Modeling Polymer Dielectric/Pentacene Interfaces: On the Role of Electrostatic Energy Disorder on Charge Carrier Mobility By Nicolas G. Martinelli, Matteo Savini, Luca Muccioli, Yoann Olivier, Fre ´de ´ric Castet, Claudio Zannoni, David Beljonne, and Je ´roˆmeCornil* 1. Introduction The ﬁeld of organic electronics has developed tremendously over the last twenty years. [1] The key advantages of organic semi- conductors compared to their inorganic equivalents are the versatility of chemical synthesis, the ease of processing (in particular over large areas), and low production costs. Among the numerous applications, organic ﬁeld-effect transistors (OFETs) are attractive devices planned to be used as building blocks in low- cost electronic components. [2–5] The optimization of their performance requires the improvement of the charge transport properties in organic layers that are quantiﬁed by the charge carrier mobility value (m). [6] FETs based on amorphous silicon yield mobilities around 1 cm 2 V 1 s 1 while intensive research has now led to values as high as 30 cm 2 V 1 s 1 for organic materials in the best cases. [7] In spite of good device performance, a complete understanding of charge transport in organic semiconductors at the micro- scopic level is still lacking. A hopping regime, in which charges jump from molecule to molecule, is typically assumed to describe charge transport in bulk disordered systems where static positional disorder (giving rise to a distribution of electronic couplings between the interacting units) and energetic disorder (giving rise to a distribution of energies for the transport levels, that is, the HOMO level in the case of hole transport, as a result of ﬂuctuations in the nature of the molecular environment around each unit) promote charge localization over individual molecules. [8] Similarly, recent theoretical studies have also suggested that the charges become strongly localized in highly ordered systems such as molecular crystals due to thermal ﬂuctuations and the resulting dynamic energetic and/or spatial disorder. [9–11] In OFETs, the charge transport mostly operates (at low bias) within the ﬁrst molecular layer in contact with the insulator layer. [3,12] The transport properties, and hence the charge mobility values, are thus expected to be further affected in the conducting channel by i) the electrostatic properties of the insulator layer. A signiﬁcant drop of the mobility by up to one order of magnitude was reported in OFETs based on polytriarylamine chains when replacing low-k polymer dielectrics by poly(methyl methacryl- ate) (PMMA). [13] Similar observations were made in the case of pentacene layers. [14,15] This deterioration of the mobility was attributed to an increase in the energetic disorder pro- moted by the polar carbonyl bonds of the PMMA chains. [13] This model is consistent with previous theoretical studies of charge transport in solids formed by molecules bearing a permanent dipole moment. [16] Shifts of the threshold voltages in OFETs upon different surface treatment of the gate insu- lator have also been explained on the same basis. [17,18] In the case of highly polarizable dielectric layers, the formation of Fro ¨lich polarons has been suggested, implying that the drift of the charges is slowed by the nuclear polarization of the dielectric layer. [19] However, this model does not appear to be applicable for polymer dielectrics. [20,21] ii) the morphology of the interfacial organic layer promoted by the dielectric substrate. The formation of grain boundaries in the conducting channel is one of the major structural FULL PAPER www.afm-journal.de [*] Dr. J. Cornil, N. G. Martinelli, Dr. Y. Olivier, Dr. D. Beljonne Laboratory for Chemistry of Novel Materials University of Mons Place du Parc 20, Mons, 7000 (Belgium) E-mail: jerome@averell.umh.ac.be M. Savini, Dr. L. Muccioli, Prof. C. Zannoni Dipartimento di Chimica Fisica e Inorganica and INSTM Universita ` di Bologna Viale Risorgimento 4, Bologna, 40136 (Italy) Dr. F. Castet Institut des Sciences Mole ´culaires, UMR CNRS 5255 Universite ´ de Bordeaux 351 Cours de la Libe ´ration, Talence, 33405 (France) DOI: 10.1002/adfm.200901077 Force-ﬁeld and quantum-chemical calculations are combined to model the packing of pentacene molecules at the atomic level on two polymer dielectric layers (poly(methyl methacrylate) (PMMA) versus polystyrene (PS)) widely used in ﬁeld-effect transistors and to assess the impact of electrostatic interactions at the interface on the charge mobility values in the pentacene layers. The results show unambiguously that the electrostatic interactions introduce a signiﬁcant energetic disorder in the pentacene layer in contact with the polymer chains; a drop in the hole mobility by a factor of 5 is predicted with PS chains while a factor of 60 is obtained for PMMA due to the presence of polar carbonyl groups. 3254 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19, 3254–3261