Fullerene-Functionalized DonorâAcceptor Block Copolymers through

Article pubs.acs.org/Macromolecules

Fullerene-Functionalized Donor−Acceptor Block Copolymers through Etherification as Stabilizers for Bulk Heterojunction Solar Cells Maria Heuken,†,‡ Hartmut Komber,† Tim Erdmann,† Volodymyr Senkovskyy,† Anton Kiriy,† and Brigitte Voit*,†,‡ †

Leibniz-Institut für Polymerforschung Dresden, e.V., Hohe Strasse 6, 01069 Dresden, Germany Organic Chemistry of Polymers, Technische Universität Dresden, 01062 Dresden, Germany

‡

S Supporting Information *

ABSTRACT: A new synthetic method for the covalent linking of fullerenes to polymers is introduced. The Bingelreaction was used to prepare bromine-functionalized fullerene building blocks that could be covalently linked to hydroxyl groups of model copolymers by the cesium carbonate promoted Williamson ether synthesis. Subsequently, block copolymers with a second block based on styrene and hydroxystyrene or hydroxyethyl methacrylate could be synthesized with a poly(3-hexylthiophene)−TEMPO macroinitiator through NMRP. Fullerene derivatives were linked to these polymers in a controlled manner and donor−acceptor block copolymers with high fullerene contents of near 50 wt % were achieved.

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time.18,19 Hence, the nanometer scale must be preserved by either cross-linking17,20−24 or adding stabilizing agents such as block copolymers. Block copolymers proved to be well suited, as was discussed in numerous reviews,25−29 as they locate at the blend interfaces. Thus, the interfacial energy is reduced and the coalescence is suppressed. Not yet understood is their influence on the PCEs, which often show worse results than without additives. Nevertheless, some improvements have been reported.18,30−32 Synthetic strategies that target at covalently linking fullerenes in charm bracelet style to polymers often revert to reactions with unmodified fullerenes. Examples employing atom transfer radical addition (ATRA), 33 reactions with polymeric azides34−36 or tosylhydrazone groups32 have been reported. Recently, the Bingel reaction with polymeric malonates has been introduced into polymer-analogous reactions as well.37 Nevertheless, all methods suffer from purification difficulties and cross-linking reactions. Only few approaches using fullerene derivatives have been published. Pioneering was the work of Fréchet’s group that prepared P3HT- and fullerenebuilding blocks with norbornene for the ring-opening metathesis polymerization. In this way, they prepared brush-type macromolecules containing 50 wt % C60. Multiblock copolymers with fullerene and P3HT in the polymer backbone were

INTRODUCTION The field of organic photovoltaics (OPVs) has received tremendous attention since Sariciftci et al. first reported the photoinduced electron transfer from a conjugated polymer to fullerenes.1 Low production costs and the processability onto flexible, lightweight substrates have contributed to the widespread research activities and a growing interest to commercialize this technology.2−4 Widely used is the combination of the π-conjugated donor poly(3-hexylthiophene) (P3HT) and the acceptor phenyl-C[61]-butyric acid methyl ester (PCBM). After optimizing processing parameters like the spin coating solvent,5 slow drying,6 thermal annealing7,8 or solvent vapor annealing,9 devices have reached more than 5% power conversion efficiency (PCE).7,10,11 The development of new materials, primarily new p-type polymers, even led to efficiencies of 8.3% (certified value)12 or even more than 9% (uncertified).13 Regardless of the material, three-dimensional interpenetrating networks are essential for the devices, forming the so-called bulk-heterojunction (BHJ) morphology. It provides a large interfacial area for the dissociation of excitons and separated pathways for the transportation of free charge carriers to the respective electrodes.14 One important requirement of the BHJ layer is that the phase dimensions should be as similar as possible to the exciton diffusion length of around 10 nm.15,16 This is obtained shortly after processing, but due to the immiscibility of P3HT and PCBM, these structures are thermodynamically instable.17 They dissociate into microscopic domains, and thus, the performance of OPVs degrades with © 2012 American Chemical Society

Received: March 7, 2012 Revised: April 20, 2012 Published: April 30, 2012 4101

dx.doi.org/10.1021/ma300473w | Macromolecules 2012, 45, 4101−4114

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phenyl-3-azahexane (TIPNO−styrene),52 and the TEMPO-functionalized P3HT-macroinitiator (P3HT-MI, DP = 55−60, Đ 1.16−1.17) as well as P3HT (DP = 55−60)51 were prepared as reported elsewhere. All reactions were performed under nitrogen atmosphere if not stated otherwise. Experimental details of the synthesis of all compounds and polymers, NMR characterization and signal assignments as well as IR spectra are given in the Supporting Information. Experimental Methods. 1H (500.13 MHz) and 13C (125.75 MHz) NMR measurements were performed with an Avance III 500 spectrometer (Bruker Biospin, Germany). CDCl3 was used as solvent and the spectra were referenced on the solvent peak (δ(1H) = 7.26 ppm, δ(13C) = 77.0 ppm). 2D spectra were recorded to verify the signal assignments. The molar masses and dispersities of the polymers were determined by gel permeation chromatography (GPC) with a Resipore column (Polymer Laboratories) with refractive index detector (polystyrene standards) and THF as eluent, and with a PL Gel MIXED-B-LS column (Agilent) with refractive index detector and UV−vis detector (Agilent) and chloroform as eluent. UV−vis absorption spectra were obtained on a Lambda 800 spectrophotometer (Perkin-Elmer, Germany). Fluorescence spectra were measured with a Fluorolog 3 spectrometer (Horiba JobinYvon). Thermogravimetric analyses (TGA) were carried out with a TGA Q5000 (TA Instruments) at a heating rate of 10 K/min under a nitrogen atmosphere. Atomic force microscopy (AFM) measurements were done in the tapping mode by a Dimension 3100 NanoScope V (Bruker-Nano, USA). Silicon-SPM-sensors (BudgetSensors, Bulgaria) with a spring constant of ca. 3 N/m and a resonance frequency of ca. 75 kHz were used with a tip radius lower than 10 nm. FT-IR measurements were performed with KBr-pellets on a Vertex 80 Vspectrometer (Bruker) in the range of 400−4000 cm−1 with 32 scans. Optical microscope images were measured with a BH2 microscope in combination with a DP71 camera (both Olympus). Experimental Details. General Polymerization Procedure (pC1, pC2, pC3). All polymerizations were performed in dry Schlenk tubes under nitrogen atmosphere at 120 °C in bulk. The respective amounts of monomers and initiator were prepared in a vial, transferred to the Schlenk tube, degassed and polymerized for 18 h. The polymer was then precipitated twice from methanol and dried. Poly(styrene-ran-hydroxystyrene) (C1, C2). The polymer was dissolved in dioxane. For each gram of polymer, 10 mL of hydrazine monohydrate was added and the solution was stirred overnight at room temperature. After evaporation of the solvent, the polymer was twice precipitated from water and dried, and the product was isolated as a white powder. Poly[styrene-ran-(hydroxyethyl methacrylate)] (C3). pC3 was dissolved in a 1:1 (v:v) mixture of tetrahydrofuran (THF) and ethanol. Several milliliters of 0.1 N hydrochloric acid were added, and the polymer precipitated immediately. The precipitate was filtered, dissolved in THF, precipitated twice from water, and dried to yield C3 as white solid. Etherification with Copolymers. The copolymer was dissolved in dimethylformamide (DMF) and purged with nitrogen. To the solution, the functionalized fullerene and cesium carbonate were added and stirred for 11 days at 50 °C. The solution was filtered and dried, the product dissolved in THF and precipitated from hot hexane until the supernatant solution was colorless. The brownish product was dried in vacuum. C1-Fu2. 0.25 g (0.14 mmol −OH) of C1, 15 mL of DMF, 0.18 g (0.14 mmol) of Fu2, 45 mg (0.14 mmol) of Cs2CO3. Yield: 0.185 g (43%) C3-Fu1. 0.126 g (0.21 mmol −OH) of C3, 65 mL of DMF, 0.20 g (0.21 mmol) of Fu1, 68 mg (0.105 mmol) of Cs2CO3.Yield: 200 mg (61%) General Procedure for Block Copolymer Synthesis. The P3HTmacroinitiator was put in a Schlenk tube, degassed, and put under nitrogen. It was dissolved in a large amount of styrene and the respective comonomer and degassed. A small amount of acetic anhydride was added before the reaction was started in a hot oil bath

prepared via a condensation reaction by Hiorns et al.38 Lee et al. used the commercially available [6,6]-phenyl-C61-butyric acid and reacted it under Steglich conditions with polymeric hydroxyethyl methacrylate to esters.39 Alkyne containing fullerenes could be attached to azide groups in block copolymers, avoiding cross-linking by mild reaction conditions.40 However, ester bonds are instable toward hydrolysis and remaining azide groups in polymers may cause insoluble networks with fullerene acting as cross-linking points. Therefore, we aimed at more stable compounds by binding the fullerenes to the polymer chain by ether linkage. The reaction between polymeric alcohols and brominated fullerene building blocks seemed promising. The cesium promoted ether synthesis which was recently used in polymer analogous reactions in our lab, seemed to be a feasible strategy for this aim.41,42 Donor−acceptor block copolymers used as blend compatibilizers are generally built using sequential, synthetic steps, namely (a) the preparation of the conductive block, (b) a postpolymerization transformation into a macroinitiator, and (c) the living/controlled polymerization of a second block. For the second block, anionic polymerization,43,44 Reversible addition−fragmentation chain transfer polymerization (RAFT), 32,45−47 atom transfer radical polymerization (ATRP),39,48 and nitroxide-mediated radical polymerization (NMRP)35,49,50 could be employed. Yet, the method suffers from incomplete end-capping when forming the macroinitiator and therefore nonfunctionalized initiator remains in the final material. A method developed by Kiriy et al. elegantly avoids this by introducing a NMRP-starting group into the catalyst complex needed for the hexylthiophene polymerization. As a result, a polymer with initiating “head” is produced in the Kumada-catalyst transfer polycondensation without requiring any further reactions steps.51 In this paper, we report the controlled synthesis of donor− acceptor block copolymers. The first studies focus on the evaluation of reaction conditions for the functionalization of styrene-based copolymers with bromine functionalized fullerene derivatives through ether formation. In a subsequent step, the synthetic strategy is transferred onto block copolymers. On the basis of a P3HT-macroinitiator, a second block is polymerized by NMRP. The subsequent functionalization with fullerene derivatives yielded donor−acceptor block copolymers that have been thoroughly characterized.

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EXPERIMENTAL SECTION

Material. Tetrahydrofuran (THF; Fluka) was distilled to remove butylhydroxytoluene (BHT) prior to use and was dried over molecular sieve (4 Å). Toluene (Acros) was dried over molecular sieve (4 Å). Styrene (Merck) was filtered over neutral aluminum oxide to remove stabilizers. (Trimethylsilyloxy)ethyl methacrylate (Aldrich) was distilled before use. 2,2-Dimethyl-1,3-dioxane-4,6-dione (Meldrum’s acid; Acros), dioxane (Acros), methanol (Acros), ethyl acetate (Acros), 6-bromo-1propanol (Aldrich), N,N′-dicyclohexylcarbodiimide (DCC; Aldrich), 4-(dimethylamino)pyridine (DMAP; Aldrich), dichloromethane (DCM; Aldrich), dimethylformamide (DMF; Aldrich), iodine (Aldrich), cesium carbonate (Aldrich), 2,2-diphenylethanol (Aldrich), chloroform (Aldrich), 1-pentanol (Aldrich), copper(I)iodide (Fluka), 1,8-diazabicycloundecene (DBU; Fluka), 3-bromo-1-propanol (Alfa Aesar), hexane (Merck), and fullerene C60 (>99.50%, MTR Ltd.) were used as received. Octadecyl hydrogen malonate, 3-bromopropyl octadecyl 3′Hcyclopropa[1,9](C60-Ih)[5,6]fullerene-3′,3′-dicarboxylate (Fu1), 6-bromohexyl octadecyl malonate,37 2,2,5-trimethyl-3-(1-phenylethoxy)-44102


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(135 °C). After the reaction was completed, it was cooled to room temperature, subjected to a Soxhlet extraction in first ethyl acetate, then in chloroform, and finally recovered by precipitation in methanol.pBC1 Poly(3-hexylthiophene)-block-poly(styrene-ran-acetoxystyrene) (pBC1, pBC2) 0.01 mmol of P3HT-MIa, 8 mol of styrene, 0.44 mol of acetoxystyrene; reaction time, 1 h. Yield: 0,327 gpBC2 0.04 mmol of P3HT-MIb, 43.2 mmol of styrene, 4.8 mmol of acetoxystyrene; reaction time, 2 h. Yield: 2,183 g Poly(3-hexylthiophene)-block-poly{styrene-ran[(trimethylsilyloxy)ethyl methacrylate]} (pBC3). 0.04 mmol of P3HT-MIb, 43.2 mmol of styrene, 4.8 mmol of (trimethylsilyloxy)ethyl methacrylate; reaction time, 4 h. Yield: 2,276 g Poly(3-hexylthiophene)-block-poly(styrene-ran-hydroxystyrene) (BC1, BC2). pBC1 (or pBC2) was dissolved in dioxane and heated to 40 °C. For each gram of polymer, 5 mL of hydrazine monohydrate was added, and the solution was stirred overnight. After evaporation of the solvent, the polymer was twice precipitated from water and dried, and the product was isolated as a purple powder. Poly(3-hexylthiophene)-block-poly[styrene-ran-(hydroxyethyl methacrylate)] (BC3). pBC3 was dissolved in a 1:1 (v:v) mixture of THF and ethanol. Several milliliters of 0.1 N hydrochloric acid were added, and the polymer precipitated immediately. The precipitate was filtered, dissolved in hot THF, precipitated twice from water, and dried to yield BC3 as purple solid. Etherification with Block Copolymers. . The block copolymer was dissolved in a small amount of chlorobenzene, then DMF was added slowly, and it was purged with nitrogen. To the solution were added the functionalized fullerene and cesium carbonate, and the reaction was stirred for 11 days at 50 °C. The filtered solution was dried; the product dissolved in THF and precipitated from hot hexane until the supernatant solution was colorless. The brownish product was dried in vacuum. BC2-Fu1. 0.1 g (0.094 mmol −OH) of BC2, 20 mL of chlorobenzene, 200 mL of DMF, 0.112 g (0.094 mmol) of Fu1, 49 mg (0.094 mmol) of Cs2CO3. Yield: 0.161 g (88%). BC2-Fu3. 0.1 g (0.094 mmol −OH) of BC2, 5 mL of chlorobenzene, 50 mL of DMF, 0.95 g (0.094 mmol) of Fu3, 49 mg (0.094 mmol) of Cs2CO3. Yield not determined. BC3-Fu1. 0.2 g (0.187 mmol −OH) of BC3, 12 mL of chlorobenzene, 70 mL of DMF, 0.224 g (0.187 mmol) of Fu1, 61 mg (0.187 mmol) of Cs2CO3. Yield: 0.196 g (58%).

Table 1. Molar Ratios, Molecular Weights, and Dispersities of Copolymers sample

comonomer

molar content of the comonomer (%)

Mn,theor (g/mol)

Mn,expa (g/mol)

Đa

pC1 C1 pC2 C2 pC3 C3

AS HS AS HS TMSHEMA HEMA

6 6 20 20 18 18

10700 10600b 11600 10800b 7300 6600b

11900 11500 13400 12500 7500 6700

1.18 1.18 1.15 1.14 1.11 1.16

a

Determined by GPC with THF (eluent) and RI-detector with PSstandard. bCalculated for complete deprotection of the precursor polymer.

Table 2. “Weight fractions of C60 in the copolymers and conversion of hydroxyl groups determined by different analytical methods” weight fraction of fullerene a

conversion (%) b

sample

theoretical

TGA

NMR

C1-Fu1 C1-Fu2 C2-Fu1 C3-Fu1

0.25 0.24 0.44 0.42

0.20 0.24 0.39 0.36

− 0.18 0.38 0.27

TGAa

NMRc

66 76 74 74

n.d. 65 70 40

Residue at 500 °C. bCalculated from hydroxyl group conversion obtained by 13C NMR, estimated error: ± 10%. cEstimated from signal intensities of C3′ and C30(27) (C1-Fu2; C2-Fu1) and of C5′ and C28 (C3-Fu1); error ±10%; n.d. = not determined. a

this was the reaction with malonates developed by Bingel.53 The simple preparation of asymmetrically substituted bismalonates allows decorating the fullerenes simultaneously with a linker and a solubilizing group. In a previous work, a bromine functionalized derivate with long alkyl chain as solubilizing moiety had been developed (Fu1).37 Two additional fullerene derivatives were developed to extend the range of possible reactions, one carrying a longer spacer between fullerene and reactive site (Fu2) and a second one with an aromatic substituent (Fu3) (Figure 1). The reaction conditions were first evaluated with copolymers instead of block copolymers in order to simplify the characterization. Hence, copolymers based on styrene and comonomers with hydroxyl functionality were prepared by controlled radical polymerization (NMRP) with 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane (TIPNO-styrene) as initiator. Because hydroxyl groups hinder NMRP, protected monomers were used in this reaction step. Aromatic alcohols were expected to react more easily in the ether formation reaction because of their lower acid constant, therefore polymers with two different ratios of acetoxy styrene (AS) (styrene/acetoxystyrene: 0.94/0.06 pC1; 0.8/0.2 pC2) were copolymerized. The degree of polymerization was around 100 and the Đs were low (pC1, 1.18; pC2, 1.15) in both polymers (see Table 1). Even though it was expected that an aliphatic alcohol might not be as reactive in the etherification, (trimethylsilyloxy)ethyl methacrylate (TMSHEMA) was copolymerized, too. Its flexible side chain was expected possessing a beneficial effect, possibly compensating the higher pKS. Though methacrylates cannot be homopolymerized easily by NMRP, copolymerization with styrene is possible.54 Again, the Đ was low (1.11) and the molecular weight was in the expected range (Mn = 7 500 g/mol).

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RESULTS AND DISCUSSION The goal of this work was to develop a new method of linking modified fullerene C60 to polymers in order to create donor− acceptor block copolymers. Therefore, suitable fullerene building blocks had to be prepared. A versatile method for

Figure 1. Structures of different Bingel fullerene derivatives used in this study. 4103


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Figure 2. TGA curves of copolymers (a) with hydroxystyrene groups before (C1 (solid black line), C2 (solid grey line)) and after fullerene modification (C1-Fu1 (dashed black line), C1-Fu2 (dotted black line), and C2-Fu1 (dashed grey line)) and (b) with HEMA groups before (C3) and after fullerene modification (C3-Fu1).

have been used in organic chemistry since the midseventies for the preparation of ethers and esters.56,57 Their so-called “cesium effect” (huge, lipophilic ion, “naked ions” in dipolar aprotic solvents) makes cesium salts very reactive and, thus, efficient under mild conditions.58,59 However, it has never been used for polymer analogous reactions of macromolecules with fullerenes. As fullerenes are highly reactive towards a variety of groups, mild conditions are needed for the polymer analogous reaction. The reaction at 50 °C for 11 days, performed in dimethylformamide to support the formation of solvated ion pairs of the cesium salt, proved to be efficient and compatible with the starting material. Bromine functionalized fullerenes Fu1 and Fu2 were linked covalently to HS as well as to HEMA. The crude product was dissolved in THF and repeatedly precipitated into hot hexane, a non-solvent for the polymer. As the fullerene derivatives are soluble in the THF/hexane mixture, unreacted starting material could be completely removed from the polymer. The characterization was performed with NMR. All 1H NMR spectra in chloroform show an significant peak broadening for atoms close to the fullerene (see Supporting Information, Figures S1 and S4) and sharp signals of the starting material were not observed proving successful purification. Unfortunately, signal overlap and broadening hamper quantification of the 1H NMR spectra. Therefore, information from 13C NMR spectra were taken into account, even though all signals were broad, a feature generally observed for the studied fullerene substituted (block) copolymers (see Supporting Information, Figure S2). For a rough estimation of the hydroxyl group conversion of C1-Fu2 and C2-Fu1, the signal integrals of the last methylene group of the octadecyl moiety (∼23 ppm) and of the meta-carbons of the non-reacted hydroxystyryl groups (∼115 ppm) were evaluated (see Table 2). 65 % of the hydroxyl groups of C1Fu2 (4.5 units per chain) had been converted into ethers and 70 % in C2-Fu1 (16 units per chain). The different spacer

Figure 3. UV−visible spectra of C2 (solid black line), C2-Fu1 (dashed black line), C3 (solid grey line), and C3-Fu1 (dashed grey line). Concentration: 1.7 × 10−4 mol/L in chloroform.

In order to remove the acetyl protecting group, the acetoxystyrene copolymers were treated with hydrazine monohydrate55 and still narrowly distributed polymers with hydroxystyrene units (HS) (Đ C1: 1.18; C2: 1.14) were isolated. For the deprotection of the TMSHEMA units, resulting in hydroxyethyl methacrylate (HEMA) units, diluted hydrochloric acid was used. The product showed a slightly broader distribution than the starting material (Đ C3: 1.16), most probably caused by interactions of the copolymer with the column material. The Williamson ether synthesis of polymers and fullerene derivatives was performed with cesium carbonate. Cesium salts 4104


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Scheme 1. Reaction Sequence of the Block Copolymer Synthesis with Styrene/Acetoxystyrene and Subsequent Ether Formation with Fu1

that the 13C NMR spectra were not measured under conditions giving quantitative signal intensities.60 Thermogravimetric analysis (TGA) was used to quantify the C60 content in the different copolymers. Figure 2 depicts the TGA thermograms of the respective precursor polymers and the modified products. All precursors show almost complete decomposition and char yields below 1 wt % at 500 °C. The decomposition behavior of C1-Fu1 and C1-Fu2 is nearly identical with a significant weight loss starting at around 400 °C as observed also for the precursor polymers but a higher char yield of 20% and 24%, respectively, attributed to residual C60 (resulting in OH group conversion of 66% and 76%). Polymer C2-Fu1 with a higher content of functionalized units in the chain shows a major loss at 400 °C. The remaining weight fraction of around 0.39 corresponds to an OH group conversion of 74%. For C3Fu1, the weight fraction of C60 is calculated to be around 0.36 indicating 74% OH group conversion. Obviously, the conversions calculated by TGA were slightly higher than those from NMR (Table 2), an effect attributed to the possibility of cross-linking during heating.32,36 TGA, probably with a smaller error in comparison to 13C NMR estimation, revealed that the cesium mediated etherification of HEMA units proceeded better than expected from 13C NMR data, qualifying it for the modification also of block copolymers. Because of interactions with the column material61 and exclusion volumes differing significantly from polystyrene standards,62 GPC traces of fullerene containing polymers did not show evaluable results. Comparing the FT-IR spectra of the products with those of the starting material, the hydroxystyrene copolymers functionalized with fullerenes show a characteristic band for CO stretching at 1746 cm−1 caused by malonates (see Supporting Information, Figure S3). At 527 cm−1, a band for C60 can be observed. For HEMA copolymers, the band for CO stretching changes its shape with an additional shoulder at 1730 cm−1 and the C60 band at 527 cm−1 appears. Both indicate

Table 3. Molar Ratios, Molecular Weights, and Dispersities of Block Copolymers sample P3HTMIa P3HTMIb pBC1 BC1 pBC2 BC2 pBC3 BC3

comonomer −

AS HS AS HS TMSHEMA HEMA

Mnb (g/mol)

Mnc (g/mol)

Đc

58/−/−

9600

17200

1.17

58/−/−

9600

19900

1.16

36800 35600 67700 65300 67600 63000

39900 37000 55800 54000 59700 18000

1.36 1.38 1.64 1.64 1.61 1.58

averaged number of monomer unitsa

58/214/29 58/214/29 58/464/58 58/464/58 58/429/62 58/429/62

a

Determined for the P3HT-MIs by end group analysis; for block copolymers the number of styrene and comonomer units was calculated from 1H NMR signal intensities based on DP = 58 for the P3HT block. bCalculated from the averaged number of monomer units. cDetermined by GPC with THF (eluent) and RI-detector with PS-standard.

lengths (propyl in Fu1 vs. hexyl in Fu2) did not seem to influence the conversion significantly. The weight fraction of C60 in the polymers should consequently be 0.18 and 0.38 in C1-Fu2 and C2-Fu1, respectively. In the 13C NMR spectrum of C3-Fu1 an extremely broadened signal of polymer bound fullerene is visible in the 152 to 137 ppm region (Figure S5, Supporting Information). The absence of sharp peaks originating from C60 or Fu1 verifies the successful linking and complete purification. The conversion for C3-Fu1 was only ∼40% (4.4 units per chain) calculated from 13C NMR signal intensities of the last methylene group of the octadecyl moiety (∼23 ppm) and of the residual hydroxymethylene groups (∼61 ppm). Though an error of around ±10% for the calculations must be considered, the reactivity for ether formation appeared to be much higher for the aromatic alcohol, as expected. It should be mentioned 4105


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Figure 4. GPC traces (refractive index detection) of (a) P3HT-MIa (solid black line), pBC1 (dashed black line) and BC1 (dotted black line) and (b) P3HT-MIb (solid black line), pBC2 (dashed black line) and BC2 (dotted black line). Solvent: THF.

Figure 5. GPC traces of P3HT-MIb (solid black line), pBC3 (dashed black line) and BC3 (dotted black line) with (a) refractive index detection and (b) UV−vis detection. Solvent: chloroform.

In the block copolymer synthesis the low-molecular weight initiator TIPNO−styrene was replaced by TEMPO-endfunctionalized P3HT-macroinitiators (P3HT-MI) introducing the donor block in the final diblock copolymer and allowing for the initiation of controlled radical polymerization of the precursor block for fullerene modification. Two macroinitiators from different batches were prepared via Kumada−catalyst transfer polycondensation (KCTP) with a degree of polymerization (DP) of around 58, calculated by NMR end group analysis.51 Typically, the polymerization of the second block by NMRP was carried out in bulk in a large amount of the monomers in order to solubilize the initiator sufficiently (Scheme 1). The DP was adjusted by the polymerization time and acetic anhydride was added in order to improve the reactivity. BC1 (second block: styrene/acetoxystyrene) was initiated with P3HT-MIa,

the successful reaction with Fu1 (see Supporting Information, Figure S6). UV−visible spectra of solutions in chloroform of the precursor copolymers C2 and C3 and the fullerene functionalized polymers C2-Fu1 and C3-Fu1 are depicted in Figure 3. While C2 and C3 display nearly no absorption, C2-Fu1 and C3-Fu1 exhibit an absorption with a shoulder at 330 nm, arising from bound fullerene, and a long tailing band, probably due to scattering. The obtained results for the etherification of OH-functionalized copolymers with fullerene derivatives proved this polymer-analogous reaction as a promising route to stable fullerene-modified polymers. The reaction was in consequence adapted to block copolymers, allowing for the synthesis of donor−acceptor materials. 4106


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Figure 6. 13C NMR spectra of (a) BC2-Fu1 and (b) BC3-Fu1 (solvent: CDCl3).

The molecular weights and Đs of the macroinitiators and block copolymers are listed in Table 3. Their determination with GPC can only be considered as estimation, due to the stiffness of P3HT that differs strongly from the coil-like PS standards used. Thus, the Mn values determined by NMR end group analysis and GPC differ significantly. Nevertheless, the GPC traces (Figure 4a and b) for the macroinitiators (P3HT-

BC2 (styrene/acetoxystyrene) and BC3 (styrene/ (trimethylsilyloxy)ethyl methacrylate) with P3HT-MIb. The purification by Soxhlet extraction and repeated precipitation from methanol yielded block copolymers as dark purple powders. For the removal of the acetyl- and TMSprotecting groups, the procedures were the same as for the copolymers. 4107


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Table 4. ”Summary of weight fractions of C60 in the block copolymers and conversion of hydroxyl groups determined by different analytical methods” weight fraction of fullerene 1

sample BC2Fu1 BC2Fu3 BC3Fu1

tion for signal assignments and spectra, Figures S7−S10). Besides structure verification, the molar ratios of the monomers and the molecular weights (Mn) were calculated from integrals of appropriate 1H NMR signals (Table 3). Comparison between pBC1 and pBC2 illustrates that the length of the second block can be varied by the reaction time (1 h/2 h) for the styrene/acetoxystyrene block copolymers. As the St/HEMA copolymer had shown slower reaction kinetics, the reaction time for pBC3 was set to 4 h in order to obtain a comparable block length. With this set of starting materials, polymer analogous etherifications with Fu1 and Fu3 were performed with polymers BC2 and BC3. The reaction conditions were adopted from the preliminary successful reactions on copolymers, the only exception being the choice of the solvent. As the block copolymers exhibit no solubility in DMF, the solvent essentially necessary when cesium salts are used, a mixed system of chlorobenzene/DMF was applied. Dissolving the polymer completely in a small amount of chlorobenzene followed by slow addition of DMF allowed working with only 9−14 vol % of chlorobenzene, thus limiting its content to the least possible. The 1H NMR spectra of the obtained polymers prove fullerene modification by the appearance of a broad and unstructured signal in the 4.0−5.0 ppm region characteristic for the malonate ester methylene protons of the polymer-bonded fullerene derivatives (Supporting Information, Figures S11 and S13), as also observed in the preliminary reactions on C1 − C3. Generally, the 1H and 13C NMR spectra are superpositions of narrow signals of the rod-like P3HT block and broad signals of the polystyrene-based fullerene containing block. The polymerbonded C60 results in an ill-resolved broad signal covering the 153−137 ppm region. Exemplarily, Figure 6 depicts the 13C NMR spectra of BC2-Fu1 with etherified hydroxystyrene units and of BC3-Fu1 with etherified HEMA units both showing the aforementioned features. Absence of narrow signals of lowmolecular-weight compounds confirms successful purification. The degree of etherification was determined from the 1H signal intensity of the α-methylene signal of P3HT (He, 2.81 ppm) and the intensity of the methyl group signal at 0.90 ppm

conversion (%)

13

1

13 C NMRb

theoretical

H NMRa

C NMRb

TGAc

H NMRa

32

17

18

32

37

40

100

35

−

25

48

n.d.

60

>100

35

15

16

19

27

30

38

TGAc

a Estimated error: ±10%. bEstimated error: ±10%. cDetermined from residue at 500 °C; n.d. = not determined.

MI) reveal a small shoulder at higher molecular weights, most probably due to termination reactions during KCTP. pBC1 and pBC2 as well as the deprotected polymers BC1 and BC2 are narrowly distributed, displaying only a small shoulder of unreacted macroinitiator. The traces of related protected and unprotected polymers have very similar shapes proving that the deprotection does not result in side reactions influencing the molecular weight distribution. Figure 5 depicts the traces of P3HT-MIb, pBC3 and BC3, with the same good results. A UV−vis detection system had been used in addition to the refractive index detector, as the P3HT-block displays a strong absorption, whereas the second block is not UV-active (Figure 5b). As the intensity diminishes significantly for the block copolymers due to the smaller fraction of absorbing material, the measurement confirms the growth of the coil block onto the rod-like macroinitiator. In comparison to macroinitiators created by polymer-analogous transformations and block copolymers thereof,39 the macroinitiators used in this work demonstrate an excellent initiator efficiency and the controlled synthesis of pure material was possible. Finally, the 1H and 13C NMR spectra unambiguously proved the successful preparation of the protected block copolymers and the complete deprotection to the OH-functionalized precursors for further modification (see Supporting Informa-

Figure 7. TGA curves of (a) hydroxystyrene block copolymers BC2 (solid black line), BC2-Fu1 (dashed black line) and BC2-Fu3 (dotted black line) and (b) HEMA block copolymers BC3-Fu1 (solid black line) and BC3-Fu1 (dashed black line). 4108


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Figure 8. UV−vis absorption spectra of macroinitiator P3HT-MIb (solid line), block copolymers BC2 (dashed line) and BC2-Fu1 (dotted line) (a) in chloroform (c = 10−5 mol/L with regard to P3HT) and (b) in thin films normalized to the maximum P3HT-absorption.

Figure 9. UV−vis absorption spectra of macroinitiator P3HT-MIb (solid line), block copolymers BC3 (dashed line) and BC3-Fu1 (dotted line) (a) in chloroform solution (c = 10−5 mol/L with regard to P3HT) and (b) in thin films normalized to the maximum P3HT absorption.

(CH2OC(O), CH) the possibility for a rough estimation resulting in about 60% conversion (25 wt % C60) (Supporting Information, Figure S12). The NMR-based conversions are given in Table 4. As a second method, TGA was employed for the quantification of the C60 content (Figure 7). Slightly higher char yields of 5.4−7.3% were identified for the unmodified block copolymers in comparison to the copolymers, originating from P3HT. The degradation behavior of the polymer with aromatic substituents (BC2-Fu3) deviated slightly from those with aliphatic substituents (BC2-Fu1), as its decomposition started earlier (Figure 7a). The substituent was possibly less stable and cleavage occurred at the sterically crowded diphenyl alkyl substituted methine carbon. The calculated C60 contents were 48 wt % (BC2-Fu3, > 100% conversion) and 32 wt % (BC2-Fu1, ∼ 100% conversion), respectively. In the case of

which results both from the methyl group of P3HT (Hk) and the methyl group of the polymer-bonded fullerene derivative Fu1 (H28 or H29). This calculation is based on the monomer composition given in Table 3. The conversion was also estimated from the 13C NMR spectra.60 Here, the signal intensities of Ch (P3HT) and of the methylene carbon beta to the methyl group of the malonate (C26(27)) were evaluated for BC2-Fu1 and BC3-Fu1. For BC2-Fu1 1H NMR gives 37% conversion (17 wt % C60) in good agreement with the 13C NMR value (40%; 18 wt % C60). The corresponding conversions are slightly lower for BC3-Fu1 (1H NMR, 27%; 15 wt % C60; 13C NMR, 30%; 16 wt % C60). The block copolymer with aromatic substituents at the malonate unit (BC2-Fu3) did not possess any proton signal evaluable for calculations. However, 13C NMR provided with signal integrals of P3HT (Cj) and the malonate unit 4109


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Figure 10. Fluorescence emission spectra (a) of P3HT-MIb (solid line), block copolymers BC2 (dashed line) and BC2-Fu1 (dotted line) and (b) of P3HT-MIb (solid line), block copolymers BC3 (dashed line) and BC3-Fu1 (dotted line) in thin films (excitation wavelength: 630 nm).

Figure 11. AFM images of thin films of BC2 (a, e), BC3 (b, f), BC2-Fu1 (c, g), and BC3-Fu1 (d, h) as cast from chlorobenzene solution (upper images) and after 1 h of annealing at 150 °C (lower images) on silica substrates.

BC2-Fu3, the 13C NMR spectrum (Supporting Information, Figure S12) does not indicate any residual nonbonded fullerene derivatives which could cause the overestimation. Thus, a higher tendency toward cross-linking during the heating process might be the reason for the very high C60 values. The HEMA block copolymer BC3-Fu1 showed 19 wt % C60 and 38% conversion (Figure 7b), the values ranging between those calculated from 1H and 13C NMR. All results from TGA may be higher than the actual values, as discussed for the functionalized copolymers. Nevertheless, the tendencies were in good agreement with NMR and high loadings are obviously viable. Unfortunately, there are no methods allowing for the determination of the C60 content of these polymers with a higher accuracy. To clarify the effect of the fullerene functionalization on the block copolymer absorption features, UV−vis measurements of the macroinitiator, the block copolymer and the functionalized polymers were investigated in solution and in thin films (Figure 8 and 9).

The absorption spectra of P3HT-MIb, BC2, and BC2-Fu1 show the typical π−π* absorption of the P3HT-block. In solution it can be found at 445 nm (BC2-Fu1) and 450 nm (P3HT-MIb and BC2-Fu1). When going from solution to solid state, a red shift in the P3HT absorption band and three vibronic absorption shoulders can be observed. The maxima of P3HT-MIb at 523, 549, and 601 nm are shifted to 516, 549, and 596 nm for BC2 and to almost the same values (516, 550, and 597 nm) for BC2-Fu1.63 Hence, the π-stacking of the P3HT causing this bathochromic shift in comparison to solution seems to be present in all studied films and was only slightly disrupted by the presence of a second block and the fullerene-containing substituent. An important feature of the BC2-Fu1 spectrum is the absorption below 400 nm, which is caused by the attached fullerene. For the HEMA-containing block copolymers, the absorption features were similar to maxima at 450 nm (P3HT-MIb) and 451 nm (BC3 and BC3Fu1) in solution (Figure 9). In thin films, the π−π* absorption bands of the block copolymers can be found at 516, 543, and 4110


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Figure 12. Optical microscope images of (a− c) P3HT/Fu blend, (d−f) P3HT/Fu/BC2-Fu1 blends, and (g−i) P3HT/Fu/BC3-Fu1 blends with 2.5 wt % block copolymer addition, respectively, as a function of annealing time at 150 °C.

596 nm (BC3) and at 519, 550, and 596 nm (BC3-Fu1). Again, the fullerene absorption below 400 nm hints at the successful etherification. To further characterize their optical properties, the thin films were examined by fluorescence emission spectroscopy (Figure 10). In comparison to P3HT-MIb (640 nm), blue shifts of the emission maxima could be observed for all block copolymers, indicating at least a slight disruption of the P3HTcrystallinity.64 As expected, this bathochromic shift was more pronounced for the fullerene functionalized polymers (BC2Fu1, 630 nm; BC3-Fu1, 630 nm) than for the pristine block copolymers (BC2, 634 nm; BC3, 637 nm). Efficient emission quenching has been observed for donor−acceptor block copolymers35,39 and double-cable polymers.65 This effect was only weakly pronounced for BC3-Fu1 (Figure 10a), but more prominent for the polymer with less fullerene groups attached (Figure 10b). Probably, the phase morphology was more beneficial for the less bulky coil block and, therefore, quenching was facilitated. Nevertheless, a higher number of available acceptor moieties should have supported this process. Atomic force microscopy (AFM) images were taken from spin-cast films of chlorobenzene solutions to investigate the morphology of thin films. In the as-cast films of unmodified block copolymers BC2 and BC3, the rod block forms bright, branched fibers, whereas the coil block assembles in dark domains (Figure 11a,b).46 Accordingly, the crystalline structure of the P3HT block is present. For BC3, the fibers seem to be more numerous and shorter. They still exist after 1 h of annealing at 150 °C in both materials (Figure 11e,f) and do not

seem to change their shape significantly. After the functionalization with Fu1, the ordered structures turn into less ordered ones. The AFM image shows that for BC2-Fu1 some small bright domains exist (Figure 11c), whereas BC3-Fu1 exhibits a structure with small round, knob-like features (Figure 11d). These structures appear in the BC2-Fu1 film after annealing, too. The nature of these knobs is not known, but probably they originate from aggregates of the fullerene containing coil block. As the polymers were synthesized for the use as blend stabilizers in organic solar cells, their ability to suppress phase separation was examined by optical light microscopy. Therefore, a blend mixture similar to those used in solar cells was employed, with P3HT and a functionalized fullerene (Fu, see Scheme1) in a 1:1 ratio. Pure blend films and blends with the addition of some percentages of block copolymers were monitored as a function of annealing time (Figure 12). Drop cast films of the pure blend mixture show large dark aggregations after just 2 h of annealing. The aggregation of fullerene after annealing, sometimes in needle-like shape, has been observed before.17,21,39,66,67 In blend films with addition of block copolymers, this aggregation was obviously suppressed. Therefore, the developed block copolymers with covalently bound fullerenes can be considered as efficient blend morphology stabilizers.

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CONCLUSION In this work, we developed a new method to prepare donor− acceptor block copolymers with covalently bonded fullerene C60. In the first step, styrene-based copolymers with either 4111


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(4) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107 (4), 1324−1338. (5) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. 2.5% efficient organic plastic solar cells. Appl. Phys. Lett. 2001, 78 (6), 841−843. (6) Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Popescu, L. M.; Hummelen, J. C.; Blom, P. W. M.; Koster, L. J. A. Origin of the enhanced performance in poly(3-hexylthiophene): [6,6]-phenyl C-61butyric acid methyl ester solar cells upon slow drying of the active layer. Appl. Phys. Lett. 2006, 89 (1). (7) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15 (10), 1617−1622. (8) Hoppe, H.; Sariciftci, N. S. Morphology of polymer/fullerene bulk heterojunction solar cells. J. Mater. Chem. 2006, 16 (1), 45−61. (9) Miller, S.; Fanchini, G.; Lin, Y.-Y.; Li, C.; Chen, C.-W.; Su, W.-F.; Chhowalla, M. Investigation of nanoscale morphological changes in organic photovoltaics during solvent vapor annealing. J. Mater. Chem. 2008, 18 (3), 306−312. (10) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat. Mater. 2005, 4 (11), 864−868. (11) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007, 317 (5835), 222− 225. (12) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar cell efficiency tables (Version 38). Prog. Photovoltaics: Res. Appl. 2011, 19 (5), 565−572. (13) Service, R. F. Outlook Brightens for Plastic Solar Cells. Science 2011, 332, 293. (14) Yang, F.; Forrest, S. R. Photocurrent Generation in Nanostructured Organic Solar Cells. ACS Nano 2008, 2 (5), 1022−1032. (15) Shaw, P. E.; Ruseckas, A.; Samuel, I. D. W. Exciton Diffusion Measurements in Poly(3-hexylthiophene). Adv. Mater. 2008, 20 (18), 3516−3520. (16) Nunzi, J.-M. Organic photovoltaic materials and devices. C. R. Phys. 2002, 3 (4), 523−542. (17) Miyanishi, S.; Tajima, K.; Hashimoto, K. Morphological Stabilization of Polymer Photovoltaic Cells by Using Cross-Linkable Poly(3-(5-hexenyl)thiophene). Macromolecules 2009, 42 (5), 1610− 1618. (18) Sivula, K.; Ball, Z. T.; Watanabe, N.; Fréchet, J. M. J. Amphiphilic Diblock Copolymer Compatibilizers and Their Effect on the Morphology and Performance of Polythiophene:Fullerene Solar Cells. Adv. Mater. 2006, 18 (2), 206−210. (19) Jörgensen, M.; Norrman, K.; Krebs, F. C. Stability/degradation of polymer solar cells. Sol. Energy Mater. Sol. Cells 2008, 92 (7), 686− 714. (20) Kim, B. J.; Miyamoto, Y.; Ma, B.; Fréchet, J. M. J. Photocrosslinkable Polythiophenes for Efficient, Thermally Stable, Organic Photovoltaics. Adv. Funct. Mater. 2009, 19 (14), 2273−2281. (21) Gholamkhass, B.; Holdcroft, S. Toward Stabilization of Domains in Polymer Bulk Heterojunction Films. Chem. Mater. 2010, 22 (18), 5371−5376. (22) Griffini, G.; Douglas, J. D.; Piliego, C.; Holcombe, T. W.; Turri, S.; Fréchet, J. M. J.; Mynar, J. L. Long-Term Thermal Stability of HighEfficiency Polymer Solar Cells Based on Photocrosslinkable DonorAcceptor Conjugated Polymers. Adv. Mater. 2011, 23 (14), 1660− 1664. (23) Zhu, Z.; Hadjikyriacou, S.; Waller, D.; Gaudiana, R. Stabilization of Film Morphology in Polymer-Fullerene Heterojunction Solar Cells. J. Macromol. Sci., Part A 2004, 41 (12), 1467−1487. (24) Drees, M.; Hoppe, H.; Winder, C.; Neugebauer, H.; Sariciftci, N. S.; Schwinger, W.; Schaffler, F.; Topf, C.; Scharber, M. C.; Zhu, Z. G.; Gaudiana, R. Stabilization of the nanomorphology of polymer-

hydroxystyrene or hydroxyethyl methacrylate as comonomers were prepared. These polymers were reacted with bromine functionalized fullerene derivatives that had been prepared by the Bingel reaction. With the cesium carbonate promoted Williamson ether synthesis, fullerene functionalized polymers with C60 content up to 39 wt %, depending on the starting material, could be isolated. On the basis of these studies, block copolymers were prepared using a P3HT-macroinitiator that carried a TEMPO group for the controlled radical polymerization of the second block. This macroinitiator, which proved to be highly effective for chain extension, was prepared though Kumada−catalyst transfer polycondensation, which did not require an additional end-functionalization step. Again, the nonconjugated block contained styrene and hydroxystyrene or hydroxyethyl methacrylate, and its length was varied by the reaction time. Block copolymers showed excellent GPC characteristics and narrow distributions. Their conversion with different bromine functionalized fullerene derivatives resulted in highly soluble donor−acceptor block copolymers. The fullerene loading was high (19−48 wt %) and the conversion proceeded without side reactions. Thin films of the unmodified block copolymers revealed structured features, whereas the morphology changed for the functionalized block copolymers. To study the stabilizing ability of the donor−acceptor block copolymers in blend films of P3HT and fullerene derivatives, samples were monitored with optical light microscopy as a function of annealing time. The block copolymers were able to efficiently suppress the formation of fullerene aggregates. The properties of the block copolymers as morphology stabilizers in polymer solar cells are currently under examination.

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ASSOCIATED CONTENT

* Supporting Information S

Further experimental details as well as additional NMR and FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: 49-351-4658590. Fax: 49351-4658565. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the European Centre for Emerging Materials and Processes (ECEMP), the European Fund of Regional Development (EFRE), and the Free State of Saxony. The authors would like to thank Liane Häußler for TGA measurements, Ulrich Oertel for UV/vis and fluorescence measurements, Robert Socher for optical light microscopy investigations, and Andreas Janke for the AFM measurements.

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REFERENCES

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