Challenges and Opportunities in the Development of Conjugated

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Challenges and Opportunities in the Development of Conjugated Block Copolymers for Photovoltaics Youngmin Lee† and Enrique D. Gomez*,†,‡ †

Department of Chemical Engineering and ‡Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Fully conjugated block copolymers that covalently link donor and acceptor units within the molecular structure have the potential to simultaneously control the mesoscale assembly and interfacial structure. As a consequence, utilizing block copolymers as the active layer provides a unique opportunity to tune charge and energy transfer processes while retaining a mesoscale scaffold for efficient charge extraction within photovoltaic devices. Recent progress in synthesis, structural characterization, modeling, and device fabrication suggest significant advances in block copolymer materials are in the near futureboth as model materials for systematic studies of charge and energy transfer and as high performance materials for photovoltaic devices. In this Perspective, we describe recent efforts, current challenges, and opportunities in the development of fully conjugated block copolymers for organic photovoltaics.

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INTRODUCTION Conjugated polymers have the potential to yield electronic materials with mechanical flexibility, solution processability, and chemical versatility.1−6 A chain backbone composed of contiguous sp2-hybridized carbon atoms creates the possibility of delocalization of π electrons and significant charge mobilities, to date up to 10 cm2/(V s).7−12 The combination of mechanical flexibility and charge conduction has led to the application of conjugated polymers as antistatic coatings. Related compounds, small molecule organic semiconductors, are employed in light-emitting diodes; other applications based on organic electronics, such as integrated circuits, sensors, and photovoltaics, are on the horizon.13−16 Unfortunately, one of the key advantages of polymeric semiconductors is also a key limitation. Weak intermolecular interactions, often governed by nonspecific van der Waals forces, allow for dissolution in a variety of organic solvents. Many organic semiconductors are specifically designed in such a manner; for example, side chains are tuned to impart solubility without severely disrupting crystallization and charge conduction. Such materials enable solution-based processing methods, such as screen printing, inkjet printing, and roll-to-toll casting, and consequently may beget applications in printed or woven electronics. Unfortunately, weak intermolecular forces also lead to a large amount of disorder, such that control of the microstructure at the molecular or mesoscale level is challenging. For instance, the coherence of the crystalline microstructure in conjugated polymers appears limited by thermal fluctuations.17−19 Lack of precise control of the structure near critical interfaces, such as at contacts or heterojunctions, severely limits the potential of organic and polymer semiconductors. One approach to enhance control of the microstructure at critical interfaces is to utilize covalent bonds. Incorporating © XXXX American Chemical Society

functional interfaces within the chemical structure of organic semiconductors provides an opportunity to define the microstructure at a molecular level without compromising solution processability. For example, block copolymer architectures can place the functional interface at the copolymer junction. Thus, the local environment of the interface is governed by the chemical composition of the junction. The development of block copolymers for optoelectronic applications has a long history, mainly focused on the control of the mesoscale structure. The microphase separation of block copolymers within thin films may lead to advances in lithography20,21 and control of the active layer morphology in organic light-emitting diodes22−24 and photovoltaics.25−35 Recently, the development of fully conjugated block copolymers has demonstrated the potential to control not only the mesoscale morphology but also the interfacial structure in organic optoelectronic devices.27,31,36−45 Fully conjugated, donor−acceptor block copolymers, where donor and acceptor blocks are covalently bonded together, offer a platform to examine fundamental questions regarding optoelectronic properties of conducting polymers. Furthermore, fully conjugated block copolymers have various advantages over donor−acceptor mixtures systems commonly employed in organic solar cells, such as directed mesoscale selfassembly with lengths scales on the order of the exciton diffusion length, pathways for charge extraction afforded by the morphology, an equilibrium microstructure that is not subject to coarsening with aging, and control of the molecular configuration and chemistry near the donor−acceptor interface. Despite the clear advantages of fully conjugated block Received: January 19, 2015 Revised: September 3, 2015

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advantages of controlling the morphology for this donor− acceptor system. Looking forward, tuning the chemistry at the junction of block copolymers, perhaps through bridging moieties, will be crucial to access the full potential of conjugated block copolymers in photovoltaic applications. In this Perspective, we focus on recent advances in fully conjugated block copolymers for photovoltaics while highlighting current challenges and opportunities. Recent reviews of the development of optoelectronic materials from block copolymers that are not fully conjugated can be found elsewhere.23,25−27,35,42,46−48 We begin with opportunities in refining the synthesis of fully conjugated donor−acceptor block copolymers and then highlight progress in predicting the selfassembly of semiflexible and rodlike block copolymers. We also describe the potential for interfacial control and close with an outlook on opportunities in the field.

copolymers for photovoltaic applications, only recently has significant device performance been demonstrated. When poly(3-hexylthiophene-2,5-diyl)-block-poly((9,9-bis(2-octyl)fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (P3HT-b-PFTBT) is utilized as the active layer, photovoltaic devices exhibit approximately 3% power conversion efficiency, 3 times higher than blend devices (Figure 1).37 Although open-circuit voltages are high, about 1.2

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PROGRESS IN THE SYNTHESIS OF FULLY CONJUGATED BLOCK COPOLYMERS Fully conjugated block copolymers have been prepared mainly through three strategies as shown in Figure 2: (i) Kumada catalyst transfer polycondensations (KCTP), also referred as Grignard metathesis polymerizations (GRIM), with sequential injection of monomers, (ii) KCTP/GRIM for the synthesis of the first block followed by a chain extension using crosscoupling polycondensations, such as Stille or Suzuki−Miyaura reactions, and (iii) coupling between donor and acceptor homopolymers, such as through click chemistry. The first approach yields materials with well-defined architectures and little homopolymer impurities but is limited in the number of compatible monomers for the KCTP/GRIM polymerization in particular, acceptors or high electron affinity moieties are challenging to incorporate. In contrast, the second approach relies on Stille or Suzuki−Miyaura coupling, resulting in a versatile synthesis; but, relying on a chain extension through a polycondensation creates difficulties in producing block copolymers with low dispersities free of homopolymer impurities that are not mixtures of diblocks and multiblocks. The third approach relies on coupling between premade donor and acceptor polymers, but unless both blocks are made through a polymerization with end-group control, the same heterogeneity in architecture of reaction products is possible as in the second approach. Clearly, opportunities abound in the synthesis of conjugated block copolymers. The quasi-living chain growth polymerization of regioregular poly(3-alkylthiophene-2,5-diyl)s (P3ATs),49,50 leading to narrow dispersities51 and well-defined end groups,52,53 makes KCTP/GRIM an ideal candidate for the synthesis of block copolymers. Through the sequential injection of monomers, fully conjugated P3AT block copolymers have been synthesized based on thiophenes or selenophenes incorporating a variety of side chains, such as butyl, octyl, ethylhexyl, and semifluorinated alkyl groups. For example, poly(3-hexylthiophene-2,5-diyl)block-poly(3-(2-ethylhexyl)thiophene-2,5-diyl) (P3HT-bP3EHT) was prepared by injecting 3HT and 3EHT monomers sequentially. P3HT-b-P3EHT with symmetric compositions exhibit fibril structures and reduced crystallinity of P3HT.54 Poly(3-butylthiophene-2,5-diyl)-block-poly(3-octylthiophene2,5-diyl) consists of two crystalline blocks. The polymer again exhibits fibril-like self-assembly, in both solution and film, but with a distinct crystal structure for each block.54 As highlighted above, polymerizations through KCTP/ GRIM of block copolymers are more amenable to donor

Figure 1. (a) Resonant soft X-ray scattering intensities versus scattering vector of P3HT-b-PFTBT block copolymer and P3HT/ PFTBT blend films in the transmission geometry. The q* and 2q* peaks for the block copolymer are indicative of lamellae. Inset: schematic illustration of block copolymer active layer consisting of vertically oriented lamellae. (b) Current density (J) as a function of applied bias (V) for solar cells fabricated with P3HT-b-PFTBT block copolymer or P3HT/PFTBT blend active layers. Device efficiencies of block copolymer devices are near 3%, while those of blend devices are 1%. Reproduced with permission from ref 37.

V, the short-circuit currents of devices are modest, near 5 mA/ cm2, reflecting the limited external quantum efficiencies (∼35%) of block copolymer devices. Resonant soft X-ray scattering experiments that probe the in-plane polymer microstructure reveal that device performance depends on the self-assembly of the block copolymer within the active layer. Although directly linking donor and acceptor moieties provides an opportunity for molecular control of interfaces, covalent bonds can also lead to strong electronic coupling between donors and acceptors, thereby potentially promoting charge recombination. Clearly, such strong recombination pathways may negate potential advantages of the block copolymer architecture. The demonstrated increase in the short circuit current of solar cells, from about 4 mA/cm2 in devices with P3HT/PFTBT blends as the active layer to 5 mA/ cm2 for P3HT-b-PFTBT devices,37 suggests that recombination is not severe enough in P3HT-b-PFTBT devices to offset the B

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This represents an important achievement, as KCTP/GRIM has been previously employed only for electron-rich monomers such as thiophenes, selenophenes, pyrroles, fluorenes, and phenylenes. The amount of control through KCTP/GRIM reactions relies on the affinity for catalyst association to the monomer.58 More stable complexes give greater control over the polymerization and typically require an electron-rich monomer.59 For example, the low affinity of fluorene to traditional Ni catalysts, such as [1,3-bis(diphenylphosphino)propane]dichloronickel(II), makes polymerization of fluorene as a second block after thiophene or selenophene challenging. Different catalysts, such as Pd(Ruphos), have less selective association energies and enable the polymerization of thiophene, fluorene, and selenophene with any sequence.60 Density functional theory predicts that [N,N′-dimesityl-2-3-(1,8-naphthyl)-1,4-diazabutadiene]dibromonickel catalysts complex strongly with electron-deficient monomers, such as benzotriazole, enabling the synthesis of poly(3-hexylthiphene-2,5-diyl)-block-poly(2-(2octyldodecyl)-2H-benzo[d][1,2,3]triazole) as a conjugated donor−acceptor block copolymer (1, Figure 3).57 The ability to synthesize donor and acceptor blocks sequentially through controlled polymerizations will likely lead to a variety of novel, well-defined, fully conjugated, donor−acceptor block copolymers that are suitable for organic photovoltaics and other optoelectronic applications. The second approach for the synthesis of fully conjugated block copolymers relies on a chain extension from a halogen end group using Yamamoto, Stille, or Suzuki−Miyaura polycondensations (Figure 2b). The use of Yamamoto polymerizations utilizes bifunctional monomers for synthesis of the second block, while Stille and Suzuki−Miyaura coupling relies on heterobifunctional monomers or two monomers with different functionalities. The diversity of compatible monomers is large, and the resulting second block can be an alternating copolymer, enabling significant tuning of the optoelectronic properties. Nevertheless, as we discuss below, this approach leads to significant chemical heterogeneity and high dispersity in block copolymer products. Ideally, the P3HT polymerization conditions are controlled to yield a single functional end group and thus minimize the amount of uncoupled homopolymer. For example, the addition of LiCl during the metal−halogen exchange of the 2,5dibromo-3-hexylthiophene monomer accelerates the Grignard reaction and minimizes the amount of unreacted Grignard agent.61 The resulting P3HT has a single Br end group in greater than 90% of the chains, as demonstrated by matrixassisted laser deposition ionization time-of-flight mass spectrometry.62−64 Utilizing KCTP/GRIM and Yamamoto coupling, a variety of fully conjugated block copolymers have been synthesized, including poly(3-hexylthiophene-2,5-diyl)-block-poly(naphthalene bisimide)-block-poly(3-hexylthiophene-2,5-diyl) (P3HT-b-PNBI-b-P3HT) triblock copolymers (2, Figure 3),65 poly(3-hexylthiophene-2,5-diyl)-block-poly(pyridinium pheneylene)-block-poly(3-hexylthiophene-2,5-diyl),66 and poly(3hexylthiophene-2,5-diyl)-block-poly(cyanophenylenevinylene)block-poly(3-hexylthiophene-2,5-diyl).67 The chain extension to yield P3HT-b-PNBI-b-P3HT, for example, is done with a dibromonaphthalene bisimide monomer and a bis(1,5-cylcooctadiene)nickel(0) catalyst;65 in this approach, a single functional group is required for coupling with P3HT and for polymerizing the acceptor monomers. After purification by

Figure 2. Approaches for the synthesis of fully conjugated block copolymers. (a) Polymerization scheme for conjugated block copolymers with KCTP/GRIM. Judicious choice of the catalyst enable the polymerization of donor−acceptor fully conjugated block copolymers.57 (b) Synthesis scheme for conjugated block copolymers with KCTP/GRIM for polymerization of the donor block followed by a Stille or Suzuki−Miyaura polycondensation to grow the acceptor block. This approach can lead to a mixture of products, including homopolymers, diblock copolymers, and triblock copolymers. (c) Scheme for the synthesis of conjugated block copolymers from premade donor and acceptor homopolymers using a coupling reaction such as click chemistry. Depending on the degree of end-group control in the homopolymers, products can contain diblock, triblock, and multiblock copolymers as well as unreacted homopolymers.

materials. As a consequence, these block copolymers have been utilized as mixtures with electron acceptors in the active layer of photovoltaics. Fully conjugated P3AT block copolymers, including semifluoroalkylthiophene-based materials such as poly(3-hexylthiophene-2,5-diyl)-block-poly(3-(4,4,5,5,6,6,7,7,7nonafluoroheptyl)thiophene-2,5-diyl), were mixed with [6,6]phenyl C61 butyric acid methyl ester (PCBM) to yield photovoltaic active layers.55 Devices produce a maximum power conversion efficiency of 0.84%. Furthermore, heterocyclo thiophene block copolymers incorporating selenophene, such as poly(3-hexylselenophene-2,5-diyl)-block-poly(3-hexylthiophene-2,5-diyl) (P3HS-b-P3HT), provide an opportunity to control optical properties due to the smaller bandgap of polyselenophene when compared to polythiophene. Indeed, P3HS-b-P3HT gives a broad optical absorption with an 80 nm red-shift relative to that of P3HT.56 Recently, Seferos and co-workers have demonstrated the synthesis of a fully conjugated donor−acceptor block copolymer from a KCTP/GRIM polymerization, as schematized in Figure 2a.57 The use of a Ni(II) diamine catalyst enabled the polymerization of an electron-deficient monomer. C

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Figure 3. Examples of fully conjugated block copolymers recently reported in the literature. From refs 57, 65, 39, 68, and 36 for copolymers 1−5, respectively.

homopolymers. The performance of the block copolymer product in devices was not reported; nevertheless, the block copolymer was utilized in an attempt to stabilize blends composed of the constituent homopolymers. Although the maximum efficiency of devices incorporating the ternary blend (near 1.2% efficiency with 17% block copolymer) is never higher than that of devices incorporating the blend without any block copolymer (near 1.5% efficiency), the device performance after annealing at high temperatures (200 °C) for 10 min is significantly higher for ternary blend devices1% when 40% block copolymer is used versus 0.6% efficiency without block copolymers in the active layer.68 Using KCTP/GRIM and Suzuki−Miyaura coupling, a variety of P3HT-based fully conjugated block copolymers have been synthesized with P3HT macroreagents and dibromo and diboronic ester monomers.36 After preparing P3HT-Br by KCTP/GRIM, a Suzuki−Miyaura reaction was used to grow a second block, either poly(9′,9′-dioctylfluorene) (PF), poly(9′,9′-dioctylfluorene-alt-benzothiadiazole) (PFBT), or poly((9,9-bis(2-octyl)fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)2,1,3-benzothiadiazole)-5′,5″-diyl) (PFTBT). In some cases, block copolymers with molecular weights in excess of 50 kg mol−1 were achieved, although many of the reported products show evidence for significant homopolymer impurities in GPC traces, greater than 10%.36,37 Nevertheless, devices with P3HTb-PFTBT (5, Figure 3) as the active layer exhibit significant device performance, near 3% power conversion efficiencies, as described earlier (Figure 1).37 The third approach to synthesize fully conjugated block copolymers relies on coupling preformed homopolymers. As shown in Figure 2c, a high degree of end-group control is important to prevent heterogeneity in the block copolymer architecture and obtain copolymers with only the desired number of blocks. The consequence of utilizing the prevalent polycondensations for conjugated polymer synthesis in the

Soxhlet extraction with acetone, gel permeation chromatography (GPC) traces show narrow and unimodal profiles for the P3HT-b-PNBI-b-P3HT block copolymer with a clear shift from the P3HT homopolymer peak. Nevertheless, P3HT can couple with itself, and this product could be challenging to discriminate from block copolymers. The authors noted that the purified products were likely a mixture of P3HT-b-PNBI-bP3HT triblock copolymers, homocoupled P3HT, and P3HT-bPNBI diblock copolymers.65 Although devices composed of only the purified P3HT-b-PNBI-b-P3HT block copolymer were not reported, photovoltaic device performance, with blends of P3HT-b-PNBI-b-P3HT and P3HT homopolymer as the active layer, yields 1.28% power conversion efficiency, open-circuit voltages of 0.56 V, short-circuit currents of 4.57 mA/cm2, and fill factors of 0.50 after thermal annealing at 200 °C. Utilizing Stille or Suzuki−Miyaura coupling eliminates the possibility of coupling the macroreagent (first block) with itself. A fully conjugated block copolymer, poly(3-hexylthiophene2,5diyl)-block-poly(diketopyrrolopyrrole-terthiophene) (P3HT-b-DPP), was synthesized with KCTP/GRIM and Stille reactions (3, Figure 3).39 The monomers for the polycondensations rely on stannyl functionalities, here bis(trimethylstannyl)thiophene, and dibromo moieties, such as materials based on dibromodithienodiketopyrrolopyrrole. GPC results demonstrate block copolymers with little or no homopolymer impurities, although the traces with UV−vis detectors may indicate significant compositional heterogeneity among the products. No device performance was reported with these materials as the active layer. Poly(3-hexylthiophene-2,5-diyl)-block-poly((4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl-alt-(9,9-bis(2-octyldodecyl)fluorene-2,7-diyl)) (P3HT-b-PTBTF) block copolymers were also prepared by KCTP/GRIM polymerizations and Stille coupling (4, Figure 3).68 Analysis of NMR data of the block copolymer product reveals P3HT and PTBTF D

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diblock, triblock, or multiblock products are produced. Indeed, most reports highlighted here note undesired byproducts. Although fractionation and purification of fully conjugated block copolymers by preparative GPC are possible,78 the impact of homopolymer impurities on device performance remains unclear. Further efforts to develop synthetic methods that yield well-defined block copolymers with low dispersities and minimal impurities are warranted. For example, enabling the synthesis of acceptor blocks through KCTP/GRIM by tuning interactions between catalysts and high electron affinity moieties may prove to be a critical breakthrough because welldefined block copolymers are possible through the sequential synthesis of donor and acceptor blocks using a controlled chain growth polymerization scheme.57

synthesis of block copolymers is that products are a mixture of homopolymers, diblocks, triblocks, and higher multiblock copolymers. Nevertheless, homopolymer coupling provides the opportunity to insert bridge units at the donor−acceptor junction of block copolymers and thereby potentially tune the donor−acceptor interface. Click chemistry provides one avenue for coupling conjugated homopolymers.69−71 For example, the synthesis of P3HT with KCTP/GRIM allows for alkyne functionalization of a single chain end. 72−74 A single difunctional monomer in a polycondensation also can lead to the end-capping of one end of each chain. 7′-Bromo-9′,9′-dioctylfluorene-2′-yl-4,4,5,5tetramethyl-[1,3,2]dioxaborolane in a Suzuki−Miyaura reaction can be capped with a tosylate group, which itself can be converted to an azide with tert-butylammonium fluoride and azidotrimethylsilane.70 Linking of the alkyne-functionalized P3HT and azide-functionalized poly(dioctylfluorene) through a Cu(I)-catalyzed azide−alkyne cycloaddition leads to poly(3hexylthiophene-2,5-diyl)-block-poly(9,9-dioctylfluorene) (P3HT-b-PFO) diblock copolymers.70 Other than homopolymer impurities due to incomplete coupling or lack of endgroup functionalization, this approach leads to only diblock copolymer products without the possibility for larger multiblock copolymers. Relying on a polycondensation, however, either limits the polymerization to a single monomer species or capitulates the selective functionalization of just one chain end if two monomer species are used (and thus leads to heterogeneity in the architecture of the block copolymer product, Figure 2c). As a consequence, acceptor blocks are more challenging to include in the block copolymer synthesis; to date, many fully conjugated block copolymers made through click chemistry, such as P3HT-b-PFO, do not include an acceptor block.69−71 Using click chemistry to link two conjugated homopolymers together introduces a linking group, or bridge moiety, at the junction between blocks. Thus, the end-functionalization of the homopolymers provides an opportunity to tune the bridging units between donor and acceptor units. As we discuss below, tuning the block copolymer junction could lead to unprecedented control of electronic coupling and charge transfer rates between donor and acceptor blocks. A variety of bridging groups have been incorporated in donor−acceptor conjugated block copolymers, including nonconjugated linkers31,33,43,75,76 and dye moieties, such as porphyrins44,77 and bis(terpyridine)ruthenium(II) complexes.44 Phosphonate endcapped poly(phenylenevinylene)s that are either decyloxy- or sulfone-substituted to serve as donor and acceptor blocks, respectively, can be linked using a dialdehyde bridging unit through condensation reactions between aldehyde and phosphonate moieties.43 Another approach relies on the formation of the bridging group, such as a ruthenium terpyridine complex, where the donor and acceptor blocks come together.44 Although the performance of devices composed of the aforementioned block copolymers is minimal, below 0.01% power conversion efficiencies, the potential to control the donor−acceptor junction could lead to future materials with unprecedented control of charge transfer processes. In Figure 2 we highlight multiple different products that are possible given current approaches for the synthesis of conjugated block copolymers. Relying on a step growth mechanism leads to larger dispersities, challenges in avoiding homopolymer impurities, and problems in controlling whether

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PREDICTING THE SELF-ASSEMBLY OF SEMIFLEXIBLE BLOCK COPOLYMERS Although one of the distinct advantages of conjugated block copolymers is the ability to control self-assembly, a complete picture of microphase separation in semiflexible and rodlike block copolymers is lacking. The self-assembly of flexible block copolymers, or block copolymers based on chains with coil-like conformations, can be described in terms of the volume fraction f and the product χN of the number of statistical segment lengths N and Flory−Huggins interaction parameter χ.79−82 When the persistence length is much larger than the size of the monomer, however, the chain stiffness can affect self-assembly by destabilizing morphologies with curved interfaces83 and by inducing nematic phases within domains84,85 and at interfaces between domains.86 Even though many of the pieces have been developed, a complete description of the self-assembly of semiflexible block copolymers is lacking. The morphology of block copolymers in the active layer has a significant impact on the performance of photovoltaic devices. Simulations of solar cells composed of mixtures or blends with model morphologies suggest that an optimum amount of interfacial area exists to maximize device performance due to the trade-offs between charge photogeneration and charge extraction efficacy.87−89 Recent work demonstrates that high performance in P3HT-b-PFTBT solar cells only occurs when block copolymer self-assembly in the active layer is apparent in resonant soft X-ray scattering data (Figure 1a).37 Devices incorporating poly(3-hexylthiophene-2,5-diyl)-block-poly(nbutyl acrylate-stat-acrylate perylene) block copolymers, where a perylene diimide acceptor is attached as a pendant group, have demonstrated significant effects with changes in the morphology of the active layer. Block copolymers with strong long-range order of cylinders in the active layer yield device efficiencies near 0.01%, while poorly ordered, short cylinders in the active layer yield device efficiencies of 0.02%.34 As the block copolymer order increases, cylinders laying down are not conducive to charge extraction. The phase behavior of semiflexible and rodlike block copolymers has been described through self-consistent field theory (SCFT).90 Utilizing the wormlike chain model in SCFT, where the persistence length introduces an orientational dependence for the chain propagator in addition to the canonical positional dependence, the phase diagram can be described in terms of the flexibility of the chain L/a (L is the contour length and a is the Kuhn length, twice the persistence length), volume fraction f, and degree of phase segregation (product χN), as shown in Figure 4. When chains are very flexible, L/a ≫ 1, the phase behavior follows the predictions for E

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Nevertheless, rod−coil block copolymers, where one block has a persistence length near the contour length, demonstrate the potential richness of the semiflexible block copolymer phase diagram. The SCFT results presented in Figure 4 ignore nematic interactions, such that liquid crystal phases are not shown. Experimentally, the phase behavior of various rod−coil block copolymers has been explored, and various liquid crystalline phases have been identified. A systematic series of poly(2,5-di(2′-ethylhexyloxy)-1,4-phenylenevinylene)-blockpoly(1,4-isoprene) block copolymers that vary in molecular weight and composition were synthesized to examine the phase behavior with X-ray scattering and electron microscopy. In addition to lamellar and hexagonally closed-packed cylinders, nematic phases were identified in between ordered morphologies and the isotropic phase.97−99 Other block copolymers based on poly(phenylenevinylene) and either poly(4-vinylpyridine)100,101 or poly(methyl methacrylate)102 also exhibit liquid crystalline phases in addition to block copolymer mesophases. Although in principle it should be possible to integrate nematic interactions98,103 and the wormlike model into SCFT,83,104 it has yet to be demonstrated. A recent approach to predict the nematic coupling parameter, which would be an input in SCFT simulations, has been reported.94 Block copolymers composed of semiflexible blocks do not always show clear evidence of liquid crystalline phases, but the self-assembly can instead be dominated by the crystallization of the constituent blocks. For example, poly(3-hexylthiophene2,5-diyl)-block-poly(2-vinylpyridine) (P3HT-b-P2VP) block copolymers can exhibit spherical, hexagonally closed-packed cylinders, and lamellar morphologies, in addition to a fiberlike motif that is a consequence of the crystallization of P3HT.105 In general, crystallization competes with block copolymer microphase separation. Crystallization of one or more of the blocks can be confined within domains, such as within the lamellae, if the degree of phase separation, or χN, is high. If χN is low, then crystals can break out, thereby destroying the mesoscale assembly.106−108 Because crystallization will be faster than mesoscale assembly due to the shorter length scales involved, processing can significantly impact the morphology. Ideally, the block copolymer mesophase is formed above the melting temperature for crystals but below the order to disorder temperature. Thus, a high degree of phase separation, or large χN, may be critical for mesoscale assembly of fully conjugated block copolymers because many of the constituent blocks will be crystalline, partly to promote charge transport but also as a consequence of the stiffer chain conformations.

Figure 4. (a) Possible morphologies for diblock copolymers composed of semiflexible chains obtained from SCFT simulations. (b)−(e) Phase behavior for chains of various flexibility L/a, where L is the contour length, a is the Kuhn length, and f is the volume fraction. Morphologies are labeled as L for lamellae, H for hexagonally closed-packed cylinders, P4 for tetragonally ordered cylinders, Q230 for a bicontinuous gyroid phase, and Q229 for body-centered spheres. Reproduced with permission from ref 83. Copyright 2013 American Physical Society.

block copolymers with Gaussian chain conformations.80,91 As the polymer chains become stiffer, the morphologies with curved interfaces, such as body-centered spheres (Q229) and hexagonally close-packed cylinders (H), destabilize, and a new morphology, tetragonally ordered cylinders (P4), emerges. The phase boundaries shift with chain stiffness, where the phase space of the lamellar morphology (L) grows and the position of the gyroid phase (Q230) moves significantly. In the rodlike limit, L/a = 1, the lamellar phase dominates the phase diagram. Clearly, the chain stiffness is an important parameter in governing the self-assembly of block copolymers. A few estimates of the persistence length of conjugated polymers have been reported. Small-angle neutron scattering experiments of P3HT in solution yield persistence length values of 3.0 nm,92 while light scattering estimates the persistence length at 2.4 nm.93 Furthermore, the persistence length of P3HT can be predicted from the dihedral potentials by numerically integrating over the probability distribution of torsional angles. Such predictions also yield about 2.8 nm,94 consistent with more detailed molecular dynamics simulations (3.2 nm).95 This approach yields persistence lengths for PFTBT of 5.9 nm.95 Both persistence lengths are similar to values obtained from the freely rotating bond model96 (2.8 and 6.6 nm for P3HT and PFTBT, respectively), suggesting that the torsional barriers are small compared to kT at room temperature and that the backbones are effectively freely rotating. Thus, P3HT above 3 kg/mol and PFTBT above 4 kg/ mol are wormlike polymers, where the chain contour length, or the length when the chain is fully extended, is at least 2 times larger than the persistence length.

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INTERFACE CONTROL PROVIDES OPPORTUNITIES FOR PHOTOVOLTAICS The morphology near the donor−acceptor interface can strongly affect device performance. Recent evidence suggests that high local charge mobilities near interfaces,109 energy cascades created at interfaces between amorphous and crystalline domains,110−115 enhanced local order and charge delocalization at interfaces,116−122 and thermal fluctuations123−126 may be responsible for efficient charge photogeneration. Fully conjugated block copolymers provide an opportunity to control the interface for systematic studies of charge and energy transfer, if the interfacial morphology can be tuned with the molecular structure. The presence of crystalline and nematic phases in addition to block copolymer morphologies will significantly affect the interface. Broad interfaces between donor and acceptor F

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Macromolecules domains have the potential to open pathways for intermolecular charge recombination not available for block copolymers with sharp interfaces;127 impurities within domains of the opposite block can have the same effect. Predicting the interfacial structure of semiflexible block copolymers where crystalline phases are present or the chains have nematic interactions remains a challenge. The width of flexible polymer−polymer interfaces is proportional to the size of the chain that has an energetic penalty on the order of kT while in the wrong phase.128,129 The interfacial width scales with b/χ1/2 (b is the statistical segment length) such that more incompatible polymers result in sharper interfaces. Interfacial widths of about 5 nm are not unusual for common polymers with moderate values of χ.130,131 When compared to systems with flexible chains, interfaces of semiflexible polymers are narrower by a factor of 1/blpχ, where lp is the persistence length.86 Furthermore, if the width of the interface approaches the persistence length, fluctuations of chains in the wrong phase are no longer random walks. Chains begin to lie parallel across the phase boundary, effectively exhibiting nematic behavior at the interface. The crossover is at κχ ∼ 1, where κ is the chain bending constant equal to the ratio of the persistence length and the statistical segment length.86 It is unclear whether conjugated polymers such as P3HT and PFTBT are stiff enough for semiflexible effects to be important for the interfacial structure because χ is currently unavailable for pairs of conjugated polymers. Measurements of the Flory− Huggins interaction parameter χ mostly rely on the presence of amorphous phases,132 making estimates of χ challenging because crystalline and nematic phases often dominate the phase diagram of conjugated polymers. Nevertheless, the opportunity to control charge and energy transfer events is clear. Defining the interface with the molecular structure enables control of the energetics and electronic coupling. For example, work on donor−bridge− acceptor molecules has demonstrated control of interfacial processes at ultrafast time scales. Using 3,5-dimethyl-4-(9anthracenyl)julolidine as the donor and naphthalene-1,8:4,5bis(dicarboximide) as the acceptor, a series of molecules with various linking, or bridge groups, were synthesized. 1,1Diphenylethene is conjugated between the phenyl rings and a double bond in the middle of the molecule, but the rings are not conjugated with each other. trans-Stilbene, on the other hand, is conjugated across the bridge molecule. Time-resolved spectroscopy experiments reveal that charge separation occurs about 30 times slower through the 1,1-diphenylethene bridge than through trans-stilbene, although charge recombination is 10 times slower through the 1,1-diphenylethene bridge.133 Another example is recent work on examining charge transfer in P3HT coupled to a single (9,9-bis(2-octyl)fluorene-2,7-diyl)(4,7-di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl unit, where either the fluorene or the dithienobenzothiadiazole moiety is directly coupled: P3HT-TBT-F or P3HT-F-TBT. As schematized in Figure 5b, transient absorption spectra of P3HT-TBT-F show fast, less than 40 fs, formation of localized charge transfer states that recombine within 700 ps. In contrast, P3HT-F-TBT does not show evidence of a localized charge transfer state and does show signatures of charge-separated states.134 Clearly, the linking chemistry makes a difference for charge photogeneration. A possible explanation for the dependence on the linking chemistry shown in Figure 5 is that fluorene can mediate charge transfer. Benzothiadiazole has the strongest electron affinity in

Figure 5. (a) Example of a polymerization scheme for conjugated block copolymers incorporating a bridge unit (D−B−A). The bridge unit could also be incorporated into block copolymers by linking two homopolymers, as in Figure 2c. (b) Example of how the linking chemistry can affect charge transfer processes. Conjugating an acceptor moiety, dithienobenzothiadiazole (TBT), directly to a P3HT donor leads to localized charge transfer states that promote recombination. Separating the high electron affinity group, benzothiadiazole, from P3HT with a large band gap moiety such as dioctylfluorene (F8) leads to the formation of charge-separated states. The hypothesis is that dioctylfluorene can act as a bridge group to prevent the formation of strongly bound charge transfer states. Reproduced with permission from ref 134.

the molecule, such that electrons in the first excited state of the system should reside near benzothiadiazole. Thus, fluorene may act as a bridge moiety, which spatially separates charges constituting the charge transfer state and therefore reduces both the electrostatic binding energy and the electronic coupling between the donor and acceptor blocks. The net result is the reduction of intramolecular recombination rates across the block copolymer junction. P3HT-b-PFTBT (5, Figure 3), which is utilized as the active layer of devices with efficiencies up to 3% (Figure 1), utilizes fluorene as the linking group between the donor and acceptor. The linking order of the acceptor block is set by the chemistry; as shown by Figures 2 and 5a, the order of the acceptor groups can be tuned by the functional groups of the monomers. But, open questions remain, such as: does the reduced intramolecular coupling also prevent intramolecular charge transfer, such that P3HT-bPFTBT37 relies on intermolecular contacts for charge photogeneration? The potential remains to tune charge transfer processes in block copolymer architectures, perhaps suppressing charge recombination without negatively affecting polaron yields by tuning the chemistry at the junction. This represents an opportunity beyond polymer/fullerene mixtures or donor/ acceptor blendsthe ability to directly control charge transfer processes through the chemical structure to both enable mechanistic studies of charge photogeneration and develop next-generation high performance organic photovoltaic materials. G

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CONCLUSIONS AND OUTLOOK Fully conjugated block copolymers offer opportunities for model studies and high performance devices. Controlling the donor−acceptor interface with the molecular structure enables systematic studies of the role of delocalization, energetics, and local environments, among other factors. Furthermore, block copolymers may also self-assemble into mesoscale morphologies that are amenable to extracting charges and providing interfaces for dissociating excitons in the active layer of organic solar cells. Many challenges remain in the field, starting from developing synthetic approaches to yield well-defined products, and include predicting the morphology and fabricating high performance devices as well. Building blocks for fully conjugated donor−acceptor block copolymers have diversified through the development of various polymerization schemes such as KCTP/GRIM, Yamamoto, Stille, and Suzuki−Miyaura coupling. The impressive performance of recent polymer blend solar cells is a consequence of the implementation of low band gap conjugated polymers, often with high charge mobilities,135−139 such as poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexanoyl)thieno[3,4-b]thiophene-)-2,6-diyl)] and poly{[N,N′-bis(2-hexyldexyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′selenophene)},137 but these homopolymers have yet to be incorporated in fully conjugated block copolymers. Furthermore, unwanted homopolymer impurities are present in many examples of conjugated block copolymers reported in the literature. The impact of these impurities remains unclear as well as the role of dispersity and heterogeneity in architecture, such as the unintended consequence of mixing diblocks and triblocks. The development of KCTP/GRIM catalysts capable of synthesizing both donor and acceptor blocks is a promising novel development that may enable well-defined conjugated block copolymers.57 In principle, once we know how the molecular structure will affect the equilibrium morphology, processing is no longer as challenging as in donor/acceptor mixtures that must be trapped outside of equilibriumall that is required is a method, such as solvent or thermal annealing, that enhances molecular motion to push the system toward equilibrium. Care must be taken to promote microphase separation before crystallization, but such protocols have been established.107,108,140,141 Nevertheless, a problem arises with the orientation of mesophases for morphologies without three-dimensional symmetry. Cylindrical morphologies and lamellar morphologies, where the cylinders or lamellae are laying down, would lead to problems with charge extraction in photovoltaic devices. Often, surfaces that are neutral with respect to the blocks are required for cylinders or lamellae to stand up perpendicular to the substrate.142−147 The soft X-ray scattering results for P3HT-b-PFTBT shown in Figure 1 are from transmission experiments, such that only lamellae that are standing up perpendicular to the substrate would contribute to the peaks. The authors did not perform any special treatments on the substrate to promote vertical lamellae.37 Recent advances in predicting the properties of semiflexible block copolymers83,94,95,98,104 have the potential to propel our ability to computationally design high performance materials. A comprehensive description is lacking, and critical morphological aspects, such as the structure at the donor−acceptor interface, remain unclear. Modeling efforts may drive the field into more

complex architectures, such as triblock or graft copolymers, which provide opportunities beyond linear diblock copolymers. Recent work describes the synthesis of graft block copolymers based on P3HT, as shown in Figure 6a.148 As shown in Figure

Figure 6. (a) Example of a fully conjugated graft block copolymer. Architectures beyond linear diblock copolymers may provide unique opportunities to control the morphology and charge transfer with the molecular structure. Reproduced with permission from ref 148. (b) An example of the consequence of graft block copolymer architecture on self-assembly. Graft block copolymers appear to promote morphologies perpendicular to the substrate, such as standing-up cylinders. Reproduced with permission from ref 149. Copyright 2010 Royal Society of Chemistry.

6b, block copolymers based on a graft architecture can promote vertically oriented cylinder mesophases.149 The tremendous advances in synthesis and modeling suggest materials capable of controlling the mesoscale morphology, interfacial structure, and charge and energy transfer are in the near future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (E.D.G.). Notes

The authors declare no competing financial interest. Biographies

Youngmin Lee was granted a B.S. in Chemical Engineering from the Pohang University of Science and Technology (POSTECH), Korea, in 2004. He received a Ph.D. degree in Chemical Engineering from H

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POSTECH under the supervision of Prof. Jin Kon Kim in 2010. He then joined Prof. Marc A. Hillmyer’s group at the University of Minnesota as a postdoctoral researcher. In 2013, he joined the group of Prof. Enrique D. Gomez at the Pennsylvania State University as a postdoctoral researcher. His research interests include synthesis of fully conjugated block copolymers and applications for photovoltaics.

Enrique D. Gomez received his B.S. in Chemical Engineering from the University of Florida and his Ph.D. in Chemical Engineering from the University of California, Berkeley. After a little over a year as a postdoctoral research associate at Princeton University, he joined the faculty in the Chemical Engineering Department of the Pennsylvania State University in August of 2009 and is currently an Associate Professor. His research activities focus on understanding how structure at various length scales affects macroscopic properties of soft condensed matter. Currently, the work in the Gomez group examines the relationship between chemical structure, microstructure, and optoelectronic properties of conjugated organic molecules. During his time at Penn State, Dr. Gomez has won multiple awards, including the Oak Ridge Associated Universities Ralph E. Powe Junior Faculty Enhancement Award, the National Science Foundation CAREER Award, and the Rustum and Della Roy Innovation in Materials Science Award.

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ACKNOWLEDGMENTS Educational discussions with Scott Milner on block copolymer self-assembly and interfacial structure and with John Asbury on charge transfer in donor−acceptor systems are gratefully acknowledged. Financial support from the Office of Naval Research under Grant N000141410532 is also gratefully acknowledged.

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DOI: 10.1021/acs.macromol.5b00112 Macromolecules XXXX, XXX, XXX−XXX