Organic memory using [6,6]-phenyl-C₆₁ butyric acid methyl ester: Morphology, thickness and concentration dependence studies

We report a simple memory device in which the fullerene-derivative [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) mixed with inert polystyrene (PS) matrix is sandwiched between two aluminum (Al) electrodes. Transmission electron microscopy (TEM) images of PCBM:PS films showed well controlled morphology without forming any aggregates at low weight percentages (<10 wt%) of PCBM in PS. Energy dispersive x-ray spectroscopy (EDX) analysis of the device cross-sections indicated that the thermal evaporation of the Al electrodes did not lead to the inclusion of Al metal nanoparticles into the active PCBM:PS film. Above a threshold voltage of <3 V, independent of thickness, a consistent negative differential resistance (NDR) is observed in devices in the thickness range from 200 to 350 nm made from solutions with 4–10 wt% of PCBM in PS. We found that the threshold voltage (V_th) for switching from the high-impedance state to the low-impedance state, the voltage at maximum current density (V_max) and the voltage at minimum current density (V_min) in the NDR regime are constant within this thickness range. The current density ratio at V_max and V_min is more than or equal to 10, increasing with thickness. Furthermore, the current density is exponentially dependent on the longest tunneling jump between two PCBM molecules, suggesting a tunneling mechanism between individual PCBM molecules. This is further supported with temperature independent NDR down to 240 K.