Article pubs.acs.org/Macromolecules
Effect of Backbone Regioregularity on the Structure and Orientation of a Donor−Acceptor Semiconducting Copolymer Louis A. Perez,†,∥,⊥ Peter Zalar,‡,∥ Lei Ying,‡,∥,▽ Kristin Schmidt,# Michael F. Toney,# Thuc-Quyen Nguyen,‡,∥ Guillermo C. Bazan,*,†,‡,∥,⊥ and Edward J. Kramer*,†,§,⊥ †
Department of Materials, ‡Department of Chemistry and Biochemistry, §Department of Chemical Engineering, ∥Center for Polymers and Organic Solids, and ⊥Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, California 93106, United States # Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States S Supporting Information *
ABSTRACT: A regioregular (RR) donor−acceptor conjugated copolymer based on cyclopenta[2,1-b:3,4-b′]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units was prepared by using polymerization reactions involving reactants specifically designed to avoid random orientation of the asymmetric PT heterocycle along the copolymer backbone. Compared to its regioirregular (RI) counterpart, the RR polymer exhibits a 2 orders of magnitude increase in hole mobility from 0.005 to 0.6 cm2 V−1 s−1. To probe the reason for this difference in mobility, we examined the crystalline structure and its orientation in thin films of both copolymers as a function of depth via grazing incidence wide-angle X-ray scattering (GIWAXS). In the RI film, the π−π stacking direction of the crystallites is mainly perpendicular to the substrate normal (edge-on orientation) while in the RR film the crystallites adopt a mixed π−π stacking orientation in the center of the film as well as near the interface between the polymer and the dielectric layer. These results demonstrate that control of backbone regularity is another important design criterion to consider in the synthesis and optimization of new conjugated copolymers with asymmetric structural units.
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because it lowers the copolymer band gap and enhances the double-bond character between repeating units. 15 D−A copolymers are typically made via cross-coupling condensation reactions with symmetrical monomer units that produce linear polymer backbone configurations. Several reports of high performing copolymers have also appeared that employ asymmetric units and therefore are capable of containing various regiochemistries between adjacent structural units along the backbone vector.16−19 It is postulated that producing asymmetry in the polymer backbone could have a significant effect because numerous studies have shown that even slight modifications of the chemical architecture of a conjugated polymer, i.e., single atom substitution and alkyl side chain length, can lead to significant changes of the copolymer’s microstructure and ultimately electronic properties.20−23 A recent example of the effects of the use of an asymmetric monomer in a D−A copolymer was shown by Ying et al. where the controlled incorporation of the asymmetric acceptor, [1,2,5]thiadiazolo[3,4-c]pyridine (PT), copolymerized with cyclopenta[2,1-b:3,4-b′]dithiophene (CDT) to produce a regioregular copolymer (RR in Scheme 1) led to a significant
emiconducting polymers are under consideration for the production of organic electronic devices via large scale highthroughput solution-deposition techniques.1,2 Such solutionprocessed devicese.g., organic field effect transistors (OFET)have several attractive features, such as low material costs and the ability to be processed at low temperatures to produce transparent, homogeneous, multilayered thin films on a variety of substrates, including flexible substrates.3,4 Widespread adoption, however, has been limited due to low charge carrier mobility and lifetime stability compared to other low cost thin film alternatives such as amorphous Si (a-Si) (mobilities ∼0.5−1.0 cm2 V−1 s−1). Nevertheless, the performance of solution-processed semiconducting polymer OFETs has improved significantly recently with several reports of hole mobility greater than 1 cm2 V−1 s−1.5−9 This figure of merit, along with improvements in encapsulation technology, strengthens the case for semiconducting polymers as viable candidates for commercial applications.10 These vast improvements in charge mobility are linked to advancements in conjugated polymer molecular structure design, which has enabled optimal charge injection and transport, coupled with optimized processing methods.11,12 A well-established method of designing semiconducting polymers is the “donor−acceptor” (D−A) motif.13 A D−A polymer is a copolymer that consists of alternating electrondeficient and -rich monomer units.14 This design is advantageous © XXXX American Chemical Society
Received: September 23, 2013 Revised: January 23, 2014
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Scheme 1. Reaction Pathway to the D−A Copolymer Structures of Regioregular (RR) and Regioirregular (RI) Polymers That Contain [1,2,5]Thiadiazolo[3,4-c]pyridine (PT) (μ = Hole Mobility)
important for most polymer thin films since they do not scatter strongly due a lack of strong long-range order and because they are usually composed of low atomic number elements.35 This disorder usually limits the ability to determine or index an exact unit cell. There is, however, strong scattering from the stacked lamellar-like layers that form due to the alkyl side chains as well as weaker scattering from the π−π stacking of the conjugated backbone, which is often nearly orthogonal to the alkyl stacking direction (Figure 1).38 We used these scattering features to distinguish whether the alkyl stacking orients perpendicular to the substrate normal (face-on) or parallel (edge-on) to it (Figure 1b,c). In addition, the choice of incident angle determines the region of the film that is probed and can reduce complications due to scattering from the substrate.39 We utilize this capability to increase the scattered intensity from local regions within the films, i.e., surface, bulk, and the interface, to highlight variation in polymer orientation as a function of depth. Thin film samples used in this study matched those used to make OFET devices prior to electrode deposition with samples spun-cast at 2000 rpm at room temperature from dilute (0.5 mL/mg) solutions of chlorobenzene onto OTS-treated SiO2/Si substrates. The samples were examined via GIWAXS, where scattered X-rays were collected by an area detector as shown in Figure 2. There is a restriction on the accessible polar angles (χ) as a function of q in the grazing incidence configuration due to the incident angle being held constant and the surface of the Ewald sphere being curved.40 The restricted GIWAXS regions are indicated by the dark blue wedge of zero intensity shown in Figures 2a,b.40,41 The line-cut profiles of RR and RI at χ = 11° and 88°which represent scattering that is mainly out-of-plane and in-plane, respectivelyare plotted as intensity (in arbitrary units) vs the scattering vector (q (nm−1)) in Figure 2c. At χ = 88° and 11°, there are several strong peaks in the low q regime (1−10 nm−1) for both RR and RI. These peaks are attributed to scattering from well-ordered alkyl lamellar stacks. The position of the peaks for each copolymer is the same at both χ values. This similarity indicates that each copolymer has a similar lamellar packing structure in both orientations. The lamellar packing distance d, however, is dependent on the regioregularity, with the first order, (100), alkyl stacking peak positions at q ∼ 2.50 nm−1 (d ∼ 2.51 nm) for RR and q ∼ 3.02 nm−1 (d ∼ 2.08 nm) for RI. This shorter alkyl chain packing distance in RI compared to RR could be due to a difference in the side-chain conformation or tilt of the molecular plane for each copolymer.
increase in hole mobility in field effect transistors (FETs).24 Compared to its regioirregular (RI) counterpart, the RR copolymer exhibited a 2 orders of magnitude increase in hole mobility, from 0.005 to 0.6 cm2 V−1 s−1, without the aid of any postprocessing or solvent additive treatments. The optical and electrochemical properties of the two copolymers, which had similar molecular weights and dispersity, did not show any appreciable differences. In addition, there were no thermal transitions observed with dynamic scanning calorimetry (DSC) measurements of either copolymer in the bulk state. It seemed reasonable that the large variation in hole mobility could be due to a preferred structural arrangement or orientation of the RR copolymer.25,26 Such considerations are reminiscent of the classic work on the impact of alkyl side chain regiochemistry on the electronic properties of poly(3-hexylthiophene).27−30 In this contribution we detail the structural differences between RR and RI via quantitative polymer morphological analyses. Such structure analysis, including the orientation distribution of polymer crystallites, is important because many materials properties (mechanical, optical, and electronic) can be anisotropic due to structural anisotropy.31,32 We also examined the crystalline structure and the crystallite orientation distribution as a function of depth via grazing incidence wide-angle X-ray scattering (GIWAXS) and report on significant differences in the crystallite orientation, where the π−π stacking direction of the crystallites is mainly perpendicular to the substrate normal (edge-on orientation) for RI, while in the RR film the crystallites adopt a mixed π−π stacking orientation in the center of the film as well as near the interface between the polymer and the dielectric interface. These results are surprising since it is often postulated that an edge-on orientation is necessary to achieve high charge carrier mobilities; however, this orientation is present in the lower performing regioisomer, RI.
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RESULTS AND DISCUSSION GIWAXS is a versatile characterization technique for thin film microstructure and crystallite orientation that has been used on a number of semiconducting polymer substrate supported thin films to elucidate structure−processing−property relationships.33−37 In the grazing configuration, the sample is irradiated at a fixed incident angle, on the order of a tenth of a degree, which increases the path length of the X-ray beam within the layer to multiples of the sample thickness (Figure 1a). More of the sample is probed and so the scattering signal increases, which is B
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Figure 1. (a) Schematic of a grazing incidence wide-angle X-ray scattering (GIWAXS) experimental setup with an area detector. Schematics of orthogonal polymer backbone stacking orientation direction relative to the substrate where the conjugated backbone plane is (b) parallel (face-on) and (c) perpendicular (edge-on) to the substrate normal.
features indicate that there is an overall higher population and larger correlation length of π−π stacking lattice planes in RR versus RI. The GIWAXS diffraction pattern and line-cut profiles show that RI predominantly adopts an orientation where the (010) or the π−π stacking direction is perpendicular to the substrate normal while RR shows contributions from both parallel and perpendicular orientations. However, as mentioned previously, the incident angle dictates what portion of the film that is probed by altering the depth-dependent amplitude of the electric field intensity within the sample.44−46 This provides an opportunity to probe the polymer structure in specific regions, namely the surface and interface, of the polymer film when the incident angle is above the critical angle for total external reflection of the substrate. We, therefore, exploit this property to quantify the crystallite orientation distribution as a function of depth within the film. The depth profiling aspect is an attractive feature for this study since it is well established that the largest contribution of charge transport in bottom gate, top-contact field effect transistors occurs within the first few nanometers of the semiconductor between the polymer layer and the dielectric interface.47,48 Several parameters of each layer in a multilayered sample, (Si/SiO2/OTS/polymer) for the samples in this study, such as thickness (h), root-mean-square roughness (r), and density (ρ), are needed to model the dependence of the electric field intensity (EFI) distribution at a chosen incident grazing angle (αi).49,50 X-ray reflectivity (XRR) was used to determine the abovementioned film parameters for both RI and RR samples (Figure 3 inset).51 The polymer films from both copolymers had similar parameters with an h of 16.1 nm (RI) and 19.1 nm (RR) and mass densities of 1.10 g/cm3 (RI) and 1.12 g/cm3 (RR), which were determined from the best-fit scattering length densities, for RI and RR, respectively. The copolymer films were also smooth with r < 0.7 nm for both. The simulated EFI as a function of depth within a sample of RR at several αi is shown in Figure 3.49,50 The simulated EFI for RI is similar and can be found in the Supporting Information. An angular divergence of 0.01° was used for the calculations. At αi = 0.10° an evanescent wave forms at the surface of the film because this αi is below the critical angle for total external reflection (αc) of the polymer film. The EFI maximum occurs at
Higher order reflectionsmultiples of the (100) stacking distanceof the alkyl stacking peak are present for both RR and RI; however, their appearance is orientation dependent. For RI, at χ = 11°, there is a second-order alkyl stacking peak (200) at q ∼ 6.04 nm−1 (d ∼ 1.04 nm) that is not present at χ = 88°. This second-order peak at χ = 11° indicates that the alkyl side chain stacking in RI is oriented mainly parallel to the substrate normal. RR has a second-order alkyl stacking peak (200) at q ∼ 5.01 nm−1 (d ∼ 1.25 nm) at χ = 11° and a second-order (200) peak at q ∼ 5.01 nm−1 (d ∼ 1.25 nm) as well as a third-order peak (300) at q ∼ 7.52 nm−1 (d ∼ 0.86 nm) at χ = 88°. This presence of higher order reflections is a strong indicator of crystallite quality and perfection, which is often thought to be a desirable structural feature for improving charge mobility.42 The GIWAXS results, however, show that the RR polymer does not order exclusively into the supposedly desirable edge-on orientation. Several features worthy of discussion are present in the high q regime, 10−25 nm−1. The broad hump from q ∼ 10 to 20 nm−1 is attributed to amorphous scattering from disordered regions within the film, scattering from the side chains, and extraneous scattering from the substrate.43 Within that broad hump is the (010) reflection or what is commonly referred to as the π−π stacking peak (see Figure 1b and c). Other features found in the region can also be attributed to scattering from ordered side chains.43 The π−π stacking peak is at q ∼ 17.4 nm−1 (d ∼ 0.36 nm) for both RR and RI at both χ = 11° and 88°. At χ = 11°, for RI, there is a possible broad hump around q ∼ 18 nm−1 that could be the π−π stacking peak, and if so, it has a small correlation length; however, a much stronger π−π stacking peak is clearly visible in RR. The appearance of a strong π−π stacking peak in RR in the out-of-plane directionimplying a stacking often referred to as “face-on”is consistent with the long-range alkyl stacking order seen in-plane. The alkyl stacking and π−π stacking peak directions are approximately orthogonal to one another and are usually envisioned as a pseudo-orthorhombic unit cell (Figure 1b,c).38 At χ = 88°, the π−π stacking peak is present for RR and RI and suggests a π−π stacking orientation that is perpendicular to the substrate normal (edge-on). It is also worth noting that if crystallite orientation effects are excluded by taking azimuthal line cuts over all polar angles, the scattered intensity of the π−π stacking peak for RR is stronger than in RI and the peak width for RR is more narrow (Figure S7). These scattering C
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Figure 3. Electric field intensity (in arbitrary units) simulation of an RR sample shows the depth dependence of the EFI on the angles of incidence 0.10°, 0.12°, and 0.13°, producing a maximum intensity at the surface, in the bulk, and at the polymer/dielectric interface, respectively. The inset contains fitted XRR plots, which were used to determine the thickness, roughness, and density of all layers in the samples.
for RR at αi = 0.10° and 0.12° do not display a significant difference qualitatively. The out-of-plane π−π (010) stacking peak and strong evidence of both in-plane and out-of-plane (100) lattice plane orientation are present at both incidence angles. The diffraction patterns for RI at αi = 0.10° and 0.12° suggest that the (100) lattice planes are ordered predominantly out-of-plane and the (010) lattice planes are in-plane. There is a significant amount of background scattering due to the thick (150 nm) SiO2 dielectric layer in both RR and RI at αi = 0.13°, which is expected from the EFI simulations where there is a large fraction of EFI in the substrate. The background scattering from the substrate makes it difficult to distinguish the π−π stacking peak at αi = 0.13°; however, the alkyl stacking peak region is unaffected. This provides an opportunity to probe specific regions of both RI and RR to determine the orientation of ordered lattice planes in conjugated copolymers as a function of depth within a conjugated copolymer near the surface, the middle, and the interface between the polymer film and substrate. The Hermans orientation parameter (S) was calculated to quantify the lattice plane orientation distribution of the firstorder alkyl stacking peak (100) at αi = 0.10°, 0.12°, and 0.13° for both RR and RI. We focused on the (100) lattice plane due to the difficulty in determining the (010) peak location and intensity due to large background scattering from the substrate at αi = 0.13°. Line cuts were taken at 1° increments and width from χ = 4° to 90° to account for all orientations of the (100) plane accessible by GIWAXS.40,52 The (100) peak and higher order reflections were fit with individual Gaussian functions for each polar angle line cut.41,53 This regime contains not only scattering from the (100) peak but also contributions from diffuse reflected intensity from the surface, which was fit with a decaying exponential.53 The average orientation of a lattice plane (pole) is represented by a molecular orientation parameter ( f⊥), which represents the orientation of the pole relative to the axis normal of the sample, and is given by:
Figure 2. GIWAXS patterns of (a) RR and (b) RI at an incidence angle = 0.12°. Insets show the region near the (100). The line-cut profiles (c) at specific polar angles are meant to represent out-of-plane (χ = 11°) and in-plane (χ = 88°) orientations.
the surface when αi = 0.10°, which allows the probing of surface features. At αi = 0.12°, which is above the αc of the polymer, the EFI maximum shifts to the middle of the polymer film. The EFI maximum shifts deeper into the polymer film with the maximum concentrated at the interface between the polymer/substrate interface at αi = 0.13°. The GIWAXS diffraction patterns of RR and RI at αi = 0.10°, 0.12°, and 0.13° are shown in Figure 4. The diffraction patterns D
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Figure 4. GIWAXS diffraction patterns of RI and RR at different incident angles that probe different portions of the semiconducting polymer film.
f⊥ =
∫0
π /2
pole figures (top of Figure 5) This shows that an edge-on orientation persists throughout the film thickness. The S values for the RR copolymer are 0.15, 0.05, and 0.08 for αi = 0.10°, 0.12°, and 0.13°, respectively. These values and the quasi-pole figure image plots (top of Figure 5) indicate the RR copolymer mainly adopts a mixed orientation in the bulk and substrate interface with a slight preference for edge-on character at the surface. Evidence of a different surface orientation compared to the bulk has also been observed in another D−A conjugated copolymer by near-edge X-ray absorption fine structure (NEXAFS) measurements.55 The orientation analysis shows that the regioirregular isomer, RI, adopts mainly an edge-on orientation while the high performance regioregular copolymer, RR, retains a mixed character. The results of the orientation analysis are different from the often accepted preferred orientation expected for high mobility OFETs, which postulates that a polymer that adopts a mainly edge-on orientation should facilitate more efficient charge transfer from the source to drain electrodes in the plane of the substrate. Several recent reports of high performance conjugated polymer based OFETs, however, indicate the appearance of a mixed orientation in the bulk of the film.7,9,56,57 It is postulated that maintaining a mixture of ordered lamellar sheets of edge-on and face-on orientation could be advantageous because there is a three-dimensional network of charge conduction.9,58 The cause for the difference in lattice plane orientation is currently not clear, but it is noted that the average stacking distance for a lamellar sheet is ∼1.5 nm; therefore, there could be restrictions on the length of a crystallite in the edge-on case since these films are thin
I(χ ) cos2(χ ) sin(χ ) dχ
∫0
π /2
I(χ ) sin(χ ) dχ
where I(χ) is the total scattered intensity that was determined as the area of the Gaussian of the (100) peak at each polar angle and the sin(χ) term is an geometric intensity correction factor.54 The S value can be calculated with the determined f⊥: S=
1 (3f − 1) 2 ⊥
An advantage of the S formalism is that a single number designates/represents the degree of orientation of a pole.54 The S value ranges from −0.5 to 1, where −0.5 corresponds to all lattice plane normals oriented perpendicular to the substrate normal while 1 corresponds to all lattice plane normals oriented parallel to the substrate normal. Quasi-pole figure plots (quasi because polar angles 0°−4° are inaccessible in the GIWAXS geometry at the wavelength used) of the (100) peak for RR and RI at αi = 0.10°, 0.12°, and 0.13° along with image plots are shown in Figure 5. The abscissa corresponds to the polar angle (χ), the ordinate is plotted as the scattering vector q (nm−1), and the color contrast corresponds to the total scattered intensity on a linear scale, corrected for sin(χ). The S value is indicated in the top right of each pole figure image plot. The S values for the RI copolymer, 0.49, 0.47, and 0.47 for αi = 0.10°, 0.12°, and 0.13°, respectively, do not vary significantly as a function of depth in the film and neither do the shapes of the E
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Figure 5. Quasi-pole figure and image plots of the (100) lattice plane of RR and RI show that RI adopts an edge-on orientation throughout the polymer film while RR has a mixture of edge-on and face-on orientations. The color contrast in the image plots corresponds to the total scattered intensity. N2-filled glovebox. RR and RI were dissolved in chlorobenzene, affording 0.5% w/v solutions. Polymer films were spun-coat at a speed of 2000 rpm for 60 s at room temperature in an N2-filled glovebox. The molecular weights of RR and RI were 34 kDa (Đ = 3.1) and 40 kDa (Đ = 2.5), respectively. X-ray Reflectivity (XRR). XRR was done on a Rigaku Smartlab high resolution diffractometer from a Cu source (λ = 0.154 18 nm). The scans were from 2θ = 0°−3.5° at a rate of 0.01° s−1 and held at each step for 60 s. The data were fit with the IGOR MotoFit package.59 Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS). GIWAXS was performed on beamline 11-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) with an X-ray wavelength of 0.9752 Å and with a MAR345 image plate detector. The sample-to-detector distance was 400 mm and was calibrated with a LaB6 standard. Samples were exposed scanned for 120 s while under a helium environment to minimize beam damage and reduce air scattering.
(
