Rational Band Engineering of Organic Double-Heterojunction for Artificial Synaptic Device with Enhanced State Retention and Linear Update of Synaptic Weight.

Herein we propose an organic double heterojunction to enable nonvolatile step modulation of the conductance of an artificial synapse; the double heterojunction is composed of N,N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), copper phthalocyanine (CuPc), and para-sexiphenyl (p-6P). The carrier confinement in the CuPc region present in the double-heterojunction structure enabled nonvolatile modulation of the postsynaptic current. The proposed organic synapse exhibited an excellent conductance change characteristic with a nonlinearity (NL) value below 0.01 in the long-term potentiation (LTP) region. Furthermore, the NL value for long-term depression (LTD) could be reduced effectively from 45 to 3.5 by a pulse modulation technique. A simple artificial neural network (ANN) was theoretically designed using the LTP/LTD characteristic curves of such organic synapses, and then, learning and recognition tasks were performed using Modified National Institute of Standards and Technology (MNIST) digit images. A four-amplitude weight update method enabled considerable enhancement of the recognition rate, from 53% to 70%. Although the designed ANN was based on a single-layer perceptron model, a high maximum accuracy of 75% was achieved. These newly studied techniques for synaptic devices are expected to open up new possibilities for the realization of artificial synapses based on organic double-heterojunctions.