Advances in Charge Carrier Mobilities of Semiconducting Polymers

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Advances in Charge Carrier Mobilities of Semiconducting Polymers Used in Organic Transistors Sarah Holliday,* Jenny E. Donaghey,* and Iain McCulloch Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K. ABSTRACT: In this review, we describe and discuss recent advances in the performance of polymeric semiconductors in organic field effect transistors (OFETs). Design concepts such as short intermolecular contacts, low conformational disorder, side chain optimization, and noncovalent interactions have all been successfully employed to improve the charge carrier mobility of polymer thin films. The relationship between the molecular design, thin film microstructure, and electrical performance has been exemplified by a range of thiophene, bridged and fused ring, diketopyrrolopyrrole (DPP), and isoindigo containing polymers which are reviewed and discussed. KEYWORDS: organic, polymer, transistor, semiconductor

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INTRODUCTION Applications for Organic Transistors. Conjugated aromatic polymers have demonstrated great promise as the thin film charge transport layer in organic field effect transistors.1−3 Their ease of solution processing, good mechanical properties, and increasingly improving charge transport properties have prompted extensive research and development activity in both academia and industry. Applications such as flexible displays,4 circuitry,5,6 and even RFID7 have been proposed, each of which have distinct electrical, stability, and cost requirements. In particular, the combination of a printed thin film transistor backplane for display applications,8 especially flexible displays, is an attractive proposition. The incumbent technology throughout most of the historical development period of organic semiconductors, amorphous silicon, had modest charge carrier mobilities and the fabrication process was not readily compatible with plastic substrates envisaged for flexible displays. A clear entry opportunity for organic transistors was identified as the switching transistor in the emerging application of electrophoretic displays for e-readers. Due to the frontplane bistability and reflectivity, slow switching speed requirements, and comparatively low resolution of these displays, the transistors used could be large and require carrier mobilities of less than 0.1 cm2/(V s), which is well within the possibility of organics. Operational stability for such applications is also quite relaxed. However, it is still not clear, at the time of writing this review, whether a successful commercial e-reader product utilizing organic transistors will emerge. This is probably due to market timing, competition from strong alternative materials, and processing yield issues rather than the intrinsic property limitations of the technology, yet it is undeniably an unfortunate and substantial setback for our field. Significant © XXXX American Chemical Society

competitive material advances also threaten the growth of organic electronics, especially low temperature metal oxides such as zinc oxide, where high electron mobilities are readily achieved at processing temperatures ever closer to plastic compatibility. An even more ambitious application target for flexible displays is the high performance printed OLED frontplane. Fully printed by additive processing, this prospective technology is now gaining attention where significant potential cost advantages are foreseen and competitive alternatives have some drawbacks. Obviously the electrical requirements for both types of transistor in an OLED backplane are significantly greater than for the e-reader, with charge carrier mobility requirements of about 5−10 cm2/(V s) and the additional requirement of highly uniform transistor performance. Encouragingly, polymer charge carrier mobilities have improved significantly over the past few years, and it is certainly conceivable, at least from a technical perspective, that the technology will be compatible with this demanding application. In recent years there has been less emphasis on potential RFID type applications for organic transistors. Possible reasons include the large-scale of manufacture immediately required for mainstream applications, the difficulties in assembling a complete system with coordinated standards, etc. Niche products based on anticounterfeit and product identification applications are still under development but, again, competitive Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 18, 2013 Revised: August 18, 2013

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technologies present tough cost and performance targets. Perhaps the most realistic opportunities in the short term exist in the area of low performance, disposable electronics, where lifetime and speed are not a prerequisite. Disappointingly, application developers have not yet been sufficiently inspired to create any pervasive products, which may be essential to reinvigorate the field. Semiconducting Polymers. Despite charge transport being observed in polyphenylene vinylene and thiophene vinylene,9 the first wave of semiconducting polymers used in OTFT applications tended to be drawn from the xerographic and light emitting fields, including polyaryl amines10 and fluorene copolymers.11 These polymers had proven ambient stability and could be processed into thin films, yet their lack of structural order in part inhibited their carrier mobilities.12 More crystalline and ordered polymers emerged exemplified by poly(3-hexylthiophene),13 often referred to as the “fruit fly” of organic semiconductors. The significant enhancement in charge carrier mobility of this polymer was attributed to the thin film microstructure.14 Specifically, planar backbones comprised of the aromatic unit thiophene assembled in closely packed lamellae, with close intermolecular interactions between neighboring backbones (π-stacking) separated laterally by more amorphous side chain layers. When these lamellae were oriented such that the plane of the aromatic backbones was orthogonal to the substrate, efficient intermolecular charge hopping was facilitated and charge transport was enhanced.15,16 Further improvements in charge carrier mobility could be realized when the assembly into ordered domains occurred within the liquid crystalline phase.17,18 Larger, more coaligned domains resulted with a corresponding improvement in mobility. Most recently, two molecular design features have contributed to the impressive increases in charge transport. Reducing the conformational energetic backbone disorder through use of bridging units and extended aromatic structures to induce coplanarity19,20 has been shown to be effective in enhancing mobility. Even when conventional analytical techniques such as XRD and DSC do not detect evidence of crystallinity, a suitable morphology for transport can be provided by small length-scale short contacts interconnected by polymer backbones where one-dimensional transport can occur. This approach of enhancing short contacts can be further facilitated through dipolar interactions between adjacent backbones, as illustrated by the diketopyrrolopyrrole (DPP) unit 21,22 where the polar pyrrole units contribute to intermolecular aggregation through noncovalent interactions. Currently, several DPP-based polymers exhibit the highest charge carrier mobilities reported to date.23−25 Figure 1 dissects the broad category of polymer semiconductor publications for transistor applications over the last 5 years. Overall, there has been an increase in the number of publications with some interesting trends. For example, research in the area of thiophene based polymers (P3HT, PBTTT, etc.) is observed to be reaching a plateau, being challenged by newer dipolar units such as DPP and isoindigo. In the past 5 years the number of publications based on the DPP unit has increased 30-fold and is expected to rise further. Bridged ring systems, such as cyclopentadithiophene (CPDT) and indacenodithiophene (IDT), are also gaining interest with resulting polymers reaching mobilities of >3 cm2/(V s).20,26 Research on fused ring systems has also been steadily increasing due to their rigid, planar structure which promotes intermolecular order and efficient charge transport. Examples include benzodithiophene

Figure 1. Graphical representation of the number of publications of polymers used as semiconducting thin films in OFETs since 2009, based on a Scifinder search. Four categories were chosen based on polymer structure: Polythiophene includes P3HT, PBTTT, and their analogues and other nonfused ring thiophene based polymers; Bridged ring includes polymers with monomers that have two or more aromatic units bridged by carbon or another heteroatom, such as fluorene, CPDT, and IDT; Fused ring includes polymers with monomers that have two or more fused aromatic units (excluding thienothiophene), such as benzodithiophene, NDI, and PDI; DPP includes any diketopyrrolopyrrole based polymers and analogues (e.g., isoDPP); Isoindigo includes any polymer based on phenyl and thienyl isoindigo and their analogues; and Other includes PPV and polymers containing nonfused units such as triphenylamine and selenophene.

(BDT), benzotrithiophene (BTT), and naphthalene diimide (NDI). Other polymers containing units such as triphenylamine and seleneophene have been successfully incorporated into high performance OFET polymers and continue to be studied. This progress in charge carrier mobility is qualitatively illustrated in Figure 2 in the context of target application

Figure 2. Progress in charge carrier mobility for the evolving classes of semiconducting polymers. Short contact polymers include intramolecular dipolar polymers such as those containing DPP and BT.

requirements. A major catalyst for the molecular design improvements and developments has unquestionably been the very recent explosion of research in new push−pull low bandgap polymers for organic solar cells. The synergistic requirements of ambient stability and control of molecular conformation, microstructure and optimization of carrier mobility, have led to a new wave of transistor materials. In this review we will describe in detail the evolution in performance of organic semiconducting polymers, paying B

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treatment prior to the dielectric SAM deposition in order to increase the electrode work function through the surface reaction with strongly electron withdrawing thiols such as pentafluorophenyl thiol. The SAM can also be used to control semiconductor morphology at the electrode surface.34 A common modification of this architecture is to fabricate from a plastic (typically polyethylene terephthalate) substrate, with a patterned gate electrode and an organic dielectric. The choice of bottom gate organic dielectric is critical to the device performance. The main concern is that the type of aggressive solvent used to deposit the semiconductor can often also dissolve the dielectric layer, resulting in a mixed interface with poor transport. Consequently, the dielectric thin film is often post-cross-linked to ensure solvent orthogonality. It is necessary to ensure that the cross-linking chemistry does not create functionalities which can act as charge traps at the dielectric interface with the semiconductor, as well as ensuring that the cross-linking conditions, whether thermal or photochemical, are compatible with the device materials and processing. Top gate transistor devices often demonstrate the highest semiconductor charge carrier mobilities, even though bottom gate dielectric interfaces are typically less rough. A key requirement when fabricating top-gate devices is that the semiconductor thin film must be inert to the solvents used during the subsequent deposition of the dielectric layer. This limits the choice of dielectric polymers to those that dissolve in orthogonal solvents, i.e., those that do not dissolve the semiconductor. As it is not normally feasible to cross-link the semiconductor, the solvent used to deposit the dielectric must be at the extremes of polarity. Fluoropolymers such as Cytop, deposited from fluorosolvents, are common top gate dielectric selections with very discrete interfaces possible due to the complete solvent orthogonality, which enhances electrical performance. These low K dielectrics are also attributed to enhancing carrier mobility but present a challenge to deposit the gate electrode or passivation layer on the very low energy surface. The dielectric layer has also been shown to act as an effective barrier layer to reactive species in the environment such as ozone which can contribute to semiconductor oxidation and doping, thus degrading performance.35 Contact resistance is a common problem with organic transistors, usually related to either an energetic mismatch between the workfunction of the electrode and the semiconductor energy levels or poor physical contacts between the electrode and the semiconductor. While SAMs are used to improve the contacts, charge injection may be improved by use of a staggered architecture which enhances the interfacial area between the semiconductor and electrodes. Direct deposition of the electrode onto the semiconductor can also be beneficial in this respect. These methods will consequently reduce the dependence of mobility on channel length. Importance of Polymer Morphology. It is clear that in order to further advance the performance of organic semiconducting thin films in transistor applications, there must be a fundamental understanding of the relationship between the molecular structure and conformation of the semiconductor, the resultant thin film microstructure, and subsequent electrical performance. Organic semiconductors exhibit varying degrees of morphological disorder, ranging from completely amorphous to polycrystalline (where crystalline and amorphous domains coexist). There have been many differing and detailed hypotheses to explain the progressive increases in polymer charge carrier mobility in the past decade or more, mainly

particular attention to molecular design criteria responsible for controlling thin film morphology and molecular electronic energy levels as these determine the charge carrier mobilities and electrochemical stability. Transistor Design, Fabrication, and Characterization. There are four general transistor architectures used to evaluate the electrical properties of organic semiconductors (Figure 3).

Figure 3. Schematic diagrams of the four common transistor architectures with the electrodes in pale gray, dielectric in blue, semiconductor in red, and substrate in dark gray.

The bottom gate, bottom contact device is prevalent in display backplanes and often used for routine screening and characterization since it only requires deposition of the semiconductor and is therefore simple to fabricate. As the semiconductor deposition is the final step in fabrication, this architecture is advantageous where the semiconductor is sensitive to processing conditions. For routine screening, a highly n-doped crystalline silicon substrate is typically employed as a common gate electrode with a thermally grown silicon-oxide layer as a gate dielectric (around 300 nm thick). Gold source and drain electrodes (for p-type devices) typically patterned by lithography complete the substrate, often with a series of differing channel lengths and widths. The SiO2 dielectric surface is of relatively high surface energy at 40−55 mN/m depending on fabrication conditions27 and comprised of a high concentration of polar species such as hydroxyl groups. This high energy surface controls the thin film morphology of the subsequently deposited semiconductor, often through rapid nucleation which leads to small crystallite size.28 Meanwhile the polar surface can template the polymer backbone orientation29,30 through specific polymer−surface interactions. Smaller crystallite sizes can result in lower charge carrier mobility especially when grain boundary effects are prominent which can be true of small molecule semiconductors. Polar dielectric surface functionalities have also been shown to act as charge traps (particularly for electrons),31 and the adsorption of moisture can lead to device hysteresis.32 To counteract these problems it is often beneficial to modify the dielectric surface with a hydrophobic self-assembled monolayer (SAM),33 comprising a hydrocarbon “tail” with a reactive silyl end group which polymerizes and covalently bonds to the polar OH groups on the dielectric surface. This generates a very low energy surface which allows enhanced self-assembly of the polymer subsequently deposited on the SAM surface. On the other hand, the low energy surface promotes dewetting, reticulation of the thin film, and often delamination. Semiconductors with low lying HOMO energy levels can present a significant energetic barrier to hole injection at the gold source electrode. This requires application of a separate SAM C

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to include a shearing step during the solvent deposition process. This can be achieved through a simple dip-coating method.42 More elegantly, it can be achieved by means of a moving substrate with respect to the deposition nozzle, a moving nozzle, a meniscus traversing the film created by surface tension,43 or application of shear forces across the film.44

focused on attaining a highly ordered crystalline structure in an attempt to emulate small molecule organic single crystals. Initial studies of polythiophene thin films14,36 served to illustrate the benefits of long-range order achieved in two-dimensional lamellar microstructures, with transport occurring by hopping within the sheets comprising the conjugated backbones. As discussed within this review, however, this paradigm has been recently challenged by the new “push−pull” or dipolar conjugated polymers where order is on a much shorter length scale, although the critical intermolecular π−π distances can still be well below 4 Å. Even polymers that had previously been reported to be highly crystalline by XRD15 in fact have recently been shown37 to exhibit significant structural disorder even within the crystallite lattice arrangements, which can be quantified by a so-called paracrystallinity parameter, g.38 This disorder is responsible for electronic localization and gives rise to traps, thus inhibiting charge carrier mobility. Enhancement of charge carrier mobility can be most effectively achieved by ensuring there are sufficient regimes of close molecular contacts,20 even within a mainly disordered morphology, or by achieving a percolation of interconnected aggregates39,20 within the thin film. These morphological regimes can be molecularly engineered by a combination of employing a high molecular weight polymer to act as “bridging molecules”39 between crystalline domains, facilitating short contacts through steric and dipolar intermolecular interactions such as in the case of DPP polymers and by use of both preaggregation40 and liquid crystalline alignment as observed for PBTTT. In addition to molecular design, the processing conditions for polymer thin films can also dictate and control thin film microstructure and are an important tool in optimizing electrical performance. As a simple starting point, thermal annealing of the as-cast thin films generally improves the degree of polymer crystallinity within a film, with subsequent improvements in performance. The nature of the substrate is also critical and can influence the orientation29 and nucleation processes28 during thin film drying. The relative surface energies at the interface, as well as dipolar effects, can promote polymer backbone orientation, while high energy surfaces and surface roughness can promote crystal nucleation which leads to smaller crystallite sizes. Choice of solvent and processing conditions is also important. High boiling solvents41 allow more time during drying for polymer ordering in the thin film. Solution rheology has been shown to influence the molecular orientation of thiophene polymers with high shear rates, with fast spin-casting speeds favoring an in-plane orientation of the polymer backbone and slower spin speed promoting an edgeon orientation. The remarkable lamellar length-scale and orientation in PBTTT thin films has also been suggested to arise, in part, from solvent effects.40 Poor or “borderline” solvents can initiate a preaggregation phase in solution, where the polymer backbone chains, with their high degree of freedom, can assemble into π-stacked lamellae which can coalesce as the solution concentrates on drying. Solvent annealing has been demonstrated as a way to reduce thin film bulk viscosity and allow enhancement of both crystalline order and crystallite size. Both molecular and crystallite orientation can be promoted through a shearing effect from an anisotropic deposition of the solution onto a substrate. It has also been shown that applying a rubbing process on the surface of a heated P3HT film can enhance orientation and lead to improved performance. A potentially more efficient way of achieving axial orientation in thin films is

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HOLE TRANSPORTING POLYMERS Polythiophenes. Historically, OFET devices have been predominantly p-type due to the generally higher charge carrier mobilities and the greater ambient stability of hole versus electron transporting materials. The first OFETs published in the late 1980s were polythiophene based, initially produced by electrochemical polymerization directly onto the source and drain electrodes.45 This method led to low molecular weight, amorphous materials with mobilities on the order of 10−5 cm2/ (V s). It was not long before alkylated polythiophenes were synthesized in order to allow solution processing (e.g., via spincoating) of the active layer.46 The improved synthesis afforded a more controlled polymerization, inducing a more favorable structural ordering which led to higher mobilities by an order of magnitude. The polymers were shown to have a semicrystalline lamellar structure with the alkyl side chains functioning as spacers between the planar polythiophene chains.47 Further developments employing a nickel catalyst and stringently inert conditions resulted in higher regioregularity (98%) P3HT.48 Similar methods using regiospecific organozinc reagents and a Ni(dppp)Cl2 catalyst achieved regioregularities >98.5%.49 These high regioregularity polyalkylthiophenes resulted in improved mobilities of 0.015−0.045 cm2/(V s) due to better ordering in the films.13 X-ray diffraction studies indicated preferential orientation of the lamellae stacked perpendicular to the substrate, allowing π−π stacking in the transport direction. Further studies attributed the remarkable increase in mobility to the formation of extended transporting/polaron states due to the improved self-organization of the polymer, in contrast to the variable-range hopping of self-localized polymers observed in less regioregular polymers.33 The reduction of grain boundaries, defects and residual doping in higher regioregularity materials was thought to reduce the number of trapping sites and hence the large increase in mobility. Regioregular P3HT was used to fabricate an all-polymer integrated device (OFET used to drive an OLED) and mobilities of 0.1 cm2/(V s) were obtained, close to those of amorphous silicon devices.33 The high mobility was partially attributed to treatment of the surface with hexamethyldisilane (HMDS) in order to replace hydroxyl groups on the SiO2 surface with nonpolar methyl groups, promoting phase segregation of the polymer at the interface. Processing conditions have been shown elsewhere to have a significant effect on the mobility of regioregular P3HT, with dip-coated films giving higher mobility (0.2 cm2/(V s)) than spin-coated films because of the slower evaporation rates.42 By controlling the P3HT molecular weight and regioregularity, it was shown that edge-on or face-on orientation could be selectively induced with a significant effect on the charge carrier mobility.50,14 Samples with low molecular weight but high regioregularity adopted a lamellar structure with the polymer backbone oriented edge-on to the substrate, and these films exhibited mobilities of up to 0.1 cm2/(V s). The high molecular weight, low regioregularity polymer, however, oriented with lamellae parallel to the substrate, and mobilities were lower here by 3 orders of magnitude. While the influence of molecular D

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low on/off ratios, as well as a positive shift in the threshold voltage.13,54 In order to increase the oxidative stability of polythiophenes, the HOMO can be lowered by judicious molecular design to decrease π-electron density and delocalization. High electron density can be created by a combination of electron resonance and inductive donation, through, for example, lone pair donation from sulfur, oxygen, or nitrogen atoms conjugated to the π-electron system or inductive effects from alkyl chains. Increasing the quinoidal character and extending the conjugation length both also contribute to increasing the electron density, leading to a high lying HOMO energy level. Effective strategies to lower the HOMO energy level include reducing the density of electron donating alkyl chains; reducing the conjugation length of the aromatic backbone (by inducing torsional twisting by steric substitutions at alpha positions to linking aromatic groups);55 introducing cross-conjugated units where the π electron system has discrete and short lengths along the backbone;56 enhancing aromatic resonance stabilization; and introducing electron withdrawing units within the conjugated system such as carboxylate groups57 and copolymerization with electron poor repeat units such as dialkylfluorene units.11 In an effort to improve the microstructural ordering of P3HT, the polymer poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) PBTTT was developed.17 In this case the repeat unit was centrosymmetric which alleviated any regioregularity disorder. Additionally, the unsubstituted thieno[3,2-b]thiophene central ring created additional free volume between the side chains which facilitated interdigitation between adjacent backbones orthogonal to the π stacking direction. It was believed that this interdigitation allowed the πstacked lamellar sheets to register in the out-of-plane direction to create a three-dimensional ordering. This is in contrast to P3HT where the side chains are significantly disordered and do not interdigitate (Figure 5). A further benefit of the side chain

weight and solution viscosity were not considered, it was presumed that this difference in charge carrier mobility arose from the out-of-plane orientation of the lamellae. Also, the increased planarity of the polymer backbone in regioregular P3HT allowed for more ordered solid state packing as well as increasing wave function delocalization along the backbone to facilitate hopping. The correlation between molecular weight and charge carrier mobility was later explored via a series of studies.39,50−52 Thin films of low molecular weight, highly regioregular P3HT (∼5 kDa) were highly crystalline, with a rod-like microstructure observed by AFM measurements in which the rod width does not exceed the length of the polymer chains (Figure 4).50

Figure 4. Atomic force micrographs of low molecular weight (a) and high molecular weight (b) films with the corresponding schematics for charge transport. In (a) there is a large concentration of grain boundaries which act as traps whereas in (b) these ordered regions are bridged by longer chains. Adapted with permission from ref 50. Copyright 2005 American Chemical Society.

Higher molecular weight P3HT (>30 kDa) with similar regioregularity appeared to be less crystalline, exhibiting small nodule like domains. Although the low molecular weight films are more crystalline, they exhibit lower mobilities. It was proposed that the higher molecular weight P3HT has well connected grains whereas the low molecular weight P3HT has more defined grain boundaries.50 An enhanced out of plane twisting in low molecular weight polymer backbone conformation has also been proposed as an explanation for the difference in mobility.52 In addition to the utilizing high molecular weight polymer to bridge between domains, directional crystallization has been shown to assist charge transport through promoting “lowangle” grain boundaries where intergrain connectivity is optimized by coalignment in the direction parallel to the polymer backbone.53 An early problem associated with very electron rich polymers such as P3HT was their oxidative instability due to a high lying HOMO level which leads to p-type doping in ambient conditions. This can result in high off currents and therefore

Figure 5. Schematic diagram comparing side chain packing of P3HT and PBTTT. The spacing of alkyl chains does not allow for any coupling whereas PBTTT is fully interdigitated. Adapted with permission from ref 36. Copyright 2007 American Chemical Society.

interdigitation was that it promoted a “mesophase” above the side chain melt temperature, where growth and coalignment of the crystalline domains was possible due to the lower bulk viscosity. Processing above this phase transition and below the backbone melt transition was shown to significantly improve ordering, resulting in very large thin film domains of greater than 200 nm (Figure 6) and had a subsequent improvement in the electrical properties. The aromatic stabilization of the thienothiophene ring along with reduced inductive electron E

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backbone (Figure 7).61,62 For as cast TG-BC devices the hole mobility of the resulting copolymers (F8-TAA μh = 2 × 10−2 cm2/(V s) and IF8-TAA μh = 4 × 10−2 cm2/(V s)) was around 1 order of magnitude greater than that recorded for the homopolymer PTAA, and they still exhibited excellent ambient photo and electrical stability (Figure 8).61,63 A logical extension to this design strategy of the IF8-TAA copolymer was to further reduce conformational disorder by reducing the backbone twisting caused by both phenyl−phenyl linkages and the presence of the two consecutive nitrogen− carbon single bonds. Thus the flanking phenyl groups of the IF were replaced with thiophene units, to give the indacenodithiophene (IDT) unit, and the TAA unit was replaced with the electron poor benzothiadiazole (BT) unit.64,65 IDT possesses a rigid, planar, conjugated backbone structure, with low conformational disorder which contributes to its high charge carrier mobility. Additionally, the electron rich nature of the peripheral thiophene units facilitates molecular orbital hybridization with electron poor comonomers, leading to low band gap polymers. The thiophene links along the backbone promote coplanarity by reducing the steric twist between adjacent aromatic units (Figure 9), enhancing π-conjugation and intermolecular π-stacking.19,64 The bridging heteroatoms allow for the incorporation of solubilizing functionalities and offer a versatile position to fine-tune the molecular orbital energy levels. In contrast to planar sp2 hybridized bridging strategies,66,67 the bridging atom is sp3 hybridized, which results in an out of plane projection of the alkyl side chains. This steric effect greatly improves polymer solubility, whereas the potential for close intermolecular contact distances does not seem to be adversely affected. IDT based polymers have been employed in both solar cell and transistor applications, affording high performance devices.19,64,68−74 The bridging heteroatom can be varied via different synthetic routes. The carbon,19,69 silicon,68,72,75 germanium,74 and nitrogen70,71,76 IDT analogues have all been reported. The IDT, SiIDT, and GeIDT units possess alkyl side chains on the bridging atom, which project out of the plane of the backbone, helping to impart solubility to the resulting polymer. The nitrogen however can fully donate its lone pair into the fused system resulting in a fully aromatic, planar unit with the alkyl chain functionalities extending coplanar to the backbone. This significantly influences microstructure and solubility of the resulting polymers.64 Transistor devices with IDT copolymers, in particular with BT as a comonomer, have shown high hole mobilities combined with ambient stability. In top-gate device architectures, a saturation mobility value of 1.2 cm2/(V s) could be achieved for IDT-BT with hexadecyl side chains, which was essentially field independent.19 Upon further improvement of the molecular weight a maximum hole mobility of 3.6 cm2/(V s) could be reached.20 An interesting aspect of the IDT-BT thin film morphology is that it does not exhibit the high degree of long-range order or optimal molecular orientation previously demonstrated to be a prerequisite for high carrier mobility in other thiophene based polymer thin films.15 Grazing incidence 2D XRD shows an absence of higher order lamellar reflections and a face-on orientation of the polymer backbone to the substrate (Figure 10).19,64 Also, no significant thermal transitions were observed by differential scanning calorimetry indicating a lack of observable crystallinity. Remarkably, the branched C2C6 chain polymer also exhibits high carrier mobility, despite an apparent lack of detectable structural

Figure 6. AFM height images of films of (a) P3HT and (b) PBTTTC12 after heating to 180 °C for 5 min. Scale bar corresponds to 200 nm. The P3HT has a nodular structure while the PBTTT films have well-defined terraces which extend laterally for up to several micrometers. Adapted with permission from ref 58. Copyright 2008 Wiley.

donation from alkyl chains contributed to lowering of the HOMO energy level of PBTTT (−5.2 eV) in comparison to P3HT (−4.8 eV) and subsequently improved the ambient stability. Recently, a copolymer of thieno[3,4-c]pyrrole-4,6-dione (TPD) with thiophene and thienothiophene (TPD-T-TT, Table 1) yielded high mobilities of 1.29 cm2/(V s) in OFETs.59 Hydrogen bonding between the lactam oxygens with α-protons on the adjacent thiophenes may help to planarize the structure, contributing to the high performance. Varying the substitution position of the alkyl chains on the polymer backbone showed a marked difference in film morphology and mobility which was attributed to the more interdigitated orientations and therefore better solid state packing available. Bridged-Ring Systems. One of the first semiconducting polymers to both possess acceptable ambient stability and charge carrier mobility greater than 10−2 cm2/(V s) (a benchmark figure, representing the minimum requirement predicted at the time for a low performance electrophoretic display backplane driving transistor) was the alternating copolymer of fluorene with bithiophene, F8T2 (Figure 7).11,60 This polymer was shown to possess a thermotropic, nematic LC phase above 265 °C, which was exploited whereby the liquid-crystalline self-organization could be used to control microstructural order of the active layer in OFETs.11 This polymer was found to orient into a monodomain when an alignment layer was employed, which led to the development of a new OFET configuration incorporating a rubbed polyimide alignment layer.11 The F8T2 chains were aligned parallel to the rubbing direction of the underlying polyimide during an annealing step in the LC phase, forming a nematic glass which was subsequently quenched, preserving the alignment of the polymer chains but suppressing crystallinity. The microstructure comprising of uniaxially aligned polymer chains and domain sizes of 0.1−1 μm allowed fast intrachain transport along the polymer backbone to be exploited, leading to hole mobilities of 0.01−0.02 cm2/(V s). Polytriaryl amines (Figure 7) had been demonstrated to be useful as a stable, easy to process, and highly soluble hole conducting layer in xerographic applications.10 However their hole mobility in OFET devices was limited,12 mainly due to their amorphous morphology and absence of close intermolecular contacts. In order to reduce the conformational energetic disorder of this amorphous polymer backbone, fluorene (F8) and indenofluorene (IF8) units, with bridged, planar aryl groups, were incorporated into the aryl amine F

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Table 1. Summary of p-Type Polymers Mentioned in This Review with Mobilities over 1 cm2/(V s)

order by GIXRD.69 This suggests that the polymer thin films are either entirely amorphous or that ordered domains exist in the films but they are not at a large enough length scale to be

observed via most conventional techniques. Unpublished recent high resolution TEM results show the presence of an appreciable degree of nanoscale domains. It is proposed that G

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results contradict the theory that edge-on alignment with substrate and large crystalline domains are a requirement if high mobilities are to be reached. The intrachain mobility is likely sensitive to molecular weight, and in fact low mobilities were observed for the NIDT containing polymers whose molecular weights were limited by poor solubility, especially for the BT copolymer. Mobilities approaching 0.1 cm2/(V s) were achieved from NIDT polymers with the difluorobenzothiadiazole comonomer (Figure 11), where the molecular weight was

Figure 7. Polymer structures based on fluorene and PTAA.

Figure 11. Polymer structures based on IDT and CPDT. Figure 8. (a) (Top) Periodic ON and OFF currents and (bottom) charge carrier mobility measurements on ambient devices stored in air over a period of ∼3 months. (b) Transfer characteristics of PIF8-TAA copolymer with L = 30 μm, W = 1000 μm.61

a little higher and the F−S short contacts may act to planarize the backbone, promoting shorter π−π intermolecular distances and enhanced transport.70 Disappointingly, the silicon containing polymers also exhibited low carrier mobilities when the BT comonomer was employed. This is attributed, in part, to the low lying HOMO energy levels, which may induce contact resistance at the electrode interface, unless very high work function electrodes, such as platinum, are employed. The highest mobilities were recorded for the SiIDT-DPP (Figure 11) polymer which exhibited ambipolar charge transport with hole and electron mobilities of 0.65 and 0.1 cm2/(V s) respectively, but the improvements in carrier mobility are most likely attributed to the strong aggregating effects of the DPP unit.68 When first synthesized and tested, the cyclopentadithiophene-co-benzothiadiazole polymer CPDT-BTa exhibited a hole mobility of 0.17 cm2/(V s).77 In order to improve the performance, efforts were made to induce directional longrange alignment via dip-coating. When measured along the dipcoating direction mobilities as high as 1.4 cm2/(V s) were recorded.78 Upon further optimization of the molecular weight, excellent hole mobilities of ∼3.3 cm2/(V s) were achieved.26 It was apparent that the crystallinity and subsequently the hole mobility increased with increasing molecular weight. X-ray diffraction (XRD) experiments were carried out on drop-cast films of two polymers with different alkyl side chains. The polymer with the linear hexadecyl side chains showed a second order XRD reflection, indicating a higher order in the bulk film. When higher molecular weight fractions of CPDT-BTa were analyzed by XRD, the diffraction peak narrowed, suggesting more pronounced long-range order which was consistent with an increase in charge carrier mobility.26 This observation was in contrast to earlier studies with P3HT and many other polymers, where the higher molecular weight led to increased

Figure 9. Overhead (left) and cross-section (right) view of both a phenyl−phenyl (a) and a thiophene−thiophene (b) coupled dimer molecule.64

Figure 10. Schematic depiction of the preferred orientation of P3HT p-stacked lamella (left) with an out of plane polymer backbone orientation and (right) the in-plane orientation of a IDT-BT copolymer domain; both have been experimentally observed by twodimensional high resolution grazing incidence X-ray diffraction.64

the holes travel via a combination of intrachain charge transport along the backbone, and interchain charge transport within the close intermolecular contacts of nanoscale domains. These H

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bulk viscosities, thus inhibiting the crystallization process. Perhaps the fractionation of the CPDT-BTa additionally had the beneficial effect of reducing the polydispersity of the high molecular weight fraction. Upon optimization of the processing conditions, hole mobilities could be further improved.79 Solvent vapor enhanced drop-casting (SVED) was employed in order to grow long polymer fibers. Charge carriers travel in the same direction as the fiber axis, which means that the fibers provide an unhindered charge carrier pathway, allowing mobilities as high as 5.5 cm2/(V s) to be reached when single fiber OFET devices were fabricated.79 Fused-Ring Systems. Another high performing class of polymers are those based on fused aromatic systems (Figure 12). In contrast to the bridged-ring systems, these units consist

Benzo[2,1-b;3,4-b′]dithiophene containing polymers have also shown promise, reaching maximum hole mobilities of 0.1 cm2/ (V s) 81 and polymers containing the more extended napthodithiophene (NDT) unit can achieve mobilities of up to 0.5 cm2/(V s) (Figure 12).87 More recently benzotrithiophene (BTT) has been successfully incorporated into p-type polymers.80,88,89 Similar to BDT, only with an extra fused thiophene ring, the BTT monomer was synthesized with the intention of further increasing the electron density and the size of the building block in order to enhance π−π interactions.88 The highest performing polymer was a copolymer with thiophene, BTT-T, where a maximum hole mobility of 0.24 cm2/(V s) was reported.88 Angular-fused anthradithiophene (aADT) serves as an extended electron rich unit comprised of anthracene flanked by two fused thiophene units. Through synthetic optimization four alkyl substituents, which are coplanar to the backbone, could be incorporated into the molecule in order to enhance solubility of the resulting polymers, while maintaining close π−π stacking.90 When copolymerized with alkylated bithiophene, PaADTT, the polymer exhibited OFET mobilities up to 0.08 cm2/(V s).90 In subsequent work the central benzene ring of aADT was replaced with thienothiophene, resulting in the 6-ring fused system, bis-thienobenzothienothiophene (DTBTBT).91 This highly rigid, planar unit required four dodecyl solubilizing groups in order to ensure solution processability. When copolymerized with thiophene, the resulting polymer, DTBTBT-T, exhibited a maximum hole mobility of 0.1 cm2/ (V s). The benzobisthiadiazole unit, with its planar triple fused ring system and strong electrostatic interactions between the hypervalent S and electron deficient N, has been shown to be promising when copolymerized with alkylated quaterthiophene (PBBTQT, Table 1).92 Very short π-stacking distances of 3.5 Å were measured between the edge-on orientated polymer chains which stacked parallel to the substrate. This highly ordered tight packing arrangement resulted in high hole mobilities of up to to 2.5 cm2/(V s) after annealing at 260 °C. DPP Polymers. Diketopyrrolopyrrole (DPP) polymers were among the first to demonstrate balanced and high mobility ambipolarity with the first OFET devices made from poly[3,6-bis-(40-dodecyl-[2,20]bithiophenyl-5-yl)-2,5-bis-(2hexyl-decyl)-2,5-dihydro-pyrrolo[3,4-]pyrrole-1,4-dione] (BBTDPP1, Figure 13) reaching mobilities of 0.1 cm2/(V s) and 0.09 cm2/(V s) for holes and electrons, respectively.93 Ambipolarity was attributed to a combination of extended HOMO and LUMO distribution along the backbone as well as donor−acceptor hybridization from alternating electron-poor and electron-rich repeat units, leading to optimized molecular orbital energy levels.94 With appropriate electrodes, chosen to optimize charge injection, the photoluminescent properties of the polymer BBTDPP1 were also demonstrated in a lightemitting transistor (LET) device. These high mobilities can be attributed to the highly aggregated, π-stacked conformation of DPP, arising from the dipolar nature of the fused lactam carbonyl units and planarity of the thiophene flanking units, as well as its ability to participate in hydrogen bonding and other short contact interactions.95 Typically DPP polymers are synthesized by Stille or Suzuki coupling of the dibrominated DPP monomer with a bis-stannyl or diboronate monomer, respectively. The flanking unit is typically dictated by availability of aromatic carbonitrile

Figure 12. Polymer structures containing fused-ring systems.

of two or more fused homo- or heterocyclic aromatic rings. This results in fully aromatic, planar systems which promote enhanced intermolecular interactions and therefore charge transport. Due to the rigid nature of these units, problems with solubility often occur and thus alkyl functionalities are required. The challenge is to minimize any detrimental effect on the packing and therefore the hole mobilities attainable.80,81 Included in this class of polymer are tetrathienoacenes which consist of four fused thiophene units that are alkylated in the βpositions to improve solubility. This unit was designed to enhance backbone planarity and improve hole mobility with respect to P3HT. When copolymerized with bithiophene (P2TDC13FT4) and tested as the hole transport layer in OFET devices, mobilities of up to 0.33 cm2/(V s) were obtained.82 Further studies on a series of tri-, tetra-, and pentathienoacenes showed that the tetrathienoacenes, which possess C2-symmetry, exhibited smaller lamellar spacing and higher hole mobilities.83 Benzodithiophene (BDT) has also shown promise for use in high performance OFET devices.84 The central benzene ring allows for the incorporation of solubilizing alkyl (and more recently, aromatic 85 functionality. Benzo[1,2-b:4,5-b′]dithiophene, when copolymerized with 3-alkylthiophene (lBDT-2T), forms well-ordered lamellae in the solid state and hole mobilities as high as 0.25 cm2/(V s) can be achieved.86 I

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ing crystallinity, and promote delocalization of the HOMO and LUMO along the backbone.69 The copolymer with thiophene initially showed predominantly p-type behavior with high hole mobilities of 1.95 cm2/(V s) and electron mobilities of 0.06 cm2/(V s), although later work demonstrated increased electron mobility to a more impressive 0.9 cm2/(V s).22 The homopolymer exhibited largely n-type character with electron mobilities of 0.3 cm2/(V s) and hole mobilities an order of magnitude lower. It was suggested that the shorter repeat unit and hence increased steric interactions between the alkylated DPP units in the homopolymer disrupted the solid-state packing, contributing to its lower performance. In comparison with modifying the flanking group, variation of the DPP comonomer tends to be synthetically simpler and more versatile. The dithienyl-DPP core has been copolymerized with a wide range of repeat units including bi-,104 ter-21 and quater-thiophene, thienothiophene,105 thiazolothiazole,106 dithienopyrrole,107 phenyl,108 and benzothiadiazole.109 Copolymers with unsubstituted thiophene repeat units have demonstrated better solid-state packing and therefore higher mobilities than their alkylated thiophene counterparts, attributed to the reduced steric interactions (PDQT, Figure 13). The loss of solubility from using unsubstituted comonomers can be compensated by using branched 2-octyldodecyl or 2-decyl-dodecyl side chains on the thienyl DPP unit.21,110 Interestingly, the choice of corepeat unit was shown to have an influence on the backbone orientation and subsequent microstructure in an analysis of thiophene and thienothiophene DPP copolymers.104 The thienothiophene copolymer was observed by 2D grazing incidence XRD and NEXAFS to exhibit edge-on orientation with respect to the substrate, whereas the thiophene analogue exhibited face-on orientation. In this case, the edge-on orientation microstructure exhibited a higher charge carrier mobility, but no general conclusions can be drawn from this isolated result. The origin of the orientation effect is also not fully concluded and may indeed be an artifact of processing conditions. A strong molecular weight dependence was observed for the DPP− quaterthiophene polymer PDQT (Figure 13), with mobilities of 0.97 cm2/(V s) for the high molecular weight materials compared to 0.39 cm2/(V s) for low molecular weights.110 It is likely that the higher molecular weights have better connected crystalline domains. As well as yielding high mobilities as a flanking unit, thienothiophene has been shown to function effectively as an electron rich comonomer, leading to some of the highest mobilities demonstrated for a semiconducting polymer25,104,24 The unsubstituted planar thienothiophene unit facilitates close intermolecular packing, promoting highly ordered microstructures. Later work on this copolymer reported with a high mobility of 10.5 cm2/(V s) (DPPT-TT, Table 1) and long-term ambient stability measured over the course of one year.25 It was noted that high molecular weight (Mn 110 kDa) was important in improving charge carrier mobility, with lower molecular weight samples (Mn = 29 KDa) exhibiting much lower mobilities of 1 cm2/(V s) which is consistent with the earlier studies of polyalkylthiophenes. The analogous polymer with thienothiophene substituted for the more electron poor, but structurally similar thiazolothiazole has also been reported.106,111 This series of polymers recently reached mobilities up to 3.43 cm2/(V s) after annealing at 120 °C (PDPPTzBT, Table 1). A short π−π stacking distance of 3.52 Å was measured by XRD, indicating efficient packing

Figure 13. DPP and isoindigo polymer structures.

reagents during synthesis of the DPP monomer. Thiophene is generally favored for the facile hole injection that arises from its high-lying HOMO, as well as the increased planarity of the unit relative to phenyl-flanked DPP. Diphenyl−DPP units, the type most commonly used in the dye industry,96 have been employed in polymers, but these generally have lower mobilities than their thiophene analogues. Steric hindrance between the lactam oxygen and phenyl α-proton results in a larger dihedral angle of 27° compared to 12° for dithienyl− DPP.22 This lack of coplanar conformation prevents good π−π stacking and hence limits charge transport capabilities. Dithienyl−DPP, on the other hand, benefits from planarizing hydrogen bonding between the pyrrole oxygen and thiophene α-proton, as has been recently verified from the crystallographic data of DPP small molecules.97 Substitution of the sulfur atom with oxygen or selenium can alter the electron density of the flanking group. Difuranyl−DPP polymers have been studied98,99 as the reduced size of an oxygen atom relative to sulfur, and a shorter C−O vs C−S bond was claimed to contribute to reducing steric hindrance between the lactam carbonyl and the heterocycle allowing for a more coplanar structure.22 Mobilities of up to 1.54 cm2/(V s) were measured for copolymers with bithiophene after annealing at 200 °C (PDBFBT, Table 1), despite observations via AFM of a disordered morphology in the solid state.100 Selenophene-flanked DPP polymers are predicted by DFT calculations to be fully planar despite the large size of the Se atom. Copolymers of diselenophenyl−DPP with thienothiophene101 and benzothiadiazole102 have demonstrated ambipolar charge transport. The stabilized (compared to thiophene) and delocalized LUMO of selenophene103 was intended to facilitate electron injection as well as improve charge transport. Annealed films of the BT copolymer PSeDPPBT (Figure 13) displayed high hole and electron mobilities of 0.46 and 0.84 cm2/(V s), respectively, larger than the corresponding dithienyl−DPP polymers. The enhanced backbone coplanarity and hence improved crystallinity of the thienothiophene copolymers led to higher hole and electron mobilities of 1.1 and 0.15 cm2/(V s), respectively (PDPPSe-TT, Table 1). Thienothiophene units were also employed as flanking groups, with the design rationale that the fused thienothiophene groups reduce conformational energy disorder, enhancJ

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studied and with various alkyl chain optimization and functionalization of the core, mobilities above 1 cm2/(V s) have been regularly achieved, and balanced ambipolar charge transport can be realized.119−122 The first report of an air-stable, high performing isoindigo polymer, IIDDT, was based on the phenyl isoindigo unit (shown in Table 1) copolymerized with bithiophene. Bottomgate/top-contact devices of the as cast film exhibited mobilities of 0.1−0.2 cm2/(V s); however, upon annealing the hole mobility increased to 0.79 cm2/(V s). IIDDT was shown by XRD to have edge-on lamellar packing in the film and large crystalline fibrillar intercalating networks as observed by AFM. This was attributed to strong intermolecular π−π interactions. The high hole mobilities are the result of the significant degree of order in the film. The excellent ambient stability reported was attributed to the low lying HOMO level of −5.7 eV as measured by cyclic voltammetry.121 A novel isoindigo based monomer was designed whereby the flanking phenyl substituents were replaced by thiophenes to form thienopyrrolone units, and then this was copolymerized with benzothiadiazole to give IGT-BT (Figure 13). 123 Replacement of phenyl linking units with thiophene has been shown to enhance polymer backbone planarity.124 This both maximizes the π-conjugation and enhances the intermolecular contacts leading to a close π-stacked microstructure. The strong donor−acceptor character increases molecular orbital hybridization between units leading to a low lying LUMO and high lying HOMO which is beneficial for ambipolarity. After annealing the hole mobility remained at 0.16 cm2/(V s) whereas the electron mobility increased from 0.006 cm2/(V s) to 0.14 cm2/(V s). This increase in electron mobility may have been a result of increased thin film order after a phase transition resulting in a more favorable microstructure, consistent with XRD studies which showed an increase in the intensity of the πstacking peak upon annealing. The choice of alkyl side chain has been shown to have a significant effect on molecular packing, thin film morphology, and therefore transistor performance.36,125 Optimization of the side chain on IIDDT led to the introduction of a siloxaneterminated solubilizing group.122 The use of this side chain was expected to improve polymer solubility, and placing the branching point further away from the polymer backbone could potentially allow the polymer chains to pack closer together, enhancing interchain interactions. The polymer bearing siloxane side chains PII2T-Si (IIDDT-Si in Table 1) were found to have a hole mobility of 2 cm2/(V s). In contrast, the reference polymer with branched 2-octyldodecyl side chains (IIDDT) gave an average mobility of 0.3 cm2/(V s). The enhanced mobility of PII2T-Si was attributed to the stronger intermolecular interactions of the polymer in the solid state. XRD data showed that PII2T-Si had a smaller π−π stacking distance in comparison to the reference contributing to the observed higher mobilities.126 Further studies on the alkyl chain functionality demonstrated that subtle changes to the conventional alkyl chains could lead to a marked improvement in device performance. The IIDDT polymer backbone was used with various alkyl chains attached, sequentially moving the branching point further away from the backbone (Table 1). The IIDDT-C3 devices exhibited an improved average hole mobility of 3 cm2/(V s) with the IIDDT-C4 films also displaying increased mobility (1.4 cm2/(V s)), but surprisingly IIDDT-C2 showed a decrease in hole mobility (0.4 cm2/(V s)).

which was attributed to the concentration of intermolecular dipolar attractions. Incorporation of vinyl linkages between adjacent aryl comonomers has been shown to lead to enhanced intermolecular interactions and high mobilities. High performing polymers with hole mobilities up to 5 cm2/(V s) were found for vinylene−selenophene−vinylene comonomers (PDPPDTSe, Table 1)112 and 8 cm2/(V s) for vinylene− thiophene−vinylene materials (PDVT-10, Table 1).113 The origin of this impressive carrier mobility has been attributed to the deviation to the backbone linearity, caused by the vinyl group, which was believed to enhance the area available for intermolecular π orbital overlap. The transfer characteristics of these devices, as well as many very high mobility DPP polymers, exhibit strangely nonlinear square root Isd vs Vg behavior113 and are often injection limited, which makes extraction of carrier mobility from the IV characteristics quite difficult. As a result, there is often some inaccuracy with the results claimed. Due to the highly aggregating nature of DPP polymers which arises from the strong intermolecular dipole interactions, it is typically necessary to use large, branched alkyl side chains on both nitrogen pyrrole groups in order to ensure sufficient polymer solubility. However, the shape and size of the side chain also has a strong influence on the polymer packing and crystallinity, and there have been several reports that demonstrate a significant effect on charge carrier mobility. Studies on the DPPT-TT copolymer referred to in Table 1 included a comparison of hexyldecyl and octyldodecyl side chains to reveal significantly higher crystallinity from the longer alkyl chains and therefore higher mobility. The longer chains were also more responsive to the thermal annealing. A dithienyl−DPP unit with siloxane-terminated alkyl solubilizing groups was copolymerized with selenophene with very high ambipolar mobilities reported (see PTDPPSe-Si, Table 1).114,115 Hole mobilities of 8.84 cm2/(V s) as well as high electron mobilities of 4.34 cm2/(V s) have been since been reported with this polymer.115 Films deposited by solutionshearing exhibited dramatically better performance than those deposited by drop-casting due to improved alignment, as can be seen from the AFMs of Figure 14. Isoindigo Polymers. The first report of push−pull polymers based on isoindigo highlighted their potential for use in OPV and OFET devices.116 Since then this electron deficient unit has been successfully incorporated into OPV devices,117,118 in some cases reaching efficiencies of over 6%.118 The use of isoindigo polymers in OFET devices has also been

Figure 14. AFM phase images of PTDPPSe deposited by (a) dropcasting and (b) solution-shearing films, both annealed at 220 °C. Adapted from with permission from ref 115. Copyright 2013 American Chemical Society. K

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injection. Also, many devices were tested in air, and if the polymer was not operationally stable under ambient conditions then the performance would rapidly decrease. It is generally believed that the ideal LUMO level for an n-type material is between −4.0 and −4.3 eV if it is to be stable under ambient conditions. Any higher and the polymer will degrade on exposure to ambient conditions, but if it is too low then the device will be difficult to turn off as the electrons will be free to flow from the electrode into the channel due to the absence of an injection barrier.99,129 Since the fundamental understanding of electron transport and injection has vastly improved, the materials used and fabrication techniques employed for OFET devices can be tailored specifically to enhance n-type polymer performance. Lower work-function electrodes, such as Al or Ca, can be utilized to reduce the injection barrier and device fabrication and testing can be carried out under an inert atmosphere. Also developments in dielectric design can remove the issue of electron trapping. The first report of an electron transporting polymer in an OFET device was the ladder-type polymer, poly(benzobisimidazobenzophenanthroline) (BBL, Figure 15)

The high hole mobilities of the IIDDT-C3 and IIDDT-C4 polymers are ascribed to the more exposed isoindigo core which can allow the polymer backbones to pack closer, enhancing the π−π stacking ability of the polymers. This was confirmed by XRD which shows that, as the branching point is moved further away, the π-stacking distance gradually decreases. DFT calculations on isoindigo copolymers with thiophene, for example, reveal that the LUMO level is mainly located on the isoindigo unit while the HOMO is fully delocalized. Functionalization of the isoindigo unit therefore allows for manipulation of both molecular orbitals simultaneously.120 Fluorination was investigated with the aim of lowering the LUMO level in order to improve ambipolarity. Electron withdrawing fluorine atoms reduce the electron density of the core further and promote molecular orbital hybridization between comonomer units. The fluorinated isoindigo was copolymerized with bithiophene (IIDDT-F, Table 1) and the LUMO level was effectively lowered, which significantly increases the electron mobility from 10−2 to 0.43 cm2/(V s), while maintaining high hole mobility up to 1.85 cm2/(V s). 2DGIXRD studies and AFM images showed that fluorination increased crystallinity which could be attributed to the planarizing interaction of the fluorine on the isoindigo and the beta hydrogen on the bithiophene. The halogenation approach has also been effectively extended to chlorine, resulting in a bithiophene copolymer which exhibited balanced, stable charge carrier transport (hole mobility of 0.53 cm2/(V s) and an electron mobility of 0.48 cm2/(V s))127 in contrast to the nonchlorinated polymer which exhibited a high hole mobility (1.27 cm2/(V s)) but low electron mobility (10−3 cm2/(V s)). The stronger electron donating ability of the biselenophene was also employed as a corepeat unit. Again, balanced ambipolarity was reported, with hole and electron mobilities of 1.05 and 0.72 cm2/(V s), respectively.127 The hole mobilities are slightly reduced in comparison to the nonchlorinated polymers, possibly due to the larger backbone twist which results from steric interactions between the chlorine atoms and beta hydrogens of the selenophene. In summary, functionalization of the isoindigo core (such as fluorination, chlorination, and substitution of the flanking phenyls for thiophenes), side chain engineering (such as use of siloxane chains or moving the branching point further away), and choice of donor unit (e.g., biselenophene instead of thiophene) can all be used to help improve the transistor performance.

Figure 15. N-type polymer structures.

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which exhibited an electron mobility of 0.1 cm2/(V s) as a spin-coated polycrystalline film.130 Further studies on this polymer showed that it was air stable: the electrical parameters (electron mobility, on/off current ratio, and threshold voltage) of the transistors in air were found to be constant over a 4 year period.131 The stability of the BBL devices was attributed to the close-packed crystalline morphology of the polymer chains which was claimed to significantly reduce the permeability and/ or diffusion of molecular oxygen into the crystalline polymer film; however, the low lying LUMO energy level undoubtedly was a major factor. N-type field effect behavior was then observed in polymers previously measured to be solely p-type materials. This was achieved by the use of a hydroxyl free dielectric consisting of a cross-linkable divinyltetramethylsiloxane−bis(benzocyclobutene) BCB.31 This could be solution processed,

ELECTRON TRANSPORTING POLYMERS Historically, there has been an emphasis on the hole transport properties of polymers for p-type transistor devices, and consequently, electron transporting polymers have been significantly underdeveloped in comparison. As the understanding of electron transport in OFETs improved it became clear that the mobility was not governed by polymer electronic structure alone; it was also largely affected by the transistor dielectric, choice of electrodes, and fabrication process. Traditionally the dielectrics employed in these devices were oxides, such as silicon dioxide, which inherently trap electrons.31,128 The choice of electrode, which was normally gold, had a huge effect on performance as the LUMOs of these n-type polymers were typically too high lying in comparison to the work function of the electrode, causing a large barrier to L

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although requiring relatively high processing temperature, yielding a high quality hydroxyl-free interface between the gate and semiconductor. It was shown, using multiple-reflection attenuated-total-reflection FTIR spectrometry (ATR−FTIR), that as a result of SiOH reduction in the n-channel, the interface could become charged by a layer of trapped electrons that pushes the gate threshold outside the measurement window. With this problem eliminated by the use of crosslinked BCB, electron mobilities as high as 0.02 cm2/(V s) could be achieved for poly(9,9-dioctylfluorene) using low work function Ca electrodes. This work established that electrons were mobile in many polymers previously believed to be only hole conductors, and with developments in dielectrics and choice of electrodes, efficient n-type OFETs could be realized.31 The first perylene based polymer which showed n-type characteristics was N,N′-dialkyl-1,7-dibromo-3,4,9,10-perylenediimide-co-dithienothiophene PDI-DTT (Figure 15). This polymer was thermally stable and had a high electron affinity.132 When tested in a bottom gate−top contact OFET device, using aluminum electrodes, a maximum saturated electron mobility of 1.3 × 10−2 cm2/(V s) was recorded. In a subsequent report the dithienopyrrole analogue (PDI-DTP, Figure 15) was synthesized and tested but it was found to have an electron mobility 1 order of magnitude lower than PDIDTT. This was attributed to the presence of the N-alkyl substituent which may disrupt packing in the solid state or dilute the electron deficiency of the polymer.133 Copolymerization of naphthalene diimide and bithiophene resulted in the n-type polymer P(NDI2OD-T2).134 This polymer was evaluated with top gate−bottom contact devices.135 Gold contacts were used, and various polymeric dielectrics which could be solution deposited were tested. When the semiconducting layer was deposited by spin-coating, electron mobilities as high as 0.45−0.85 cm2/(V s) were recorded in ambient conditions. This study demonstrated a combination of good ambient stability, performance, and versatility of this n-type polymer.135 Further studies probed the microstructural order of the spincast polymer thin films.136 To determine the molecular packing grazing incidence, specular X-ray diffraction measurements were carried out to investigate the out of plane and in plane order, respectively.136 These measurements revealed an unexpected packing order of the polymer chains, which were aligning with the backbone parallel to the substrate in a face-on orientation (Figure 16). This was an interesting result as it was generally believed that efficient charge transport in OFETs required an edge-on orientation to allow for 2D transport via an interchain charge hopping mechanism as observed for P3HT and PBTTT. It was proposed that the face on packing would allow for at least 2−3 layers of conjugated backbone to populate the accumulation layer at the dielectric interface, which is where the largest contribution to charge transport occurs. Charges could effectively also hop through these layers via π−π stacking interactions in the vertical direction, providing an additional route for charge transport in contrast to lamellar structures (such as those exhibited by P3HT and PBTTT) with insulating alkyl sheets. Percolation to the electrode along the polymer backbone can also occur, resulting in a more 3D charge transport mechanism.136 To further elucidate the influence of the face-on packing, in relation to OFET performance, polymer thin films were annealed to the melt point and then slowly cooled to ambient

Figure 16. Representation of face-on alignment in P(NDI2OD-T2). Adapted with permission from ref 136. Copyright 2010 Wiley.

temperature.38 X-ray diffraction techniques were employed which showed a shift from face-on to edge-on lamellae, as well as a twofold increase in crystallinity and a decrease in intracrystallite disorder (Figure 17). As expected this change

Figure 17. 2D grazing incidence X-ray scattering patterns of P(NDI2OD-T2) before (left) and after (right) annealing to the melt and slowly cooling the sample. Adapted with permission from ref 38. Copyright 2011 American Chemical Society.

in alignment had a large influence on the electron-only diode current density out of plane, but remarkably there appeared to be little effect on the OFET electron mobility. This indicated that the molecular ordering with regards to the substrate may play a secondary role in the resultant polymer performance, or the bulk order was mainly edge-on and only the surface is changing.38 A solution processable n-type semiconducting polymer based on alternating naphthalene diimide and biselenophene units, PNDIBS (Figure 15), was tested in OFET devices, and when a high molecular weight, phenyl end-capped polymer was tested, a maximum mobility of 0.24 cm2/(V s) was achieved.137 The combination of improved dielectric materials and the development of polymers with more stabilized LUMO energy levels and more delocalized wave functions has prompted the emergence of high performing ambipolar polymers. DPP and isoindigo polymers often have the optimal combination of close contacts from dipolar interactions, low lying LUMO energy levels to allow facile electron injection from even relatively high work function metals, and extended hole and electron wave functions. Single devices employing carefully designed DPP polymers can now exhibit carrier mobilities of over 1 cm2/(V s) M

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for both holes and electrons, with isoindigo polymers now not far behind. The challenge is to find a unique application to exploit this.

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CONCLUSIONS Since 2011, there has been a new wave of progress in the performance of semiconducting polymers for p-type OFET applications, with n-type not far behind. To a large extent, these advances have been driven by an improved understanding of the influence of short, noncovalent intermolecular contacts, arising from dipolar and steric effects, acting as connections between domains. Larger fused and bridged aromatic units, with low conformational disorder are being employed as polymer repeat units, including indacenodithiophene, DPP, and isoindigo, with outstanding results. Headline mobilities are now well in the range of most OLED transistor requirements.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.H.). *E-mail: [email protected] (J.E.D.). Notes

The authors declare no competing financial interest. Biographies Iain McCulloch graduated in 1989 with a PhD in Polymer Chemistry from Strathclyde University, UK. He was a Research Manager at Hoechst AG, US, and then with Merck KGaA, UK. He was appointed Professor of Polymer Materials in the Department of Chemistry at Imperial College London in 2007. He has coauthored over 230 papers, with an h index of 48, is coinventor on over 60 patents, and has edited one book and five book chapters. Jenny E. Donaghey is a research associate in the Department of Chemistry at Imperial College London. She received her PhD from the same institution in 2012. Her current research is focused on the design and synthesis of novel electron poor units for incorporation in to π-conjugated polymers, aimed at organic field effect transistor and solar cell applications. Sarah Holliday completed her undergraduate studies at the University of Edinburgh before joining the Plastic Electronics Doctoral Training Centre at Imperial College London in 2011. She has since been working in the McCulloch group as a doctoral student, focusing on the synthesis of electron-deficient small molecules for OPV, OFET, and OLED applications.

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