Charge Transport in Organic Diodes and OFETs: A Comparison

Abstract

Charge carrier mobility can be measured in the bulk of fullerene films using the Charge Extraction by Linearly Increasing Voltage (CELIV) technique and at the interface with insulators using Organic Field-Effect Transistors (OFET). Time-resolved non-equilibrium electron mobility and relaxation of photo-generated charge carriers are measured using photo-CELIV. Electric field, carrier concentration and temperature dependences of the electron mobility, measured using both methods, are compared. The electron mobility is at least two orders of magnitude higher than for hole transport in the C60 films prepared by thermal evaporation. More than one order of magnitude higher charge carrier mobility values are measured in OFET configuration due to high charge carrier concentrations at the quasi 2D transport near the dielectric interface. The Meyer-Neldel Rule 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 is observed in both device structures but the charge carrier mobility is much higher in OFETs. The Meyer-Neldel energy, which is interpreted as disorder parameter, is the same in both device geometries, which suggests that the level of disorder is similar in the bulk of fullerene films and at the interface with insulators. The over one order of magnitude higher electron mobility in OFETs in comparison to diodes is explained by the much higher carrier concentrations in the channel compared to transport in the bulk.