited state across ordered P3HT πstacking chains.61,64 The intensity of λπ−π of P3HT-b-sPP diblock copolymers with the shorter sPP segment (P1−P3) are similar, implying the shorter coil-like sPP segment does not hinder the π−π interaction of P3HT block. However, the λπ−π intensity of P4 is slightly decreased in contrast to P1−P3, indicating the longer sPP segment may fold a more planar form or different orientation of helical form to prevent the π−π stacking of P3HT. The absorption coefficient profiles of P3HTb-sPP thin films are shown in the inset of Figure 7, which has the order of P2 > P1 > P3 > P4. The P2 thin film shows the highest absorption coefficient suggests that it has much ordered P3HT packing structure, which could lead to the enhancement of carrier transport properties.63 The lowest absorption coefficient of P4 due to the hindrance to the P3HT π−π stacking resulted from the longer sPP segment. The influence of crystal structure and orientation of sPP segments will be discussed later. Molecular Stacking Structure of P3HT-b-sPP Diblock Copolymers. Crystalline structure of diblock copolymer is a significant factor for controlling the FETs performance. For conjugated-insulated diblock copolymers, e.g., P3HT-b-PS, P3HT-b-PE, and P3HT-b-sPP, a well-defined π−π stacking structure of P3HT can provide a good hole channel to enhance the field-effect mobility. Grazing incidence wide-angle X-ray scattering (GIWAXS) is a powerful tool to probe the microstructure of P3HT-b-sPP diblock copolymers and the influence of the sPP segment on the P3HT block. Twodimensional GIWAXS patterns of sPP (Mw: 14 000 g/mol), P2, and P4 thin films are shown in Figure 8. The sPP presents many crystalline structures, the most stable structure proposed by Lotz and Lovinger is the chains in s(2/1)2 helical conformation packed in the orthorhombic unit cell having axes a = 14.5 Å, b = 11.2 Å, and c = 7.4 Å.65−69 The GIWAXS pattern of sPP thin film exhibits two orientations of helical form. The major orientations of sPP crystalline domain (helical form) deposited on substrate are developed to (020)-axis normal to the substrate (form I) and (020)-axis parallel to substrate (form II). The schematic of sPP molecular packing is presented in Figure 9. The calculated intersheet spacing of sPP (200) (dsPP(200)) and sPP (020) (dsPP(020)) are 7.23 and 5.61 Å, respectively, which are identical with Lotz and Lovinger reported.70 The GIWAXS patterns of P1−P4 (P1 and P3 are shown in Figure S10) all exhibit evident P3HT out-of-plane lamellar-layer structure ((100), (200), and (300)) along the qz direction and π−π interchain stacking (010) along the qxy direction, corresponding to that of P3HT with 16.20 Å for intersheet spacing (dP3HT(100)) and 3.82 Å for π−π stacking distance (dP3HT(010)) of P3HT-b-sPP, respectively.71−75 Moreover, the pole figure (Figure S11) illustrates the P3HT (100) diffraction peak of thin film prepared by P1−P4 and P3HT. The pole figure of P2 shows quite narrower fwhm (full width at half-maximum), which is around 28°, than P3HT (∼35°), P1

Figure 6. DSC curves of P3HT-b-sPP block copolymers and P3HT and sPP6K homopolymers. The measuring conditions are operated at the heating rate of 5 °C/min and cooling rate of 10 °C/min under a nitrogen atmosphere.

P3HT domain, both Tc and Tm of P3HT segments are shifted toward lower temperature with the increase of sPP block length. This can be explained by the disturbed P3HT interpacking and the further confined P3HT crystallization while the sPP block with longer chain length would need more space for stretching. Two different crystalline forms of sPP are observed in DSC curves, where the higher melting peak and the lower temperature peak are attributed to the helical form and the planar form of sPP, respectively.60 The content of the planar form sPP increases with the longer sPP block length, suggesting under this condition sPP has more ability to develop the planar form, whereas the helical form sPP maintains the majority. On the other hand, the copolymers exhibit higher Tm of sPP while increasing sPP block length, indicating that the longer sPP block length leads to the higher quality of the sPP crystallite. A significant increase in the Tc of sPP accompanied by the broadening exotherms is observed with increasing sPP block length in the copolymers; therefore, the higher crystallization ability of sPP is again confirmed. Absorption spectra of conjugated polymers would provide useful information on interchain interaction (π−π interaction) and effective conjugation length.61,62 Figure 7 shows the normalized absorption spectra and absorption coefficient profiles of P3HT-b-sPP thin films. The polymer thin films are prepared from polymer solution using chloroform as the

Figure 7. Normalized absorption spectra of P3HT-b-sPP polymer films prepared from polymer solution using CF as solvent. The inset shows the absorption coefficient profiles of the prepared films. 3010

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Figure 9. Schematic of molecular packing of sPP and P3HT-b-sPP on substrate. The orientations of major structure of sPP crystallites (helical form) deposited on substrate could develop to (020)-axis normal to the substrate (form I) or (020)-axis parallel to substrate (form II). The P3HT-b-sPP diblock copolymer deposited on substrate with sPP segment stacking to form I is the most possible molecular structure (form III). When sPP segment develops to form II on the substrate, the ability of molecular stacking of P3HT is restrained (form IV).

especially for P4. It indicates that the long sPP segment has more possibility to develop form II crystalline domain and further inhibits the packing of P3HT. The favorable chain length of the sPP segment could develop the most form I structure to improve the ability of the P3HT packing. Morphologies of Thin Film Prepared from P3HT-b-sPP Diblock Copolymers. The effect of crystallized coil block (sPP) on the surface structure of P3HT-b-sPP block copolymers is observed by TEM and AFM. The distinct nanofiber structure presented in Figure 10 for P3HT-b-sPP can be attributed to a balance between thermodynamics and kinetics, which is the interplay between microphase separation of the incorporated blocks (sPP and P3HT) and the selfassembly inside the individual crystalline blocks. The Flory− Huggins interaction parameter, χ, is generally employed to predict the microphase separation in the block copolymer (P3HT-b-sPP).76 The interaction parameter (χ) between the assigned polymer block and the solvent for sPP block (χsPP‑CF) is 0.394, which is much higher than the value obtained for P3HT block (χP3HT‑CF) (down to 0.031).76−78 This result suggests that sPP block has a greater incompatibility with CF, resulting in higher crystallizing speed and capability of sPP compared to P3HT. Because of the heavy atom of P3HT, the dark region in the TEM images (Figure 10a−d) can be directly related to the stacking of the P3HT domains. Therefore, P2 shows the thickest nanofibers with a width of about 5.3−6.7 nm, while those with P1, P3, and P4 exhibit quite thinner ones with widths of 3.3−4.3, 4.2−5.1, and 2.8−4.0 nm, respectively.

Figure 8. Two-dimensional GIWAXS patterns of sPP (Mw: 14 000 g/ mol), P2, and P4.

(∼35°), P3 (∼34°), and P4 (∼41°). Thus, it suggests that the P2 can form a more ordered stacking structure of P3HT domain. Based on the concept of lattice matching, compare the molecular stacking structures of sPP and P3HT in the diblock copolymer deposited on substrate; the sPP segment stacking to form I shows suitable d-spacing (dsPP(200) and dsPP(020)) to fit the P3HT packing structure (dP3HT(100) and dP3HT(010)) and to further enhance the stacking ability of P3HT.32−36 The schematic molecular structure of P3HT-b-sPP is shown in Figure 9 (form III). When the sPP segment develops to form II on the substrate, the ability of P3HT molecular packing is restrained (form IV) (Figure 9). The GIWAXS pattern of P1 exhibits only the diffraction peak of P3HT domain due to the shorter sPP segment. The major crystalline structure of sPP domain displays in P2−P4 is form I (sPP (020) along the qz direction and sPP (200) along the qxy direction), suggesting the major crystalline structure of P3HT-b-sPP is form III. As increasing the sPP segment length of P3HT-b-sPP, the sPP diffraction peak of form II (sPP (200) along the qz direction and sPP (020) along the qxy direction) becomes quite clearer, 3011

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FET Performances of P3HT-b-sPP Thin Films. Bottom gate and top contact FET devices are fabricated by spin-coating onto the 300 nm SiO2/Si wafers (100, n+2) with Au as the source and drain electrodes. The field effect mobility (μ) is measured by the transfer curves in the saturation regime and can be determined from the slope of (−Ids)1/2 versus Vgs. The threshold voltage (Vt) of the devices are estimated from a linear relationship between (−Ids)1/2 versus Vgs by extrapolating the measured data to −Ids = 0 A. The field effect mobility is measured by the equation Ids = μ(WC/2L)(Vgs − Vt)2, where W, L, and C are channel width, channel length, and capacitance of gate insulator, respectively. Figure 11a shows the transfer

Figure 11. (a) Transfer characteristics of FETs fabricated from P3HTb-sPP block copolymers and P3HT homopolymer. (b) Output characteristics of transistor of P2 thin film. All FETs are fabricated by spin-coating and using CF as solvent.

Figure 10. TEM (a−d) and AFM (e−h) images of P3HT-b-sPP thin films.

characteristics of FETs fabricated from P1−P4 and P3HT using CF as the processing solvent at drain-source voltage (Vds) −100 V. The FET output characteristics of the P2 thin film exhibits good current modulation and well-defined saturation regions, as shown in Figure 11b. The gate leakage is negligible in the studied device since the positive drain current at zero drain voltage is always much smaller in modulus than the drain saturation current at the respective gate voltage.79 The details of the electrical properties are summarized in Table 2. The average data of FETs performance are obtained from at least 10 devices of two batches. The P1−P3 thin films present higher mobilities (4.15 × 10−3 cm2 V−1 s−1 for P1, 4.16 × 10−2 cm2 V−1 s−1 for P2, and 3.95 × 10−3 cm2 V−1 s−1 for P3) than the P3HT thin film (1.43 × 10−3 cm2 V−1 s−1), where the P3HT shows corresponding FET performance to literatures.80,81 The feature of the increase in FET mobility could be realized by the molecular stacking structure suggested from the results of the

These results clearly suggest that the sPP blocks in the diblock copolymer play a significant role in the crystallinity of P3HT nanofibers. On the other hand, as shown in Figure 10c,g for P3 and Figure 10d,h for P4, with increasing sPP segment length in block copolymers P3 and P4, the scale and intensity of sPP crystallite are increased. However, the most intense nanofibers are observed for P2 (Figure 10b,f). It suggests that the longer block length of sPP (P3 and P4) would induce relatively extensive content of form II to suppress the latter arrangement of P3HT. Oppositely, the P3HT stacking is only slightly affected in P1 due to the shorter sPP block length. Agreeable to GIWAXS results, P2 has the most moderate sPP block length to pack form I structure; hence, the P3HT stacking (form III) is further improved. 3012

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Table 2. FET Performances of P3HT-b-sPP Block Copolymer with Different Chain Length of sPP Segment μavg (cm2 V−1 s−1) P1 P2 P3 P4 P3HT

(4.15 (4.16 (3.95 (7.53 (1.43

± ± ± ± ±

0.41) 0.34) 0.11) 1.57) 0.28)

× × × × ×

−3

10 10−2 10−3 10−4 10−3

on/off ratio 2.40 2.91 1.36 6.35 1.46

× × × × ×

4

10 104 104 104 104

± ± ± ± ±

*E-mail: [email protected] (W.-C.C.); [email protected] (J.-C.T.).

0.80 1.36 3.87 8.82 0.87

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The financial support from National Science Council of Taiwan and National Taiwan University Excellent Research program is highly appreciated. We thank Professor S. H. Tung for the helpful discussions on the GIWAXS and morphologies.

GIWAXS, TEM, and AFM images. The sPP segments of P1− P3 could develop almost form I structure on the polymer/gateinsulator interface; this structure may constrain the molecular motion of P3HT segment in the process of solvent-evaporating and further improve both the ability of P3HT packing and the carrier mobility. These results could be also recognized from the morphologies of P1−P3 which display the clear fiber-like crystalline domain of P3HT. As increasing the sPP chain length, such as for P3 and P4, the sPP segments of P3HT-b-sPP block copolymer can stack to form II structure, resulting in the formation of larger sPP crystalline domain and the diminution of the hole mobility. Notably, P2 exhibits the best FET performance (μ: 4.16 × 10−2 cm2 V−1 s−1, on/off ratio: 2.91 × 104) due to its applicable chain length of sPP segment to develop the well-packed structure of P3HT-b-sPP (form III) and the larger, well-defined fibrillar crystalline domain of P3HT.

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CONCLUSIONS In summary, the crystalline-stereoregular crystalline diblock copolymers P3HT-b-sPP have been successfully synthesized and further investigated the correlations between the molecular packing structures, morphologies, and their field-effect transistor characteristics. The stereoregular crystalline block sPP developed different types of molecular stacking structure and further affect the P3HT domain packing and the field-effect mobility. In agreement with the lattice matching concept, it is found that the most suitable packing structure for P3HT-b-sPP diblock copolymers is form III structure, which assembles when sPP segments develop to form I structure and lead to the lamellar edge-on structure of P3HT domain. Consequently, this well-oriented packing structure of sPP domain (form I) can enhance the molecular packing of P3HT domain and further improve the field-effect mobility. Therefore, the molecular packing structures and morphology results could reasonably explain the FET hole mobilities, which of P1−P3 thin films prepared by pure CF were up to 1 order of magnitude higher than that of the parent P3HT thin film. This study of crystalline-stereoregular crystalline diblock copolymers P3HTb-sPP demonstrates that a lattice matching concept of tuning both molecular stacking structure and morphology could effectively improve the charge transporting characteristics of semiconducting polymers.

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AUTHOR INFORMATION

Corresponding Author

Vth (V) 13.36 −14.94 −2.56 5.03 −5.39

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing GPC and NMR spectra of Br-capped sPP, P3HT-b-sPP diblock copolymers; two-dimensional GIWAXS patterns of P1 and P3; pole figures of the P3HT (100) diffraction peak of thin film prepared by P1−P4 and P3HT homopolymer. This material is available free of charge via the Internet at http://pubs.acs.org. 3013

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