Novel conjugated polymers based on bis-dithieno[3,2-b;2′,3′-d]pyrrole vinylene donor and diketopyrrolopyrrole acceptor: side chain engineering in organic field effect transistors

Abstract

Two new donor–acceptor conjugated polymers, i.e. PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈, were prepared by the Stille coupling of (E)-1,2-bis(4-dodecyl-6-(trimethylstannyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)ethene (Sn-B(C₁₂DTP)V) with 3,6-bis(5-bromothiophen-2-yl)-2,5-didodecylpyrrolo[3,4-c]pyrrole-1,4-dione (Br-DTDPP-C₁₂) and 3,6-bis(5-bromo-thiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-dione (Br-DTDPP-C₈C₁₂), respectively. The molecular weight and thermal properties of the obtained polymers were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and thermogravimetry (TG). Their optical and electrochemical properties were analyzed by UV-vis-NIR spectroscopy and cyclic voltammetry (CV). The morphological and molecular-packing features of the polymers were characterized by optical microscopy and atomic force microscopy (AFM), UV polarization spectroscopy and grazing incidence X-ray diffraction (GIXD) to determine the relationship between the structural morphology and the performance. PB(C₁₂DTP)V-DTDPP-C₁₂C₈ with branched C₁₂C₈ side chains on DPP units shows much higher hole mobility (1.2 cm² V⁻¹ s⁻¹) than 0.69 cm² V⁻¹ s⁻¹ of PB(C₁₂DTP)V-DTDPP-C₁₂.

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Introduction

Donor–acceptor (D–A) conjugated copolymers have attracted extensive scientific research interest because of their promising applications in organic field effect transistors (OFETs),¹ organic solar cells,² light-emitting diodes³ and memory devices.⁴ In developing high-performance OFET D–A polymers, manipulating the D and A configuration is a most common strategy to improve the length of intramolecular effective conjugation, the coplanarity of the backbone, the intermolecular π–π stacking order, the energy level and the band gap.^1,5

Highly planar D–A polymers providing sufficient intramolecular π conjugation and tight intermolecular π–π stacking could be advantageous for intra- and inter-chain charge transport over poly(3-hexylthiophene-2,5-diyl) (P3HT) with a high degree of crystallinity.^5,6 The design of the coplanarity in the D–A backbones hence becomes particularly important.¹ In preceding work, highly planar diketopyrrolopyrrole (DPP), naphthalene diimide (NDI), and isoindigo (IID) were incorporated into D–A copolymers mostly as the acceptor units, but the coplanarity of the entire backbone was decreased by torsionable linkages generated on inducing the thiophene donors.^7–9 Recent research efforts have, therefore, focused on improving the inter-annular rotation and the torsion angle of the D–A backbone.^10–15 Our laboratories demonstrated an induced multifused structure, including tricyclic and tetracyclic arenes, on DPP-based backbones to avoid the inter-annular rotation of the backbones.^11,12 Moreover, using a vinylene linkage between two thiophene donors can reconcile the torsion angle of the backbones, resulting in highly coplanar conjugated structures and a superior OFET performance.^10,13–15

Flexible side chains play a compensating role in the fine adjustment of not only the solubility of the copolymers in organic solvents during solution processes but also the intermolecular packing of the rigid backbone during dewetting processes.^16–19 Many side-chain engineering approaches, e.g. types,²⁰ lengths,²¹ appended position²² and grafted densities,²³ have thus become a significant issue in improving the electro-optic properties of the conjugated copolymers. Kang et al. demonstrated that moving the branching position of alkyl side chains away from the DPP unit of poly(selenophene-vinylene-selenophene-diketopyrrolopyrrole) (PSVS-DPP) backbones sufficiently decreased the steric hindrance of the side chains, increased the π–π interactions, adjusted the distance of the vertically adjacent layer and promoted long-range order.²⁴ Lee et al. reported that a branched side chain on the benzotriazole (BTz) unit of thienylene-vinylene-thienylene-benzotriazole polymers (PTVT-BTz) promoted the formation of a backbone structure more tilted than a linear side chain, resulting in an improved overlap of π-electron densities and high hole mobility.²³ These conditions signify that selecting suitable side chains is as critical as the D–A backbone for the design of next-generation D–A copolymers, but the effect of the side chains on the intra- and interlayer packing stability of backbones in thin films of D–A polymers is unclear.¹⁹

In this study, we synthesized two new D–A type conjugated polymers, composed of an extended bis-dithieno[3,2-b:2′,3′-d]pyrrole (DTP) vinylene and dithiophene tethered DPP units. In this molecular design of donor units, extending two DTP molecules linked by the vinylene group can maintain the backbone coplanarity and a small band gap, but might also result in instability of the inter- or intra-chain packing and insolubility. To probe efficient meso-structures for high-performance OFET devices,^5,16 we, therefore, designed branched alkyl chains symmetrically grafted on the DPP units of the B(DTP)V-DTDPP backbone, named PB(C₁₂DTP)V-DTDPP-C₁₂C₈, in addition to common linear alkyl chains for PB(C₁₂DTP)V-DTDPP-C₁₂. We contend that the long and rigid backbone of D–A polymers with the matching side-chain topology renders a new route for the molecular design of π-conjugated polymers to improve the OFET performance.

Experimental

Materials

All chemicals were purchased from Aldrich, Tokyo Chemical Industry (TCI), and Acros, and used as received unless otherwise specified. Monomers, N-dodecyl dithieno[3,2-b:2′,3′-d]pyrrole, 2,5-bis-(2-octyldodecyl)-3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (DTDPP-C₁₂C₈) and 2,5-didodecyl-3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (DTDPP-C₁₂) were prepared according to literature methods.^25,26

N-Dodecyldithieno[3,2-b:2′,3′-d]pyrrole-2-carbaldehyde (C₁₂-DTP-CHO). A two-necked flask (25 mL) was charged with C₁₂-DTP (3.00 g, 8.63 mmol), DMF (1.76 mL) and anhydrous dichloromethane. After the mixture was cooled to 0 °C, POCl₃ (1.64 mL) was added dropwise to this flask. The mixture was kept at 0 °C for 2 h, and then stirred at 40 °C for 4 h. After saturated NaHCO₃ (80 mL) was added, the mixture was stirred for another 2 h. The mixture was extracted with dichloromethane. The collected CH₂Cl₂ solution was dried over anhydrous MgSO₄. After the removal of the solvent, the crude product was purified on a silica-gel chromatograph using a mixture of dichloromethane and hexane (5 : 1) to give the final compound, C₁₂-DTP-CHO, as a yellow liquid (2.80 g, 86%). ¹H NMR (400 MHz, CDCl₃): δ 9.88 (s, 1H), 7.65 (s, 1H), 7.38 (d, J = 5.6 Hz, 1H), 7.02 (d, J = 5.6 Hz, 1H), 4.23 (t, J = 6.8 Hz, 2H), 1.89 (m, 2H), 1.31 (m, 21H),0.86 (t, J = 6.8 Hz, 3H) ppm.

(E)-1,2-Bis(4-dodecyl-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)ethene (B(C₁₂DTP)V). In a dry three-necked round-bottomed flask, TiCl₄ (6.2 mL) was added in dry THF (110 mL) at 0 °C. Zinc (6.60 g) was added to this mixture, which was then refluxed for 1 h. After the mixture was cooled to 0 °C, C₁₂-DTP-CHO (2.80 g, 7.45 mmol) was added and stirred at 30 °C for 20 h. The mixture was extracted with ethyl acetate and HCl aqueous solution. The ethyl acetate solution was dried over anhydrous MgSO₄. After the removal of the solvent, the crude product was purified on a silica-gel chromatograph with a mixture of dichloromethane and hexane (1 : 3) to give the final compound, B(C₁₂DTP)V, as an orange solid (1.00 g, 37%) with a T_m of 87 °C. ¹H NMR (400 MHz, CDCl₃): δ 7.12 (d, J = 5.2 Hz, 2H), 7.06 (s, 2H), 6.97 (d, J = 4.4 Hz, 2H), 6.93 (s, 2H), 4.15 (t, J = 6.8 Hz, 4H), 1.85 (m, 4H), 1.27 (m, 42H), 0.87 (t, J = 6.8 Hz, 6H) ppm. ¹³C NMR (CDCl₃): δ 145.2, 144.7, 140.5, 127.5, 124.0, 117.9, 115.1, 110.5, 109.8, 47.3, 32.0, 30.3, 29.6, 29.5, 27.0, 22.7, 14.1 ppm. Mass (m/z): 718, 359.

(E)-1,2-Bis(4-dodecyl-6-(trimethylstannyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)ethene (Sn-B(C₁₂DTP)V). To a solution of (B(C₁₂DTP)V) (500 mg, 0.695 mmol) in dry THF (6 mL) at −78 °C was added n-BuLi (0.9 mL, 1.73 mmol). After the mixture was stirred for another 1 h, trimethyltin chloride (4 mL, 3.48 mmol) was added at −78 °C. The mixture was stirred near 23 °C for 20 h before extraction with diethyl ether. The crude product was obtained on precipitation in methanol. The orange crystals were collected and recrystallized from methanol and ethyl acetate. Yield: 0.35 g, 48%. ¹H NMR (400 MHz, CDCl₃): δ 7.05 (s, 2H), 6.98 (s, 2H), 6.90 (s, 2H), 4.15 (t, J = 6.8 Hz, 4H), 1.85 (m, 4H), 1.27 (m, 42H), 0.87 (t, J = 6.8 Hz, 6H), 0.40 (t, J = 7.1 Hz, 18H) ppm.

3,6-Bis(5-bromothiophen-2-yl)-2,5-didodecy pyrrolo[3,4-c]pyrrole-1,4-dione (Br-DTDPP-C₁₂). To a solution of DTDPP-C₁₂ (0.600 g, 0.942 mmol) in CHCl₃ (72 mL), N-bromosuccinimide (NBS) (0.427 g, 2.40 mmol) was added slowly. The mixture was stirred under darkness for 48 h before extraction with dichloromethane. Purple crystals were collected and recrystallized from methanol and dichloromethane. Yield: 0.715 g, 95%. T_m: 168 °C. ¹H NMR (400 MHz, CDCl₃): δ 8.62 (d, J = 8.1 Hz, 2H), 7.21 (d, J = 7.3 Hz, 2H), 3.98 (t, J = 8.1 Hz, 4H), 1.71 (m, J = 7.3 Hz, 4H), 1.42–1.27 (m, 34H), 0.89 (t, J = 6.1 Hz, 6H) ppm.

3,6-Bis(5-bromothiophen-2-yl)-2,5-bis-(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-dione (Br-DTDPP-C₁₂C₈). To a solution of DTDPP-C₁₂C₈ (0.600 g, 0.70 mmol) in CHCl₃ (60 mL), NBS (0.260 g, 1.48 mmol) was added slowly. The mixture was stirred under darkness for another 48 h before extraction with dichloromethane. Purple crystals were collected and recrystallized from methanol and dichloromethane. Yield: 0.440 g, 67%. T_m: 97 °C. ¹H NMR (400 MHz, CDCl₃): δ 8.62 (d, J = 4.2 Hz, 2H), 7 92 (d, J = 4.2 Hz, 2H), 3 92 (t, J = 7.8 Hz, 4H), 1.90–1.85 (m, 2H), 1.34–1.26 (br, 64H), 0.90–0.86 (m, 12H) ppm.

PB(C₁₂DTP)V-DTDPP-C₁₂ polymer. A solution of Br-DTDPP-C₁₂ (0.090 g, 0.113 mmol), Sn-B(C₁₂-DTP)V (0.118 g, 0.113 mmol), Pd₂(dba)₃ (4.14 mg, 0.005 mmol) and P(o-tolyl)₃ (13.8 mg, 0.045 mmol) in dry chlorobenzene (5 mL) was deoxygenated with nitrogen for 1 h. The mixture was stirred at 120 °C for another 48 h. The crude polymer was obtained on precipitation in methanol (400 mL), and purified by Soxhlet extraction with acetone and hexane to remove the low molar mass portion. The remaining polymer and Si–thiol in hot toluene were stirred at 60 °C for 12 h. The purified polymer was obtained on precipitation in methanol; yield: 45 mg, 29%.

PB(C₁₂DTP)V-DTDPP-C₁₂C₈ polymer. A solution of Br-DTDPP-C₁₂C₈ (0.140 g, 0.133 mmol), Sn-B(C₁₂-DTP)V (0.129 g, 0.133 mmol), Pd₂(dba)₃ (4.9 mg, 0.005 mmol), and P(o-tolyl)₃ (16.2 mg, 0.050 mmol) in dry chlorobenzene (5 mL) was deoxygenated with nitrogen for l h. The mixture was stirred at 120 °C for 48 h. The crude polymer that was obtained on precipitation in methanol (400 mL) was purified by Soxhlet extraction with acetone and hexane to remove the low molar mass portion. The remaining polymer and Si-thiol in hot toluene were stirred at 60 °C for 12 h. The purified polymer was obtained on precipitation in methanol; yield: 93 mg, 44%.

OFET fabrication and characterization of polymer thin films

OFETs were fabricated on highly doped silicon substrates with a bottom-gate top-contact structure. A SiO₂ layer (thickness 300 nm) was deposited on silicon substrates and it served as a gate dielectric. The SiO₂ surface was modified with octadecyltrichlorosilane (ODTS). The PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ solutions were prepared at a concentration of 10 mg mL⁻¹ in chlorobenzene (CB) or 1,2-dichlorobenzene (ODCB). The highly aligned polymer films (thickness ∼40 nm) were fabricated with the off-center spin-coating (OCSC) method,^27,28 in which the polymer solution was injected 3 cm away from the center of the spin-coater at a spin rate of 2000 or 2500 rpm under nitrogen. The thin-films were annealed at various temperatures (50, 180, 200 and 220 °C) for 10 min under nitrogen. Subsequently, gold electrodes as the source and drain electrodes were thermally evaporated onto the polymer thin films through a shadow mask. The source-to-drain direction was parallel to the aligning direction of the polymer chains. The OFET performances were measured by using an Agilent B1500 series parametric analyzer. The hole mobilities (μ_hs) were calculated from the data in the saturation regime with the equation:

μ = (2I_DSL)/(WC(V_G − V_th)²)

where I_DS is the saturation drain current, L (100 μm) is the channel length, W (1000 μm) is the channel width, C (10 nF cm⁻²) is the capacitance per unit area of the gate dielectric layer, V_G is the gate voltage, and V_th is the threshold voltage. Average data were calculated from the analysis of 30 independent devices. For comparison, the OFETs from spin-coated polymer thin films (∼40 nm) were prepared under the same conditions.

Instrumentation and theoretical calculations

Nuclear magnetic resonance (NMR) spectra were recorded (Varian Unity-300 and Agilent Unity-400 spectrometers) with samples in deuterated dichloromethane solution near 23 °C. The chemical shifts of signals in these spectra were recorded in ppm. Coupling parameters (J) were obtained in hertz. The resonance multiplicity is described as s (singlet), d (doublet), t (triplet), and m (multiplet). The molar mass (M_n) and polydispersity index (PDI) were obtained at 25 °C by gel permeation chromatography (GPC) (polystyrene standards; eluent: 1,2-dichlorobenzene). Thermogravimetric analysis (PerkinElmer model Pyris 1 TGA) and differential scanning calorimetry (DSC) measurements (TA Instruments DSC Q200) were performed under a nitrogen flow of 50 mL min⁻¹ at a scan rate of 10 °C min⁻¹. UV-vis-NIR spectra were obtained by using a Hitachi U-4100 spectrophotometer. Geometry optimizations and normal-mode calculations were performed at the B3LYP/6-311G(d,p) and PCM model (chloroform, ε = 4.71; 1,2 dichlorobenzene, ε = 9.99) using the Gaussian 09. Electrochemistry measurements (CH Instruments model 61 ID) were performed near 23 °C in a classical three-electrode cell; the working, reference and counter electrodes were a glassy carbon electrode with polymer films, Ag/AgCl/CH₃CN/(nC₄H₉)₄NPF₆ and a Pt wire, respectively. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc⁺), measured under the same conditions, was located at 0.09 eV vs. the Ag/AgCl electrode; the redox potential of Fc/Fc⁺ has an absolute energy level of −4.80 eV to vacuum. The HOMO and LUMO energy levels were then calculated according to equations,

E_HOMO = −(φ_ox − φ_{ox(ferrocene)} + 4.80) (eV)

E_HOMO = −(φ_re − φ_{re(ferrocene)} + 4.80) (eV)

where φ_ox is the onset oxidation potential vs. Ag/AgCl and φ_re is the onset reduction potential vs. Ag/AgCl. The film thickness and topography of the polymer thin films were examined with an atomic force microscope (AFM, tapping mode, Bruker). The polymer films were characterized by cross-polarized optical microscopy (Zeiss) and UV-vis-NIR polarization spectroscopy. Grazing-incidence wide-angle X-ray scattering (GIXD) was measured (X-ray wavelength 1.0332 Å, 12.0 keV) at beamline 13A of Taiwan Light Source in National Synchrotron Radiation Research Center (NSRRC), at a distance of 460 mm from the sample to the detector. With an incident angle of 0.2°, GIXD was performed; the pattern was collected with an image plate (Mar345). The scattering wave vector was calibrated with silver behenate and silicon powders. According to the Scherrer equation, the crystalline correlation length (L_c)^29–33 was calculated with

L_c = 0.9λ/(β_hkl cos(θ))

where λ is the X-ray wavelength, β_hkl is the full width at half maximum of (hkl) diffraction in radians, and θ is the Bragg diffraction angle.

Results and discussion

Molecular design, synthesis and characterization

Scheme 1 outlines the synthetic procedure for the PB(DTP)V-DTDPP based polymers. Both PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ were prepared by a Stille coupling of (Sn-B(C₁₂DTP)V) with Br-DTDPP-C₁₂ and Br-DTDPP-C₈C₁₂, respectively. The obtained polymers were Soxhlet extracted with acetone and hexane to remove their small molar mass portion. The structure and purity of each molecule were evaluated with ¹H NMR spectra and mass spectra, as shown in Fig. S1–S3.† The molar mass (M_n) and polydispersity index (PDI) were controlled at 38 kDa and 2.2 for PB(C₁₂DTP)V-DTDPP-C₁₂ and 16 kDa and 1.3 for PB(C₁₂DTP)V-DTDPP-C₁₂C₈, respectively. The decomposition temperatures of both copolymers are ca. 365 and 342 °C, respectively (Fig. S4†). The copolymers were heated up to 320 °C to avoid thermal degradation in the DSC analysis (Fig. S5†). Phase transition was not obviously found for both polymers below 320 °C, which indicated that the melting point of both polymers should be very close to their decomposition temperature. Both polymers displayed good solubilities in chloroform, chlorobenzene and 1,2-dichlorobenzene. Solutions with concentrations of 2 mg mL⁻¹ and 10 mg mL⁻¹ were respectively prepared at room temperature and 60 °C. The solutions prepared at room temperature and 60 °C were able to pass through a 0.45 μm pore size PTFE filter. Hence, chloroform, chlorobenzene and 1,2-dichlorobenzene were used as solvents in the experiments.

Scheme 1 Synthetic route to PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈. Reagents and conditions: (i) DMF, POCl₃, dichloroethane, 0 °C to 40 °C; (ii) TiCl₄, Zn, THF, 30 °C; (iii) n-BuLi, Me₃SnCl, −78 °C to 23 °C; (iv) Pd₂(dba)₃, P(o-tolyl)₃, chlorobenzene, 120 °C.

For further understanding the theoretical planarity of the synthesized polymers, Fig. 1 shows the conformational simulation of the designed DTP-based conjugated backbones by density functional theory (DFT). Although the conformational defects of conjugated polymers in the bulk state are difficult to obtain exactly from the DFT calculation, the DFT simulation can provide a reference of theoretical planarity for revealing the relationship between the molecular design of the synthesized polymers and the OFET device performance. The optimized structures were obtained using the B3LYP/6-311G(d,p) and PCM model (chloroform, ε = 4.71; 1,2 dichlorobenzene, ε = 9.99) in the Gaussian 09 package.

Fig. 1 Front and side views of the model; (a) B(DTP)V-DTDPP, (b) DTP-DTDPP obtained from DFT at B3LYP 6-311G(d,p) and PCM model (chloroform), and (c) B(DTP)V-DTDPP dimer obtained from DFT at B3LYP 6-311G(d,p) and PCM model (chloroform; 1,2 dichlorobenzene). The dihedral angles listed in top and bottom in (c) are obtained by PCM models from chloroform and 1,2 dichlorobenzene, respectively. (d) DFT-optimized geometries and charge-density isosurfaces for the B(DTP)V-DTDPP dimer with a visualization of HOMO and LUMO molecular orbitals (isovalue 0.02).

In the B(DTP)V-DTDPP monomer (Fig. 1a), the linkage between the DTP unit and the vinyl group shows a dihedral angle θ₁ of about 3.2°. Notably, the two DTP units linked with the vinyl group greatly decrease the dihedral angle θ₂ between DTP and thiophene (4.3°) relative to an angle of 7.3° in the single DTP and thiophene case³⁴ (as shown in Fig. 1b). From the side view the monomer backbone of B(DTP)V-DTDPP evidently exhibits great planarity. As shown in Fig. 1c, a dimer of B(DTP)V-DTDPP displays a small angle, ca. 8.1°, between two repeating units, indicating that the polymers can maintain small steric hindrance between the intramolecular units and remain planar. In Fig. 1d, the electron densities of the HOMO and LUMO are well delocalized over the conjugated repeat units of the PB(DTP)V-DTDPP backbones. The coplanarity and delocalized electronic structure of the B(DTP)V-DTDTPP backbone might correspond to a requirement of fabricating highly efficient OFET D–A polymers.

Optical and electrochemical properties

Fig. 2a shows that the normalized UV-vis-NIR spectra exhibit broad dual-band absorptions in the region of 400–1200 nm for the two cases in both solution (in chloroform) and thin film states. The absorption features at high and low energies are attributed to a π–π* transition and intramolecular charge transfer (ICT) between DTP and DPP segments, respectively.^10,35 Relative to the solution state, the broader absorption bands in the film state result from aggregation or orderly π–π stacking, which is helpful to improve the charge transport of devices.³⁵ Interestingly, compared to the λ_max of PB(C₁₂DTP)V-DTDPP-C₁₂C₈, the absorption of PB(C₁₂DTP)V-DTDPP-C₁₂ blue-shifted by 80 nm, which suggested the increase in H-aggregation.^10,21,36 The 0-1 vibration peak is observed in the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film and the 0-0/0-1 c ratio for the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film is larger than the unity, which manifests the existence of stronger intrachain interactions or J-aggregation during solidification and annealing.^20,36 Both polymers featuring identical conjugated backbones, therefore, display similar optical energy bandgaps (E^opt_g), −1.23 and −1.24 eV, estimated from the absorption onsets at a long wavelength, for PB(C₁₂DTP)V-DTDPP-C₁₂C₈ and PB(C₁₂DTP)V-DTDPP-C₁₂, respectively (Table 1). In Fig. 2b, the cyclic voltammograms reveal that the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels are −4.83/−3.53 eV for PB(C₁₂DTP)V-DTDPP-C₁₂ and −4.97/−3.58 eV for PB(C₁₂DTP)V-DTDPP-C₁₂C₈.

Fig. 2 (a) Normalized ultraviolet-visible absorption spectra of PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ in solution and film states. (b) Cyclic voltammograms of PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ films at a scan rate of 100 mV s⁻¹.

Table 1 Molar masses, and optical and electrochemical properties of PB(DTP)V-DTDPP polymers

Polymers	λ ^solmax ^a (nm)	λ ^filmmax ^b (nm)	E ^optg ^c (eV)	E HOMO/ELUMO ^d (eV)	E ^elecg ^e (eV)
a Solution absorption spectra (10^−5 M in trichloromethane). b Thin-film absorption spectra from films spin-cast from TCE solution (5 mg mL^−1). c Optical energy gap estimated from the onset of film absorption. d Cyclic voltammograms were recorded with Fc/Fc^+ (EHOMO = −4.8 eV) as the external reference. e E ^elecg = ELUMO − EHOMO.
PB(DTP)V-DTDPP-C12	765	762	1.24	−4.83/−3.53	1.30
PB(DTP)V-DTDPP-C12C8	841	845	1.23	−4.97/−3.58	1.39

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Fabrication of OFET and I–V characterization

In order to evaluate the clear side chain impact on the intra- and inter-molecular charge transport, off-center spin-coating (OCSC)^27,28 was used to align the polymer chains to form a highly orientated polymer thin film. The OCSC PB(DTP)V-DTDPP devices were fabricated in a bottom-gate top-contact (BGTC) configuration, where the source–drain axis was parallel to the polymer chains’ alignment. The fabrication process is described in the Experimental section. After experiments with various solvents, spin rates, and annealing temperatures (Table 2 and Table S1†), the best device performance was found at an annealing temperature of 220 °C for PB(C₁₂DTP)V-DTDPP-C₁₂ and at 180 °C for PB(C₁₂DTP)V-DTDPP-C₁₂C₈. 1,2-Dichlorobenzene is a better solvent for both polymers compared to chlorobenzene. The OCSC device performance mainly contributed to the intramolecular charge transport behavior of polymer chains. Both the best transfer and output characteristics of PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ devices are depicted in Fig. 3; the hole mobility (μ_h), threshold voltage (V_th) and on/off current ratios (I_on/I_off) are summarized in Table 2. The saturation-regime mobilities were calculated from the slope of a plot of the square root of the drain current (I_DS^−0.5) as a function of gate voltage (V_G). Small I_on/I_off values result from the ionization energies of the two polymers.^8,37 The maximum μ_h attained is 0.69 cm² V⁻¹ s⁻¹ for PB(C₁₂DTP)V-DTDPP-C₁₂ devices and 1.2 cm² V⁻¹ s⁻¹ for PB(C₁₂DTP)V-DTDPP-C₁₂C₈ devices. The average μ_h of the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ device was calculated to be 1.0 cm² V⁻¹ s⁻¹, which is twice that of a PB(C₁₂DTP)V-DTDPP-C₁₂ device (0.52 cm² V⁻¹ s⁻¹). The PB(C₁₂DTP)V-DTDPP-C₁₂C₈ devices had 3 times higher mobility than the PDTP-DTDPP polymers bearing only one DTP unit reported in the literature.¹⁸ As the polymer chains were perpendicular to the source–drain axis, the performances of the OFET devices drastically decreased (Table 2). Compared with OCSC films, the spin-coated (SC) films exhibited a smaller μ_h, 0.061 cm² V⁻¹ s⁻¹, for the PB(C₁₂DTP)V-DTDPP-C₁₂ device and 0.11 cm² V⁻¹ s⁻¹ for the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ device, which resulted from the combined effect of intra-/inter-molecular charge transport and possible defects. The excellent OCSC device performance of PB(C₁₂DTP)V-DTDPP-C₁₂C₈ indicates its superior intramolecular charge transport property and corresponds to the results of UV-visible-NIR spectroscopy (Fig. 2a).

Fig. 3 Current–voltage (I–V) characteristics of OFET. (a) Transfer curves of PB(C₁₂DTP)V-DTDPP-C₁₂ with μ_h = 0.69 cm² V⁻¹ s⁻¹. Transfer curves taken at V_DS = −80 V. (b) Output curves of PB(C₁₂DTP)V-DTDPP-C₁₂ with μ_h = 0.69 cm² V⁻¹ s⁻¹. (c) Transfer curves of PB(C₁₂DTP)V-DTDPP-C₁₂C₈ with μ_h = 1.2 cm² V⁻¹ s⁻¹. Transfer curves taken at V_DS = −80 V. (d) Output curves of PB(C₁₂DTP)V-DTDPP-C₁₂C₈ with μ_h = 1.2 cm² V⁻¹ s⁻¹.

Table 2 Bottom-gate top-contact (BGTC) OFET device performances for B(DTP)V-DTDPP polymers measured under nitrogen

Polymers	Coating method	Coating direction	T a ^a (°C)	μ h, adv(μh, max)^b (cm^2 V^−1 s^−1)	V th ^c (V)	I on/Ioff
a T a indicates the annealing temperature. b Average mobilities and maximum values of hole mobility are shown in parentheses (more than 30 devices were tested for each polymer). c Averaged value of 30 devices. SC indicates spin-coating. All thin films were prepared using ODCB solvent at a spin rate of 2000 rpm.
PB(DTP)V-DTDPP-C12	OCSC	Parallel	180	0.10(0.21)	−33	10^1–10^2
	OCSC	Parallel	200	0.27(0.35)	−28	10^1–10^2
	OCSC	Parallel	220	0.52(0.69)	−27	10^1–10^2
	OCSC	Vertical	220	0.008(0.012)	−27	10^1–10^2
	SC	—	220	0.037(0.061)	−22	10^1–10^2
PB(DTP)V-DTDPP-C12C8	OCSC	Parallel	180	1.0(1.2)	−19	10^1–10^2
	OCSC	Vertical	180	0.032(0.036)	−23	10^1–10^2
	OCSC	Parallel	200	0.53(0.75)	−20	10^1–10^2
	OCSC	Parallel	220	0.12(0.43)	−24	10^1–10^2
	SC	—	180	0.062(0.11)	−25	10^1–10^2

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Chain alignment of OCSC films

Fig. 4a, b, d and e show images, obtained using a polarized optical microscope (POM), of two polymer OCSC thin films. The brightest images are observed only when the aligning direction of the films is tilted 45° to either the polarizer or analyzer. This condition indicates that, with the OCSC process, the backbones are oriented mostly along the alignment direction.^6,28,38 To quantify the degree of alignment, we derived the optical dichroic ratio D = A_∥/A_⊥ from the polarized UV spectra (Fig. 4c and f); A_∥ and A_⊥ denote the absorbance maximum of a vibrational signal in polarized and depolarized UV curves respectively.⁶ The PB(C₁₂DTP)V-DTDPP-C₁₂C₈ exhibits a larger dichroic ratio (D = 2.18) than PB(C₁₂DTP)V-DTDPP-C₁₂ (D = 1.97), revealing that the designed branched side chains can support a greater degree of chain alignment.

Fig. 4 Polarized optical-microscopy images of an off-center spin-coated (a–b) PB(C₁₂DTP)V-DTDPP-C₁₂ film after annealing at 220 °C and (d–e) PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film after annealing at 180 °C. P and A indicate the axes of the microscope polarizer and the plane of light vibration, respectively. Crystallites with polymer backbones parallel to one polarizer remain dark whereas those at 45° are the brightest. Polarized UV spectra of an off-center spin-coated (c) PB(C₁₂DTP)V-DTDPP-C₁₂ film after annealing at 220 °C and (f) PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film after annealing at 180 °C.

Surface topology of OCSC films

Surface roughness has been demonstrated to affect the OFET performance in some conjugated copolymers.^6,10,37 In Fig. 5, AFM height images reveal the surface topographies of the optimal devices for realizing the roughness of PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ films relative to their device performance. Their phase images exhibit fiber-like textures (domain size, a few hundred nm) as shown in Fig. S6.† The thickness of the OCSC polymer thin films is about 40 nm. Roughness parameters (R_t) of the top surface of the films, estimated from the full area of the AFM height images, are 1.89 and 0.44 nm for the PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ devices, respectively. Roughness parameters (R_b) of the bottom surface were obtained from both the films peeled off the substrate of the optimal devices as shown in the AFM height images (Fig. 5d and e). The R_b is 1.13 nm for the PB(C₁₂DTP)V-DTDPP-C₁₂ film and 0.44 nm for the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film.

Fig. 5 Tapping-mode AFM height images of the (a, d) PB(C₁₂DTP)V-DTDPP-C₁₂ film after annealing at 220 °C and (b, e) PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film after annealing at 180 °C. (c) The cross-section profile of the dashed line on the top surface (a) and (b). (f) The cross-section profile of the dashed line on the bottom surface (e) and (f).

The statistical roughness parameters and 1D height profiles extracted arbitrarily from the AFM height images (Fig. 5c and f) obviously reveal that the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ has a smaller roughness value than the PB(C₁₂DTP)V-DTDPP-C₁₂. We also found the same result from the device fabricated using different solvents and spin rates (Fig. S7†). The smaller top and bottom surface roughness parameters of the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film mean that the thin film is much smoother and fits on the substrate. The smoother interface between the semiconductor and dielectric layers on bottom-gate top-contact (BGTC) devices decreases the opportunity of charge trapping, as a result PB(C₁₂DTP)V-DTDPP-C₁₂C₈ films exhibit higher charge transport.^6,10,37 Higher surface roughness in PB(C₁₂DTP)V-DTDPP-C₁₂ may be attributed to the nature of the materials, e.g. the thermal expansion coefficient, or larger stress relaxation after the thermal annealing.

Side-chain effect on mesostructures

To determine the characteristics of the molecular packing and the degree of packing order in the microstructures of the PB(DTP)V-DTDPP films, we performed grazing incidence X-ray diffraction (GIXD) with the incident X-ray beam respectively parallel and perpendicular to the aligning direction of the optimal OCSC devices, as shown in Fig. 6 and S8.† The GIXD patterns exhibit lamellar-like diffractions, e.g. (100), (200) and (300), along the out-of-plane direction for the two cases of the PB(DTP)V-DTDPP polymers (Fig. 6c and d). As shown in Table 3, the lamellar thickness (d₁₀₀ = 2.03 nm) for the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ OCSC film larger than that for the PB(C₁₂DTP)V-DTDPP-C₁₂ case (d₁₀₀ = 1.85 nm) can be ascribed to the steric bulk of the branched C₁₂C₈ side chains. The isotropic scattering halo represents the mean distance between liquid-like alkyl side chains. The (010) diffraction corresponding to the π–π stacking distance emerged along the in-plane direction and partly overlapped with the isotropic halo, attributed to disordered alkyl side chains, when the incident X-ray beam was parallel to the aligning direction of the OCSC films (Fig. 6e and f), but weakened when the incident X-ray beam was perpendicular to the aligning direction of the OCSC films (Fig. 6e, f, S6a and b†) in the two cases. This result indicates that, with the OCSC process, the backbones displayed high orientation along the aligning direction. The lamellar thickness (d₁₀₀) is smaller than the lateral dimension of the backbones with extended side chains (3.8 nm), indicating that the inter-digitation of the alkyl side chains might emerge within the lamellae. The π–π stacking distance, estimated from the Gaussian fitting peak, of PB(C₁₂DTP)V-DTDPP-C₁₂ is shorter than that of PB(C₁₂DTP)V-DTDPP-C₁₂C₈ (Fig. S9,†Table 3). According to the peak width δb of the (100) diffraction calculated with the Scherrer equation,^29–33,39 the grain size (L_c) of PB(C₁₂DTP)V-DTDPP-C₁₂C₈ (20.2 nm) is 3.2 nm smaller than that of PB(C₁₂DTP)V-DTDPP-C₁₂, although the former has greater mobility (as manifested in Fig. 3). This result is in contrast to a common conclusion that a small grain size might produce a large grain boundary resulting in many traps or barriers to slow down the charge transport. Notably, sharp diffractions at (100), (200) and (300) but weak (010) diffraction in the two cases indicate the paracrystalline nature of the backbone packing existing within the highly aligned lamellae.^29–33 The packing order of backbones within each layer is thus expected to become a decisive factor in charge transport.

Fig. 6 2D GIXD patterns of the (a) PB(C₁₂DTP)V-DTDPP-C₁₂ OCSC film after annealing at 220 °C and (b) PB(C₁₂DTP)V-DTDPP-C₁₂C₈ OCSC film after annealing at 180 °C. Out-of-plane profiles extracted from the corresponding 2D GIXD patterns of (c) PB(C₁₂DTP)V-DTDPP-C₁₂ and (d) PB(C₁₂DTP)V-DTDPP-C₁₂C₈ films. GIXD in-plane profiles extracted from the corresponding 2D GIXD patterns of (e) PB(C₁₂DTP)V-DTDPP-C₁₂ and (f) PB(C₁₂DTP)V-DTDPP-C₁₂C₈ films.

Table 3 Assignments for the out-of-plane GIXD intensity profiles obtained from PB(DTP)V-DTDPP polymer thin films after thermal annealing

Polymers	Coating method	d (100) (Å)	d (010) ^a (Å)	L p(h00) (Å)	N̄(h00)^b	g (h00) (%)
a π–π stacking distance estimated from the Gaussian fitting peak. b Average number of the diffraction planes, calculated from Lp = N̄d. c Paracrystalline distortion parameter of the (h00) plane. SC indicates spin-coating.
PB(DTP)V-DTDPP-C12	OCSC	18.5	3.8	226.2	12.3	4.4
	SC	20.5	4.0	—	—	—
PB(DTP)V-DTDPP-C12C8	OCSC	20.3	4.0	182.8	9.0	3.9
	SC	20.4	4.0	—	—	—

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For paracrystalline systems, the distortion parameter (g) of paracrystals is determined from the slope (= g²π²/d) of the δb–h² plot, in which a larger g value means more highly distorted paracrystals.^29–33 The true paracrystalline size (L_p) can be obtained from the intercept with a linear extrapolation to h = 0; d and h are the lamellar thickness and the order of diffraction, respectively. Fig. 7 shows the δb–h² plot extracted from the GIXD data of the two cases. The resultant paracrystalline parameters are listed in Table 3. The distortion parameter of PB(C₁₂DTP)V-DTDPP-C₁₂C₈ (g = 3.9) is smaller than that (g = 4.4) of PB(C₁₂DTP)V-DTDPP-C₁₂, revealing that high packing distortion of the backbones exists in the lamellar structure of PB(C₁₂DTP)V-DTDPP-C₁₂.

Fig. 7 δb–h² curves of the annealed PB(C₁₂DTP)V-DTDPP-C₁₂ and PB(C₁₂DTP)V-DTDPP-C₁₂C₈ thin films extracted from the GIXD analysis of OCSC films. The error bar is the standard deviation from various measurements.

An order parameter obtained from the dichroic ratio is also related to the packing order of backbones within the lamellae. Based on the assumption of an optical dipole moment parallel to the copolymer backbone, the 2D structural-order parameter Φ can be expressed in terms of the dichroic ratio as Φ = (D − 1)/(D + 1).³⁸ The Φ value is between 0 and 1, corresponding to the isotropic orientation to the perfect uniaxial orientation. The order parameter of the PB(C₁₂DTP)V-DTDPP-C₁₂C₈ film (Φ = 0.37) is greater than that of the PB(C₁₂DTP)V-DTDPP-C₁₂ film (Φ = 0.32). This result, consistent with the distortion parameter g, indicates that PB(C₁₂DTP)V-DTDPP-C₁₂C₈ features a high packing order of backbones within the layers. Furthermore, the deviation of the lamellar thickness (d₁₀₀) and backbone stacking distance (d₀₁₀) values arises from the different film-forming processes in the linear C₁₂ side chain case, whereas both values are constant for the OCSC and spin-coating processes in the branched C₁₂C₈ side chain case (Fig. 6 and S6,†Table 3).

The above-mentioned results for the side-chain effect, featuring steric confinement and increased side-chain density, strongly indicate that the branched C₁₂C₈ side chains maintain the backbone coplanarity and act like latches between crystalline layers to stabilize the lamellar packing of the bulky backbones. Nevertheless, the flexible linear C₁₂ side chains cause a slight twist in the backbone and lack steric hindrance, resulting in a dynamic variability of intra- and inter-chain distances. Although the thin films of PB(C₁₂DTP)V-DTDPP-C₁₂ show a larger grain size, as illustrated in Fig. 8, the packing order of their backbones within the lamellae might strongly influence their charge transport, even with just a small difference between the meso-structures in the bulky backbone system. We therefore suggest that appropriate side chains play a key role in maintaining the backbone coplanarity and stabilizing the packing order of bulky backbones and their charge-transport characteristics.

Fig. 8 Schematic illustration of meso-structures with edge-on backbone orientation for OFET devices of (a) PB(C₁₂DTP)V-DTDPP-C₁₂ and (b) PB(C₁₂DTP)V-DTDPP-C₁₂C₈. Yellow arrows in (a) and (b) indicate the charge transfer. The purple and blue blocks in the bottom represent crystal grains. a, b and c axes are lattice parameters.

Conclusions

Two PB(DTP)V-DTDPP based conjugated polymers with linear C₁₂ and branched C₁₂C₈ side chains were prepared and characterized. Both polymers show strong absorption in the 600–1000 nm region. An off-center spin-coating method was used to fabricate the polymer thin films and OFET devices. Strong anisotropy for the polymer thin film was observed in polarized optical microscopy and polarized UV spectroscopy. The OFET results demonstrate that the π-extended PB(DTP)V-DTDPP polymers displayed a hole mobility 3 times higher than that of the PDTP-DTDPP polymers bearing only one DTP unit reported in the literature. The PB(DTP)V-DTDPP with branched C₁₂C₈ side chains exhibits a maximum hole mobility of 1.2 cm² V⁻¹ s⁻¹, which is about twice that of the linear C₁₂ side-chain case (0.69 cm² V⁻¹ s⁻¹). This improved performance is attributed to the branched C₁₂C₈ side chains, causing steric hindrance, that can maintain the backbone coplanarity and act like latches between crystalline layers to efficiently stabilize the π–π stacking of backbones and the 2D packing order of the paracrystalline structure. For a long conjugated polymer backbone, we suggest that a rigid backbone with matching side-chain density is necessary to adjust the stacking order within the layer for an impressive OFET performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The Ministry of Science and Technology of Taiwan, ROC (MOST 101-2221-E009-026) provided financial support.

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Footnote

† Electronic supplementary information (ESI) available: Full characterization of all new compounds, as well as details of physicochemical properties and molecular-packing features. See DOI: 10.1039/c7py01340j

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