Organic semiconductors that can be fabricated by simple processing techniques and possess excellent electrical performance, are key requirements in the progress of organic electronics. Both high semiconductor charge-carrier mobility, optimized through understanding and control of the semiconductor microstructure, and stability of the semiconductor to ambient electrochemical oxidative processes are required. We report on new semiconducting liquid-crystalline thieno[3,2-b ]thiophene polymers, the enhancement in charge-carrier mobility achieved through highly organized morphology from processing in the mesophase, and the effects of exposure to both ambient and low-humidity air on the performance of transistor devices. Relatively large crystalline domain sizes on the length scale of lithographically accessible channel lengths ( approximately 200 nm) were exhibited in thin films, thus offering the potential for fabrication of single-crystal polymer transistors. Good transistor stability under static storage and operation in a low-humidity air environment was demonstrated, with charge-carrier field-effect mobilities of 0.2-0.6 cm(2) V(-1) s(-1) achieved under nitrogen.

Organic semiconductors are emerging as a viable alternative to amorphous silicon in a range of thin‐film transistor devices. With the possibility to formulate these p‐type materials as inks and subsequently print into patterned devices, organic‐based transistors offer significant commercial advantages for manufacture, with initial applications such as low performance displays and simple logic being envisaged. Previous limitations of both air stability and electrical performance are now being overcome with a range of both small molecule and polymer‐based solution‐processable materials, which achieve charge carrier mobilities in excess of 0.5 cm2 V−1 s−1, a benchmark value for amorphous silicon semiconductors. Polymer semiconductors based on thienothiophene copolymers have achieved amongst the highest charge carrier mobilities in solution‐processed transistor devices. In this Progress Report, we evaluate the advances and limitations of this class of polymer in transistor devices.

This work describes a new design methodology that allows the preparation of air stable, semiconducting thiophene polymers with high charge carrier mobilities. The incorporation of thieno[2,3-b]thiophene into a polythiophene backbone introduces cross-conjugated double bonds that disfavor full delocalization, leading to high ionization potential in comparison to a fully conjugated polythiophene, with no reduction in charge carrier mobility. The resulting solution processable polymers exhibit charge carrier mobilities up to 0.15 cm2/V s and on/off ratios greater than 105 when measured in air. Transistors exhibit lifetimes of several months in air with no encapsulation necessary.

Charge modulation spectroscopy ͑CMS͒ is an electro-optical spectroscopic technique that allows the charge carriers present in the conducting channels of field-effect transistors ͑FET's͒ to be studied in situ. We use this technique to study the charge carriers present in regio-regular poly͑3-hexylthiophene͒ P3HT that has been shown to exhibit high field-effect mobilities of up to 0.1 cm 2 /Vs, similar to that observed for amorphous silicon. We demonstrate that the CMS spectra of charge carriers in high-mobility regio-regular P3HT FET's are independent of charge density, modulation frequency, and temperature. This is evidence for the presence of a single, intrinsic charge carrier that we identify as a singly charged polaronic species. The spectral features attributed to the charged species show a lack of vibronic structure that is in contrast to the vibronic structure present in the bleaching of the main -* absorption of the neutral chains. The transition energies observed in regio-regular P3HT cannot be understood as an extrapolation of charge-induced transitions in isolated short-chain oligomers to long conjugation lengths. Our results give evidence that interchain coupling in highly ordered P3HT is sufficiently strong so that the charge carriers cannot be considered to be confined to a single chain, rather, they now exhibit quasi-two-dimensional characteristics.

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