Jia-Hao Wu

Highly transparent and flexible polyimide–AgNW hybrid electrodes with excellent thermal stability for electrochromic applications and defogging devices

In this study, highly transparent and flexible electrodes with the highest thermal stability were successfully prepared from silver nanowire (AgNW)–polyimide (PI) hybrid solutions by facile solution casting on insoluble polyimide substrates without any troublesome transferring process. The prepared highly flexible AgNW–PI electrodes exhibit a low resistance of 25 Ω sq−1 and high transmittance up to 86% at a wavelength of 550 nm. Thus, by introducing high performance polyimide as a binder, the obtained colorless AgNW electrodes show improved adhesion properties between the AgNWs and the substrates as well as excellent thermal stabilities and high glass transition temperatures (Tg) above 300 °C. Furthermore, the resulting AgNW–PI hybrid colorless electrodes could maintain the conductivity even after folding more than 1000 times. Thus, these optically transparent AgNW–PI hybrid electrodes have extremely high potential to operate at high temperatures in the working environment or post processing.

Triphenylamine-based luminogens and fluorescent polyimides: effects of functional groups and substituents on photophysical behaviors

We prepared four series of triphenylamine (TPA)-based luminogens with various functional groups (diphthalic imide, tetracarboxylic acid, diphthalic anhydride, and pristine TPA) and substituted groups (–H, –Br, –CHO, and –CN), and corresponding fluorescent polyimides (PIs) were prepared from triarylamine-based dianhydride monomers with various aromatic and aliphatic diamine monomers for the investigation of their photophyscial behaviors. In the solution state, the introduction of strong electron acceptors such as formyl and cyano substituents in the luminogens induced strong emissions due to hybridized local and charge-transfer transitions (HLCTs). However, aggregated molecules containing these pendant electron-accepting groups resulted in quenching fluorescence behavior due to intermolecular interactions and energy transfers. Furthermore, the competition of aggregation-induced enhanced emission (AIEE) and aggregation-caused quenching (ACQ) effects was investigated and is discussed.

Novel thermally stable and soluble triarylamine functionalized polyimides for gas separation

A novel series of solution-processable aromatic polyimide membranes with trimethyl-substituted triphenylamine units for gas separation were prepared from a newly synthesized dianhydride, N,N-bis(3,4-dicarboxyphenyl)-2,4,6-trimethylaniline dianhydride (2), and various diamines via one-step high-temperature solution polymerization. The corresponding polyimides derived from structurally related dianhydrides with different pendant groups such as phenyl, naphthyl, and pyrenyl moieties were also prepared for comparison. All the polyimides were readily soluble in many polar solvents and showed useful levels of thermal stability associated with high glass-transition temperatures (Tg, 324–455 °C) and high char yields (higher than 58% at 800 °C in nitrogen). The noncoplanar triphenylamine-containing polyimides IV with pendant trimethyl-phenyl moieties exhibited an effective improvement of gas permeability with a minor decrease in permselectivity.

Flexible memory devices with tunable electrical bistability via controlled energetics in donor–donor and donor–acceptor conjugated polymers

Flexible nonvolatile memory devices were fabricated from benzodithiophene-based donor–donor and donor–acceptor conjugated polymers with thermally/non-thermally recoverable memory behaviors.

Substituent and Charge Transfer Effects on Memory Behavior of the Ambipolar Poly(triphenylamine)s

A series of poly(triphneylamine)s (CN-PTPA, 2CN-PTPA, 3CN-PTPA, and NO2-PTPA) with pendent acceptors (cyano, dicyanovinyl, tricyanovinyl, and nitro) have been readily synthesized by oxidative coupling polymerization using FeCl3 as oxidant. The tunable memory properties of the ITO/polymer/Al sandwiched memory devices including DRAM, SRAM, and WORM could be achieved by introducing substituent acceptors with different extent of electronic delocalization and electron-withdrawing intensity into the poly(triphenylamine)s. The highly fluorescent CN-PTPA exhibited volatile DRAM memory characteristic due to the large band gap and weak intramolecular charge transfer capability. 2CN-PTPA and 3CN-PTPA showed volatile SRAM memory property with retention time of 5 and 14 min, respectively, depending on electron-withdrawing capability of the acceptors. Furthermore, NO2-PTPA afforded nonvolatile WORM memory behavior attributed to the charge could be trapped into the nonconjugated nitro group even though the dipole moment and electron-withdrawing capability of nitro group were weaker than cyanovinyl groups. Moreover, except NO2-PTPA, all the devices derived from cyano-containing ambipolar polymers including CN-PTPA, 2CN-PTPA, and 3CN-PTPA could be switched to the ON state and exhibited WORM memory behavior in positive unipolar I-V switching. This phenomenon indicated that the Al atoms preferentially interact with poly(triphneylamine)s containing cyano than nitro substituents.

High-efficiency fluorescent polyimides based on locally excited triarylamine-containing dianhydride moieties†

Three novel high fluorescent polyimides (PIs) were readily synthesized from the polycondensation of triarylamine-based tetracarboxylic dianhydride monomers with a commercially available aliphatic diamine monomer. The photoluminescence (PL) intensity of the solid film and the nanofiber fabricated by solution casting and electrospinning methods revealed high quantum yields of up to 32% and 35%, respectively. Furthermore, in order to investigate the fluorescent transition mechanisms of the PIs, a series of model compounds corresponding to the repeat units of PIs were also synthesized, and density functional theory calculation results were also used to support the deduction. Moreover, the competition of aggregation enhanced emission (AEE) and aggregation caused quenching (ACQ) effects was investigated and demonstrated.

High performance polymers and their PCBM hybrids for memory device application

Three series of high ON/OFF ratio (∼109) memory devices were prepared from OHTPA-based high-performance polymers with various amounts of PCBM. The memory behaviors of these devices can be tuned in a wide range from insulator through DRAM, SRAM to WORM by the concentration of the electron-acceptor PCBM.

The steric effect of α- and β-substituted anthraquinone units on high performance polymeric memory devices

The new classes of anthraquinone-substituted triarylamine-based high-performance polymers have been successfully synthesized and fabricated for memory device application. The results show that the conformational change can effectively affect the retention time and memory behavior, providing a strategy for tailoring the memory characteristics through structural modulation.

CHAPTER 4:High Performance Polyimides for Resistive Switching Memory Devices

Polymeric materials for memory device applications have attracted a large amount of interest due to their numerous advantages, such as low cost, high mechanical strength, solution processability, and the ability to develop three-dimensional stacking devices. Considering the heat resistance during the device fabrication process and operation, polyimides are one of the most attractive polymers for memory applications due to their high thermal/dimensional stability and mechanical strength. Recently, a large amount of research has reported that most memory properties from volatile to non-volatile can be produced by optimizing the chemical structure of the polyimides. In this chapter, we summarize the most widely used mechanisms in polyimide resistive memory devices, charge transfer, space charge traps, and filamentary conduction. In addition, recent studies of functional polyimides for memory device applications are reviewed, compared and differentiated based on the mechanisms and structural design methods.