Improved Resistive Switching WORM Memory Behavior in D-π-A Architectures by Modifying the Terminal Donor Units

Abstract

A series of D-π-A, D-π-D, and A-π-A based push–pull compounds with triarylamine and benzophenone were designed and synthesized for nonvolatile memory applications. All of the compounds showed good solubility in common organic solvents, which permits solution processability. D-π-A based compounds exhibited write-once-read-many (WORM) memory applications, and the compound with a methoxyphenyl substituent exhibited switching with a low threshold voltage of −0.82 V, an ON/OFF current ratio of 102, and a long-lasting retention time of 103 s. The effect of differently functionalized triarylamines on memory behavior was explored by optical, electrochemical, and computational studies. The highest HOMO levels of around ∼5.0 eV and irreversible anodic peaks (0.7–1.3 V) obtained for the compounds facilitate charge injection and switching behavior. Besides, electrochemical and density functional theory studies disclose the charge-transfer mechanism of the D-π-A systems, which is related to the bistability of the devices.

Strategical design and synthesis of D–A–D-based quinolines for improved WORM memory performance

“…Observing the trend in the on/off ratios, it is noticed that the compounds with electron-withdrawing substituents at the triarylamines have lower on/off ratios and higher threshold voltages compared to the ones with electron-donating substituents. 29 This is true compared to the observations made in the earlier work suggesting equal strength of donors and acceptors to be incorporated in the molecules for having a balanced charge transfer leading to better memory performance. 29 The electron-donating tert -butylphenyl groups and triarylamine with no substituents gave the maximum on/off ratio of 10 4 and a threshold voltage as low as −1.10 and −1.25 V, respectively.…”

“…29 This is true compared to the observations made in the earlier work suggesting equal strength of donors and acceptors to be incorporated in the molecules for having a balanced charge transfer leading to better memory performance. 29 The electron-donating tert -butylphenyl groups and triarylamine with no substituents gave the maximum on/off ratio of 10 4 and a threshold voltage as low as −1.10 and −1.25 V, respectively. The electron-donating substituents formulate the charge conduction pathway much easier contributing to the maximum ICT and thereby formulating a smooth memory operation.…”

Tailoring the Resistive Switching WORM Memory Behavior of Functionalized Bis(triphenylamine)

“…These features point to an easy charge transfer from bis-TPA (HOMO) to terminal acceptors (LUMO), forming charge-transfer complexes. Subsequently, after the charge transfer process, the formed holes may generate a pathway that aids in increasing the current abruptly, and the device switches from the OFF to the ON state. ,,, …”

“…Subsequently, after the charge transfer process, the formed holes may generate a pathway that aids in increasing the current abruptly, and the device switches from the OFF to the ON state. 10,11,21,38 In contrast, compounds 5b and 5c have orbitals evenly distributed throughout the molecules, with the HOMO and LUMO having orbitals over the bis-TPA part, which appears detrimental to the charge transfer process. Compound 5a, on the other hand, shows a charge separation in the HOMO− LUMO energy levels but fails to display resistive switching property due to its higher band gap (Figure S25).…”

“…This sharp electrical transition from a high-resistance state (HRS) to a low-resistance state (LRS) can be considered the writing process in digital memory applications. 38 The devices could retain their ON state for a successive negative scan from 0 to −3 V (sweep 2) and even after applying a positive voltage sweep from 0 to 3 V (sweep 3), corresponding to the reading process. Thus, the devices demonstrated a nonvolatile binary WORM memory behavior with ON/OFF ratios ranging from 10 1 to 10 4 .…”