Inverted resistive switching mechanism in polycrystalline PBTTT-C14 polymer devices based on contact geometry and molecular packing for neuromorphic memory

Abstract

Resistive switching (RS) devices switch resistance based on the applied voltage and hence find their application in memory devices for neuromorphic computing. Here we report for the first time an inverse RS that is opposite to the reported RS mechanisms in organic semiconductor (OSC) based memory devices. The inverse RS phenomena from high to low current state occurred in polycrystalline Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14) based two-terminal devices by modifying the contact geometry. A hexagonal shaped perforation on the bottom metal electrodes is used to create high local electric field regions in the device which causes high local current paths and hence Joule heating which modifies the molecular packing in the OSC polymer. To support our proposed mechanism, we performed same measurements with another OSC material which is an amorphous polymer that is, Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDI-2T or N2200). The changes in the molecular packing in the grains and grain boundaries due to high current in PBTTT-C14 devices are found to be responsible for the inverted RS phenomena. The application of voltage to the device lead to the formation of initial current conducting channels which sustain up to a current limit and then a sudden breakage of that channel occurs due to the high current density heating which changes the molecular packing from regio-regular to regio-random state. The high current density heating causes the deformation of molecular packing in the polymer around the conducting channels, including grain and grain boundaries. This phenomenon was absent in N2200 based devices due to the amorphous nature of the polymer as any changes in morphology due to Joule heating result in the same amorphous molecular packing/regio-random morphology. The inverted RS mechanism due to morphological changes in the PBTTT-C14 polycrystalline polymer can be utilized for memory application and neuromorphic computing.