Polymer-Based Memories

While many of the first conductive polymers degraded rapidly in normal atmosphere, poly(3,4-ethylenedioxythiophene), or PEDOT, is a highly air-stable conductive polymer. Moreover, it can by synthesized in an aqueous dispersion (Baytron® P, H.C. Starck) with polystyrene sulfonate (PSS), from which it is readily coated into thin films for electronic devices. The research group of Professor Stephen Forrest (then in Princeton Electrical Engineering) found that the passage of a high current density through PEDOT:PSS films caused an irreversible increase in resistivity by as much as six orders of magnitude, though macroscopically the film appears unchanged.  This feature allows PEDOT:PSS to be used as a “fuse”, which in turn forms the basis for a write-once, read-many times (WORM) memory. A high-density, all-electronic WORM memory would have significant simplicity and size advantages over, for example, a CD drive. In collaboration with Forrest’s group, we traced the mechanism of this resistivity increase to a migration of PSS during device operation, creating an insulating layer at the interface.

To expand the range of accessible materials, we also synthesized other PEDOT:polyelectrolyte dispersions, as well as adding post-coating processing steps, both aimed at increasing the ratio of conductivities in the “as spun” vs. “blown” states, as this ratio ultimately defines the maximum size of the memory array.

An aqueous PEDOT:PSS dispersion
An aqueous PEDOT:PSS dispersion synthesized in our laboratory.  The conductive form of PEDOT is blue (a deep blue, in the photo); the actual PEDOT concentration is only 1 wt%.

Supported by the National Science Foundation (Electronics, Photonics, and Device Technologies Program)

Group Members Involved, and Their Project Titles:

Hongwei Zhang PDRA – "Synthesis of New PEDOT Dispersions"
Babaniyi Babatope (visitor) – "Characterization of PEDOT:PSSH"
Xin Xu PhD EE *10 – “Organic Photodetectors and Their Applications for Hemispherical Imaging Focal Plane Arrays”

Relevant Group Publication:

X. Xu, R.A. Register, and S.R. Forrest, “Mechanisms for Current Induced Conductivity Changes in a Conducting Polymer”, Appl. Phys. Lett., 89, 142109 (2006).