Optimizing the Performance of Conjugated Polymers in Organic

Perspective pubs.acs.org/Macromolecules

Optimizing the Performance of Conjugated Polymers in Organic Photovoltaic Cells by Traversing Group 16 Malika Jeffries-EL,* Brandon M. Kobilka, and Benjamin J. Hale Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States ABSTRACT: Conjugated polymers are organic semiconductors that have emerged over the past few decades as promising alternatives to inorganic semiconductors for use in a range of electronic applications. Since the performance of these materials in various devices is dependent on many variables, there have been numerous reports on the design and synthesis of novel materials. In this Perspective, we focus specifically on the use of heteroatom substitution to tune the properties of conjugated polymers. This strategy is very promising as the subtle modification in the polymer’s structure can often result in significant changes in the optical and electronic properties of the materials. Given the current interest in renewable energy, we are highlighting the impact these modifications have on the performance of these conjugated polymers in photovoltaic cells. Based on the popularity of thiophene-based conjugated polymers, the focus of this work is the group 16 elements oxygen, sulfur, selenium, and tellurium. In recent years, we have seen large number of papers utilizing this approach with mixed results. In addition to summarizing the work in this area, we will also identify opportunities and challenges.

1. INTRODUCTION 1.1. Background. Since Wallace Carothers’s pioneering research and the dawn of modern polymer chemistry, scientists have strived to synthesize new materials to replace those derived from natural sources. Although the properties of synthetic polymers are not identical to the naturally derived materials, synthetic polymers can be structurally modified, often affording materials with superior properties. During the past 40 years the field of synthetic polymers has expanded to include conjugated polymers (CP)s−materials that feature a continuous backbone of alternating sp2 and/or sp hybridized atoms which, as a result of the extensive delocalization of electrons, possess semiconductive properties.1 In contrast to conventional semiconductors which are based on inorganic materials, organic semiconductors are made from carbon-based molecules and offer several advantages over inorganic materials including relative abundance, low fabrication costs, and properties that are readily tuned via chemical synthesis. Accordingly, CPs are finding use in several applications including organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs),2,3 organic field effect transistors (OFETs),4 and sensors.5 Since the ability to efficiently harvest solar energy is a critical component of future global energy production, there is a tremendous interest in the development of OPVs. Currently the most efficient OPVs are those in which a bulk-heterojunction (BHJ) is formed between an electron-accepting nanomaterial such as a fullerene derivative and an electron-donating CP.6,7 In order to optimize the overall performance of these OPVs, the CP should have a narrow band gap for efficient solar energy harvesting,7 a low-lying highest occupied molecular orbital (HOMO) to maximize the open circuit voltage (VOC),8 and a © 2014 American Chemical Society

lowest unoccupied molecular orbital (LUMO) level that is appropriately offset above the acceptor’s LUMO to drive charge separation while minimizing energy loss.6 Although 0.3 eV is the commonly accepted minimum for the latter, the exact offset required is still the subject of debate.9,10 Additionally, charge carrier mobility and polymer film morphology also influence performance. Alas, concurrent optimization of all of these variables can be challenging. In recent years, the synthesis of polymers composed of alternating electron-rich and electronpoor moieties along the backbone has emerged as an effective way to tune the optical and electronic properties of CPs.11 By varying the strength of the donor and acceptor components of the copolymer, the intramolecular charge transfer (ICT) between these components can be tuned, facilitating manipulation of the polymer’s HOMO and LUMO levels.12 Additionally, substituents on the monomers, such as fluorine atoms or side chains of varying length, can further modify the properties of these materials. However, these substituents can also influence polymer morphology and potentially have an adverse effect on the overall performance of the CP. In the case of CPs containing heterocycles, the heteroatoms can be switched to further modulate their properties. This atomic engineering is particularly promising as such changes can considerably impact the electronic, optical, and physical properties of the resultant materials. Among building blocks used in the synthesis of CPs, sulfur-containing heterocycles are predominant. This is largely a result of the early success of Received: June 14, 2014 Revised: September 10, 2014 Published: September 30, 2014 7253

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regioregular poly(3-hexylthiophene) (P3HT),13,14 the commercial availability of functional thiophenes, and the synthetic ease in which such molecules can be modified. However, the past few years has seen a surge in the number of papers on the development of novel heterocycles comprising other group 16 elements, their incorporation into CPs, and their use in OPVs.15,16 In this Perspective we will survey some of the work on the synthesis, properties, and OPV performance of analogous CPs containing group 16 elements. 1.2. The Chemistry of Group 16 Heterocycles. Group 16 of the periodic table contains the elements oxygen, sulfur, selenium, tellurium, the radioactive polonium, and the synthetic element ununhexium (Uuh). As freshman chemistry teaches, the chemical and physical properties of these elements will be similar and will vary as you traverse the period. The first four members of group 16 are often referred to as the chalcogens, all of which have been successfully incorporated into five-membered heterocycles. A comparison of these molecules provides some insight into the potential impact of heteroatom substitution (Table 1).

mobilities.23 Additionally, furan-based materials are highly fluorescent due to the lack of a heavy atom.24 Although the values can vary depending on the methods used, the aromaticity of conjugated heterocycles is typically evaluated by comparing the resonance energy. Historically, this was done using the empirical resonance energy (ERE), which is the experimentally determined deficiency of a conjugated system is compared to the nonconjugated cyclic system,30,31 the Dewar resonance energy (DRE), which is based on a theoretical comparison to the linear polyene,30,32,33 or the Hess Shad resonance energy (HSRE), which is a variant of the DRE adjusted to the Hückel molecular orbital model.34,35 Recently, the I5 index, which is based upon the statistical degree of uniformity of the bond orders of the ring periphery, has emerged as an alternative approach.36,37 These values have been summarized in Table 1. The ERE, DRE, HSRE, and I5 values for benzene are 15.77 kJ mol−1, 25.0 kJ mol−1, 0.065β, and 100, respectively.17,36 Thus, the trend in aromaticity is benzene > thiophene > selenophene > tellurophene > furan.17,36 The aromaticity of conjugated systems can impact the formation of extent of delocalization along the polymer backbone, influencing the material’s the materials energy levels and band gap.28,38 As a result of the lower aromaticity of furan, polyfuran has a larger optical band gap (2.83 eV theoretical,39 2.35 eV experimental)40 than polythiophene (2.03 eV theoretical,39 2.0 eV experimental).41 Oligofurans also have similar hole mobilities as oligothiophenes, which is consistent with theoretical predictions and likely a result of the larger quinoid character.42−44 Conversely, polyselenophene has a slightly smaller optical band gap (1.85 eV theoretical, 1.9 eV experimental)45 than polythiophene due to selenium’s stabilization of the LUMO level.19 Thiophene,46 furan,47 and their derivatives can be prepared using numerous methods which have been reviewed previously. Furan and several derivatives can also be prepared from biomass, making them green building blocks.48 The use of selenophene, tellurophene, and their derivatives in CPs has risen over the past two decades, primarily due to an increased number of methods for their preparation.29 All of these chalcogenophenes are πexcessive heterocycles and will undergo electrophilic substitution reactions faster than benzene. Substitution usually occurs at the α-position unless those positions are already substituted, and the relative reaction rates are furan > tellurophene > selenophene > thiophene.17 However, ring-opening side reactions can occur in furans in the presence of aqueous Brønsted acids and during certain oxidations.49 Thiophenes can also experience oxidation of sulfur to form S,S-dioxides and desulfurization reactions,30 while tellurophenes can be halogenated to form dihalotellerides.29,50,51

Table 1. Key Properties of Five-Membered Heterocycles Containing Group 16 Elements

Increasing the size of the heteroatom results in lengthening of the X−C(2) bond and a decrease in the C(2)−X−C(5) bond angle.17 Thus, the smaller atomic radius of the oxygen results facilitates planarity between neighboring heterocycles,18 whereas intermolecular interactions between neighboring selenium atoms can also increase planarity, despite the steric effects.19 The larger more polarizable radius of selenium and tellurium leads to stronger intermolecular interactions and higher degree of rigidity.19 The difference in the electronegativity of the chalcogens impacts the incorporation of electrons into the heterocycle’s π-system and its dipole moment. Because of the low electronegativity of tellurium, carbon−tellurium bonds are polarized Teδ+−Cδ−; conversely, oxygen−carbon bonds are polarized Oδ−−Cδ+.20 The electrons on sulfur, selenium, and tellurium are also more readily incorporated into the conjugated system than those on oxygen are. As a result, furan has the largest reported dipole moment and tellurophene the smallest.21,22 This difference in dipole moment is likely the reason that furan-based materials exhibit better solubility in polar solvents than analogous thiophene-based materials. However, the stronger intermolecular interactions in selenium- and tellurium-based materials reduce their solubility but increase the overlap of πelectrons, which is beneficial for improving field-effect

2. ALL DONOR POLYMERS Poly(3-alkylthiophene)s bearing various side chains have been synthesized by both chemical and electrochemical methods, and their chemistry has been reviewed previously.14,52,53 As a result of the asymmetry of the 3-alkylthiophene monomer, polymerization can result in two different types of polymers: regiorandom 201 in which the orientation of the alkyl group is not controlled and regioregular 202 in which the alkyl chains are consistently positioned along the polymer backbone. The degree of regioregularity has major implications on the performance of the material. Regioregular poly(3-alkylthiophene)s are among the most widely studied CPs due to their excellent solubility, oxidative stability, crystalline morphology, and good charge carrier mobility.13,14 Regioregular poly(3-hexylthiophene) (rr7254

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P3HT) (202a) was the one of the first polymers to be successfully used in organic electronics with reported hole mobilities (μh), as high as 0.4 cm2 V−1 s−1 and, after numerous optimizations, power conversion efficiencies (PCE)s above 5.0% in BHJ-OPVs with the fullerene derivative [6,6]-pheny-C61butyric acid methyl ester (PCBM).54−56 Unfortunately, the performance of P3HT:PCBM in OPVs is intrinsically limited by its relatively wide band gap (∼1.9 eV), which impacts the harvesting of solar energy. Additionally, rr-P3HT’s HOMO of −4.8 eV as determined via ultraviolet photoelectron spectroscopy (UPS)57 limits the open-circuit voltage (VOC) to ∼0.6 V.7,58 Lastly, rr-P3HT also has a LUMO of −2.7 eV,57 while PCBM has a LUMO energy level of −3.7 eV which results in additional energetic losses.58 Although some groups have reported increased performance through the use of new electron acceptors such as indene-C60 bisadduct (ICBA),59 the difficulty associated with the synthesis and preparation of these compounds limits their utility. The facile methods of synthesis of rr-P3HT make it very appealing for large-scale synthesis, and its large degree of crystallinity results in favorable morphology when blended with fullerenes. Thus, chalcogen substitution has been explored to improve upon the properties of rr-P3HT, while minimally changing the polymer’s structure. Poly(3-alkylfuran)s have been prepared by chemical and electrochemical oxidation as well as nickel-catalyzed cross-coupling reactions.60,61 Regiorandom poly(3-hexylfuran) (203a) and poly(3-dodecylfuran) (203c) both had optical band gaps of 2.4 eV when prepared via electrochemical oxidation62 and optical band gaps of 2.3 eV when prepared via chemical oxidation,60 while regioregular poly(3octylfuran) (204b) prepared via nickel-catalyzed cross-coupling reaction had a band gap of 2.2 eV.61 Polymer 204b exhibited a quasi-reversible oxidation, E1/2ox, of 0.32 eV vs the ferrocene/ ferrocenium couple, which corresponds to a HOMO level of 5.2 eV,63 which is slightly lower than the reported values for 202a (5.1 eV) under similar conditions.7 To date, the use of poly(3alkylfurans) in OPVs or OFETs has not been reported. Poly(3alkyltellurophene)s bearing hexyl, ethylhexyl, or dodecyl side chains have been synthesized using both oxidative coupling and nickel-catalyzed cross-coupling reactions.64,65 The optical band gaps of 208a and 208c, determined by onset of absorption, are 1.44 eV, whereas 208d has an optical band gap of 1.57 eV. All of these band gaps are smaller than those of 202a (1.9 eV) and 206a (1.7 eV). Regiorandom poly(3-hexyltellurophene) (207a) prepared via electrochemical oxidation had an absorbance maximum at 599 nm with a well-defined shoulder at ∼750 nm. Regioregular poly(3-hexyltellurophene) 208a, the dodecyl derivative 208c, and the ethylhexyl analogue 208d have maximum absorption peaks at 558, 545, and 512 nm, respectively.64 These values are all red-shifted relative to 202a (455 nm) and the selenium analogue 206a (500 nm).13 A recent study by Seferos et al. has shown that the introduction of the heavy atom in the polymer backbone facilitates intersystem crossing, resulting in an increased chance of the primary photoexcitations being converted into triplet states in materials containing tellurium and selenium.66 To date, only 208 has been used in an OPV, affording a PCE of only 1.1%.65 Regiorandom poly(3-alkylselenophene)s 205 featuring side chains of various lengths have been prepared by chemical and electrochemical oxidation,19,80−85 whereas nickel-catalyzed cross-coupling reactions have been employed to prepare regioregular poly(3-hexylselenophene) rr-P3HS 206a.86 Both regiorandom P3HS (205) and regiorandom poly(3-dodecylselenophene) (205c) have an optical band gap of 2.4 eV,80 whereas

Figure 1. Generic structures of poly(3-alkylchalcogenophene)s.

206a has an optical band gap of 1.6 eV and an electrochemical band gap of 1.9 eV.86 Both values are 0.3 eV lower than those attained for analogous thiophene derivatives under the same conditions.86 The HOMO level of 206a as determined by UPS is 4.8 eV, which is identical that reported for 202a.57 Thus, as expected, replacing sulfur with selenium only influences the polymer’s LUMO level, as result of the lower HOMO level of selenium in comparison to sulfur. BHJ-OPVs made using 206a and (PC61BM) had a PCE of 2.7%, which is slightly lower than that of 202a:PC61BM device (3.0%).68 The broader absorption profile of 206a results in higher JSC; however, the lower mobility of 206a (4.0 × 10−6−7.0 × 10−5 cm2 V−1 s−1), in comparison to 202a (4.0 × 10−4 cm2 V−1 s−1) under the same conditions, reduced its overall performance. The reduced performance of 206a relative to 202a in OPVs is also a result of differences in the molecular order and morphology within thin films of the polymer/fullerene blend.87 A detailed study of these films using Raman spectroscopy and X-ray diffraction has shown that 206a has a higher fraction of material in a disordered phase which prevents the formation of micrometer-sized PCBM aggregates in blend films during thermal annealing. As a result, OPVs made using 206a:PCBM have smaller initial short-circuit current (JSC) than those made from 202a:PCBM. However, the JSC of devices made from 206a:PCBM increases more upon thermal annealing.87 The high hole mobility of 202a is a result of its ability to form lamellar structures with parallel layers of thiophene backbones separated by interdigitated side chains. This three-dimensional 7255

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Table 2. Properties of Poly(3-alkylchalcogenophene)s polymer 67

Mn [kDa]

PDI

202a 206a67,68 206d69 208c64,65 20967,70

34 24.1 19.4 20.0 26.8

1.99 1.20 1.4 2.4 1.2

210a70

22.8

1.18

210e69 21271,72 21373,74 21475 21575 21676 21777,78 21879 21979

24.5 46.0 22.1 11.0 11.2 18.5 23.4 29.6 22.5

1.1 2.0 1.5 1.7 2.0 1.4 3.0 2.2 2.5

λmax [nm] (film)

HOMO [eV]

LUMO [eV]

442 60768

−5.2 −5.4a

−3.3 −3.8c

545

−5.2a −5.16a

−3.7c −3.6c

−5.06b −5.1a −5.20a −5.17a −5.11a −5.10b −5.39b −5.30b

−3.41b −2.95a −3.47c −3.31c −3.53c −3.05b −3.32b −3.16b

52

535 545 603 673 787 525 531 541

a

c

VOC [V] 0.6 0.54 0.72 0.66 0.60 0.80 0.59 0.70 0.70 0.71 0.67 0.57 0.44 0.75 0.75 0.83 0.81

JSC [mA cm−2] 8.29 7.06 6.75 4.29 5.91 2.11 8.52 7.74 8.08 3.30 4.30 8.60 9.77 6.72 5.72 3.71 2.24

FF 0.66 0.56 0.44 0.39 0.61 0.67 0.62 0.56 0.60 0.44 0.47 0.48 0.51 0.35 0.39 0.39 0.39

PCE [%] d

3.3 2.1d 1.9e 1.1d 2.2d 1.1e 2.7d 3.1e 3.3d 1.0d 1.4d 2.4d 2.2e 1.8d 1.7d 1.2d 0.8d

μh [cm2 V−1 s−1] 5.6 × 10−4 f 7.0 × 10−5 f

2.2 × 10−4 f

0.18f 0.02f 6.0 × 10−3 f 0.05f 2.6 × 10−3 f 1.1 × 10−3f 1.3 × 10−4 f 3.0 × 10−3 f

a

Determined via cyclic voltammetry using Fc/Fc+. bDetermined via cyclic voltammetry using Ag/Ag+. cEstimated from HOMO-Egopt. dPC61BM was used as the acceptor. ePC71BM was used as the acceptor. fObtained from bottom contact OFETs. gObtained via space charge limited current (SCLC) measurements of fullerene/polymer films.

and 206d are 232 and 210 °C, respectively, while 210d has a melting transition at 211 °C. The fact that the melting temperatures of copolymer 210d and 202d are similar further supports the compatibility of the two monomers. OPVs using 210d as the donor and ICBA as the electron acceptor outperformed those made using either the homopolymer 206d or the analogous block copolymer with hexylselenophene 210a.69 This improvement is a result of both the favorable morphology of the polymer films and its broad absorption profile. Collectively, the broader absorption profile and favorable stacking indicate that copolymerization is a promising approach for designing new OPV materials. Introducing nonalkylated heterocycles as spacers is another approach that has been used to improve transport properties. Poly(3,3‴-didodecylquarterthiophene) (PQT-12) 212 can undergo self-assembly under appropriate conditions, forming three-dimensional lamellar π-stacking structures.94 Furthermore, the unsubstituted thiophenes have a higher degree of rotational freedom. As a result, 212 has a HOMO of −5.24 eV versus the −5.00 eV measured for 206 under the same conditions.95 Although 212 exhibits excellent oxidative stability and hole mobility around 0.2 cm2 V−1 s−1,72 to date the highest PCE obtained was 1.4% for a 212:PC61BM device upon thermal annealing.96 The incorporation of selenophenes has been utilized to improve the charge carrier mobility and OPV performance with mixed results. OFETs using 213 exhibited a lower hole mobility than those made from 212;74 the SCLC behavior at various temperatures indicates better charge transport is occurring in 213 than 212.73 Nonetheless, the OPV performance of 213 was similar, with a maximum PCE of 1.4% for a 213:PC61BM device.73,97 A related series of polymers in which the amount of selenophenes along the polymer backbone was varied have also been evaluated.75 The d-spacings with the thin films are 19.3 and 24.0 Å for 214 and 215, respectively. The dspacing of 214 is similar to that previously reported for 212 (17.2 Å), indicating the side chains are likely interdigitated.94 Conversely, the spacing of 215 is similar to that of poly(3dodecylthiophene), 206c (26.2 Å), which exhibits end-to-end packing of the side chains.88 Despite the difference in packing,

structure is often referred to as the type 1 polymorph of 202a, and layer thickness of this phase varies almost linearly with the length of the aliphatic side chains.88 A second polymorph (type 2) that has substantially shorter layer thickness has been reported for the longer side chain derivatives such as 202b and 202s.89 The purely type 2 phase has not been observed in 202a; however, it is readily obtained for 206a.90,91 This difference in crystallization behavior demonstrates the potential of 206a to drive distinct types of self-assembled nanostructures in conjugated polymer films and has inspired research on both block 209,67,70,92 statistical 210,69,70 and gradient 21193 rrP3HT/rr-P3AS copolymers. Although the block copolymer is composed of two different blocks, the external quantum efficiency (EQE) spectra, hole mobility, fill factor (FF), short circuit current (JSC), and PCE exceed those of a physical mixture of the two homopolymers. As a result, the block copolymer 209 performs better in OPVs than 206a or a blend of 206a and 202a (PCE = 2.1%).67 However, the OPV performance of 209 is worse than that of 202a under the same conditions. The statistical copolymer 210a spontaneously undergoes phase separation, while copolymer 209 does not.70 Though, the ability to form nanofibrils does not correlate with better OPV performance. Furthermore, the self-assembly of these polymers in the presence of fullerenes is very different and strongly influenced by processing conditions.70 The best OPVs using 209 were obtained using PC71BM and standard solution processing conditions, whereas the best OPVs using 210a were obtained using ICBA and solvent annealing. The composition of the polymer also influences the polymer’s packing as evidenced by changes in their melting temperatures as the random copolymer 211r has a Tm = 236 °C, the gradient polymer 211g has a Tm = 238 °C, and the block copolymer 210 has a Tm = 242 °C.93 To date, neither 211r nor 211g has been used in OPVs or OFETs. To further evaluate the self-assembly of the poly(3-alkylthiophene)-stat-poly(3-alkylselenophene), 206d has been synthesized and characterized.69 The d-spacing in 206d is 17.1 Å, which is approximately 1.6 Å larger than 206a90 and 1.7 Å larger than 210d.69 This suggests that the two monomers are more compatible with each other. The melting transitions for 202a 7256

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Figure 2. Structures of benzodichalogenophene homopolymers.

selenium is beneficial to intermolecular interactions with polymer films, improving charge transport.110 The incorporation of BDF into a conjugated polymer and its photovoltaic properties were first reported in 2008.111 The monomer was synthesized in analogous fashion as the thiophene derivative starting with 3furancarboxylic acid. The copolymer was composed of 4,8bis(ethylhexyloxy)benzo[1,2-b:4,5-b′]difuran and the electronwithdrawing unit 4,7-dithienyl-2,1,3-benzothiadiazole. The resulting polymer 301111 had an optical band gap of 1.60 eV and an electrochemical band gap of 1.9 eV, which are similar to the values reported for the sulfur analogue 302.112 However, the HOMO level of 301 is 0.3 eV lower than that of 302. Furthermore, OPVs made from 301 had PCEs as high as 5.0%,111 whereas devices made from 302 only achieved PCEs of 2.0%, despite the fact that 301 has a lower molecular weight than 302. The superior performance is likely due to the combination of higher FF, JSC, and hole mobility of 301. Replacing the alkyl chains on the flanking thiophenes of 302 with electron-rich alkoxy chains affords 303, which has a higher HOMO level than 302. As a result, the VOC of 303 is half that of 302, diminishing its OPV performance, despite the fact that 303 has a higher hole mobility than 302.112 Similarly, the alkoxy substitutents typically present on the central benzene ring of the BDF and BDT moieties can also raise the HOMO level. For this reason several groups have synthesized monomers featuring various substituents at this position.113,114 In 304 the alkoxy chains on the benzene ring of 301 have been replaced with alkyl furans.115 This modification had a negligible effect on the HOMO level but lowered the LUMO level by 0.37 eV. Replacing the alkoxy chains with alkylthiophenes had a similar effect as 305 has the same HOMO level as 301, but its LUMO is 0.16 eV lower. Conversely, replacing alkoxy chains on the benzene ring of the BDT derivative 302 with alkyl furans lowers both the HOMO and LUMO levels by 0.44 and 0.31 eV, respectively, whereas thiophene substitution lowered the HOMO and LUMO levels by 0.46 and 0.11 eV, respectively.115 Of the series, 305 exhibited the highest hole mobility and also had the best OPV performance with a PCE of 4.4% when 3% diiodoctane (DIO) was used as an additive in the device fabrication.115 Interestingly, the isomeric polymers 304 and 306 gave similar results, whereas 307, which contained only thiophenes, had the lowest PCE and mobility. This difference in performance may be a result of the processability of the polymers as 308, which is structurally similar to 307, but has an additional alkyl chain on the thiophene substituents, had a PCE of 5.7% under similar conditions.116

OPVs fabricated using 214 and 215 performed similarly with PCE’s of 2.4% and 2.2%, respectively.75 Poly(pentaselenophene) 216 features an unsubstituted selenophene flanked by two dodecyl-substituted selenophenes on each side.76 SCLC measurements show that 216 has good hole mobility (2.6 × 10−3 cm2 V−1 s−1); however, its performance in OPVs was modest at 1.8%. This is likely a result of the formation of an unfavorable morphology when blended with fullerene. The fused benzo[1,2-b:4,5-b′]dithiophene (BDT) moiety features a planar conjugated structure that facilitates π−π stacking, improving hole mobility.98,99 Furthermore, the symmetry of this molecule eliminates the need for regiochemical control. Replacing sulfur with oxygen is advantageous as the smaller atomic radius of the oxygen atom reduces the steric interactions with neighboring groups, increasing planarity and conjugation.18 Introducing phenylethynyl substituents onto benzodithiophene results in two perpendicular conjugated pathways which can increase delocalization within the system.78,100 The BDT homopolymer 217 had a similar HOMO level and band gap as 206; however, when used in OPVs it only yielded a PCE of 1.7%.77 Replacing sulfur with oxygen lowered the both the HOMO and LUMO levels; as a result, 218 has a band gap similar to 217.79 Despite the lower HOMO level, the OPV performance of 218 was worse than that of 217, likely a result of the lower mobility. Combining the two monomers afforded 219, which has a broader absorption profile than either of the two homopolymers.79 However, this polymer gave the poorest OPV performance of the set, which is likely a result of nonideal thin film morphology.

3. ELECTRON-RICH MONOMERS FOR USE IN DONOR−ACCEPTOR COPOLYMERS 3.1. Benzodichalcogenophenes. The electron-rich BDT moiety has been used in the synthesis of some the best performing polymers in OPVs to date.101−107 However, many of these materials have high-lying HOMO levels, reducing the VOC and limiting their overall performance.108,109 One approach for lowering the HOMO level is replacing the sulfur atoms within BDT with oxygen or selenium, affording the analogous benzo[1,2-b:4,5-b′]difurans (BDF)s and benzo[1,2-b:4,5-b′]diselenophenes (BDSe)s, respectively. The higher electronegativity of oxygen relative to sulfur is expected to stabilize the HOMO level of BDF, whereas incorporating selenium generally reduces the band gap by lowering the LUMO level. Additionally, the larger, more polarizable atomic radius of 7257

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Figure 3. Benzodichalogenophene donor−acceptor copolymers with thienylbenzothiadiazole.

Table 3. Data for Benzodichalogenophene Donor−Acceptor Copolymers with Thienylbenzothiadiazole polymer

Mn [kDa]

PDI

λmax [nm] (film)

HOMO [eV]

LUMO [eV]

VOC [V]

JSC [mA cm−2]

FF

PCE [%]

μh [cm2 V−1 s−1]

301111 302112 303112 304115 305115

6.3 23.2 19.1 5.2 4.0

1.6 1.3 1.2 2.5 2.9

613 404/531 451/625 604 600

−5.10a −4.80a −5.20a −5.11b −5.08b

−3.23a −3.25c 3.65b −3.60b −3.39b

7.6 6.8 27.4 5.3 6.0

3.0 4.8 1.8 1.2 2.5

604 604 596/627 683 616

−5.24b −5.26b −5.31b −5.33a −5.30a

−3.54b −3.34b −3.44b −3.58a −3.57a

11.77 6.28 5.27 5.84 8.86 9.94 8.41 5.83 16.70 9.17 9.31

0.54 0.37 0.61 0.56 0.57 0.60 0.40 0.36 0.57 0.58 0.60

5.0d 2.0d 1.3d 2.6e 4.0e 4.4e,f 2.5e 1.9e 5.7e 3.3e 4.0e

8.6 × 10−4 h 1.8 × 10−7 h 2.5 × 10−5h 3.0 × 10−4 h 9.0 × 10−3 h

306115 307115 308116 309117 310117

0.77 0.84 0.40 0.80 0.79 0.70 0.85 0.88 0.92 0.62 0.72

6.1 × 10−4 h 2.5 × 10−5 h 1.8 × 10−2 h 5.0 × 10−2 h

a

Determined via cyclic voltammetry using Fc/Fc+. bDetermined via cyclic voltammetry using Ag/Ag+. cEstimated from HOMO-Egopt. dPC61BM was used as the acceptor. ePC71BM was used as the acceptor. fAdditives were used; check reference for details. gObtained from bottom contact OFETs. h Obtained via SCLC measurements of fullerene/polymer films.

including replacing the hydrogen atom with a fluorine, replacing the ester group with a ketone, and varying the nature of the side chains, a PCE of 7.7% has been obtained for 319101 in a standard cell, whereas a PCE of 9.2% has been achieved using 316104 in an inverted structure. Although several examples have been reported as listed in Table 3, direct comparisons can be made only be made between the fluorine-containing polymers 311 and 316, the hydrogen-containing polymers 313 and 318, and the two-dimensional polymers 324 and 325. The LUMO level of 311 is 0.3 eV lower than that of 316, but the HOMO levels are similar. As are result, 311 has a narrower band gap and a broader absorption profile. Additionally, the molecular weight of 311 was twice as high as 316. Nonetheless, 316 had an better performance overall in OPVs with a PCE of 6.1% and an EQE exceeding 60% at the maximum solar flux (680 nm),106 while the best device based on 311 had a PCE of 4.2% and an EQE around 50% at the maximum solar flux under similar conditions.118 In comparison to 318, the BDF analogue 313 had a lower HOMO (−5.27 vs −5.12 eV) and LUMO (−3.69 vs −3.55 eV), a slightly higher molecular weight, and a hole mobility an order of

However, a direct comparison is misleading as the molecular weight of 308 is also significantly higher, which impacts film formation and morphology. Additionally, 308 exhibited a broad absorption that results in high EQE (∼50%) and internal quantum efficiency (IQE) (∼80%). In donor−acceptor copolymers, the LUMO level is largely determined by the strength of the electron-accepting moiety.11 Thus, 309 and 310 both have lower LUMO levels than their non-fluorinated analogues 301 and 302. Interestingly, both 309 and 310 have similar HOMO and LUMO levels, despite the heteroatom substitition. However, 310 has a broader absorption profile and a higher hole mobility than 309. When blended with fullerene, 310 produced smoother polymer films with smaller domain sizes than 309; as a result, it exhibited a better OPV performance.117 Among BDT-based polymers, Yu’s copolymers with thieno[3,4-b]thiophene (TT) have exhibited the highest PCEs to date.102,106 These polymers feature a low band gap, good hole mobility, and excellent solubility in organic solvents.102 As a result, OPVs containing these materials have both high EQE and IQE.101,102,106 After several optimizations on the TT moiety 7258

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Figure 4. Benzodichalogenophene donor−acceptor copolymers with thieno[3,4-b]thiophene.

Table 4. Data for Benzodichalogenophene Donor−Acceptor Copolymers with Thieno[3,4-b]thiophene polymer

Mn [kDa]

PDI

λmax [nm] (film)

HOMO [eV]

LUMO [eV]

692

−5.03

−3.63

1.7

671

−5.07a

−3.61a

44.0

2.1

642

5.27a

3.69a

314118

45.3

2.4

700

−5.11a

−3.60a

315122

14.0

3.0

−4.90

−3.18

316106

19.3

1.3

−5.1a

−3.31a

317102

97.5

2.1

−5.15a

−3.31a

318123 319101 320121 321121 322121 323121 324122

30.0

1.8

74.0 15.0 41.0 52.1 4.2

2.1 1.7 2.7 1.8 3.6

672 702 712 697 660/700

−5.12a −5.22a −5.00a −5.05a −5.04a −5.04a −5.21a

−3.55a −3.45a −3.13a −3.27a −3.26a −3.26a −3.20b

325120 326124

32.4 20.0

2.7 3.2

692

−5.28a −5.11

−3.67b −3.25

118

40.7

1.7

312118

61.0

313119

311

627/682

a

a

VOC [V] 0.56 0.54 0.63 0.61 0.68 0.66 0.63 0.63 0.63 0.63 0.76 0.74 0.75 0.74 0.70 0.76 0.64 0.59 0.64 0.64 0.79 0.78 0.79 0.74

JSC [mA cm−2] 6.41 13.13 12.04 15.75 6.39 10.45 12.51 13.88 12.51 13.88 9.20 13.0 13.52 14.50 14.7 15.2 14.3 15.4 16.8 14.6 9.79 13.04 16.74 17.48

FF 53.42 60.03 54.78 53.8 54.2 63.8 57.34 59.81 57.34 59.70 0.45 0.61 59.23 68.97 0.64 0.67 0.66 0.54 0.64 0.66 58.76 61.55 0.63 58.7

PCE [%]

μh [cm2 V−1 s−1]

e

1.9 4.2e,f 4.2e 5.2e,f 2.4e 4.4e,f 4.5e 5.2e,f 4.5e 5.2e,f 3.1d 6.1d,f 6.0e 7.4e,f 6.6e 7.7e 5.7e,f 5.5e,f 6.9e,f 6.1e,f 4.5 6.2e,f 8.8e,f 7.6e,f

5.4 × 10−3 h

5.8 × 10−2 g 7.7 × 10−4 g

2.4 × 10−4 g 7.0 × 10−4 h 4.0 × 10−4h 3.2 × 10−4h 1.4 × 10−3 h 3.5 × 10−4 h 0.128h 2.8 × 10−4 h 0.27g 0.011h

a

Determined via cyclic voltammetry using Fc/Fc+. bDetermined via cyclic voltammetry using Ag/Ag+. cEstimated from HOMO-Egopt. dPC61BM was used as the acceptor. ePC71BM was used as the acceptor. fAdditives were used; see reference for details. gObtained from bottom contact OFETs. h Obtained via SCLC measurements of fullerene/polymer films.

The two-dimensional polymer 324 had a similar HOMO (−5.21 vs −5.28 eV), a higher LUMO (−3.20 vs −3.67 eV), and a significantly lower molecular weight than the sulfur analogue 325. The BDF analogue 324 had a hole mobility that was 3 orders of magnitude higher than the BDT analogue 325. Although this two-dimensional BDF polymer 324 had a PCE of

magnitude higher than 318. Notwithstanding these differences, 318 outperformed 313 in all categories, resulting in a PCE of 6.6%. Interestingly, the PCE of 313 almost doubled when 3% diiodooctane was used to prepare the films,119 suggesting that optimization of the film morphology may result in better performances within these BDF polymers. 7259

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Figure 5. Benzodichalogenophene donor−acceptor copolymers with amide- and diimide-based comonomers.

Table 5. Data for Benzodichalogenophene Donor−Acceptor Copolymers with Amide- and Diimide-Based Comonomers HOMO [eV]

LUMO [eV]

VOC [V]

JSC [mA cm−2]

FF

PCE [%]

μh [cm2 V−1 s−1]

−5.41b

−3.44c

−5.29b

−3.44c

2.4

−5.21b

−3.43c

3.1

−5.29b

−3.41c

331126

12.7

3.2

−5.29b

−3.44c

332127 333127 334128

17.4 33.1 90.0

1.9 2.3 2.7

403/612 415/653 635/425

−5.7a −5.7a −5.74a

−3.8a −3.8a −3.91a

335129,130

61.4

3.0

634

−5.35

−3.40

11.2 11.2 11.1 11.2 3.3 7.8 3.5 11.1 3.1 8.7 1.3 1.6 2.8 3.15 11.2

0.68 0.57 0.66 0.61 0.51 0.42 0.44 0.58 0.39 0.48 0.36 0.49 0.61 0.60 0.55

7.4e 6.6e,f 7.0e 7.1e,f 1.5e 3.0e,f 1.5e 6.5e,f 1.2e 4.7e,f 0.30d 0.59d 1.3d 1.4e 6.0d,f

2.3 × 10−4 h 5.9 × 10−5 f,h 1.9 × 10−4 h 2.5 × 10−5 e,h 1.9 × 10−4 h

16.4

0.97 0.97 0.94 0.94 0.90 0.90 1.0 1.0 1.0 0.98 0.64 0.74 0.74 0.73 0.97

336129

61.8

2.1

672

−5.33

−3.48

0.89

13.7

0.56

6.8d,f

Mn [kDa]

PDI

327125

13.4

3.0

328125

14.8

3.3

329126

14.5

330126

polymer

λmax [nm] (film)

685

4.7 × 10−4 h 2.4 × 10−4 h

1.0g 2.0 × 10−3 h 1.4 × 10−2 h

a Determined via cyclic voltammetry using Fc/Fc+. bDetermined via ultraviolet photelectron spectroscopy. cEstimated from HOMO-Egopt. dPC61BM was used as the acceptor. ePC71BM was used as the acceptor. fAdditives were used; check reference for details. gObtained from bottom contact OFETs. hObtained via SCLC measurements of fullerene/polymer films.

levels of the selenium-containing polymers were all the same and slightly lower than the reported values for the sulfur analogue. The increased selenium content was beneficial in improving charge carrier transport within these polymers as 322 had the highest hole mobility of the series with a PCE of 6.9% obtained through the use of additives. The morphology of all the polymer films was studied in detail, and it was found that 322 had the smallest and most uniform domain size, which in combination with its broad absorption and high hole mobiliy led to the best OPV performance.121 These results suggest that introducing selenium into the polymer backbone is a promising approach to designing OPV materials. Copolymers composed of BDT and the electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety represent another efficient system for use in OPVs. Like BDT, TPD also has a symmetric, planar structure that is beneficial for promoting intrachain and interchain interactions along and between coplanar polymer chains.131 TPD is a strong electron-with-

6.2%, which is the best for the group, the thiophene analogue 325 performed better with a PCE of 8.8%.120 On the basis of the extensive morphological studies presented in the paper along with fact that 325 performed better than 326, which does not contain fluorine, the authors concluded that the exceptional performance of 325 is a result of the optimized energy levels of the polymer along with improvement in the polymer/fullerene interactions and nanoscale morphology of the polymer film due to the high fluorine content.120 These results suggest that additional optimization of 324 may be beneficial to see if the performance can be further improved. Yu et al. evaluted the impact of selenium substitution within the BDT moiety, the TT moiety, or both.121 In comparison to the all-sulfur-containing polymer 320, the absoprtion spectra for the selenium-containing polymers exhibited a bathochromic shift and 322, with the highest selenium content, had the highest absorption maxium, while there was only a slight difference in the absorption spectra of 321 and 323. Interestingly, the energy 7260

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levels of the polymers were similar (−5.35 vs −5.33 eV), but the LUMO of 336 was slightly lower than that of 355 (−3.40 vs −3.48 eV), and as a result, 336 had a smaller electrochemical band gap (1.95 vs 1.85 eV). OFETs based on 336 had a mobility of ∼1.0 cm2 V−1 s−1, whereas those made from 335 only had a mobility of 0.014 cm2 V−1 s−1. A device configuration of (ITO/ PEDOT:PSS/polymer:PC71BM(1:4)/Bis-C60/Ag using a 1:4 polymer:fullerene (w/w) ratio resulted in a PCE of 6.0% for 335 and 6.8% for 336.129 The devices from both polymers showed excellent stability under ambient conditions. These results suppport the hypothesis that introducing selenium is beneficial in producing better polymers for use in OPVs.

drawing group, which when combined with BDT affords narrow band gap polymers with low-lying HOMO levels. Lastly, the side chains are readily varied to improve the solubility and processability of the resulting copolymers.131,132 The BDT− TPD copolymer 328 was first prepared by Leclerc et al., who obtained a PCE of 5.5%.131 Shortly thereafter, Fréchet et al. reported several analogues and obtained a PCE of 6.8% for 328 with the use of additives.132 Recently, Beaujuge et al. reported the synthesis of both the BDT−TPD 328 and BDF−TPD copolymers 327.125 Both polymers have similar molecular weights, which is ideal for comparison. Polymer 327 had a deeper HOMO level than 328 (−5.4 eV vs −5.29 eV), but the same LUMO level; as a result, 327 has a wider band gap (1.97 vs 1.85 eV). However, 327 has one of the highest reported PCE’s (7.4%) for a BDF-based polymer. This is due to the combination of the high VOC and JSC for this polymer. The thiophene analogue also had high VOC and JSC, resulting in a PCE of 7.1%.125 The superior performance of the furan analogue is a result of its higher EQE in 360−600 nm range, combined with its higher hole mobility. Collectively, these results indicate that the combination of TPD and BDF is very promising for developing OPV materials. Beaujuge et al. have also evaluated the impact of heteroatom substitution on the two-dimentional BDT monomer.126 In contrast to the previous set of materials, the furan-substituted polymer 329 has the smallest band gap, whereas the thiophene analogue 330 has the largest. The selenophene analogue 331 has the same HOMO level as the thiophene analogue while the furan analogue has a slightly higher HOMO. The most remarkable aspect of these materials is that they all yielded VOC’s of around 1.0 when used in OPVs with PC71BM.126 Of the series, the thiophene analogue 330 gave the highest performance with a PCE of 6.5%. The diminished performance of the other polymers in this series is result of the differences in their morphology.126 Jeffries-EL et al. utilized a iodine-mediated cyclization reaction to synthesize a dithienylbenzodifuran monomer.127 This approach offers the benefits of easily prepared starting materials, inexpensive catalysts, and the opportunity to generate a variety of new substituted BDFs from a common intermediate via crosscoupling reactions. Additionally, the alkyl chains on the flanking thiophene enhance the solubility of the resulting polymers, although care must be taken regarding their placement. The molecular weight of 333 is higher than that of 332 as a result of the steric hindrance of the “outward-facing” side chains or those closest to the isoinidgo moiety. The thin film absorption spectrum of 333 is also bathochromically shifted relative to 332 as a result of the more planar backbone. The analogous BDT polymer 334 had a similar HOMO level and a slightly lower LUMO level (−3.9 vs −3.8 eV) than the BDF polymers.128 All of these polymers exhibited poor performances when incorporated in OPVs with PC61BM. This is likely a result of the small offset between the LUMO of the donor polymer and the LUMO of the fullerene. The indacenodithiophene (IDT) moiety is a promising electron-donating group due to its planar structure which can enhance charge carrier mobility. A copolymer of IDT and benzothiadiazole (BT) units 335 has a reported hole mobility approaching ∼1.0 cm2 V−1 s−1 when used in OFETs and a PCE of 6.4% in a device with PC61BM.130,133 The synthesis of the analogous selenium-containing polymer has also been reported.129,133,134 A comparison of 335 and 336 of similar molecular weights shows that the absorbance maximum of 336 is red-shifted 38 nm relative to 335 (634 vs 672 nm). The HOMO

Figure 6. Indacenodichalcogenophene copolymers.

4. ELECTRON-DEFICIENT MONOMERS 4.1. Diketopyrrolopyrroles. While the electron-withdrawing diketopyrrolopyrrole (DPP) unit has been incoporated into polymers since the early 90s,135 the first reported application of a DPP-containing polymer in OPVs has only come within the last six years.136 When the DPP unit is substituted with 6-membered rings at the 3- and 6-positions, steric interactions force the molecule to twist, causing a shortened conjugation length. As a result, 5-membered rings are preferred, as they alleviate steric hindrance and extend the effective conjugation length through increased coplanarity. Heteroatom substitution of the 5membered rings flanking the DPP core was first reported in 2010 by Fréchet et al. with substitution of the thiophene (TDPP) with furan (FDPP) to give 402137 as a comparison to the TDPP− thiophene copolymer 401(Figure 7).138 402 showed slightly higher weight relative to 401, along with a deeper HOMO (−5.4 vs −5.2 eV) and LUMO (−3.80 vs −3.6 eV) and a bathochromically shifted absorption (789 vs 850 nm). A device configuration of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al resulted in similar parameters, with the exception of VOC, which is likely due to the decreased HOMO of 402. Unfortunately, a direct comparison of 401 and 402 is misleading due to the different alkyl chains on the DPP units. When considering the degree of polymerization (DPn), it appears 402 is nearly twice the chain length of 401 (114.8 vs 64.9), which results in higher charge transfer along the polymer backbone.139 Shortly after, a study making direct comparisons between thiophene and furan was published by Janssen et al.140 Both FDPP and TDPP were polymerized with thiophene and furan as donor units, and all weights and degrees of polymerization were similar. The furan copolymers, 403 and 405, possessed very similar electrochemical and optical properties along with device parameters yet show a significant difference in PCEs (1.9% vs 2.6%). This can likely be attributed to the higher hole and electron mobilities of 405. 7261

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Figure 7. Structures of DPP- and APD-based materials.

with selenophene substitution having the largest effect. The fabrication of OPVs with the architecture ITO/PEDOT:PSS/ polymer:PC71BM/Ca/Al showed a decrease in VOC across the series of polymers that correlates to the destabilization of the HOMO. 410 and 411 showed increased JSC compared to 409 due to their smaller band gaps and higher crystallinity. The high PCE of 411 can be attributed to its improved phase separation and noticeable fibril features that were not present in 409 or 410 when the BHJ blends were examined under transmission electron microscopy, along with a more planar polymer backbone that can enhance intermolecular π−π stacking. When FDPP, TDPP, and SeDPP were polymerized with thienylbenzodithiophene, the respective resulting polymers 412, 413, and 414 show the same trend of improved efficiencies going from FDPP to TDPP to SeDPP along with a diminishing band gap.143 While 414 showed a slightly lower VOC than 412 or 413, the very high photocurrent of 16.8 mA cm−2 led to a PCE of 7.2% in single junction OPVs with the architecture ITO/ PEDOT:PSS/polymer:PC71BM/Ca/Al. The increase in photocurrent and subsequent increase in PCE are believed to be due to higher mobilities, preferential morphology in the blended films,

When comparing the thiophene copolymers, 404 and 406, the same patterns hold, with 406 having a slightly lower VOC of 0.68 V than the 0.73 V of 404. The electron and hole mobilities of 401 and 406 are independent of Mn, suggesting the difference in PCE (2.7 vs 4.7%) between the two is due to morphology of the BHJ blends and not due differences in chain length. Polymerization with thiophene-flanked BDF gave 407 and 408 with similar optical and electrochemical properties.141 Substitution of the S for O resulted in a lower JSC (7.4 mA cm−2), yet higher VOC (0.69 V), FF (0.60), and PCE (2.9%). One possible explanation for the reduced FF of 408 is the higher series resistance (16.3 Ω cm2) and lower shunt resistance (495.2 Ω cm2) compared to 407 (13.1 and 695.0 Ω cm2, respectively). Unfortunately, much like 401 and 402, a direct comparison of the two materials is misleading due to the drastically different DPn of 407 and 408 (40 vs 17). Further investigation of heteroatom influence has led to the incorporation of selenophene into the DPP (SeDPP) unit. Polymerization of FDPP, TDPP, and SeDPP with tetrathienoacenes have afforded 409, 410, and 411, respectively, all with similar weights and DPn.142 The progression of furan to thiophene to selenophene is shown to narrow the band gap, 7262

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Table 6. DPP- and APD-Based Materials polymer

Mna [kDa]

PDI

DPn

λfilm max [nm]

HOMO [eV]

LUMO [eV]

VOC [V]

JSC [mA cm−2]

FF

PCEc [%]

146

54.0 66.0 13.0 15.0 16.0 10.0 28.9 9.5 20.6 28.1 26.3 35.2 40.7 38.4 13.0b 11.0 13.0

3.2 2.1 2.8 3.5 2.6 2.4 1.9 2.5 3.0 3.2 2.9 2.1 2.1 2.1 2.6 1.9 2.3

64.9 114.9 16.6 18.8 19.6 12.0 40 17 15.1 20.2 17.7 29.8 33.5 29.4 18.3 15.9 17.2

850 789 775 800 790

−5.17 −5.4 −5.32 −5.29 −5.28 −5.30 −5.5 −5.6 −5.31 −5.26 −5.19 −5.26 −5.30 −5.25 −5.56 −5.65 −5.51

−3.61 −3.8 −3.85 −3.84 −3.86 −3.74 −3.7 −3.8 −3.72 −3.74 −3.76 −3.64 −3.63 −3.70 −3.75 −3.79 −3.75

0.65 0.74 0.66 0.73 0.64 0.68 0.69 0.66 0.63 0.60 0.58 0.77 0.73 0.69 0.85

11.8 11.2 5.4 4.3 6.4 6.3 7.0 7.4 10.6 11.8 15.6 10.9 13.7 16.8 9.8

0.60 0.60 0.54 0.63 0.63 0.63 0.60 0.47 0.58 0.64 0.62 0.56 0.65 0.62 0.66

4.7 5.0 1.9 2.0 2.6 2.7 2.9 2.3 3.8 4.6 5.7 4.7 6.5 7.2 5.5

401 402147 403140 404140 405140 406140 407141 408141 409142 410142 411142 412143 413143 414143 415144,145 416145 417145

774 752 792 800 821 757 769 808 610 581 635

a

All weights were obtained using size exclusion chromatography (SEC) with chloroform as eluent, unless otherwise noted. bObtained from hightemperature SEC using 1,2,4-trichlorobenenze at 140 °C as eluent. cMaximum PCE reported.

respectively; however, these studies were limited to mostly to OLED applications.154−159 More recently, Seferos et al. synthesized an analogous set of CPDT-containing donor− acceptor copolymers and compared how 2,1,3-benzotelluradiazole (BTe) incorporation altered the optical and electronic properties when compared with BT and BSe.160 While no OPVs were made from the aforementioned copolymers, the results show that BTe incorporation gives significantly narrower band gaps but suggests it may also lead to reduced charge separation. The first report of benzochalcogenodiazole (BC) heteroatom substitution in OPVs was by Leclerc et al. in 2008 when they copolymerized an alkylated carbazole comonomer with thiophene-flanked BO, giving polymer 418.161 The effect of oxygen heteroatom substitution was directly compared with the analogous BT polymer 419 as well as in the pyridylthiazoleand pyridyloxazole-containing copolymers 420 and 421. They expected this substitution to exhibit polymers with both deeper HOMO and LUMO levels, resulting in better air stability and a higher VOC. While no significant difference was observed between the HOMO levels between the analogous polymers, the oxygen-containing polymers 418 and 420 dislpayed a VOC of ∼0.1 V greater than the sulfur-containing polymers 419 and 421 when used in OPVs. Even with this increase in VOC, devices made from 418 still lagged in overall performance, giving a PCE of 2.4% compared with those of 419 PCE of 3.6% primarily due to a JSC around half of what 419 displayed. This result could possibly be explained by 419 displaying higher structural organization when polymer thin films were examined by X-ray diffraction. One of the workhorses among DA copolymers is composed of BT and PCPDTBT (422), with PCEs reported of up to 5.5− 6.1% for BHJ single-junction cells.150,151,162 A concurrent study of both 422 and its BO analogue, 423, was undertaken by Janssen et al. It was found that substituting a BO for a BT in the polymer backbone affects the HOMO level, observed by a relative decrease of the oxidation potential by 0.23 eV, as well as the LUMO level, which experienced a drop of 0.15 eV.153 This resulted in a VOC increase 0.12 eV in OPVs for the oxygensubstituted polymer 423 over the BT analogue 422. This boost to VOC, along with a simultaneous increase in fill factor, results in a better PCE for solar cells based on 423 (2.5%) than for those of

broader absorption range, and higher EQE when compared to 412 and 413. 4.2. Aryl[3,4-c]pyrrole Diones. Aryl[3,4-c]pyrrole dione is an electron-deficient monomer that has gained popularity, with the thiophene-based thieno[3,4-c]pyrrole dione (TPD) unit being the most widely studied. The synthesis of the BDT−TPD copolymer 415 by Leclerc et al. was one of the early uses of TPD as an acceptor in copolymers.144 Very little has been done, so far, on the effect of heterosubstitution of the 5-membered aryl ring, with only one instance of furo- (FPD) or selenopheno- (SePD) copolymers with BDT being reported, 416 and 417, respectively.145 Relative to 415, a hypsochromic shift was observed in 416 and a bathochromic shift was seen in 417, yet the optical effect of changing the heteroatom was small in both cases (∼0.1 eV). 416 showed a stabilization of the HOMO and destabilization of the LUMO, while 417 showed a destabilization of the HOMO but, unlike many selenophene-containing compounds, showed no stabilization of the LUMO. Further studies with both FPD and SePD are needed to gauge their utility in OPVs. 4.3. 2,1,3-Benzochalcogenodiazoles. As an electrondeficient monomer, 2,1,3-benzothiadiazole (BT) has seen widespread use in high-efficiency conjugated polymer-based OPVs over the past decade.148−151 The popularity of BT is not only due to number of beneficial, intrinsic properties related to its electron-withdrawing nature and its ability to adopt the quinoid resonance form but also its short synthetic pathway and commercial availability.152 Through some early theoretical estimations, Janssen et al. suggested that devices with an active layer composed of poly[2,6-(4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and PC71BM are limited to a theoretical efficiency of ∼6.2% due to a rather narrow band gap of 1.40 eV and the relative position of the LUMO level.153 They also illustrated that widening the band gap and manipulating the energy levels, perhaps by heteroatom substitution in BT, could result in better device performance. Several early studies of the effect of heteroatom subsitution into BT were done by replacing the sulfur with either oxygen or selenium, giving 2,1,3benzooxadiazole (BO) or 2,1,3-benzoselenadiazole (BSe), 7263

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422 (1.9%), despite an increase in the JSC of 422-based devices. However, the lower molecular weight 422 may also contribute to the poorer performance of its devices, making this comparison less than definitive. In an attempt to improve the photon harvesting capabilities of CPDTBT, Yang et al. substituted BSe for BT, giving polymer 424.163 While they succeeded in obtaining lower optical band gaps, 1.39 eV for 424 vs 1.43 eV for 422, this resulted in a detrimental decrease in VOC to 0.52 V for 424-based devices. These devices also displayed a poor FF (0.34), tentatively attributed to imbalanced charge transport as measured by the SCLC method, which may have ultimately lead to much lower PCE (0.9%) than those of their oxygen- and sulfur-based counterparts. Shim, Yoo, and co-workers also reported similar OPVs of 424.164 Perhaps a more apt comparison of the BHJ-OPV devices with two analogous copolymers of BO (425) and BT (426) is with the CPDT analogue dithienosilole (DTS), as they have similar molecular weights, from two separate reports by Bazan et al.151,165 Influenced by the presence of oxygen, 425 has a more stabilized HOMO than sulfur-bearing 426 (−5.50 vs −5.35 eV), leading to a 0.11 eV improvement in VOC when the polymers are used with PC71BM in the active layer of BHJ-OPVs. However, after optimization of the polymer/PC71BM ratio, solvent, and solvent additives, devices made with 426 provided a better JSC (17.3 mA cm−2) than those from 425 (13.0 mA cm−2), leading to a better overall device performance (5.9 vs 4.9%). In a more recent study on fullerene mixing behavior with DA copolymers, Bazan et al. report that BO/CPDT and BO/DTS copolymers show reduced donor/acceptor interfaces when compared with similar BT-based copolymers as a result of poorer mixing of BO copolymers with PC71BM.166 A similar trend holds for OPVs using copolymers of thiophene and alkoxy-functionalized BO (427) and BT (428) with PC61BM, where the devices of 427 have a higher VOC by 0.15 eV, but those composed of 428 have better overall performance resulting from greater JSC and better FF.167,168 One of the few examples of heteroatom substitution in BCs that compares the effects of oxygen, sulfur, and selenium was done by studying OPVs consisting of copolymers of hexylthiophene-flanked BCs and hexylphenothiazine by Lin et al.169 In this particular set of copolymers, there was not a significant difference in the HOMO levels, with only a 0.05 eV drop from 431 to 430 and again from 430 to 429, resulting in a negligible difference between the VOC of the subsequent OPVs based on a 1:1 w/w blend polymers and PC61BM. The only significant difference observed between the OPVs was in the JSC values, of which the highest was fabricated from the BT-based polymer 430. Unfortunately, only optimization of the 430-based devices was attempted, making it difficult to draw conclusions about the effect of heteroatom substitution in BCs in this case. A fair number of examples exist of analogous BC copolymers with the widely used donor molecule BDT. Of these, two different sets of copolymers are based on thiophene-flanked BO and BT: 432 (BO) and 433 (BT) with 4,8-alkoxy-BDT and 434 (BO) and 435 (BT) with 4,8-alkylthienyl-BDT.170−172 While oxygen-containing 434 has an expectantly lower lying HOMO than its sulfur analogue 435 (−5.25 vs −5.15 eV), both 432 and 433 have nearly identical HOMO levels (ca. −5.20 eV). The similarity 432 and 433 may not be completely valid, as both sets of polymers display a higher VOC for the BO-based polymers than for the BT-based polymers by ∼0.1 V when fabricated into OPVs. Interestingly, OPVs of both BO-based polymers also experienced higher JSC and gave higher PCEs. While analogous

polymers 432 and 433 do have the same backbone, they differ in their alkyl chain length, have vastly different molecular weights, and are reported by different research groups. However, 434 and 435 were published in the same study, and of the two, the BObased copolymer was found to have better hole mobility via the SCLC method as well as a better nanostructure blend morphology, implying more favorable percolation pathways. When comparing the effects of selenium substitution into BT−BDT copolymers, 437 (BSe/BDT) displayed a narrower electrochemical band gap than 436 (1.55 vs 1.91 eV) owing to a concurrent increase in the HOMO and a decrease in the LUMO.173 Despite the broader absorbance that this smaller band gap provides by the selenium-substituted BC in 437, it performed worse in every category when fabricated into OPVs than did 436. Another noteworthy comparison of selenium substitution in BC/BDT polymers lies in 438 and 439, thiophene-flanked BT and BSe copolymers, respectively.174 Again, devices from a 439/PC71BM blended active layer gave a lower VOC of about 0.1 V when compared with those of 438, coinciding with a ∼0.1 eV difference in the HOMO levels of each polymer. The performances of both devices are roughly the same with PCEs of 5.01% for 438-based devices without solvent additives and 5.18% of 439-based devices with 3% DIO as a solvent additive. The advantage in hole mobility, obtained from fabricated OFETs, and in JSC also goes to the devices based on the BSe-containing polymer. The above-mentioned trends in BC/ BDT polymeric devices also generally hold in an analogous series OPVs fabricated from alkoxy-functionalized BO (440), BT (441), and BSe (442) copolymers with 4,8-bis[(2-ethylhexyl)oxy]-functionalized BDT.175−177 In these devices, the Vocs decrease by ∼0.1 V from BO to BT and from BT to BSe, with those made from 440 outperforming those of either 441 or 442, helped by improved short-circuit currents. Another analogous pair of BO/BT copolymers with a naphtho[1,2-b:5,6-b′]dithiophene (zNDT) donor display a performance pattern similar to those of copolymers with BDT when incorporated into OPVs as reported by Li et al.178 True to form, devices made from the BO−zNDT copolymer (443) had a more stabilized HOMO and a larger VOC as compared with those of BT−zNDT (444), on par with other similar BDT-containing analogues. After optimization of solvent additives (DIO), thermal annealing conditions, and polymer/PC70BM blend ratios, 443 displayed higher currents (10.35 vs 8.88 mA cm−2), FFs (0.66 vs 0.61), and hole mobility (4.4 × 10−2 vs 3.3 × 10−3 cm2 V−1 s−1), resulting in significantly higher PCEs (5.1 vs 3.2%). The improved performance of 443-based devices is in agreement with improved surface roughness and a higher degree of crystallinity as compared with 444; however, the significantly higher molecular weight of 443 may also play a role in the improvement of these device properties as well. Conversely, it was also found by Li et al. that when switching from BDT to BDF, OPVs from a copolymer of BT and BDF (445) outperformed those of a copolymer of BO and BDF (446).179 This performance enhancement (4.5% vs 2.9%) is despite 445-based devices displaying the larger VOC typically seen with BO incorporation and the higher molecular weight of 445 over 446. It was found by AFM and TEM analysis that the polymer/PC71BM blend showed evidence of a nanoscale bicontinuous phase separation morphology and more ideal surface roughness for 446 that was significantly lacking in 445. These morphological effects could be largely responsible for the higher hole mobility values seen in 446 blends by the SCLC method and the larger JSC values observed in 446-based OPVs. 7264

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Figure 8. Structures of benzochalcogenodiazoles copolymers.

4.4. Benzobisazoles. Jointly known as benzobisazoles, benzobisoxazole and benzobisthiazole are electron-deficient heterocycles that feature a planar fused structure that facilitates π-stacking and intramolecular interactions. The incorporation of benzobisazoles into conjugated materials is beneficial as it can enhance charge transport, photoluminescence, and third-order nonlinear optical properties.180−183 Since the benzobisazole moiety is typically formed via condensation chemistry, the synthesis often requires strong acids or oxidants and high temperatures, limiting the types of substituents that can be incorporated.184,185 Accordingly, the synthesis of a donor−

acceptor−donor triad composed of a benzobisazole ring sandwiched between two alkylthiophenes is promising as it enables the synthesis of conjugated polymers using transitionmetal-catalyzed cross-coupling reactions.182,183,186−188 Both the Jeffries-EL and Jenekhe groups have synthesized benzobisazole− quarterthiophene copolymers and benzobisazole−thiophene− benzo[1,2-b:4,5-b′]dithiophene terpolymers. 175−178 The LUMO levels for the isoelectronic quarterthiophene polymers 447 and 448 are the same, and the HOMO level for the sulfur analogue is slighty lower (−5.2 vs −5.3 eV). The performance of these polymers in OPVs was modest, with the oxygen analogue 7265

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Table 7. Properties of Benzochalcogenodiazoles Copolymers polymer 161

418 419161 420161 421161 422153 423153 424163 425151 426165 427167 428168 429169 430169 431169 432170 433171 434172 435172 436173 437173 438174 439174 440175 441176 442177 443178 444178 445179 446179

Mn [kDa] 26.0 36.0 4.5 4.0 15.0 21.0 22.0 41.0 44.0 35.2 30.6 38.5 40.7 51.3 23.9 62.9 41.3 16.7 18.5 16.9 22.0 20.1 62.0 25.7 20.0 50.0 32.8 27.0 7.0

PDI 2.0 1.5 1.3 1.4 1.5 2.0 1.6 3.0 2.0 1.5 1.7 1.7 1.9 2.0 2.8 1.5 1.7 1.5 1.7 2.1 2.1 2.1 4.1 2.7 2.4 1.5 1.8 1.2 2.0

λfilm max [nm]

HOMO [eV]

LUMO [eV]

−5.47 −5.45a −5.55a −5.53a +0.16h −0.07h −4.90 −5.50a −5.36 −5.47 −5.41 −5.47a −5.42a −5.38a −5.20a −5.23a −5.25a −5.15a −5.10 −4.88 −5.26a −5.18a −5.27a −5.17a −5.20 −5.30 −5.15 −5.19 −5.11

−3.65 −3.60a −3.93a −3.80a −1.52i −1.67i −3.28 −3.7a −3.55 −3.59 −3.54 −3.60a −3.47a −3.55a −3.60a −3.57a −3.28a −3.30a −3.19 −3.33 −3.50a −3.48a −3.16a −3.61a −3.51 −3.38 −3.44 −3.49 −3.38

a

783

662 652 553 552 582 590 606 592 640 591 641 646 680 600 569 637 639 580 583

a

VOC [V]

JSC [mA cm−2]

0.96 0.86 0.85 0.71 0.66 0.78 0.52 0.68 0.57 1.02 0.87 0.69 0.67 0.65 0.75 0.68 0.84 0.75 0.68 0.55 0.70 0.60 0.86 0.76 0.70 0.60 0.74 0.82 0.69

−3.7 −6.8 −1.4 −2.9 6.5 5.4 5.0 13.0 17.3 9.0 10.5 1.24 1.92 1.43 5.25 3.48 11.45 10.29 2.97 1.05 10.43 13.58 10.4 8.69 9.37 8.88 10.46 5.04 9.87

FF 0.60 0.56 0.60 0.32 0.44 0.60 0.34 0.55 0.61 0.49 0.65 0.29 0.32 0.31 0.58 0.47 0.61 0.64 0.44 0.32 0.65 0.64 0.64 0.59 0.48 0.61 0.66 0.70 0.65

PCEh [%]

μh [cm2 V−1 s−1]

c

2.4 3.6c 0.8c 0.7c 1.9d 2.5d 0.9d 4.9d 5.9d 4.5c 5.9c 0.3c 0.4c 0.3c 2.9c 1.1c 5.9d 4.9d 0.90 0.2 5.0d 5.2d 5.7c 4.0d 3.2 3.2 5.1 2.9 4.5

4.0 × 10−4 f 3.0 × 10−4 f 6.0 × 10−5 f

2.5 × 10−5 g

9.7 × 10−4 g 2.2 × 10−5 g

2.0 × 10−5 g 4.9 × 10−1 g 2.4 × 10−1 g

2.6 × 10−3 f 5.4 × 10−3 f 1.7 × 10−4 g 7.1 × 10−3 g 1.1 × 10−5f 3.3 × 10−3 g 4.4 × 10−2 g 2.3 × 10−4 g 6.7 × 10−2 g

a Determined via cyclic voltammetry using Fc/Fc+. bEstimated from HOMO-Egopt. cPC61BM was used as the acceptor. dPC71BM was used as the acceptor. eMaximum PCE reported. fObtained from bottom contact OFETs. gObtained via SCLC measurements of fullerene/polymer films. g Maximum PCE reported. hElectrochemical oxidation potential vs Fc/Fc+. iElectrochemical oxidation potential vs Fc/Fc+.

Figure 9. Structures of benzobisazole terpolymers.

giving a better result than the sulfur-containing one.189 The absorbance maximum for 449 is red-shifted by almost 80 nm relative to 448. This is a result most likely due to the different location of the alkyl chains on the thiophenes flanking the benzobisazole ring. The alkyl chains on the flanking thiophenes of 449 are facing toward the benzobisazole (inside), and the alkylthiophenes on the bithiophene comonomer have a tail-totail orientation.186 Conversely, the thiophenes flanking the

benzobisazole ring of 448 are pointed away from it (outside), and the alkylthiophenes of the bithiophene comonomer have a headto-head orientation. While it is not clear if inside or outside orientation is more favorable for OPV performance, the head-tohead orientation of the bithiophene is known to decrease planarity and conjugation, increasing the optical band gap.190 Interestingly, 451 has a lower LUMO (−3.5 vs −3.2 eV) than 450 and a slightly higher HOMO (−5.2 vs −5.3 eV); as a result, 7266

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Table 8. Properties of Benzobisazole Terpolymers polymer 189

447 448189 449186 450191 451191 452192

Mn [kDa] 8.2 5.0 15.4 10.9 5.3 19.9

PDI 1.4 1.7 1.9 2.1 1.5 2.8

λmax [nm] (film)

HOMO [eV]

LUMO [eV]

475 462 540 487 518 512

−5.2 −5.3b −5.2a −5.3a −5.2a −5.71a

−3.1 −3.1c −3.3 −3.2 −3.5 −3.3

b

VOC [V]

c

0.73 0.53 0.66 0.71 0.75 0.81 0.63

Jsc [mA cm−2] 5.13 3.52 7.08 7.95 3.85 3.42 7.22

FF 0.31 0.32 0.45 0.52 0.49 0.63 0.64

PCE [%] d

1.1 0.6d 2.1d 2.8e 1.6e 1.8e 2.9e,f

μh [cm2 V−1 s−1] 8.9 × 10−5 g 1 × 10−4 g 2.0 × 10−3g

9.3 × 10−3

a

Determined via cyclic voltammetry using Fc/Fc+. bDetermined via ultraviolet photoelectron spectroscopy. cEstimated from HOMO-Egopt. PC61BM was used as the acceptor. ePC71BM was used as the acceptor. fObtained from bottom contact OFETs. gObtained via SCLC measurements of fullerene/polymer films.

d

were attributing benefits to simple heteroatom substitution while ignoring other structural differences within the polymers that they were comparing. It is important that we make appropriate comparisons before reaching conclusions. Similarly, noting the methods used to establish HOMO, LUMO, and band gap is also needed to make valid comparison. Accordingly, since device fabrication is also dependent on the technique, comparing data from different groups should always be done with caution. To that end, making analogous sets of materials is often the best approach as measurements vary less within laboratories. In summary, this use of heteroatom substitution is a promising approach toward altering the optical and electronic properties of conjugated polymers. Despite the large number of papers on the topic thus far, additional work is required in the design, synthesis, and device optimization of conjugated polymers for use in OPVs.

the band gap is much smaller. Nonetheless, the oxygen analogue 450 outperformed the sulfur one, 451 affording a PCE of 2.8% when utilized in an OPV with PC71BM.191 The benzobisthiazole polymer 452 gave the best performance of the series with a PCE of 2.9%, despite its wide band gap. To date, the benzobisoxazole analogue has not been reported.192

5. CONCLUSIONS AND OUTLOOK New developments in synthetic organic chemistry have provided increased access to functional heterocycles containing group 16 elements, facilitating the development of new conjugated polymers for use in organic electronics. Although furan-based conjugated polymers are potentially green materials, if they can be made from naturally derived sources, replacing sulfur with oxygen in all donor polymers is detrimental due to the widening of the band gap that occurs as a result of the stabilization of the HOMO level. Conversely, the introduction of selenium into all donor polymers is beneficial due to the stabilization of the LUMO level that occurs, reducing the polymers bang gap. Additionally, the intramolecular interactions between the selenium atoms induce order into the polymers, affording nanostructures that cannot be easily attained in thiophene-based polymers. This is a particularly promising venue toward controlling nanoscale self-assembly in conjugated polymers worth pursuing further. In donor−acceptor polymers, it is evident that the incorporation of oxygen improves the solubility of the resulting polymers, enabling the synthesis of high molecular weight materials. Additionally, in linear systems, replacing sulfur with oxygen generally lowers the HOMO level, increasing the VOC. Despite these benefits, when direct comparisons are made to the sulfur and selenium analogues, the oxygen-containing polymers tend to have the inferior performance. One contributing factor is the difference in the morphology of the polymers, suggesting that optimization of the polymer thin films could lead to better results. Although there are fewer examples of donor−acceptor copolymers containing selenium, these results indicate that there is some benefit toward incorporating selenium into the monomers. Specifically, the reduction of the band gap of the polymer and the favorable thin film morphology can contribute to good performance. However, given the modest gains and the challenging synthesis, the pursuit of some of these analogues may not be worthwhile, as the best conjugated polymers for use in OPVs are still based on thiophene. Nonetheless, additional work is required to optimize these new systems before conclusions can be reached. Moving forward one major consideration is consistency. During the course of preparing this Perspective we worked diligently to compare relevant examples from the literature to each other. It was alarming to find that in many papers authors

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest. Biographies

Malika Jeffries-EL is an Associate Professor of Chemistry at Iowa State University. She received BA degrees in Chemistry and Africana Studies from Wellesley College and Master’s and Ph.D degrees in chemistry from The George Washington University. After working as a postdoctoral fellow under the direction of Professor Richard D. McCullough at Carnegie Mellon University, she joined the faculty in the Chemistry Department at Iowa State University. She has won numerous awards including the 3M nontenured faculty award (2008), the Lloyd Ferguson Award from the National Organization of Black Chemist and Chemical Engineers (2009), the NSF CAREER award (2009), the ACS-Women Chemist Committee Rising Star award (2012), and the Iota Sigma Pi Agnes Fay Morgan Award (2013). Her 7267

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research focuses on the development of organic semiconductors for use in photovoltaic cells, transistors, and light-emitting diodes.

Brandon Kobilka received his B.S. in Chemistry from the University of WisconsinMadison in 2005. In 2013, he received his Ph.D. in Chemistry from Iowa State University under the guidance of Prof. Malika Jeffries-EL where he investigated the synthesis of conjugated polymers for electronic applications. He is currently a postdoctoral researcher in the lab of Prof. Trisha Andrew at the University of WisconsinMadison.

Benjamin Hale received his B.S. in Chemistry from the University of Northern Iowa in 2009. He is currently a graduate student at Iowa State University under the guidance of Prof. Malika Jeffries-EL.

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ACKNOWLEDGMENTS We acknowledge the National Science Foundation (DMR0846607 and DMR-141088) for partial support of this work We thank Mr. Sean Bernard for designing the graphical abstract.

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REFERENCES

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dx.doi.org/10.1021/ma501236v | Macromolecules 2014, 47, 7253−7271