Charge Carrier Mobility Measurements in Tetracene Single Crystals
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Charge Carrier Mobility Measurements In Tetracene Single Crystals Jens Pflaum, Jens Niemax, and Ashutosh Kumar Tripathi 3. Physics Department, University of Stuttgart, 70550 Stuttgart, Germany ABSTRACT We present data on the transport of charge carriers in the organic semiconductor tetracene. Comparative measurements by time-of-flight (TOF) spectroscopy and measurements in fieldeffect transistor (FET) geometry reveal hole mobilities of about 1 cm2/Vs. Whereas for FETs only hole transport can be detected, from TOF a strong dispersive transport for negative charge carriers is observed. This observation is mainly caused by deep-level trapping of electrons. By fitting the temperature dependent hole mobility to a model of multiple-trapping and release of charge carriers the trap energy and the relative trap density can be adjusted to 130 meV and 5ยท10-3, respectively. Comparative chemical and structural analysis of inhomogeneities show that the traps affecting the transport are mainly caused by chemical defects rather than by structural imperfections.
INTRODUCTION Beside their potential application as semiconducting layers in organic based electronic devices, polyaromatic hydrocarbons offer an interesting new material class in solid state physics [1]. In contrast to covalently bound inorganic semiconductors such as Si, crystalline organic semiconductors are defined by a wide band gap (~eV) accompanied by a small energy band dispersion (~100 meV) [2]. Both phenomena together with a strong spatial anisotropy of the charge carrier transport can be explained by the weakly van-der-Waals bonding between the molecular entities. As a result of the wide energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively identified with the valence band and the conduction band in the following, the susceptibility of the electrical transport to chemical or structural defects causing trap states in the mid-gap is extremely high. Even at a relative impurity concentration of 10-6 a significant drop of the hole as well as of the electron mobility might be observed [3]. In this context, transport studies on thin film samples on weakly interacting substrates, such as SiO2, are a priori affected by rotational domains acting as carrier traps by the variation of polarization energy at their grain boundaries [2,4]. Moreover, effects at the metal-organic contact interfaces might result in a non-ohmic behaviour together with limited carrier injection. Therefore, the substantial demand on the chemical and structural quality can only be fulfilled by samples made of ultra-pure molecular compounds and with the highest degree of structural order, namely organic single crystals. As an example, we present measurements on tetracene, a polyaromatic hydrocarbon consisting of four linearly condensed benzene rings. The resulting properties of crystals prepared by two different growth techniques are discussed. In addition, the reliability of charge carrier mobilities obtained by contact-
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