DOI:
10.1039/C5RA24266E
(Paper)
RSC Adv., 2015,
5, 107970-107976
Controlling morphology and crystalline structure in poly(3-hexylselenophene) solutions during aging†
Received
17th November 2015
, Accepted 9th December 2015
First published on 11th December 2015
Abstract
Poly(3-hexylselenophene) (P3HS) is much less studied compared with its close analogue poly(3-hexylthiophene) despite its excellent electronic properties. In this work, we report the crystallization behavior of P3HS in aged solution and the effects of aging time, solution concentration, and solvents on the self-assembly of P3HS are explored. A novel bundlelike spherulitic morphology is observed, which is so far scarcely observed in polyselenophenes. These P3HS spherulites consist of nanoribbons in which the polymer chains adopt a flat-on orientation associated with form II crystals. The formation mechanism and kinetics of P3HS spherulites are discussed in detail. Overall, this work offers an effective way to demonstrate how the P3HS crystallize in solution and the correlation between its morphology and crystalline structure, which improves our understanding of P3HS crystallization and facilitates the ongoing exploration of using such spherulites in organic electronics.
Introduction
Conjugated polymers have received great attention in both academic and industrial areas for their excellent optoelectronic properties as well as their good solubility, processability and low-cost.1 Among various conjugated polymers, poly(3-hexylthiophene) (P3HT), with a typical hair-rod configuration, is one of the most widely studied systems due to its excellent charge carrier mobility, and has been used in organic photovoltaics (OPVs),2 field-effect transistors (FETs),3 and so on. From a device standpoint, increasing the crystallinity is an important strategy to improve the charge carrier mobility, in which the direction of crystalline domains plays a key role in determining their electrical performances in various types of devices.4,5 For OPVs, the charge transport pathway is from cathode (anode) to anode (cathode) which is normal to the photoactive layer and substrate. Therefore, the ideal alignment of P3HT crystallites should be vertical to the substrate with “face-on” or “flat-on” orientation.4 For FETs with the charge transport from source to drain, the P3HT crystallites with “edge-on” orientation are more favored.6
Despite thousands of papers were published on polythiophenes and their derivatives, polyselenophenes, which are a close analogue of polythiophenes, have been much less studied.7–13 By replacing S atom in the thiophene ring with the lower electronegative and more polarizable Se atom, polyselenophenes are expected to have some advantages over polythiophenes, such as having lower oxidation and reduction potentials, better interchain charge transfer, and a narrower HOMO–LUMO gap, etc., which make them promising alternatives for photovoltaic applications. Since devices properties are greatly affected by film morphology, crystalline structure, and orientation type, controlling the morphology and crystal structure of P3HS is an important issue which facilitates charge transfer and transport that influences device function. However, to the best of our knowledge, there are only a few reports on the phase structure and crystallization properties of polyselenophenes so far. Seferos et al. discovered a pure type-2 crystal phase in low molecular weight poly(3-hexylselenophene) (P3HS), which was very distinct from that in P3HT.14 Introducing P3HS into block copolymer systems is a good strategy to manipulate the phase structure through microphase separation. Based on this point, Seferos et al. also designed a new class of selenophene–thiophene block copolymers, poly(3-hexylselenophene)-b-poly(3-hexylthiophene) (P3HS-b-P3HT), and discovered that phase separation as well as the optical properties could be controlled by the heterocycle in the polymer chain.15,16 Lilliu et al. investigated the effects of thermal annealing upon the blends of P3HS and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) by grazing incidence X-ray diffraction and found the melting point of P3HS crystallites was decreased by the presence of PCBM.17 Compared with the heavily studied polythiophenes, the crystallization behavior of polyselenophenes remains less explored and understood and needs extensive investigation.
Up to now, controlling the crystalline structure of polythiophenes is mostly based on two aspects, post-treatment approach, such as application of electric field,18 thermal19 or solvent annealing,20,21 and the approach without post treatment. For the latter, the polythiophene nanostructures can be efficiently and directly tailored in solutions by changing the solvent selectivity,22–24 ultrasonic treatment,25 aging,26–28 adding additives,29–31 or ultrafiltration,32 etc., which is more attractive. One might naturally think these strategies can also be applicable to polyselenophene solutions which may exhibit some new structures that have not been explored. Of particular interest to us is that the difference in their heterocycle of P3HS compared with P3HT may lead to some novel and intriguing self-assembled structures which may be promising in optoelectronic devices. Motivated by these thoughts, we investigated the morphological evolution of P3HS in solutions during the aging process and tried to control its solution structures, crystallization and dynamics. A strikingly new bundlelike spherulitic morphology of P3HS was observed with the formation process, crystalline structure and orientation type characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD). The effects of aging time, solution concentration, and solvents on the self-assembly of P3HS were investigated. To the best of our knowledge, this is the first report of the crystallization of P3HS in aged solution, which can improve our understanding of the crystallization behavior of P3HS and make a further step to its application in the device.
Experimental section
Materials
The monomer 2,5-dibromo-3-hexylselenophene was synthesized according to the literature.15 Isopropylmagnesium chloride (i-PrMgCl, 2.0 M in tetrahydrofuran) and (1,3-bis(diphenylphosphino)-propane)-dichloronickel(II) (Ni(dppp)Cl2) were purchased from Aldrich and used as received. The other reagents and solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (SRC). Tetrahydrofuran (THF) was freshly dried over sodium benzophenone ketyl, and all other solvents were used as received.
P3HS synthesis and solution preparation
P3HS (Mn = 5600, Mw/Mn = 1.34) was synthesized by a catalyst-transfer Kumada polycondensation.33,34 The detailed synthetic procedures, monomer and polymer characterization were presented in the ESI (Fig. S1–S3†). P3HS was dissolved in different solvents with the concentrations ranging from 0.1 to 5 mg mL−1 by heating at 80 °C for 3 h to reach sufficient dissolution and cooled to 25 °C. Dichloromethane (CH2Cl2), anisole, and toluene were used as solvents to dissolve P3HS. The solutions were then aged at 25 °C for different times for the crystallization of P3HS.
Characterization
TEM images and SAED patterns were obtained on a Tecnai G2 20, FEI electron microscope operated at 200 kV. All the solutions were drop-cast onto carbon-coated copper grids, followed by evaporation of the solvent at ambient. Atomic force microscopy (AFM) images were obtained on a Multimode 8 AFM Nanoscope IV in tapping mode. Thin films for AFM were prepared by spin-coating P3HS solutions on pre-cleaned silicon wafers at 3000 rpm for 60 s. Field emission scanning electron microscopy (FESEM) images were obtained on a field emission SEM (Ultra 55, Zeiss) under 5 kV accelerating voltage. UV-vis spectroscopy was carried out with a Perkin-Elmer Lambda 750 equipment. 1H NMR spectra were recorded on a AVANCE III HD 400 MHz spectrometer in CD2Cl2 with tetramethylsilane (TMS) as the internal standard.
Results and discussion
Novel morphology of P3HS in solution
As shown in Fig. 1a, the P3HS crystallized into bundlelike spherulitic morphology obtained from CH2Cl2 solution which was aged at 25 °C for 54 h. The lengths of the bundlelike spherulites ranged from 2 to 5 μm. The zoomed-in image indicated that the spherulitic structures were actually composed of a number of nanoribbons (Fig. 1b). The crystallographic structure of P3HS could be proved by the SAED pattern, which showed four clear diffraction rings assigned to crystallographic (100), (200), (300), and (020) planes, respectively (Fig. 1c). It suggests that the electron beam goes along (001) direction, which proves that in these spherulites, the P3HS chains mainly adopt flat-on orientation with the polymer backbones perpendicular to the substrate.35 Some devices like polymer solar cells, require high charge-carrier mobility in the direction normal to the photoactive layer so that electrons and holes can be transported to the electrodes before recombination.35 For this purpose, the orientation of rigid polymer backbones perpendicular to the substrate is highly favorable, serving as transportation pathways for free charge carriers. Therefore, such new structure with flat-on orientation may have some potential applications in optoelectronic devices. The d-spacing of the (100) plane is 12.0 Å and the π–π stacking distance is 4.6 Å, which could be attributed to form II crystal of P3HS with interdigitating side chains.14 As further measured by XRD (Fig. S4†), the P3HS spherulites showed diffraction peaks at 2θ = 6.1° and 7.6°, with a d-spacing of 14.4 Å and 11.6 Å, respectively. These two peaks were corresponding to (100) reflections of form I and II, respectively. Compared with the pronounced peak associated with from II, the peak associated with form I was relatively weak. The TEM and XRD results show that in P3HS spherulitic structure, P3HS chains mainly adopt flat-on orientation associated with form II crystals, accompanied by the presence of small amount of form I crystals. The absence of diffraction ring associated with form I in SAED may due to the thick center part of the spherulites, which could not be penetrated by electronic beam.26 As measured by AFM, the height and width of the center part of spherulites were around 200 nm and 450 nm, respectively (Fig. 1d). Large area of such bundlelike structure was also measured by FESEM, as shown in Fig. 1e. It should be noted that to avoid the growth of P3HS structures during solvent evaporation, a filter paper was used to blot the excess solvent surrounding the copper grid within seconds. Therefore, the P3HS structure on the copper grid represented the structure in the solution. Besides, since the solvent evaporation rates during the preparation of AFM and TEM samples were different, the consistent morphology measured by both modes further indicated the spherulitic structure existed in the initial solution. Since such bundlelike spherulitic structure was small and didn't develop into complete spherulites, the birefringence under polarized light was not observed by polarized optical microscopy.
 |
| Fig. 1 (a) TEM image of P3HS bundlelike spherulitic structure drop-cast from CH2Cl2 solution (1 mg mL−1) after aging at 25 °C for 54 h. (b) A magnified TEM image of the spherulite and (c) the corresponding SAED pattern. (d) AFM image along with line profile of P3HS spherulitic structure. (e) FESEM image of P3HS bundlelike spherulitic structure in a large area. | |
Growth process of bundlelike spherulitic structure
The morphology and the growth kinetics of P3HS bundlelike spherulitic structure from the aged CH2Cl2 solution was characterized by TEM, as illustrated in Fig. 2. After heating the solution and cooling to 25 °C without aging, P3HS dissolved in CH2Cl2 well and formed a flat surface with some nanodots on TEM grid (Fig. 2a). It proved that without aging, P3HS chains formed amorphous phase. Aged at 25 °C for 5 h, some lamellar crystalline structures appeared with an average length and width of ∼240 and 35 nm, respectively (Fig. 2b). With increased aging time, small bundlelike spherulitic structures appeared and grew, coexisting with lamellar structures homogenously distributed in the film (Fig. 2c and d). Further increasing the aging time led to the disappearance of lamella and formation of more and more spherulitic structures (Fig. 2e). These spherulites almost did not further grow after aging for 120 h, indicating that their development was nearly complete (Fig. 2f).
 |
| Fig. 2 TEM images showing the growth process of P3HS bundlelike spherulitic structure drop-cast from (a) freshly prepared CH2Cl2 solution (1 mg mL−1) followed by aging at 25 °C for (b) 5 h, (c) 12 h, (d) 24 h, (e) 54 h, and (f) 120 h. | |
To further investigate the optical properties of P3HS solution during aging and find out the structure–property relationship, its UV-vis absorption spectra were characterized. As shown in Fig. 3, the freshly prepared P3HS solution showed a single absorption peak with a maximum at 476 nm, which was due to the intrachain π–π* transition of P3HS. In this case, the polymer chains were dissolved in the solution well and exhibited the coil conformation. During aging, an absorption peak at 665 nm appeared and its intensity increased gradually, indicating the occurrence of ordered aggregates associated with interchain π–π stacking in the solution. Not surprisingly, P3HS chains undergo a transition from flexible coils to rigid rods, during which P3HS backbones with increased planarization tend to aggregate in a side-by-side manner. Such optical variations were concomitant with the aforementioned growth of bundlelike spherulites captured by TEM.
 |
| Fig. 3 UV-vis absorption spectra of freshly prepared P3HS CH2Cl2 solution (1 mg mL−1). The inset shows that the gradual formation of P3HS aggregates in the solution with the increased aging time. | |
Effect of the P3HS concentration and solvent
From the above discussion, it is shown that P3HS in CH2Cl2 can form bundlelike spherulitic structure during the aging process. To examine the effect of P3HS concentration on the spherulites, in addition to the previously discussed concentration of 1 mg mL−1, two other concentrations, 0.1 and 5 mg mL−1 were investigated with all of them aging at 25 °C for 24 h. It showed that in 0.1 mg mL−1 CH2Cl2 solution, only nanodots were observed similar to the dewetting pattern (Fig. 4a), while in the 5 mg mL−1 system (Fig. 4b), bundlelike spherulites with similar size but much higher density were observed compared with the case of 1 mg mL−1. It indicated that higher concentration of P3HS led to more instead of larger spherulites.
 |
| Fig. 4 TEM images of the P3HS structure drop-cast from CH2Cl2 solution with different concentrations aging at 25 °C for 24 h. (a) 0.1 mg mL−1 and (b) 5 mg mL−1. | |
The solvent also played an important role in the solution structure of P3HS during aging process. In addition to CH2Cl2, anisole and toluene were also used and the P3HS in these two solvents were aged at 25 °C for a certain time. In contrast to CH2Cl2, nanoribbons with width and height at 70–130 nm and 5–12 nm were observed in anisole (Fig. 5a–c). While in toluene, nanofibers were formed with the width and height at 25–40 nm, and 4–8 nm, respectively (Fig. 5d–f). Confirmed by the SAED pattern in the insets, these P3HS nanoribbons and nanofibers adopt flat-on orientation associated with form II crystals, which have the same crystalline type as that of spherulitic structure in CH2Cl2. The above results indicated that P3HS exhibited different morphologies in different solvents but the polymer chains adopt the same orientation.
 |
| Fig. 5 (a) TEM, (b) AFM along with line profile and (c) FESEM images of P3HS nanoribbon structure in anisole solution (1 mg mL−1) after aging at 25 °C for 24 h. (d) TEM, (e) AFM along with line profile and (f) FESEM images of P3HS nanofiber structure in toluene solution (1 mg mL−1) after aging at 25 °C for 24 h. The insets in (a) and (c) are the corresponding SAED pattern. | |
Formation mechanism of bundlelike spherulitic structure
Combining all the above results, we propose a possible model in the following and illustrate it in Fig. 6 to explain how the spherulitic structure is formed. Similar to P3HT, P3HS has a hair-rod molecular configuration, which can self-organize into semicrystalline nanofibers through strong anisotropic π–π interactions between planar rigid backbones and weak van der Waals interactions between their pendent alkyl side chains. As Han et al.36 mentioned, two steps were involved in P3HT crystallization from solution: intrachain mechanism of coil-to-rod conformational transition to form lamellae, and interchain mechanism of lamellae stacking into ordered structure. We propose that the crystallization of P3HS behaves in a similar manner in solution. After heating P3HS solution for complete dissolution, P3HS is molecularly dissolved and exhibits flexible coil characteristic (Fig. 6a). During the cooling and aging process, P3HS chains transform from coil to rod and planarized conformation since CH2Cl2 is a marginal solvent for P3HS. The lamellae are formed initially from the aging process, which act as seed nuclei for further P3HS crystallization (Fig. 6b). These lamellae aggregate with each other and some lamellae individual split apart in which the splitting may be due to the lamellae receiving impurities such as coil or amorphous P3HS chains at the growth front (Fig. 6c). Consequently, a sheaf-like structure is formed (Fig. 6d). These sheaf-like structures guide the crystallization at the later stage. After consuming more and more P3HS molecules from their surrounding during crystallization, the sheaf-like structure become longer and larger to form a complete bundlelike spherulitic structure (Fig. 6e). It should be noted that the growth of lamellae results from the synergistic effect between π–π interaction and alkyl interaction. According to Zhai et al.37 and Yang et al.,38 after the growth of P3HT nanofibers driven by π–π interaction reached its equilibrium, a further growth promoted by alkyl interaction led to the 2D nanoribbon. In similar P3HS system, at first, the π–π interaction of P3HS leads to the growth of nanofibers along (020) direction. Then the alkyl interaction promotes the growth along (100) direction which leads to the formation of nanoribbons, in which the P3HS chains adopt flat-on orientation with the polymer backbones perpendicular to the film plane (Fig. 6e). To support the proposed interactions between alkyl chains, the P3HS in CD2Cl2 at different aging time (0 h and 48 h) was recorded by 1H NMR spectroscopy (Fig. 7). Recent work shows that 1H NMR can detect self-aggregation of complex molecules in solution, thereby providing insight into the mechanism of aggregation.39 Controlled experiments verified that P3HS also formed similar bundlelike structure in CD2Cl2. It shows that the signals δ = 2.66 and 0.84 ppm, which are assigned to protons of the methylene group connected to the thiophene ring and the protons of the terminal methyl group of the hexyl side chains, respectively, are weaker after aging, compared to those before aging. It is indicative of the decreased rotational mobility of hexyl chains during self-assembly process due to strengthened alkyl interaction. In addition, the peak at δ = 7.09 ppm, which is assigned to the proton of the P3HS polymer backbone, also decreased after aging, which is due to the strengthened π–π interaction. These results prove the increased π–π interaction and alkyl interaction during aging to form aggregates of P3HS. The different morphologies formed in different solvents may be due to the competition between these two interactions. In toluene, the π–π interaction is the dominant force, resulting in the nanofibers. While in CH2Cl2 and anisole, both π–π interaction and alkyl interaction contribute a lot, leading to the 2D nanoribbons. The different P3HS structures in CH2Cl2 and anisole may be due to the different solvent selectivity. CH2Cl2 is more selective to P3HS compared with anisole, therefore, it contains more P3HS coil chains than those in anisole. These coil chains may act as impurities at the lamellar growth front, which induce the splitting of lamellae and the formation of bundlelike spherulitic structure. Research about the solvent effects on the P3HS crystallization during aging is still ongoing and will be revealed elsewhere.
 |
| Fig. 6 Schematic illustration to the formation of P3HS bundlelike spherulitic structure during the aging process. | |
 |
| Fig. 7 1H NMR spectra of P3HS in CD2Cl2 solution after aging for 0 h and 48 h. | |
Conclusions
In summary, we have studied the crystallization behavior of P3HS during aging and successfully prepared P3HS bundlelike spherulitic structure. These spherulites are composed of nanoribbons, in which P3HS chains mainly adopt flat-on orientation associated with form II crystals analyzed by TEM and XRD. Initially, P3HS chains in freshly prepared CH2Cl2 solution are flexible coil characteristic. By cooling the P3HS solution and subsequently aging, P3HS chains transform from coil to rod and form lamellae which aggregate with each other, split apart, and grow into bundlelike spherulites finally. It is interesting to show that the solvent influences the P3HS morphology in solutions greatly and nanofibers and nanoribbons are obtained when using toluene and anisole as solvents respectively. Such morphological differences are due to competition between π–π interaction and alkyl interaction. Overall, the significance of this work is to demonstrate how the P3HS crystallize during aging and the correlation between its morphology and crystalline structure. We expect these P3HS crystalline structures can be used in electronic devices.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21274029) and National Basic Research Program of China (2011CB605700).
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Footnote |
† Electronic supplementary information (ESI) available: Experimental conditions, detailed characterization results. See DOI: 10.1039/c5ra24266e |
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