Uniaxial Alignment of Conjugated Polymer Films for High‐Performance Organic Field‐Effect Transistors

Polymer semiconductors have been experiencing a remarkable improvement in electronic and optoelectronic properties, which are largely related to the recent development of a vast library of high‐performance, donor–acceptor copolymers showing alternation of chemical moieties with different electronic affinities along their backbones. Such steady improvement is making conjugated polymers even more appealing for large‐area and flexible electronic applications, from distributed and portable electronics to healthcare devices, where cost‐effective manufacturing, light weight, and ease of integration represent key benefits. Recently, a strong boost to charge carrier mobility in polymer‐based field‐effect transistors, consistently achieving the range from 1.0 to 10 cm² V^?1 s^?1 for both holes and electrons, has been given by uniaxial backbone alignment of polymers in thin films, inducing strong transport anisotropy and favoring enhanced transport properties along the alignment direction. Herein, an overview on this topic is provided with a focus on the processing–structure–property relationships that enable the controlled and uniform alignment of polymer films over large areas with scalable processes. The key aspects are specific molecular structures, such as planarized backbones with a reduced degree of conformational disorder, solution formulation with controlled aggregation, and deposition techniques inducing suitable directional flow.     

Exploiting Ionic Coupling in Electronic Devices: Electrolyte‐Gated Organic Field‐Effect Transistors

Currently there is great interest in using organic semiconductors to develop novel flexible electronic applications. An emerging strategy in organic semiconductor materials research involves development of composite or layered materials in which electronic and ionic conductivity is combined to create enhanced functionality in devices. For example, we and other groups have employed ionic motion to modulate electronic transport in organic field‐effect transistors using solid electrolytes. Not only do these transistors operate at low voltages as a result of greatly enhanced capacitive coupling, but they also display intriguing transport phenomena such as negative differential transconductance. Here, we discuss differences in operation between traditional (e.g., SiO₂) and electrolyte‐based dielectrics, suggest further improvements to currently used electrolyte materials, and propose several possibilities for exploiting electrolytes in future applications with both organic and inorganic semiconductors.     

Self‐Assembled Monolayers as Patterning Tool for Organic Electronic Devices

The patterning of functional materials represents a crucial step for the implementation of organic semiconducting materials into functional devices. Classical patterning techniques such as photolithography or shadow masking exhibit certain limitations in terms of choice of materials, processing techniques and feasibility for large area fabrication. The use of self‐assembled monolayers (SAMs) as a patterning tool offers a wide variety of opportunities, from the region‐selective deposition of active components to guiding the crystallization direction. Here, we discuss general techniques and mechanisms for SAM‐based patterning and show that all necessary components for organic electronic devices, i.e., conducting materials, dielectrics, organic semiconductors, and further functional layers can be patterned with the use of self‐assembled monolayers. The advantages and limitations, and potential further applications of patterning approaches based on self‐assembled monolayers are critically discussed.     

Halogenated Tetraazapentacenes with Electron Mobility as High as 27.8 cm2 V−1 s−1 in Solution‐Processed n‐Channel Organic Thin‐Film Transistors

Molecular engineering of tetraazapentacene with different numbers of fluorine and chlorine substituents fine‐tunes the frontier molecular orbitals, molecular vibrations, and π–π stacking for n‐type organic semiconductors. Among the six halogenated tetraazapentacenes studied herein, the tetrachloro derivative (4Cl‐TAP) in solution‐processed thin‐film transistors exhibits electron mobility of 14.9 ± 4.9 cm² V^?1 s^?1 with a maximum value of 27.8 cm² V^?1 s^?1, which sets a new record for n‐channel organic field‐effect transistors. Computational studies on the basis of crystal structures shed light on the structure–property relationships for organic semiconductors. First, chlorine substituents slightly decrease the reorganization energy of the tetraazapentacene whereas fluorine substituents increase the reorganization energy as a result of fine‐tuning molecular vibrations. Second, the electron transfer integral is very sensitive to subtle changes in the 2D π‐stacking with brickwork arrangement. The unprecedentedly high electron mobility of 4Cl‐TAP is attributed to the reduced reorganization energy and enhanced electron transfer integral as a result of modification of tetraazapentacene with four chlorine substituents.