Semi-conjugated acceptor-based polyimides as electrets for nonvolatile transistor memory devices

We report new polyimides as electrets for pentacene-based organic field-effect transistor (OFET) memory devices, consisting of different electron-accepting dianhydride moieties (PI(BPDA-DAP), PI(PMDA-DAP), and PI(ODPA-DAP)), and varied aliphatic spacer lengths (PI(PMDA-DAH) and PI(PMDA-DAD)). The effects of the molecular structures on the charge mobility and OFET memory characteristics were systematically investigated in terms of the related frontier energy levels and morphologies. All the devices showed bistable nonvolatile memory characteristics but with a distinct charge storage behavior. The OFET memory devices using the PI(BPDA-DAP) electret exhibited a larger memory window of 81 V compared to those of PI(PMDA-DAP) and PI(ODPA-DAP), mainly attributed to its low-lying LUMO energy level that results from its more extended conjugation. Both the high retention time (around 104 s for the ON/OFF states) and steady write-read-erase-read (WRER) cycles suggested good stability and read/write properties of the studied devices. As the aliphatic spacer length increased from PI(PMDA-DAP) to PI(PMDA-DAH) and PI(PMDA-DAD), the memory characteristics changed from flash to WORM (write once read many times) behaviour. Furthermore, the memory window was reduced with the increased aliphatic spacer length, due to the reduced density of the conjugated electron-trapping sites. This study demonstrated that the electrical characteristics and charge mobility of transistor memory devices can be effectively modulated through the extent of conjugation of the electron accepting moiety and spacer length in polymer electrets.