Effect of Film Morphology on Charge Transport in C 60 -based Organic Field Effect Transistors
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1270-II06-68
Effect of film morphology on charge transport in C60 based Organic Field Effect Transistors Mujeeb Ullah1, A. K. Kadashchuk2, 3, P. Stadler4, A. Kharchenko5, A. Pivrikas4, C. Simbrunner1, N. S. Sariciftci4 and H. Sitter1 1 Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Austria. 2 IMEC, Kapeldreef 75, B-3001, Heverlee, Belgium. 3 Institute of Physics, National Academy of Sciences of Ukraine, Prospect Nauky 46, 03028 Kyiv, Ukraine 4 Linz Institute of Organic Solar Cells (LIOS), Johannes Kepler University Linz, Austria. 5 PANalytical B.V, Lelyweg 1, P.O.Box 13, 7600 AA Almelo, The Netherlands. ABSTRACT The critical factor that limits the efficiencies of organic electronic devices is the low charge carrier mobility which is attributed to disorder in organic films. In this work we study the effects of active film morphology on the charge transport in Organic Field Effect Transistors (OFETs). We fabricated the OFETs using different substrate temperature to grow different morphologies of C60 films by Hot Wall Epitaxy. Atomic Force Microscopy images and XRD results showed increasing grain size with increasing substrate temperature. An increase in field effect mobility was observed for different OFETs with increasing grain size in C60 films. The temperature dependence of charge carrier mobility in these devices followed the empirical relation named as Meyer-Neldel Rule and showed different activation energies for films with different degree of disorder. A shift in characteristic Meyer-Neldel energy was observed with changing C60 morphology which can be considered as an energetic disorder parameter. INTRODUCTION The field of organic field-effect transistors (OFETs) has gained considerable attention because of its potential for emerging flexible electronics and active matrix backplanes for displays [1]. Considerable efforts have been devoted to increase the performance of OFETs [2, 3] which critically depends on charge carrier transport properties in organic semiconductor layers. Although the charge mobility in OFETs has been improved significantly in the last years, it still remains by far lower than that of conventional inorganic devices and, along with stability issues, makes a current bottleneck for the large scale industrial application of organic electronic devices. Charge mobility is primarily determined by the disordered nature of conventional thin organic films, therefore methods capable of improving organic thin film morphologies are thus of considerable scientific as well as technological interest. In amorphous disordered thin organic films the electronic states are localized and energetically distributed, so that transport occurs via incoherent thermally-assisted hopping [4-6] in the density-of-states (DOS) distribution most often described by a Gaussian disorder model of Bässler [4].
2 2 3
µ (T , E ) = µ 0 exp − σˆ exp[− C (σˆ 2 − Σ 2 )E 1 / 2 ]
(1)
σˆ = σ / k B T and Σ are parameters characterizing energetic disorder and positional d
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