Side Chain Engineering in Solution-Processable Conjugated

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Perspective

Side Chain Engineering in Solution-Processible Conjugated Polymers for Organic Solar Cells and Field-Effect Transistors Jianguo Mei, and Zhenan Bao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4020805 • Publication Date (Web): 29 Aug 2013 Downloaded from http://pubs.acs.org on September 14, 2013

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Chemistry of Materials

Side Chain Engineering in Solution-Processable Conjugated Polymers Jianguo Mei, Zhenan Bao* Department of Chemical Engineering, Stanford University, Stanford, California 94305 KEYWORDS: Conjugated polymers, side chain engineering, organic field-effect transistors, solar cells.

ABSTRACT: Side chains in conjugated polymers have been primarily utilized as solubilizing groups. However, these side chains have roles that are far beyond. We advocate using side chain engineering to tune a polymer’s physical properties, including absorption, emission, energy level, molecular packing and charge transport. To date, numerous aromatic units suitable for constructing conjugated polymers have been reported. In this Perspective article, we detail how the side chain engineering approach can advance better designs for next-generation conjugated polymers to improve the new materials’ processability, fabrication and performance.

1. INTRODUCTION Conjugated polymers are plastics that can be designed to absorb/emit light and conduct electrical currents. They have been incorporated into various organic electronics, including organic photovoltaics (OPVs), field-effect transistors (FETs), light-emitting diodes, electrochromic devices, and others.1-4 The inception of next-generation flexible and printed electronics has energized the field of conjugated polymer. Consequently, the number of new conjugated polymers has exponentially increased in recent years. The performance of these materials has steadily improved, and commercial applications are seemingly imminent. For instance, conjugated polymer-based bulk-heterojunction solar cells have repeatedly exhibited power conversion efficiencies (PCEs) of up to 8-10%.5-7 Polymerbased field-effect transistors have been gradually surpassing their inorganic amorphous silicon (α-Si) counterparts, with 110 cm2/V-1s-1 mobilities.2,8-11 Our better understanding of materials design, processing and device fabrication has contributed to these performance advancements. In this article, however, we will only discuss material design and focus on side chain engineering for conjugated polymers. Selecting the side chains is as important as selecting the conjugated backbones when designing conjugated polymers. Solution-processable conjugated polymers generally contain two parts: π-conjugated backbones and peripheral flexible solubilizing side chains. π-conjugated backbones determines the optoelectronic properties of the resulting polymers. Consequently most research efforts have focused on the conjugated backbones.1-2,12-14 The side chains have not been fully exploited even though numerous side chain substituents have been tested over the years. Some recent studies have begun to focus on side chain engineering, collectively observing that this approach could greatly enhance the charge carrier mobilities of the OFETs and PCEs in OPVs. This Perspective article will emphasize the significance of this strategy. In this article, we will introduce some representative side chains used in conjugated polymers. Next, some recent examples in which side chain engineering has been successfully employed to improve device performance will be highlighted.

Finally, we will discuss the future of side chain engineering and how it may address critical issues affecting the design of next-generation conjugated polymers for OPVs and OFETs. 2. SIDE CHAIN TOOLBOX Various flexible chains have appeared as the side chains of conjugated polymers. Specifically, there is currently a relatively large side chain toolbox. For the sake of simplicity, these side chains are categorized in Chart 1 based on their compositions, namely, alkyl, hybrid, oligoether, fluoroalkyl and latently reactive side chains. 2.1 Alkyl Side Chains Alkyl chains have a -CnH2n+1 molecular formula, where n is the number of carbon atoms, and are the most commonly used side chains in conjugated polymers. There are two forms of alkyl chains: linear and branched alkyl chains. Of the linear alkyl chains, hexyl, octyl and dodecyl chains are most common. In recent years, longer side chains, such as tetradecyl and octadecyl, were also introduced in donor-acceptor conjugated polymers.15 Statistically, linear alkyl chains with even numbers of carbon atoms are more popular than those with oddnumbered carbon atoms, presumably due to their commercial availability. The odd-even effect of side chains is known for small molecules16 but remains unreported for conjugated polymers. It has been observed that properly chosen and placed linear alkyl chains can promote interchain interdigitation.17-19 Compared to linear alkyl chains, branched alkyl chains generally preclude interchain interdigitation because their bulkiness usually hinders interchain interactions. On the same account, branched alkyl chains usually impart better solubility when both chains have identical molecular formulas. ,2Ethylhexyl (-C2C6), 2-hexyldecyl (-C6C10), 2-octyldodecyl (C8C12) and 2-decyltetradecyl (-C10C14) are the most commonly used branched alkyl chains. Branched alkyl chains with more than one methylene group past the branching point were recently reported.20-21 These side chains are interesting because they provide good solubilizing capability without disrupting the π-π interactions. These capabilities are particularly important when designing conjugated polymers for charge

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35 rr-P3ATs have a homogenous polymer backbone with minimal steric hindrance from the adjacent alkyl chains. However, regiorandom P3ATs have heterogeneous and randomly twisted backbones caused by the steric hindrance arising from the interactions between neighboring alkyl chains. This structural difference is reflected by their physical properties and performance in devices. For instance, doped regioregular poly(3dodecylthiophene) has an average conductivity of 600 S cm–1 that is two orders of magnitude higher than those of the regiorandom materials.36 The charge carrier mobilities were measured using thin film transistors and are also two to three orders of magnitude higher for rr-P3AT relative to the regiorandom polythiophene.37 Tuning the alkyl chain positions alters the properties of polymers. However, the positions available for alkylation are limited on conjugated backbones. In contrast, changing the alkyl chains’ bulkiness and length enable more versatility. Several investigations have studied the impact of alkyl chains’ bulkiness and length.38-41 Recent studies have attempted to correlate the side chains’ bulkiness and length with material properties and device performances. You and coworkers prepared six polymers P1 with identical polymer backbones and alkyl chains with varied bulkiness, as shown in Figure 1.39 The relationship between side chain bulkiness and the measured open circuit voltage (Voc) was intriguing; the Voc increases with the bulkiness of the side chains. P1-b contained two bulky branched side chains and exhibited a 0.83 V Voc ; P1-c had two slim linear side chains and generated a much lower Voc (0.39 V). The authors suggested that the weak π-π interactions among the conjugated polymer backbones enhanced the Voc. However, the charge transport was adversely affected when the π-π-interactions were weakened, generating a smaller short circuit current (Jsc). Therefore, the best overall power conversion efficiency (PCE) was obtained for polymers with intermediate side chain bulkiness and Voc.

Chart 1. Side chains in conjugated polymers. transport. Chiral alkyl chains have also been investigated.22-27 Notably, the above-mentioned branched side chains are chiral, even though they are racemic. Introducing an enantiomeric pure chiral side chain should induce the formation of chiral nanostructures via self-assembly. Cyclic side chains (e.g., the cyclohexyl group) are another special class of branched side chains. When they are directly attached to the sp2 carbons along the polymer backbone, their bulkiness can disrupt the backbone’s planarity and inhibit interchain interactions, consequently affecting the polymer’s optical and electrical properties. For instance, poly(3-cyclohexylthiophene) is a green emitter, while poly(3-hexylthiophene) is a red emitter.28 In a different scenario, using cyclic side chains effectively reduces steric hindrance: when these cyclic side chains form by connecting sp3-hybridized bridgehead carbon or silicon atoms (e.g., dithieno silole-based polymers).29 While the type of alkyl chain affects a polymer’s conformations and molecular packing, the chains’ position on the conjugated backbones also modulates these parameters.30-32 The regioregular poly(3-aklylthiophene) (rr-P3AT) and regiorandom counterparts are well-known examples of this effect.33-

Figure 1. Characteristic J-V curves for the bulk heterojunction solar cells made from P1 (PNDTDTBT) with illumination under 1 sun while using AM1.5 conditions. Reproduced with permission from Ref 39. Copyright 2010 American Chemical Society.

The trend was different for the P2 (PBDTTPD)-based solar cells: the Voc remained nearly unchanged at 0.85±0.04 V, regardless of the side chains’ bulk, as exhibited in Figure.38 Instead, a noticeable impact was observed on Jsc. P2-a contained a bulky ethylhexyl chain; a smaller Jsc (5.5 mA/cm2) was observed. The Jsc almost doubled (up to 10.6 mA/cm2) for P2-c, which has a linear octyl side chain. The high Jsc observed for P2-c was attributed its smaller π-π distance. More recently, by replacing the octyl group with a heptyl group, the PCE further increased to 8.5% and the Voc was high (0.96 V). 42


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Figure 2. Characteristic J−V curves for the bulk heterojunction solar cells fabricated from P2 (PBDTTPD) under AM 1.5 G, 100 mW/cm2 illumination. Reproduced with permission from Ref 38. Copyright 2010 American Chemical Society.

Marks, Chen and Yu et al. also studied side chain bulkiness using P3 (PTB), which was based on thieno[3,4-b]thiophene (TT) and benzodithiophene (BDT), as illustrated in Figure 3.41 They reported two interesting findings: (i) a positive correlation between the π–π distance of the polymer backbone, the OPV device fill factor (FF) and Voc; (ii) high performance devices may be realized without nanometer-sized PCBM domains, as indicated by grazing incident wide angle X-ray scattering (GIWAXS) experiments.

Figure 3. a) The structure of P3 (PTB). b) Table listing the PCE values, the π–π stacking scattering vectors and linewidths (in parentheses), and π–π stacking distances for the PTB polymers. The linear (L) and branched (B) aliphatic side chains on the TT and BDT moieties are also marked. c) The π–π stacking scattering vector peaks for the seven P3 polymers. d) Correlation between OPV fill factor and the π–π stacking distance. Reproduced with permission from Ref 41. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effects of side chain length have also been systematically explored.43-46 For example, Gadisa and coworker studied the charge transport and photovoltaic characteristics of regioregular poly(3-alklylthiophene) (P3AT), including P3BT, poly(2-pentylthiophene) (P3PT), and P3HT.44 Solar cells with polymer/PCBM blends delivered similar Jsc, while their FF were 0.529, 0.624 to 0.675 for P3BT, P3PT and P3HT, respectively. The Voc of the solar cells increased slightly from 0.539 0.549, to 0.574 for P3AT V when the side-chain length increased. The authors proposed that the longer side chains facilitated the formation of the PCBM clusters by establishing effi-

cient electron-percolation pathways. Essentially, balanced electron/hole mobilities were obtained with proper phase separation, leading to a higher fill factor. In a study of longer alkyl chains in P3ATs by Sariciftci,40 this trend was not observed. Instead, they concluded that longer side chains in P3ATs may facilitate PCBM diffusion into the polymer matrix, leading to greater phase separation after thermal annealing . This phenomenon greatly decreased the performance of the photovoltaic devices because the reduced interfacial area caused lower photocurrent generation. Bao and coworkers investigated poly(3,4-alkthiophene) (P34AT) (e.g., dihexyl (PDHTT), dioctyl (PDOTT) and didodecyl (PDDTT)).43 In contrast to the P3ATs, the P34ATs did not exhibit strong π-π interactions, as indicated by the barely visible π−π stacking peaks by synchrotron X-ray diffraction. The Voc of the solar cells increased slightly from 0.716 to 0.754 and 0.771 V when the P34AT side chains lengthened, while the FF remained almost constant at 0.66 ± 0.01; the Jsc dropped significantly from 8.54 to 7.96 and 6.42 mA/cm2 with increasing side chain lengths. The Jsc trend was attributed to the lowered diode hole mobility that was caused by the increasing alkyl chain length. The above examples suggested that the bulkiness and length of the side chains affected the π-π interactions and interchain interdigitation, leading to further modulation of the charge transport, Voc, Jsc and FF. These studies also indicate that the π-π interactions can be fine-tuned using the appropriate side chains. Short π-π distances facilitate the charge transport in OFETs, while adversely affecting the Voc in OPVs. Unfortunately, the complex morphological behaviors of these polymers and the effects of molecular weight and purity make quantifying the impacts and predicting the proper chain bulkiness and length for a given conjugated backbone difficult. Advanced theoretical computation would help synthetic chemists identify the appropriate side chain moieties. 2.2 Hybrid Side Chains The hybrid side chains described in this Perspective are mostly composed of (CH2)x segments and decorated with various functional groups that render specific properties. Hybrid side chains are further divided into two categories based on their connectivity to conjugated backbones. The first category encompasses side chains with functional moieties directly connected to the conjugated backbones, while the second category contains terminally functionalized side chains. The first category may be divided into three subgroups: electrondonating, electron-withdrawing and conjugated side chains. 2.2.1 Electron-donating side chains Electron-donating side chains can donate some electron density to the conjugated polymer backbones through connecting moieties, such as alkoxyl (-OR),47-54 alkylthio (-SR),55-58 alkylamino (-NHR and –NRR’),59 acetate (-OCOR) and amide (NHCOR) groups, etc. These side chains were primarily chosen to raise the HOMO (highest occupied molecular level) energy levels. In addition, these side chains utilized intramolecular interactions (e.g., hydrogen bonding and S(thienyl)O(alkoxy)) to lock the backbone into a co-planar structure. However, when the moieties were bulkier than a methylene unit, the π-π interactions were disrupted unless other intramolecular interactions were present.49 Alkoxyl side chains are the most frequently used electrondonating side chains. One such example (P4) is shown in Figure 4.60 From branched ethylhexyl (P4-a), linear dodecyl



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Figure 4. The chemical structures of P4.

(P4-c) to linear electron-donating dodecyloxy (P4-e) side chains, the HOMO levels of the thin film were -5.42, -5.32 and -4.84 eV, respectively. The LUMO levels for P4-a, P4-c and P4-e were almost unchanged, as dictated by the electrondeficient naphthalene diimide (NDI) unit. In contrast, P4-b and P4-d exhibited intermediate HOMO and LUMO levels. The trend was nicely reflected in the UV-vis spectra as well. The bulky 2-ethylhexyl side chains on P4-a caused a severe twist in its polymer backbone, leading to an amorphous film. P4-c had less bulky n-dodecyl side chains and exhibited a large red-shift (~87 nm) from the solution to the solid state, indicating a transition from a twisted state to a planar conformation. The absence of clear π-π stacking diffraction suggested that P4-c was amorphous as a thin film. In contrast, the dodecyloxy side chains not only increased the HOMO level but also helped to planarize the backbone through its electrostatic attractions between the ether oxygen and thienyl sulfur atoms.49 The fine absorption structures present in the solution spectra suggested P4-e had a rigid backbone. In addition, Jackson and coworkers performed computational studies on the ‘through-space’ oxygen−sulfur interactions and suggested that the oxygen−sulfur interactions were insufficient stabilizing forces for backbone planarization.61 Electron-donating side chains are usually introduced onto donor (electron-rich) units in donor/acceptor-type conjugated polymers. These side chains may be placed on acceptor (electron deficient) moieties. For instance, Zhang and coworkers mounted two octyloxyl chains on the benzothiazole ring to create a new soluble electron acceptor.53 This approach can generate new electron acceptors with good solubilities. 2.2.2 Electron-withdrawing side chains Electron-withdrawing side chains are solubilizing chains capable of pulling electron density from the π-conjugated backbones and lowering the material’s energy levels. A few electron withdrawing groups have been explored in this category: acetyl (-COR),62 ester (-COOR),63-64 amide (-CONHR),65 boron (-BR2)66 and sulfonyl (-SO3R)67 groups. Fluoroalkyl chains will be discussed separately in the next section. Similar to electron-donating moieties, electron-withdrawing side chains can also generate intramolecular interactions, such as hydrogen bonding and S(thienyl)-CO(acetyl) interaction.48,68 Andersson and coworkers prepared a set of thiophene– quinoxaline polymers (P5) with identical backbones modified with different electron-withdrawing groups adjacent to the thiophene rings, as illustrated in Figure 5.69 The LUMO energy levels were shifted to lower levels when the functional groups were varied between CH2, CF2, C=O and malononitrile. The HOMO levels displayed a similar trend. Unfortunately, this study did not describe the effects of these side chains on the film’s morphology or device performances.

Figure 5. HOMO (bottom pairs) and LUMO (top pairs) levels derived from square wave voltammetry peak onset (solid lines) and modeling of P5. Reproduced with permission from Ref 69 Copyright 2013 Elsevier Ltd.

2.2.3 Conjugated side chains Using conjugated side chains differs strategically from the previous two cases. These side chains usually affect both the HOMO and LUMO levels of the polymers, while broadening the absorption spectrum in the visible region.70-71 Hou and Li utilized this strategy to generate high-performance polythiophenes71. The absorption profile of the resulting polymers (P6) was tuned by controlling the ratio of the conjugated vs. linear side chains, as illustrated in Figure 6. The authors extended the scope of this method to include a donor/acceptor-type conjugated polymer by replacing the alkoxy group with an alkylthienyl group on the donor unit (benzodithiophene (BDT)).72 This structural modification significantly enhanced both the hole mobility and photovoltaic properties.

Figure 6. Absorption spectra of the P6 and P3HT polymer films . Reproduced with permission from Ref 71. Copyright 2006 American Chemical Society.

Jen and coworkers proposed using the electron acceptors as part of the conjugated side chains.73-74 Strictly speaking, these acceptor-conjugated side chains cannot be considered solubilizing groups because they usually require additional flexible


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chains make them processable in solution. These conjugated polymers are called “two-dimensional conjugated polymers,” and increasing interest in them was recently observed.75-82 2.2.4 Terminally functionalized hybrid side chains In the first hybrid side chain category, the functional moieties are directly connected to the conjugated backbones. Terminally functionalized hybrid side chains have chain-end modifications.83 Because these functional moieties are far away from the conjugated backbones, they usually do not affect the polymers’ electronic properties directly. Instead, they affect the film’s morphology. Jen and co-workers designed this type of polymer (P7-I) by attaching a terminal cyano group to a hexyl group (Figure 7).83 In the P7-I/PC71BM blends, different morphologies were observed. P7-I/ PC71BM mixed well due to the components’ similar surface energies relative to the parent polymer without cyano groups. Swager and coworkers prepared a series of sidechain-functionalized poly(thiophene)s (P7-II), as presented in Figure 7.84 The authors used these polymers as additives in P3HT/PCBM bulk heterojunction solar cells and found that the additive content significantly affected the PCEs of the ternary blends. At higher additive loadings (>5 wt %), a detrimental nanoscale phase separation was observed; the solar cells exhibited high series resistances and low overall PCEs. At low loadings, the short circuit current of the corresponding devices increased, while the series resistances decreased. These devices exhibited better PCEs than the P3HT/PC61BM reference cells. The authors proposed that P7-II could selectively localize at the interface between the rr-P3HT and PCBM phases. They also suggested that aromatic moieties at the sidechain termini introduced a dipole at the polymer–fullerene interface, lowering the rate of bimolecular recombination and enhancing the charge collection across the active layer. Because the authors did not measure the surface energy of these polymer additives (given a PCE of 5.3% by P7-II-f as an additive), it remains unclear whether the better mixing of P7-II with PCBM is the result of the surface energy (similar to the previous study) or the stronger interactions between end-chain aromatic moieties and PCBM. , these two studies suggested that the side chain termini may affect film morphology and effectively tune the interactions between polymers and fullerenes. 2.3 Ionic Side Chains Combining ionic side chains with conjugated backbones forms conjugated polyelectrolytes (CPEs)85 that are usually soluble in water or polar organic solvents, such as dimethylformamide and methanol.86 Therefore, conjugated polyelectrolytes can be processed using orthogonal solvents directly in contact with conventional conjugated polymers, facilitating surface and interface tuning. Conjugated polyelectrolytes were also investigated extensively in various fluorescence-based sensor applications87-89 and electrochemical transistors90. In photovoltaic applications, conjugated polyelectrolytes were used as a light absorbent in the active layer.91-92 More importantly, conjugated polyelectrolytes were used as an interfacial layer to tune electrode work functions or to act as charge transporting (blocking) materials.5,93-95 Bazan, Heeger, Cao and others studied the functional mechanism of utilized by the conjugated polyelectrolytes in OPVs and OFETs.96-97 An example using conjugated polyelectrolytes as an interfacial layer for solar cells is presented in Figure 8.98 The CPE layers (P8-I/II) were directly spin-casted onto the

Figure 7. Molecular structures of polymers (P7-I/II) with terminally functionalized alkyl chains.

solar cell in a methanol solution, increasing both the Jsc and Voc. The methanol-treated control device also exhibited an enhanced Voc, while the Jsc remained unchanged. The authors suggested that the Voc enhancement might be caused by the swelling and redistribution of BHJ components at the interface. Further studies concluded that the decreased the cathode work function and the reduced recombination of holes at the cathode contributed to the enhanced device performances observed when CPE layers were used.93

Figure 8. Molecular structures of the materials (P8)

The ionic side chains’ counterions are important for tuning the electronic properties of conjugated polyelectrolytes.99-101 Bazan and coworker prepared two conjugated polyelectrolytes (P9-II/III) with narrow band gaps and studied their charge transport properties (Figure 9). The ionization potential and electron affinity were measured by ultraviolet photoelectron spectroscopy (UPS); these experiments revealed more positive values for P9-III (4.91 and 3.53 eV) and P9-II (4.70 and 3.31 eV) relative to the neutral P9-I (4.49 and 3.05 eV), indicating the polyelectrolytes were less prone to oxidation. The weaker pyridinium/BIm4– ion-pairing may lead to a stronger electrostatic effect near the conjugated backbone. The neutral poly-



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mer P9-I exhibited p-type characteristics. However, P9-III exhibited n-type charge transport behaviors, and P9-II demonstrated no FET behaviors. The authors speculated that ionic functionalities might provide electrostatic dipoles at the electrode, reducing the barriers for charge injection. The lowered orbital energy levels might stabilize the radical anion (polaron), which is unavailable for the neutral conjugated polymers. Clearly, the mechanism is not clear at this stage. However, this study indicates that donor-acceptor-type conjugated polyelectrolytes with low band gaps generate new routes toward novel functional π-conjugated materials with unique properties that are otherwise inaccessible.

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decyl chains to study these materials’ self-assembly behavior at the water/air interface.112 The conductivity of these polymers was ~1 to 50 S cm-1 after iodine doping and was one to two orders of magnitude lower than that of polydodecylthiophene. However, Patil and coworkers prepared a copolymer (P10) based on diketopyrrolopyrrole–diketopyrrolopyrrole (DPP-DPP) where one DPP has a triethylene glycol chain and the other has a branched alkyl chain, as displayed in Figure 10. The electron mobilities approached 3 cm2 V–1 s–1 with a current on/off ratio on the order of 104; these values were some of the best for n-channel charge transport.113 This study unveiled a promising future for this combination.

Figure 10. Transfer characteristics of the top-gate–bottom-contact (TG–BC) transistors based on P10. Reproduced with permission from Ref.110 Copyright 2012 American Chemical Society. Figure 9. Molecular structures (P9) used for field-effect transistors.

2.4 Oligoether Side Chains The interest in oligoether side chains is mainly attributed to their hydrophilicity, their ability to render solubility in polar solvents without using ionic groups, and their tendency to complex with metal ions. A few oligoether-containing polymers were reported, including polythiophene,102-104 poly(phenylene vinylene),105 and polyfluorene.96,106-107 Roncali and coworkers prepared and studied poly[3-(3,6dioxaheptyl)thiophene] (PDHT).102 Although the ether groups were physically decoupled from the conjugated backbone by a -(CH2CH2)- spacer, the ethylene oxide chain at the β position caused observable changes in both the electrochemical and optical properties of PDHT: specific electrochemical responses toward lithium cations, significantly increased hydrophilicity, and a 30-nm bathochromic shift in the absorption maximum, relative to the alkyl analogue poly(3-heptylthiophene). Despite these features, conjugated polymers with only oligoether side chains were uncommon in OPVs and OFETs. Presumably, these polymers have a high affinity for moisture and ionic impurities because they are compatible with polar solvents, including water. This tendency inevitably causes poor stability and performance. Xiao and coworkers prepared poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methyl thiophene) and investigated its charge transport properties.103 Hole mobilities of 10−5 cm2 V−1 s−1 were observed and were several times lower than their alkyl analogues.22,108 In addition, the film’s morphology was unstable under ambient conditions because oligoether chains typically have a low glass transition temperature (below room temperature). Combining alkyl chains with oligoether chain might be a viable solution.109-111 McCullough and coworkers prepared amphiphilic polythiophene with alternating triglyme and do-

Hayward, Emrick and coworkers utilized a different strategy: using oligoether chains as side chains to obtain hierarchical functional structures.114 They prepared a completely conjugated polythiophene di-block copolymer (P11) containing nonpolar (hexyl) and polar (triethylene glycol) side chains (Figure 11). The triethylene glycol chains provided a large change in solubility, modulating the crystallization mechanisms. Further, complexing TEG side chains with K+ ions formed superhelical structures. This study revealed that the morphologies of materials could be tailored by choosing incompatible side chains. These structures might be suitable for optoelectronic devices, such as double-cable-like polymer solar cells.115

Figure 11. Molecular structure of P3HT-b-P3(TEG)T (P11) diblock copolymers and schematic representation of their assembly into superhelical structures via crystallization with potassium ions. The TEM images of P11 after KI addition revealed helical ribbons with a regular pitch. Reproduced with permission from Ref 114, Copyright 2011 American Chemical Society.

2.5 Fluoroalkyl Side Chains Fluoroalkyl side chains were used in conjugated polymers because they have unique properties, such as hydrophobicity, rigidity and thermal stability, as well as chemical and oxida-


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tive resistance and self-organizing ﬂuorinated chains.116-127 Roncali, Lemaire and coworkers synthesized semi-fluoroalkyl thiophene monomers.128 Leory and coworkers synthesized perfluoroalkyl thiophene derivatives.129 Collard and coworkers produced various fluoroalkyl-substituted polythiophenes over the past two decades, including alternating polythiophenes bearing hydrocarbon and fluorocarbon side chains and their block copolymers.116-122,130 In contrast to the previously mentioned combinations of side chains (i.e., alkyl and oligoether), alkyl and fluoroalkyl segments are both hydrophobic. When fluoroalkyl and alkyl chains are simultaneously introduced to a polymer, the two segments usually segregate, producing a highly ordered crystalline material.116,120,125 This behavior allows control over the molecular packing. Using fluoroalkyl chains is hindered by their limited solubility in common organic solvents, creating problems during purification and processing. One solution to this problem may be to use supercritical carbon dioxide (scCO2), which is both environmentally benign and fluorophiphilic.121,131-132 Even though it is not mentioned during conjugated polymer studies, fluoroalkyl chains cause concern over their potential environmental impact. Because fluorocarbons are extremely stable and are bioaccumulative.133-134 Currently, the use of fluoroalkylated conjugated polymers for OFETs and OPVs is rare.120,124-125 Collard and coworkers prepared a series of regioregular poly(3-alkylthiophene) (m = 5, 8, 11; n = 4; p = 4, 8, 12) (P12), with alternating alkyl and semifluoroalkyl substituents to study these materials’ selfassembly and charge transport properties, as illustrated in Figure 12.120 The authors suggested that the amphiphilic regioregular alternating copolymers self-assembled into a highly ordered bilayer lamellar structure in the solid state because the hydrocarbon and fluorocarbon chains were immiscible. Hole mobilities approaching 1.45 × 10−2 cm2 V-1 s-1 were observed in field-effect transistors using P12 as the semiconducting layer, similar to regioregular poly(3-alkylthiophenes).22

In an inverted organic solar cell, the block copolymer was placed on top of a P3HT/PCBM blend to form an interfacial dipole that shifted the work function of the anode. Simultaneously, the P3HT block could interact with the P3HT donor. When using this self-assembled hole transport layer to align the energy levels, the device performances improved (50% PCE increase over a PEDOT:PSS control device), and longterm stability (>300 h in air) was achieved.124 2.6 Latently Reactive Side Chains The need for polymer films with improved morphological stability (e.g., solvent resistance, thermal and photochemical stabilities) and patterning capabilities has driven the development of conjugated polymers that can be treated to remove the flexible side chains. Additionally, removing excess insulating side chains might benefit the charge transport g properties of the polymer. Using thermally or photo- cleavable side chains is therefore attractive. Various latently reactive side chains, including oxetanes,135-138 nitrenes,139 vinyl groups,140 azides,106,141-142 bromides,143-145 acrylates,146 Diels-Alder components,147 thermally cleavable esters,148-152 replaceable activated ester groups,153 ketal substituents,154 tetrahydropyranyl groups (THP),155-156 acetyl groups,157 and silyl groups, have been reported.158-159 Based on the mechanism for solubility loss, these side chains may be divided into three groups. The first group can form crosslinking networks. To achieve homogeneous and crack-free layers, the crosslinking process should have desirable features, such as little volume shrinkage, conversion times and quantitative yields. Of the various functional groups, azide-, bromide- and oxetane-containing side chains meet these requirements, making them the most popular.135-138,141-142,144-145 An example with an azide unit appended to P3HT (P13) is displayed in Figure 13.141 Crosslinked P3HT-azide copolymers had enhanced solvent resistance and morphological stability. The bulk-heterojunction OPVs containing P3HT-azide copolymers demonstrated efficiencies above 3.3% after 40 h annealing at 150°C.

Figure 12. Schematic image of the lamellar structure of alternating copolymer poly(3-alkylthiophene)s (P12).

Polythiophenes with alternating alkyl and semifluoroalkyl substituents tend to self-assemble, making these polymers very attractive when forming self-assembled monolayers (SAMs) for ionization energy tuning. Tajimi and coworkers described the surface segregation behavior of regioregular poly[40dodecyl-3-(1H,1H,2H,2H-perﬂuorooctyl)-2,20-bithiophene] (P3DDFT) on the surface of poly(3-dodecylthiophene) (P3DDT).125 The semifluoroalkyl chains aligned at the air/solid interface to form a large molecular dipole moment that continuously shifted the ionization energy. Block copolymers containing P3HT and poly(3fluoroalkylthiophene) also formed self-assembled monolayers.

Figure 13. Structures of P13 and its application as hole transporting materials for solvent-resistant OTFTs and in situ compatibilizer for thermally stable OPVs. Reproduced with permission from Ref.141 Copyright 2012 American Chemical Society.

The second group allows the removal of entire side chains. Some examples are displayed in Figure 14. The silyl groups (P14-I) and tertiary esters (P14-II) were removed by acid and thermal cleavage, respectively.158 Clearly, polymer films formed using this approach might present the largest optical density achievable for a given thickness. However, the disadvantage of this approach is also obvious. The film quality is



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usually poor due to the generation of small molecules as side products during the cleavage process.

Figure 14. Two representative polymers (P14-I/II) with removable side chains and their respective cleavage methods.

The third group utilizes side chains that may be partially cleaved. This partial cleavage causes drastic changes in solubility.151,154-155,157 For instance, a tetrahydropyranyl group (THP) group was with acid.155 The resulting alcohol became scarcely soluble in the original solvent, as illustrated in Figure 15. Holdcroft and coworkers applied this strategy to pattern P15-a using chemically amplified photolithography and soft lithography.155

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3. SIDECHAIN ENGINEERING ENABLES HIGH PERFORMANCE CONJUGATED POLYMERS Our group has actively pursued side chain engineering. In this section, we share two cases in which side chain engineering was effective for obtaining high performance conjugated polymers. The first case applied side chain engineering to generate high performance conjugated polymers for OFETs, as presented in Figure 16. Pei and coworkers first reported hole mobility over 1 cm2 V-1s-1 in an isoindigo-based conjugated polymer (P16-I) incorporated in air-stable field-effect transistor devices.160 Based on these results, we hypothesized that charge transport might improve if the π−π stacking distance between the conjugated backbones were shortened. Consequently, siloxane hybrid chains were designed. Because we had previous experience with isoindigo molecules,161-163 We used isoindigo-based polymers as a model system. The designed polymer with a hybrid side chain (P16-II) exhibited improved charge transport properties and hole mobilities approaching 2.48 cm2 V-1s-1.9 We also demonstrated that this approach shortened the π−π stacking distance in diketopyrrole(DPP)-based conjugated polymers.164 Pei and coworkers subsequently moved the branching site from the 2-position to the 3-, 4- and 5-positions and observed that the π−π stacking distances of the polymers (P16-III) do not decrease beyond the 3-position.20 Their effort led to the hole mobility of

Figure 15. a) Scheme depicting the chemically amplified photolithographic process and b) a micrograph of patterned polymer. Reproduced with permission from Ref.155 Copyright 2002 American Chemical Society.

The introduction of crosslinking bridges, the elimination of protective groups, or the removal of entire side chains from conjugated polymers usually destroys/disturbs the polymer’s molecular packing and morphology, dramatically reducing their performance. This is important to consider when designing latently reactive side chains. Developing conjugated polymers with ideal latently reactive side chains remains a challenge. These side chains have not been successfully implemented in high performance conjugated polymers yet, especially for OPV or OFET applications. However, for future commercialization of organic electronics with required stability and reliability, these developments are crucial.

Figure 16. A representative case of side chain engineering leading to high performance conjugated polymers for OFETs.

3.62 cm2 V-1s-1, four times higher than the originally reported polymer. Yang and Oh used the siloxane hybrid side chains in


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another DPP-polymer using diketopyrrolopyrrole–selenophene as the backbone (P16-IV),165 generating a hole mobility of 3.97 cm2 V-1s-1; this polymer also revealed a high electron mobility (2.20 cm2 V-1s-1). When using the siloxane hybrid side chains while tuning the spacers in the diketopyrrolopyrrole–selenophene polymers, Yang and Oh achieved record values for hole mobility (8.84 cm2 V-1s-1) and electron mobility (4.34 cm2 V-1s-1).166 This case clearly demonstrated the power of side chain engineering for tuning charge transport. For the second case, side chain engineering generates processable polymers without sacrificing performance. Specifically, low molecular weight polystyrene (PS Mn =1,300 g/mol) side chains containing isoindigo monomers were incorporated into isoindigo conjugated polymers (P17) via random copolymerization, as illustrated in Figure 17. Incorporating the PS side chains imparted better solubility relative to their counterparts without the PS side chains, leading to improved batch-tobatch reproducibility and thin film processability after recycling gel permeation chromatography. The bulk heterojunction solar cell devices fabricated with these PS-containing (10 mol%) copolymers demonstrated significantly improved performances: the maximum PCE was >7%, and the open circuit voltages (Voc) were ≥0.95 V versus the highest reported performances (PCE = 6.3% and Voc = 0.70) by similar isoindigocontaining polymers.167 This study reinforces the utility of the side chain engineering approach. Currently, our lab is expanding the scope of these designs to other conjugated systems.

Figure 17. A representative case of side chain engineering leading to high performance conjugated polymers for OPVs.168

4. SUMMARY Side chains have primarily been utilized as solubilizing groups in conjugated polymers. They have largely been underestimated when developing conjugated polymers, even though various side chains have been introduced in recent years. The selected studies clearly illustrated the promising future of side chain engineering. Obviously, the applicability of side chain engineering extends beyond the examples discussed here. This Perspective article advocates expanding the role of polymer’s side chains during the development of high performance conjugated polymers for organic solar cells and fieldeffect transistors. Side chains engineering is critical for solving issues in material synthesis, processing and device fabrica-

tion, such as poor batch-to-batch reproducibility and poor thin film processability. Combining different flexible chain types and introducing polymer chains might provide solutions for these problems. Side chain engineering is also important for designing new functional conjugated polymers, such as conjugated polymer additives, conjugated polyelectrolytes and cleavable/crosslinkable conjugated polymers. Finally, side chain engineering may be important for future designs of stretchable and self-healing conjugated polymers.169

AUTHOR INFORMATION Corresponding Author [email protected]

ACKNOWLEDGMENT This work was supported in part by the Center for Advanced Molecular Photovoltaics made by King Abdullah University of Science and Technology (KAUST). We also acknowledge the support of the Global Climate and Energy Program at Stanford and the Air Force Office for Scientific Research (FA9550-12-1-0190). We are grateful for the reviewers’ comments, and thank Dr. Jeffrey B.-H. Tok for his valuable input.

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Short Biography Jianguo Mei is currently a Postdoctoral Fellow in Professor Zhenan Bao’s group at Stanford University. A native of China, he received his college education from the Hefei University of Technology, and obtained his Bachelor’s and Master’s Degree in Chemical Engineering. He later moved to University of Florida, where he received his doctoral training (2005-2010) with Professor John R Reynolds. There, he gained expertise in design, synthesis and characterization of solution-processable organic electroactive materials for electronic devices. During 2011-2012, he was a Principal Investigator at DuPont Research and Development Center in Shanghai, China. Prior to that, he was a Camille and Henry Dreyfus Environmental Chemistry Postdoctoral Fellow at Stanford University. Zhenan Bao is a Professor of Chemical Engineering andby courtest professor of Chemistry and Materials Science & Engineering at Stanford University. Prior to joining Stanford in 2004, she was a Distinguished Member of Technical Staff in

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Bell Labs, Lucent Technologies from 1995–2004. She has over 280 refereed publications and over 40 US patents. She is a Fellow of SPIE, ACS and AAAS and a recipient of the ACS Polymer Division Carl S. Marvel Creative Polymer Chemistry Award 2013, ACS Author Cope Scholar Award 2011, Royal Society of Chemistry Beilby Medal and Prize 2009, IUPAC Creativity in Applied Polymer Science Prize 2008, American Chemical Society Team Innovation Award 2001, and R&D 100 Award 2001.

TOC


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Side Chain Engineering in Solution-Processable Conjugated

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