Spatial summation of the short-term plasticity of a pair of organic heterogeneous junctions

Abstract

Recent studies have found that responses to electrical stimulations in organic semiconductor and/or electrolyte heterogeneous junctions possess features in common with synaptic plasticity in neural networks. Spatial summation of short-term plasticity was then studied using a pair of such junctions, i.e., Pt/Mg-doped polyethylene oxide (PEO)/Pt and Pt/Mg-doped PEO/poly(3-hexylthiophene-2,5-diyl) (P3HT)/Pt devices. The former displayed short-term depression for charging peaks and short-term facilitation (STF) for discharging peaks, while the latter displayed STF for both the charging and discharging peaks. A simple integration of parallel connection showed that the system displayed frequency selectivity in the weight modification of the charging peaks, i.e., it facilitated below a frequency threshold but depressed at a higher frequency. The frequency threshold varied with input numbers from about 60 Hz to 100 Hz. In contrast, only STF was observed in the weight modifications of the discharging peaks. In addition, the weight modification could be linearly summed from those of the two source devices though the absolute peak currents could not. Our study demonstrates that synaptic computation are feasible for parallel connection system, depending on both input frequency and linear summation of weight modifications. Finally, we suggest that directional selectivity might be realized using the parallel system.