Ion Transport and Switching Speed in Redox-Gated 3-Terminal Organic Memory Devices

The dynamics of redox gated organic memory devices based on dynamic doping of polythiophene were examined in detail in order to improve “write” and “erase” speed and determine ultimate performance. A 3-terminal geometry similar to a field effect transistor provided a source/gate circuit which reversibly oxidized a polythiophene polymer to cause a large increase in conductance between the source and drain electrodes. The devices were cycled for >1000 complete R/W/R/E cycles, and operated at relatively low voltage compared to commercial “flash” memory. The “write” and “erase” speeds were improved by a factor of >100 by using a spin-coated electrolyte layer and by small increases in device temperature. The influence of charging current and polaron propagation on response time were determined to be minor, with the rate limiting process being identified as the rate of conducting polaron generation. The main factor determining the W/E time was the mobility of ions in the polyethylene oxide electrolyte layer, which resulted in resistance losses during the application of the S-G “write” pulse. Response time was strongly dependent on the atmosphere, with water or acetonitrile vapor significantly increasing the rate of polaron generation. The results are important for design of molecular memory devices based on dynamic doping, and indicate likely avenues for further performance improvements.