8
Ambipolar Charge-Carrier Transport in Molecular Field-Effect Transistors
8.1 Introduction
Molecular or polymeric materials used as organic semiconductors in thin-film devices are traditionally reported as electron- or hole-transporting materials [1, 2]. Most of the polymeric materials, like poly-phenylenevinylenes or poly-thiophenes, and some classes of molecular materials, like acenes or phthalocyanines, are typically p-conducting materials, whereas fullerenes or fluorinated acenes or phthalocyanines are mainly n-conducting materials. In contrast, bipolar transport was reported for single crystals as obtained by time-of-flight (TOF) measurements [3]. In this technique, electron–hole pairs are generated by light absorption close to one electrode and the drift of one charge carrier type toward the counter electrode is measured in an electric field. The sign of this applied voltage determines the charge carrier type that is transported through the whole crystal and is analyzed by the transit time.
In thin-film devices, charge carriers are generated either by the field effect or by injection. Here the misalignment of the metal work function to the respective transport levels induces more or less large injection barriers. Furthermore, the transport in field-effect transistors (FETs) can be limited by trap states at the semiconductor/insulator interface. Especially, the often-used gate oxides, like silica or alumina, have electron traps at their ...
