Effects of the acceptor conjugation length and composition on the electrical memory characteristics of random copolyimides

Resistive-switching memories based on copolyimides (coPIs), PI-NTCDIX and PI-BTCDIX, with different compositions of 4,4-diamino-4-methyltriphenylamine (AMTPA), 4,4-(hexafluoroisopropylidene)diphthalic anhydride, and N,N-bis-(4-aminophenyl)-1,8:4,5-naphthalenetetracarboxydiimide (NTCDI) or N,N-bis-(4-aminophenyl)-1,2:4,5-benzenetetracarboxydiimide (BTCDI) have been developed. By varying the feed ratio of monomers, PI-NTCDIX and PI-BTCDIX showed tunable optical and electronic properties through the charge transfer (CT) between AMTPA and NTCDI or BTCDI. The memory devices based on PI-NTCDIX exhibited the tunable electrical bistability from the volatile dynamic random access memory to nonvolatile write once read many memory characteristics as the NTCDI composition increased. The OFF/ON electrical switching transition was mainly attributed to the CT mechanism for the charge separated high conductance, based on the analysis of model compounds and density functional theory calculation. Also, the volatility of the memory device depended on the stability of CT complex. The long conjugation and high electron affinity of the NTCDI moiety stabilized the radical anion generated in the CT complex and prevented the recombination of segregated radical species even through applying the high positive or negative voltage. On the other hand, the memory devices based on PI-BTCDIX showed a rather unique behavior compared with those based on PI-NTCDIX. At the low BTCDI composition, the device exhibited volatile memory property. However, no switching behavior was observed at the high BTCDI composition due to the low highest occupied molecular orbital energy level of BTCDI. Combining these results and our previous study on perylenebisimide (PBI), we concluded that memory characteristics could be tailored by changing the conjugation length (PBI > NTCDI > BTCDI) and the acceptor composition in random coPIs. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013