Controlling Poly(3-hexythiophene) Hierarchical Polymer/SWCNT Nanohybrid Shish-Kebab Morphologies in Marginal Solvents

Abstract

In organic optoelectronic devices, the self-assembly behavior of the conjugated polymer poly(3-hexylthiophene) (P3HT) into structured aggregates significantly influences the device’s performance, with processing conditions playing a key role. Incorporating carbon nanotubes (CNTs) into a P3HT solution can form hierarchical supramolecular structures known as nanohybrid shish-kebabs (NHSKs). These structures alter the morphology of polymer aggregates and provide an alternative pathway for improved charge transport in thin film devices. Herein, we investigated the impact of solvent quality using different combinations of chloroform and anisole during the quasi-isothermal crystallization of P3HT:CNTs. We found that NHSKs of different nanowire lengths can be formed through changing solvent quality while maintaining a constant P3HT:SWCNT ratio and a constant SWCNT concentration. Optical absorbance measurements showed that increasing the amount of the good solvent (chloroform) to 10.19% (v/v) reduced the exciton bandwidth by 36.4% compared to the NHSK solution that only contained ~2.37% (v/v). This observation demonstrates the importance of solvent quality and how this processing parameter directly leads to the enhanced crystallization of supramolecular structures.

1. Introduction

Conjugated polymers, specifically poly(3-hexylthiophene) (P3HT), are a well-studied class of materials that have been researched extensively for their use in organic field effect transistors [1,2,3,4], organic photovoltaic cells [5,6], and organic light-emitting diodes [7,8]. The tendency for P3HT to self-assemble into π-stacked lamellar structures plays an integral role in the performance of optoelectrical devices when used as an active channel material [9,10]. Processing conditions of these supramolecular structures significantly affect the degree of structural order [11]. For example, defects induced during the crystallization process will lead to trapped charge states, which inhibit transport within the P3HT film [12]. An important parameter to consider in the case of regioregular P3HT is the crystalline aggregate stacking characteristics, which can typically be characterized through the order across the polymer chain (H-aggregates) or along the polymer chain (J-aggregates) [13]. Spano et al. [9] developed an HJ-aggregate model for a P3HT thin film that correlates spectral signals with H-aggregate (positive) and J-aggregate (negative) electronic coupling. Thus, achieving optimal optoelectrical performance relies heavily on increasing the fraction and interconnectivity of ordered regions within the film [14,15]. The formation of aggregates is affected by various processing parameters, including the P3HT molecular weight, solvent types, crystallization temperatures, UV exposure, ultrasonication, and solvent quality [16,17,18,19,20].

An alternative approach to minimizing defects and promoting order is utilizing nanocomposites with carbon nanotubes (CNTs)—specifically, developing hierarchical supramolecular structures. P3HT, with a zigzag chain conformation, can self-assemble to form periodic crystals on the CNT surface due to π-π interactions inducing a soft-epitaxial heterogeneous crystallization process [21,22]. These structures are formally known as nanohybrid shish-kebabs (NHSKs) [23]. The inclusion of CNTs to modify the ordering of P3HT chains while also serving as an alternate path for enhanced charge transport [24] makes NHSKs a promising candidate for an active channel material in optoelectrical devices. Single-walled carbon nanotubes (SWCNTs) and P3HT composite films have been explored for their use in optoelectronic devices. Bergemann et al. [25] developed a highly responsive device made from P3HT spin-coated on aligned SWCNTs. In addition, Bahrami et al. [26] fabricated a solution-processable thin-film transistor with good photoresponsitivity. The composite film was composed of SWCNTs (previously deposited onto a silicon wafer) and submerged in a P3HT solution. Both of these studies show promising results but provide limited details on the crystallinity of the P3HT aggregates and the morphology of the film. In addition, both methods are unlikely to form hierarchical supramolecular structures, i.e., NHSKs.

In the P3HT:SWCNT NHSK system, the growth mechanism of crystalline structures is reported to be an epitaxial scenario involving two stages. In the first stage, the P3HT chains align along the SWCNT axis through surface-mediated stretching of the polymer chains [27]. The alignment of P3HT chains along the SWCNT surface acts as the nucleation site for P3HT crystallization [27,28]. The second stage involves the periodic nucleation of P3HT crystalline lamellae stimulated by the availability of polymer chains [27]. P3HT stacking occurs perpendicular to the SWCNT long axis, forming the supramolecular structures [29,30,31]. Other possible factors controlling the growth of NHSKs are the geometric confinement of the absorbed P3HT chains [28,32] and the P3HT fold surface free energy [33], the latter being extremely sensitive to the P3HT:SWCNT concentration ratio. A common method to fabricate NHSKs follows a quasi-isothermal solution crystallization of P3HT:SWCNT, as depicted in Figure 1. In this method, a good or marginal solvent with respect to P3HT is used to drive the aggregation and self-assembly of the polymer chains through the heating and subsequent cooling of the solution.

Several reported factors influence the morphology and order of the NHSKs’ structure, including CNT curvature/diameter, CNT concentration, the P3HT molecular weight, solvent type, crystallization temperature, and the P3HT:SWCNT ratio [21,28,33,34,35]. A key limiting factor in optimizing the hybrid P3HT:SWCNT film in optoelectrical devices is the lack of a comprehensive understanding of the solvent vehicle to develop these supramolecular structures. Researchers have shown that the solvent quality (the ratio of good to poor solvent) controls the microstructural self-assembly of P3HT chains to achieve a higher degree of crystallinity in homogeneous P3HT nanofibers [18,20]. We hypothesize that applying this understanding of P3HT nanofibers to the hybrid P3HT:SWCNT will improve the ordering of the supramolecular structures. In this work, P3HT:SWCNT NHSKs were formed from different combinations of a mixed solvent vehicle composed of chloroform (good solvent) and anisole (poor solvent) to comprehensively understand the influence of solvent quality on nanowire formation and morphology. P3HT is highly soluble in chloroform, containing a solubility limit of 38 mg/mL [36], and anisole is a commonly used marginal solvent that induces the aggregation of P3HT. It was used as a marginal solvent due to aggregate ordering and was characterized through UV-Vis spectroscopy and compared to its associated morphology via electron microscopy. We demonstrated that the nanowire length distribution and order of polymer aggregates in the NHSKs can be directly optimized by increasing the percentage of good solvents within a co-solvent vehicle. These new findings can lead to the improved performance of P3HT/SWCNT composite materials for optoelectronic applications.

2. Materials and Methods

2.1. Preparation of SWCNT Supramolecular Structures

All solvents and reagents used were purchased from Sigma-Aldrich, including toluene (99.8%), anisole (99.7%), chloroform (99.8%), and acetone (99.5%). CoMoCAT (6,5) chirality SWCNT (773735) with an average diameter of 0.78 nm was also purchased from Sigma-Aldrich. Regioregular Poly(3-hexylthiophene-2,5-diyl) (P3HT) was purchased from Luminescence Technology.

The preparation of the P3HT:SWCNT NHSK structures utilized a similar procedure adopted from Liu et al. [21]; see Figure 1. The as-received SWCNTs were redispersed using P3HT with a weight ratio of 1.7:1 in chloroform. We prepared four SWCNT dispersions: 0.4, 0.25, 0.1, and 0.05 mg/mL. Each solution was tip sonicated at a frequency of 20 kHz, a duty cycle of 50%, and an output of 30% for 45 min. During sonication, the solution was kept in an ice bath. Afterward, the mixture was centrifuged at 7570× g for 1.5 h to remove large bundles/aggregates. The dispersed solutions were bath sonicated (40 kHz) for an additional 30 min to ensure homogeneity post centrifugation.

A quasi-isothermal solution crystallization method was used to form the NHSK supramolecular structures (see Figure 1B) by dissolving 0.2 mg of P3HT in anisole and heating it to 70 °C. The required amount of dispersed SWCNTs was added to the heated solution to create a final mass ratio (between P3HT and SWCNT) of 6 and a final SWCNT concentration of 9.25 × 10⁻³ mg/mL. These specific values were selected as they were determined to form the NHSK structure experimentally. The combined solution was air-cooled to room temperature and left undisturbed for 12 h.

Table 1. Comparison of NHSK solutions.

Sample ID	SWCNT Dispersion		NHSK Solution
	SWCNT Conc. (mg/mL)	P3HT: SWCNT	Anisole Amt. (mL)	Chloroform Amt. (mL)	Chloroform % (v/v)	P3HT:SWCNT Mass Ratio	SWCNT Conc. (mg/mL)
NHSK-0.4 mg/mL	0.40	1:1.7	3.9075	0.0925	2.37	6	0.00925
NHSK-0.25 mg/mL	0.25	1:1.7	3.8520	0.1480	3.84	6	0.00925
NHSK-0.1 mg/mL	0.10	1:1.7	3.6300	0.3700	10.19	6	0.00925
NHSK-0.05 mg/mL	0.05	1:1.7	3.2600	0.7400	22.70	6	0.00925

provides a detailed composition summary of the different NHSK solutions. The ratio of chloroform (good solvent) and anisole (poor solvent) was varied in the NHSK solution to assess the effect of mixed solvent vehicles with disparate solvent quality.

2.2. Characterization

The NHSK solutions were characterized via UV-Vis spectroscopy with quartz cuvettes. Absorption spectra from 350 to 800 nm were recorded on a PerkinElmer Lambda 10 UV/Visible Spectrophotometer (Shelton, CT, USA). TEM analysis was performed on a Talos L120C (Thermo Fisher, Waltham, MA, USA) using a 4 K × 4 K Ceta 16M camera using a low dose mode (~48 e/Å) to minimize irradiation of the samples. TEM grids were prepared by drop-casting 10 µL of the NHSK solution onto a Quantifoil R1.2/1.3 + 2 nm C continuous grid. The grids were previously treated to a plasma glow discharge to create a hydrophilic surface.

The NHSK morphology deposited onto 300 nm SiO₂ substrates was characterized using scanning electron microscopy (SEM) with a Zeiss 1530 electron microscope(Dublin, OH, USA) operating at an accelerating voltage of 5 kV. The films’ surface morphologies were characterized on a Bruker Dimension Icon atomic force microscope (Camarillo, CA, USA) with the tapping mode.

3. Results

3.1. Optical Properties

The differences in the P3HT aggregates formed onto SWCNTs were further investigated through UV-Vis spectroscopy of the resultant solutions. Figure 2 shows the UV-Vis spectra (performed in solution) of the NHSK samples and control samples of P3HT in anisole and chloroform. Similarly to the NHSK solutions, the control samples had a P3HT concentration of 0.05 mg/mL and were heated to 70 °C and aged for 24 h prior to testing. All the NHSK and P3HT/anisole samples had a maximum absorption peak at ~520 nm wavelength with two vibronic peaks at 552–560 nm and 602–610 nm. The peaks in the lower energy domains signify that the aggregates are predominantly H-aggregates with face-to-face oriented chains. The height of the lowest energy peak, 602–610 nm, is defined as (A_0–0), and the preceding peak, 552–560 nm, is (A_0–1). The ratio between the A_0–0 and A_0–1 directly corresponds to excitonic coupling [37]. Specifically, the exciton bandwidth (W) can be estimated using Equation (1), proposed by Spano et al. [9,37]:

$${\displaystyle \frac{A_{0–0}}{A_{0–1}}}=({{\displaystyle \frac{1-\frac{0.24W}{E_p}}{1+\frac{0.073W}{E_p}}})}^2$$

(1)

where E_p is the C=C symmetric stretch and is assumed to be 0.18 eV [37,38].

Table 2. Absorbance ratios of lowest energy peaks.

Sample ID	A0–1/A0–2	A0–0/A0–1	Exciton Bandwidth (meV)
P3HT/Ansiole	0.742	0.508	177.1
NHSK-0.4 mg/mL	0.918	0.724	88.8
NHSK-0.25 mg/mL	0.858	0.767	73.6
NHSK-0.1 mg/mL	0.973	0.817	56.5
NHSK-0.05 mg/mL	0.973	0.917	24.6

shows the absorbance ratio of the lower energy peaks alongside the calculated exciton bandwidth. The A_0–0/A_0–1 peak ratio is larger for higher concentrations of chloroform (i.e., 0.05 mg/mL prepared NHSK solution). Similarly, the exciton bandwidth decreases with the addition of the good solvent chloroform. The 0.1 mg/mL NHSK sample with 10.19% (v/v) chloroform and the 0.05 mg/mL NHSK sample with 22.70% (v/v) chloroform had a 36.4% and 72.3% reduction in exciton bandwidth, respectively, compared to the 0.4 mg/mL NHSK sample. The decrease in W indicates increasing intrachain order and longer conjugation lengths [37]. In addition, with the increased amount of chloroform in the marginal solvent, significant peak broadening can be observed in the spectra (350–650 nm). The broadening of the absorption peaks in the UV range is a desired characteristic to enhance the power conversion efficiency of optoelectronic devices [39,40]. However, it is worth noting that the maximum absorbance of the 0–1 and 0–2 vibronic peaks was reached with the 0.1 mg/mL NHSK solution, indicating that excess chloroform (beyond ~10.19% (v/v)) can impede the extent of crystallization of the polymer. On the other hand, the P3HT/chloroform only has a singular peak at ~450 nm, signifying that the polymer chains are predominantly in an amorphous state due to the higher quality solvent. An additional feature, seen in Figure 2, is the increase in light scattered at higher wavelengths (650–800 nm). This indicates that there are more SWCNTs that are individually dispersed when using the more diluted CNT dispersions (i.e., the 0.05 mg/mL) as aggregates of CNTs will cause light to scatter within this wavelength regime. This coincides with our observation of the CNT dispersions before being added to the heated anisole solutions.

3.2. Morphology and Structure of P3HT:SWCNT NHSKs

The NHSK solutions were drop-casted onto SiO₂ substrates to analyze the surface morphology of the hybrid films. Figure 3 shows SEM images of the NHSK structures formed from altering the solvent quality. The 0.4 mg/mL NHSK sample has the largest NHSK structures in terms of polymer nanowire length nucleating from SWCNTs. However, at these lower chloroform concentrations (<3.8% (v/v)), the supramolecular structures are irregular and begin to resemble an interconnection of one-dimensional P3HT crystalline nanofibers. As the concentration of chloroform increases, the morphology predominantly resembles the typical NHSK structure but with reduced polymer nanowire lengths. Morphological surface profiles of the different films were investigated further through atomic force microscopy (AFM); see Figure 4. From these images, we can see that the large NHSK structures found in the lower chloroform concentrations appear to dry on the SiO₂ in an isolated manner. Conversely, the smaller NHSK structures (0.05 and 0.1 mg/mL NHSK) tend to overlap and form networks. In addition, the height measurement of the supramolecular structures appears to be the same between the smaller and larger NHSKs, with the highest height (~12 nm) achieved directly over top of the SWCNTs themselves.

TEM was conducted on the 0.25 and 0.1 mg/mL NHSK samples as this marked an interesting transition from forming large nanowire-length NHSKs to smaller ones. Figure 5A,B are TEM images of the two different NHSK solutions depicting the supramolecular structures with P3HT nanowires growing perpendicular to the SWCNTs. The P3HT nanowires are similar in both solutions, having a width of ~14–15 nm and a similar periodicity of ~8–10 nm between nanowires. The measured nanowire widths imply that P3HT chains are folding during crystallization, a common phenomenon observed in previous reports [28,41,42]. The width of the neat P3HT nanowires was also found to be ~14 nm, implying that differences in solvent quality and inclusion of SWNTs did not affect the molecular arrangement of the polymer. The main difference between the two NHSK samples is the polymer nanowires’ length distributions, as shown in Figure 6. Specifically, the 0.25 mg/mL sample (86.9 ± 59.3 nm) was much more varied than the 0.1 mg/mL (45.7 ± 24.3 nm). The longer nanowire lengths found in low chloroform concentrations (~200 nm) comply well with the NHSKs created by Liu. et al. [21] using a P3HT:SWCNT mass ratio of 10.

4. Discussion

Altering the solvent quality in the formation of P3HT:SWCNT supramolecular structures significantly affects the exciton bandwidth and conjugation lengths of the polymer nanowires. In our experiment, we observed a change in the exciton bandwidth from 89.6 meV to 25.3 meV simply by modifying the amount of chloroform from approximately 2.37% to 22.70% (v/v). This trend of increasing the ratio of good solvent within a mixed solvent vehicle leading to a higher degree of intra- and interchain order of the polymer aggregates has been observed in homogeneous P3HT nanofiber processing [20], and our work further suggests that the mechanisms remain persistent in heterogeneous crystallization. The main consequence of adding SWCNTs to the P3HT self-assembly process is altering the location of crystallization nucleation sites to favor the nanotube surface.

In addition, the solvent quality drastically changes the morphology of the supramolecular structures. We can either form uniform NHSKs with shorter nanowire lengths or more varied and irregular NHSKs with longer nanowire lengths by modifying the solvent vehicle. The morphology that results in shorter and uniform nanowire lengths is characterized as higher aggregate quality and order based on the reduction in the exciton bandwidth. It is higher quality due to the increased amount of chloroform that causes a larger number of shorter P3HT chains to remain dissolved in the marginal solution [20]. This results in nanowires within these NHSKs being composed of longer P3HT chains, which reduces chain end defects during polymer chain folding [20]. As the chloroform content is reduced to 3.84% (v/v) and below, the NHSK solution will have more P3HT chains available for crystallization, leading to NHSKs with longer nanowire lengths. However, the excess of available polymer does not translate to an improved order due to the simultaneous formation of irregular NHSKs. These irregular NHSKs are hypothesized to be a result of supramolecular structures composed of shorter P3HT chains with more chain end defects. Manipulation of the solvent quality can be a simple method to further improve the molecular ordering of polymer crystals (needed for optoelectrical devices) on CNTs without the need for filtering or removal of irregular morphologies.

While our findings contribute to the understanding of the P3HT:SWCNT morphological changes due to solvent quality, we acknowledge the potential challenges when fabricating devices. Specifically, determining the optimal way to deposit these hybrid materials with a high enough density to form a percolation network while retaining the desired crystalline structure. Future work involves investigating different deposition techniques, such as printing technologies, to improve composite film thickness and density. In addition, alternative solvent vehicles outside of anisole and chloroform should be explored to improve the ordering of the NHSK structures further.

5. Conclusions

Our findings underscore the importance of solvent quality and its role in increasing the intrachain order of NHSK structures. Specifically, we showed that we can synthesize NHSKs of different nanowire lengths by only changing solvent quality while maintaining a constant P3HT:SWCNT ratio and a constant nanotube concentration. Despite having the ability to form large NHSKs, these structures are characterized as having more defects and disorders. In optoelectrical devices, P3HT:SWCNT NHSKs with shorter, more uniform nanowire lengths are desired as they present enhanced crystallinity and reduction in the exciton bandwidth. These findings highlight a previously overlooked parameter—solvent quality—which can be tuned to design novel hybrid active channel materials with improved optoelectrical properties.