Letter Fullerene-based bistable devices and associated negative differential resistance effect Himadri S. Majumdar a, * , Jayanta K. Baral a , Ronald O ¨ sterbacka a , Olli Ikkala b , Henrik Stubb a a Department of Physics, A ˚ bo Akademi University, Porthansgatan 3, Turku 20500, Finland b Optics and Molecular Materials, Helsinki University of Technology, Finland Received 4 May 2005; received in revised form 25 June 2005; accepted 27 June 2005 Abstract We have observed bistability in single layer devices made from fullerenes (C60) mixed with polystyrene (PS) and sandwiched between two Al electrodes. By merely changing the concentration of C60 in PS we found three distinctly different device properties, namely a true insulator, a bistable device switching between an OFF and an ON state having ON–OFF current ratios larger than 10 4 and a write-once, read-many times (WORM) device. An additional negative differential resistance (NDR) was observed in the ON state of both the bistable and WORM devices leading to multi- level switching capability of the devices. This opens up a wide range of application possibilities of such devices in dis- posable printable electronics. Ó 2005 Elsevier B.V. All rights reserved. PACS: 73.61.Wp; 85.65.+h; 73.40.Sx; 84.37.+q Keywords: Fullerene; Conductance switching; Memory applications; Negative differential resistance; Tunneling 1. Introduction The past few years have seen an upsurge in fabrication and characterization of two-terminal nonvolatile memory devices using organic semi- conductors and polymers [1–7]. These devices can switch between a high- and a low-impedance state upon application of an electric field. Different active materials and device architectures have been suggested for this purpose. Small molecules and conjugated polymers used as semiconductors in a metal/semiconductor/intermediate metal/semicon- ductor/metal (MSIMSM) structure reported 1566-1199/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2005.06.005 * Corresponding author. Tel.: +358 2 2154606; fax: +358 2 2154776. E-mail address: himadri.majumdar@abo.fi (H.S. Majum- dar). Organic Electronics 6 (2005) 188–192 www.elsevier.com/locate/orgel