Immobilization Strategies for Organic Semiconducting Conjugated

Review Cite This: Chem. Rev. 2018, 118, 5598−5689

pubs.acs.org/CR

Immobilization Strategies for Organic Semiconducting Conjugated Polymers Jan Freudenberg,†,‡ Daniel Jan̈ sch,†,‡ Felix Hinkel,† and Uwe H. F. Bunz*,† †

Chem. Rev. 2018.118:5598-5689. Downloaded from pubs.acs.org by UNIV OF TOLEDO on 09/30/18. For personal use only.

Organisch-Chemisches Institut and Centre of Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225 and 270, 69120 Heidelberg, FRG ‡ InnovationLab, Speyerer Str. 4, 69115 Heidelberg, FRG ABSTRACT: This Review details synthetic routes toward and properties of insoluble polymeric organic semiconductors obtained through desolubilization strategies. Typical applications include fixation of donor−acceptor bulk-heterojunction morphologies in organic photovoltaic cells, cross-linking of charge transport materials and active emitters in light emitting diodes or similar devices, and immobilization of morphologies in field effect transistors. A second important application is the structuring of organic semiconductors, using them as photoresists. After desolubilization, removal of the nonirradiated resist leads to elevated, micron-sized features of the semiconductor. In this Review, different strategies for desolubilization are covered. By photochemical or thermal cleavage of solubility-mediating groups such as esters, sulfonium salts, amides, ethers, and acetals or by retro-Diels−Alder reactions, volatile elimination products and the insoluble semiconductor are formed. In another case, desolubilization is achieved by cross-linking via functional groups present in the polymer side chains including vinyl, halide, silicone, boronic acid, and azide functionalities, which polymerize thermally or photochemically. Alternatively, small molecular additives such as photoacids, oligothiols, or oligoazides result in network formation in combination with compatible functional groups present in the immobilizable polymers. Advantages and disadvantages of the respective methods are discussed.

CONTENTS 1. Introduction 1.1. General Considerations 1.2. Scope and Limitations 2. Thermal Cleavage 2.1. Temperature Tolerance of Organic Electronic Devices 2.2. Classic Sulfonium and Related Species 2.3. Thermal Ester Cleavage 2.4. Thermal Amide Cleavage 2.5. Ether and Acetal Cleavage 2.6. Retro-Diels−Alder Reactions 3. Photochemical Cleavage 3.1. Photoacid Generators (PAGs) 3.2. Cleavage of Nitrobenzylic Esters 3.3. Cleavage of Other (Aromatic) Esters 4. Thermal Cross-linking 4.1. Vinyl Polymerizations 4.2. Azide-Containing Species 4.3. SiO- and BO-Containing Species 4.4. Diels−Alder Reactions 4.5. Free Radical Approaches 4.6. Nucleophilic Cross-linking 5. Photochemical Cross-linking 5.1. Cross-linking with Electron Beams 5.2. Oxetane as Cross-linker 5.3. Oxiranes as Cross-linker © 2018 American Chemical Society

5.4. Externally Added Diazides as Cross-linkers 5.5. Thiol−Ene Click Chemistry as Cross-linker and Disulfide Formation 5.6. Photochemical Acrylate and Olefin Polymerization 5.7. Cinnamic Acid-, Coumarin-, and Uracil-Based Photodimerizations 5.8. ω-Bromoalkenes as Cross-linkers 5.9. Comparative Studies 5.10. Comments on Radiation-Based Desolubilization Methodologies 6. Ionic Cross-linking 7. Miscellaneous Methods 8. Conclusions and Perspectives Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Dedication References

5599 5599 5604 5604 5604 5605 5613 5621 5624 5629 5632 5632 5632 5634 5635 5636 5640 5644 5644 5646 5647 5650 5651 5652 5659

5659 5665 5667 5670 5673 5676 5678 5678 5679 5680 5680 5680 5680 5680 5680 5681 5681 5681

Received: January 30, 2018 Published: May 30, 2018 5598

DOI: 10.1021/acs.chemrev.8b00063 Chem. Rev. 2018, 118, 5598−5689

Chemical Reviews

Review

Chart 1. Structures and Abbreviations of Commonly Used Materials in Organic Electronic Devices

1. INTRODUCTION

exploitation of orthogonal solvents, which leave the previously deposited film undissolved. The disadvantages of chemical vapor deposition (CVD) are the immense material losses (>70%) due to the requirement for masking and limitation to relatively low molecular weight materials, which are still sublimable. This method excludes

1.1. General Considerations

Immobilization strategies for organic semiconductors concern a wide scientific audience from the fields of synthetic organic and polymer chemistry, to physical chemistry, to material science, and to device engineering and device physics. This Review intends to guide researchers in selecting and implementing the right immobilization strategy for polymeric conjugated organic semiconductors, as there are numerous well developed synthetic concepts for immobilization of semiconductors. This Review aims to be a comprehensive, authoritative, critical, and readable contribution and a hand-guide toward desolubilization of organic semiconductors. Organic semiconductors have been at the center of attention for more than 25 years. The function and performance of organic electronic devices such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaic (OPV) cells is not only governed by the molecular structure of the organic semiconductors. Processing and the resulting morphology is of comparable or perhaps even higher significance. Most modern devices contain multiple layers of organic materials (commonly used materials are depicted in Chart 1), be it OLEDs or OPV cells. A significant challenge is the consecutive deposition of the organic layers. This process requires sharp physical separation of the different, subsequently deposited layers, e.g. emissive, hole blocking, electron injection, etc. layers in an OLED. There should only be minimal or ideally no intermixing at the interfaces, as this leads to terminal device failure or at least to significantly reduced performance. The classic approaches to this problem are (1) vacuum sublimation, i.e. gas phase deposition of different layers of organic materials, (2) inorganic and therefore completely insoluble layers plus one solution processed layer, and (3)

Figure 1. Color evolution during a standard Gilch synthesis within 5 min after addition of KOtBu and precipitation in methanol. Adapted with permission from ref 43. Copyright 2009 John Wiley and Sons.

processing of conjugated and other polymers, as they are nonvolatile even in high vacuum. Conjugated polymers, however, offer the possibility of both inter- and intrachain transport of charge carriers through longer percolation pathways, paired with beneficial properties such as stretchability1−4 and self-healing capabilities,5 allowing for future application in 5599


Chemical Reviews

Review

Chart 2. Desolubilization Methods Summarized in This Review

flexible electronics and wearables. Consequently, there is and has been an urgent need to develop solution processable organic polymeric semiconductors that form homogeneous thin films, which can be desolubilized, to permit deposition of solution

processed organic layers on top of each other. This approach is even more important in printing processes. Here, obviously one also needs strategies in place that allow the consecutive deposition of organic layers, without partially dissolving the 5600


Chemical Reviews

Review

Table 1. Reviews that Cover Aspects of Immobilization Strategies for Organic Semiconductors ref

Authors

8

Ho, S.; Liu, S.; Chen, Y.; So, F.

9 10

Huang, F.; Cheng, Y.-J.; Zhang, Y.; Liu, M. S.; Jen, A. K.Y. Kraft, A.; Grimsdale, A. C.; Holmes, A. B.

11

Li, Y.; Zou, Y.

12

Manouras, T.; Vamvakaki, M.

13

Mei, J.; Bao, Z.

14

Rumer, J. W.; McCulloch, I.

15 16

Wantz, G.; Derue, L.; Dautel, O.; Rivaton, A.; Hudhomme, P.; Dagron-Lartigau, C. Zhong, C.; Duan, C.; Huang, F.; Wu, H.; Cao, Y.

17

Zuniga, C. A.; Barlow, S.; Marder, S. R.

Title

Citation

Review of recent progress in multilayer solution-processed organic lightemitting diodes. Cross-linkable hole-transporting materials for solution processed polymer light-emitting diodes. Electroluminescent Conjugated PolymersSeeing Polymers in a New Light. Conjugated Polymer Photovoltaic Materials with Broad Absorption Band and High Charge Carrier Mobility. Field responsive materials: Photo-, electro-, magnetic- and ultrasoundsensitive polymers. Side Chain Engineering in Solution-Processable Conjugated Polymers.

J. Photon. Energy 2015, 5, 57611. J. Mater. Chem. 2008, 18, 4495. Angew. Chem. Int. Ed. 1998, 37, 402−428. Adv. Mater. 2008, 20, 2952− 2958. Polym. Chem. 2017, 8, 74− 96. Chem. Mater. 2014, 26, 604−615. Mater. Today 2015, 18, 425−435. Polym. Int. 2014, 63, 1346− 1361. Chem. Mater. 2011, 23, 326−340. Chem. Mater. 2011, 23, 658−681.

Organic photovoltaics: Cross-linking for optimal morphology and stability. Stabilizing polymer-based bulk heterojunction solar cells via crosslinking. Materials and Devices Toward Fully Solution Processable Organic LightEmitting Diodes. Approaches to Solution-Processed Multilayer Organic Light-Emitting Diodes Based on Cross-linking

previous ones, vital for fully solution processed OLEDs and printed circuits. A somewhat different, yet related field of immobilization covers fixation of specific morphologies which are a prerequisite for stable device performance. Such a task is necessary in the

production of blends for bulk heterojunctions in OPVs. Once the active material is processed into the desired phase-separated morphology, chemical, photochemical, radiation induced, or thermal cross-linking or immobilization by removal of side chains

Figure 2. (a) Structures of xanthate precursors for photopatterend PPV films. (c) Schematic of photolithographic process with and without PAG addition. Optical (a and f−h) and fluorescence (e and i) microscopy images of patterned PPV (d and e), 3.19 (f and h), and 3.20 (g and i). The numbers indicate the feature size and spacing (in microns). Adapted from refs 43 and 44 with permission of The Royal Society of Chemistry. 5601


Chemical Reviews

Review

Scheme 1. Overview over Syntheses of PPV

Scheme 2. Synthesis of Various Monomers for Polymerization to PPV

Figure 3. Positive and negative resist approaches toward patterning PPVs.

5602


Chemical Reviews

Review

Scheme 3. Synthesis of Different Poly(arylene)vinylenes and Their Bis-dithiocarbamate or Dithiocarbonate Monomers

etc. fixes such a morphology and, thus, improves the long-term stability of OPV elements. A further application of desolubilization of organic semiconductors comprises photoresists. Here, the irradiation of a thin semiconductor film through a mask or spatially limited laserwriting patterns a specific area. Removal of the unirradiated, still soluble semiconductor by washing leaves the structured, insoluble organic semiconductor on a micron-size scale. All of these applications depend upon successful cross-linking and other desolubilization strategies provided by organic synthesis. As a consequence, while the immobilization may not look like a glamorous task, it is a critical tool for advanced organic electronics, when dealing with solution processable materials. As an important application, “all-printed” organic electronics is

envisioned. Such devices would be less expensive with respect to processing and production on large scale. Our twist on this topic will be to concentrate upon and to illuminate the organic synthetic strategies and aspects of making solution processable organic semiconductors insoluble. We comprehensively look at different immobilization techniques of conjugated organic polymers (Chart 2). Besides informing our readership about the developed strategies, we will discuss their strengths and their weaknesses, deriving general rules to allow decisions about which techniques or strategies are the most appropriate for given problems. We look at what kind of chemical strategy is useful for a specific synthetic problem in semiconductor desolubilization and which effect this immobilization strategy imposes on device performance. 5603


Chemical Reviews

Review

only insofar as they relate to the synthetic organic issues within the reports. This approach dissects and discusses the role of the preparative synthetic effortsit is also valuable as a divining rod for all scientists that will work in this area to select suitable immobilization strategies. This Review covers the literature from 1990 to 2017 with specific emphasis upon the developments in the past decade. Into this context, seminal older papers are embedded to give a sense of the achievement of the authors and conceptual issues, but for example, PPV-type synthesis using sulfonium salts and related routes will not be covered in full detail, as that alone would entail a large number of papers. Instead, the more modern developments that have changed the field are highlighted, focusing on materials applied and applicable in organic electronic devices.

Scheme 4. Two Potential Degradation Pathways Yield Thiophene Carbaldehydes

2. THERMAL CLEAVAGE 2.1. Temperature Tolerance of Organic Electronic Devices

Prior to covering thermally induced desolubilization strategies, via either cleavage of solubility-mediating groups (this chapter) or cross-linking reactions (section 4), a general discussion about thermal stability of components within organic electronic devices and their fabrication process is required. For fully solution processed flexible organic devices, high-speed and large-area manufacturing via roll-to-roll (R2R) processes is the future direction. Traditionally, three types of substrates are compatible to R2R processes: metal foils (steel,18 titanium,19 etc.), thin glass, and plastics. Although planar and rigid substrates such as glass and metal tolerate harsh conditions, i.e. even temperatures above 300 °C, those substrates are not compatible with flexible organic devices available through R2R processes. Plastics, however, show the desired combination of low costs, high smoothness, transparency, and high flexibility but also lowest tolerance to temperatures due to low glass transitions/melting points, deformation, and material migration.20−22 From the material’s perspective, flexible substrates as the foundation of flexible

1.2. Scope and Limitations

This Review is focused on the immobilization of conjugated polymers by network formation or modulation and manipulation and removal of side chains or auxiliary substituents. The synthetic effort necessary to achieve this goal and the efficiency and the quality of these strategies are examined. As immobilization strategies for small molecules or nonconjugated polymers have been reviewed recently (see Table 1), they will not be discussed (comprehensively). 3D printing is also beyond the scope of this Review, as this topic has been reviewed by Mülhaupt et al.6 and Wegener and Barner-Kowollik.7 While discussing success metrics, the materials science and device performance aspects of the publications are looked upon, but

Scheme 5. Synthesis of 9-[(tert-Butyldimethylsilyl)oxy][2.2]paracyclophan-1-ene (5.4) and Its ROMP to PPV and BlockCopolymer 5.10 Combining a Living Polymerization System with Mild Conversion of Precursor Polymers

5604


Chemical Reviews

Review

Figure 6. (a) Schematic self-assembly and (b) thermolysis of oligothiophenes on a solid surface. (c) 10 μm × 10 μm AFM images of T6 films spun-cast from chloroform and then heated for 20 min at the indicated temperatures. The initially microcrystalline film transforms into crystalline molecular terraces after thermolysis. Adapted with permission from refs 66−68. Copyright 2004 and 2005 American Chemical Society. Figure 4. (a) Schematic of immobilization by PAG-initialized elimination of water from a ROMP-obtained prepolymer to give patterned PPV. (b) Photograph of the emission of PPV50-blockpolyNBE25 in a 15 by 15 array of alternating 125 and 250 μm spots. (c) Fluorescence photomicroscopy of PPV40-block-polyNBE70 under 200fold magnification. (d) Schematic cross section of circular, patterned LEDs. Adapted with permission from ref 62. Copyright 1997 John Wiley and Sons.

expensive poly(ether imide) foil (Ultem B, General Electric, Tg 217 °C), and polyarylate foil (Arylite A200HC, Ferania, Tg 330 °C). Zardetto et al. investigated PET and polyethylene naphthalate (PEN) as well as their heat-stabilized (HS) derivatives and indium tin oxide-coated conductive films and tested their optical properties in addition to their mechanical flexibility under stress, solvent resistance, and stability with respect to temperature treatment. The authors demonstrated that the limiting factor for the use of the substrates was the physical deformation of the plastic samples: A radius of curvature of 1 m was observed for HS-PET and HS-PEN at 180 °C. For ITO-coated PET films, fixation during thermal treatment led to decreased deformation with still good electrical and mechanical stability for temperatures up to 150 °C.21 Alternatively, regenerated cellulose and nanopaper are flexible, transparent, and biodegradable substrates,24−26 tolerating temperatures as high as 150 and 200 °C, respectively. However, there still remain challenges concerning morphology/roughness after processing and interfacial adhesion.27 The curing process of bulk heterojunction organic solar cells often requires temperatures above 200 °C for up to 2 h (P3HT:PCBM system), even without any desolubilization.28 Annealing steps that render a conjugated polymer insoluble often require extended times and/or higher temperatures which are not applicable to R2R production lines. As an illustrative example, a thermal treatment for 30 min at a speed of 1 m s−1 requires a heated production line of at least 1.8 km in length. This implies that the desolublization strategy for organic semiconductors has to be designed by chemists to be as mild as possible: To compete in the field of flexible plastic electronics in future applications, processes should stay below the critical ceiling treatment temperatures of around 170 to 180 °C on lowcost substrates (e.g., on PEN, nanopaper) with as short annealing periods as possible to be compatible with roll speeds and maximum allowed oven lengths in R2R processes.20

Figure 5. (a) 3D representation of the optical field strength in a thin film placed below the tip of an apertured SNOL probe. (b) Tapping mode AFM scan of institution’s logo. (c) magnification and (d) array of dots with exposure times of 50, 100, 200, 500, and 1000 ms from left to right by columns. (e) Cross section through this feature showing fwhm of approximately 55 nm. Adapted with permission from ref 65. Copyright 2010 John Wiley and Sons.

electronics have to be thermally and dimensionally stable and possess a low coefficient of thermal expansion as well as good solvent resistivity and barrier properties to keep out humidity and oxygen.23 Common and commercial examples are polyethylene terephthalate foil (biaxially oriented PET, Tenolan OAN, Tg 98 °C), polycarbonate foil (Makrofol, Bayer, Tg 148 °C), the more

2.2. Classic Sulfonium and Related Species

Elimination of solubilizing groups as volatile species produces insoluble organic semiconductors. Poly(p-phenylene-vinylene) (PPV) is a good electrical conductor after chemical or electrical doping.29−31 Early attempts to synthesize unsubstituted, high 5605


Chemical Reviews

Review

Scheme 6. Synthesis of Ester Substituted Oligothiophenes as Precursors for Alkene Terminated Oligothiophenes

molecular weight PPV by Wittig reaction of p-xylenebis(triphenylphosphonium chloride) with terephthalaldehyde,32 Gilch reaction of p-xylylidene dihalides,33 or polymerization of pxylylenebis(dimethylsulfonium tetrafluoroborate), as explored by Kanbe and Okawara,34 resulted in oligomers only, with no more than 9 to 11 monomer units per chain (Figure 1). Precursor routes, in which a soluble precursor polymer is formed, are an option to introduce sufficient processability. Wessling and Zimmerman investigated the polymerization of bis-sulfonium salts, yielding high-molecular weight, water-soluble polyelectrolytes.35−37 Treatment with an excess of base or thermal exposure in solid form generated PPV directly as insoluble and robust fibers, coatings, and foams. This concept has been applied by Karasz et al. 3 8 who polymerized p-xylylene-bis(dimethylsulfonium chloride) into a water-soluble sulfonium salt polyelectrolyte (Scheme 1). Thermal elimination of Me2S and HCl from precursor films cast from such an aqueous solution gave free-standing PPV films of good mechanical quality. The degree of conversion could be controlled and the electronic and conducting properties of doped samples related to the conjugation length of the resulting PPVs. In all of these precursor polymerization routes, the actual monomer (the p-quinodimethane system) is formed in situ by elimination. After polymerization a soluble precursor polymer is

formed, which subsequently is converted to the conjugated PPV.39 The reaction mechanism was elucidated from various monomers in different studies and involves either anionic and/or radical species.40−44 A real breakthrough in the field of organic electronics was reached by Friend et al.,45 who employed the Wessling method35 (Scheme 1) to prepare a water-soluble precursor-polymer applied for fabrication of large-area light-emitting diodes (LED). Homogeneous thin films (100 nm) of this polymer were spin-coated from methanol, and pyrolyzed at 250 °C in vacuum to give PPV. Tetrahydrothiophene as leaving group apparently decreased the temperature of the elimination reaction by 100 °C, due to suppression of side reactions by the removal of the more stable alicyclic sulfide.46−48 Air stable PPV films resulted, which remained uniform after thermal treatment and were used as the active element in large-area LEDs with quantum efficiencies up to 0.05%. The combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green−yellow region of the spectrum render PPV an attractive emitter. For synthetic work, Vanderzande contributed toward the development of precursor routes toward PPV and its derivatives. One specific case employs asymmetric monomers with a polarizer group (Pol = sulfonyl or sulfinyl) in the α′-position 5606


Chemical Reviews

Review

Scheme 7. Synthesis of the 7.6 Precursor Oligothiophene

Scheme 8. Synthesis of Carboxylate Substituted Polythiophenes

Figure 7. (a) Self- assembly of carboxylate substituted polythiophenes as connecting layer between titanium dioxide and P3HT in a solar cell stack. (b) Schematic of the formation of the dipole layer at the TiO2/ polymer interface due to acid−base interaction. (c) Flat-band energy diagram of the TiO2/polymer structure. Adapted with permission from ref 70. Copyright 2006 American Chemical Society.

and α-chloro-α′-alkyl-(aryl)sulfonyl-p-xylene were polymerized at −20 °C with sodium hydride as base.40,42,49−53 Another case employed bis-dithiocarbamate monomers (Scheme 1 (6)), which were polymerized by the addition of lithium diisopropyl amide (LDA) as strong base. The monomer synthesis is not as difficult as the synthesis of the ones employed in the sulfinyl route40,42,49−53 and proceeds without losing quality and processability of the precursor polymer. The prepolymer converts thermally when heated from ambient temperature to 350 °C (2 °C/min) under a continuous flow of N2. The thermal conversion into the PPVs was studied by in situ UV−vis and FT-IR spectroscopy, while heating the spincoated precursor polymer. The dithiocarbamate route is an alternative to the Wessling and Gilch methods, tackling the disadvantages associated with these routes, such as side reactions, gelation, and limited solubility of the precursor polymer and high dispersities observed for the xanthate route41 (vide supra). According to the authors,54 polymers of superior quality with lower defect levels are produced compared to the already existing sulfinyl route. This method allowed Vanderzande et al. to incorporate various aromatic and heteroaromatic cores (Scheme 3).55 The synthesis of a fluoranthenylenevinylene polymer

and a leaving group in the α-position of p-xylene (Scheme 1 (5) and Scheme 2). The investigated α-chloro-α′-alkyl-(aryl)sulfinyl5607


Chemical Reviews

Review

Figure 8. (a) Structure of bilayer heterojunction OPV cells. (b) Short circuit current for a device operated in vacuum under accelerated conditions with continuous illumination at 1 kW m−2, AM 1.5G, 72 °C. (c) Two-stage process for thermal cleavage. (d) Thermogravimetric data in the temperature range 25−475 °C at 50 °C min−1. The theoretical weight losses for the two processes are 32% (from PT1/9.5 to P3CT/9.6) and 14% (from P3CT/9.6 to PT), while the observed values are 22% and 13%, respectively. (e) 13C CP/MAS spectra (left) of the ester-functionalized polythiophene ((PT1/ 9.5)), heated to 210 °C (P3CT/9.6) and heated above 300 °C (PT) with expansions of the aromatic/aliphatic region to the right. (f) Schematic of solid state self-assembly of carboxylated polythiophene. Figure 8b and f reprinted from ref 72, copyright 2008, with permission from Elsevier. Figure 8d−e adapted with permission from ref 74. Copyright 2007 American Chemical Society.

Scheme 9. Synthesis of 13C-Labeled Ester Substituted Polythiophene

Figure 9. (a) Illustration of the device architecture. (b) Freshly prepared cell (top) and same cell after 150 h, 1000 W m−2, AM1.5G, 72 ± 2 °C, ambient atmosphere, 35 ± 5% relative humidity, showing bleaching and cracking (bottom). Reprinted from ref 77, copyright 2008, with permission from Elsevier.

(PFV) was demonstrated.56 The monomer 3.11 was constructed through a combination of a Knoevenagel condensation followed 5608


Chemical Reviews

Review

Figure 10. (a) Chemical transformation of PT1 (8.4) to P3CT at ∼200 °C and decarboxylation to PT at ∼300 °C and structures of [60]PCBM, [70]PCBM, and bis[60]PCBM. (b) Efficiency versus annealing temperature for the cells with PT1 and [60]PCBM (●) or [70]PCBM (red □) mixture measured under 1000 W m−2, AM1.5G, 70 °C conditions. The vertical lines show the onset of cleaving temperatures. (c) Simple photovoltaic test for solar cell efficiency. (d) Fabrication and immobilization of the polythiophenes to give P3CT using dissipative heating with an LED. Adapted with permission from refs 75, 78, and 80 Copyright 2008−2010 American Chemical Society.

by a Diels−Alder-retro-Diels−Alder sequence (Scheme 3). The initially formed diester 3.8 was transformed into the bisthiocarbamate 3.11, which, after addition of LDA, gave the neutral prepolymer. Conversion to PFV starts around 100 °C and is complete at 200 °C; the absorption profile at 420 nm reveals that degradation of PFV, as observed with thermal gravimetric analysis (TGA), does not start until 325 °C. Electrochemical conversion of the dithiocarbamate prepolymers leads to a polymer in which the fluoranthene groups are connected by ethano-bridges.57 The authors also studied the electrochemistry and optoelectronic properties of these PFVs. The use of cyclopenta-fused nonalternant hydrocarbons significantly increases the electron affinity of the resulting materials, which in contrast to other PPVs renders them as n-type semiconductors and a potential substitute to PCBM.56,57 The method of Vanderzande et al. is not only useful to make pure hydrocarbon based vinylene-connected polymers but it also works for thiophene-based monomers.55,58 Poly(2,5-thienylene vinylene) derivatives (PTVs), low band gap conjugated polymers, are attractive but require a tedious synthesis. The dithiocarbamate route was introduced as a synthetic approach toward PTV, but access to alkyl-PTV derivatives failed when

Figure 11. AFM image of PT layers from the thermal cleaving at 300 °C (surface roughness RMS ~3.0 nm) (top) and prepared by trichloroacetic acid (TCA) cleavage (RMS ~15 nm) (bottom). Reprinted from ref 82, Copyright 2016, with permission from Elsevier.

using LDA as the base in the polymerization reaction. Lithium bis(trimethylsilyl)amide (LHMDS), a non-nucleophilic, sterically hindered base, however, allows smooth synthesis of alkyl5609


Chemical Reviews

Review

Scheme 10. Synthetic Route to the Polymers 10.3 and 10.4

Scheme 11. Synthesis of a Carbamate Protected Bisthiophenyl-Pyrrole Unit and Copolymerization with Benzothiadiazole

Figure 12. Schematic of solvent vapor annealing of 8.4 films using CS2 and corresponding AFM images: a) formation of CS2/water droplets (d), b) creation of holes by CS2/water droplets on the surface (e), c) formation of fractal-like structures (f). Reprinted by permission from Springer Nature: ref 83 (2013). Figure 13. Thiophene-benzothiadiazole copolymers with different number of thiophenes per repeat unit as well as side chains.

PTV derivatives. An ordered structure forms for the 3-alkyl derivatives, suggesting a regioregular polymerization. An important recent application for precursor polymers that are thermally rendered insoluble is the simple, direct photopatterning of PPV, demonstrated by various groups. Johnson et al. patterned a xanthate precursor 3.16, first reported by Vanderzande et al.,41 well soluble in common organic solvents, and easily spun cast into thin films of high quality (Figure 2).59 Upon irradiation, partial photoelimination occurs, and the

untreated precursor polymer was washed away. The samples were baked at 275 °C under a flow of nitrogen for 30 min to fully convert the polymer backbone. By optimizing the photolithographic conditions, a spatial resolution of one micron is obtained, with minimal impact to the properties of the photopatterned PPV. In an extension of the xanthate method, the authors also 5610


Chemical Reviews

Review

Scheme 12. Synthesis of Two Isomers of Ester-Substituted Bisthiophenyl-Benzothiadiazole

Scheme 13. Synthesis and Thermal Desolubilization of Conjugated Polymers Based on Thiophene, Benzothiadiazole, and Pyrrole

Scheme 14. Synthesis of Low-Bandgap Thiophene-Benzothiadiazole Copolymers

5611


Chemical Reviews

Review

PTV. Then, the irradiation was continued through a mask in the presence of oxygen. The exposed parts were oxidized under chain breakage, leaving soluble fragments (Scheme 4), which dissolved and could be removed (Figure 3). This complementary method using photoresist approaches accesses negative and positive PPV features. Bazan et al.62 exploited ring-opening metathesis polymerization (ROMP) of 9-[(tert-butyldimethylsilyl)oxy][2.2]paracyclophan-1-ene (5.4), reported earlier.63 ROMP offers some advantages compared with other methods, i.e. the living polymerization allows control of the average degree of polymerization (Scheme 5). By the para-cyclophene method, block copolymers can be obtained tuning the solubility and viscosity.64 The elimination of the hydroxyl groups is performed at comparatively mild conditions (thermally at 110 °C, with HCl at 50−60 °C). Block copolymer 5.9 is spin coated together with the PAG triphenylsulfonium triflate on indium tin oxide (ITO)/ glass substrates and was irradiated through a mask, and a latent image of 5.10 is formed. Removing the soluble prepolymer develops the image when an optimum ratio of x/y is met. This approach was used to prepare an LED with emission from a 15 by 15 array of alternating 125 and 250 μm spots. The OLEDs were fabricated immediately after development; the smaller squares are approximately 25 μm on the side (Figure 4). Credgington et al.65 demonstrated, using known systems, discussed previously in this section, the fabrication of high resolution nanostructures from a PPV precursor (see Scheme 1 (4)) by scanning near-field optical lithography. The authors drew complex, reproducible structures with 65000 pixels and a lateral resolution below 60 nm (3900 h under accelerated testing.73 After one year, the efficiency went down to 20% of the initial value (Figure 8b). When the cell was exposed to oxygen, C60 but not the carboxylated polythiophene P3CT was sensitive toward degradation.71,72,74

Figure 15. GIWAXS patterns of 20.8a:PCBM thin films on PEDOT:PSS (a) before and (b) after thermocleavage at 200 °C. Insets show the small-range scattering range with the interlayer reflection indicated by an arrow. Schematic illustrations of the surface organization obtained from 20.8a after fabrication with (c) edge-on and (d) face-on arrangement after thermal annealing. Reprinted with permission from ref 98. Copyright 2015 American Chemical Society.

Scheme 21. Synthesis and Thermal Treatment of an n-Type Semiconducting Polymer

5615


Chemical Reviews

Review

were studied.74 At 210 °C the mass loss is in accordance with earlier findings by Frechet et al.69 (Figure 8c) and the alkyl signals vanish. At 285 to 350 °C carbon dioxide is lost, as supported by IR and NMR of the 13C-labeled material (Figure 8d).72,74 The carboxylic moiety can be removed completely, leaving the native PT without a trace of the solubilizing groups.74,75 Solid-state 13C NMR of 9.6 suggests formation of dimeric rods in the solid state. The extraordinary stability of this system is attributed to its rigid nature, hydrogen-bonds in the network (Figure 8f), and presence of a partially oxidized polythiophene, i.e. self-doping through the carboxylic acid.72 The combination of thermocleavable polythiophene ester 8.4, which converts into the insoluble carboxylate P3CT, with solubilized ZnO nanoparticles allows for the simple fabrication of low-cost, fullerene-free, stable PV cells.76 The morphologies of the device films and materials were characterized by TEM, featuring well dispersed particles of homogeneous size. A solution based silver back electrode (Figure 9a) obviates vacuum steps for processing an inverted geometry device. The polymer solar cell can be stored under ambient conditions in the dark for 6 months without notable degradation in performance. The accelerated lifetime in air was tested for 150 h, resulting in bleaching and cracking (Figure 9b).77 The long-term stability is inferior to that of inorganic photovoltaics, but the technology compares well and competes with small batteries in terms of capacity. Additionally, bulk heterojunctions of the introduced poly(thiophene)s and the fullerene derivatives [60]PCBM, [70]PCBM, or bis[60]PCBM (Figure 10a) were prepared by standard film-forming processes from solution.75,78 The device with a bulk heterojunction of PT/[70]PCBM showed the best efficiency as well as a slow decay of the performance over 500 h of continuous illumination, identifying PT as an attractive semiconductor (Figure 10b).75 This finding was supported by investigations of the exciton diffusion length of 8.4, P3CT, and PT. With 13 ± 2 nm, PT is the best candidate for construction of solar cells among the polymers studied.79 To gain high open circuit voltage, four types of bulk-heterojunction solar cells with different photoactive layers were investigated.78 More than 300 solar cells were produced under comparable conditions, varying only the material combination of the photoactive layer. [60]PCBM or bis[60]PCBM as electron acceptors and polymer 8.4 or PT as donors were used and compared to P3HT-based solar cells. Replacing PCBM with bis[60]PCBM increases the open circuit voltage by 100 mV for 8.4 and 200 mV for PT solar cells (Figure 10c). Treatment of the soluble precursor 8.4 with intense visible light, matching the absorption range of PT, produces local heating. Irradiation with a large area high-power LED array converts 8.4 to P3CT (∼200 °C) and then to PT (∼300 °C).80 The light-induced cleavage does not affect the flexible polymeric substrate. The power usage was ∼1.6 kW for the high-power LED array, while a hot-air drier requires about 34 kW to reach 250 °C. The authors employed a full roll-to-roll process (Figure 10d) to fabricate large-area polymer solar cell modules on polyimide substrate (Kapton). The active layer 8.4/[60]PCBM was slot-die-coated and converted to PT by irradiation, and finally poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) solution was coated on top. Neither ITO nor vacuum processing were needed.80 To decrease the cleavage temperature, Krebs et al.81 investigated acid assisted cleavage of primary, secondary, and tertiary ester substituents from thiophene polymers (Scheme 10).81 They find considerable

Scheme 23. Synthesis of Ester Substituted 3,4Propylenedioxythiophene Monomers, Polymerization to 23.9−23.11 and 23.16, and Ester Cleavage to the Corresponding Electrochromic Alcohols 23.12−23.14 or Carboxylic Acid 23.17

A 13C-labeled polythiophene was synthesized in a multistep reaction, following published procedures with minor modifications for isotopic labeling (Scheme 9). The morphological mobility of the polythiophene 9.5 was “switched-off” by thermal treatment to eliminate the ester groups, leaving an insoluble carboxy-substituted polythiophene 9.6. The overall two-stage weight loss mechanisms (25−475 °C) 5616


Chemical Reviews

Review

Figure 17. (a) On the left-hand side is the color of the alcohol 23.14, while on the right-hand side is the color of the ester 23.11. (b) Illustration of the difference in solubility observed before and after saponification of the thin films and coloration of oxidized and reduced polymer 23.17. Figure 17a reprinted with permission from ref 106, Copyright 2007 American Chemical Society. Figure 17b adapted with permission from refs 106 and 108. Copyright 2010 John Wiley and Sons.

Figure 18. Illustration of induced coplanarity by thermal removal of tBoc groups. Reproduced from ref 109 with permission of The Royal Society of Chemistry.

lowering of the cleavage temperatures for 8.4 of both the side chains (200−220 °C → 22−50 °C) and the subsequent decarboxylation (250−300 °C → 120−150 °C), when employing trifluoromethanesulfonic acid. For 10.3 with secondary ester groups the cleavage temperature of the side chain increases (65− 120 °C), and for primary ester 10.4, the cleavage of the side chain happens at 140−170 °C, but the decarboxylation reaction remains the same as expected. This allows for in situ ester cleavage on polymer films printed by a roll-to-roll-process on flexible polyethylene terephthalate (PET). Krebs et al.82 compared P3HT with PT obtained by two different methods. 8.4 first eliminates an alkene and then CO2 at 300 °C whereas the silyl groups of 10.5 could be cleaved at room temperature by acid treatment (Figure 11). The authors found that even though the roughness was different for PT obtained from either species, their property for use in PV cells is roughly the same. Sun et al.83 used 8.4 to prepare fractal-like structures via CS2 vapor annealing in high humidity. The formed structures are subsequently immobilized by thermal curing (Figure 12). To explain the formation of the holes and the fractal-like structures, a mechanism was proposed that involves solvent-covered water droplets. Similar conditions form breath figures.84 All these fundamental experiments were performed with polythiophene derivatives. For tuning of the optical and electronic properties donor−acceptor polymers are of great interest. Frechet et al.85 studied design, synthesis, and characterization of benzothiadiazole-based copolymers 11.8 and 13.2,

Figure 19. Synthesis and immobilization of a conjugated polymer containing isoindigo units with cleavable and persistent side chains. Adapted from ref 113 with permission of The Royal Society of Chemistry.

whose solubility and bandgap plummet after thermal treatment of their thin films. The cleavable side chains were introduced to the monomers either as carbamate at the pyrrole units of 11.6 (Scheme 11) or at the thiophene units of 12.4 (Scheme 13). The monomers were copolymerized by Stille coupling (Scheme 11 and Scheme 13). GPC in THF versus polystyrene standard indicates a Mn = 11.4 kg mol−1; Đ = 1.96 for 11.8 and Mn = 10.2 kg mol−1; Đ = 2.23 for 13.2. The TGA confirms thermolysis, as well as the excellent thermal stability of the deprotected polymers 11.9 and 13.3. Polymer 13.2 exhibits a 35% mass loss (42% calcd) with an onset at 240 °C. Infrared spectroscopy before and after thermal treatment at 280 °C of a thin film of 11.8 confirms the predicted structure 11.9 with the loss of the carbamate protecting group. 5617


Chemical Reviews

Review

Scheme 24. Synthesis of Indigo, Isoindigo, and Diketopyrrolopyrrole Monomers

Krebs et al.86 described the synthesis of four thiophenebenzothiadiazole copolymers (Figure 13) and their photovoltaic performance in blends with PCBM. The polymers differ in the number of thiophenes per repeat unit as well as in the side chains. The ester-substituted polymer 14.1 was synthesized by Stille coupling of 12.9 and 6.6 (Mw = 17.3 kg mol−1; Đ = 2.6). When heated to 215 °C, elimination desolubilizes 14.1 (Scheme 14). Devices based on a blend of 14.2 with PCBM performed better after thermocleavage, as indicated by an increase in current and fill factor, resulting in power conversion efficiencies up to 0.42%. Copolymer 13.4 was investigated further (Scheme 15).87 The ester groups are located in different positions of the

bisthiophenyl-benzothiadiazole unit (Scheme 12). Both polymers, 14.3 and 13.5, show band gaps in the range of 1.66−2.03 eV and were explored in polymer photovoltaic devices as mixtures with soluble methanofullerenes. The position of the ester groups is significant despite the identical conjugated backbone. At 200 °C (TGA) elimination of the ester side chains is followed by loss of CO2 at 300 °C. Thermal cleavage of the active layer films was optimized at 265 °C. The 13.5:PCBM solar cells maintained efficiencies up to 1.5%. This concept is further expanded by copolymers from a dithienylthiazolo[5,4-d]thiazole unit 16.4 and silolodithiophenes 16.4a and 16.4b (Scheme 16).88 The polymers 16.5a and 16.5b 5618


Chemical Reviews

Review

Scheme 25. Synthesis and Thermolysis of DPP and Indigo- and Isoindigo-Based Donor−Acceptor Polymers

show optical band gaps in the range of 1.64−1.80 eV in thin films. The ester groups eliminate at 200 °C, improving the photochemical stability of thin films. The photovoltaic performance of 16.5a increases after thermal treatment, whereas films of untreated 16.5a are unstable during accelerated photochemical aging in air under 100 solar intensities. Thus, the unfavorable effect of solubilizing groups on the photochemical stability of conjugated polymers is emphasized. In a further extension of their project Krebs et al.89 used a 2,3diphenyl-thieno(3,4-b)pyrazine as acceptor unit in their donor− acceptor polymers and prepared multilayer tandem polymer solar cells by solution processing without vacuum coating steps. Polythiophene 8.4 and the newly synthesized thermocleavable low band gap polymer 17.6 were employed. Dried films (210 °C for 2 min) of 8.4 transform into P3CT, and 17.6 is converted to 17.7, under color change from burgundy to bright red (Scheme 17). The unencapsulated devices could be stored in the dark under ambient conditions without degradation. The thermo-

cleavable materials give rise to very good operational stability in air (Figure 14). A number of thienopyrazine-based, ester carrying conjugated polymers were obtained by condensation of a diaminoterthiopene 17.2 with phenylester substituted diketones.90 Bromination and Stille coupling led to the target polymer 18.9 carrying different ester groups (Scheme 18). Upon heating, alkenes eliminate and leave the carboxylated copolymers. Elimination occurs at 220−240 °C for tertiary esters, giving the free acid with a good degree of control over the chemistry in the thermocleaved product. Photovoltaic cells with ITO/PEDOT/polymer:PCBM/aluminum geometry were studied under continuous illumination (1000 W m−2, AM1.5G, 72 °C). However, the power conversion efficiency (PCE) of 0.4% was inferior to the performance of P3HT and PT. In an extension of this concept, low-band gap alternating polymers based on dithienylthienopyrazine, bearing thermocleavable esters on the pyrazine ring, and different donor segments (dialkoxybenzene, fluorene, thiophene, cyclopentadi5619


Chemical Reviews

Review

Scheme 26. Electropolymerization of Bisthienyl Substituted and t-Boc Protected Indigo and Chemical and Electrochemical Reduction/Oxidation Process

Figure 20. Schematic representations of π−π stacking 27.2 as cast and after annealing. Reproduced from ref 114. Published by The Royal Society of Chemistry.

orientation before cleavage and face-on orientation after cleavage with respect to the surface (Figure 15), impairing the device performance upon thermal treatment. The lowest cleavage temperature of 140 °C is achieved for 20.8c with a cyclic fivemembered ring in the α-position to the carbonate group. In an extension of this concept, Hamburger et al.99 prepared ntype semiconducting polymers with naphthalene tetracarboxydiimide (NDI) units and thermally cleavable side chains (Scheme 21). The carbonate was attached to the NDI core with a short aliphatic linker, or a tertiary carbamate was linked directly to the imide nitrogen. Pyrolysis results in homogeneous, insoluble films. Variation of curing temperature and time govern the amount of side chains in the film and influence the FET device performance. The side chains split off at 180−230 °C and lead to the expected weight loss for polymers 21.2a−c with slight deviations attributed to fragments not fully evaporated during TGA measurement. Independent of the curing time the removal efficiency is higher at 220 °C compared to 200 °C, while channel resistance is best for films thermally treated for 20 min at 180 °C. Built-up mechanical stress due to washed-out side chains and a potentially porous morphology affects the electrical properties and results in a decrease in device performance, which is compensated for by longer curing times at lower temperatures. In the same vein, this group prepared a carbonate-based benzothiadiazole-fluorene copolymer 22.4 by Suzuki coupling (Scheme 22).100 Thermolysis of 22.4 at 200 °C led to insoluble polymer 22.5. If employed as an active layer for OLEDs, incorporation of the intermixed electron transport material 1,3bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7), and additional 4,7-diphenyl-1,10-phenanthroline (BPhen) as hole blocking layer (device D), the devices with 22.5 (Figure 16) achieved a current efficiency of 5.3 cd A−1 (at L = 1000 cd m−2).

thiophene) were prepared.91 The ester groups cleave at around 200 °C after 1 min but decarboxylation only takes place under decomposition at ∼400° (Scheme 18). The devices (blends with PCBM) performed worse after thermocleavage due to a drop in the current density, attributed to phase segregation of the polymer and PCBM upon annealing. Farinola et al. expanded the scope for ester cleavage (Scheme 19) and prepared diketopyrrolopyrrole (DPP)-based alternating donor−acceptor polymers with oligoethylene glycol side chains by statistical Stille coupling and direct heteroarylation polymerization (DHAP).92 The triethylene glycol (TEG) groups enhance packing in solid state and improve device performances.93−97 The ester groups were cleaved at 300 °C within 30 s, a rather harsh, however, fast process. Decarboxylation takes place at temperatures around 400 °C, as measured by TGA. Significant reduction of solubility in chloroform was detected upon cleavage of the ester side chains. Hamburger et al.98 developed an alternative method for the desolubilization of polythiophene using carbonate-based, thermolabile groups instead of esters. Thermal cleavage leaves a hydroxyl function on the polymeric backbone, changing polarity and solubility (Scheme 20). Cleavage temperature can be tuned by changing the structure of the solubilizing side chain, approaching the desired range between 140 and 200 °C, lower than for carboxylic esters. They described the fabrication of organic solar cells and field-effect transistors with robust, solventresistant active layers. Polythiophene 20.8a exhibits an edge-on 5620


Chemical Reviews

Review

Scheme 27. Synthesis and Thermal Conversion of Novel Diketopyrrolopyrrole Polymers

Reynolds et al. reported a series of electrochromic poly(3,4propylenedioxythiophene)s101−105 including the cleavable polymers 23.9−23.11 obtained by oxidative polymerization (Scheme 23).106 These exhibited promising electrochromic properties, switching in under 1 s with a contrast ratio of 63% and a composite coloration efficiency (CE) value of 660 cm2 C−1. The tetraester 23.11 gave a high CE value at 703 cm2 C−1, but with slower switching speeds and lower contrast ratios. The onset of color change occurs at −0.6 V, similar to other alkylenedioxythiophene polymers. Polymer 23.10 dissolved upon switching due to the solubility of the oxidized form in propylene carbonate. The ester derivatives were saponified with KOH in methanol, resulting in insoluble films. Spray cast films of 23.11 exhibited a burgundy color, which changed to blue upon conversion to the alcohol 23.14 (Figure 17). Similarly, the hydrolyzable benzothiadiazole copolymer (Mn = 11.1 kg mol−1; Đ = 1.34) was obtained by base-free Suzuki coupling107 and could be used as electrochromic material with attractive color changes.108

Electrochromic polymers of this class, which can be directly patterned, are discussed in section 5.6 (Scheme 75). 2.4. Thermal Amide Cleavage

In this section we introduce carbamates as removable substituents. Numerous compounds suitable as electron acceptors in D−A-polymers originate from dyes and pigments, and several contain nitrogen atoms in a lactam or imide motif. As an example of a carbamate protection, the naphthalene diimidebased polymer 21.2c was shown in Scheme 21. Cleavage of the carbamate leads to the formation of strong H-bonds and thus massively decreases the solubility. Typical examples for such latent hydrogen-bonds were reported by Guo et al.109 and Liu et al.,110 who prepared copolymers of indigo, isoindigo, and diketopyrrolopyrrole monomers with t-Boc-protected amides (Scheme 24) and the donor monomers 25.1 and 25.4 by Stille coupling (Scheme 25). The donor−acceptor polymer 25.2 was used as an active layer in organic thin film transistors (OTFTs).109 Heating above 170 °C removes the t-Boc groups to give the highly coplanar polymer 5621


Chemical Reviews

Review

Scheme 28. Synthesis and Thermal Cleavage of an Amide Protected DPP Polymer as Organic Semiconductor

25.3 (Figure 18). Whereas films from the protected polymer 25.2 exhibit no field-effect, films of 25.3 prepared at 200 °C showed an electron mobility of up to 5.7 × 10−3 cm2 V−1 s−1 in OTFTs, a 5-fold increase compared to that of the indigo-based polymers reported111 at that time. Similarly, thermal treatment of copolymer films of 25.5−25.7 at 200 °C for 10 min deprotects the amide in nearly quantitative yield, resulting in N−H···OC hydrogen bonds, enhanced coplanarity, and intermolecular ordering of the indigo, isoindigo, or diketopyrrolopyrrole units.110 Side groups with latent hydrogen bonds provide a viable strategy to construct conjugated polymers that attain ordered intermolecular stacking by thermal treatment resulting in increased field-effect mobility. Głowacki et al.112 directly electropolymerized the bisthienylsubstituted and t-Boc-protected indigo 24.5 and then converted the resulting polymer 26.1 into its highly stable hydrogen bonded form 26.2 (Scheme 26). The resulting polymer can be reversibly oxidized and reduced over hundreds of cycles while remaining immobilized on the working electrode surface. The very stable polymer shows a pronounced photoconductivity response in a diode device geometry with a dark resistivity of ∼10 TΩ cm and an increase in conductivity by 4 orders of magnitude upon irradiation. Guo et al.113 employed the isoindigo dibromide 24.7 for protection with either a t-Boc group or a polyisobutylene (PIB) telechele. Combination of these two monomers with bistin-

Figure 21. (a) Synthesis of diketopyrrolopyrrole polymer 27.8. (b) Thermocleavable polymer based on DPP and (c) schematic illustration of solution-shearing technique. Figure 21 adapted with permission from ref 115. Copyright 2012 John Wiley and Sons.

substituted thiophene 6.6 gave the conjugated polymer 26.5 with different ratios of t-Boc and PIB substituents (Figure 19). Upon 5622


Chemical Reviews

Review

Scheme 29. Synthesis and Immobilization Conjugated Polymer for a Field Effect Transistor Layer That Senses Ammonia and Amines

Figure 22. Schematic representation of the fabrication process of patterned electrochromic films. Reprinted with permission from ref 118. Copyright 2014 American Chemical Society.

heating to 180 °C, loss of the t-Boc group is observed, while at 400 °C the PIB group leaves. Postfilm-casting thermal treatment removed t-Boc groups generating a solvent resistant H-bonded cross-linked polymer network with organic field-effect transistor properties, even after the films were immersed in organic solvents. Multilayered films of the polymers could be fabricated using multiple “casting−annealing−casting−annealing” cycles. In a similar approach, t-Boc-protected DPP and isoindigo units were randomly copolymerized by Stille cross-coupling with monomers, bearing octyldodecyl chains to ensure solubility to obtain conjugated polymers 27.2, 27.3, 27.5, and 27.6 in high yields and average molecular weights in the range of Mn = 16−22 kg mol−1 and Đ = 1.7−1.8 (Scheme 27). H-bonding is turned on upon heating up to 220 °C (Figure 20).114 Cast polymer films

exhibited considerable red-shifted UV−vis absorption spectra, and a further red-shift was observed in the thermally annealed films (at 220 °C, 30 min), which reflected the increased crystallinity of the hydrogen-bonded polymers (Figure 20). The t-Boc-protected soluble DPP-based donor−acceptor polymer 27.8 is a p-channel semiconductor, while upon removal of the t-Boc group at 200 °C, the polymers develop a hydrogenbonded network and turn from p- to n-type transport.115 The synthesis is straightforward and employs Suzuki coupling for polymerization, resulting in 27.8 (Mn = 13.5 kg mol−1; Đ = 3.56) (Figure 21). Switching from p- to n-channel semiconductors obtained from the same synthetic approach is attractive. Rather different polymer 28.3 was obtained by a Stille coupling of the amide-protected dibromo-dithienyl-DPP 28.2 with 5623


Chemical Reviews

Review

Scheme 30. Synthesis and Desolubilization of a Series of Novel Conjugated Polymers Containing Hydrogen-Bond Capable Monomers

(Scheme 31).119,310,311Suzuki coupling with fluorene 30.1 gives the soluble precursor polymers 31.6 and 31.13 with latent hydrogen-bonding. Heating to 200 °C releases isobutene and renders the polymers 31.7 and 31.14 insoluble. The process was followed by weight loss through TGA and IR spectroscopy. The hydrogen-bonded polymer 31.14 is an air stable n-type semiconductor with electron mobility of 0.01 cm2 V−1 s−1, 40 times higher than the mobility of its precursor polymer 31.13. Fang et al.120 prepared a t-Boc-protected ladder polymer (Figure 23) by “condensation followed by annulation”121 of a 4,4′-diamino-dialkylfluorene 31.15 with dimethyl-1,4-cyclohexandione-2,5-dicarboxylate 31.16 to give t-Boc-substituted, processable 31.20. The conjugated polymer 31.20 is synthesized without any transition-metal-catalyzed couplings. Processing of 31.20 and heating relieves isobutene to obtain 31.21. The t-Boc groups are cleaved off at 200 °C while the alkyl residues are removed at around 400 °C. The quinacridone units of 31.21 have multiple self-complementary hydrogen motifs, so that the formed pyrolyzed films were resistant toward organic solvents. In conclusion, thermal cleavage of ester groups renders precursor polymers insoluble, allowing for patterning, and the immobilized products often show improved device characteristics in organic solar cells and field effect transistors. Liberation of carboxylic acid and amide groups forms stabilizing hydrogen bonds that may improve the film morphology, order, and density. As the protected precursors are soluble, they are readily processed from solution and converted in the solid state.

bisstannylated dithiophene 6.8 (Scheme 28). Weight loss observed at ∼200 °C (TGA) corresponds to the removal of the solubilizing side groups.116 Thin films of the deprotected polymer 28.4 are morphologically stable, resistant to solvents, show a hole mobility of up to 0.078 cm2V−1 s−1, and have a favorable band gap for light harvesting (Scheme 28). The stability originates from the formation of intermolecular hydrogen bonds, as reported in previous examples. Beyond the release of the lactam-nitrogen, forming strong hydrogen bonds, the carboxylate carrying DPP polymer 29.5 (Scheme 29) was obtained by Zhang et al. via Stille coupling of 29.2 with 29.3 and thermal removal of gaseous isobutylene from films of the soluble precursor 29.4, accompanied by the formation of nanopores.117 These allow the diffusion and interaction of ammonia and amines with the carboxylic acid substituted polymer, leading to high sensitivity and fast response for this material in a FET-based sensor. Detection of ammonia and amines down to a 10 ppb threshold is possible with the immobilized polymer 29.5. Other analytes do not interfere. Zhu et al. described patternable conjugated polymers with latent hydrogen-bonding motifs (Figure 22) by performing a Suzuki coupling of dihexylfluorene bisboronic acid acetal 30.1 with a series of t-Boc-protected amide-type monomers 30.2− 30.4 and 24.13 (Scheme 30).118 A spin-coated film was photodeveloped, and after lift-off of the mask, the unirradiated polymers were washed away and the irradiated parts had lost isobutene and remained. In an electrochromic experiment, the dibromophenanthridinone-based polymer 30.5 showed red coloration at the anode, different from widely reported coloring at the cathode. Patterned electrochromic devices were fabricated with patterns of different length features (25 to 125 μm). The same group also synthesized dihalogenated, t-Bocprotected benzodipyrrolidone 31.5 and naphthodipyrrolidone 31.12 by condensation and electrophilic aromatic substitution

2.5. Ether and Acetal Cleavage

While esters are one logical choice for the thermal elimination, other functional groups, such as acetals, fare better when the deprotection has to occur at lower temperatures. The group of Holdcroft122 prepared tetrahydropyrane (THP) protected 2bromothiophenes 32.3 and 32.7a−c (Scheme 32), which were deprotonated by LDA and then transmetalated by MgBr2. 5624


Chemical Reviews

Review

Scheme 31. Synthesis and Pyrolysis of Benzodipyrrolidone and Naphthodipyrrolidone Containing Polymers

Subsequent GRIM123−126 polymerization furnished the substituted, acetal protected polythiophenes 32.4, 32.9, and 32.10. This method is also useful to prepare copolymer 32.19 with hexylthiophene units. The formed polymers were thermally deprotected above 240 °C, while addition of camphorsulfonic acid (CSA) decreased elimination temperatures to 50−75 °C (for 32.4) to give insoluble polythiophene 32.20 with a high degree of planarity. This method is complementary to Hamburger’s carbonate pyrolysis.98 In an extension of this concept Holdcroft et al. combined EDOT (32.11) and thiophene monomers with hydroxyalkyl side chains of different lengths to obtain the protected polymers 32.15−32.17 after GRIM (Scheme 32).127 Although the longchain substituted polythiophene is reluctant to undergo the acidcatalyzed deprotection reaction because of its crystallinity, deprotection occurs readily at elevated temperature in the absence of acid and for the shorter chain derivative at much lower

temperatures in the presence of catalytic amounts of acid. Copolymer 32.17 can also be patterned by chemically amplified photolithography and soft lithography. In a similar design, microscopic images reveal smooth films after thermolysis of 32.10 removing the THP groups.127 Deprotection results in significant changes to the surface topology, which may account for the loss of crystallinity. The absorption profiles were similar to that of P3HT. Polythiophene 32.10 exhibited a high order in thin films according to XRD. Although such properties are good for PV devices, these systems proved to be inferior to P3HT:PCBM. A low-bandgap donor polymer 33.6 with THP terminated side chains was prepared by a Stille polymerization (Scheme 33).128 The authors cleaved the acetal functionality from polymer 33.6 at 140 °C in the presence of CSA. The morphology of films made from blends of the polymer with PCBM is investigated before (33.6:[60]PCBM) and after (33.7: [60]PCBM) cleaving and compared with that of the reported 5625


Chemical Reviews

Review

Figure 23. Synthesis and immobilization of a quinacridone containing ladder polymer. Reprinted from ref 120, copyright 2017, with permission from Elsevier.

topologies of the π-conjugated polymers.130 They could also directly pattern their precursor 32.4 (Scheme 32)133 as shown in Figure 25, when employing a formulation in which an NIR cyanine dye is added and the confined heating is performed with an NIR laser. The soluble, unreacted polymer is washed away and leaves the patterns with a resolution of less than 0.01 mm. Large surface areas up to 1 m2 can be structured at relatively high throughput at laser scan speeds of 0.6 m s−1 and with micrometer size resolution. A variant of laser-induced thermal patterning employs the NIR-dye as a second layer (Figure 26).134 The dye and native polymer 35.8 are removed after patterning. The THP groups are lost above 210 °C for the employed polymers 35.6, 35.4, 35.3 (Scheme 35) and 35.8. The deacetalization can also be employed to prepare the ketosubstituted organic semiconductor 36.6 (Scheme 36).135 Improved field-effect transistor performance of a benzotrithiophene polymer is achieved through ketal cleavage in the solid state. Thermal annealing and conversion of 36.2 to 36.3 resulted in a significantly improved hole mobility from 7.0 × 10−4 cm2V−1s−1 to 0.01 cm2V−1s−1, presumably due to better crystalline ordering. The deketalization starts at around 300 °C, as ketals are more stable than acetals. Itami et al. described an elegant method to prepare polyarylenes with a low side chain density and to pattern these.136 An ether-protected, nonaromatic kinked diiodide was

PTB4.129 Phase separation is suppressed at micro-, and nanoscale in the 33.7:PCBM films, while large micron-sized PCBM aggregates develop during thermal annealing in P3HT:PCBM, 33.8:PCBM, and 33.6:PCBM films. Thermal cleavage results in stable performance of photovoltaic cells. Some polyfluorenes and their copolymers with thiophene or benzothiadiazole were reported130,131 showing blue, green, and red luminescence (Scheme 34 and Scheme 35). Thermolytic removal of the THP group from polymer films renders them insoluble, yet emissive. Purely thermal elimination starts at around 250 °C, while the addition of CSA lowers the temperature to 120−150 °C, where the acid catalyzed elimination starts. In a further expansion Holdcroft et al. showed that nanostructured morphologies and topologies are achieved from thermally reactive blends of poly(methyl methacrylate) (PMMA) and a conjugated polymer bearing solubilizing tetrahydropyranyl groups followed by thermally induced, solidphase deprotection of the conjugated polymer and subsequent dissolution of PMMA (Figure 24).132 Regioregular polythiophene 32.4 and poly(fluorene) 34.4 were selected as archetypical conjugated materials. Porous films of the insoluble conjugated polymer resulted. With the same method and poly(hydroxyethylthiophene) obtained from 32.4 mixed with PMMA, Holdcroft et al. fabricated organic solar cells with nano- and microsized 5626


Chemical Reviews

Review

Scheme 32. Synthesis of THP-Substituted Thiophene Monomers, GRIM Polymerization to Polythiophenes, and Their Deprotection

coupled to a fluorene bisboronic acid. The soluble precursor polymer was then spin coated with a PAG. A mask was placed over the polymer, and the irradiated areas were deprotected under aromatization with concomitant reduction of solubility. The undeveloped polymer was washed away to give patterns that were well resolved on a 50 μm scale (Figure 27). Various synthetic approaches for the synthesis of poly(paraphenylene) (PPP) as prototype of a conjugated polymer were reported.137−145 Müllen et al. established a synthetic route toward unsubstituted, structurally well-defined, high-molecular weight PPP (Figure 28).146 Kinked monomers 36.9 and 36.10 containing disubstituted cyclohexadienylene moieties, previously

developed for [n]-cyclo-para-phenylenes,147 were polymerized in a Suzuki coupling to yield the soluble, linear precursor polymer 36.11 (Mw = 11.1 kg mol−1; Đ = 2.1) after fractionated precipitation. Additive-free aromatization of 36.11 in thin films (∼20 nm) by thermal treatment (300 °C) leads to PPP with ∼75 phenylenes unrivaled in length and purity compared to the previously reported precursor routes or direct aryl−aryl coupling protocols. Acetals offer advantageous cleavage temperature compared to esters. In the presence of catalytic amounts of acid, the temperature can be even lowered, and when PAGs or NIR dyes are applied, photopatterning is possible on the micro- to 5627


Chemical Reviews

Review

Scheme 33. Synthesis of 33.6 and 33.7 and Structure of the Employed Polymer 33.8

Scheme 34. Synthesis of Copolymers 34.4, 34.6, 34.8, and 34.10 for Thermal or Acid Catalyzed THP Cleavage

5628


Chemical Reviews

Review

Scheme 35. Synthesis of Fluorene-Based Low-Bandgap Copolymers 35.3, 35.4, and 35.6 for Thermal or Acid Catalyzed THP Cleavage

nanometer scale. Ketal and ether cleavages, however, are rarely

2.6. Retro-Diels−Alder Reactions

used for thermal cleavage in the literature, as these protecting

Retro-Diels−Alder (DA) reactions are a convenient tool to desolubilize organic semiconductors. The retro-DA reaction is Noncatalytic, thermal and leaves gaseous products, depending on the protecting group. Most commonly, acenes, in particular

groups require higher cleavage temperatures due to their higher stability. Chart 3. Comparison of the Methods Discussed in Section 2

5629


Chemical Reviews

Review

Figure 25. (a) Structure of the used NIR dye. (b) Schematic diagram for the direct thermal patterning of a πCP 32.4 using a NIR laser. Bright field micrographs of laser patterned film of polymer 32.4 and NIR dye. (c) Line and pixilated structures (inset: surface profile). (d) Higher magnification. (e) 50 μm pixilated structures. Reprinted with permission from ref 133. Copyright 2007 American Chemical Society.

Figure 24. (a) Schematic illustration of formation of micro- and nanometer-sized features. AFM topography images of 32.4/PMMA (b and c) and 34.4/PMMA (d and e) in the presence of acid as cast (b and d) developed holey films after removal of PMMA (c and d). Reprinted with permission from ref 132. Copyright 2007 John Wiley and Sons.

Hodge et al. utilized the Diels−Alder adduct from anthracene and dimethylmaleate to prepare conjugated polymers in which the anthracene acts as solubilizing group for hexa-1,3,5-triene units (38.6 and 38.8)149 or as part of the conjugated polymers 38.14 and 38.16.150 The halogenated monomers 38.3a and 38.3b were prepared from 38.1 by an indirect approach via reduction with LiAlH4 to a diol and selective oxidation with PPC to the corresponding dialdehyde 38.2, which then was olefinated by Wittig reaction with p-halogenbenzyltriphenylphosphonium chloride. Nickelcatalyzed polymerization of 38.3a and 38.3b resulted in polymer

anthracene and pentacene, are found as the diene partner. The early work of Müllen et al. used this reaction (Scheme 37) to obtain pentacene 37.3 after elimination of ethylene.148 Ethanobridged pentacene 37.2 undergoes retro-Diels−Alder reaction at around 180 °C to give pentacene, which could be used in an OFET with a mobility of 0.1 cm2 V−1 s−1. 5630


Chemical Reviews

Review

Figure 26. (a) Synthesis and thermal deprotection of 35.8. (b) Schematic illustration of thermal patterning with a bilayer technique. Fluorescence microscopy images of patterned 35.8 at different scan speeds of 0.3, 0.6, and 0.9 m s−1. Adapted with permission from ref 134. Copyright 2008 John Wiley and Sons.

38.5 with Mn in the ranges of 4.5−5.7 kg mol−1 (Đ ∼ 2.5) and 3.0−3.5 kg mol−1 (Đ ∼ 2.0), respectively. Dibromide 38.3b was copolymerized with phenyl-bisboronic ester 38.4 in a Suzuki coupling to give polymer 38.7 in 80% yield. Elimination of anthracene from polymers 38.5 and 38.7 occurs at 200 °C and leaves well-defined, insoluble copolymers of polyacetylene and PPP comprised of hexa-1,3,5-triene segments separated by bi-pphenylene (38.6) or ter-p-phenylene (38.8) segments.149 Dibromo-anthracene 38.11, prepared from diaminoanthrachinone 38.9, was reacted with maleic anhydride and subsequently esterified to obtain monomer 38.12 (Scheme 38). Yamamoto coupling of 38.12 furnishes the homopolymer 38.13, while Suzuki coupling with a phenyl-bisboronic ester 38.4 gives copolymer 38.15. Colorless films of 38.13 and 38.15 heated to 260 °C turn yellow under elimination of dimethylmaleate.150 Similarly, one can synthesize anthracene-based donor− acceptor copolymers with thermally removable groups that are useful for PV (Scheme 39).151 The maleic ester approach gives well soluble anthracene-based precursor copolymers, containing

Scheme 36. Thermal Deketalization of a Conjugated Polymer at 300 °C

Figure 27. (a) Synthesis and immobilization and (b) structuring of a polyarylene with low density of side chains. Reprinted with permission from ref 136. Copyright 2010 American Chemical Society. 5631


Chemical Reviews

Review

In a similar vein, Uemura et al. synthesized semiconducting polymers through soluble precursor polymers with thermally removable groups (Scheme 40).152 A series of DA-adducts were obtained from dibromoanthracene 38.2 and N-phenyl-maleimide, N-hexyl-maleimide, or diethyl azodicarboxylate and copolymerized with bisstannylated thiophene or bithiophene in a Stille coupling. The precursor polymers of the maleimide adduct underwent retro-DA in the range of 275−295 °C, whereas the cleavage temperature for the azodicarboxylate adducts was considerably lower (∼200 °C). Heating of 40.3c and 40.2c resulted in colored polymer films that were useful as active layers in thin film transistors with field-effect mobility of 0.015 cm2 V−1 s−1. The use of retro-Diels−Alder reaction for solubility modulation of conjugated polymers at present is rather limited, and it is applied for anthracene-based polymers only. However, as it is an additive free, purely thermal approach it might find future applications. A summary of the methods discussed in section 2 is given in Chart 3.

3. PHOTOCHEMICAL CLEAVAGE Photochemical cleavage is discussed in two sections: (1) photoacids that provide acidic protons under light irraditation (see section 3.1) and (2) cleavage of other aromatic esters (see section 3.2). These methods have different criteria for the processing of polymer materials and specific wavelengths for irradiation of the materials’ films. 3.1. Photoacid Generators (PAGs)

PAGs provide protons under illumination and are often used in thin-films. We will discuss examples employing proton sources in the upcoming section (see 5); the most commonly used derivatives along with their synthesis and excitation wavelengths are shown in this subsection together with an application to demonstrate their utility. For further applications of photoacids, e.g. for Meerholz’ oxetane cleavage,153−157 the reader is referred to the following sections, ordered by functional groups as well as to reviews in the literature.17,158 Meerholz et al. use iodonium salts to initiate the protonmediated cross-linking of oxetane groups (see 5.2). Irradiation with light generates a proton when the diaryliodonium salt 41.1 is reduced toward the iodoarene in a radical mechanism (Scheme 41). The quantum yield for the photolysis of the diaryliodonium compound (Y = CH3, MXn = AsF6−) is 0.2 for irradiation at 313 nm in acetonitrile. The rate of photolysis of these compounds does not depend on the aromatic substituent and the counteranions. Typically, the reaction is performed in a polar solvent such as THF or, when mixed with a oxetane-bearing polymer, in thin films.17,158

Figure 28. (a) Synthesis of kinked monomers 36.9 and 36.10 and precursor-polymer 36.11, and thermal conversion to PPP/36.12. (b) Photographs of spun cast films of 36.11 (left) and PPP (right) after thermal treatment. Reprinted with permission from ref 146. Copyright 2016 American Chemical Society.

Scheme 37. Synthesis of Pentacene According to Müllen

3.2. Cleavage of Nitrobenzylic Esters

The cleavage of nitrobenzylic esters by light is an attractive method to decrease solubility and obtain organic dyes and semiconductors with different functionality after the removal of the caging group 42.7.159 Thomas et al. demonstrated with a distyrylbenzene (DSB) derivative 42.1 that uncaging (Scheme 42) by UV-light transformed a nonfluorescent, organo-soluble precursor into a pH-sensitive, highly fluorescent dye with pHdependent solubility.160 This approach is effective at tuning the properties of conjugated materials with light after traditional synthetic operations and has potential for use in photoactivatable fluorophores or solution processable multilayer devices.

benzodiathiazole (39.3 and 39.5) or DPP (39.7 and 39.9) units. Upon heating to 230−250 °C, retro-DA reaction furnishes the planarized, highly conjugated polymers 39.10−39.13 (Figure 29). In PV devices, the best PCE of 2.2% was achieved with copolymer 39.13. 5632


Chemical Reviews

Review

Scheme 38. Synthesis and Thermolysis of Polymeric DA-Adducts

Chart 4. Comparison of the Methods Discussed in Section 3

The same group prepared a protected polythiophene 43.6 by Stille coupling into thiophene-based materials that undergo photoinduced aggregation or precipitation upon irradiation with UV light (Scheme 43).161 The only solubilizing side chains on these materials are photocleavable by connection through photolabile nitrobenzyl esters. Quaterthiophene oligomers yield diacids that remain soluble in dichloromethane at micromolar concentrations after irradiation with UV light; the polymeric analog shows both red-shifted absorbance and heavily quenched fluorescence, consistent with aggregation after photochemical uncaging. Thin films of this polymer resisted dissolution in organic solvent upon irradiation, suggesting applicability in the construction of multilayer solid-state devices.

Similarly, the authors prepared an aryl substituted polythiophene 43.4, which upon irradiation loses a substituted 2nitrosobenzaldehyde to give an insoluble polythiophene 43.7 patterned onto a surface (Figure 30).162 The authors found that ethers instead of esters make the photocleavage more efficient with a higher photoyield of immobilized polymer. This concept was further expanded by creating stimuliresponsive side chains for polythiophenes.163 Smith et al. discuss photopatterning and solution-based fabrication of multilayer conjugated polymer films without orthogonally soluble materials. Hamburger et al.164 developed photocleavable conjugated polymers exploiting carbamate appended nitroaromatics, 44.5a and 44.5b. Upon irradiation with UV light (λ = 350 nm) these species eject CO2 and nitrostyrenes and leave a polymer with a 5633


Chemical Reviews

Review

Scheme 39. Synthesis of Anthracene-Based Monomers and Precursor Copolymers

Figure 29. Synthesis of conjugated anthracene containing polymers using a retro Diels−Alder reaction and schematic processing. Reprinted with permission from ref 151. Copyright 2014 American Chemical Society.

3.3. Cleavage of Other (Aromatic) Esters

shortened, hydroxyl-group terminated side chain. Cleavage of

In a further extension of this concept Müllen et al. prepared a photocleavable polyfluorene derivative 45.6, carrying a hydroxycinnamic acid as side chain (Scheme 45).165 Irradiation expels coumarin 45.8 and leaves the polyfluorene insoluble. The formed

side groups leads to a turn-on of the fluorescence, while solubility of the π-conjugated materials is drastically reduced (Scheme 44). 5634


Chemical Reviews

Review

Scheme 40. Synthesis of Thermally Generated Conjugated Polymers with Anthracene Unitsa

Scheme 41. Mechanism of the Cleavage of an Iodonium Salt with the Anion Typically Being AsF6−

a Reprinted (adapted) with permission from ref 152. Copyright 2013 American Chemical Society.

coumarin, encapsulated in the polyfluorene, increases its performance in an OLED device. Employing acidic cleavage of the esters, Holdcroft et al. employed an acetate substituted polythiophene in combination with a PAG (Figure 31).166 Spin-casting, photoimaging, removal, and reirradiation, followed by development, lead to structured, patterned, and nonsoluble films of 2-hydroxyethyl-P3HET. The hydrophilic hydroxy group facilitates wetting and protonic doping of the polymer by acidic solutions. P3AcET can be selectively exposed to render a spatially modified surface bearing hydroxy functionalities. These can be subsequently modified by postreaction processes. This strategy allows the spatial functionalization of poly(3-alkylthiophenes). In summary (Chart 4), photochemical cleavage is a valuable concept to immobilize organic thin-films simply by irradiation with light. As a requirement, the materials have to be photostable and, at least for some examples, resistant against acids. In section 3.2, the cleavage of nitrobenzylic esters has been shown to be an efficient reaction, however leaving the cleaved remains in the films can lead to a turn-on in fluorescence of the polymer films. In section 3.3, other examples of light-induced cleavage of esters are shown to be effective even for lithography. The performances of the built devices are stable and could, in case of the work of Mü l len et al., even be increased upon photochemical desolubilization.

Figure 30. Illustration of the patterning process of a phenol-substituted polythiophene by uncaging. Adapted with permission from ref 162. Copyright 2015 American Chemical Society.

(vide inf ra)the polymers bear functional groups accessible by a stimulus to react with each other without loss of cleaved groups but rearrangement of covalent bonds (formation of novel bonds). Different functionalities achieve this goal. Typically styryl groups, vinyl groups, and azides are useful in that regard. For the first two cases, one exploits basic concepts of polymer chemistry by introduction of classical monomeric units into the side chains of conjugated polymers. Since these monomers with functionalities such as styrene and terminal olefins survive metalcatalyzed polymerization conditions, they are attractive as groups for postfunctionalization.

4. THERMAL CROSS-LINKING Thermal cross-linking is an desolubilizing method for organic materials in thin-films, whereinin contrast to cleaving methods 5635


Chemical Reviews

Review

Scheme 42. Photochemical Uncaging of a Distyrylbenzene Derivative: (a) End-Capped Oligomer and Photochemical Cleavage of the 2-Nitrobenzyl Group; (b) Mechanism of the Cleaving Reaction

Scheme 43. (a) Synthesis and (b) Photo-deprotection of a Caged Polythiophene

Azides are a well-investigated class of thermal cross-linkers that can be introduced before or after polymerization of the polymeric backbone, depending upon polymerization conditions. Their high reactivity can lead to degradation under Suzuki conditions. Their cross-linking mechanism is described by Young et al.168 They state that employing a stimulus (thermal or photolysis) of pentafluorophenyl azide releases singlet (pentafluorophenyl)nitrene, which can insert into a CH bond, ring expand to a ketenimine, or undergo intersystem crossing (XSC) to the triplet state of the nitrene, creating reactive species that cross-link the polymer chains. Other methods for thermal cross-linking are Diels−Alder reactions ([4 + 2] cycloadditions) and Si−O or B−O functionalized polymers.

Figure 31. Synthesis and photoconversion of a polythiophene acetate into its corresponding alcohol: P3AcET (layer B) was first spin coated onto a wafer (layer C) and a positive-tone photoresist (layer A) spin-cast on top. Following irradiation through a photomask, the exposed photoresist was removed (1) and the imaged film immersed in sulfuric acid (2). The remaining resist was stripped with acetone, leaving a conjugated polymer film in which regions were electronically conducting (light blue) by acid doping (3). When the relief image was treated with chloroform, the nonacid hydrolyzed region of P3AcET dissolved, leaving a selectively deposited acid-doped polymer (5). If the relief image was treated with base and the unexposed polymer not removed, the acid-doped regions were neutralized to afford the neutral red colored polymer (4).

4.1. Vinyl Polymerizations

A simple method of cross-linking was described by Klärner and Miller (Scheme 46).169 An end-capper or a comonomer containing styryl groups was added to the polymerization mixture to give polyfluorenes 46.3 and 46.6 that cross-linked by 5636


Chemical Reviews

Review

Scheme 44. Synthesis of Polyfluorenes with Photocleavable Side Chains Employing 1-Nitro-2-vinylbenzene Leaving Groups

Chart 5. Comparison of the Methods Discussed in Section 4

thermal polymerization of styrene at 150 °C gives both higher polymers and a mixture of dimers in largely independent reactions. The dimerization is mostly a nonchain reaction, giving 1,2-diphenylcyclobutane and 1-phenyltetralin. Miller et al. used cross-linked layers of polymeric arylamines and polyfluorenes (Scheme 47) to produce efficient OLEDs.171 The layers of crosslinkable materials were spin-coated from cyclohexanone solution. Curing at 200 °C (15 min) in a nitrogen atmosphere forms an insoluble layer. The triarylamine polymer 47.3 serves as

heating of thin films to 150−200 °C for 10−60 min. Multilayering of organic polymers in light-emitting devices using spin-casting techniques was demonstrated. The crosslinking immobilizes the polymer chains and hampers their ability to π-stack, an effect which leads to suppression of the red-shifted emission attributed to excimer formation and results in color-fast blue-light emitting devices. A plausible explanation of the reaction mechanism of crosslinking styrene units is given by Mayo et al.170 They state that 5637


Chemical Reviews

Review

Scheme 45. Synthesis and Immobilization of a Polyfluorene Derivative 45.6 under Expulsion of an Auxiliary Fluorophore for OLED Applications

Scheme 46. Simple Thermal Cross-linking Using Styrene Functionalities: (a) End-Capped Poly-fluorenes 46.3 and Poly-hexylfluorene 46.5; (b) Side Chain-Functionalized Polyfluorene 46.6

Scheme 47. Chemical Structure of the Employed Polymer 47.3a

a

hole injection layer, while the polyfluorene emits. Single-, double-, and triple-layer polymer light-emitting diodes (PLEDs) with indium−tin oxide ITO anodes and Ca cathodes were fabricated. For the latter case, an electron transporting oxadiazole trimer was employed.172 These triple-layer PLEDs showed the highest efficiency (maximum external quantum efficiency >1%), brightness, and saturation of blue emission (maximum luminescence 1526 cd m−2). Thus, the thermal cross-linking allowed the solution-based construction of multilayer devices without intermixing of the layers.173 External quantum efficiency increases by 2 orders of magnitude as a result of blending 10% of a hole-transporting triarylamine into the polyfluorene, altering the morphology by cross-linking, and modifying the relative injection efficiencies of the electrodes. However, with the understanding that the single layer OLED is doubly injection limited, it is clear that enhancing the injection of either carrier improves the quantum efficiency.174,175 This approach can be employed to cross-link other types of polymers such as PPVs. A processable PPVD 48.4 with 3vinylbenzene as the end group was synthesized by a Wittig reaction (Scheme 48).176 The thermal cross-linking of the

The end group is the cross-linkable styrene group.

polymer was verified by DSC measurements as well as by UV−vis and photoluminescence spectra before and after solvent rinsing. The cross-linked film was uniform, as indicated by AFM and single- and double-layer PLEDs fabricated from the cross-linked PPVD. 2-(4-Biphenyl)-5-(4-tert -butylphenyl)-1,3,4-oxadiazole (PBD) as an electron-transporting layer improved the maximum luminance efficiency of the diode to 0.7 cd A−1, 12 times that of the single-layer device. The cross-linking of vinylogous triarylamine-type hyperbranched polymers is facile, as the formed product 49.2 contains already multiple styryl groups (Scheme 49).177 Heating under dynamic vacuum for 2 h at 150 °C cross-links 49.2 into insoluble films, upon which a light-emitting polymer, poly[2-methoxy-5(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), was spun-cast. The LEDs fabricated using the cross-linked 49.2 (HB-BVPA) exhibited superior performance (104 cd m−2) to conventional devices with PEDOT−PSS as hole-transport layers.178 5638


Chemical Reviews

Review

Miyanishi et al. created a hexenyl-substituted polythiophene in a GRIM-approach (Scheme 51).181 This polythiophene 51.4 (P3HNT) shows a PV-performance after short thermal annealing (250 °C) comparable to that of P3HT due to the high crystallinity of the polymer. Thermal cross-linking suppresses aggregation of PCBM. A more stable device performance results in the case of P3HNT as compared to that of P3HT; that is, the cross-linking affixed the original morphology. The slight increase in VOC and the faster decrease in FF in the case of P3HNT are not fully understood. Crosslinking at the side chains might cause strain in the polymer backbone to change conformation and stacking. Cao et al. prepared two series of oxetane- or styrenefunctionalized polyfluorenes and compared their cross-linking behavior (Scheme 52).182 For the photo-cross-linking polymers 52.1 (PFN-OX) and 52.3 (PF-OX), protons were provided by the PAG 2-(4-methoxystyryl)-4,6-bis(trichloro-methyl)-1,3,5triazine and the cross-linking proceeds by heating the halfdried films at 150 °C under fluorescence lamp illumination for 20 min. For the thermally cross-linked polymers 52.2 (PFN-S) and 52.4 (PF-S), the half-dried films were held at 185 °C for 30 min. Fabricated OLED devices with a structure of ITO/cross-linked EICP/EML/MoO3/Al achieve high LE and brightness comparable to that of conventional structure counterparts. The maximum luminance and LE of an ITO/PFN-OX/P-PPV/ MoO3/Al device were ca. 14.7 × 103 cd m−2 and 14.8 cd A−1, respectively, which were substantially higher than 7.0 × 103 cd m−2 and 13.5 cd A−1 for the conventional structure counterpart ITO/PEDOT:PSS/P-PPV/PFN-OX/Al. Similarly, inverted OLED devices183 using cross-linked PFN-S as the electron injection layers displayed slightly higher luminescence efficiency than conventional OLED devices (11.6 cd A−1 vs 9.7 cd A−1). PFN-S and PF-S OLED and inverted OLED devices showed lower maximum luminance and luminescence efficiency than the PFN-OX devices with oxetanes as cross-linkers. Waters et al. prepared statistical copolymers 53.5a and 53.5b containing cyclopentadithiophene and benzothiadiazole units by Suzuki couplings (Scheme 53).184 Introduction of bithiophene units carrying terminal hexenyl groups allows thermal cross-

Scheme 48. Synthesis of Thermally Cross-linkable of PPVDerivative 48.4

In a similar vein, a fluorene monomer containg two vinyl groups gave rise to a hyperbranched polyfluorene with vinyl groups at the periphery, 50.2 and 50.4 (Scheme 50).179 Thermal cross-linking led to enhanced fluorescence efficiency after curing at 150 °C. Two-layer EL devices (ITO/PEDOT/HPFV or LPFV/Ca/Al) were fabricated and their optoelectronic properties investigated. The maximal luminance and luminance efficiencies of HPFV devices were 1480 cd m−2 and 0.18 cd A−1, respectively; both were much greater than those of linear LPFV devices (352 cd m−2 and 0.06 cd A−1). The luminance efficiency of HPFV (0.18 cd A−1) is about three times that of LPFV (0.06 cd A−1). Improved EL performance in the HPFV device was attributed to its hyperbranched structure with terminal cross-linkable vinyl groups and higher fluorescence quantum yield.180

Scheme 49. Cross-linkable Polymer 49.2 for Hole Injection Layers in PLEDs

5639


Chemical Reviews

Review

Scheme 50. Cross-linking of a Heck-Based Vinylated Polyfluorene for Application in OLEDsa

a

HPFV = Hyperbranched poly-flourene-vinylene and LPFV = linear poly-flourene-vinylene.

to be 150 °C for 40 h with 2.5% of vinyl groups. This optimized PBDTTPD-V0.025/PCBM device shows a PCE of 6.06% after thermal cross-linking. Thermally cross-linked polymers bearing styrene or olefin functionalities are useful in creating insolubilized layers. However, optimization of parameters such as duration and temperature of the thermal treatment of the polymer films as well as the amount of cross-linkable functionalities needs to be carefully adjusted to optimize device performance. An increasing amount of thermal cross-linker increases the device stability whereas the device performance (i.e., PCE in PV, luminescence in OLEDs) can decrease.

Scheme 51. Synthesis of a Thermally Cross-linkable HexenylSubstituted Polythiophene

linking. Polymer−fullerene OPVs with PCEs of up to 3.7% result.185,186 Cross-linking at 180 °C enhanced the lifetime of the OPV. A different approach was employed by Chen and Li, connecting a PCBM-type material with a vinylated, conjugated polymer (Scheme 54).187 Thermal annealing of the vinylfunctionalized polymer 54.6 (PBDTTT-V) and fullerene 54.8 (PCBD) induced cross-linking, confirmed by the insolubility of the films, with developed and stable morphologies after thermal annealing at 150 °C for 8 h. The stability of PBDTTT improved by the intercross-linked network formed between vinyl-functionalized donor and acceptor; however, if the concentration of vinyl groups is too high, efficiency falls from 3.6% for the non-crosslinked PBDTTT:PCBM device to 2.5%. The same authors varied the building blocks in their crosslinkable donor−acceptor conjugated polymers 55.4 (PBDTTPD-V) (Scheme 55).188 Again, the vinyl content impacts cross-linking effectiveness and device performance. A high content of vinyl units is favorable for effective cross-linking, but device efficiency deteriorates here also; the optimum seems

4.2. Azide-Containing Species

Bunz et al. prepared poly phenylene-ethynylene polymers (PPEs) from azide-containing monomers, which tolerate Sonogashira polymerization conditions. Breath figures were formed (Figure 32).189 The azide-PPE 55.8 was heated to 300 °C to retain the breath figure structure forming picoliter beakers, resistant to solvent treatment. The intermediately formed nitrenes cross-linked the PPE and retained the micrometer features in the breath figure pattern of the film. Polymers can be postfunctionalized by introduction of azides (Scheme 56). Friend et al. copolymerized a monomer with a protected hydroxyl group via GRIM and removed the THP group.190 The alcohol is transformed into an azide in a polymer analogous reaction. Thermal decomposition of the azide is achieved by heating films of polymers 56.5 to 200 °C under vacuum for 30 min; the azide decomposes above 185 °C. Loss of nitrogen results in formation of a nitrene, which cross-links the polythiophene chains by insertion and/or addition to the alkyl chains. A slightly blue-shifted absorption was observed. These 5640


Chemical Reviews

Review

Scheme 52. List of Polyfluorenes, Cross-linkable by Acidic Ring Opening or Thermal Polymerization

Scheme 53. Synthesis of Cross-linkable Conjugated Polymers Carrying Vinyl Groups for Thermal Cross-linking

presume that several different mechanisms of cross-linking occur, depending on the presence of triplet or singlet nitrenes. Singlet nitrenes undergo insertion reactions while triplet (diradical) nitrenes abstract available hydrogen atoms, producing radical species, which will combine with other radicals to form a crosslinked polymer network. Azide substituents can also affix fullerene types according to Holdcroft et al. (Scheme 58).193 Regioregular P3HT (58.1) was synthesized using GRIM, brominated partially toward 58.2, and converted into the TEMPO (1%)-P3HT macroinitiator 58.4. Grafting resulted in the precursor P3HT-1%graft-(ST-statCMS) 58.5, which was converted into the functionalized polymer 58.6 containing around 1% azide groups, sufficient for stabilization of the bulk heterojunction. The chlorine atoms were quantitatively substituted by azide groups to form the azido-

authors investigated the conformational changes in regioregular polythiophenes after thermal cross-linking, in a related paper.191 Polythiophenes with higher azide content 56.5c showed higher degree of cross-linking upon annealing and remained in a permanently disordered conformation; polymers with lower azide contents (56.5a, 56.5b) recover an ordered conformation upon cooling. A different postfunctionalization approach by Huyal et al. transformed a polyfluorene with brominated side chains into an azide-containing polyflourene 57.2 (PF6-azide) (Scheme 57).192 Heating to 220 °C resulted in cross-linked polymers according to DSC, suggesting a reactive nitrene species. Thin films of the cross-linked polymers were combined with an InGaN UV diode acting as a broad-spectrum downconverter of energy. White light emission of the hybrid LED results. The authors 5641


Chemical Reviews

Review

Scheme 54. Synthetic Routes toward Precursors of Co-cross-linkable Conjugated Polymers and Fullerene Derivatives

size of the nanoparticles is tunable in the range of 40−210 nm, simply by adjusting the feed amount of the oil phase or surfactant in the mini-emulsion cross-linking. The nanoparticles display multiple neutral surface hydroxyl groups, which show good water dispersibility, bright fluorescence, good optical stability, and low cytotoxicity. BSA and γ-globulin were chosen as the representative examples to study their interactions with CP− HPG. Here, fluorescence remains unchanged when the protein concentrations are increased from 0 to 300 μg mL−1. LLS measurements further confirm that there is no significant difference in size and size distribution of CP−HPG in aqueous solution before and after the addition of BSA and γ-globulin, suggesting good antiprotein adhesive property of CP−HPG. Kim et al. prepared 58.16 as solvent resistant material in the fabrication of cross-linked OPV cells (Figure 35).199 After 40 h of thermal annealing at 150 °C, a PCE of more than 3.3% was obtained from an OPV device containing 15% thiophenes with azide groups as cross-linker. Photo-cross-linking of 58.16 dramatically improves the solvent resistance of the active layer, without disrupting the molecular ordering and charge transport. Notably, the choice of stimulus (thermal or photolysis) does not seem to heavily influence the resulting films in azide polymers. In both cases, well-performing, immobilized layers of the organic

functionalized polymer 58.6. The cycloaddition of the azide group to PCBM was performed in the solid state after spincoating a blend of copolymer 58.6 and PCBM. FTIR spectroscopy confirmed the cycloaddition. Beginning of-life PCEs for 58.6/PCBM blend devices were lower than that of native P3HT/PCBM blends, the rate of degradation of performance is slower, decreasing by only 50% over a 3 h period (compared to 80% for P3HT/PCBM films) as a consequence of the covalent connection between PCBM and the polymer. Hawker et al. employed a thermal-cross-link/orthogonal functionalization approach, employing a polystyrene decorated with azide and alkyne groups (Figure 33).194 Polymers containing both azides and alkynes 58.11, randomly incorporated along the backbone, were prepared by free radical copolymerization of a three component mixture of styrene, propargyloxy-styrene, and vinylbenzyl azide.195−197 Upon deprotection and heating, cross-linking occurs (58.12); the residual azide and/or alkyne groups can be further functionalized by suitable reagents. This approach allows printing of several different channels on top of each other. To obtain stable organic yet water-dispersible multihydroxy conjugated polymer nanoparticles with tunable size, Liu et al.198 prepared an alkyne terminated dendrimer, which was combined with an azide containing polyfluorene 58.14 (Figure 34). The 5642


Chemical Reviews

Review

Scheme 55. Synthesis of Cross-linked Polymers for OPV Applications

Scheme 56. Synthesis of a Regio-Regular Polythiophene with Subsequent Transformation into an Azide-Functionalized Polymer

Figure 32. (a) Synthesis and (b) pyrolysis of an azide-substituted poly(para-phenyleneethynylene). Adapted with permission from ref 189. Copyright 2004 American Chemical Society.

thermal annealing on polymer structure and PV device properties to develop guidelines for optimizing device processing conditions. The exposure to UV light did not degrade the optical or electrical properties of the systems; blends with PCBM crosslinked without any problems fixating the bulk heterojunction geometry of the OPV cells. The UV-cross-linked 59.5:PCBM blend solar cells retained 65% of the initial PCE (∼1.5%) after 40 h at 110 °C, as compared to commercial non-cross-linked P3HT:PCBM devices whose PCE decreased to 28% of their initial value (∼2.2%). In comparison, the control devices made using solely thermally cross-linked 59.5:PCBM blends also exhibited improved thermal stability, likely due to P3HT thermal cross-linking by azide groups activated at 110 °C. In 59.5:PCBM devices (both thermal and UV cross-linked), JSC and VOC remain

polymers are obtained. It is assumed that one can select the fabrication method most convenient to the device architecture. Both types of cross-linking (thermal and photolytic) of azides can affect polymer packing morphologies and influence the materials’ electronic properties, as high loading of functional groups or small molecule additives often interfere with polymer diffusion and phase behavior. Grubbs et al.200 (Scheme 59) prepared azide-containing polythiophenes by GRIM polymerization of a mix of monomers, followed by postfunctionalization. They fabricated photo-cross-linkable 59.5 (P3HT-N5 with 5% azide):PCBM BHJ solar cells in air and also studied the effects of 5643


Chemical Reviews

Review

Scheme 57. Synthesis of an Azide-Carrying Polyfluorene 57.2

Scheme 58. (a) Synthesis and (b) Stabilization of the Morphology of a PV Cell Using a Lightly Azide-Substituted Polythiophenea

largely unchanged during the entire 40 h anneal, with the decrease in PCE due to a 20% reduction in the FF that stabilized after 5 h at 110 °C. Thelakkat et al.201 (Scheme 60) reported low band gap polymers containing around 10% of azide-equipped thiophene units, which tolerate Stille or Suzuki reactions. UV−vis irradiation cross-links the polymers into solvent-resistant films. Cross-linking has a negligible effect upon optical and electrochemical properties of the polymers but renders them insoluble. In organic field effect transistors, the introduction of 10% azidefunctionalized monomer does not considerably influence hole transport mobility (0.20−0.45 cm2 V−1 s−1). The azide-functionalized materials are highly reactive, easily accessible, and versatile in their application. However, the reaction is hard to control since there are many examples that state their ability to immobilize thin-films without detrimental loss in device performance. 4.3. SiO- and BO-Containing Species

Si−O and B−O-containing species are attractive to render organic semiconductors insoluble and affix them into a desired morphology. Kippelen et al.202 prepared an allylated triarylamine 61.2, which upon Pt catalyzed hydrosilylation gave a trifold tristrichlorosilane species 61.3 (Scheme 61). Hydrolysis and subsequent condensation gave the trifold trimethoxysilane species 61.4. A combined spin-coating/siloxane cross-linking approach to OLED charge transport layers provided a high throughput route to robust, pinhole-free, adherent thin films with covalently interlinked, glassy structures of readily controllable thickness. Using these interlayers, hybrid OLEDs with brightnesses of up to ∼1700 cd m−2, external quantum efficiencies of ∼0.2%, and minimal leakage currents were constructed. Alq3 was evaporated as emitter material. Li et al.203 reported (Scheme 62) the synthesis of defined boronic acid terminated oligofluorenes and a doubly boronic acid substituted carbazole. In both cases, heating led to the loss of water under cross-linking into amorphous solid-state materials that form at 130 °C and which could be used as OLED emitters. 62.4a (F3BA) or 62.4b (F4BA) are blue emitters. Efforts to improve device performance continue with the focus on the role of the chemical structure of the hole-transporting layer, adding an appropriate electron-transporting layer. The absorption and emission spectra of these thin films are thermally stable, showing

a

Adapted with permission from ref 193. Copyright 2010 American Chemical Society.

no long wavelength emissions after being heated to 150 °C for 24 h. The described approaches for Si−O and B−O cross-linking are valuable for immobilizing layers in electronic devices. When comparing vinyl or azide cross-linking, it is obvious that the resulting films are not as stable since they are sensitive to hydrolysis or oxygen. 4.4. Diels−Alder Reactions

Thompson et al.204 described new thermally cross-linkable polymers as hole-transporting layer for solution processed 5644


Chemical Reviews

Review

Figure 33. (a) Synthesis of postfunctionalized and cross-linked emissive polystyrenes. (b) Schematic representation of the thermal azide−alkyne cycloaddition chemistry and (c) secondary functionalization. Confocal fluorescence images of different polymer films each patterned with either (left) azide-, (center) alkyne-, or (right) amine-bearing fluorescent dyes by thermal microcontact printing mediated covalent bond formation. Adapted with permission from ref 194. Copyright 2011 American Chemical Society.

Figure 34. Synthesis of conjugated nanoparticles. The obtained nanoparticle are easily dispersed in water and display orange emission with high fluorescence quantum yields of 23% ± 2%. Adapted with permission from ref 198. Copyright 2012 American Chemical Society. 5645


Chemical Reviews

Review

Figure 35. An azide-containing polythiophene for solvent-resistant OTFTs and thermally stable OPV-elements. Adapted with permission from ref 199. Copyright 2012 American Chemical Society.

emitter aggregation indicated by shifts in emission maxima, it was hypothesized that cross-linking the emissive dopant with the host polymer would lead to deaggregation. In thin films of 63.6 (PFO (X)) and 63.8 (bE-BTD(X)), a bathochromic shift of 16 nm is observed in the emission maximum when the dopant concentration is increased from 1% to 8%, suggesting that the dopant is aggregating. In similar films where a passive cross-linker (PC) is included and the film is heated to effect cross-linking, a comparable 16 nm shift in the emission maximum is observed, indicating that aggregation is still occurring and not affected by the heating step. When an electron transport layer is used in 63.6/63.7 devices, luminance and luminous efficiency increase by 190% and 490% relative to devices without an electrontransport layer. The results showed that this Diels−Alder crosslinkable system is amenable to multilayer deposition by solution methods. When 63.8 is included as the dopant emitter, similar increases in luminance and luminous efficiency are observed.

Scheme 59. Synthesis of Azide Containing PT P3HT-N5

4.5. Free Radical Approaches

An alternative for the cross-linking through defined functional groups is the generation of free radicals through addition of a radical starter such as di-tert-butyl peroxide to the semiconductor. Black et al.207 treated P3HT with di-tert-butyl peroxide and found that the cross-linked polythiophene showed an increased Voc (40%, from 0.25 to 0.34 V) and Jsc (2.5 times, 0.58 mA cm−2 to 1.5 mA cm−2). The mechanism of this procedure was postulated to be dimerization of radicals or similar processes. Alternatively, Tsang et al.208 explored hyperthermal hydrogen induced cross-linking (HHIC) to give solvent-resistant conjugated polymer thin films at elevated temperatures with excellent thermal stability. HHIC cleaves C−H bonds; the amount of cleaved C−H bonds is sufficient for cross-linking but not enough to be detected by FTIR. This treatment has no impact on the electronic properties of the organic thin film in organic field effect transistors (OFET), but the stability of OFETs has significantly improved with the HHIC treatment. HHIC can be applied in organic donor−acceptor blend structures with enhanced thermal

multilayer OLEDs. A vinylated benzocyclobutene was copolymerized with a suitable triarylamine monomer (Scheme 63). The thermal cross-linking of benzocyclobutene occurs between 180 °C and 250 °C through ring opening of the four-membered cyclobutene ring followed by irreversible cycloaddition to form a cyclooctadiene ring.205 Thus, in an analogous manner, upon heating this polymer to 170 °C, the benzocylcobutene ring opens into an orthoquinodimethane and cross-links by dimerization and/or other reactions to give a solvent resistant film of the hole injection layer 63.5. Using this polymer 63.4, efficient solution-processed multilayer green emitting phosphorescent devices with an EQE of 10.4% at 350 cd m−2 were demonstrated. Reynolds et al.206 prepared a Diels−Alder cross-linkable host polymer (Figure 36) for an improved PLED. They employed a furan-substituted polyfluorene and a maleinimid-substituted donor−acceptor trimer, which reacts under Diels−Alder crosslinking. Because such systems often suffer from quenching and 5646


Chemical Reviews

Review

Scheme 60. Synthesis of Novel Azide-Containing Donor−Acceptor Polymers

shows a lowered work function and increased hydrophobicity compared to pristine ITO. Inverted polymer solar cells (IPSCs) using this modified ITO display remarkable enhanced power conversion efficiency (9.4%) compared to those made with pristine ITO (6.8%). In summary, cross-linking through vinyl polymerization (section 4.1) has the advantage of the synthetic availability of the polymers and the complete immobilization of the polymer films. Furthermore, vinyl polymerization does not lead to sideproducts that would remain in the film resulting in stable device performance. On the other hand, harsh conditions (>200 °C) ensure cross-linking that, in the worst case, degrades the organic material. In section 4.2, a different approach is shown with the

stability. Compared to the conventional chemically driven crosslinking approaches, the HHIC requires no synthetic preprocedure semiconductor-modification and therefore has a great potential. 4.6. Nucleophilic Cross-linking

Lin et al. used cross-linking to modify the metal/metal oxide electrode and to fabricate high-performance multilayer organic optoelectronic devices as a consequence of its ability to resist solvent erosion (Figure 37).209 Robust cross-linked thin films were prepared from an epoxy-functionalized conjugated polymer 63.9 (PFEX) and an amine 63.10 (TAA). The cross-linking occurs under mild heating at 80 °C due to the highly efficient amine-epoxide reaction. ITO, modified by the cross-linked films, 5647


Chemical Reviews

Review

Scheme 61. Synthesis of a Cross-linked Triphenylamine Charge Injection Layer for OLEDs

Scheme 64. Synthesis of an Electron-Beam Cross-linkable Polythiophene 64.3

Scheme 65. Electron-Beam Cross-linking of Emissive PPVDerivatives 65.1−65.4a

Scheme 62. Synthesis of Amorphous Solvent Resistant Networks from Oligofluorenesa

a

Top: PPVs investigated. Bottom: Cross-linking mechanism upon excitation with an electron beam.

introduction of azide functionalities into semiconducting polymers. The reactivity of these groups makes the cross-linking unselective and the reaction control difficult, but the crosslinking yield is excellent and the materials are attractive for photolysis. Attempt to control the cross-linking reactions is also demonstrated by adjusting the quantity of azide groups in a given

a

Under thermal treatment, the boronic acid trimerizes to build polymeric networks.

Scheme 63. Synthesis of a Cross-linked Hole Transport Layera

a

Thermal ring-opening cycloaddition of the benzocyclobutene leads to the formation of eight-membered rings. 5648


Chemical Reviews

Review

Figure 36. Synthesis and cross-linking of a polyfluorene using Diels−Alder chemistry. Host polymer PFO(X), maleimide containing passive cross-linker and the red emitting oligomeric dopant bE-BTD(X). Illustration of the Diels−Alder reaction between furan and maleimide and the cross-linking that would occur in a film composed of a cross-linkable polymer and a separate cross-linking agent. Adapted from ref 206 with permission of The Royal Society of Chemistry.

Figure 37. IPSCs from an epoxy-functionalized conjugated polymer (63.9, PFEX) and an amine-based small molecule (63.10, TAA). Reprinted from ref 209. Copyright 2017, with permission from Elsevier.

polymer. The polymer thin-films are stable, and in some cases, one can even see an improvement in electrical parameters. The mild conditions in section 4.3 described methods including boronic acids or siloxanes that are attractive for sensitive device architectures. However, their potential instability against air and moisture as well as the synthetic effort do not make this method the number one choice when deciding on a cross-linkable material. Diels−Alder reactions, as demonstrated in section 4.4 are easily controllable by the reaction temperature. However, while beneficial for the device processing step, the reversibility of the reaction at higher temperature can limit the device working temperature. Nevertheless, the synthetic availability of the monomers and polymers, the stability, and the performance of the fabricated devices make the method attractive. Free radical

reactions (section 4.5) increase device performance without any modification of the polymer material. The concept was shown on the basis of P3HT. Radical reactions, which are hard to control, could be applied to other higher performing polymer materials. Finally, the multicomponent, nucleophilic addition reactions described in section 4.6 increase the performance of OPVs with a highly efficient reaction of simple polymers mixed with a commercial cross-linker. All described methods (sections 4.1−4.6, Chart 5) are easily applicable to any high-performing organic semiconductor. However, there are disadvantages such as harsh conditions or sensitivity of some methods against external influences, yet the electronic properties of the resulting devices are remarkable and 5649


Chemical Reviews

Review

Figure 38. Investigation of the mechanism of photopolymerization of oxetanes: (a) Structures. (b) Photoinduced electron transfer. (c) Follow-up reactions. Reprinted with permission from ref 155. Copyright 2011 American Chemical Society.

Scheme 66. Synthesis of Novel Cross-linkable Polyfluorene-Based Polymers 66.8 by a Privileged Oxetane-Containing Building Block 66.2 (highlighted) and Structure of the PAG 66.9 for Cross-linking216

5. PHOTOCHEMICAL CROSS-LINKING Application of a photochemical stimulus (either for direct excitation of the active material and subsequent photochemical cleavage of solubility-mediating groups or for activation of a photoacid as a proton source to initiate (thermal) follow-up

often show great improvement compared to non-cross-linked

devices. 5650


Chemical Reviews

Review

Scheme 67. Synthesis of Oxetane-Based Nonconjugated Polymers as Charge Transport Layers: (A) Homopolymers; (B) Polymers with Varying Cross-linker Ratio157

reactions) is a versatile complement to thermal methods. Similar to its thermal counterpart, usage of a cold (for purely photochemical processes) or a hot (with additional curing effects) light source leads to in situ cross-linked polymeric networks by judicious choice of cross-linkable functional groups or addition of photoactivatable small molecule cross-linkers. However, careful optimization of irradiation parameters (wavelength and dose) is vital as excitation energies in the UV range (250−350 nm, corresponding to 480−340 kJ/mol) do not only induce cross-linking reactions but also degrade and photobleach organic materials. Although not being a photochemical method, we will start by looking at early examples of unselective and potentially highly destructive electron-beam cross-linking to motivate the development of more sophisticated and selective photochemical approaches.

Scheme 68. Bis-oxetane Substituted Donor−Acceptor Oligomer for OPV

irradiation additionally leads to an increase of conductivity by 4 orders of magnitude. Hikmet and Thomasson adapted this concept by synthesizing PPV-derivatives 65.1−65.4 and irradiated them with an electron beam, forming free radicals and hydrogen atoms (Scheme 65).211 The free radicals recombine in the film to give a cross-linked PPV-semiconductor. This cross-linked fraction becomes insoluble and increases with radiation dose. Cross-linking reduces hole mobility within the polymer but does not influence quantum efficiency of devices. Patterning with a mask produces multicolored OLEDs. While cross-linking via electron beam is applicable to any kind of polymer and no additional washing steps are necessary

5.1. Cross-linking with Electron Beams

High energy electron beams cross-link polymers unselectively via radical generation and recombination. Early on, Kock et al. (Scheme 64) prepared an acrylate substituted regiorandom polythiophene 64.3 by iron trichloride-assisted oxidative polymerization.210 Reasonable yields of the polythiophene with high molecular weights were obtained (after 4 min, Mn = 18−30 kg/mol). A high Đ was observed due to inherent cross-linking. The copolymers act as negative resists in electron beam lithography and form conductive patterns. Irradiation of films with a 40 Mrad electron beam dose leads to desolubilization. Addition of iodonium salts before film formation followed by 5651


Chemical Reviews

Review

Figure 39. (a) Synthesis of bithiophene-fluorene copolymer 67.15. (b) Structuring of a bithiophene-fluorene copolymer by cospin-casting with polystyrene, development, and removal of the polystyrene. (c) A crosslinkable bithiophene-fluorene copolymer as a microstructuring tool for the fabrication of nanotowers of donor polymers for PV-cells. (d) Photoacid generator for cross-linking. Figure 39b reprinted with permission from ref 220. Copyright 2009 American Chemical Society. Figure 39c adapted from ref 221 with permission of The Royal Society of Chemistry.

Figure 40. (a) Synthesis and (b, c) AFM-obtained morphology of GRIM-based and post-cross-linked polythiophene copolymers 67.19. (d) Structures of thiophene-based comonomers 67.20−67.22. Figure 40b,c reprinted from ref 222, copyright 2012, with permission from Elsevier.

Suzuki polymerizations or GRIM metatheses and allows for preparation and purification of the as-functionalized monomers. Usually, heating is necessary after photoinitiation with a PAG, to enhance the mobility of the oxetane units in thin-films and complete polymerization. As impurities, unreacted oxonium species or protons could remain in the thin films, which may drift in the electric field to adjacent layers in OLEDs. Although small molecule semiconductors are excluded from this Review, we will first have a brief look at the photo-crosslinking mechanism as disclosed by Meerholz and Feser155 before we turn our focus onto immobilization of polymers. The authors carefully investigated the mechanism of cross-linking using triphenylamine dimers. In addition to cross-linking chemical redox doping of the layers occurs. Persistent doping charges were generated during the initiation process. OFETs were used to quantify this unintentional side-reaction, and the mechanism and effects were investigated. Treating cross-linkable and non-crosslinkable triarylamine derivatives with a photoacid generator (PAG) or stoichiometric oxidants (redox salt, nitrosyl cation) led to detailed information about charge transport of doped cross-linked films, in which the PAG induces a stoichiometric

(thermal curing is required), careful control of the applied dose is vital. Cross-linking occurs via material degradation and recombination of the generated, highly reactive intermediates (radicals). Recombination is unspecific without the directing aid of functional groups. To retain the desired optoelectronic properties in immobilized films, optimization of the irradiation dose is critical for each polymer. The method may not be suitable for every material, as unforeseen reaction products may decrease device performance. 5.2. Oxetane as Cross-linker

Oxetane-functionalized organic semiconductors, both small molecules,154,212 oligomers,213 or polymeric semiconductors,153 developed by Meerholz et al. enjoy great popularity for the fabrication of multilayer OLEDs/PLEDs as well as for photolithography. The desolubilization is based on the cationic ringopening polymerization (CROP) of oxetanes initiated by a photoacid generator (PAG).214,215 The lower reactivity of oxetanes compared to their more strained analogues, the oxiranes, allows them to tolerate cross-coupling conditions like 5652


Chemical Reviews

Review

Figure 41. (a) Synthesis of oxetane-substituted regiorandom polythiophenes and structure of PAG. (b) UV−vis absorption spectra of 68.8 solution and films before and after CROP. (c) Optical (left) and SEM image (right) after irradiation and etching with THF. Figure 41b,c reprinted with permission from ref 225. Copyright 2009 American Chemical Society.

Figure 42. (a) Synthesis of a cross-linked electrochromic oligomer 68.15 for oxetane-based CROP. (b) Optical micrographs of photopatterned polymer (A), the shadow mask (B), and large area photopatterning (C). Figure 42b reprinted with permission from ref 226. Copyright 2014 American Chemical Society.

5653


Chemical Reviews

Review

Scheme 69. Synthesis of Cross-linkable Polyfluorenes 69.6 with Different Comonomers to Obtain Highly Emissive Polymers for OLED Applications

Scheme 70. Follow-up Reactions upon Singlet Nitrene Generation244,a

a

F symbolizes perfluorination.

roughnesses of

Immobilization Strategies for Organic Semiconducting Conjugated

Recommend Documents