Conductivity Switching and Electronic Memory Effect in Polymers with Pendant Azobenzene Chromophores

Electronic memory devices having the indium−tin oxide/polymer/Al sandwich structure were fabricated from polymers containing pendant azobenzene chromophores in donor−acceptor structures. The reversibility, or rewritability, of the high-conductivity (ON) state was found to be dependant on the terminal moiety of the azobenzene chromophore. While the polymers with electron-accepting terminal moieties (−Br or −NO2) in the pendant azobenzene exhibit write-once, read-many-times (WORM) type memory behavior, those with electron-donating terminal moieties (−OCH3) exihibit rewritable (FLASH) memory behavior. The WORM memory devices have low switching ("write") voltages below −2 V and high ON/OFF current ratios of about 104−106. The polarity of the "write" voltage can be reversed by using an electrode with a higher work function than Al, thus excluding metallic filamentary conduction as a cause of the bistable switching phenomenon. The FLASH memory devices have low "write" and "erase" voltages of about −1.7 to −1.8 V and 2.0 to 2.2 V, respectively, and ON/OFF current ratios of about 103−104. The electrical bistability observed can be attributed to charge trapping at the azobenzene chromophores, resulting in the charge-separated, high-conductivity state. The proposed mechanism is supported experimentally by a red shift and peak broadening in the UV−visible absorption spectra of the polymer films resulting from the OFF-to-ON electrical transition.