Meyer-Neldel rule for charge carrier transport in fullerene devices: A comparative study

Charge transport is comparatively studied in the bulk and at the interface of disordered fullerene films fabricated using physical vapour deposition. Charge carrier concentration and temperature dependent electron mobilities are comparatively studied using charge extraction by linearly increasing voltage (CELIV) technique and organic field-effect transistors (OFET) measurements. Electron mobility is at least two orders of magnitude higher than hole mobility in the fullerene films. Lower mobility values and stronger concentration dependence in diodes is observed. Carrier concentration dependent activation energy is experimentally measured in both types of devices. Larger activation energy for electron transport is required at lower carrier concentrations. Meyer-Neldel rule (MNR) for electron mobility is observed in both the bulk of the fullerene films and in the transistor channel at the interface. Meyer-Neldel energy (E-MN = 35 meV), which is interpreted as disorder parameter, is the same in both device geometries, which suggest that the level of disorder and energetical landscape for charge transport is similar in the bulk of fullerene films and at the interface with the insulator. Disorder formalism is used to qualitatively explain that either carrier concentration or the nature of non-equilibrium charge carrier transport in CELIV compared to steady-state OFET measurements is responsible for observed transport properties. (C) 2010 Elsevier B. V. All rights reserved.