Polycyclic aromatic hydrocarbons (PAH) have been widely used as solid carbon sources for the synthesis of graphene at low temperatures. The inevitable formation of structural defects, however, has significantly limited the quality of the synthesized graphene. This article describes a low-temperature chemical vapor deposition method that effectively mitigates defect formation in graphene by heterogeneous solid carbon sources containing a mixture of aromatic and aliphatic carbon on a Cu substrate. The addition of small amount of aliphatic carbon sources to the PAH significantly decreases the defect density of graphene synthesized at 400 ≤ T ≤ 600 °C by incorporating small aliphatic carbon fragments into defect sites. The carrier mobility of graphene grown using this heterogeneous solid carbon source is more than five times that of graphene synthesized using only PAH. Two mechanisms are also proposed by which vacancies can be generated during graphene growth using PAH sources on Cu, defect generation due to the disordered packing and the geometric limitation of PAH molecules. This low-temperature method of synthesizing graphene reduces the degree of defect density using heterogeneous solid carbon sources promises to provide wide utility in electronics applications.

The polymer-supported wet transfer of chemical vapor deposition-grown graphene provides high-quality large-area graphene on a target substrate. The transfer-induced defects that result from these processes, such as micrometer-scale folds and cracks, have been regarded as an inevitable problem. Here, the transfer processes are thoroughly examined stage-by-stage and it is found that lamination wrinkles, which cause defects in the graphene, are generated as a result of the high contact angles of the trapped transfer medium liquids. Systematic theoretical and experimental studies demonstrate that a liquid droplet with a low surface tension trapped between the polymer/graphene film and the substrate minimizes lamination wrinkles during the transfer process by completely wetting the target substrate, regardless of the surface energy. In connection with these results, a simple and broadly applicable transfer method is developed using an organic liquid with a low surface tension to uniformly transfer high-quality graphene onto arbitrary substrates, even onto superhydrophobic substrate. The graphene obtained using the proposed organic liquid transfer method displays better electrical and mechanical properties than the graphene transferred by the conventional method using water. This effective and practical transfer method provides an approach to obtaining high-quality graphene for use in graphene-based devices.

Inkjet printing of semiconducting polymers is desirable for realizing low-cost, large-area printed electronics. However, sequential inkjet printing methods often suffer from nozzle clogging because the solubility of semiconducting polymers in organic solvents is limited. Here, it is demonstrated that the addition of an insulating polymer to a semiconducting polymer ink greatly enhances the solubility and stability of the ink, leading to the stable ejection of ink droplets. This bicomponent blend comprising a liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole) (PQTBTz-C12), and an insulating commodity polymer, polystyrene, is extremely useful as a semiconducting layer in organic field-effect transistors (OFETs), providing fine control over the phase-separated morphology and structure of the inkjet-printed film. Tailoring the solubility-induced phase separation of the two components leads to a bilayer structure consisting of a polystyrene layer on the top and a highly crystalline PQTBTz-C12 layer on the bottom. The blend film is used as the semiconducting layer in OFETs, reducing the semiconductor content to several tens of pictograms in a single device without degrading the device performance. Furthermore, OFETs based on the PQTBTz-C12/polystyrene film exhibit much greater environmental and electrical stabilities compared to the films prepared from homo PQTBTz-C12, mainly due to the self-encapsulated structure of the blend film.

Despite the considerable efforts applied toward developing stretchable electronics, few intrinsically stretchable semiconductors have been reported that retain the original electrical characteristics under stretching. This study introduces an intrinsically stretchable and transparent organic semiconducting layer by blending self-assembled nanowires (NWs) of an organic semiconductor with an elastomeric and transparent polymer. Blends of poly(3-hexylthiophene) (P3HT) NWs and poly(dimethylsiloxane) (PDMS) yield P3HT NW networks embedded in the PDMS matrix. Interestingly, it is found that the vertical distribution of P3HT NWs in the blend films is sensitive to the surface characteristics of the underlying substrate. Compared to the P3HT NWs distributed on a Si substrate with vertical gradation, the P3HT NWs are evenly distributed throughout the PDMS matrix on a PDMS substrate. Organic transistors prepared with the blend active layers with various P3HT ratios exhibit device performances comparable to those of a device prepared with homo-P3HT NWs, even at 1 wt% P3HT, due to the formation of percolated networks of the P3HT NWs with a high crystallinity and a large aspect ratio. This blend active layer shows the superior electrical and mechanical properties during stretching at high strains unlike the homo-P3HT NW system.

Despite remarkable advances in the performances of organic field-effect transistors (OFETs) in recent years, the bias stability of OFET devices remains a critical obstacle to their commercial use. The microstructural origins of charge traps inside OFET devices are not yet clearly understood, and investigating these origins presents an important challenge. The unique electrical properties of an n-type semiconducting 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)), are explored here to study the correlation between the microstructures of polymer semiconductor thin films and the bias stability of an OFET. It is demonstrated that although the charge carrier mobilities in a series of devices may be similar, the bias stress stabilities can differ significantly, depending on the molecular orientations of the semiconducting thin films. A higher degree of bias stress stability is attained in the P(NDI2OD-T2) FETs prepared with face-on thin-film structures compared to the bias stress stability attained in the edge-on film structures. Further experimental evidence suggests that the aliphatic alkyl chains in edge-on-oriented P(NDI2OD-T2) films present a hurdle to vertical charge transport and induce large numbers of bipolarons during bias stress, in contrast with the face-on structured thin films.

A new and simple strategy for enhancing the stability of organic solar cells (OSCs) was developed by using self-passivating metal top electrodes. Systematic investigations on O₂ permeability of Al top electrodes revealed that the main pathways for oxidation-induced degradation could be greatly suppressed by simply controlling the nanoscale morphology of the Al electrode. The population of nanoscale pinholes among Al grains, which critically decided the diffusion of O₂ molecules toward the Al–organic interfaces that are vulnerable to oxidation, was successfully regulated by rapidly depositing Al or promoting lateral growth among the Al grains, accompanied by increasing the deposition thickness. Our observations suggested that the stability of OSCs with conventional architectures might be greatly enhanced simply by controlling the fabrication conditions of the Al top electrode, without the aid of additional secondary treatments.

Solution-processable organic semiconductor nanowires (NWs) offer a potentially powerful strategy for producing large-area printed flexible devices. Here, the fabrication of lateral organic solar cells (LOSC) using solution-processed organic NW blends on a flexible substrate to produce a power source for use in flexible integrated microelectronics is reported. A high photocarrier generation and an efficient charge sweep out are achieved by incorporating 1D self-assembled poly(3-hexylthiophene) NWs into the active layer, and an MoO₃ interfacial layer with high work function is introduced to increase the built-in potential. These structures significantly increase the carrier diffusion/drift length and overall generated photocurrent in the channel. The utility of the LOSCs for high power source applications is demonstrated by using interdigitated electrode patterns that consist of multiple devices connected in parallel or in series. High photovoltage-producing LOSC modules on plastic substrates for use in flexible optoelectronic devices are successfully fabricated. The LOSCs described here offer a new device architecture for use in highly flexible photoresponsive energy devices.

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the “carrier selectivity” at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution-processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non-Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier-selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier-selectivity-controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions.

Here, a highly crystalline and self-assembled 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pentacene) thin films formed by simple spin-coating for the fabrication of high-performance solution-processed organic field-effect transistors (OFETs) are reported. Rather than using semiconducting organic small-molecule–insulating polymer blends for an active layer of an organic transistor, TIPS-Pentacene organic semiconductor is separately self-assembled on partially crosslinked poly-4-vinylphenol:poly(melamine-co-formaldehyde) (PVP:PMF) gate dielectric, which results in a vertically segregated semiconductor-dielectric film with millimeter-sized spherulite-crystalline morphology of TIPS-Pentacene. The structural and electrical properties of TIPS-Pentacene/PVP:PMF films have been studied using a combination of polarized optical microscopy, atomic force microscopy, 2D-grazing incidence wide-angle X-ray scattering, and secondary ion mass spectrometry. It is finally demonstrated a high-performance OFETs with a maximum hole mobility of 3.40 cm² V⁻¹ s⁻¹ which is, to the best of our knowledge, one of the highest mobility values for TIPS-Pentacene OFETs fabricated using a conventional solution process. It is expected that this new deposition method would be applicable to other small molecular semiconductor–curable polymer gate dielectric systems for high-performance organic electronic applications.

Printing technologies are instrumental to the fabrication of low-cost lightweight flexible electronic devices and circuits, which are necessary to produce wearable electronic applications. However, attaining fully printed devices on flexible films over large areas has typically been a challenge. Here, the fabrication of fully drawn all-organic field-effect transistor (FET) arrays on mechanically flexible substrates using a capillary-pen printing method is demonstrated. A highly crystalline organic semiconductor (active layer), a smooth insulating polymer (dielectric layer), and a conducting polymer (source, drain, and gate electrodes) are deposited from solution sequentially. The bottom-gate bottom-contact FETs drawn onto flexible substrates exhibit superior field-effect mobilities of up to 0.54 cm² V⁻¹ s⁻¹, good reproducibility, operational stability, and mechanical bendability. Furthermore, to emphasize the methodological advantages of the capillary-pen printing, an organic FET (OFET) array on a curvilinear substrate of a plastic straw and the repairing concept for a broken electrical circuit are demonstrated. These results indicate that capillary pen printing shows promise as a manufacturing technique for a wide range of large-area electronic applications.

Recent studies of the bias-stress-driven electrical instability of organic field-effect transistors (OFETs) are reviewed. OFETs are operated under continuous gate and source/drain biases and these bias stresses degrade device performance. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. There are three main charge-trapping sites: the semiconductor, the dielectric, and the semiconductor-dielectric interface. The charge-trapping phenomena in these three regions are analyzed with special attention to the microstructural dependence of bias instability. Finally, possibilities for future research in this field are presented. This critical review aims to enhance our insight into bias-stress-induced charge trapping in OFETs with the aim of minimizing operational instability.

Understanding the vertical phase separation of donor and acceptor compounds in organic photovoltaics is requisite for the control of charge transport behavior and the achievement of efficient charge collection. Here, the vertically phase-separated morphologies of poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend films are examined with transmission electron microtomography, dynamic secondary ion mass spectroscopy, and X-ray photoelectron spectroscopy. The 3D morphologies of the processed films are analyzed and how the solvent additive causes vertical segregation is determined. The photocurrent–voltage characteristics of the vertically segregated blend films are strongly dependent on the 3D morphological organization of the donor and acceptor compounds in the photoactive layer. This dependence is correlated with asymmetric carrier transport at the buried interface and the air surface in the vertically segregated blend films.

A series of four polymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole (2FBT), PBDT2FBT, PBDT2FBT-O, PBDT2FBT-T, and PBDT2FBT-T-O, are synthesized with their four different side chains, alkyl-, alkoxy-, alkylthienyl-, and alkoxythienyl. Experimental results and theoretical calculations show that the molecular tuning of the side chains simultaneously influences the solubilities, energy levels, light absorption, surface tension, and intermolecular packing of the resulting polymers by altering their molecular coplanarity and electron affinity. The polymer solar cell (PSC) based on a blend of PBDT2FBT-T/[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) exhibits the best photovoltaic performance of the four PBDT2FBT derivatives, with a high open-circuit voltage of 0.98 V and a power conversion efficiency of 6.37%, without any processing additives, post-treatments, or optical spacers. Furthermore, PBDT2FBT-T-O, which has a novel side chain alkoxythienyl, showed promising properties with the most red-shifted absorption and strong intermolecular packing property in solid state. This study provides insight into molecular design and fabrication strategies via structural tuning of the side chains of conjugated polymers for achieving highly efficient PSCs.

The high-precision deposition of highly crystalline organic semiconductors by inkjet printing is important for the production of printed organic transistors. Herein, a facile nonconventional lithographic patterning technique is developed for fabricating banks with microwell structures by inkjet printing solvent droplets onto a polymer layer, thereby locally dissolving the polymer to form microwells. The semiconductor ink is then inkjet-printed into the microwells. In addition to confining the inkjet-printed organic semiconductor droplets, the microwells provide a platform onto which organic semiconductor molecules crystallize during solvent evaporation. When printed onto the hydrophilic microwells, the inkjet-printed 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN) molecules undergo self-organization to form highly ordered crystalline structures as a result of contact line pinning at the top corner of the bank and the outward hydrodynamic flow within the drying droplet. By contrast, small crystallites form with relatively poor molecular ordering in the hydrophobic microwells as a result of depinning of the contact line along the walls of the microwells. Because pinning in the hydrophilic microwells occurred at the top corner of the bank, treating the surfaces of the dielectric layer with a hydrophobic organic layer does not disturb the formation of the highly ordered TIPS_PEN crystals. Transistors fabricated on the hydrophilic microwells and the hydrophobic dielectric layer exhibit the best electrical properties, which is explained by the solvent evaporation and crystallization characteristics of the organic semiconductor droplets in the microwell. These results indicate that this technique is suitable for patterning organic semiconductor deposits on large-area flexible substrates for the direct-write fabrication of high-performance organic transistors.

Rice leaves can directionally shed water droplets along the longitudinal direction of the leaf. Inspired by the hierarchical structures of rice leaf surfaces, synthetic rice leaf-like wavy surfaces are fabricated that display a tunable anisotropic wettability by using electrostatic layer-by-layer assembly on anisotropic microwrinkled substrates. The nanoscale roughness of the rice leaf-like surfaces is controlled to yield tunable anisotropic wettability and hydrophobic properties that transitioned between the anisotropic/pinned, anisotropic/rollable, and isotropic/rollable water droplet behavior states. These remarkable changes result from discontinuities in the three-phase (solid–liquid–gas) contact line due to the presence of air trapped beneath the liquid, which is controlled by the surface roughness of the hierarchical nanostructures. The mechanism underlying the directional water-rolling properties of the rice leaf-like surfaces provides insight into the development of a range of innovative applications that require control over directional flow.

A novel strategy for analyzing bias-stress effects in organic field-effect transistors (OFETs) based on a four-parameter double stretched-exponential formula is reported. The formula is obtained by modifying a traditional single stretched-exponential expression comprising two parameters (a characteristic time and a stretched-exponential factor) that describe the bias-stress effects. The expression yields two characteristic times and two stretched-exponential factors, thereby separating out the contributions due to charge trapping events in the semiconductor layer-side of the interface and the gate-dielectric layer-side of the interface. The validity of this method was tested by designing two model systems in which the physical properties of the semiconductor layer and the gate-dielectric layer were varied systematically. It was found that the gate-dielectric layer, in general, plays a more critical role than the semiconductor layer in the bias-stress effects, possibly due to the wider distribution of the activation energy for charge trapping. Furthermore, the presence of a self-assembled monolayer further widens the distribution of the activation energy for charge trapping in gate-dielectric layer-side of the interface and causes the channel current to decay rapidly in the early stages. The novel analysis method presented here enhances our understanding of charge trapping and provides rational guidelines for developing efficient OFETs with high performance.

Herein is demonstrated that the polymer chain ends of polymer gate- dielectrics (PGDs) in organic field-effect transistors (OFETs) can trap charges; the bias-stress stability is reduced without changes in the mobilities of the transistor devices as well as the morphologies of the organic semiconductors. The bias-stress stabilities of OFETs using PGD with various molecular weights (MWs) are investigated. Under bias stress in ambient air, the drain current decay and the threshold voltage shift are found to increase as the MW of the PGD decreases (MW effect). This MW effect is caused by the variation in the density of polymer chain ends in the PGDs with MW: the free volumes at the polymer chain ends act as charge-trap sites, resulting in drain current decay during bias stress. The free volumes at polymer chain ends are sufficiently large to allow the residence of water molecules, the presence of which significantly increases the density of charge-trap sites. In contrast, polymer chain ends without trapped water molecules do not allow charge trapping and so bias-stress stability is independent of the MW of the PGD. It is also found that the hydrophilicity/hydrophobicity of the chain ends of the PGD can affect bias-stress stability; carboxyl-terminated polystyrene exhibits a much higher trap density and lower bias-stress stability than hydrogen-terminated polystyrene when these devices are exposed to humid nitrogen.

The phase-separation characteristics of spin-cast difluorinated-triethylsilylethynyl anthradithiophene (F-TESADT)/poly(methyl methacrylate) (PMMA) blends are investigated with the aim of fabricating transistors with a high field-effect mobility and stability. It is found that the presence of PMMA in the F-TESADT/PMMA blends prevents dewetting of F-TESADT from the substrate and provides a platform for F-TESADT molecules to segregate and crystallize at the air–film interface. By controlling the solvent evaporation rate of the spin-cast blend solution, it is possible to regulate the phase separation of the two components, which in turn determines the structural development of the F-TESADT crystals on PMMA. At a low solvent evaporation rate, a bilayer structure consisting of highly ordered F-TESAT crystals on the top and low-trap PMMA dielectric on the bottom can be fabricated by a one-step spin-casting process. The use of F-TESADT/PMMA blend films in bottom gate transistors produces much higher field-effect mobilities and greater stability than homo F-TESADT films because the phase-separated interface provides an efficient pathway for charge transport.

Bulk heterojunction solar cells based on blends of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PC₆₁BM) are fabricated using self-assembled P3HT nanowires in a marginal solvent without post-treatments. The interconnected network structures of self-organized P3HT nanowires create continuous percolation pathways through the active layer and contribute to enhanced carrier mobility. The morphology and photovoltaic properties are studied as a function of ageing time of the P3HT precursor solution. Optimal photovoltaic properties are found at 60 h ageing time, which increases both light absorption and charge balance. Multilayered solar cells with a compositionally graded structure are fabriacted using preformed P3HT nanowires by inserting a pure P3HT donor phase onto the hole-collecting electrode. Applying optimized annealing conditions to the P3HT buffer layer achieves an enhanced hole mobility and a power conversion efficiency of 3.94%. The introduction of a compositionally graded device structure, which contains a P3HT-only region, reduces charge recombination and electron injection to the indium tin oxide (ITO) electrode and enhances the device properties. These results demonstrate that preformed semiconductor nanowires and compositionally graded structures constitute a promising approach to the control of bulk heterojunction morphology and charge-carrier mobility.

A structured polymer solar cell architecture featuring a large interface between donor and acceptor with connecting paths to the respective electrodes is explored. To this end, poly-(3-hexylthiophene) (P3HT) nanorods oriented perpendicularly to indium tin oxide (ITO) glass are fabricated using an anodic aluminum oxide template. It is found that the P3HT chains in bulk films or nanorods are oriented differently; perpendicular or parallel to the ITO substrate, respectively. Such chain alignment of the P3HT nanorods enhanced the electrical conductivity up to tenfold compared with planar P3HT films. Furthermore, the donor/acceptor contact area could be maximised using P3HT nanorods as donor and C60 as acceptor. In a photovoltaic device employing this structure, remarkable photoluminescence quenching (88%) and a seven-fold efficiency increase (relative to a device with a planar bilayer) are achieved.

Fabrication of organic field-effect transistors (OFETs) using a high-throughput printing process has garnered tremendous interest for realizing low-cost and large-area flexible electronic devices. Printing of organic semiconductors for active layer of transistor is one of the most critical steps for achieving this goal. The charge carrier transport behavior in this layer, dictated by the crystalline microstructure and molecular orientations of the organic semiconductor, determines the transistor performance. Here, it is demonstrated that an inkjet-printed single-droplet of a semiconducting/insulating polymer blend holds substantial promise as a means for implementing direct-write fabrication of organic transistors. Control of the solubility of the semiconducting component in a blend solution can yield an inkjet-printed single-droplet blend film characterized by a semiconductor nanowire network embedded in an insulating polymer matrix. The inkjet-printed blend films having this unique structure provide effective pathways for charge carrier transport through semiconductor nanowires, as well as significantly improve the on-off current ratio and the environmental stability of the printed transistors.

Enhanced performance of an inverted-type polymer solar cell is reported by controlling the surface energy of a zinc oxide (ZnO) buffer layer, on which a photoactive layer composed of a polymer:fullerene-derivative bulk heterojunction is formed. With the approach based on a mixed self-assembled monolayer, the surface energy of the ZnO buffer layer can be controlled between 40 mN m⁻¹ and 70 mN m⁻¹ with negligible changes in its work function. For the given range of surface energy the power conversion efficiency increases from 3.27% to 3.70% through enhanced photocurrents. The optimized morphology obtained by surface energy control results in the enhanced photocurrent and transmission electron microscopy analysis verifies the correlation between the surface energy and the phase morphology of the bulk heterojunction. These results demonstrate that surface energy control is an effective method for further improving the performance of polymer solar cells, with potentially important implications for other organic devices containing an interface between a blended organic active layer and a buffer or an electrode layer.

We toughened poly(butylene terephthalate) (PBT) by loading core–shell rubber (CSR) type impact modifiers, consisting of a rubbery poly(n-butyl acrylate) core and a rigid poly(methyl methacrylate) shell. To optimize the dispersion of CSR particles into the PBT matrix during melt compounding, the shell surface was modified with different grafting ratios of glycidyl methacrylate (GMA) reactive with PBT chain ends. In PBT blends with a 20 wt % CSR loading, the dispersed rubbery phases showed discernible shapes depending on the grafted GMA content, from predetermined spheres with 0.25 ± 0.05 μm diameters to their aggregates in the 2–3 μm diameter range. As a result, the interparticle spacing (τ) could be controlled from 0.25 to 4.0 μm in the PBT blends containing the fixed rubber loading. The Izod impact strengths of these samples increased significantly below τ = 0.4 μm. Additional thermal and morphological analyses strongly supported the hypothesis that the marked increase in toughness of the blends was related to less ordered lamellar formation of the PBT matrix under the confined geometry. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

With the aim of enhancing the field-effect mobility of self-assembled regioregular poly(3-hexylthiophene), P3HT, by promoting two-dimensional molecular ordering, the organization of the P3HT in precursor solutions is transformed from random-coil conformation to ordered aggregates by adding small amounts of the non-solvent acetonitrile to the solutions prior to film formation. The ordering of the precursor in the solutions significantly increases the crystallinity of the P3HT thin films. It is found that with the appropriate acetonitrile concentration in the precursor solution, the resulting P3HT nanocrystals adopt a highly ordered molecular structure with a field-effect mobility dramatically improved by a factor of approximately 20 depending on the P3HT concentration. This improvement is due to the change in the P3HT organization in the precursor solution from random-coil conformation to an ordered aggregate structure as a result of the addition of acetonitrile. In the good solvent chloroform, the P3HT molecules are molecularly dissolved and adopt a random-coil conformation, whereas upon the addition of acetonitrile, which is a non-solvent for aromatic backbones and alkyl side chains, 1D or 2D aggregation of the P3HT molecules occurs depending on the P3HT concentration. This state minimizes the unfavorable interactions between the poorly soluble P3HT and the acetonitrile solvent, and maximizes the favorable π–π stacking interactions in the precursor solution, which improves the molecular ordering of the resulting P3HT thin film and enhances the field-effect mobility without post-treatment.

Solution-processable functionalized acenes have received special attention as promising organic semiconductors in recent years because of their superior intermolecular interactions and solution-processability, and provide useful benchmarks for organic field-effect transistors (OFETs). Charge-carrier transport in organic semiconductor thin films is governed by their morphologies and molecular orientation, so self-assembly of these functionalized acenes during solution processing is an important challenge. This article discusses the charge-carrier transport characteristics of solution-processed functionalized acene transistors and, in particular, focuses on the fine control of the films' morphologies and structural evolution during film-deposition processes such as inkjet printing and post-deposition annealing. We discuss strategies for controlling morphologies and crystalline microstructure of soluble acenes with a view to fabricating high-performance OFETs.

We have demonstrated the influence of evaporation-induced flow in a single droplet on the crystalline microstructure and film morphology of an ink-jet-printed organic semiconductor, 6,13-bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring-deposited films. However, self-aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface-tension-driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field-effect transistors fabricated with these self-aligned crystals via ink-jet printing exhibit significantly improved performance with an average effective field-effect mobility of 0.12 cm² V^–1 s^–1. These results demonstrate that with the choice of appropriate solvent ink-jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct-write fabrication of high-performance organic electronics.

Charge carrier transport in organic electronic devices is influenced by the crystalline microstructure and morphology of the organic semiconductor film. Evaporation behavior during drying plays a vital role in controlling the film morphology and the distribution of solute in inkjet-printed films. On p. 229, Kilwon Cho and co-workers demonstrate the influence of the evaporation-induced flow in a single droplet on the crystalline microstructure and film morphology of inkjet-printed 6,13-bis((triisopropylsilylethynyl) pentacene. The results provide an excellent method for direct-write fabrication of high-performance organic electronics.

We have demonstrated the influence of evaporation-induced flow in a single droplet on the crystalline microstructure and film morphology of an ink-jet-printed organic semiconductor, 6,13-bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring-deposited films. However, self-aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface-tension-driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field-effect transistors fabricated with these self-aligned crystals via ink-jet printing exhibit significantly improved performance with an average effective field-effect mobility of 0.12 cm² V^–1 s^–1. These results demonstrate that with the choice of appropriate solvent ink-jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct-write fabrication of high-performance organic electronics.

We report on the room-temperature self-organizing characteristics of thin films of the organic small-molecule semiconductor triethylsilylethynyl-anthradithiophene (TES-ADT) and its effect on the electrical properties of TES-ADT-based field-effect transistors (FETs). The morphology of TES-ADT films changed dramatically with time, and the field-effect mobility of FETs based on these films increased about 100-fold after seven days as a result of the change in molecular orientation from a tilted structure in the as-prepared film to a well-oriented structure in the final film. We found that the molecular movement is large enough to induce a conformational change to an energetically stable state in spin-coated TES-ADT films, because TES-ADT has a low glass-transition temperature (around room temperature). Our findings demonstrate that organic small-molecule semiconductors that exhibit a low crystallinity immediately after spin-coating can be changed into highly crystalline structures by spontaneous self-organization of the molecules at room temperature, which results in improved electrical properties of FETs based on these semiconductors.

Toughening behavior of semicrystalline polymers was investigated using syndiotactic polystyrene (sPS)/polyamide 6(PA-6) blends compatibilized with maleic-anhydride functionalized poly (styrene-b-(ethylene-co-butylene)-b-styrene) SEBS-MA triblock copolymer. The effect of interparticle distance and crystal microstructure near the particle/matrix interface of the blends were studied. The morphology studies revealed that the size and interparticle distance of the dispersed PA-6 particles decreased with increasing SEBS-MA concentration. sPS/PA-6 blends exhibited sharp brittle-ductile transitions at a critical interparticle distance of 0.25 μm. With the increase of the compatibilizer concentration beyond a certain level, it was observed that the further addition resulted in decreased impact strength. This could be attributed to the formation of a separate phase in the matrix by the additional SEBS added. The TEM studies showed that when the interparticle distance is below 0.25 μm, the matrix ligaments between the inclusions will be filled with well-oriented crystalline material of reduced plastic resistance. From DSC and X-ray diffraction studies of model thin films, it was found that the fraction of small and imperfect crystallites near the particle/matrix interface increased with decreasing interparticle distance. This resulted in decreased yield stress of the whole matrix with increasing concentration of SEBS-MA accompanied by changes in the fracture mode from brittle to tough. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

We investigated the effects of solvent type (mono-ester vs. di-ester solvent) and aging on the structural development in the poly(vinyl chloride)/butyl benzoate (PVC/BB) and PVC/dibutyl phthalate(DBP) gels, as well as on their viscoelastic and mechanical behaviors. It was found that aged PVC/DBP gels held at RT for 7 days exhibit an improvement of about 100% in storage modulus (G′) compared to fresh gels, with a sudden drop in G′ around 50 °C, whereas the storage moduli of the PVC/BB gels decrease monotonically with temperature, irrespective of the postaging time. These different behaviors of the PVC/BB and PVC/DBP gels arise mainly because of the difference in the network structure produced by the formation of the polymer-solvent complex between the CO groups of the solvent and the polarized hydrogen moieties of PVC, as was confirmed with small angle X-ray scattering and uniaxial tensile experiments. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 263–271, 2008