High‐Performance Solution‐Processable Poly(p‐phenylene vinylene)s for Air‐Stable Organic Field‐Effect Transistors

The influence of the substitution pattern (unsymmetrical or symmetrical), the nature of the side chain (linear or branched), and the processing of several solution processable alkoxy‐substituted poly(p‐phenylene vinylene)s (PPVs) on the charge‐carrier mobility in organic field‐effect transistors (OFETs) is investigated. We have found the highest mobilities in a class of symmetrically substituted PPVs with linear alkyl chains (e.g., R¹, R² = n‐C₁₁H₂₃, R³ = n‐C₁₈H₃₇). We have shown that the mobility of these PPVs can be improved significantly up to values of 10^–2 cm² V^–1 s^–1 by annealing at 110 °C. In addition, these devices display an excellent stability in air and dark conditions. No change in the electrical performance is observed, even after storage for thirty days in humid air.     

High‐Performance and Stable Organic Thin‐Film Transistors Based on Fused Thiophenes

A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.     

2,7‐Carbazolenevinylene‐Based Oligomer Thin‐Film Transistors: High Mobility Through Structural Ordering

We have fabricated organic field‐effect transistors based on thin films of 2,7‐carbazole oligomeric semiconductors 1,4‐bis(vinylene‐(N‐hexyl‐2‐carbazole))phenylene (CPC), 1,4‐bis(vinylene‐(N′‐methyl‐7′‐hexyl‐2′‐carbazole))benzene (RCPCR), N‐hexyl‐2,7‐bis(vinylene‐(N‐hexyl‐2‐carbazole))carbazole (CCC), and N‐methyl‐2,7‐bis(vinylene‐(7‐hexyl‐N‐methyl‐2‐carbazole))carbazole (RCCCR). The organic semiconductors are deposited by thermal evaporation on bare and chemically modified silicon dioxide surfaces (SiO₂/Si) held at different temperatures varying from 25 to 200 °C during deposition. The resulting thin films have been characterized using UV‐vis and Fourier‐transform infrared spectroscopies, scanning electron microscopy, and X‐ray diffraction, and the observed top‐contact transistor performances have been correlated with thin‐film properties. We found that these new π‐conjugated oligomers can form highly ordered structures and reach high hole mobilities. Devices using CPC as the active semiconductor have exhibited mobilities as high as 0.3 cm² V^–1 s^–1 with on/off current ratios of up to 10⁷. These features make CPC and 2,7‐carbazolenevinylene‐based oligomers attractive candidates for device applications.     

Effect of Mesoscale Crystalline Structure on the Field‐Effect Mobility of Regioregular Poly(3‐hexyl thiophene) in Thin‐Film Transistors

Regioregular poly(3‐hexyl thiophene) (RR P3HT) is drop‐cast to fabricate field‐effect transistor (FET) devices from different solvents with different boiling points and solubilities for RR P3HT, such as methylene chloride, toluene, tetrahydrofuran, and chloroform. A Petri dish is used to cover the solution, and it takes less than 30 min for the solvents to evaporate at room temperature. The mesoscale crystalline morphology of RR P3HT thin films can be manipulated from well‐dispersed nanofibrils to well‐developed spherulites by changing solution processing conditions. The morphological correlation with the charge‐carrier mobility in RR P3HT thin‐film transistor (TFT) devices is investigated. The TFT devices show charge‐carrier mobilities in the range of 10^–4 ～ 10^–2 cm² V^–1 s^–1 depending on the solvent used, although grazing‐incidence X‐ray diffraction (GIXD) reveals that all films develop the same π–π‐stacking orientation, where the <100>‐axis is normal to the polymer films. By combining results from atomic force microscopy (AFM) and GIXD, it is found that the morphological connectivity of crystalline nanofibrils and the <100>‐axis orientation distribution of the π–π‐stacking plane with respect to the film normal play important roles on the charge‐carrier mobility of RR P3HT for TFT applications.     

Microstructural Control of Charge Transport in Organic Blend Thin‐Film Transistors

The charge‐transport processes in organic p‐channel transistors based on the small‐molecule 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (diF‐TES ADT), the polymer poly(triarylamine)(PTAA) and blends thereof are investigated. In the case of blend films, lateral conductive atomic force microscopy in combination with energy filtered transmission electron microscopy are used to study the evolution of charge transport as a function of blends composition, allowing direct correlation of the film's elemental composition and morphology with hole transport. Low‐temperature transport measurements reveal that optimized blend devices exhibit lower temperature dependence of hole mobility than pristine PTAA devices while also providing a narrower bandgap trap distribution than pristine diF‐TES ADT devices. These combined effects increase the mean hole mobility in optimized blends to 2.4 cm²/Vs – double the value measured for best diF‐TES ADT‐only devices. The bandgap trap distribution in transistors based on different diF‐TES ADT:PTAA blend ratios are compared and the act of blending these semiconductors is seen to reduce the trap distribution width yet increase the average trap energy compared to pristine diF‐TES ADT‐based devices. Our measurements suggest that an average trap energy of <75 meV and a trap distribution of <100 meV is needed to achieve optimum hole mobility in transistors based on diF‐TES ADT:PTAA blends.     

n‐Channel,Ambipolar, and p‐Channel Organic Heterojunction Transistors Fabricated with Various Film Morphologies

The relationship between the performance characteristics of organic field‐effect transistors (OFETs) with 2,5‐bis(4‐biphenylyl)bithiophene/copper hexadecafluorophthalocyanine (BP2T/F₁₆CuPc) heterojunctions and the thickness of the BP2T bottom layer is investigated. Three operating modes (n‐channel, ambipolar, and p‐channel) are obtained by varying the thickness of the organic semiconductor layer. The changes in operating mode are attributable to the morphology of the film and the heterojunction effect, which also leads to an evolution of the field‐effect mobility with increasing film thickness. In BP2T/F₁₆CuPc heterojunctions the mobile charge carriers accumulate at both sides of the heterojunction interface, with an accumulation layer thickness of ca. 10 nm. High field‐effect mobility values can be achieved in continuous and flat films that exhibit the heterojunction effect.     

Inside Front Cover: High‐Performance and Stable Organic Thin‐Film Transistors Based on Fused Thiophenes (Adv. Funct. Mater. 3/2006)

A series of new organic semiconductors for organic thin‐film transistors using dithieno[3,2‐b:2′,3′‐d]thiophene as the core have been synthesized. In work reported by Liu, Zhu, and co‐workers on p. 426, the phenyl‐substituted compound exhibited a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. Weekly shelf‐life tests of the transistors based on the bis(diphenyl)‐substituted thiophene under ambient conditions showed that the mobility was almost unchanged after more than two months, demonstrating potential for applications in future organic electronics. A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.     

Air‐Stable n‐Type Organic Field‐Effect Transistors Based on Carbonyl‐Bridged Bithiazole Derivatives

An electronegative conjugated compound composed of a newly designed carbonyl‐bridged bithiazole unit and trifluoroacetyl terminal groups is synthesized as a candidate for air‐stable n‐type organic field‐effect transistor (OFET) materials. Cyclic voltammetry measurements reveal that carbonyl‐bridging contributes both to lowering the lowest unoccupied molecular orbital energy level and to stabilizing the anionic species. X‐ray crystallographic analysis of the compound shows a planar molecular geometry and a dense molecular packing, which is advantageous to electron transport. Through these appropriate electrochemical properties and structures for n‐type semiconductor materials, OFET devices based on this compound show electron mobilities as high as 0.06 cm² V^?1 s^?1 with on/off ratios of 10⁶ and threshold voltages of 20 V under vacuum conditions. Furthermore, these devices show the same order of electron mobility under ambient conditions.     

Oligothiophene‐Functionalized Truxene: Star‐Shaped Compounds for Organic Field‐Effect Transistors

Organic field‐effect transistors (OFETs) based on oligothiophene‐functionalized truxene derivatives have been fabricated for use as novel star‐shaped organic semiconductors in solution‐processible organic electronics. The electronic and optical properties of compounds 1 – 3 , with increasing numbers of thiophene rings at each of the three branches, have been investigated using scanning electron microscopy (SEM), X‐ray diffraction measurements, and ultraviolet–visible (UV‐vis) and photoluminescence spectroscopies. The results show that with a stepwise increase of the thiophene rings at every branch, a transition from a polycrystalline to an amorphous state is observed. The characteristics of compounds 1 , 2 , and 3 used for OFETs exhibit a significant difference. The mobility depends greatly on the morphology in the solid state, and decreases in going from 1 to 3 . Mobilities up to 1.03 × 10^–3 cm² V^–1 s^–1 and an on/off ratio of about 10³ for compound 1 have been achieved; these are the highest values for star‐shaped organic semiconductors used for OFETs so far. All the results demonstrate that the truxene core of the oligothiophene‐functionalized truxene derivatives not only extends the π‐delocalized system, but also leads to high mobilities for the compounds.     

Vertically Segregated Structure and Properties of Small Molecule–Polymer Blend Semiconductors for Organic Thin‐Film Transistors

A comprehensive structure and performance study of thin blend films of the small‐molecule semiconductor, 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (diF‐TESADT), with various insulating binder polymers in organic thin‐film transistors is reported. The vertically segregated composition profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X‐ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X‐ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly(α‐methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composition profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution‐processed bottom‐gate/bottom‐contact thin‐film transistors.     

Cover Picture: n‐Channel,Ambipolar, and p‐Channel Organic Heterojunction Transistors Fabricated with Various Film Morphologies (Adv. Funct. Mater. 3/2007)

Simultaneous introduction of short‐range repulsive interactions between dissimilar colloidal particles and attractive interactions between like particles provides a general new route to fabricating self‐organizing bipolar devices. By identifying combinations of conductive device materials between which short‐range repulsive forces exist in the presence of an intervening liquid, electrochemical junctions can be self‐formed, as reported by Chiang and co‐workers on p. 379. The relationship between the performance characteristics of organic field‐effect transistors (OFETs) with 2,5‐bis(4‐biphenylyl)bithiophene/copper hexadecafluorophthalocyanine (BP2T/F₁₆CuPc) heterojunctions and the thickness of the BP2T bottom layer is investigated. Three operating modes (n‐channel, ambipolar, and p‐channel) are obtained by varying the thickness of the organic semiconductor layer. The changes in operating mode are attributable to the morphology of the film and the heterojunction effect, which also leads to an evolution of the field‐effect mobility with increasing film thickness. In BP2T/F₁₆CuPc heterojunctions the mobile charge carriers accumulate at both sides of the heterojunction interface, with an accumulation layer thickness of ca. 10 nm. High field‐effect mobility values can be achieved in continuous and flat films that exhibit the heterojunction effect.     

Transistor Paint: Environmentally Stable N‐alkyldithienopyrrole and Bithiazole‐Based Copolymer Thin‐Film Transistors Show Reproducible High Mobilities without Annealing

New solution processable 4‐(2‐hexyldecan)‐4H‐bisthieno[2,3‐d:3′,2′‐b]pyrrole and 4,4′‐dialkyl‐2,2′‐bithiazole‐based copolymers (PBTzDTPs) are synthesized with excellent FET performance. These novel copolymers have considerable potential in printable electronics as they have high charge carrier mobilities, excellent air stability, good solution processibility, and no requirement for post‐deposition thermal annealing, all requirements for this field of application. The thin film transistors fabricated from PBTzDTPs achieve field effect mobilities as high as 0.14 cm² V^?1 s^?1 with current on/off ratios up to 10⁶ without thermal annealing. In addition, the devices exhibit stable performance in air, showing no significant degradation over 60 days. Moreover, the polymers described here provide an excellent example of the systems in which higher mobility performance does not require higher crystalline, long‐range ordered structures. Such a system appears to be particularly promising for rapid fabrication techniques, where kinetic conditions usually prevent the development of long‐range order.     

Temperature‐Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers

Previous investigations of the field‐effect mobility in poly(3‐hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium‐MW: M_n = 7 200 g mol^?1; high‐MW: M_n = 27 000 g mol^?1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and organic field‐effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra‐grain mobility depends only weakly on MW (being in the order of 10^?2 cm² V^?1 s^?1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature‐dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium‐MW P3HT undergo a “pre‐melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short‐chain P3HT is strongly inhibited by the presence of disordered domains, while in high‐MW P3HT the low‐mobility disordered zones are bridged via inter‐crystalline molecular connections.     

Morphology and Field‐Effect‐Transistor Mobility in Tetracene Thin Films

The growth of vacuum‐sublimed tetracene thin films on silicon dioxide has been investigated from the early stages of the process. The effects of deposition flux and substrate silanization on film morphology and electrical properties have been explored. Tetracene shows an island growth, resulting in films with a granular structure. Both an increase in the deposition flux and the substrate silanization determine a decrease of the grain size and an improvement of the connectivity of the film in direct contact with the substrate. The hole mobility in field‐effect transistors based on tetracene thin films, which also generate electroluminescence, increases with the deposition flux and values as high as 0.15 cm² V^–1 s^–1 are obtained.     

Controlling Polymorphism in Poly(3‐Hexylthiophene) through Addition of Ferrocene for Enhanced Charge Mobilities in Thin‐Film Transistors

Crystalline organic molecules often exhibit the ability to assemble into multiple crystal structures depending on the processing conditions. Exploiting this polymorphism to optimize molecular orbital overlap between adjacent molecules in the unit lattice is an effective method for improving charge transport within the material. In this study, grazing incident X‐ray diffraction was employed to demonstrate the formation of tighter π‐π stacking poly(3‐hexylthiophene‐2,5‐diyl) polymorphs in films spin coated from ferrocene‐containing solutions. As a result, the addition of ferrocene to casting solutions yields thin‐film transistors which exhibit approximately three times higher source‐drain currents and charge mobilities than neat polymer devices. Nevertheless, XPS depth profiling and NMR analyses of the active layer reveal that all ferrocene is removed during the spin coating process, which may be an essential factor to achieve high mobilities. Such insights gleaned from ferrocene/poly(3‐hexylthiophene‐2,5‐diyl) mixtures can serve as a template for selection and optimization of other small molecule/polymer systems with greater baseline charge mobilities.     

The Influence of Film Morphology in High‐Mobility Small‐Molecule:Polymer Blend Organic Transistors

Organic field‐effect transistors (OFETs) based upon blends of small molecular semiconductors and polymers show promise for high performance organic electronics applications. Here the charge transport characteristics of high mobility p‐channel organic transistors based on 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl) anthradithiophene:poly(triarylamine) blend films are investigated. By simple alteration of the film processing conditions two distinct film microstructures can be obtained: one characterized by small spherulitic grains (SG) and one by large grains (LG). Charge transport measurements reveal thermally activated hole transport in both SG and LG film microstructures with two distinct temperature regimes. For temperatures >115 K, gate voltage dependent activation energies (E_A) in the range of 25–60 meV are derived. At temperatures <115 K, the activation energies are smaller and typically in the range 5–30 meV. For both film microstructures hole transport appears to be dominated by trapping at the grain boundaries. Estimates of the trap densities suggests that LG films with fewer grain boundaries are characterized by a reduced number of traps that are less energetically disordered but deeper in energy than for small SG films. The effects of source and drain electrode treatment with self‐assembled monolayers (SAMs) on current injection is also investigated. Fluorinated thiol SAMs were found to alter the work function of gold electrodes by up to ～1 eV leading to a lower contact resistance. However, charge transport analysis suggests that electrode work function is not the only parameter to consider for efficient charge injection.     

Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors

Polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. The recent development of semicrystalline polymers with strongly improved charge transport properties forces a review of the current understanding of the charge transport mechanisms and how they relate to the polymer's chemical and structural properties. Here, the charge density dependence of field effect mobility in semicrystalline polymer semiconductors is studied. A simultaneous increase in mobility and its charge density dependence, directly correlated to the increase in average crystallite size of the polymer film, is observed. Further evidence from charge accumulation spectroscopy shows that charges accumulate in the crystalline regions of the polymer film and that the increase in crystallite size affects the average electronic orbitals delocalization. These results clearly point to an effect that is not caused by energetic disorder. It is instead shown that the inclusion of short range coulomb repulsion between charge carriers on nanoscale crystalline domains allows describing the observed mobility dependence in agreement with the structural and optical characterization. The conclusions that are extracted extend beyond pure transistor characterization and can provide new insights into charge carrier transport for regimes and timescales that are relevant to other optoelectronic devices.     

Enhancement of Interconnectivity in the Channels of Pentacene Thin‐Film Transistors and Its Effect on Field‐Effect Mobility

With the aim of improving the field‐effect mobility of transistors by promoting the interconnectivity of the grains in pentacene thin films, deposition conditions of the pentacene molecules using one‐step (total thickness of layer 50 nm: 0.1 Å s^–1) and two‐step (first layer 10 nm: 0.1 Å s^–1, second layer 40 nm: 4.0 Å s^–1) depositions are controlled. Significantly, it is found that the continuities of the pentacene thin films vary with the deposition conditions of the pentacene molecules. Specifically, a smaller number of voids is observed at the interface for the two‐step deposition, which results in field‐effect mobilities as high as 1.2 cm² V^–1 s^–1; these are higher by more than a factor of two than those of the pentacene films deposited in one step. This remarkable increase in field‐effect mobility is due in particular to the interconnectivity of the pentacene grains near the insulator substrate.     

Anisotropy of Charge Transport in a Uniaxially Aligned and Chain‐Extended,High‐Mobility,Conjugated Polymer Semiconductor

Charge transport in the ribbon phase of poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT)—one of the most highly ordered, chain‐extended crystalline microstructures available in a conjugated polymer semiconductor—is studied. Ribbon‐phase PBTTT has previously been found not to exhibit high carrier mobilities, but it is shown here that field‐effect mobilities depend strongly on the device architecture and active interface. When devices are constructed such that the ribbon‐phase films are in contact with either a polymer gate dielectric or an SiO₂ gate dielectric modified by a hydrophobic, self‐assembled monolayer, high mobilities of up to 0.4 cm² V^?1 s^?1 can be achieved, which is comparable to those observed previously in terrace‐phase PBTTT. In uniaxially aligned, zone‐cast films of ribbon‐phase PBTTT the mobility anisotropy is measured for transport both parallel and perpendicular to the polymer chain direction. The mobility anisotropy is relatively small, with the mobility along the polymer chain direction being higher by a factor of 3–5, consistent with the grain size encountered in the two transport directions.     

The Effect of Gate‐Dielectric Surface Energy on Pentacene Morphology and Organic Field‐Effect Transistor Characteristics

The effects of the surface energy of polymer gate dielectrics on pentacene morphology and the electrical properties of pentacene field‐effect transistors (FETs) are reported, using surface‐energy‐controllable poly(imide‐siloxane)s as gate‐dielectric layers. The surface energy of gate dielectrics strongly influences the pentacene film morphology and growth mode, producing Stranski–Krastanov growth with large and dendritic grains at high surface energy and three‐dimensional island growth with small grains at low surface energy. In spite of the small grain size (≈ 300 nm) and decreased ordering of pentacene molecules vertical to the gate dielectric with low surface energy, the mobility of FETs with a low‐surface‐energy gate dielectric is larger by a factor of about five, compared to their high‐surface‐energy counterparts. In pentacene growth on the low‐surface‐energy gate dielectric, interconnection between grains is observed and gradual lateral growth of grains causes the vacant space between grains to be filled. Hence, the higher mobility of the FETs with low‐surface‐energy gate dielectrics can be achieved by interconnection and tight packing between pentacene grains. On the other hand, the high‐surface‐energy dielectric forms the first pentacene layer with some voids and then successive, incomplete layers over the first, which can limit the transport of charge carriers and cause lower carrier mobility, in spite of the formation of large grains (≈ 1.3 μm) in a thicker pentacene film.