Thioalkyl-Substituted Benzothiadiazole Acceptors: Copolymerization

Article pubs.acs.org/Macromolecules

Thioalkyl-Substituted Benzothiadiazole Acceptors: Copolymerization with Carbazole Affords Polymers with Large Stokes Shifts and High Solar Cell Voltages Abby Casey, Raja Shahid Ashraf, Zhuping Fei, and Martin Heeney* Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K. S Supporting Information *

ABSTRACT: Copolymers of carbazole and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (dTBT) incorporating thioalkyl (−SR) and alkoxy (−OR) solubilizing groups on the 2,1,3benzothiazdiazole (BT) unit are synthesized and compared. The introduction of −SR and −OR groups onto the BT unit of the polymer was found to have different effects on the electronic properties of the polymers as well as the conformation of the polymer backbone. Large conformational changes between the ground state (GS) and excited state (ES) geometries of the polymers with −SR groups led to very large Stokes shifts of up to 224 nm. The polymer with −OR groups was found to have approximately double the photovoltaic efficiency at ∼4% compared to the polymers with −SR groups (PCE ∼ 2%). However, polymers with −SR groups were found to give very high open circuit voltages (VOC) of over 1 V. Changing the −SR chain length from ethyl to dodecyl was found to have little influence on the solar cell performance of the polymer or the magnitude of the Stokes shift.

1. INTRODUCTION Bulk heterojunction (BHJ) solar cells consisting of a mixed blend of electron donor material (usually a conjugated polymer) and an electron acceptor material (typically PCBM) have attracted a lot of attention in recent years.1−4 The principal interest in these devices lies in their potential to provide low-cost energy through high throughput fabrication. Large area, solution processed devices could potentially be produced using techniques such as roll-to-roll printing.5 The solubility of these materials is therefore pivotal to the commercial success of BHJ solar cells. The solubility of conjugated polymers is commonly achieved by the incorporation of long alkyl chains on the backbone periphery. Besides improving solubility and film forming properties, the use of different solubilizing groups can also drastically influence the optoelectronic properties of polymers.6,7 Alkoxy groups have been widely investigated as solubilizing substituents for conjugated polymers for a variety of applications, in part due to the prevalence of readily available hydroxyl functionalized precursors. In the area of photovoltaic donor polymers, a particularly successful example has been polymers utilizing dialkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) comonomers which have achieved power conversion efficiencies (PCEs) of 7−8.5%.8−10 However, one drawback of the alkoxy group in comparison to other solubilizing groups such as alkyl chains is that the electrondonating nature of the alkoxy group results in a reduction in polymer ionization potential, resulting in relatively low open circuit voltages (VOC) between 0.5 and 0.7 V.11 © 2014 American Chemical Society

The use of thioalkyls as solubilizing groups has received far less attention than their alkoxy counterparts.12−16 The thioalkyl group is less electron donating than the corresponding alkoxy group, and with this in mind, thioalkyl-substituted BDT units have been recently synthesized to reduce electron donation into the donor unit and therefore increase the ionization potential of the polymer in comparison to the analogous dialkoxy polymer.17,18 Polymers containing dithioalkyl-substituted BDT units have therefore led to improved VOC values of up to 0.99 V.19 The incorporation of strongly electron-accepting monomers into conjugated polymer backbones has been a highly successful strategy to control polymer backbone and optoelectronic properties. Within the class of acceptor comonomers, 2,1,3-benzothiadiazole (BT) has been widely studied, with copolymers with a range of electron-rich monomers affording promising device performance in many applications. However, one limitation of BT is the absence of solubilizing groups, which can lead to conjugated polymers with poor processability. A common solution has been the incorporation of alkoxy groups onto the electron-accepting BT unit.20,21 However, the strongly electron-donating nature of alkoxy groups reduces the electron-accepting character of the BT unit, resulting in a reduction of intramolecular charge transfer and therefore a widening of the polymer band gap.20 We were Received: January 14, 2014 Revised: March 3, 2014 Published: March 17, 2014 2279

dx.doi.org/10.1021/ma5000943 | Macromolecules 2014, 47, 2279−2288

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Figure 1. Chemical structures of (a) PCDTBT, (b) HXS-1, and (c) PCDTBT-8.

Scheme 1. Synthesis of P1 by Suzuki Polymerization

Scheme 2. Synthetic Procedure for Monomers C and D and Polymers P2 and P3

processing additives has helped control nanoscale morphology, leading to efficiencies of over 7%.31 Another advantage of using PCDTBT derivatives is that several alkoxy-substituted BT analogues have already been reported and found to exhibit high fill factors and promising efficiencies. For example, a variation on PCDTBT called HXS1, incorporating octyloxy groups on the electron-withdrawing BT unit and a straight N-octyl chain on the carbazole unit (see Figure 1b),21 has a reported efficiency of 5.4%. Interestingly, HXS-1 was actually more crystalline than PCDTBT, most likely due to the absence of branched chain solubilizing groups. However, the close packed structure of the polymer resulted in even lower solubility compared to PCDTBT, with only hot chlorinated solvents allowing dissolution. A more soluble PCDTBT derivative, “PCDTBT-8”, was prepared through functionalization of the BT moiety with octyloxy groups, while maintaining the branched heptadecyl side chain on the

therefore interested to investigate the use of less electrondonating thioalkyl groups as the solubilizing substituent on the BT unit and to compare the performance of these acceptors to the commonly used alkoxy equivalent. In order to examine the differences between thioalkyl (−SR) substituted BT units and alkoxy (−OR) substituted BT units, we have chosen to synthesize analogues of the well-studied carbazole copolymer poly[N-9′-heptadecanyl-2,7-carbazole-alt5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT; see Figure 1). Carbazole is a promising and readily available building block,22,23 and PCDTBT displays an attractive combination of good chemical stability, high charge carrier mobility, and relatively high ionization potential, leading to solar cells with a high open circuit voltage and highly promising efficiencies.24−27 Further studies have shown that an optimized PCDTBT:PC71BM blend morphology is vital to increase charge generation and extraction28−30 and recently the use of 2280


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carbazole (Figure 1c).20 The resulting polymer had a wider band gap compared to PCDTBT (due to the reduction in the electron accepting nature of the BT unit); however, device efficiencies of 4.22% were still achieved. Herein we present our systematic studies on the use of the thioalkyl (−SR) groups as solubilizing substituents on the BT unit and make direct comparison to the alkoxy analogue (PCDTBT-8). We report the synthesis of the novel carbazolebased copolymers utilizing thioalkyl chains of different length and discuss how changes in molecular structure influence optoelectronic properties and solar cell performance. Using DFT calculations, we have related the optoelectronic properties to differences in the conformation of the polymer backbone. The polymers with thioalkyl groups were found to have extremely large Stokes shifts of up to 224 nm. The origin of these Stokes shifts were investigated by examining the ground state (GS) and excited state (ES) geometries using TD-DFT calculations. Polymers with large Stokes shifts are potentially useful in applications such as OLEDs, solar concentrators, and lasers as they minimize self-quenching. Finally, we report the OPV device performance of blends with PCBM, demonstrating that high open circuit voltage over 1 V can be obtained.

difluoro-2,1,3-benzothiadiazole (2) with dodecanethiol was found to proceed readily in the presence of K2CO3 in a mixed solvent of DMF/THF with gentle heating (Scheme 2a). However, under these conditions, small amounts of the product resulting from displacement of a thienyl bromide were also formed, which complicated purification and an overall yield of 58% was obtained. A simple solution was to displace the fluorine substituents of the nonbrominated 4,7-bis(thiophen-2yl)-5,6-difluoro-2,1,3-benzothiadiazole (1), which improved yields to 70% by preventing unwanted substitution of bromine with an thioalkyl group. Monomer D was subsequently obtained by displacing the fluorine substituents from the precursor 1 with ethanethiol groups to produce molecule 3, which was subsequently brominated with NBS in 85% yield (Scheme 2b). Monomers C and D were then polymerized with monomer B using Suzuki coupling in a mixture of toluene and aqueous sodium carbonate in the presence of the phase transfer agent aliquot 336. After 3 days reflux, all of the polymers were endcapped in situ with phenylboronic acid and then bromobenzene to limit the possible detrimental effects of the residual halogen and/or boronic acid groups.34 Following precipitation, and removal of low weight oligomers and catalyst residues by Soxhlet extraction, the crude polymers were dissolved in chloroform and washed with aqueous sodium diethyldithiocarbamate dihydrate solution to remove palladium residues.35 Following a final precipitation the polymers were isolated as dark red polymers in high yields (86−89%) (Scheme 2c). Gel permeation chromatography (against polystyrene standards) in chlorobenzene at 80 °C was used to determine both the molecular weights and polydispersity (PDI) values of P1, P2, and P3 in chlorobenzene (see Table 1). P1 and P2 both

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of Monomers and Polymers. Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt(5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (P1), poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(dodecylthio)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (P2), and poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(ethylthio)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (P3) were synthesized in high yield (87, 89, and 86%, respectively) via Suzuki coupling. The alkoxy-substituted 2,1,3-benzothiadiazole was prepared following the literature procedure.32 Briefly, this involved the alkylation of catechol, followed by double nitration, reduction, and then formation of the benzothiadiazole ring by reaction with N-thionylaniline. Subsequent bromination and cross-coupling affords the monomer A, which was subsequently polymerized with monomer B to afford P1 (Scheme 1). The synthesis of thioalkyl-substituted 2,1,3-benzothiadiazole has not been previously reported in the literature; however, the use of an analogous procedure to the alkoxy monomer was not attractive due to the high cost of benzene-1,2-dithiol and the propensity for the thioalkyl groups to oxidize during the subsequent synthetic steps. Therefore, we developed a simple route to thioalkyl-substituted monomers C and D through the convenient and mild displacement of fluorine groups from a previously reported flourinated precursor (Scheme 2a,b) under SNAr conditions. This late stage introduction of the thioalkyl group also facilitates the investigation of side chain effects, since varying thiols can be readily added. The requisite fluorinated precursor 1 (dTdFBT) was synthesized according to a slightly modified literature procedure in which 2-thienylzinc bromide was used to couple to 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole, instead of the previously reported trimethyl(thiophen-2-yl)stannane.33 We found this not only avoided the use of a highly toxic precursor but also significantly improved the cross-coupling yield to 97%. Subsequent bromination to monomer 2 followed an identical procedure to the literature. The displacement of the fluorine substituents on 4,7-bis(5-bromothiophen-2-yl)-5,6-

Table 1. GPC Data Giving Molecular Weights versus Polystyrene, PDI, and Degree of Polymerization polymer

Mn (kDa)

Mw (kDa)

DPna

DPwb

PDI

P1 P2 P3

25.4 28.7 26.5

45.9 67.5 58.2

26 26 32

48 61 70

1.80 2.35 2.20

a

Degree of polymerization determined by Mn. bDegree of polymerization determined by Mw.

had similar degrees of polymerization and polydispersity as isolated, whereas P3 had a slightly higher molecular weight of 36.6 kDa with a large polydispersity of 4.35. Therefore, P3 was fractionated using preparative GPC to allow for effective comparison with P1 and P2. The fraction of P3 used had an Mn of 26.5 kDa, giving a comparable degree of polymerization (DP) to both P1 and P2 (see Table 1). All three polymers had high solubility in room temperature chloroform and also nonchlorinated solvents such as toluene and THF. Film morphology was investigated using DSC (differential scanning calorimetry) and XRD (X-ray diffraction). All three polymers appeared to be amorphous as there were no obvious peaks suggesting phase transitions in the DSC traces (up to 300 °C) or any clear peaks in the XRD spectra. 2.2. Optical Properties. 2.2.1. Absorption Spectra. The absorption spectra of P1, P2, and P3 in chloroform solution and as a thin film are shown in Figure 2. P1 had two peak absorptions at 386 and 516 nm in solution and 390 and 531 nm in thin film, similar to that previously reported.20 P2 and P3 had almost identical absorption spectra showing peaks at 374− 2281


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Figure 2. Normalized UV−vis absorption spectra of P1 (black), P2 (red), and P3 (blue) in chloroform solution (a) and (b) as-spun thin films (see Table 2 for absorption coefficients).

Table 2. Optical Properties of P1, P2, and P3 in Chloroform Solution and as Thin Film polymer

λabs,max, nm (α, M−1 cm−1)a solution

λabs,max, nm (α, μm−1)b film

Eg(opt), eV

λem,max(sol), nm

Δλ(sol), nm (eV)

λem,max(film), nm

Δλ(film), nm (eV)

P1 P2 P3

386 (3.2 × 104), 516 (3.7 × 104) 374 (5.8 × 104), 488 (2.5 × 104) 375 (4.6 × 104), 488 (2.0 × 104)

390 (9.90), 531 (9.44) 374 (9.44), 519 (4.88) 375 (14.04), 519 (6.22)

1.99 2.03 2.03

647 712 712

131 (0.49) 224 (0.80) 224 (0.80)

663 697 702

132 (0.47) 178 (0.61) 183 (0.62)

a

Absorption coefficient in solution. bAbsorption coefficient in thin film.

Figure 3. Absorbance and photoluminescence spectra of P1 in chloroform (a) and P2 (red)/P3 (blue) in chloroform (b).

375 and 488 nm in solution and 383−384 and 519 nm in thin film. The similarity of the absorption spectra for P2 and P3 shows that changing the length of the −SR group from ethyl to dodecyl has had little effect on the optical properties of the polymer. The similarity of the solid state absorption spectra also suggests that reducing the length of the −SR group from dodecyl to ethyl had little effect on the solid state packing. The data obtained from the absorbance spectra are summarized in Table 2. A 15 nm red-shift was observed between the solution and thin-film absorption spectra of P1, while a greater red-shift of 31 nm was measured between the solution and thin-film absorption spectra of P2 and P3. This suggests a greater amount of structural rearrangement occurs between the solution and solid state for the polymers with −SR groups (P2/P3) in comparison to the polymer with −OR groups (P1). The most notable difference between the absorption spectra of P1 (−OR groups) and P2/P3 (−SR groups) is the difference

in intensity of the longer wavelength absorption (500−600 nm) relative to the shorter wavelength absorption (300−400 nm). While the absorption maximum (λmax) occurs at the longer wavelength in the UV spectra of P1, the introduction of −SR groups in P2/P3 reduces the relative intensity of this peak such that the shorter wavelength absorption is now λmax. The solution and thin film absorption coefficients of P2 and P3 measured at the longer wavelength transition are also reduced in comparison to P1 (see Table 2 and Figure S4). The difference in relative absorbance may be due to a reduction in the strength of the intramolecular charge transfer (ICT) character for the polymers with −SR groups, potentially due to a more localized LUMO level (see section 2.3). Comparing P1 and P2/P3 to PCDTBT, which absorbs at 547 and 392 nm in chloroform and 570 and 396 nm in thin film,20 shows that the addition of −OR and −SR groups onto the BT moiety blue-shifts the absorption in both thin-film and solution absorption spectra. This corresponds to a widening in 2282


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Figure 4. HOMO and LUMO electron density plots and optimized geometries of (a) P1 and (b) P2/P3. B3LYP level of theory, basis set of 631G(d) (hydrogen atoms omitted for clarity). (c) Side view of the central T−BT−T unit of the trimer for P1 and P2/3.

(see Figures S2 and S3). The emission solvachromism suggests that the excited state has polar character, in agreement with the lowest energy absorption and emission having ICT character. 2.3. Relating Absorption Spectra to Polymer Conformation Using DFT Calculations. The HOMO and LUMO molecular orbital energies, electron density plots, and optimized geometries of P1 and P2/P3 were calculated using DFT (density functional theory) with B3LYP36 level of theory and a basis set of 6-31G(d). In all cases trimers of the carbazole-dTBT repeat unit with methyl groups instead of longer side chains were employed as models in order to simplify the calculations. For each polymer structure trans (thiophenes facing the −OR or −SR groups) and cis (thiophenes facing away from −OR or −SR groups) conformations were allowed to relax to the equilibrium geometry; the lowest energy conformations are shown. Frequency calculations were performed on the lowest energy conformations to ensure the geometry was not the result of a local minima. The optimized ground state geometries of trimer molecules of P1 and P2/P3, calculated by DFT, show that both P1 and P2/P3 are nonplanar (see Figure 4). There are deviations from coplanarity between the carbazole and thiophene units (C−T) and between the thiophene and benzothiadiazole units (T−BT). For both P1 and P2/P3 the C−T dihedral angle (taken between the central carbazole and thiophene units of the trimer) remains the same at 28°−29°. However, the inclusion of either −OR or −SR groups on the BT unit was found the drastically alter the dihedral angle and therefore, planarity, between adjacent thiophene and BT units. The inclusion of −SR groups (P2/P3) was found to increase the T−BT dihedral angle in comparison to −OR groups (P1), leading to a less planar polymer backbone. To show these dihedral angles more clearly, the dTBT derivative unit has been cut from the center of both P1 and P2/P3 trimers and is shown side on in Figure 4c. A T−BT dihedral angle of 8.85° was calculated when −OR groups were present (P1), while a much more acute T−BT dihedral angle of 46.36° was calculated when −SR groups were present (P2/P3). The more acute T−BT dihedral angle present in P2/P3 may be explained by increased steric clash caused by the large sulfur atom in the −SR groups in comparison to the smaller oxygen atom in the −OR groups.

the optical band gap. This increase in band gap is probably due to a mixture of steric effects, with the long chains disrupting πstacking and backbone planarization, and electronic effects as the oxygen and sulfur atoms donate into the electronwithdrawing BT moiety. This electron donation into the BT unit will reduce its electron-accepting characteristics, therefore reducing the strength of ICT between the donor and acceptor moieties along the backbone and widening the band gap. Comparing the solution and solid state absorption spectra of P1 and P2/P3, a significant blue-shift of 28 nm in solution and 12 nm in thin film is observed as −OR groups (P1) are replaced with −SR groups (P2/P3). Since −SR groups are less donating than −OR groups,17−19 we believe this widening in the band gap between P1 and P2/P3 is not the result of increased donation into the BT unit but instead due to a reduced conjugation length because of backbone twisting. Optimized ground state (GS) geometries elucidated by DFT calculations reveal a highly twisted backbone conformation in P2/P3 due to the steric bulk of the large sulfur atoms, reducing the conjugation length (see section 2.3 for full details). 2.2.2. Emission Spectra. The photoluminescence (PL) spectra of P1, P2, and P3 in chloroform solution and in thin film are shown in Figure 3. All three polymers are red emitters with solution spectra maxima at 647, 712, and 712 nm for P1, P2, and P3, respectively (see Table 2). Large Stokes shifts of 131 nm (0.49 eV) for P1 and 224 nm (0.80 eV) for P2 and P3 were therefore observed between the absorption and emission spectra in solution. The solid state emission spectra for P1 was red-shifted to 663 nm; however, both P2 and P3 showed small blue-shifts compared to solution with maxima at 697 and 702 nm respectively (see Figure S1). The Stokes shifts between the absorption and emission spectra in the solid state were therefore reduced to 175 and 183 nm for P2 and P3. The Stokes shift for P1 in the solid state remained very similar to that observed in solution at 132 nm. The emission spectra of P1 and P3 were also measured in toluene and compared to the chloroform emission spectra. Solutions of P1 and P3 were kept at the same concentration in each solvent (1.67 × 10−2 g dm−3). Both polymers showed a blue-shifted emission maxima with an increased intensity in toluene, a solvent of reduced polarity compared to chloroform 2283


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Figure 5. GS and ES optimized geometries of P1 and P2/P3 (hydrogen atoms omitted for clarity).

Table 3. T−BT and C−T Dihedral Angles (deg) Calculated for Figure 5 Using DFT for GS and TD-DFT for ES polymer

dihedral angle (T−BT) GS

dihedral angle (C−T) GS

dihedral angle (T−BT) ES

dihedral angle (C−T) ES

Δdihedral angle (T−BT) GS → ES

Δdihedral angle (C−T) GS → ES

P1 P2/P3

8.38 54.66

31.16 32.29

1.16 10.05

0.23 1.45

7.22 44.61

30.93 30.84

P1 had a calculated HOMO of −4.75 eV and LUMO of −2.84 eV, giving a band gap of 1.91 eV. P2/P3 had a calculated HOMO of −5.03 eV and LUMO of −2.95 eV, giving a band gap of 2.08 eV. The theoretical band gaps agree well with the optical band gaps of P1, P2, and P3 which were measured by the absorption onset in the thin film absorption spectra and had values of 1.99, 2.03, and 2.03 eV, respectively. From the electron density plots in in Figure 4 it can be seen that the HOMO orbital of P1 is slightly more delocalized than that of P2/P3 probably due to an increased conjugation length as the polymer backbone is more planar. The increased torsional disorder in the polymer backbone of P2 and P3 will reduce the effective conjugation length of the polymers, resulting in the observed lowering of the HOMO level in comparison to P1 and explaining the blue-shift in absorption observed for P2/P3. The more delocalized HOMO of P1 explains the red-shifted absorption spectra in comparison to P2/P3 and the slightly lower band gap. The LUMO orbital of P2/P3 also appears to be slightly more localized than that of P1. This may explain the reduced intensity of the ICT band observed in the absorption spectra, as the overlap between the HOMO and LUMO orbitals will be reduced. The ionization potentials of P1, P2, and P3 were measured as thin films using photoelectron spectroscopy in air (PESA), and values of −5.24, −5.52, and −5.52 eV (±0.05 eV) were found, respectively. The measured values are in excellent agreement with the trends predicted by DFT calculations, where the inclusion of the thioalkyl groups was predicted to result in an increase in ionization potential of 0.28 eV over the alkoxy-substituted polymer. The LUMO levels were estimated by adding the optical band gap to the ionization potentials determined by PESA and had values of −3.25, −3.49, and −3.49 eV for P1, P2, and P3, respectively. It should be noted that this method only affords an approximation of the LUMO energy, since it does not account for the exciton binding energy. 2.4. Relating Emission Spectra to Polymer Conformation Using TD-DFT Calculations. In order to investigate the origin of the large Stokes shifts observed for these polymers, TD-DFT calculations with the CAM-B3LYP level of theory and a basis set of 6-31G(d) were used to predict the excited state (ES) geometries. CAM-B3LYP uses a long-range corrected functional, making it suitable for modeling electron excitations

to higher orbitals.37 Because of computational restrictions, we were not able to carry out optimization and frequency calculations on the ES of trimer molecules (i.e., AB−AB−AB where A = carbazole derivative and B = dTBT derivative), and therefore ES geometries of monomers (i.e., AB) were calculated to give insight into the ES geometry of the polymer. In both cases 2-propyl groups were utilized on the carbazole monomer and methoxy or methylsulfanyl on the BT unit. Monomers of P1 and P2/P3 show a much more planar ES geometry in comparison to their GS geometries (see Figure 5). However, because of the higher deviation from planarity in the P2/P3 GS geometry, a much greater conformational change occurs as the backbone planarizes in the ES. The change in dihedral angle between the thiophene and benzothiadiazole (T−BT) groups as the monomer moves from the GS to the ES is therefore much greater for the polymers with −SR groups (see Table 3). There is a change in T−BT dihedral angle of 44.61° for −SR polymers between the GS and ES compared to 7.22° for the −OR analogue. Both −OR and −SR polymers have similar changes in the dihedral angle between the carbazole and thiophene (C−T) groups as the monomer moves from the GS to the ES. It should be noted that the GS dihedral values in Table 3 vary slightly from those quoted in section 2.3. This is because the dihedral angles in section 2.3 were taken between central units of a trimer molecule, in comparison to a monomer molecule in section 2.4. The larger Stokes shifts observed for P2/P3 is therefore likely to be due to the greater change in conformation between the GS and ES in comparison to P1, assuming the conformational changes calculated for the monomers are a good estimate for the polymers themselves. 2.4. OPV Performance. The photovoltaic performance of P1, P2, and P3 in a device configuration of glass/ITO/ PEDOT:PSS/polymer:PC71BM/Ca/Al were measured. Blends of polymer:PC71BM in chlorobenzene were tested in weight ratios of 1:2, 1:3.5, and 1:4. A blend ratio of 1:3.5 was found to give the best results for P2 and P3 while a blend ratio of 1:4 gave the best results for P1. Chlorobenzene (CB) was found to be the best solvent for P1, while 1,2-dichlorobenzene (DCB) gave marginally better results for P2/P3. Figure 6 shows the J−V curves for the best devices made from 10 mg/mL solutions of P1, P2, and P3 in CB (P1) and DCB (P2/P3) at the optimized blend ratio. The device 2284


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and P3 in comparison to P1. We note however that relatively few conjugated polymer blends with PCBM have reported device voltages over 1 V,38−42 and that to the best of our knowledge these are the first examples of a carbazole containing polymers with such high voltage. We further note many reported polymers with high voltage have efficiencies similar to or less than 2%, suggesting copolymers of thioalkyl bezothiadiazoles with other comonomers may be a useful strategy for imparting high device voltage.

3. CONCLUSIONS Three carbazole and thienylbenzo[1,2,5]thiadiazole based polymers were synthesized in order to investigate the effect of −SR in comparison to −OR solubilizing groups on polymer structure and optoelectronic properties. A facile synthetic approach was used to introduce different thioalkyl groups by displacing the fluorine substituents on the common precursor T−dFBT−T, allowing the effect of side chain length to be readily studied. All three polymers were found to be amorphous; however, the introduction of the −SR was found to have a significant effect on the optoelectronic properties. The −SR polymer exhibited a higher ionization potential by approximately 0.3 eV, an increased optical band gap, and a reduced intensity of the lowest energy charge transfer absorption. These effects were rationalized by computational modeling, which demonstrated that the −SR groups created much more torsional disorder between the BT unit and the adjacent thiophene monomers than the corresponding −ORsubstituted BT, which was almost coplanar. This increased torsional disorder reduced the conjugation length resulting in a higher band gap and is also likely to reduce the ability of the polymer backbones to pack closely in the solid state. In solar cell devices, the polymer with −OR groups had almost double the overall efficiency (∼4%) of the polymers with −SR groups (∼2%), although both −SR polymers had very high open circuit voltages over 1 V. The main reason for the reduced performance of the −SR polymers is due a significantly lower photocurrent as well as a reduced fill factor. The reduced photocurrent can be explained by reduced absorption of the −SR polymers in the low-energy region as a result of the backbone torsion. Drastically changing the length of the linear −SR group (C12 vs C2) was found to have minimal effect on the optical or electronic properties of the polymers, and both gave very similar solar cell performance. For OPV applications the addition of thioalkyl solubilizing groups to f used units such as BDT is more suitable since planarity of the backbone is maintained.17,18 When freely rotating thiophene groups are adjacent to the unit containing the thioalkyl groups, as is the case with P2/P3, steric hindrance created by the large sulfur atoms forces the thiophenes out of plane. The significant backbone torsional disorder also results in a large Stokes shift for the −SR containing polymers, in both

Figure 6. J−V curves for P1 (black), P2 (blue), and P3 (red).

performance of the three polymers are summarized in Table 4. All three polymers exhibited a high VOC, in particularly P2 and P3 which both had a VOC of over 1 V, which we attribute to their large ionization potentials (∼5.5 eV). However, the gain in voltage for P2 and P3 versus P1 (ca. 0.1 and 0.05 V, respectively) is less than the measured increase in ionization potential for P2/P3 over P1 for the pristine films (ca. 0.3 V). This might relate to differences in the polymer conformation between the pristine and blend films as well as differences in charge recombination for the three materials. P1 performed significantly better in devices than P2 or P3, giving almost double the efficiency at ∼4%, compared to ∼2% for P2/3. Annealing films of P1 (120 °C) was found to slightly increase the average JSC from 8.89 to 9.36 mA cm−2 which led to a slight improvement in PCE from 3.84% to 3.90%. Film annealing was found to have no effect on devices using P2 and P3, however. The performance of P1 in our hands compares very similarly to that reported by Iraqi and Lidzey,20 who observed the best performance with 1:4 blend ratios spun from CB. Their devices had a PCE of 4.2%, with a Voc of 0.96 V, a Jsc of 9.38 mA cm−2, and a fill factor of 47%. The lower performance of P2/3 is mainly due to a reduced photocurrent, which can be related to reduced light absorption for these polymers in the 400−600 nm region, as well as a reduced fill factor. Varying the length of the thioalkyl chain from C12 (P2) to C2 (P3) had very little effect on device performance. However, changing the oxygen atom in P1 to a sulfur atom (as in P2/P3) was found to halve the performance. As previously discussed, steric hindrance caused by the large sulfur atoms results in reduced planarity of the backbone of P2/ P3. This leads a shorter conjugation length and therefore increased band gap as well as reduced intramolecular charge transfer due to a more localized LUMO and might also be expected to reduce solid-state π-interactions. The combination of these factors is likely to explain the reduced JSC and FF of P2

Table 4. Performance Parameters of P1, P2, and P3 in a Device Configuration of Glass/ITO/PEDOT:PSS/Polymer:PC71BM/ Ca/Al polymer P1 P2 P3 a

solvent CB DCB DCB

polymer/PC71BM (weight ratio) 1:4 1:3.5 1:3.5

annealing temp (°C) 120

JSC (mA/cm−2)

VOC (V)

FF (%)

0.98 ± 0.01 (0.97) 9.35 ± 0.05 (9.36)b 0.42 ± 0.01 (0.43)b 1.05 ± 0.02 (1.07)b 5.40 ± 0.03 (5.42)b 0.32 ± 0.01 (0.33)b 1.03 ± 0.02 (1.02)b 6.30 ± 0.10 (6.24)b 0.31 ± 0.02 (0.33)b b

PCE (%) 3.85 ± 0.06a (3.90)b 1.81 ± 0.05a (1.92)b 2.08 ± 0.05a (2.13)b

Average device efficiency over five devices. bBest device efficiency. 2285


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(188 mg, 0.163 mmol), and a stirrer bar were added to a 20 mL high pressure microwave reactor vial. The vial was then sealed with a septum and flushed with argon before 2-thienylzinc bromide solution in THF (13 mL of a 0.5 M solution, 6.5 mmol) was added. The whole solution was then degassed again for 20 min before the reaction was heated to 100 °C for 30 min in the microwave. After cooling, the reaction mixture was diluted with THF and passed through a silica plug (10 × 5 × 5 cm), with further washing with THF. After removal of the solvent under reduced pressure, the residue was recrystallized from chloroform to afford red needle-like crystals (0.952 g, 2.83 mmol). Yield: 97%; mp 210−211 °C. 1H NMR (400 MHz, CDCl3) δ: 8.30 (dd, J = 3.8, 1.1 Hz, 2H), 7.63 (dd, J = 5.1, 1.1 Hz, 2H), 7.30− 7.26 (m, 2H). MS (EI) m/z = 336 (M+). 5,6-Bis(ethylsulfanyl)-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (3). Ethanethiol (0.204 mL, 2.75 mmol) and K2CO3 (1.52 g, 11 mmol) were added to a solution of 5,6-difluoro-4,7-di(thiophen-2-yl)2,1,3-benzothiadiazole (1) (370 mg, 1.10 mmol) in anhydrous THF (30 mL) under argon. The solution was stirred at 50 °C for 20 h. After cooling, the solution was filtered, the THF removed under reduced pressure, and the crude product recrystallized from petroleum ether (60−80 °C) to afford orange needle-like crystals (324 mg, 0.77 mmol). Yield: 70%. 1H NMR (400 MHz, CDCl3) δ: 7.60 (dd, J = 5.1, 1.1 Hz, 2H), 7.53 (dd, J = 3.6, 1.1 Hz, 2H), 7.26 (dd, J = 3.6, 5.1 Hz, 2H), 2.76 (q, J = 7.4 Hz, 4H), 1.17 (t, J = 7.4 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ: 154.32, 142.38, 137.41, 130.60, 130.43, 127.54, 126.86, 31.36, 14.51. MS (CI) m/z = 421 ([M + H]+). 4,7-Bis(5-bromothiophen-2-yl)-5,6-bis(dodecylthio)-2,1,3-benzothiadiazole (Monomer C). To a solution of 4,7-bis(5-bromothiophen2-yl)-5,6-difluoro-2,1,3-benzothiadiazole16 (2) (500 mg, 1.01 mmol) in anhydrous THF (15 mL) and anhydrous DMF (15 mL) under argon was added 1-dodecanethiol (0.613 g, 3.03 mmol) and K2CO3 (1.05 g, 7.575 mmol). The mixture was stirred at 50 °C for 20 h. The CH2Cl2 was then removed under reduced pressure, and the resulting mixture in DMF was poured into cold water. The product was then extracted using CH2Cl2. The extracts were combined, dried, and concentrated under reduced pressure. The crude product was further purified by column chromatography using hexane followed by ethyl acetate/hexane 1:99 (v/v). The product was then recrystallized twice from hexane to yield a fibrous yellow solid (500 mg, 0.608 mmol). Yield 58%; mp 81−83 °C. 1H NMR (400 MHz, CDCl3) δ: 7.40 (d, J = 3.9, 2H), 7.16 (d, J = 3.9, 2H), 2.76 (t, J = 7.3, 4H), 1.54−1.42 (m, 4H), 1.31−1.16 (m, 36H), 0.88 (t, J = 6.9, 6H). 13C NMR (101 MHz, CDCl3) δ: 153.85, 142.52, 138.68, 131.43, 129.67, 129.46, 115.24, 77.33, 77.02, 76.70, 37.73, 31.92, 29.66, 29.64, 29.59, 29.51, 29.36, 29.26, 29.13, 28.77, 22.69, 14.12. MALDI-MS: m/z = 858.2 (M+). 4,7-Bis(5-bromothiophen-2-yl)-5,6-bis(ethylsulfanyl)-2,1,3-benzothiadiazole (Monomer D). A solution of 5,6-bis(ethylsulfanyl)-4,7di(thiophen-2-yl)-2,1,3-benzothiadiazole (2) (305 mg, 0.725 mmol), N-bromosuccinimide (NBS) (271 mg, 1.523 mmol), and chloroform (20 mL) were stirred at room temperature in the absence of light for 16 h. The reaction mixture was poured into a saturated solution of sodium sulfite and extracted with chloroform. The organics were combined, dried (MgSO4), filtered, and concentrated under reduced pressure. The residue was crystallized from isopropanol to afford the product as orange solid. Yield 85%; mp 139−141 °C. 1H NMR (400 MHz, CDCl3) δ: 7.40 (d, J = 3.9 Hz, 2H), 7.17 (d, J = 3.9 Hz, 2H), 2.80 (q, J = 7.4 Hz, 2H), 1.17 (t, J = 7.4 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ: 153.75, 142.04, 138.64, 131.41, 129.80, 129.53, 115.26, 31.66, 14.35. MS (ES): m/z = 578.82 (M+). Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (P1). Monomer 1 (155.6 mg, 0.218 mmol), 9-(9-heptadecanyl)-9Hcarbazole-2,7-diboronic acid bis(pinacol) ester (143.2 mg, 0.218 mmol), Pd(PPh3)4 (5.0 mg), and a stirrer bar were added to a 20 mL high pressure microwave reactor vial. The vial was then sealed with a septum and flushed with argon, before degassed toluene (3 mL), degassed aqueous 1 M Na2CO3 (0.7 mL), and 2 drops of Aliquat 336 were added. The whole solution was then degassed again for 30 min before the reaction was heated to 120 °C for 3 days. To end-cap the polymer, a solution of phenylboronic acid (6 mg in 0.2 mL, 0.051

solution and the solid state. This could be explained as a result of the large difference in excited state geometry compared to the ground state. Such a large Stokes shift may be useful in applications where minimal self-quenching/absorption is required, such solar concentrators, lasers, and molecular imaging. Here copolymerization of thioalkyl containing acceptors with high fluorescent comonomers may be an interesting approach.

4. EXPERIMENTAL SECTION General. All solvents and chemicals, including monomer B, were purchased from Sigma-Aldrich and used without further purification. 5,6-Difluoro-4,7-diiodo-2,1,3-benzothiadiazole, 4,7-bis(5-bromothiophen-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (2) and monomer A were prepared according to the literature procedures.32,33 Microwave reactions were performed in a Biotage initiator V 2.3. in constant temperature mode. Nuclear magnetic resonance (NMR) spectra were recorded on Bruker AV-400 (400 MHz) spectrometers in chloroformd solutions. Number-average (Mn) and weight-average (Mw) molecular weights were determined with an Agilent Technologies 1200 series GPC in chlorobenzene at 80 °C using two PL mixed B columns in series and calibrated against narrow polydispersity polystyrene standards. A customer-built Shimadzu recSEC system was used to purify the polymers. The system comprises a DGU-20A3 degasser, an LC-20A pump, a CTO-20A column oven, an Agilent PLgel 10 μm MIXED-D column, and a SPD-20A UV detector. UV−vis absorption spectra and steady state photoluminescence spectra were measured using a Shimadzu UV-1800 UV−vis spectrophotometer and a Horiba Jobin Yvon Fluorolog-3 fluorometer, respectively. Solutions (5 mg/ mL) of the polymers in chloroform were used to spin-coat thin films at 1000 rpm for 60 s; measurements were taken at room temperature and were excited at the maximum of the long wavelength absorption peak. Solutions and films were not deoxygenated for these measurements. Film thicknesses used to calculate thin film absorption coefficients were measured using a Veeco Dektak profilometer. The absorption coefficient of the film was calculated from (optical density × ln 10)/ film thickness, using four different film thicknesses. Differential scanning calorimetry (DSC) traces were measured using a TA Instruments DSC Tzero Q20 instrument. Ionization potentials were measured by photoelectron spectroscopy in air (PESA) on a Riken Keiki AC-2 PESA spectrometer. Polymer thin films were prepared by spin-coating from 5 mg/mL polymer solutions in chloroform onto glass substrates. The PESA samples were run with a light intensity of 5 nW and data processed with a power number of 0.5. Density functional theory using a B3LYP36 functional and basis set of 631G(d) was used to calculate the ground state geometries and electron density plots of trimer molecules in section 2.3. Ground state and excited state geometries of monomer molecules in section 2.4 were calculated using the long-range functional CAM-B3LYP37 and a basis set of 6-31G(d). DFT was used to obtain ground state geometries, while TD-DFT was used to obtain excited state geometries. All calculations were carried out using Gaussview 5.0.43 Organic Photovoltaic Device Fabrication. ITO-coated glass substrates were washed by ultrasonication in acetone and isopropyl alcohol then dried before undergoing an oxygen plasma treatment. A t h i n l a y e r o f p o l y ( 3 , 4 - e t h y l e n e - d i ox y t h i o p h e n e ) : p o l y (styrenesulfonate) (PEDOT:PSS) was spin-coated onto the substrate and dried at 150 °C for 30 min. The active layer, consisting of polymer (10 mg/mL) and PC71BM at different blend ratios, was deposited by spin-coating from either chlorobenzene (P1) or dichlorobenzene (P2/ P3) solutions onto the PEDOT:PSS layer. A Ca (30 nm)/Al (100 nm) cathode layer was then deposited by thermal evaporation under vacuum through a shadow mask to complete the BHJ cell. Current− voltage (J−V) characteristics were measured under AM1.5 solar illumination using a xenon lamp. Five devices were measured for each polymer and the average efficiency calculated. Synthesis of Monomers and Polymers. 5,6-Difluoro-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (1). 5,6-Difluoro-4,7diiodobenzo[c][1,2,5]thiadiazole (1.24 g, 2.92 mmol), Pd(PPh3)4 2286


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mmol) was injected, and the reaction stirred for 2 h at 120 °C. Bromobenzene (8 mg, 1.49 mmol) was then added, and the reaction heated for a further 2 h. The mixture was then cooled to room temperature, precipitated in methanol (100 mL), and stirred for 30 min and filtered through a Soxhlet thimble. The polymer was then extracted (Soxhlet) using methanol, acetone, hexane, and chloroform in that order under argon. The chloroform fraction was collected and concentrated to ∼70 mL, to which a solution of aqueous sodium diethyldithiocarbamate dihydrate solution (∼100 mg in 70 mL) was added. The RBF was equipped with a condenser, and the two layers were stirred vigorously at 60 °C for 60 min to extract the palladium. The two layers were then separated, and the chloroform layer washed thoroughly with water (3 × 100 mL). The chloroform fraction was dried (MgSO4), filtered, and concentrated to ∼10 mL before being precipitated into methanol (100 mL), stirred for 30 min, and filtered. This precipitation was repeated again to yield P1 as a red solid (182 mg, 87%). Chloroform fraction: Mn = 25 400, Mw = 45.900, Mw/Mn (PDI) = 1.8. 1H NMR (400 MHz, CDCl3) δ: 8.62 (br, 2H), 8.13 (br, 2H), 7.93 (br, 1H), 7.75 (br, 1H), 7.65 (br, 2H), 7.57 (br, 2H), 4.69 (br, 1H), 4.26 (br, 4H), 2.44 (br, 2H), 2.08 (br, 6H), 1.50−1.09 (br, 44H), 0.89 (t, J = 6.6 Hz, 6H), 0.79 (t, J = 6.8 Hz, 6H). Anal. Calcd for C59H79N3O2S3: C, 73.93%; H, 8.31%; N, 4.38%. Found: C, 73.84%; H, 8.22%; N, 4.33%. Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(dodecylthio)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5diyl] (P2). Monomer 6 (220.7 mg, 0.257 mmol), 9-(9-heptadecanyl)9H-carbazole-2,7-diboronic acid bis(pinacol) ester (169.7 mg, 0.257 mmol), and Pd(PPh3)4 (5.9 mg) were reacted using the same procedure reported for P1. The polymer was cleaned via a Soxhlet extraction using methanol, acetone, hexane, and chloroform under argon. The chloroform fraction was washed with aqueous sodium diethyldithiocarbamate dehydrate to extract the palladium as above. The chloroform fraction was then dried with MgSO4 and concentrated to ∼10 mL before being precipitated into methanol (100 mL). This precipitation was repeated again to yield P2 as a red solid (250 mg, 88%), GPC: Hexane fraction: Mn = 4.10, Mw = 6.42, PDI = 1.56. Chloroform fraction: Mn = 28 700, Mw = 67 500, Mw/Mn (PDI) = 2.35. 1H NMR (400 MHz, CDCl3) δ: 8.11 (br, 2H), 7.89 (br, 1H), 7.71 (br, 1H), 7.63 (br, 4H), 7.54 (br, 2H), 4.66 (br, 1H), 2.86 (br, 4H), 2.38 (br, 2H), 2.01 (br, 2H), 1.55 (br, 6H), 1.37−1.04 (m, 58H), 0.86 (t, J = 6.8 Hz, 6H), 0.80 (t, J = 6.5 Hz, 6H). Anal. Calcd for C67H95N3S5: C, 72.97%; H, 8.68%; N, 3.81%. Found: C, 72.93%; H, 8.79%; N, 3.76%. Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(ethylthio)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (P3). Monomer 7 (168.9 mg, 0.257 mmol), 9-(9-heptadecanyl)-9Hcarbazole-2,7-diboronic acid bis(pinacol) ester (148.6 mg, 0.257 mmol), and Pd(PPh3)4 (5.9 mg) were reacted using the same procedure reported for P2. The polymer was cleaned via a Soxhlet extraction using methanol, acetone, hexane, and chloroform under argon. The chloroform fraction was washed with aqueous sodium diethyldithiocarbamate dehydrate to extract the palladium. The chloroform fraction was then dried with MgSO4 and concentrated to ∼10 mL before being precipitated into methanol (100 mL). This precipitation was repeated again to yield P2 as a red solid (182 mg, 86%). The polymer (182 mg) was then fractionated using a preparative GPC running in chlorobenzene to obtain 48 mgs of P3 with Mn of 26.5 kDa, Mw of 58.2 kDa, and Mw/Mn (PDI) = 2.20. 1H NMR (400 MHz, CDCl3) δ: 8.11 (br, 2H), 7.89 (br, 1H), 7.71 (br, 1H), 7.63 (br, 4H), 7.54 (s, 2H), 4.65 (br, 1H), 2.90 (br, 4H), 2.37 (br, 2H), 2.00 (br, 2H), 1.36−1.06 (br, 30H), 0.80 (t, J = 6.5 Hz, 6H). Anal. Calcd for C47H55N3S5: C, 68.65%; H, 6.74%; N, 5.11%. Found: C, 68.76%; H, 6.64%; N, 5.05%.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the UK’s Engineering and Physical Sciences Research Council (EPSRC) for financial support via the Doctoral Training Centre in Plastic Electronics EP/G037515/1 (A.C., M.H.) and the EPSRC platform grant EP/G060738/1 (Z.F., M.H.). We gratefully acknowledge Dr. Scott E Watkins (CSIRO) for the PESA measurements and Dr. Alexandra Simperler (Imperial College London) for discussions on computational calculations.

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S Supporting Information *

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Thioalkyl-Substituted Benzothiadiazole Acceptors: Copolymerization

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