Synthesis, Purification, and Characterization of Well-Defined All

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Synthesis, Purification, and Characterization of Well-Defined AllConjugated Diblock Copolymers PF8TBT-b-P3HT Michael Sommer,*,†,# Hartmut Komber,‡ Sven Huettner,§ Rhiannon Mulherin,§ Peter Kohn,∥ Neil C. Greenham,§ and Wilhelm T. S. Huck†,⊥ †

Melville Laboratory for Polymer Synthesis, Lensfield Road, Cambridge CB2 1EW, U.K. Leibniz-Institute of Polymer Research Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany § Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, U.K. ∥ Biological and Soft Systems, Department of Physics, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, U.K. ⊥ Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, Netherlands ‡

S Supporting Information *

ABSTRACT: We present the synthesis, purification, and characterization of all-conjugated block copolymers comprising poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)2,1,3-benzothiadiazole]-2′,2″-diyl) (PF8TBT) and poly(3-hexylthiophene) (P3HT). Suzuki step-growth polycondensation is used for the synthesis of PF8TBT, which is subsequently terminated via the addition of narrow-distributed, monobrominated P3HT-Br. Purification via preparative GPC is carried out to reduce polydispersity and to remove excess P3HT. Wavelength-dependent GPC and careful NMR end group analysis, assisted by model compounds, reveal pure diblock copolymers of PF8TBT-b-P3HT. Insight into structure formation is given by temperature-dependent UV−vis absorption, DSC, and X-ray scattering. These indicate that PF8TBT-b-P3HT does not microphase-separate within the investigated range of composition and molecular weight. The critical role of introducing sufficient dissimilarity between the segments in all-conjugated block copolymers in order to induce phase separation is discussed, with the conclusion that careful tuning of side chains is crucial for achieving selforganization.

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(KCTP)13−15 is often used for the polymerization of electron-rich, thiophene-based donor homopolymers16 and block copolymers;17−22 however, the successful KCTP of acceptor monomers had not been reported.23 A major breakthrough was very recently published by Senkovskyy et al., who demonstrated the first controlled polymerization of electron-accepting monomers based on naphthalene diimide and thiophene.24 The current progress in elevating Suzuki polycondensation from step growth to chain growth makes this method a promising alternative to the intensively investigated KCTP.25,26 However, until such advanced techniques are available for simple one-pot procedures of well-defined donor−acceptor all-conjugated block copolymers, traditional step-growth polycondensations need to be employed and combined with other polymerization techniques. Only a few papers dealing with the synthesis of all-conjugated donor/ acceptor block copolymers have been presented during the past years, in which mostly Suzuki or Yamamoto polymerizations were terminated with monobromo-functionalized poly(3-

INTRODUCTION Block copolymers (BCPs) are intriguing materials that selfassemble into well-ordered nanoscopic structures on length scales typically between 5 and 50 nm, provided that the chemical dissimilarity and the chain length of the two blocks are sufficiently large.1,2 Owing to the similar scale of phaseseparated block copolymer nanostructures and exciton diffusion lengths, block copolymers are ideal candidates for applications in organic photovoltaics (OPVs).3−6 Here, a network with continuous percolation paths of an electron accepting and a donating material is required between the electrodes, without the distance to an interface of the two materials ever greatly exceeding the short 5−10 nm length scale of exciton diffusion.7−10 However, the chemistry of conjugated materials makes the preparation of well-defined block copolymer (BCP) topologies synthetically challenging, and few donor and acceptor materials have so far been utilized and combined to make conjugated BCPs. Controlled radical polymerization of monomers with electronic function has been reported, yielding fully functionalized donor−acceptor block copolymers with side-chain architecture.11,12 The controlled chain-growth polymerization of conjugated polymers via the newly discovered Kumada catalyst transfer polycondensation © 2012 American Chemical Society

Received: March 16, 2012 Revised: April 27, 2012 Published: May 7, 2012 4142

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hexylthiophene).27−32 The strength of terminating step-growth polycondensations with preformed polymers is that any monomer combination can be used in the first polycondensation step, thus yielding a potentially wide variety of allconjugated block copolymers in which the electronic properties of complementary building blocks can be fine-tuned. However, this approach is not without difficulties. First, contamination with one or more homopolymers is likely, and tedious purification is therefore necessary. For instance, a recent paper from Mulherin et al. reported the synthesis and use of an all-conjugated block copolymer PFTBTT-b-P3HT in solar cells. While microphase separation was clearly observed, the investigated material was a mixture of block copolymer and mainly P3HT homopolymer.32 Second, the polydispersities of conjugated polymers prepared via step-growth polycondensation are generally broad, which can be a drawback at the stage of structure formation. Third, it is not always entirely clear whether diblock or triblock copolymers or mixtures thereof are obtained after end-capping. The preparation, purification, and characterization of all-conjugated block copolymers has therefore remained a great challenge. Here we present the synthesis, purification, and characterization of all-conjugated block copolymers comprising PF8TBT and P3HT. With PF8TBT/P3HT blends among the most efficient all-polymer solar cells,33,34 well-defined block copolymers PF8TBT-b-P3HT of substantial molecular weight are materials that are likely to enable further improvements in OPV performance via enhanced morphology control, either in the form of pristine components or as compatibilizers32,35 in ternary blends. We use Suzuki step-growth polycondensation for the synthesis of PF8TBT followed by the in situ addition of various chain lengths of monobrominated P3HT-Br. This procedure, which leads to pure and low-polydispersity diblock copolymers of PF8TBT-b-P3HT after purification with preparative GPC, is proposed to be generally applicable to the synthesis of all-conjugated block copolymers. Structure formation of PF8TBT-b-P3HT is analyzed via several methods and is finally discussed with respect to the critical role of introducing sufficient dissimilarity between the segments in allconjugated block copolymers in order to achieve well-defined interfaces of donor and acceptor components.

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extraction was carried out using methanol, acetone, n-hexane, dichloromethane, and chloroform. General Procedure of Suzuki Polycondensation Followed by EndCapping with Br-P3HT. 68.3 mg (0.109 mmol) of 1, 70 mg (0.109 mmol) of 2, 97 mg (0.703 mmol) of K2CO3, a few droplets of Aliquat 336, and 5 mg of Pd(PPh3)4 were placed into a 10 mL Schlenk tube. The tube was sealed with a septum, evacuated, and backfilled with argon (3×). Degassed water (1 mL) and toluene (2 mL) were added via syringe, and the mixture was stirred at 80 °C for 6 h. 100 mg of BrP3HT 5a in 2 mL of degassed toluene was subsequently added via syringe, and the whole solution was stirred for 12 h (BCP 6, 7) and 24 h (BCP 8), respectively. Solutions of P3HT-Br with higher molecular weight had a lower concentration and were slightly heated to avoid gelation during addition to the polymerization mixture. Workup was first carried out in analogy to PF8TBT homopolymerization. The raw materials were then further purified on a preparative GPC (concentration ∼ 100 mg in 1 mL of CHCl3); fractions were collected manually. This procedure was repeated several times, and the same fractions were combined afterward. The chloroform solutions were filtered over a plug of silica gel and concentrated to dryness. Methanol was added, and the materials were collected as film and dried. Instrumentation. NMR Spectroscopy. 1H (500.13 MHz) and 13C (125.77 MHz) NMR spectra were recorded on a Bruker Avance III spectrometer using a 5 mm 1H/13C/19F/31P gradient probe. All spectra were recorded at 303 K (unless otherwise noted) using CDCl3 as solvent and were referenced to the residual solvent peak (δ(1H) 7.26 ppm; δ(13C) 77.0 ppm). Analytical gel permeation chromatography (GPC) measurements were carried out on two PL gel 5 μm mixed C columns, with pore sizes ranging from 10 to 105 Å (Polymer Laboratories), connected in series with a SPD-M20A prominence diode array detector (Shimadzu) and a differential refractometer/viscometer model 200 (Viscotek). Calibration was done using polystyrene standards, THF was used as eluent and the flow rate was 1.0 mL/min. Preparative GPC was carried out on a modular system from Shimadzu equipped with a UV detector in chloroform at flow rates of 1.5−1.0 mL. Two columns with a guard column from Jordi (250 × 22 mm, cross-linked polystyrene, 5 μm particle size, 104 Å pore size) were used. MALDI-ToF mass spectra were recorded on an Applied Biosystems 4700 Proteomics Analyzer. Solutions of the analyte were prepared in THF (10−4−10−5 M), and dithranol was used as the matrix (0.1 M in THF). Equal amounts of both solutions were mixed and spotted onto the MALDI plate. UV−vis Absorption Measurements. Samples were spin-coated (∼60 nm) from chloroform solution, annealed above their melting temperature to erase sample history, and subsequently cooled down at a constant rate of ∼10 K/min to ensure comparable annealing protocols for recrystallization and to avoid nonequilibrium microstructures. All sample preparation was carried out under inert gas within a glovebox. For the in situ measurements the substrate was mounted into a Linkam hot stage, and the film was heated over the melting temperature under nitrogen. During cooling at 10 K/min down to room temperature, spectra were taken using an Agilent diode array UV−vis spectrometer. Dif ferential scanning calorimetry (DSC) was carried out on a DSC Q 2000 from TA Instruments. Heating and cooling rates were 10 K/min under nitrogen. The mass of the sample was approximately 1−2 mg. X-ray Scattering. Bulk X-ray patterns were recorded at the beamline I22 at the Diamond Light Source in Oxford, UK. The energy of the photons was 12.400 keV. Aluminum disks with holes of 0.8 mm diameter were used as sample holders. The materials were pressed into the holes, thermally annealed ∼10 K above the melting point of P3HT for 10 min, and then cooled down at 10 K/min. The disks were mounted onto a Linkam hot stage with conducting heat paste under nitrogen, and X-ray patterns were recorded. After heating the polymers into the melt and cooling them down at 10 K/min again, further X-ray patterns were taken. Typical exposure times were 10 s. SAXS and WAXS detectors were used simultaneously in order to cover a large

EXPERIMENTAL SECTION

Materials. 4,7-Bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (Br-TBT-Br, 1),36 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)-9,9-dioctylfluorene (BE-F8-BE, 2),37 and monobrominated P3HT (5)38 were synthesized as described elsewhere. The synthesis of the model compound TBT-F8-P3HT is described in the Supporting Information. Pd(PPh3)4 was purchased from STREM. Toluene was purchased from Aldrich in anhydrous grade and degassed prior to use. Potassium carbonate and Aliquat 336 were purchased from Aldrich and used as received. Synthesis. General Procedure of PF8TBT Homopolymerization. 244.4 mg (0.39 mmol) of 1, 250.7 mg (0.39 mmol) of 2, 391 mg (2.84 mmol) of K2CO3, a few droplets Aliquat 336, and 16 mg (0.014 mmol) of Pd(PPh3)4 were placed into a 10 mL Schlenk tube. The tube was sealed with a septum, evacuated, and backfilled with argon (3×). Degassed water (1.5 mL) and toluene (5 mL) were added via syringe, and the mixture was stirred at 80 °C for 2 days. Degassed bromobenzene (0.5 mL) was added, and the mixture was stirred overnight. The mixture was cooled down and diluted with chloroform and water. The organic phase was dried, filtered over a plug of silica gel, and concentrated. The concentrated solution was precipitated into methanol and filtered into an extraction thimble. Sequential Soxhlet 4143

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Scheme 1. Synthesis of (a) Homopolymer PF8TBT 4, (b) Model Compound TBT-F8-P3HT 12, and (c) Block Copolymer PF8TBT-b-P3HT 6−8 via in Situ End-Capping of PF8TBT Polycondensation

range of scattering vectors between 0.1 and 30 nm−1. Silver behenate, p-bromobenzoic acid, and silicon were used to calibrate the detectors. Atomic Force Microscopy. AFM was performed using a Veeco Dimension 3100 AFM, operated in tapping mode.

methane fraction); in some cases chloroform fractions with Mn,GPC = 180 kg/mol were isolated. End-capping with bromobenzene did not result in phenyl-capped PF8TBT 4 under these conditions. Instead, 1H NMR end group analysis indicated the formation of hydrogen end groups at the TBT end, seen at 8.02 ppm by the doublet with a small 4JHH coupling of 1.4 Hz (Scheme 1a and Figure 1c). This hydrogen end group of 4 was assigned with the help of the model compound TBTF8-P3HT 12, in which the chemical shifts of the terminal protons are almost identical (see Scheme 1a,b, Figure 1b,c, and Figure SI-3). Hydrogen termination at the fluorene end of 4 is observed as well, obviously as a result of deborylation. This side reaction, which is more commonly observed with electron-rich boronic ester-functionalized substrates,40,41 was confirmed by the appearance of a signal at 7.35 ppm and the comparison with the previously published model compound F8TBT-P3HT (Scheme 1a and Figure 1c).42 In addition, hydroxyl termination of the fluorene end was observed in some cases (Scheme 1a, Figure 1c, and Figure SI-5). The 1H chemical shifts of hydroxylterminated PF8TBT 4 fit very well with data calculated from

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RESULTS AND DISCUSSION Suzuki Step-Growth Polycondensation of PF8TBT. Suzuki step-growth polycondensation is a very versatile and simple method to prepare conjugated polymers from two symmetric monomers. In order to obtain high molecular weights, pure monomers, a precise 1:1 stoichiometry and minimized side reactions are required.39 Protocols for obtaining PF8TBT from 4,7-bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (1) and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (2) were reported by Cao et al.36 In order to successfully terminate such a polycondensation with P3HT-Br, a high end group fidelity must be preserved. The application of the standard Suzuki polymerization conditions using Pd(PPh3)4 as the catalyst and K2CO3 as the base in a water/toluene mixture for 2 days gave satisfactorily high molecular weights between Mn,GPC = 30−70 kg/mol (dichloro4144

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Figure 1. 1H NMR spectra (aromatic protons region) with end group assignments. (a) Br-terminated P3HT 5a block used in the in situ end-capping reaction. The tail-to-tail defect arising from polymerization via Ni(dppp)Cl2 is distributed along the chain. For simplicity, this tail-to-tail defect in (b), (d), and (e) is depicted in the formulas only at the chain end. (b) Model compound 12 with one TBTF8 moiety mimicking the end groups of the PF8TBT block. (c) PF8TBT homopolymer 4. (d) PF8TBT-b-P3HT BCP 6_2 with assignments of the backbone repeat units. (e) Enlarged region of PF8TBT-b-P3HT BCP 7_1 with assignments of the TBTF8 and P3HT end group signals. The dashed lines in (a) and (b) depict the signal shift of the characteristic thiophene end groups carrying bromine in (a) and fluorene in (b). Asterisks at 7.35 and 6.82 ppm in (c) indicate H and hydroxyl termination of PF8TBT, respectively.

Table 1. Molecular Weights, Polydispersities, and Weight Fractions of Homopolymer PF8TBT 4, End-Cappers Br-P3HT 5, Block Copolymers PF8TBT-b-P3HT 6-8, and Model Compound TBT-F8-P3HT 12

a

sample

Mn,GPC (kg/mol)

Mn,NMR (kg/mol)a

PDI

wt %BB P3HTb

wt %EG P3HTa

PF8TBT 4 P3HT 5a BCP 6_1 BCP 6_2 P3HT 5b BCP 7_1 BCP 7_2 BCP 7_3 P3HT 5c BCP 8_1 BCP 8_2 BCP 8_3 TBT-F8-P3HT 12

31.0 3.7 28.7 13.8 10.7 66.4 37.5 23.0 11.6 56.7 29.6 16.4 5.3

3.1 30.2 14.1 8.1 45.6 42.2 25.2 9.6 77.8 44.5 33.8 4.0

2.0 1.18 1.49 1.52 1.13 1.39 1.40 1.34 1.12 1.16 1.17 1.14 1.09

0 100 10.7 23.1 100 21.7 26.4 52.2 100 19.6 35.2 70.0

0 100 12.1 27.1 100 21.2 24.9 42.3 100 18.6 30.7 49.3

DPEG n/ma −/18 31/22 12/23 −/48 42/58 37/63 17/64 −/58 74/87 36/82 20/100 1/18

Determined from 1H NMR end group signals. bDetermined from 1H NMR backbone signals.

the chemical shifts of the hydrogen termination using 1H chemical shift increments of the OH group in benzenes and is in accordance with results from Kappaun et al.43 The proton of the OH group was observed at 4.8 ppm as a broad signal (Figure SI-5). We assume that unintentional introduction of oxygen leads to Pd−O2 intermediates44 and finally to hydroxylterminated PF8TBT. Since monomer purity and stoichiometry

had already been optimized, we rationalized that shorter reaction times might result in a higher degree of end group fidelity needed for successful end-capping. Therefore, decreased reaction times of 8 h were used before carrying out endcapping. In Situ End-Capping of Suzuki Step-Growth Polycondensation with P3HT-Br. The use of Suzuki poly4145

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Figure 2. GPC and UV−vis analysis of fractions purified by preparative GPC. (a) One-dimensional GPC elution curves of BCP 7_1, BCP 7_2, BCP 7_3, and P3HT-Br 5b in THF recorded at 258 nm. (b) UV−vis absorption in THF solution of P3HT 5b, BCP 7_3, BCP 7_2, BCP 7_1, and PF8TBT 4. (c) Two-dimensional GPC contour plots (y-axis: wavelength; x-axis: elution time; z-axis: intensity), from top to bottom: as-synthesized material BCP-7, fractions BCP 7_1, BCP 7_2, and BCP 7_3, and the pure end-capper P3HT-Br 5b.

2a and the UV−vis solution spectra in Figure 2b give only limited information on block copolymer purity, two-dimensional GPC contour plots of wavelength-dependent intensity vs elution volume are measured (Figure 2c). The top-right contour plot shows the as-synthesized material BCP 7, in which the broad polydispersity of the PF8TBT block in PF8TBT-b-P3HT and the excess of 5b are observed. The next three contour plots shown in Figure 1c depict BCP 7_1−3, which are obtained after fractionation of the as-synthesized material via preparative GPC. In addition, the corresponding end-capper 5b is shown as the last contour plot. P3HT homopolymer is efficiently removed in fractions BCP 7_1 and BCP 7_2. Fraction BCP 7_3, however, contains P3HT homopolymer. This is clearly seen from the similar absorption spectrum and elution volume to P3HT-Br 5b. Although some block copolymers were monomodal in the 1D GPC curves, the GPC contour plots did indicate the presence of P3HT homopolymer (see Table 1 and Figure SI-8a). Hence, the wavelength-dependent GPC contour plots demonstrate facile detection of P3HT homopolymer, which is of particular importance when the homopolymer appears at similar elution volumes as the main product itself. In principle, individual measurements at selected wavelengths can give the same information; however, the use of the contour plots allows for fast collection and handling, and comprehensive inspection of the data, as there is no need to compare 1D elution curves with the absorption spectra of the individual blocks. Moreover, all wavelength-dependent measurements are taken at the same time, which eliminates the need of having identical conditions during each single measurement. Having fractionated the raw material and identified fractions free of P3HT homopolymer, 1H NMR spectroscopy was employed for further characterization (Figure 1). Integration of backbone signals generally measures the relative total amounts

condensation followed by end-capping with P3HT-Br has been applied in several instances;28,29,32 however, less attention has been paid to donor−acceptor systems relevant to OPV devices32 and the identification of end groups. Having identified potential sources of end group degradation during the polymerization of PF8TBT and accordingly decreased the reaction time, several batches were polymerized for 8 h under the same conditions and end-capped with different molecular weights of P3HT-Br (see Table 1). Scheme 1c depicts the procedure of PF8TBT-b-P3HT synthesis. An excess of P3HTBr was used to further minimize the formation of PF8TBT homopolymer. Different molecular weights of end-cappers 5a− c were added as solutions in toluene; for higher molecular weights the solutions were slightly heated to avoid gelation. End-capping was carried out for 12−24 h. Afterward, in order to remove any nonreacted P3HT, the as-synthesized block copolymer/P3HT mixtures were fractionated using preparative GPC. This procedure simultaneously decreases the polydispersity (PDI) of the PF8TBT block. It is worth noting that the two hexyl chains at the thiophene rings in PF8TBT greatly enhance solubility and thereby facilitate this purification step. Thus, various fractions of PF8TBT-b-P3HT with varying chain length of PF8TBT and similar P3HT lengths were obtained during fractionation, resulting in BCP 6−8 (see Table 1). Figure 2a shows typical GPC curves of all fractions obtained from BCP 7 together with the used end-capper 5b. GPC molecular weights up to 70 kg/mol and polydispersities of 1.2−1.5 were obtained. Clearly, all fractions except BCP 7_3 are monomodal and exhibit higher molecular weights than the end-capper 5b. In these materials, the first indication for successful end-capping is gained from UV−vis in THF solution (Figure 2b). All fractions BCP 7_1−3 show superpositions of the pristine components P3HT and PF8TBT, with increasing P3HT content from BCP 7_1 to BCP 7_3. As the one-dimensional GPC curves in Figure 4146

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important information, we compared the weight fraction obtained from the backbone signal wt %BB P3HT with the weight fraction obtained from the degree of polymerization, referred to as wt %EG P3HT, which again has been calculated from the end group intensity. Values of wt %EG P3HT that are larger than wt %BB P3HT are therefore caused by the presence of PF8TBT homopolymer in materials free of P3HT homopolymer. However, as can be seen from Table 1, most of the values wt %EG P3HT are in good agreement with wt %BB P3HT when assuming an error of ±5% for end group integration. This indicates that amounts of PF8TBT homopolymer can be neglected in these samples. Only for BCP 6_1 and BCP 6_2 a slight discrepancy is observed which is outside the estimated error (for an estimation see Table SI-1). Here, the small amount of PF8TBT homopolymer can be inferred from the difference of wt %BB P3HT and wt %EG P3HT. Conversely, when wt %EG P3HT is smaller than wt %BB P3HT, P3HT homopolymer is still present. Samples BCP 7_3, BCP 8_2, and BCP 8_3 show such a behavior; here P3HT homopolymer amounts to ∼10, 5, and 20 wt %, respectively. This is fully in accordance with the contour plots shown in Figure 2c and Figure SI-7. It is worth mentioning that sample BCP 8_2 is an illustrative example that exhibits a well-defined 1-D GPC curve with a narrow polydispersity of 1.17; however, NMR end group analysis and the GPC contour plots clearly reveal a minor fraction of ∼5 wt % P3HT homopolymer (Table 1 and Figure SI-7). Overall, we find that the weight fractions obtained from backbone integration and end group integration are generally in good agreement and that the small differences reveal the presence of either PF8TBT or P3HT homopolymer. It is clear that the contribution of P3HT homopolymer increases with increasing elution volume during preparative GPC, as the molecular weight of the eluated product approaches that of the P3HT end-capper. In the present study the preparation of pure PF8TBT-b-P3HT with symmetric composition is limited by the number of columns (Table 1). Further removal of P3HT homopolymer requires better separation using more columns or repeated cycling.48 The phase separation of PF8TBT-b-P3HT was investigated in the bulk using differential scanning calorimetry (DSC). No melting or recrystallization was observed in scans at 10 K/min for P3HT weight fractions up to 20%, i.e., the first fractions. The second fractions BCP 6−8_2, having compositions higher than 20 wt%, did show a melting peak Tm with small melting enthalpies ΔHm up to 3 J/g arising from melting of P3HT (PF8TBT is amorphous). These values are much lower than what could be expected if P3HT was able to crystallize completely in PF8TBT-b-P3HT. For instance, BCP 8_2 has a ΔHm of 3.2 J/g. When compared to its weight fraction of 35% and the melting enthalpy of a 100% crystalline sample ΔH∞ m = 37 J/g,49 this gives a degree of crystallinity of ∼25%. Compared to its corresponding P3HT-Br end-capper 5c which exhibits ΔHm = 21.7 J/g, this indicates that only ∼50% of P3HT crystallizes (see Table SI-2). This is consistent with smaller P3HT crystals in BCP 8_2 compared to the P3HT-Br endcapper used, as shown by the smaller melting temperature of the block copolymer (213 °C vs 221 °C, see Table SI-2). The fact that no recrystallization peak is seen in BCP 8_2 might indicate that this process is slow, possibly caused by the stiff connectivity to the PF8TBT block. Only fractions containing P3HT homopolymer, i.e., fractions BCP 7_3 and 8_3, exhibited a recrystallization peak at 10 K/min. These results point to the fact that PF8TBT-b-P3HT presented here is not

of constituent polymers in a sample, irrespective of whether homopolymer or block copolymer is present. All block copolymers PF8TBT-b-P3HT, seen to be free of P3HT homopolymer by the GPC contour plots, exhibited the characteristic P3HT backbone signal at 6.98 ppm, which confirmed successful end-capping (Figure 1c). Hence, the integration of signals 3, 6, 10, or 9/13 for PF8TBT and 16 for P3HT allows simple determination of the molar and, accordingly, weight fractions (referred to as wt % P3HTBB, see Table 1). In order to extract information on potential homopolymer contributions, careful end group analysis was performed, which additionally yields the degrees of polymerization of both blocks. Methods that allow an estimation of absolute values of molecular weight and homopolymer impurities29 are much needed, since GPC usually overestimates molecular weight of rigid conjugated polymers. Neither could MALDI-ToF MS give satisfactory results in our experiments. The model compound TBT-F8-P3HT 12 was synthesized in order to assist in assigning the signals of end groups and the block junction to P3HT (Scheme 1b). 12 exhibits the same block junction F8-3HT to P3HT and the same TBT end group as present in PF8TBT-b-P3HT. The spectrum of the pristine P3HT end-capper 5a (Figure 1a) shows the typical signal pattern for Ni(dppp)Cl2-initiated P3HT with the terminal bromine located at a TT defect at the chain end (signal b) or next to regioregular P3HT with the TT defect anywhere else (signal group a).45 This signal pattern shifts by ∼0.3 ppm to lower fields (see dashed lines) when P3HT is covalently linked to the fluorene unit in the model compound 12 (Figure 1b) or in PF8TBT-b-P3HT (Figure 1e). Signals 3′ and 1′ of the terminal thiophene unit in the TBT end group of 12 were observed at 8.00 and 7.05 ppm, respectively. Signal 3′ was used for the determination of the degree of polymerization of the PF8TBT block, DPPF8TBT, at which intensity contributions of neighboring backbone signals were eliminated by a special baseline correction in this narrow signal region (Table 1). For the determination of DPP3HT in PF8TBT-b-P3HT, the wellknown H-end group signal at 6.9 ppm was used (signal 18″). Fractions that are free of P3HT homopolymer exhibit a good 1:1 ratio of the two end group signals 3′ and 18″, which further demonstrates successful end-capping. Importantly, the very similar intensities of these two end groups also prove the formation of a diblock copolymer (as opposed to a triblock copolymer).46 This is observed for fractions in which GPC showed the absence of P3HT homopolymer. Conversely, fractions that did contain P3HT homopolymer had intensities of signal 18″ larger than 3′. In these cases, DPP3HT in PF8TBTb-P3HT was corrected by assuming a 1:1 ratio of signals 3′ and 18″, and the difference gave the amount of P3HT homopolymer. The thus-obtained values for DP are summarized in Table 1. On the basis of these values, new P3HT weight fractions of all block copolymers were calculated (referred to as wt % P3HTEG, see Table 1). Note that the already mentioned end groups arising from 2hydro-terminated PF8TBT and 2-hydroxyl-terminated PF8TBT at ∼7.35 and ∼6.82 ppm, respectively (see ∗ in Figure 1c), were observed in some of the block copolymers as well. This indicates small amounts of PF8TBT homopolymer. Attempts to quantify the amount of PF8TBT in PF8TBT-bP3HT using these two end groups were unsuccessful, since both signal intensities were rather low and additionally superimposed by 13C satellites of backbones proton signals.47 However, as the amount of PF8TBT homopolymer is 4147

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microphase separated. Small-angle (SAXS) and wide-angle Xray scattering (WAXS) data of bulk samples confirm this assumption (Figure 3). As can be seen, the SAXS region

Figure 3. Small-angle (a) and wide-angle (b) X-ray scattering of thermally annealed bulk samples: 5b (black), 7_3 (green), 7_2 (red), 7_1 (blue). Patterns were recorded at room temperature after a controlled cooling step from the melt at 10 K/min.

(Figure 3a) between q = 0.2 and 2 nm−1 is featureless, confirming that no microphase separation occurs (only the corresponding end-cappers P3HT-Br show the known long period LP arising from alternating amorphous and crystalline lamellae38). The P3HT reflections in the WAXS region are observed; however, their intensities in all block copolymer samples are low, which confirms the observations from DSC of reduced degrees of crystallinity (Figure 3b). This hints that incompatibility between PF8TBT and P3HT may not be sufficiently large to induce microphase separation here, probably caused by the presence of the two 3-hexylthiophene units in PF8TBT. The comparison of these results with a recent report from Verduzco et al. is interesting.29 Using smaller molecular weights and larger P3HT weight fractions in P3HTb-PFO and P3HT-b-PFBT, the authors observed crystallization-induced microphase separation upon crystallization of the P3HT block. This suggests that for the materials presented here microphase separation might occur for larger P3HT weight fractions. Obviously, a larger fraction of P3HT would increase the overall block copolymer molecular weight and also balance composition. Additional light is shed on the crystallization behavior of the P3HT block in PF8TBT-b-P3HT using temperature-dependent UV−vis absorption in film (Figure 4). For this purpose, temperature-dependent spectra are taken from a P3HT homopolymer with similar molecular weight, BCP 8_2 and the analogous blend having the same molecular weights and weight fractions. The solution spectra of the block copolymer

Figure 4. Thin film absorbance of P3HT homopolymer, PF8TBT-bP3HT block copolymer, and PF8TBT/P3HT blend films after different conditions of preparation. (a) Temperature-dependent thin film UV−vis absorption of BCP 8_2 during cooling from the melt in steps of 10 K. (b) Comparison of thin film spectra of BCP 8_2, the analogous blend and the P3HT homopolymer after cooling from the melt at 10 K/min. (c) Comparison of BCP 8_2 and the analogous blend after spin-coating from chlorobenzene with 1% 4-bromoanisole as additive (black, green) and after cooling from the melt at 10 K/min.

and the blend demonstrate that the weight fractions in the two solutions used for spin-coating are indeed equal (see Figure SI10). The temperature-dependent absorption spectra of the P3HT homopolymer with similar molecular weight as used in BCP 8_2 show how the vibronic progressions at 530, 560, and 610 nm develop as the material crystallizes from the melt (Figure SI-11a).50 No such clear trend is observed in the temperature-dependent thin film spectra of BCP 8_2 (Figure 4a). Although a red-shift is observed upon cooling, distinct vibronic bands are poorly developed in BCP 8_2. For 4148

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Figure 5. AFM images of films spun from chlorobenzene with 1 wt % 4-bromoanisole as additive. (a) P3HT/PF8TBT blend with 34 wt % P3HT asspun and (b) after annealing at 240 °C. (c) PF8TBT-b-P3HT BCP 8_2 with 34% P3HT as-spun and (d) annealed at 240 °C.

comparison, the spectrum of BCP 8_2 is plotted together with the analogous blend and the P3HT homopolymer spectrum after cooling (Figure 4b, for the temperature-dependent spectra see Figure SI-11b). The two vibronic progressions at 560 and 610 nm in the blend film are slightly better resolved compared to BCP 8_2, but it is evident that P3HT crystallization is also hindered in the blend film.51 Given the inability of P3HT to fully crystallize in nonmicrophase-separated PF8TBT-b-P3HT, we set out to utilize selective solvents in order to induce structure formation in solution. A recent paper from Liu et al. has highlighted the use of 4-bromoanisole as a selective solvent for PF8TBT in PF8TBT/P3HT blends.52 Films of BCP 8_2 and the analogous blend are spin-coated from chlorobenzene with a small amount of 4-bromoanisole (1 wt %), which induces a greater degree of P3HT crystallinity in the as-spun films than spinning with chlorobenzene alone.52 UV−vis spectra are taken from the asspun films and after the films are heated to the melt and cooled down to room temperature at 10 K/min. Figure 4c compares the spectra of BCP 8_2 and the blend before and after thermal treatment. To ease comparison, the spectra are normalized at 370 nm. After the thermal treatment, both the blend and the block copolymer film show a slightly lower intensity at 610 nm and a slightly higher intensity between 400 and 500 nm. This indicates that the amount of amorphous P3HT is reduced by the thermal treatment. These results show that spin-coated films with 4-bromoanisole as additive exhibit a higher P3HT crystallinity than films slowly cooled from the melt. We assume that miscibility of P3HT in PF8TBT in the melt erases any solvent additive-induced aggregation in solution and during

spin-coating. Hence, the lower intensity in the room temperature spectra at 610 nm of both the block copolymer and the blend reflect the inability of P3HT to fully crystallize, in accordance with DSC and X-ray scattering. Figure 5 shows atomic force microscopy (AFM) images of the corresponding spin-coated films of BCP 8_2 and the blend after spin-coating from chlorobenzene containing 1 wt % of 4bromoanisole and after heating to the melt followed by controlled cooling. After spin-coating with 1 wt % of 4bromoanisole, the surface topography of both the block copolymer and the blend film is essentially flat and featureless (Figures 5a and 5c), which is not the case after heating the films into the melt (Figures 5b and 5d). After annealing, the blend shows significant coarsening and domain sizes around ∼1 μm, which is interpreted as macrophase separation (Figure 5b). While we note that the block copolymer film exhibits a slight coarsening as well, macrophase separation is impeded in the block copolymer film due to the covalent connectivity of the two blocks. As a result, the large domains as observed in Figure 5b are absent in Figure 5d. Interestingly, while both the blend and block copolymer films are flat and featureless after spincoating, the UV−vis absorption shows that they have a higher degree of P3HT crystallinity before rather than after thermal treatment. After thermal annealing, a blue shift accompanied by a slight reduction in intensity at 610 nm is observed, which indicates increased disorder of the P3HT phase. This means that in the blend film the domains become larger and less pure with heating above the melting temperature of P3HT. The slight increase in disorder of P3HT is seen for the block copolymer film as well as the blend film. The most probable 4149

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most efficient all-polymer solar cells. Several temperaturedependent characterization techniques indicate that microphase separation does not occur for the molecular weight range and composition synthesized. P3HT crystallization was largely impeded and did not drive microphase separation. This behavior is tentatively ascribed to the presence of the two side chains at the thiophene rings in PF8TBT, which enhance solubility of P3HT in PF8TBT. The critical role of the side chain pattern in the important electron-accepting polymer PF8TBT is emphasized, and the effect on structure formation and electronic properties is currently being investigated in detail.

explanation is that P3HT becomes partially soluble in PF8TBT when in the melt. At this point the question arises to determine which parameter is responsible for the considerable miscibility of P3HT in PF8TBT. Mori et al. have observed micrometersized domains in blends of P3HT and PF8TBT without side chains at the thiophene units after spin-coating from chlorobenzene by AFM, while the use of the low-boilingpoint solvent chloroform led to finer morphologies and optimum performance.34 This is not the case in P3HT/ PF8TBT blends with hexyl chains at the thiophene units, which show their best performance when processed from the highboiling-point solvent xylene.33 All these observations lead to the conclusion that miscibility of P3HT in PF8TBT is substantially promoted by the hexyl chains at the thiophene units in PF8TBT, as used here. The recently reported similar block copolymer PFTBTT-b-P3HT without side chains at the thiophene rings did in fact lead to the observation of two distinct phases.32 However, in addition to the unsubstituted TBT building block in PFTBTT-b-P3HT, doubly branched alkyl substituents at the fluorene unit were used. Hence, a contribution from the different fluorene substitution pattern to microphase separation is possible as well.32 Indeed, using conjugated block copolymers in which branched and linear alkyl substituents were used on the two blocks while maintaining the same backbone has shown phase-separated structures.17 As we have used the hexyl side chains on the TBT building block for introducing excellent solubility as a base to carry out purification and characterization easily, this study emphasizes the delicate balance of solubility and sufficient dissimilarity of the two blocks in all-conjugated block copolymers through proper choice of side chains. It is noteworthy to mention the different block junction of PFTBTT-b-P3HT32 and the herein presented PF8TBT-bP3HT. While the P3HT block is connected to the TBT group in PFTBTT-b-P3HT, it is attached to the fluorene unit in PF8TBT-b-P3HT. This subtle difference is likely to influence charge dynamics at the donor−acceptor interface, with appropriately designed block junctions posing a possibility to overcome geminate recombination. As a first step, the photophysics of the model compounds F8-TBT-P3HT42 and TBT-F8-P3HT 12 are currently being investigated in detail. To investigate and to harvest any such effects in block copolymers, a phase-separated structure with distinct donor−acceptor interfaces is a key prerequisite. Hence, next to fine-tuning composition and optimizing molecular weight, side-chain engineering will be important to induce microphase separation in all-conjugated block copolymers.

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ASSOCIATED CONTENT

S Supporting Information *

Both additional NMR and 1H and 13C NMR signal assignments, model reactions, GPC curves, thermal data, and UV vis solution spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +49-761-2036424; e-mail [email protected]. Present Address #

Makromolekulare Chemie, Universität Freiburg, Stefan-MeierStraße 31, 79100 Freiburg, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S., S.H., N.C.G., and W.T.S.H. greatly acknowledge the EPSRC for funding. R.M. is grateful to the Commonwealth Scholarship and Fellowship Program in the United Kingdom for support.

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REFERENCES

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CONCLUSION We have synthesized, purified, and thoroughly characterized allconjugated block copolymers comprising poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (PF8TBT) as the acceptor polymer and poly(3-hexylthiophene) (P3HT) as the donor polymer. We conclude that in situ end-capping of Suzuki step-growth polycondensation with prepolymerized P3HT-Br is a general and versatile approach for obtaining well-defined and pure diblock copolymers of PF8TBT-b-P3HT. This is drawn from the combination of purification via preparative GPC, wavelength-dependent GPC, and end group NMR analysis assisted by model compounds. To the best of our knowledge, this is the first report of pure all-conjugated donor−acceptor block copolymers where the corresponding blend is among the 4150

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