Electrical behavior of memory devices based on fluorene-containing organic thin films

We report on switching and negative differential resistance (NDR) behaviors of crossed bar electrode structures based on Al/organic layer/Al devices in which the organic layer was a spin-coated layer of 7-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-9,9-dihexyl-N,N-diphenyl-fluoren-2-amine. The addition of gold nanoparticles (0.5 wt %) did not change the switching behavior of thicker film structures; however, devices incorporating the nanoparticles showed more reproducible characteristics. In most cases, a "forming" process, in which a large positive voltage was applied to the top Al electrode, was required before the NDR and conductivity switching were observed. Three different electrical conductivity mechanisms have been identified: Poole-Fretikel conductivity in unformed structures, linear current versus voltage characteristics for the ON state in formed devices, and superlinear current versus voltage behavior for the OFF state in formed devices. Models based on metallic filaments or on the injection and storage of charge do not explain all our experimental observations satisfactorily. Instead, an explanation based on the formation of nanocrystalline regions within the thin film is suggested. The devices can be used as two-terminal memory cells operating with unipolar voltage pulses. (C) 2008 American Institute of Physics.