Dipole Moment Effect of Cyano-Substituted Spirofluorenes on Charge Storage for Organic Transistor Memory

As a fascinating information storage device, organic transistor memory based on molecular charge storage elements (MCSEs) has attracted great research interest. However, the charge storage mechanism of MCSEs is ambiguous due to their complex charge dynamic behaviors. Herein, the dipole moment effects on the charge trapping process and the performance of transistor memory are revealed based on cruciform spiro[fluorene-9,9′-xanthene] (SFXs), incorporating cyano moieties, as the typical electron-withdrawing substitution. The characterization of electrostatic potential (ESP) calculation, UV-vis, photoluminescence, and crystallography of SFXs shows the SFXs MCSEs with weaker dipole moment through symmetrical substitution. A series of prototype transistor memories based on SFXs exhibit an erasable type feature with smart photoresponsive behavior. The weaker dipole moment ones possess larger memory window (∼40 V), higher charge trapping density (>1 × 10¹³ cm^-2), and higher programming speed (10¹⁴-10¹¹ cm^-2 s^-1). The hole trapping process is dominated by the dipole moment rather than the charge dissipation when compared with different SFXs at the same HOMO level. Rather good charge retention property (>10⁴ s) and large on/off ratio (∼10⁴) are obtained by blending SFXs with polymer dielectrics in optimized devices. The dipole moment effects on the charge trapping behavior provide not only the design of high performance transistor memory but also the smart information encryption in future data storage.