Synthetic Metals 160 (2010) 1787–1792 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Modeling of mobility in organic thin-film transistor based octithiophene (8T) S. Mansouri a,∗ , G. Horowitz b , R. Bourguiga a a Laboratoire de Physique des Matériaux: Structure et Propriétés, Groupe Physique des Composant et Dispositifs Nanométriques, Facultés des sciences de Bizerte, 7021 Jarzouna-Bizerte, Tunisia b ITODYS, CNRS-UMR 7086, University Denis-Diderot, 1 rue Guy de la brosse, 75005 Paris, France article info Article history: Received 25 January 2010 Received in revised form 21 June 2010 Accepted 22 June 2010 Available online 21 July 2010 Keywords: Octithiophene TFTs Charge carrier mobility Output characteristic abstract Thin-film organic field-effect transistor was made with vapor-deposited polycrystalline octithiophene on silicon oxide insulator layers. In conventional field-effect transistors, the extracted mobility does not take into account the distribution of charge carriers. However, in disordered organic field-effect transistors, the local charge carrier mobility decreases from the semiconductors/insulator interface in to the bulk, due to its dependence on the charge carrier density. It is demonstrated that the conventional field-effect mobility is a good approximation for the local mobility of the charge carriers at the interface. In this paper we present a new approach to the mobility in organic thin-film transistor (OTFTs), and with a new procedure we extract the electrical parameters of organic TFTs that possible to reproduce very well the device characteristic and mobility. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Organic field-effect transistors are currently experiencing a growing interest and gaining attention in both academic and indus- trial [1]. Organic materials constitute a possible alternative to conventional semiconductors in applications where large areas are needed, such as flat panel displays [2–4]. They could also be advan- tageous in devices built on polymeric substrates, because they can be fabricated at low temperature. Besides their technological inter- est, OFET are also one of the most reliable means to study charge transport in organic semiconductors and they are still subject of large debates. It is generally agreed that charge transport in organic materials occurs via hopping of self-trapped polarons. Evidence for hopping transport mostly relies on the fact that the field- effect mobility of organic semiconductors is thermally activated. The behavior of I–V characteristics of organic thin-film transistors, OTFTs, has been frequently represented using the same expressions as for MOSFETs [5], to which specific effects present in OTFTs as ohmic or non-ohmic resistance at drain and source contacts are added [6,7]. The presence of leakage current through the dielectric layer [8], or across the channel is other features that have also been considered. In this paper we show that some organic OTFTs can be very accurately modeled using the same expression for mobility as for amorphous silicon TFTs. The transistors model also included series resistance and non-ohmic contacts. Among the advantages ∗ Corresponding author. Tel.: +216 99484459. E-mail address: mansourislah@yahoo.fr (S. Mansouri). of this method are the simple and precise extraction procedure that can be used to determine all model parameters from the electrical characteristics of the devices. At the same time, model parameters are related to physical properties of the device. In this paper we present a new approach to model mobility in organic TFTs that allows the extraction of DOS parameters of the organic semiconductor used in the device and gives a new procedure to extract electrical parameters from measured I–V to reproduce with this model the devices characteristic and mobil- ity. Finally, we study the effect of contact metal/semiconductor of organic thin-film transistor based on octithiophene as shown in Fig. 1. 2. Model of transport in organic thin-film transistors The multiple trapping and release model (MTR) have been used to account for gate voltage dependent mobility in hydrogenated amorphous silicon [9]. The same model has been used to describe the temperature dependent mobility of sexithiophene. The model assumes that charge transport occurs in extended states, but that most of carriers injected in the semiconductor are trapped in states localized in the forbidden gap. A crucial feature for the model is the determination of the density of trap states (DOS). A method for determining the DOS has been developed by Spear and Lecomber for a-Si:H and by G. Horowitz for sexithiophene [10,11]. A critical point of the method is the knowledge of the spa- tial charge distribution in the accumulation layer which is related to the thickness of the conducting channel in the transistor. Cal- culating this thickness actually consists of estimating the charge distribution across the semiconductor film, which is done by solv- 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.06.024