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2010-01-22 - Colloque/Présentation - communication orale - Anglais - 32 page(s)

Olivier Yoann , "Influence of a Polymeric Dielectric Layer on Charge Carrier Mobility in Field-Effect Transistors." in 1st European Symposium on Computing pi-Conjugated Compounds, Valencia, Espagne, 2010

  • Codes CREF : Physico-chimie générale (DI1320), Chimie quantique (DI1321)
  • Unités de recherche UMONS : Chimie des matériaux nouveaux (S817)
  • Instituts UMONS : Institut de Recherche en Science et Ingénierie des Matériaux (Matériaux)

Abstract(s) :

(Anglais) Organic field-effect transistor (OFET) has attracted considerable attention as the central component of printed electronics. The control of the charge flow through electronic circuits is crucial in electronic and is achieved by transistors. Applications such as active-matrix displays have shown the importance of this transistor feature. The light going out of each pixel of a screen as well as the time it stays switched on, is controlled by the amount of charge provided by the transistor to the pixel. The performances of active-matrix displays are thus found to be higher than passive ones. OFETs are known to have two distinct operating modes, linear (for reasonably low drain voltage compared to gate voltage) and saturation (for comparable drain and gate voltage) regimes. In both regimes, the current collected at the electrode under field-effect conditions (for gate voltage different than 0) is directly proportional to the charge carrier mobility μ. In order to improve OFET performances, charge carrier mobility should be maximized. Model materials like single crystals have shown so far the largest charge mobility (in the order of 40 cm²/Vs for pentacene [1] and rubrene [2]). However, typical materials used for conventional devices in commercial applications form amorphous or polycrystalline thin-films which decrease mobility in the range of 10-3 to 1 cm²/Vs. In the past, theoretical and experimental proofs have been given to show the crucial dependence of the charge carrier mobility in parameters such as the temperature [3,4], the electric field [5,6] as well as molecular packing [7,8]. Recently, several groups have shown that the charge mobility depends crucially on the dielectric constant of the gate insulator and especially increases when decreasing the value of the dielectric constant [9,10]. A molecular picture was clearly needed to shed light on the origin of such dependence. To do so, we have modeled the interface of four layers of pentacene in a single crystal configuration with two different polymeric dielectrics namely polystyrene (PS) and polymethyl-methacrylate (PMMA) by means of molecular dynamics and investigate the influence of such interfaces on the charge carrier mobility [11]. We have found that the hole mobility in the first layer close to the interface is highly disturbed by the presence of the dielectric compared to the upper layers due to the larger interface energetic disorder felt by the pentacene molecules. Moreover, the hole mobility in pentacene in contact with PMMA appears to be much lower (about a factor 12) than in the PS case due to the presence of strong dipoles in PMMA leading to larger interface energetic disorder. Since charge transport in OFETs occurs in the first few layers on top of the insulator, the interface properties are crucial. Other aspects such as: - the presence of grain boundaries and deposition conditions [12,13] - the presence of Self-Assembled Monolayers (SAMs) [10] - the presence of an inorganic oxide dielectric [14] have not been assessed in this study and will be discussed on the basis of recent works and examples coming from the literature.