Effects of branching position of alkyl side chains on ordering structure and charge transport property in thienothiophenedione- and quinacridone-based semiconducting polymers

Abstract

To investigate the effect of the branching position of the alkyl groups on the side chain of semiconducting polymers, we synthesized two series of semiconducting polymers based on thienothiophene-2,5-dione (PTTD4Ts) and quinacridone (PQA2Ts). 2-Decyltetradecyl, 3-decylpentadecyl, 4-decylhexadecyl, and 5-decylheptadecyl groups were used, and the branching position was systematically varied from the second carbon from the backbone to the fifth carbon. These branched side chains are introduced into the thiophene ring for PTTD4Ts and the quinacridone unit for PQA2Ts. The polymer thin films exhibited small but clear differences in their optical absorption spectra, suggesting that the intermolecular interaction in the solid state varied based on the branching position. The grazing incident X-ray diffraction study revealed that the π–π stacking d-spacing of both polymers decreased when the branching position was moved away from the backbones, indicating that the intermolecular interaction was enhanced. Therefore, regardless of the core where the alkyl groups were introduced, the branching position effectively improved the ordering structure of the polymers, which was most likely due to suppressed steric hindrance. Although PTTD4Ts did not exhibit a clear correlation between the branching position and charge carrier mobility, the mobility of PQA2Ts gradually increased as the branching position moved away from the backbone.

Introduction

Solution-processable semiconducting polymers have been extensively studied as low-cost, flexible, and lightweight electronic materials for potential applications, including organic field-effect transistors (OFETs),¹ organic light-emitting diodes,² and organic photovoltaics.³ In the development of semiconducting polymers, the design of the conjugated backbone is of primary importance because it substantially affects the molecular orbital energies,^{4, 5, 6} bandgaps,^{7, 8} and ordering structure^{9, 10} that determine the device performance of the polymers. To date, enormous effort has been focused on the exploration of backbone structures (that is, new conjugated building units and their combinations). In parallel, flexible side chains are also important components of semiconducting polymers, and these side chains have a role in determining the ordering structures, the electronic properties, and their solubility in organic solvents. Pioneering studies of the side chains have been performed by McCullough^{11, 12} and Rieke,¹³ in which they successfully controlled the regiochemistry of the alkyl side chains in poly(3-alkylthiophene)s (P3ATs) during the polymerization of asymmetrical 3-alkylthiophene monomers to yield head-to-tail regioregular P3ATs (rrP3ATs). In fact, rrP3ATs exhibited highly ordered structures with lamellar and π–π stacking structures in a thin film. However, regiorandom P3ATs (raP3ATs) exhibited an amorphous-like or disordered structure, which resulted in rrP3ATs possessing high electrical properties, such as conductivity and hole mobility.¹⁴ The placement, length, and topology of the side chain are also crucial factors for the ordering structure^{15, 16, 17} and the backbone orientation, such as edge-on and face-on, which affect the direction of charge transport.¹⁸

Recently, branched alkyl groups have been widely used as the side chain in semiconducting polymers because they provide better solubility than simple linear alkyl groups. In particular, for semiconducting polymers with donor–acceptor backbones that have strong intermolecular interactions, branched alkyl groups are necessary for solubility in organic solvents.^{19, 20} Although such bulky branched alkyl groups efficiently increase the solubility, they suppress the aggregation or close interchain π–π stacking of the polymers relative to the linear alkyl groups, resulting in inefficient charge transport.²⁰ A recent study demonstrated that moving the branching position in the branched alkyl group away from the conjugated backbone reduced the steric effect and π–π stacking distances, which enhanced the charge carrier mobilities in OFET devices.^{21, 22, 23, 24, 25} However, in most of these studies, branched alkyl groups are introduced into the N-position of limited cores, such as isoindigo and diketopyrrolopyrrole.^{26, 27, 28}

Here, to further systematically study the branching position effect, we introduced a series of branched alkyl groups with different branching positions (that is, 2-decyltetradecyl (2DT), 3-decylpentadecyl (3DP), 4-decylhexadecyl (4DH), and 5-decylheptadecyl (5DH) groups; Figure 1) into the 3-position of the thiophene ring and the N-position of QA (quinacridone). Then, the 3-alkylthiophenes were combined with the TTD (thienothiophene-2,5-dione) unit, which was copolymerized with the unsubstituted bithiophene unit (PTTD4Ts), and the alkylated QA units were also copolymerized with the unsubstituted bithiophene unit (PQA2Ts; Figure 1). The electronic properties, microstructure, and the OFET performance of the polymer thin films were characterized, and the correlation with the branching positions is discussed.

Figure 1

Chemical structures of PTTD4Ts, PQA2Ts, and branched alkyl side chains. A full color version of this figure is available at Polymer Journal online.

Results and Discussion

The syntheses of the branched alkyl groups are shown in Scheme 1. The 2DT, 3DP, 4DH, and 5DH groups with branching positions at the second, third, fourth, and fifth carbon from the backbone were chosen for the side chains. In all of the cases, the branched parts are decyl and dodecyl groups. 3-Decylpentadecan-1-ol (3DP-OH), 4-decylhexadecan-1-ol (4DH-OH), and 5-decylheptadecan-1-ol (5DH-OH) were synthesized from 2-decyl-1-tetradecyl bromide (2DT-Br) via the formation of the Grignard reagent followed by treatment with paraformaldehyde,²⁹ ethylene oxide,³⁰ and oxetane,³¹ respectively. These alkyl alcohols were brominated to afford the corresponding bromides (that is, 3-decyl-1-pentadecyl bromide (3DP-Br), 4-decyl-1-hexadecyl bromide (4DH-Br), and 5-decyl-1-heptadecyl bromide (5DH-Br)). These alkyl bromides were successfully introduced into the thiophene rings in the TTD-based monomers (TTD2TBr2s) and QA-based monomers (QABr2s) according to previously reported procedures.^{32, 33} These monomers were copolymerized with 5,5′-bis(trimethylstannyl)-2,2′-bithiophene using the Migita–Kosugi–Stille coupling reaction in the presence of tetrakis(triphenylphosphine)palladium(0) to yield the corresponding polymers (PTTD4Ts and PQA2Ts), as shown in Scheme 2. The polymers were washed with methanol, hexane, and chloroform using a Soxhlet apparatus to remove the low molecular weight fraction. Then, the purified polymers were collected by chlorobenzene.

By moving the branching point away from the backbone, the polymer solubility slightly decreased in both cases. The number-average molecular weights (M_n) of the polymers ranged from 39 to 50 kDa for PTTD4Ts and 9 to 13 kDa for PQA2Ts based on gel permeation chromatography using o-dichlorobenzene as the eluent at 140 °C (Table 1). The small deviation of M_n in PTTD4Ts and PQA2Ts would reduce the effects of the molecular weight and should provide a fair comparison of the polymer properties. The large polydispersity indices of the polymers could be an artifact of the bimodal peaks in the gel permeation chromatography curves, which are most likely caused by strong aggregation of the polymers in solution (Supplementary Figure S1).³² On the basis of differential scanning calorimetry, all of the polymers were thermally stable because they did not exhibit a melting peak below 300 °C (Supplementary Figure S2).

Table 1 Molecular weights of polymers^a

To investigate the electronic properties of the polymers, we first evaluated the molecular orbital energies via cyclic voltammetry using polymer thin films (Figure 2). In all the cases, reversible oxidation and reduction signals were observed. The redox potentials and energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are summarized in Table 2. Both the HOMO and LUMO energy levels (E_HOMOs and E_LUMOs) shifted upward from 3DP to 4DH when the branching position moved away from the backbone. The shift in the E_HOMO was significant relative to that in the E_LUMO. The E_HOMO of PTTD4T-2DT and -3DP was approximately −5.2 eV, and the E_HOMO of PTTD4T-4DH and -5DH was approximately –5.1 eV. Similarly, the E_HOMO of PQA2T-2DT and -3DP was approximately –5.0 eV, and the E_HOMO of PQA2T-4DH and -5DH was approximately –4.9 eV. These E_HOMOs were consistent with those evaluated by photoemission yield spectroscopy (Supplementary Figure S3).

Figure 2

Cyclic voltammograms of the polymers. (a) PTTD4Ts, (b) PQA2Ts. A full color version of this figure is available at Polymer Journal online.

Table 2 Physicochemical properties of the polymer films

Next, we measured the UV–Vis–NIR (ultraviolet–visible–near infrared) absorption spectra of the polymer thin films (Figure 3) and the polymer solution (Supplementary Figure S4). The spectra of the thin films and solutions were similar, indicating that the polymers aggregated even in solution. The absorption maximum wavelengths and optical bandgaps extracted from the film absorptions are summarized in Table 2. In both the cases, the polymers with different side chains exhibited similar absorption spectra with strong absorption bands at approximately 600–1000 nm for PTTD4Ts and 400–500 nm for PQA2Ts, and, therefore, similar optical bandgaps (E_gs) of approximately 1.2 eV and 2.0 eV, respectively, were obtained. However, the peak tops shifted slightly. In PTTD4Ts, PTTD4T-3DP, -4DH, and -5DH exhibited clearer vibronic structures than PTTD4T-2DT. By moving the branching position away from the backbone, the absorption maximum (λ_max) of 818 nm for PTTD4T-2DT was red-shifted to 858 nm for PTTD4T-3DP, 876 nm for PTTD4T-4DH, and 871 nm for PTTD4T-5DH. This shift implies that PTTD4T-3DP, -4DH, and -5DH possess better intermolecular interaction or ordering in the solid state than PTTD4T-2DT. A similar trend for the spectral difference as a function of the branching position was also observed for PQA2Ts. PQA2T-3DP, -4DH, and -5DH exhibited clearer vibronic structures than PQA2T-2DT, and the λ_max of 456 nm for PQA2T-2DT was red-shifted to 481 nm for PQA2T-3DP, 484 nm for PQA2T-4DH, and 483 nm for PQA2T-5DH. Although the change in the E_g may not directly correlate with a change in the energy levels, the change in the vibronic structure may qualitatively be in agreement.

Figure 3

UV–Vis–NIR absorption spectra of the polymer thin films after annealing at 300 °C. (a) PTTD4Ts and (b) PQA2Ts. Insets show the close-up of the peaks. UV–Vis–NIR, ultraviolet–visible–near infrared.

The microstructures of the polymer thin films were investigated using grazing incident X-ray diffraction (GIXD) measurements. Figure 4 shows the two-dimensional GIXD patterns of the polymer thin films annealed at 300 °C, and these films were prepared on octadecyltriethoxysilane-treated Si/SiO₂ substrates in the same manner as the fabrication of the OFET devices. In the two-dimensional GIXD patterns, both the PTTD4Ts and PQA2Ts thin films provided strong spot-like diffractions up to the fifth-order along the quasi-q_z axis, which correspond to a lamellar structure (h00), and slit-like diffractions along the q_xy axis, which correspond to a π–π stacking structure (010). This result indicates that all of the polymers are crystalline. In addition, the polymer backbones are oriented in an edge-on manner on the substrate, which is regarded as a favorable thin film structure for lateral charge transport.

Figure 4

2D-GIXD patterns of the polymer thin films on ODTS-treated SiO₂/Si substrates annealed at 300 °C. (a) PTTD4T-2DT, (b) PTTD4T-3DP, (c) PTTD4T-4DH, (d) PTTD4T-5DH, (e) PQA2T-2DT, (f) PQA2T-3DP, (g) PQA2T-4DH, and (h) PQA2T-5DH. 2D, two-dimensional; GIXD, grazing incident X-ray diffraction; ODTS, octadecyltriethoxysilane. A full color version of this figure is available at Polymer Journal online.

The lamellar and π–π stacking d-spacings were extracted from the out-of-plane and in-plane profiles of the two-dimensional GIXD images (Supplementary Figure S5, Supplementary Table S1) and are plotted as a function of the branching position from the backbone, as shown in Figure 5. In both cases, the lamellar d-spacing increased monotonically as the branching position increased. In PTTD4Ts, the π–π stacking d-spacing was 3.61 Å for PTTD4T-2DT and -3DP, which decreased to 3.56 Å for -4DH and 3.55 Å for -5DH. In PQA2Ts, the π–π stacking d-spacings was 3.69 Å for PQA2T-2DT, which decreased to 3.50 Å for -3DP and further decreased to 3.47 Å for -4DH and -5DH. This result indicates that the intermolecular interactions of the polymers increased, which was most likely due to reduced steric hindrance with the distance of the branching point regardless of the π-cores where the alkyl group was introduced. This change in the ordering structure is consistent with the change in the electronic properties described above.

Figure 5

Lamellar (filled circle) and π–π stacking (open square) d-spacings of the polymers calculated from the GIXD profiles along the ~q_z and q_xy axes, respectively, as a function of the branching position (the number of carbon from the backbone). (a) PTTD4Ts and (b) PQA2Ts. GIXD, grazing incident X-ray diffraction.

We also performed atomic force microscopy on the polymer thin films (Figure 6). The atomic force microscopy images indicated that all of the thin films had smooth surfaces (root-mean-square roughness <2 nm) and similar step-and-terrace surface morphologies with step heights of approximately 2~3 nm, which likely correspond to the lamellar spacing. This result also suggests that the polymers formed lamellar structures with edge-on orientations, and the morphology was independent of the branching position of the side chains.

Figure 6

AFM height images and root-mean-square (RMS) roughness of the polymer thin films. (a) PTTD4T-2DT, (b) PTTD4T-3DP, (c) PTTD4T-4DH, (d) PTTD4T-5DH, (e) PQA2T-2DT, (f) PQA2T-3DP, (g) PQA2T-4DH, and (h) PQA2T-5DH. AFM, atomic force microscopy. A full color version of this figure is available at Polymer Journal online.

To examine the charge transport properties of the polymers, bottom-gate-top-contact OFET devices using the polymer thin films were fabricated and characterized. The polymer thin films were deposited on octadecyltriethoxysilane-treated Si/SiO₂ substrates via spin-coating the polymer solutions followed by annealing at 300 °C in a glove box. Then, the source and drain gold electrodes, which have a channel length of 40 μm and a channel width of 1500 μm, were vacuum deposited onto the polymer thin films. The typical transfer and output characteristics of the devices measured in air are shown in Figure 7. The PTTD4T-based devices exhibited V-shape transfer curves under both p- and n-channel operations, which are typical of ambipolar OFETs. However, the PQA2T-based devices worked only under p-channel operation. All the PTTD4T- and PQA2T-based devices exhibited good saturation behaviors in the output curves. The hole (μ_h) and electron (μ_e) mobilities of the OFET devices in the saturation regime (V_g=−60 V for p-channel operations or +60 V for n-channel operation) and the threshold voltages were extracted from the transfer curves and are summarized in Table 3.

Figure 7

Transfer (a–d, i–l) and output (e–h, m–p) characteristics of typical OFET devices based on PTTD4Ts (a–h) and PQA2Ts (i–p). OFET, organic field-effect transistor. A full color version of this figure is available at Polymer Journal online.

Table 3 OFET characteristics of the polymers

In PTTD4Ts, the average μ_h and μ_e ranged from 0.11 to 0.34 cm² V⁻¹ s⁻¹ and 0.054 to 0.17 cm² V⁻¹ s⁻¹, respectively, whereas in PQA2Ts, the μ_h ranged from 0.15 to 0.33 cm² V⁻¹ s⁻¹. In PTTD4Ts, a clear correlation was not observed between the mobility and the ordering structure. The polymers with a branching point farther from the backbone did not necessarily exhibit higher mobilities even though the π–π stacking was enhanced. It is important to note that the device performance is sensitive to many other factors.^{34, 35} Because the energy levels that determine the charge injection and morphology are similar regardless of the alkyl groups. We speculate that some other factors, such as interfacial structures, may affect the OFET performance more than the bulk ordering structure in PTTD4Ts. By contrast, the OFETs based on PQA2Ts exhibited a clear correlation between the μ_h and the ordering structure. The μ_h tended to increase as the π–π stacking d-spacing decreased even though the mobility variation was moderate. Because the π–π stacking d-spacing also exhibited a clear correlation with the branching position, the branching position is an important design parameter for developing high performance polymers.

Conclusion

We synthesized two series of semiconducting polymers (PTTD4Ts and PQA2Ts) bearing 2-decyltetradecyl (2DT), 3-decylpentadecyl (3DP), 4-decylhexadecyl (4DH), and 5-decylheptadecyl (5DH) groups as the side chain, in which the branching position was systematically moved away from the backbone. While the alkyl groups were introduced into the large π-core (QA unit) in PQA2Ts, they were introduced into the thiophene ring in PTTD4Ts. In the absorption spectra, although the absorption edge that corresponds to E_g did not change, the vibronic structure gradually changed (that is, the low-energy peak intensified) as the branching distance in both polymer systems increased. This result suggests an increased order in the polymer packing structure in the thin film. The GIXD measurement revealed that the π–π stacking d-spacing gradually decreased as the branching position moved away from the backbone, indicating that the intermolecular interactions were enhanced due to suppression of the steric hindrance. Although a clear correlation between the branching position and the charge carrier mobility was not observed in PTTD4Ts, the charge carrier mobility increased as a function of the branching distance in PQA2Ts. These results indicate that the branching position affects the intermolecular interaction of the polymer backbones regardless of the substitution position or the π-cores where the alkyl groups were introduced. Therefore, careful choice of the branching position is required for semiconducting polymers to optimize the ordering structure and the charge transport property.

Synthesis of branched alkyl groups. THF, tetrahydrofuran.

Synthesis of polymers.