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

Abstract

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 electronwithdrawing 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 × 10 13 cm −2 ), and higher programming speed (10 14 −10 11 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 (>10 4 s) and large on/off ratio (∼10 4 ) 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. ■ INTRODUCTIONOrganic memory devices have attracted extensive attention for the future information storage toward the era of big data and flexible electronics on account of their good scalability, light weight, and low-cost fabrication process. 1−4 The nonvolatile organic field-effect transistor (OFET) memory is one of the most striking devices with nondestructive read-out and multibit storage property, whose conductivity can be easily manipulated by tuning the charge behavior between the channels. 5−7 Furthermore, the open channel of FET configuration facilitates the charge tuning through the external stimulation such as light irradiation and gas or biomaterial exposure, leading to remarkable smart responsive behavior. 8−11 The thorny challenge in OFET memory is the design of trapping element and getting deep insight into the trapping mechanism with high density and long-term charge maintenance.The molecular charge storage elements (MCSEs) have great superiority in charge density over the well-investigated nanofloating gate 12−15 with a designable molecular structure, flexible synthesis, and tunable bandgap. 16,17 However, the memory performance is dominated not only by the F−N (Fowler−Nordheim) tunneling barrier between the transport layer and MCSEs but also by several other factors. 18−20 Because of the polymorphic behavior in the condensed state, the polymer-based MCSEs are ambiguous in charge trapping mechanism; meanwhile, they have several drawbacks such as multistep synthesis and uncontrollable molecular we...

Excellent Charge‐Storage Properties of Polystyrene/SFXs Electret Films by Repeated Contact with an AFM Probe

“…The PS/SFXs electret films, including PS/SFX, PS/CNSFX, PS/DNSFX, PS/TCNSFX, have been used as charge storage elements in transistors memory . They are prepared with the same method as in our previous papers . PS and SFXs with a molar ratio of SFXs/PS = 0.1 are mixed in toluene (5 mg ml −1 ).…”

“…Thus, AFM methods for the contact electrification attract more and more attention because they can measure charge distributions at the nanoscale with a higher lateral resolution . Recently, the effects of the atmospheric moisture, tip biases, contact time, and temperature on the trapped charges of the films have been explored by electrostatic/Kelvin probe force microscopy (EFM/KPFM) . For example, Cunningham reported the dynamics of microscopic patches of enhanced surface potential when the AFM tip contacts the insulated surface.…”

9,10‐Imide‐Pyrene‐Fused Pyrazaacenes (IPPA) as N‐Type Doping Materials for High‐Performance Nonvolatile Organic Field Effect Transistor Memory Devices

“…For instance, the PPA-NO 2 and IPPA-NO 2 have very close LUMO levels, but the MW of PPA-NO 2 is much smaller than IPPA-NO 2 . [10,31] Same reason can also explain the fact that PPA-Cl based devices showed much smaller MW than that of the IPPA-Cl based devices, as the PPA-Cl have larger dipole moment than IPPA-Cl. As shown in Figure 4, PPA-NO 2 has larger dipole moment than that of IPPA-NO 2 .…”

“…Electron. [10,12] Herein the IPPA-based device showed larger MW, suggesting that n-type doping can be a more efficient way to achieve high memory performance. 2019, 5, 1800598 www.advelectronicmat.de (Supporting Information).…”

“…[10,31] Large dipole moment may cause rapid dispassion of the trapped charges in electret layer and reduce the memory performance. [10,31] Large dipole moment may cause rapid dispassion of the trapped charges in electret layer and reduce the memory performance.…”