Relation Between Charge Transport and the Number of

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Relation Between Charge Transport and the Number of Interconnected Lamellar Poly(3-Hexylthiophene) Crystals Binghua Wang,† Jingbo Chen,† Changyu Shen,† Günter Reiter,‡ and Bin Zhang*,† †

School of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, People’s Republic of China Institute of Physics, University of Freiburg, Freiburg 79104, Germany

‡

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S Supporting Information *

ABSTRACT: We have investigated the lateral charge transport in a multilamellar network of crystalline structures of poly(3-hexylthiophene) (P3HT) embedded in a matrix of small crystallites and amorphous polymers resulting from crystallization at temperatures close to the melting point. Removing the matrix by an appropriate washing process allowed us to observe a power-law increase of the maximum current (Imax) passing from the conductive tip across the polymer crystalline structure into the substrate with increasing number (N) of interconnected edge-on lamellae: Imax ∼ N 2/3. Varying isothermal crystallization temperature and crystallization time allowed to tune the morphological structure and to increase charge transport through the network of lamellar crystals by up to a factor of twenty. Our results suggest that the efficiency of charge transport can be increased by the cumulative contributions of a large number of interconnected crystalline lamellae, indicating a potential pathway for improving the performance of various organic electronics devices and related applications. the presence of a thin insulating SiO2 film limited the carrier transport through the substrate, they proposed that C-AFM can measure the lateral charge transport taking place within crystalline islands. When the tip of C-AFM is in contact with a conductive organic crystalline structure, various pathways are available for conduction. Before tunneling through the substrate, carriers can travel along the directions of the various connected π−π stacked aggregates existing within this structure.27 Consequently, electric charges could travel over large distances, many micrometers away from the point of injection.28 This allows using C-AFM to determine simultaneously the crystalline structures of conjugated polymers and their lateral charge transport properties. Due to the co-existence of amorphous and crystalline phases within thin films of conjugated polymers, direct observation of intrinsic charge transport properties of the crystalline structures remains challenging. In this work, after isothermal crystallization of P3HT in thin films, the amorphous fraction together with less stable small crystallites were selectively removed by employing a suitable washing procedure. Subsequently, topography and current maps of P3HT crystals have been characterized via PeakForce tunneling atomic force microscopy (PF-TUNA). By varying time and temperature of melt crystallization, the morphology of P3HT crystals can easily be controlled, varying from individual lamellae to a

1. INTRODUCTION Because of their solution processability, their flexibility and their remarkable optoelectronic properties, conjugated polymers are promising materials and thus have attracted significant research interest in the past two decades.1−3 One of the most prominent conjugated polymers is poly(3hexylthiophene) (P3HT), which has emerged in a multitude of applications, including field-effect transistors, sensors, lightemitting diodes, and solar cells.4−6 Moreover, P3HT has also been regarded as a semicrystalline conjugated model polymer for fundamental studies relating structural and electrical properties.7 A large amount of research works show that the charge carrier transport in organic semiconductors is related to the crystalline morphology, representing a hierarchy of ordering processes and alignment of conjugated polymers at multiple length-scales, which is controlled by molecular parameters but also processing and post-processing procedures.8−15 Although previous studies have suggested that tiemolecules that connect small aggregates of polymers can provide an efficient charge transport pathway,16 the understanding of the relationship between multiscale ordered structures (i.e., structural ordering achieved across the molecular scales, mesoscales, and macroscales) and longrange charge transport is not yet understood in depth.17−22 Conductive atomic force microscopy (C-AFM) provides a means of probing correlations between the morphological structures and electrical properties.23−26 Salmeron and coworkers27 have studied electrical conduction properties in monolayers of oligothiophenes on a SiO2/Si substrate. Since © XXXX American Chemical Society

Received: June 4, 2019 Revised: July 13, 2019

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DOI: 10.1021/acs.macromol.9b01146 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Topography and current maps of P3HT thin films, probed by PF-TUNA. (a, d) As-spun film. (b, e) Crystallized film, after isothermal crystallization at 185 °C for 600 s and subsequent cooling to the room temperature. (c, f) Crystallized film after selective washing in anisole at 69 °C for 5 s. The size of the scale bar is 1 μm. (g−i) Cross-sectional profiles along the directions indicated by the green, red, and blue lines in the current images, respectively. Note the difference in the scales of the current. 2.2. Crystallization Temperature Protocol and Washing Procedure. The temperature protocol applied to melt and crystallize the P3HT films is shown in Figure S1 and consisted of the following steps. First, P3HT thin films were heated at a rate of 30 °C/min from room temperature to 240 °C and held there for 1 min. Then, specimens were cooled at a rate of 40 °C/min to the preset isothermal crystallization temperature (T c) and kept there for varying crystallization times (tc). Subsequently, the samples were immediately quenched to room temperature. The experimental temperature was controlled by a Linkam THMS 600 hot stage (Linkam Scientific Instruments, Tadworth, UK). Here, it should be pointed out that all specimens were prepared in the dark and in a nitrogen environment. To facilitate a clear mapping of the surface topography and electrical properties of P3HT lamellae, it was necessary to remove the amorphous portions and less stable (small) crystallites that were formed during cooling or quenching.33,35 Consequently, we have washed the P3HT films by dipping the samples into a hot selective solvent, that is, for 5 s in anisole at 69 °C. Finally, the specimens were retracted from the solvent and dried in a flow of nitrogen. A typical comparison of the surface morphology and current images of the initial sample and the sample after washing is shown in Figure S2. 2.3. Fabrication and Characterization Techniques. Conductive atomic force microscopy (C-AFM) measurements were performed under nitrogen conditions using a scanning probe microscope (Dimension Icon, Bruker, USA) equipped with the PeakForce TUNA module. PF-TUNA has proven to be a useful tool for revealing nanoscale morphology and detecting the conductivity on the same location, establishing a direct correlation between morphology and conductivity.36−38 In this work, PF-TUNA has been employed for measuring lateral charge transport, which was derived from the current flowing from the conductive tip, passing

network of lamellae. Our observation signifies that at an applied DC bias (+1 V) P3HT edge-on lamellae represented preferential pathways along which charge carriers travelled before passing into the substrate. We used PF-TUNA to determine the current distribution within connected crystallites and lamellar structures. Furthermore, we will discuss the interplay and contributions of various ordered structures with a multitude of length-scales on long-range charge transport properties.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. For our studies, region-regular poly(3-hexylthiophene) (region-regularity > 95%, purity ≈ 99.995%, and melting point ≈ 220 °C) with an average number molecular weight of 5330 g/mol and a dispersity of Đ = 2.61 was used as received from Luoyang Microlight Material Technology Co. Ltd.26,29 P3HT was dissolved in chlorobenzene at a concentration of 1.0 wt % and heated in darkness for 6 h at 70 °C. After cooling to room temperature, the solution was filtered through a 0.22 μm polytetrafluoroethylene (PTFE) syringe filter and spin cast onto UV/ ozone-treated silicon substrates (100, p-doped, resistivity ≈ 0.001 Ω· cm) at a spin speed of 3000 rpm for 30 s.30−32 The spin-coating device used was a commercial spin coater (KW-4A, Institute of Microelectronics, Chinese Academy of Sciences, China).33,34 The film thickness determined by a spectroscopic ellipsometer (Alpha-SE ellipsometer, J.A.Woollam, USA) was approximately 30 nm. The thickness of the oxide layer on top of the highly doped silicon substrates was about 2 nm. All P3HT solutions and films were prepared under nitrogen protection. B


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Macromolecules through P3HT edge-on lamellae and eventually into the substrate (see in Figure S3).27,28 We used a conductive cantilever with a Pt/Ircoated tip (model: PF-TUNA; spring constant: 0.4 N/m; tip radius: 25 nm). During the AFM measurements, topographic and current images of the same series of samples were acquired with the same probe at an applied bias voltage +1 V. Optical microscopy observations were carried out using an Olympus (BX 51, Japan) polarizing optical microscope. Grazing incidence wide-angle X-ray scattering (GIWAXS) was performed with the Nano-inXider SAXS/WAXS system (Xenocs, France, equipped with a microfocus sealed tube and an area detector, Cu Kα, λ = 1.54 Å) to characterize the molecular packing and crystallite orientation in P3HT thin films. All GWAXS measurements were performed in a vacuum chamber to minimize beam damage and to reduce diffuse scattering. A fixed grazing incidence angle of 0.2°, slightly larger than the critical angle, was used to assure full penetration of the X-rays into the film. The data collection time was 1800 s. The diffracted intensity was corrected with respect to background scattering and was normalized by the incident photon flux. Fourier transform infrared (FTIR) spectra were measured on a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific, USA) in the attenuated total reflection (ATR) geometry mode between 450− 4000 cm−1 with 32 accumulated scans at a resolution of 4 cm−1. The concentrated P3HT solutions were used to prepare the thin films by drop-casting onto silicon wafers (double-side polishing, resistivity > 2000 Ω·cm) covered by a natural oxide layer. Subsequently, the samples were placed in the dark and under vacuum for drying. The thickness of all samples was about 200 nm. UV−vis absorption spectra (from 200 to 800 nm) were acquired on a Gary 4000 UV−vis spectrophotometer (Agilent Technologies, USA) with a spectral resolution of 1 nm in the transmission mode. All film samples were spin cast on quartz substrates (25 × 25 mm2). The thickness of samples was about 40 nm.

approaching 3 pA (see Figure 1f,i). We can conclude that the network of P3HT lamellae allowed for a higher conductivity than the small crystallites and the amorphous surrounding. We note that the substrate (SiO2/Si layer) was partially exposed after the washing process (Figure 1c,f) and exhibited a much lower current than P3HT lamellae. We speculate that the higher current measured on the lamellae is related to the lateral conduction along the crystalline lamellae of highly ordered molecules, which increased the effective electrical contact area. As indicated in Figure S3, on the bare SiO2/Si substrate, the current flow across the SiO2-layer occurs only over the contact area (a few tens of nm2) of the tip. By contrast, on the network of P3HT lamellae the current can flow laterally along crystalline lamellae over distances of micrometers before tunneling through the SiO2-layer. As a result, we obtained an increased area (in the μm2 range) for potential tunneling from tip to substrate.28 Although AFM observations provide useful structural and morphological information, this information is limited to the top surface of a film. Thus, we additionally performed GIWAXS measurements to investigate the influence of isothermal crystallization on structural arrangements (e.g., crystallinity, chain stacking, etc.) within thin P3HT films. Twodimensional GIWAXS images corresponding to the various samples of Figure 1 are shown in Figure 2a−c. Firstly, the out-

3. RESULTS AND DISCUSSION By using PF-TUNA, electrical current images of the P3HT crystals can be obtained simultaneously with topography images.39−41 The surface of as-spun P3HT films showed an isotropic nodule-like morphology (see Figure 1a), which was composed of an amorphous fraction and a large number of (small) crystallites.42−44 The isothermally crystallized P3HT film was prepared by heating the as-spun film from room temperature to 240 °C, holding this temperature for 1 min and cooling the sample down to 185 °C, where it was crystallized for 600 s and subsequently quenched to the room temperature. It can be clearly seen that the crystallized film exhibited welldeveloped lamellae, which were embedded in an amorphous phase containing small crystallites. By comparing the current maps in Figure 1d,e, the conductivity of the as-spun film was extremely low with an average current (as obtained from the cross-section profile, see Figure 1g) of about 0.15 pA only. The low conductivity was probably caused by the large number of randomly oriented (small) crystallites without connecting pathways for charge transport.5 However, the isothermally crystallized P3HT films exhibited a significant improvement in conductivity (see Figure 1e). As quantitatively confirmed from the cross-sectional profile (see Figure 1h), the average current was approximately 1.5 pA, representing an increase of one order of magnitude in conductivity compared with the as-spun film. To explore the morphology and conductivity of P3HT crystals formed during isothermal crystallization, we performed a selective washing treatment on these crystallized films, which revealed a crystalline network composed of numerous lamellae (see Figure 1c). After such washing, the conductivity was enhanced by ca. a factor of two, with an average current

Figure 2. GIWAXS investigations of P3HT thin films. Twodimensional GIWAXS patterns from the samples shown in Figure 1. (a) As-spun film, (b) isothermally crystallized film, and (c) crystallized film after selective washing on a silicon wafer. (d) Onedimensional scattering profiles (out-of-plane) along qz obtained from the 2D GIWAXS images in (b) (black squares) and (c) (red circles). The blue triangles resulted from a point-by-point subtraction of the other two scattering profiles. The inset in (d) shows a schematic model indicating the orientation of the P3HT molecules within superposed crystalline lamellae, emphasizing three main length scales. (e) Normalized intensity of the (100) reflection (at qz = 0.38 Å−1) as a function of the polar angle (χ).

of-plane diffraction peaks at scattering vectors qz ≈ 0.38, 0.77, and 1.15 Å−1, corresponding to the primary (100), secondary (200), and tertiary (300) peaks, respectively, indicating that all three films showed preferential orientation of P3HT with edgeon oriented chains. For this texture, the side chains (a-axis) are along the substrate normal, while the π−π stacking direction (b-axis) is in the plane of the film (see the inset of Figure 2d). In addition, as expected for an isothermally crystallized film, C


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within thin films, we performed further experiments at various crystallization temperatures (Tc) and differing crystallization times (tc). Based on such insight, we hope to unveil the processes responsible for high lateral charge transport in networks of P3HT lamellae with a broad distribution of the orientation of crystalline regions. As shown in Figure 3a−c, the morphology of P3HT crystals changed significantly with tc. The isothermally grown P3HT

the intensity of the primary (100) peak was significantly higher as compared to the as-spun film (see the comparison of 1D GIWAXS curves in Figure S4a), indicating a substantial increase in crystallinity. Furthermore, the corresponding FTIR and UV−vis absorption spectra of P3HT thin films before and after crystallization are shown in Figure S4b,c. On the basis of the GIWAXS results, the increase in the FTIR band at 816 cm−1 after crystallization may be related to the increased crystallinity of this sample.45,46 Moreover, UV−vis spectra exhibited an absorption peak at 610 nm, which is attributed to π−π interaction between P3HT chains. After crystallization, this peak increased in intensity in accordance with an increased degree of interchain order.5,47 By comparing Figure 2c and Figure 2d, we can observe a decrease in the intensity of diffraction peaks after selective washing, indicating the removal of less stable (small) crystals. Thus, the measured intensities of the X-ray diffraction peaks in crystallized P3HT films resulted from two contributions, from crystalline P3HT lamellae (created by isothermal crystallization at elevated temperatures) and from small crystallites (less stable crystals formed during quenching to room temperature). The shape of Bragg peaks provides information on the orientation distribution of crystallites. If perfectly crystalline structures have a narrow distribution in the alignment of the individual crystallites, that is, the (h00) direction of the majority of crystallites is normal to the silicon substrate, a narrow Bragg peak (spot or elliptical) is expected, whereas misaligned crystallites exhibiting large differences in their orientation will yield an arc-shaped pattern.48 As shown in Figure 2b, the diffraction patterns of the crystallized P3HT film contained ellipses (or spots) with stronger intensity and arcs of weaker intensity. This observation implies that, resulting from the melt crystallization process, the film contained a population of crystallites oriented along the substrate normal. However, the diffraction patterns of crystallized films after washing showed arcs of intensity, suggesting that the corresponding network of P3HT lamellae had a broad distribution of the orientation of lamellae (see Figure 2c). Therefore, the spots or ellipses of intensity in diffraction patterns of crystallized P3HT films are probably corresponding to the multitude of preferentially oriented but small crystallites of low stability (can be removed by selective washing), which were formed during the quenching step after isothermal crystallization. In order to quantify the relative contribution of such preferentially oriented P3HT crystallites, we plotted the intensity of the scattering as a function of the polar angle (χ).49 For the analysis of the out-of-plane (100) peak, an angular slice was taken from qz = 0.31 Å−1 to 0.45 Å−1, varying χ from 0° to 90°. As the sample was isotropic in the plane of the substrate (see Figure S5b), the intensity vs χ curve shown in the Figure 2e can be interpreted as the distribution in orientation of the P3HT crystallites.48,50 After washing, we can see that the crystallites in the network of edge-on lamellae exhibited a misalignment smaller than 25°, whereas crystallites in the removed part were more preferentially oriented (distribution smaller than 15°). In view of this, we propose that the network of lamellae was built up by lamellar branching, which led to a rather compact and space-filling structure. Thus, during lamellar growth, for some parts of the P3HT crystals, the orientation of the (h00) direction was not perpendicular to the silicon substrate. To gain a deeper understanding of the processes responsible for the resulting morphological structure of P3HT crystals

Figure 3. Topography (a−c) and current maps (d−f) of washed P3HT thin films, that is, after having removed amorphous portions and less stable crystallites surrounding P3HT edge-on lamellae. P3HT crystals were grown in 30 nm thick films via crystallization at 185 °C for different crystallization times tc: (a, d) 2 s, (b, e) 10 s, and (c, f) 600 s. (g, h) Height and current histograms for the P3HT edge-on lamellar structure are shown in the corresponding AFM images, respectively. The size of the scale bar is 2 μm.

crystals in thin films differed from the nanowhiskers or nanofibers typically found in the solution.17 Here, after crystallization at Tc = 185 °C for 2 s, P3HT crystals exhibited a highly branched morphology composed of lamellae with many additional outgrowing lamellae of different lengths. For the longer tc = 10 s, P3HT crystals revealed hundreds of branches (see Figure 3b). The longer lamellae were connected by shorter lamellae, which constructed an interconnected network of lamellae. As tc increased, the branching density of P3HT crystals increased also (see Figure 3c, tc = 600 s). The histograms in Figure 3g show two overlapping Gaussian peaks. The one centered at about 0 nm refers to the substrate, the other one represents the height of the edge-on lamellae, with maximum values of about 11 nm, 20 nm, and 25 nm for tc increasing from 2 s to 600 s, respectively. For the discussion, we distinguish between networks of lamellae of low branching density (LB) crystal (e.g., tc = 2 s at 185 °C), high branching density (HB) crystal (e.g., tc = 10 and 100 s at 185 °C), and ultrahigh branching density (UHB) crystal (e.g., tc = 600 s at 185 °C), respectively. As shown in Figure 3d−f, the HB D


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time, but also on crystallization temperature.51 Therefore, we performed the isothermal growth of P3HT lamellar crystals at temperatures from 180 to 205 °C, yielding a variety of patterns (see Figure S7). For Tc = 190 °C, we visualized P3HT crystals by AFM after crystallization for 10 and 25 s, shown in Figure 5a,b. In comparison with observations for Tc = 185 °C (Figure 2a), we observed fewer bridged lamellae at Tc = 190 °C. At even higher Tc = 200 °C, long isolated P3HT lamellae could be observed (see Figure 5e, tc = 60 s), similar in shape to the nanowhiskers grown from the solution. However, when extending the crystallization time, the morphology showed the addition of lamellar side-branches growing away from a central lamella (see Figure 5f, tc = 180 s). We conclude that, for a given tc, the branching density in networks of P3HT lamellae decreased with increasing Tc, in agreement with observations of the branching of lamellar crystals of other polymers, for example, as reported by Li et al.52,53 for lamellae grown in poly(bisphenol A-co-decane) (BA-C10) thin films. At high degrees of supercooling, they found many lamellar branches randomly emerging from a dominant lamella. However, at crystallization temperatures near the melting point, only few lamellar branches were observed. For sufficiently long crystallization times (e.g., tc = 6000 s at Tc = 200 °C, see Figure 5g), a network of numerous interconnected lamellae could be achieved, similar to the ones obtained for Tc = 185 °C or Tc = 190 °C. However, the network grown at 200 °C showed a lower lateral conductivity, due to its lower branching density. Interestingly, the data for this sample is exactly on the Imax ∼ N 2/3 line, see the green circle in Figure S8. The established relation between Imax and N indicates that lateral charge transport is related to the areal density of lamellae in the crystallized films, allowing charges to be injected in and transported through an interconnected network of P3HT lamellae. The formation of lamellar branches allows to establish bridges between neighboring lamellae, resulting in the possibility of high lateral interlamellar charge transport. However, if the connectivity of interlamellar structures is destroyed (see the broken lamellae in Figure S9), the noninterconnected lamellae exhibit a low conductivity. Besides the interlamellar structures of connected lamellae, we aimed to investigate if the lateral conductivity of P3HT

network exhibited a higher lateral conductivity than that the LB network. The highest lateral conductivity was found for the UHB network. The histograms of the current (see Figure 3h) showed a narrow Gaussian peak centered at about 0 pA, which was assigned to the background current of the substrate. A second broader Gaussian peak at higher current values corresponds to the current gathered within the network of P3HT lamellae, with a value of the maximum reaching up to 4 pA for the UHB network. We attribute the high lateral conductivity of the UHB network to the large available area for improved charge carrier tunneling to the substrate. With the aim to establish a relationship between morphological structure and lateral charge transport, we show the dependence of the height, width, branching density (N), and current of P3HT crystals on tc in Figure 4a. At Tc =

Figure 4. Dependence of current on the branching density of the network of lamellae. (a) Branching density (N) and maximum of current (Imax) of the lamellae as a function of tc for Tc = 185 °C. The inset shows the dependence of height and width of corresponding P3HT lamellae on tc. (b) Dependence of Imax on the branching density (N). The inset shows the area (A) of lamellae as a function of N.

185 °C, we can see that the width of P3HT crystals was roughly constant, while the height of lamellae increased with tc. The branching density and the maximum of the current showed a similar monotonous increase with tc, suggesting a relation between these two parameters. As shown in Figure 4b, we found Imax ∼ N 2/3. Since the area (A) covered by lamellae also increased with lamellar branching (see the inset in Figure 4b), we could also establish a relationship between Imax and A (see Figure S6). The morphology of polymer crystals, especially when confined in thin films, not only depends on crystallization

Figure 5. Dependence of height and current maps of P3HT lamellae on crystallization temperature (Tc) and crystallization time (tc). P3HT lamellae were grown in 30 nm-thick films via crystallization at (a−c) 190 °C and (e−g) 200 °C for different crystallization times tc: (a) 10 s, (b) 25 s, (c) 1800 s, (e) 60 s, (f) 180 s, and (g) 6000 s, respectively. (d, h) Corresponding current maps for the samples shown in (c, g), respectively. The size of the scale bar is 2 μm. E


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Figure 6. Topographic and current images of the networks of crystalline P3HT lamellae with different interlamellar and intralamellar order. (a, b) Initial network of lamellae grown in 30 nm-thick films via crystallization at 185 °C for 600 s and the subsequent selective washing in anisole at 69 °C for 5 s. (c, d) Same network of P3HT crystalline lamellae after annealing at 230 °C for 3 min followed by a quench to room temperature. The insets in (a, c) are the corresponding 2D GIWAXS patterns. (e) Height and current (inset) histograms of the network of lamellae, obtained from the corresponding AFM images, respectively. (f) 1D GIWAXS curves obtained from 2D diffraction patterns. The inset is the comparison of azimuthal profiles taken at the value of qz for the (100) peak. The size of the scale bar is 2 μm. (g, h) Schematic diagrams indicating the flow of the current through a crystalline lamella before and after the additional melting−quenching step. The three main length scales indicate the orientation of the P3HT molecules within superposed crystalline lamellae.

crystals depended also on intralamellar structures (defects) within P3HT lamellae. To this end, we have performed a melting−quenching treatment of a washed network of lamellae, as shown in Figure 6. The initial topography and current images of the washed network of P3HT lamellae with high lateral conductivity are shown in Figure 6a,b, respectively. As shown in Figure 6c,e, annealing at 230 °C (well above the melting point of 220 °C) for 3 min reduced the height of P3HT network visibly, indicating that P3HT chains in the molten state diffused toward the underlying silicon substrate. Interestingly, although the network morphology still existed after the melting−quenching step (see Figure 6c), the corresponding lateral conductivity was significantly lower than the starting value (see Figure 6d,e). We probed the changes in microstructure within the P3HT network caused by the melting−quenching step using GIWAXS. 2D GIWAXS diffraction patterns of the initial and the annealed sample are shown in the insets of Figure 6a,b. In stark contrast to the diffraction arcs of the initial network of lamellae, the diffraction pattern of the molten-quenched sample showed spots or ellipses, revealing that the distribution in orientation of edgeon crystallites within the network of lamellae has narrowed down.

Intriguingly, in spite of the enhanced crystallinity and narrowed distribution in the orientation of crystallites (see Figure 6f), the molten-quenched P3HT network displayed rather a low lateral charge transport, suggesting that conductive connections (bridges) between lamellae have been broken and intracrystallite defects within lamellae network may have been introduced during the melting−quenching step. We tentatively assume that during melting at 230 °C, P3HT lamellae were only molten partially. Upon quenching to room temperature, due to the high supercooling and some remaining crystalline parts acting as seeds, many independent crystallites grew from the molten P3HT network. As a result, a significant number of defects and randomly oriented boundaries and depletion zones between crystalline structures were generated (see Figure 6h), which hindered the efficient charge transport between the crystallites. In conclusion, without connecting conductive paths over long distances between crystalline structures like lamellae, an efficient lateral charge transport is not possible. However, during slow crystallization at elevated crystallization temperatures, interconnected lamellar structures with a low degree of defects and a large number of conductive pathways between lamellae are formed (see Figure 6g). F


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4. CONCLUSIONS Using an appropriate washing process allowed to reveal a network of multilamellar crystalline structures of poly(3hyxylthiophene) resulting from crystallization at temperatures close to the melting point. Based on a detailed analysis of the crystalline structure by GIWAXS and lateral charge transport in conjugated polymer crystals determined by C-AFM, we could establish a relationship between the complex morphology and efficiency of lateral charge transport. The network of lamellae with an ultrahigh branching density exhibited the highest charge transport efficiency. We observed a power-law increase of the maximum current (Imax) passing from the CAFM tip into the substrate with increasing number (N) of interconnected lamellae: Imax ∼ N 2/3. Applying a melting− quenching treatment to the network adversely affected the effective lateral conductivity due to the destruction of the connecting conductive paths over long distances by introducing defects and randomly oriented boundaries and depletion zones between crystalline structures. Choosing an appropriate isothermal crystallization temperature and varying the crystallization time allowed to tune the morphological structure, and accordingly, the charge transport through P3HT crystals. Thus, we have demonstrated a possible route to improve the electrical conductivity of conjugated polymers, potentially improving the performance of various organic electronics devices and related applications. The multiscale ordered structure−conduction property relationship presented here may present the foundation for future microstructure design of organic semiconductors.

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China Postdoctoral Science Foundation (nos. 2016 M592302 and 2018 T110741), Outstanding Young Talent Research Fund of Zhengzhou University (1521320004), and Startup Research Fund of Zhengzhou University (1512320001).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01146. Temperature and time protocols for melt crystallization experiments, AFM height and current images of P3HT lamellae before and after a selective washing procedure, schematic representation of the lateral conduction measurement by PF-TUNA, 1D GIWAXS integral curves, ATR-FTIR and UV−vis absorption spectra for a P3HT as-spun film and a crystallized film, out-of-plane and in-plane X-ray diffractions of dependence of the current on the area covered by lamellae, optical micrographs showing P3HT crystals grown at different temperatures, dependence of Imax on the branching density (N) for the sample of Figure 5h, AFM height and current maps of the broken lamellae (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Günter Reiter: 0000-0003-4578-8316 Bin Zhang: 0000-0002-8293-1321 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Science Foundation of China (nos. 51773182, 11872338, U1804144 and 11372284), G


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