Natural gas is the cleanest fossil fuel source. However, natural gas wells typically contain considerable amounts of CO₂, with on-site CO₂ capture necessary. Solid sorbents are advantageous over traditional amine scrubbing due to their relatively low regeneration energies and non-corrosive nature. However, it remains a challenge to improve the sorbent's CO₂ capacity at elevated pressures relevant to natural gas purification. Here, the synthesis of porous carbons derived from a 3D hierarchical nanostructured polymer hydrogel, with simple and effective tunability over the pore size distribution is reported. The optimized surface area reaches 4196 m² g⁻¹, which is among the highest of carbon-based materials, with abundant micro- and narrow mesopores (2.03 cm³ g⁻¹ with d < 4 nm). This carbon exhibits a record-high CO₂ capacity among reported carbons at elevated pressure (i.e., 28.3 mmol g⁻¹ total adsorption at 25 °C and 30 bar). This carbon also shows good CO₂/CH₄ selectivity and excellent cyclability. Molecular simulations suggest increased CO₂ density in micro- and narrow mesopores at high pressures. This is consistent with the observation that these pores are mainly responsible for the material's high-pressure CO₂ capacity. This work provides insights into material design and further development for CO₂ capture from natural gas.

The effects of the incorporation of semiconducting single-walled nanotubes (sc-SWNTs) with high purity on the bulk heterojunction (BHJ) organic solar cell (OSC) based on regioregular poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C₆₁-butyric acid methyl ester (rr-P3HT:PCBM) are reported for the first time. The sc-SWNTs induce the organization of the polymer phase, which is evident from the increase in crystallite size, the red-shifted absorption characteristics and the enhanced hole mobility. By incorporating sc-SWNTs, OSC with a power conversion efficiency (PCE) as high as 4% can be achieved, which is ≈8% higher than our best control device. A novel application of sc-SWNTs in improving the thermal stability of BHJ OSCs is also demonstrated. After heating at 150 °C for 9 h, it is observed that the thermal stability of rr-P3HT:PCBM devices improves by more than fivefold with inclusion of sc-SWNTs. The thermal stability enhancement is attributed to a more suppressed phase separation, as shown by the remarkable decrease in the formation of sizeable crystals, which in turn can be the outcome of a more controlled crystallization of the blend materials on the nanotubes.

A series of naphthalene diimide-based conjugated polymers are prepared with various molar percentage of low molecular weight polystyrene (PS) oligomer of narrow polydispersity as the side chain. The PS side chains are incorporated through preparation of a macromonomer by chain termination of living anionic polymerization. The effects of the PS side chains amount (0–20 mol%) versus overall sidechain on the electrical properties of the resulting polymers as n-type polymer semiconductors in field-effect transistors are investigated. We observe that all the studied polymers show similarly high electron mobility (≈0.2 cm² V⁻¹ s⁻¹). Importantly, the polymers with high PS side chain content (20 mol%) show a significantly improved device stability under ambient conditions, when compared to the polymers at lower PS content (0–10 mol%). By comparing this observation to the physical blending of the conjugated polymer with PS, we attribute the improved stability to the covalently attached PS side chains potentially serving as a molecular encapsulating layer around the conjugated polymer backbone, rendering it less susceptible to electron traps such as oxygen and water molecules.

Stretchable electronics exhibit unique mechanical properties to expand the applications areas of conventional electronics based on rigid wafers. Intrinsically stretchable thin film transistor is an essential component for functional stretchable electronics, which presents a great opportunity to develop mechanically compliant electronic materials. Certain elastomers have been recently adopted as the gate dielectrics, but their dielectric properties have not been thoroughly investigated for such applications. Here, a charging measurement technique with a resistor–capacitor circuit is proposed to quantify the capacitance of the dielectric layers based on elastomers. As compared with conventional methods, the technique serves as a universal approach to extract the capacitance of various elastomers under static conditions, irrespective of the charging mechanisms. This technique also offers a facile approach to reliably quantify the mobility of thin film transistors based on elastomeric dielectrics, paving the way to utilize this class of dielectrics in the development of intrinsically stretchable transistors.

The isolation of semiconducting single-walled carbon nanotubes (sc-SWNTs) with ideal diameter and high purity is highly desired for high-performance electronic devices. However, current sorting methods for large-diameter sc-SWNTs suffer from either low purity (<99%) or long processing time (>20 h). Here, a backbone-engineering strategy is reported for the polymer used for sorting to improve the purity of sorted sc-SWNTs. Six diketopyrrolopyrrole (DPP)-based conjugated polymers are used to systematically investigate their sorting ability for sc-SWNTs. It is found that incorporation of more thiophenes building blocks in the repeating units of DPP polymer backbone leads to increased selectivity and yield for sc-SWNTs. The DPP polymers can disperse sc-SWNTs with 1.4–1.6 nm in diameter and high purity of 99.6% by a processing time as short as 1 h. Furthermore, a scalable film coating method named “solution shearing” is used to fabricate SWNT network thin-film transistors (TFTs). The TFT devices exhibit both high mobilities over 50 cm² V⁻¹ s⁻¹ and high on/off ratios over 10⁵, which are among the highest performance for solution-processed SWNT network TFTs.

Semiconducting polymers, in contrast to inorganic silicon, are solution processable and can potentially be printed cost efficiently on flexible large-area substrates. However to do so it is of paramount importance to formulate the polymeric semiconductors into inks with specific viscosities. Herein, the synthesis of a new highly soluble isoindigo monomer and its incorporation into low bandgap semiconducting polymers is presented. Non-conjugated flexible linkers are introduced into the conjugated backbone in order to modulate the materials processability. The viscoelastic properties of the new polymers are studied in detail by means of rheometry and dynamical mechanical analysis. The solution viscosity is directly proportional to the content of non-conjugated linkers in the polymer backbone. In organic field-effect transistors maximum hole mobilities of 1.7 cm² V⁻¹ s⁻¹ are achieved with the new polymers. Due to the enhanced solubility all-polymer solar cells are fabricated by solution shearing, reaching power conversion efficiency values of 3.7%.

The n-doping of several conjugated co-polymers based on perylene diimide (PDI) and napthalene diimide (NDI) acceptors co-polymerized with ethynylene, ethylene, and bithiophene by the dimeric dopant (2-Cyc-DMBI)₂ is reported. The n-doping reactions are confirmed in solution by UV–Vis–NIR spectroscopy and in thin films with photothermal deflection spectroscopy (PDS). The reduced species of the ethynylene-linked polymers are found to be more delocalized along the polymer backbone than the bithiophene-linked polymers by comparison of the absorption spectra. A high conductivity of 0.45 S cm⁻¹ is measured for the ethynylene-linked polymer P(PDI2OD-A). In contrast, neither of the two bithiophene-linked polymers, P(NDI2OD-T2) and P(PDI2OD-T2), achieve conductivities greater than 4 × 10⁻³ S cm⁻¹. Furthermore, there is no correlation between the conductivity of the doped films and the mobility of the pure films used in this study. Grazing incidence X-ray diffraction (GIXD) measurements of the films find that a similar doped phase forms irrespective of the crystallinity of the pure host polymer. In absence of a significant difference in morphology, these results suggest a link between the polaron delocalization length of the polymers and the conductivity, and more fundamentally between the backbone structure of the polymer and the polaron delocalization length.

Sorting of semiconducting single-walled carbon nanotubes (SWNTs) by conjugated polymers has attracted considerable attention recently because of its simplicity, high selectivity, and high yield. However, up to now, all the conjugated polymers used for SWNT sorting are electron-donating (p-type). Here, a high-mobility electron-accepting (n-type) polymer poly([N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)) is utilized for the sorting of high-purity semiconducting SWNTs, as characterized by Raman spectroscopy, dielectric force spectroscopy and transistor measurements. In addition, the SWNTs sorted by P(NDI2OD-T2) have larger diameters than poly(3-dodecylthiophene) (P3DDT)-sorted SWNTs. Molecular dynamics simulations in explicit toluene demonstrate distinct linear or helical wrapping geometry between P(NDI2OD-T2) and different types of SWNTs, likely as a result of the strong interactions between the large aromatic core of the P(NDI2OD-T2) backbone and the hexagon path of SWNTs. By using high-mobility n-type P(NDI2OD-T2) as the sorting polymer, ambipolar SWNT transistors with better electron transport than that attained by P3DDT-sorted SWNTs are achieved. As a result, flexible negated AND and negated OR logic circuits from the same set of ambipolar transistors are fabricated, without the need for doping. The use of n-type polymers for sorting semiconducting SWNTs and achieving ambipolar SWNT transistor characteristics greatly simplifies the fabrication of flexible complementary metal-oxide-semiconductor-like SWNT logic circuits.

A general impact of solution shearing on molecular orientation correlation is observed in polymer:fullerene organic solar cells in which one of the components forms fibrils or aggregates. Further investigation with polarized soft X-ray scattering reveals that solution shearing induces more face-to-face orientation relative to the interface of two components compared to spin-coating. This impact is shearing speed dependent, that is, slow shearing speed can induce more face-to-face orientation than a fast shearing speed. These results demonstrate that solution shearing is an effective method to control the relative molecular orientation. Solution shearing can also modify the domain size and average composition variations.

A series of isoindigo-based conjugated polymers (PII2F-C_mSi, m = 3–11) with alkyl siloxane-terminated side chains are prepared, in which the branching point is systematically “moved away” from the conjugated backbone by one carbon atom. To investigate the structure–property relationship, the polymer thin film is subsequently tested in top-contact field-effect transistors, and further characterized by both grazing incidence X-ray diffraction and atomic force microscopy. Hole mobilities over 1 cm² V⁻¹ s⁻¹ is exhibited for all soluble PII2F-C_mSi (m = 5–11) polymers, which is 10 times higher than the reference polymer with same polymer backbone. PII2F-C₉Si shows the highest mobility of 4.8 cm² V⁻¹ s⁻¹, even though PII2F-C₁₁Si exhibits the smallest π–π stacking distance at 3.379 Å. In specific, when the branching point is at, or beyond, the third carbon atoms, the contribution to charge transport arising from π–π stacking distance shortening becomes less significant. Other factors, such as thin-film microstructure, crystallinity, domain size, become more important in affecting the resulting device's charge transport.

The mechanical flexibility and structural softness of ultrathin devices based on organic thin films and low-dimensional nanomaterials have enabled a wide range of applications including flexible display, artificial skin, and health monitoring devices. However, both living systems and inanimate systems that are encountered in daily lives are all 3D. It is therefore desirable to either create freestanding electronics in a 3D form or to incorporate electronics onto 3D objects. Here, a technique is reported to utilize shape-memory polymers together with carbon nanotube flexible electronics to achieve this goal. Temperature-assisted shape control of these freestanding electronics in a programmable manner is demonstrated, with theoretical analysis for understanding the shape evolution. The shape control process can be executed with prepatterned heaters, desirable for 3D shape formation in an enclosed environment. The incorporation of carbon nanotube transistors, gas sensors, temperature sensors, and memory devices that are capable of self-wrapping onto any irregular shaped-objects without degradations in device performance is demonstrated.

Dimers of 2-substituted N,N′-dimethylbenzimidazoline radicals, (2-Y-DMBI)₂ (Y=cyclohexyl (Cyc), ferrocenyl (Fc), ruthenocenyl (Rc)), have recently been reported as n-dopants for organic semiconductors. Here their structural and energetic characteristics are reported, along with the mechanisms by which they react with acceptors, A (PCBM, TIPS-pentacene), in solution. X-ray data and DFT calculations both indicate a longer CC bond for (2-Cyc-DMBI)₂ than (2-Fc-DMBI)₂, yet DFT and ESR data show that the latter dissociates more readily due to stabilization of the radical by Fc. Depending on the energetics of dimer (D₂) dissociation and of D₂-to-A electron transfer, D₂ reacts with A to form D⁺ and A⁻ by either of two mechanisms, differing in whether the first step is endergonic dissociation or endergonic electron transfer. However, the D⁺/0.5 D₂ redox potentials—the effective reducing strengths of the dimers—vary little within the series (ca. −1.9 V vs. FeCp₂^+/0) (Cp=cyclopentadienyl) due to cancelation of trends in the D^+/0 potential and D₂ dissociation energy. The implications of these findings for use of these dimers as n-dopants, and for future dopant design, are discussed.

High-performance flexible energy-storage devices have great potential as power sources for wearable electronics. One major limitation to the realization of these applications is the lack of flexible electrodes with excellent mechanical and electrochemical properties. Currently employed batteries and supercapacitors are mainly based on electrodes that are not flexible enough for these purposes. Here, a three-dimensionally interconnected hybrid hydrogel system based on carbon nanotube (CNT)-conductive polymer network architecture is reported for high-performance flexible lithium ion battery electrodes. Unlike previously reported conducting polymers (e.g., polyaniline, polypyrrole, polythiophene), which are mechanically fragile and incompatible with aqueous solution processing, this interpenetrating network of the CNT-conducting polymer hydrogel exibits good mechanical properties, high conductivity, and facile ion transport, leading to facile electrode kinetics and high strain tolerance during electrode volume change. A high-rate capability for TiO₂ and high cycling stability for SiNP electrodes are reported. Typically, the flexible TiO₂ electrodes achieved a capacity of 76 mAh g^–1 in 40 s of charge/discharge and a high areal capacity of 2.2 mAh cm^–2 can be obtained for flexible SiNP-based electrodes at 0.1C rate. This simple yet efficient solution process is promising for the fabrication of a variety of high performance flexible electrodes.

Using non-chlorinated solvents for polymer device fabrication is highly desirable to avoid the negative environmental and health effects of chlorinated solvents. Here, a non-chlorinated mixed solvent system, composed by a mixture of tetrahydronaphthalene and p­-xylene, is described for processing a high mobility donor-acceptor fused thiophene-diketopyrrolopyrrole copolymer (PTDPPTFT4) in thin film transistors. The effects of the use of a mixed solvent system on the device performance, e.g., charge transport, morphology, and molecular packing, are investigated. p-Xylene is chosen to promote polymer aggregation in solution, while a higher boiling point solvent, tetrahydronaphthalene, is used to allow a longer evaporation time and better solubility, which further facilitates morphological tuning. By optimizing the ratio of the two solvents, the charge transport characteristics of the polymer semiconductor device are observed to significantly improve for polymer devices deposited by spin coating and solution shearing. Average charge carrier mobilities of 3.13 cm² V⁻¹ s⁻¹ and a maximum value as high as 3.94 cm² V⁻¹ s⁻¹ are obtained by solution shearing. The combination of non-chlorinated mixed solvents and the solution shearing film deposition provide a practical and environmentally-friendly approach to achieve high performance polymer transistor devices.

Pressure and touch sensitivity is crucial for intuitive human-machine interfaces. Here, we investigate the use of different microstructured elastomers for use as dielectric material in capacitive pressure sensors. We use finite element modeling to simulate how different microstructures can reduce the effective mechanical modulus. We found that pyramidal structures are optimal shapes that reduce the effective mechanical modulus of the elastomer by an order of magnitude. We also investigate the dependence of spacing of the pyramidal microstructures and how it impacts mechanical sensitivity. We further demonstrate the use of these elastomeric microstructures as the dielectric material on a variety of flexible and stretchable substrates to capture touch information in order to enable large area human-computer interfaces for next generation input devices, as well as continuous health-monitoring sensors.

Although solution processing has been hailed as a more industrially relevant route for the deposition of organic semiconductors to be used in devices, the quality and performance of solution-deposited thin films of these materials have not been able to reach that of vapor-grown single crystals until recently. Single crystals and single-crystalline films benefit from the absence of grain boundaries and the reduction of crystalline defects that impede effective charge transport through the solid. Although solution-phase deposition techniques for high-quality single-crystal growth have been reported, high-throughput, large-area deposition methods for single-crystalline films remain challenging. However, a shift toward this direction has emerged in recent years and has produced a diversity of techniques to grow, deposit, and pattern organic semiconductor single crystals and thin films. In this review, we survey the physical basis for a variety of solution-based single-crystal growth and deposition methods and discuss their advantages and shortcomings in the hopes of inspiring new ways to approach these challenges.

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

The photoelectronic characteristics of single-crystalline nanowire organic phototransistors (NW-OPTs) are studied using a high-performance n-channel organic semiconductor, N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI), as the photoactive layer. The optoelectronic performances of the NW-OPTs are analyzed by way of their current–voltage (I–V) characteristics on irradiation at different wavelengths, and comparison with corresponding thin-film organic phototransistors (OPTs). Significant enhancement in the charge-carrier mobility of NW-OPTs is observed upon light irradiation as compared with when performed in the dark. A mobility enhancement is observed when the incident optical power density increases and the wavelength of the light source matches the light-absorption range of the photoactive material. The photoswitching ratio is strongly dependent upon the incident optical power density, whereas the photoresponsivity is more dependent on matching the light-source wavelength with the maximum absorption range of the photoactive material. BPE-PTCDI NW-OPTs exhibit much higher external quantum efficiency (EQE) values (≈7900 times larger) than thin-film OPTs, with a maximum EQE of 263 000%. This is attributed to the intrinsically defect-free single-crystalline nature of the BPE-PTCDI NWs. In addition, an approach is devised to analyze the charge-transport behaviors using charge accumulation/release rates from deep traps under on/off switching of external light sources.

The introduction of new substrate materials into the world of electronics has previously opened up new possibilities for novel applications and device designs. Here, the use of ion-exchanged sodium aluminosilicate (NAS) glass is presented as a new type of substrate that is not only highly damage resistant, but also allows the fabrication of high performance organic electronic devices. The smoothness of the NAS glass surface enables favorable growth of the semiconductor layer, enabling high charge carrier mobilities for typical organic semiconductors, such as pentacene or C60, and a polymer semiconductor. No degradation of the device performance is observed as a result of ion migration into the active device region, and no compromise in substrate strength due to the processing conditions is made. This work suggests the possibility of new, highly durable electronic devices on glass in large area format.

Previous routes to polymers with mono-alkylated bithiophenes have proceeded through polymerization of monoalkyl-2,2′-bithiophene monomers through oxidative or AB-type cross-coupling polymerizations. The resulting polymer regiochemistry affects both the location and orientation of the polymer side-chains. In contrast, AABB-type cross-coupling polymerizations can control the location and in some cases the orientation of the side-chains. To study how this control can impact polymer properties, two poly(monodecyl-2,2′-bithiophene) polymers have been synthesized through Stille AABB-type polycondensations of 2,5-bis(trimethylstannyl)thiophene with different monomers. The alkyl side-chains are located on every other thiophene, but polymer 1 consists of both head-to-tail and head-to-head dyads, whereas polymer 2 is made up of only head-to-head dyads. ¹H NMR, ¹³C NMR, and heteronuclear single quantum correlation spectroscopy are used to confirm and contrast the polymer regiochemistries. The physical properties of the two polymers are analyzed using UV–vis spectroscopy, differential scanning calorimetry, and grazing-incidence X-ray diffraction. Polymer 2 is found to display significantly more aggregation in solution than 1, and it displays different thermal properties. The film properties of polymers 1 and 2, however, are very similar, with nearly identical UV–vis profiles and d-spacing values as determined by grazing incidence X-ray diffraction. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

The development of organic transistors for flexible electronics requires the understanding of device behavior upon the application of strain. Here, a comprehensive study of the effect of polymer-dielectric and semiconductor chemical structure on the device performance under applied strain is reported. The systematic change of the polymer dielectric results in the modulation of the effects of strain on the mobility of organic field-effect transistor devices. A general method is demonstrated to lower the effects of strain in devices by covalent substitution of the dielectric surface. Additionally, the introduction of a hexyl chain at the peripheries of the organic semiconductor structure results in an inversion of the effects of strain on device mobility. This novel behavior may be explained by the capacitative coupling of the surface energy variations during applied strain.

Highly conductive and transparent poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) films, incorporating a fluorosurfactant as an additive, have been prepared for stretchable and transparent electrodes. The fluorosurfactant-treated PEDOT:PSS films show a 35% improvement in sheet resistance (R_s) compared to untreated films. In addition, the fluorosurfactant renders PEDOT:PSS solutions amenable for deposition on hydrophobic surfaces, including pre-deposited, annealed films of PEDOT:PSS (enabling the deposition of thick, highly conductive, multilayer films) and stretchable poly(dimethylsiloxane) (PDMS) substrates (enabling stretchable electronics). Four-layer PEDOT:PSS films have an R_s of 46 Ω per square with 82% transmittance (at 550 nm). These films, deposited on a pre-strained PDMS substrate and buckled, are shown to be reversibly stretchable, with no change to R_s, during the course of over 5000 cycles of 0 to 10% strain. Using the multilayer PEDOT:PSS films as anodes, indium tin oxide (ITO)-free organic photovoltaics are prepared and shown to have power conversion efficiencies comparable to that of devices with ITO as the anode. These results show that these highly conductive PEDOT:PSS films can not only be used as transparent electrodes in novel devices (where ITO cannot be used), such as stretchable OPVs, but also have the potential to replace ITO in conventional devices.

It is demonstrated that bifunctionalized polythiophenes involving thiol and azide end-functional groups can be synthesized by chain-growth Suzuki-Miyaura type polymerization. The bifunctionalized polythiophenes are successfully characterized by 1H NMR, gel permeation chromatography (GPC), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF). Furthermore, the azide end-group reacts with DNA via “click chemistry” to form a polythiophene/DNA hybrid structure, which is characterized by ESI-MS. The described synthetic approaches will lead to the synthesis of novel multi-block copolymers as well as biomolecule-based conjugated polymer structures.

High charge carrier mobility solution-processed n-channel organic thin-film transistors (OTFTs) based on core-chlorinated naphthalene tetracarboxylic diimides (NDIs) with fluoroalkyl chains are demonstrated. These OTFTs were prepared through a solution shearing method. Core-chlorination of NDIs not only increases the electron mobilities of OTFTs, but also enhances their air stability, since the chlorination in the NDI core lowers the lowest unoccupied molecular orbital (LUMO) levels. The air-stability of dichlorinated NDI was better than that of the tetrachlorinated NDIs, presumably due to the fact that dichlorinated NDIs have a denser packing of the fluoroalkyl chains and less grain boundaries on the surface, reducing the invasion pathway of ambient oxygen and moisture. The devices of dichlorinated NDIs exhibit good OTFT performance, even after storage in air for one and a half months. Charge transport anisotropy is observed from the dichlorinated NDI. A dichlorinated NDI with −CH₂C₃F₇ side chains reveals high mobilities of up to 0.22 and 0.57 cm² V⁻¹ s⁻¹ in parallel and perpendicular direction, respectively, with regard to the shearing direction. This mobility anisotropy is related to the grain morphology. In addition, we find that the solution-shearing deposition affects the molecular orientation in the crystalline thin films and lowers the d(001)-spacing (the out-of-plane interlayer spacing), compared to the vapor-deposited thin films. Core-chlorinated NDI derivatives are found to be highly suitable for n-channel active materials in low-cost solution-processed organic electronics.

Analogous to conventional inorganic semiconductors, the performance of organic semiconductors is directly related to their molecular packing, crystallinity, growth mode, and purity. In order to achieve the best possible performance, it is critical to understand how organic semiconductors nucleate and grow. Clever use of surface and dielectric modification chemistry can allow one to control the growth and morphology, which greatly influence the electrical properties of the organic transistor. In this Review, the nucleation and growth of organic semiconductors on dielectric surfaces is addressed. The first part of the Review concentrates on small-molecule organic semiconductors. The role of deposition conditions on film formation is described. The modification of the dielectric interface using polymers or self-assembled mono­layers and their effect on organic-semiconductor growth and performance is also discussed. The goal of this Review is primarily to discuss the thin-film formation of organic semiconducting species. The patterning of single crystals is discussed, while their nucleation and growth has been described elsewhere (see the Review by Liu et. al).1 The second part of the Review focuses on polymeric semiconductors. The dependence of physico-chemical properties, such as chain length (i.e., molecular weight) of the constituting macromolecule, and the influence of small molecular species on, e.g., melting temperature, as well as routes to induce order in such macromolecules, are described.

Core-chlorinated naphthalene tetracarboxylic diimides (NDIs) with fluoroalkyl chains are synthesized and employed for n-channel organic thin-film transistors (OTFTs). Structural analyses of the single crystals and thin films are performed and their charge-transport behavior is investigated in terms of structure–property relationships. NDIs with two chlorine substituents are shown to exhibit a herringbone structure with a very close π-plane distance (3.3–3.4 Å), a large π-stack overlap (slipping angle ca. 62°), and high crystal densities (2.046–2.091 g cm⁻³). These features result in excellent field-effect mobilities of up to 1.43 cm² V⁻¹ s⁻¹ with minimal hysteresis and high on–off ratios (ca. 10⁷) in air. This is similar to the highest n-channel mobilities in air reported so far. Despite the repulsive interactions of bulky Cl substituents, tetrachlorinated NDIs adopt a slip-stacked face-to-face packing with an interplanar distance of around 3.4 Å, resulting in a high mobility (up to 0.44 cm² V⁻¹ s⁻¹). The air-stability of dichlorinated NDIs is superior to that of tetrachlorinated NDIs, despite of their higher LUMO levels. This is closely related to the denser packing of the fluorocarbon chains of dichlorinated NDIs, which serves as a kinetic barrier to the diffusion of ambient oxidants. Interestingly, these NDIs show an optimal performance either on bare SiO₂ or on octadecyltrimethoxysilane (OTS)-treated SiO₂, depending on the carbon number of the fluoroalkyl chains. Their synthetic simplicity and processing versatility combined with their high performance make these semiconductors highly promising for practical applications in flexible electronics.

Grazing incidence X-ray scattering (GIXS) is used to characterize the morphology of poly(3-hexylthiophene) (P3HT)–phenyl-C61-butyric acid methyl ester (PCBM) thin film bulk heterojunction (BHJ) blends as a function of thermal annealing temperature, from room temperature to 220 °C. A custom-built heating chamber for in situ GIXS studies allows for the morphological characterization of thin films at elevated temperatures. Films annealed with a thermal gradient allow for the rapid investigation of the morphology over a range of temperatures that corroborate the results of the in situ experiments. Using these techniques the following are observed: the melting points of each component; an increase in the P3HT coherence length with annealing below the P3HT melting temperature; the formation of well-oriented P3HT crystallites with the (100) plane parallel to the substrate, when cooled from the melt; and the cold crystallization of PCBM associated with the PCBM glass transition temperature. The incorporation of these materials into BHJ blends affects the nature of these transitions as a function of blend ratio. These results provide a deeper understanding of the physics of how thermal annealing affects the morphology of polymer–fullerene BHJ blends and provides tools to manipulate the blend morphology in order to develop high-performance organic solar cell devices.

Organic electronic devices have demonstrated tremendous versatility in a wide range of applications including consumer electronics, photovoltaics and biotechnology. The traditional interface of organic electronics with biology, biotechnology and medicine occurs in the general field of sensing biological phenomena. For example, the fabrication of hybrid electronic structures using both organic semiconductors and bioactive molecules has led to enhancements in the sensitivity and specificity within biosensing platforms, which in turn has a potentially wide range of clinical applications. However, the interface of biomolecules and organic semiconductors has also recently explored the potential use of natural and synthetic biomaterials as structural components of electronic devices. The fabrication of electronically active systems using biomaterials-based components has the potential to produce a large set of unique devices including environmentally biodegradable systems and bioresorbable temporary medical devices. This article reviews recent advances in the implementation of biomaterials as structural components in organic electronic devices with a focus on potential applications in biotechnology and medicine. Copyright © 2010 Society of Chemical Industry

The search for low-cost, large-area, flexible devices has led to a remarkable increase in the research and development of organic semiconductors, which serve as one of the most important components for organic field-effect transistors (OFETs). In the current review, we highlight deposition techniques that offer precise control over the location or in-plane orientation of organic semiconductors. We focus on various vapor- and solution-processing techniques for patterning organic single crystals in desired locations. Furthermore, the alignment of organic semiconductors via different methods relying on mechanical forces, alignment layers, epitaxial growth, and external magnetic and electric fields are surveyed. The advantages, limitations, and applications of these techniques in OFETs are also discussed.

The organic field-effect transistor (OFET) has proven itself invaluable as both the fundamental element in organic circuits and the primary tool for the characterization of novel organic electronic materials. Crucial to the success of the OFET in each of these venues is a working understanding of the device physics that manifest themselves in the form of electrical characteristics. As commercial applications shift to smaller device dimensions and structure/property relationships become more refined, the understanding of these phenomena become increasingly critical. Here, we employ high-performance, elastomeric, photolithographically patterned single-crystal field-effect transistors as tools for the characterization of short-channel effects and bias-dependent parasitic contact resistance and field-effect mobility. Redundant characterization of devices at multiple channel lengths under a single crystal allow the morphology-free analysis of these effects, which is carried out in the context of a device model previously reported. The data show remarkable consistency with our model, yielding fresh insight into each of these phenomena, as well as confirming the utility of our FET design.

In organic thin film transistors (OTFTs), charge transport occurs in the first few monolayers of the semiconductor near the semiconductor/dielectric interface. Previous work has investigated the roles of dielectric surface energy, roughness, and chemical functionality on performance. However, large discrepancies in performance, even with apparently identical surface treatments, indicate that additional surface parameters must be identified and controlled in order to optimize OTFTs. Here, a crystalline, dense octadecylsilane (OTS) surface modification layer is found that promotes two-dimensional semiconductor growth. Higher mobility is consistently achieved for films deposited on crystalline OTS compared to on disordered OTS, with mobilities as high as 5.3 and 2.3 cm² V⁻¹ s⁻¹ for C₆₀ and pentacene, respectively. This is a significant step toward morphological control of organic semiconductors which is directly linked to their thin film charge carrier transport.

A series of compounds from the tetraceno[2,3-b]thiophene and the anthra[2,3-b]thiophene family of semiconducting molecules has been made. Specifically, synthetic routes to functionalize the parent molecules with bromo and then hexyl groups are shown. The bromo- and hexyl-functionalized tetraceno[2,3-b]thiophene and anthra[2,3-b]thiophene were characterized in the top-contact thin-film transistor (TFT) geometry. They give high mobilities, ranging from 0.12 cm² V⁻¹ s⁻¹ for α-n-hexylanthra[2,3- b]thiophene to as high as 0.85 cm² V⁻¹ s⁻¹ for α-bromotetraceno[2,3-b]thiophene. Notably, grain size increases, going from the shorter anthra[2,3-b]thiophene core to the longer tetraceno[2,3-b]thiophene core, with a corresponding increase in mobility. The transition from undesirable 3D to desirable 2D thin-film growth is explained by the increase in length of the molecule, in this case by one benzene ring, which results in an increase in intralayer interactions relative to interlayer interactions.