Article pubs.acs.org/Macromolecules
Surface Structure of Semicrystalline Naphthalene Diimide− Bithiophene Copolymer Films Studied with Atomic Force Microscopy Mario Zerson,*,† Martin Neumann,† Robert Steyrleuthner,‡ Dieter Neher,‡ and Robert Magerle*,† †
Fakultät für Naturwissenschaften, Technische Universität Chemnitz, Chemnitz, Germany Institute of Physics and Astronomy, University of Potsdam, Potsdam-Golm, Germany
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ABSTRACT: The crystallization behavior, the surface structure, and the nanomechanical properties of a semiconducting polymer play a crucial role in understanding the charge injection process, the transport of the charge carriers and the processability of the material. Here we study the semiconducting copolymer poly([N,N′-bis(2-octyldodecyl)-11naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′12 bithiophene)) (P(NDI2OD-T2)) and investigate the influence of annealing conditions on its surface structure through intermittent contact mode atomic force microscopy (AFM) and AFM-based measurements of amplitude−phase− distance (APD) curves. For spin-cast thin films as well as for films annealed at temperatures up to 320 °C, we find that the edges of crystalline lamellae are exposed at the surface. A 1.2 nm thick layer of alkyl side chains covers the film surface as indicated by the tip indentation. This suggests that charge injection into P(NDI2OD-T2) films is not hindered by a surface layer of amorphous material. In 5 nm thick films, corresponding to two monolayers of P(NDI2OD-T2), after annealing at 320 °C, crystalline lamella also orient perpendicular to the film plane with the (100) surfaces oriented parallel to the film plane. The lamellae form ∼100 nm large areas (terraces) with uniform lamella height. The step height between adjacent terraces is 2.2 nm, and we attribute it to monomolecular steps between the molecular thin layers of edge-on-oriented polymer chains. This welldefined molecular conformation at the film surface with the chain backbone and the π-stacking direction oriented in the film plane is presumably an important factor contributing to the exceptional performance of P(NDI2OD-T2) in bottom-gate organic field-effect transistors.
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Detailed knowledge about the molecular packing in the films and at the film−electrode interfaces is essential for understanding the charge injection and transport in thin films, which both determine the device performance. It has been shown that the orientation of the lamellae and the polymer chains with respect to the film plane depends on the film formation process, the solvent used,10 and the postdeposition treatments.13 For example, moderate thermal annealing above 200 °C can increase the bulk electron mobility of P(NDI2OD-T2) films due to the coalescence of preaggregated chains, thus forming larger crystallites.11,13 Annealing above the melting point, however, leads to a structural change in the bulk, changing from predominantly face-on-oriented polymer backbones (with respect to the film plane) to almost completely edge-on packing, with the chain backbone and the π-stacking direction oriented in the film plane. This structural change is accompanied by a decrease in the electron mobility in the vertical direction.8 On the other hand, the mobility measured in field-effect top-gate transistors, which is sensitive to the surface
INTRODUCTION Organic materials for electronic circuits are currently a focus of interest due to their favorable combination of electronic properties, ease of processability, and low costs. Some applications are organic field-effect transistors (OFETs), organic light-emitting diodes, and organic solar cells.1−3 Poly([N,N′-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-12 bithiophene)) (P(NDI2OD-T2)), known as ActivInk N2200 (Polyera Corporation), is an n-type semicrystalline polymer with very high electron mobility4−6 and good solubility, which makes it suitable for printed electronics. Its high electron mobility is mainly attributed to the molecular packing of the polymer chains in the crystallites, in particular, the intermolecular πstacking of the conjugated backbone,7 which is favored by the alternating donor−acceptor chain architecture, the high degree of backbone coplanarity, and the regioregularity of P(NDI2OD-T2). This enables the formation of crystalline lamellae8 which act as efficient pathways for electron transport.9−11 Computer simulations of P(NDI2OD-T2) provide insight into the molecular conformations of the polymer backbone and the alkyl side chains within the crystalline lattice.12 © XXXX American Chemical Society
Received: May 25, 2016 Revised: July 19, 2016
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DOI: 10.1021/acs.macromol.6b00988 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. IC-mode AFM height images and phase images of 40 nm thick films of P(NDI2OD-T2) after different annealing treatments. The left column shows the height images; the middle column shows the corresponding phase images. Magnifications of the areas marked with dashed rectangles are displayed in the right column. The area marked with the white square was investigated with APD measurements, and the results are shown in Figure 4.
length scales, the surface morphology of the films is studied with conventional intermittent contact (IC) mode AFM. Performing these studies on samples with different thickness and annealing conditions provides a conclusive picture of the effect of annealing on the surface structure. We first present the results obtained from 40 nm thick films, since this film thickness compares well to those studied in previous work.6,8,11 The same sample preparation and annealing conditions were used as in the previous work, allowing our results on the surface structure of P(NDI2OD-T2) films to be directly compared with the results obtained with grazing incidence X-ray diffraction, transmission electron microscopy optical polarized light microscopy, optical absorption spectroscopy, and measurements of electron mobility.6,8,11 For comparison, we also studied 5 nm thick films, which correspond to only two monolayers of P(NDI2OD-T2) chains. These experiments reveal the orientation of the crystalline lamellae, the conjugated polymer backbone, and the alkyl side chains relative to the film plane.
of the film, was only weakly affected by the reorientation of polymer chains in the bulk of the film.14 This is attributed to the fact that before and after annealing P(NDI2OD-T2) shows a similar backbone orientation at the surface, with the alkyl chains pointing out of the film plane.15 This corresponds to an edge-on orientation of polymer backbones at the film surface. Furthermore, two different crystalline forms of P(NDI2ODT2), which differ in the packing of the naphthalene diimide (NDI) and the bithiophene (T2) groups, were found in thin films.13,16 On larger length scales, polarized optical microscopy images of P(NDI2OD-T2) films show ∼1 μm large domains8 that resemble the spherulite morphology of semicrystalline polymer films. The molecular surface structure in thin films is of fundamental interest, since for most cathode materials, the device current is injection limited.6,17 An open question is whether the crystalline lamellae extend to the surface or whether the surface is covered with a thin amorphous layer, as in the case of regioregular poly(3-hexylthiophene) (P3HT)18 and in polypropylene with a low degree of crystallinity.19 Here we investigate the nanoscale surface structure of thin films of P(NDI2OD-T2) with regard to the arrangement of the polymer chains in the crystalline lamellae at the film surface, the presence of an amorphous surface layer, and the orientation of the alkyl side chains at the film surface. Since the glass transition temperature of P(NDI2OD-T2) is −70 °C,14 the amorphous regions between the crystalline lamellae are compliant at room temperature. Therefore, the thickness of an amorphous surface layer can be measured via the tip indentation into this compliant surface layer. To this end, we measure amplitude−phase−distance (APD) curves, reconstruct multiset point intermittent contact (MUSIC) mode AFM images of the film surface, and quantify the nanomechanical surface properties, in particular the tip indentation.20 On larger
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RESULTS AND DISCUSSION Surface Morphology of 40 nm Thick Films. Figure 1 shows the IC-mode AFM height images and phase images of 40 nm thick P(NDI2OD-T2) films directly after spin-casting from chlorobenzene solution and after different thermal annealing treatments. Thermal annealing up to 260 °C has no visible effect on the surface morphology. Changes are observed only after annealing at 320 °C, which is above the melting temperature TM = 305 °C of P(NDI2OD-T2). The AFM height images (Figure 1a,d,g) of the as-cast film and of the films annealed up to 260 °C show a flat surface with a root-mean-square (RMS) roughness of 0.6 nm and a texture of stripes. This morphology was also observed for thicker films
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DOI: 10.1021/acs.macromol.6b00988 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules and for films on other substrates.5,8,13,14 The corresponding AFM phase images (Figure 1b,e,h) show structures on two length scales. On the micrometer scale, bright and dark domains are visible (marked by 1 and 2, respectively). The different average phase values reflect local differences in the tip−sample interactions and indicate different mechanical properties of the surface. Such a microstructure was also observed by Rivnay et al. for samples annealed up to 180 °C.8 On the nanometer scale, bright stripes are visible within both types of the micrometer-large domains, with a mean distance of 10−20 nm between the stripes. Magnified AFM phase images (Figure 1c,f,i) clearly reveal the stripe structure in both types of domains. We attribute the bright stripes to upright-standing crystalline lamellae. This surface morphology has been reported previously.5,8,14 The shape of the micrometer-large domains and the arrangement of the crystalline lamellae within the domains resemble the spherulites in the thin films of semicrystalline polymers.8,21 The AFM height images (Figure 1j,m) of the samples annealed at 320 °C (above the melting temperature) show ∼50−100 nm wide bundles of fine stripes, as revealed by the corresponding AFM phase images (Figure 1k,l,n,o), which show no domains in the micrometer range but a very fine texture with parallel, ∼100 nm long stripes with a mean distance of 8−10 nm between the stripes for both the fast and the slowly cooled sample. The RMS roughness is 1.5 nm for the fast cooled sample and 6 nm for the slowly cooled sample. Figure 2 shows an enlarged detail of the AFM phase image shown in Figure 1l. The parallel aligned bright stripes are clearly visible.
On the basis of our results from 5 nm thick films (see below) and from the findings of McNeill et al.14 based on the surfacesensitive measurements of the near-edge X-ray adsorption fine structure (NEXAFS), we attribute the bright stripes to crystalline lamellae oriented perpendicular to the film plane, with the (100) surface oriented parallel to the film plane. Figure 3 shows a diagram of this arrangement as well as the orientation of the polymer chains within the crystalline lamellae and the unit cell of the crystalline lattice. The orientation of the lattice directions is also shown in Figure 2. The crystalline lamellae stand upright on the substrate. The repeat distance between the crystalline lamellae is ∼8−10 nm, and the lamellae have a length of several hundred nanometers. These values are inferred from the AFM phase image shown in Figure 2. The repeat distance between the crystalline lamellae agrees with the average size and distance between crystalline lamellae inferred from the TEM images of P(NDI2OD-T2) films (Figure 5 in ref 11). The polymer backbones within the crystalline lamellae are indicated by the blue and orange dashed lines. Between the crystalline lamellae, the polymer chains are depicted as dark gray lines. From the molecular weight of the P(NDI2OD-T2) we calculate a backbone length of 150 nm for an extended polymer chain, which is much larger than the lamella width (
