Hung-Ju Yen

Solution-processable triarylamine-based electroactive high performance polymers for anodically electrochromic applications

This review focused on high-performance polymers with anodically electrochromic triarylamine units built into main chains or as pendants. The main aim of the review is to present the most important and common trends regarding polymer synthesis from the point of view of the introduction of triarylamine moiety into polymer chains and to demonstrate how the polymer structures influence their properties, which leads them to be suitable for optoelectronic device construction. This review also covers a majority of the works in the recent decade involving the synthesis and characteristic evaluation of the resulting electrochromic polymers as well as their structural design by using the respective monomers.

Highly flexible and optical transparent 6F-PI/TiO2 optical hybrid films with tunable refractive index and excellent thermal stability

In this study, a synthesis route was developed for preparing highly transparent novel polyimide-nanocrystalline-titania hybrid optical films with a relatively high titania content (up to 50 wt%) and thickness (20–30 μm) from a soluble polyimide 6F-poly(p-hydroxy-imide) (6FPI). It has been demonstrated that the introduction of bulky CF3 groups into polyimide backbones enhanced both solubility and optical transparency. Furthermore, the attachment of hydroxyl groups provided the organic–inorganic bonding and resulted in homogeneous hybrid solutions by controlling the mole ratio of titanium butoxide : hydroxyl groups. AFM, SEM, TEM, and XRD results indicated the formation of well-dispersed nanocrystalline-titania. The flexible hybrid films revealed excellent optical transparency in the visible range, good surface planarity, high thermally dimensional stability, and tunable refractive index. A three-layer anti-reflection coating based on the hybrid films was prepared and revealed a reflectance of less than 0.7% in the visible range, indicating its potential optical applications.

Novel triphenylamine-containing ambipolar polyimides with pendant anthraquinone moiety for polymeric memory device, electrochromic and gas separation applications

Novel electron-donating triphenylamine (TPA)-containing electroactive functional polyimides with electron-withdrawing pendant anthraquinone moiety were designed and prepared for memory devices, electrochromic and gas separation applications. These high-performance polymers exhibited two conductivity states and could be swept negatively with a high ON/OFF current ratio of 109. The ON state of OAQ-6FPI polyimide remained around 8 min after removing the applied voltage, while the AQ-6FPI polyimide quickly returned to the OFF state during the backward sweep in a dual sweep process, implying that devices based on OAQ-6FPI and AQ-6FPI reveal static random access memory (SRAM) and dynamic random access memory (DRAM) behaviors resulting from the isolated donor–acceptor (D–A) and non-isolated D–A structures, respectively. Meanwhile, the polymer films exhibited electrochromic characteristics with a color change from neutral colorless or pale yellowish to blue and green at applied potentials, ranging from 0.00 to 1.40 V. Furthermore, the presence of a bulky and carbonyl-containing anthraquinone unit could enhance effectively CO2 permeability coefficients (PCO2) and permeability selectivity (PCO2/PCH4) for these polyimide membranes in the range of 21.6–69.9 and 28.8–31.7, respectively, and permeability increased with higher efficiency by directly attaching the bulky group to the backbone than incorporating via a spacer.

Programmable digital nonvolatile memory behaviors of donor-acceptor polyimides bearing triphenylamine derivatives: Effects of substituents

Two aromatic polyimides bearing triphenylamine (TPA) derivatives with reasonably high molecular weights were synthesized: poly(N-(2,4,6-trimethylphenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA-Me3 PI) and poly(N-(4-dimethylaminophenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA-NMe2 PI). These polymers were thermally and dimensionally very stable, providing high-quality nanoscale thin films using a conventional solution coating process. The film densities, optical properties, and electrochemical properties were determined. The polymers displayed a different nonvolatile memory behavior that depended on the substituents of the TPA unit. The 6F-TPA-Me3 PI film showed a unipolar write-once-read-many-times (WORM) memory behavior, whereas the 6F-TPA-NMe2 PI film revealed unipolar and bipolar switching memory behavior. All PI films displayed excellent retention in both the OFF- and ON-states, even under ambient conditions. The ON/OFF current ratio was high, up to 108–109. All memory behaviors were governed by a mechanism that involved trap-limited space charge limited conduction and local filament formation. The memory characteristics may originate from the electron-donating TPA and substituents and from the electron-accepting hexafluoroisopropylidenyl and imide units in the polymer backbone, which acted as effective charge-trapping sites. The film density was found to significantly influence the memory behavior. This study demonstrated that the thermally and dimensionally stable 6F-TPA-Me3 and 6F-TPA-NMe2 PIs are suitable active materials for the low-cost mass production of high-performance programmable memory devices that can be operated with very low power consumption. Moreover, the memory mode and its polarity may be tuned by changing the substituent on the TPA unit.

Novel programmable functional polyimides: preparation, mechanism of CT induced memory, and ambipolar electrochromic behavior

In this study, the new functional polyimides 9Ph-ODPI, 9Ph-DSPI, and 9Ph-PMPI consisting of electron-donating starburst triarylamine units and different dianhydrides were synthesized and used for memory device applications along with 9Ph-6FPI. To investigate the effects of donor moieties in polyimides on their memory behavior, the corresponding 3Ph- series polyimides (3Ph-PIs) and 5Ph-PIs were also discussed. With the intensity of electron donation increasing from 3Ph-PIs to 5Ph-PIs to 9Ph-PIs, the retention time of the memory device shows a systematic increase. The retention time of the memory device also increases with the increasing electron-withdrawing intensity of the dianhydrides. Furthermore, the in situ UV-vis absorption spectra of the memory devices during switching-ON were utilized as direct evidence to confirm the relationship between charge transfer (CT) complex stability and memory retention time. In electrochromic (EC) applications, the novel ambipolar 9Ph-PMPI exhibited a high contrast ratio and electroactive stability during EC operation with multiple colors at different applied potentials, due to its four oxidation and two reduction states. Finally, flexible colorful EC and volatile memory devices were fabricated for practical flexible electronics applications in the future.

Transmissive to black electrochromic aramids with high near-infrared and multicolor electrochromism based on electroactive tetraphenylbenzidine units

A series of near-infrared (NIR) electrochromic aromatic polyamides with tetraphenylbenzidine (TPB) units were prepared by phosphorylation polyamidation from a newly synthesized aromatic diamine monomer, N,N′-bis(4-aminophenyl)-N,N′-di(4-methoxyphenyl)-4,4′-biphenyldiamine, and various dicarboxylic acids. These polymers were highly soluble in many organic solvents and showed useful levels of thermal stability associated with high glass-transition temperatures and char yields. These anodically polymeric electrochromic materials showed reversible electrochemistry and electrochromism with high contrast ratio both in the visible range and NIR regions. Notably, copolymerization of TPB-based and TPPA-based aramids (TPPA: tetraphenyl-p-phenylenediamine) showed a good combination of the individual homopolymers in electrochemical and electrochromic properties, producing a transmissive-to-black electrochromic polymer with high NIR and multicolor electrochromism.

Enhanced near-infrared electrochromism in triphenylamine-based aramids bearing phenothiazine redox centers

A series of organosoluble polyamides based on N-phenothiazinylphenyl redox units showing anodically electrochromic characteristics both in the near-infrared (NIR) and visible light regions were prepared from the phosphorylation polyamidation reactions of a newly synthesized diamine monomer, 4,4′-diamino-4′′-N-phenothiazinyltriphenylamine, with various dicarboxylic acids. These polymers were readily soluble in many polar solvents and showed useful levels of thermal stability associated with relatively high glass-transition temperatures (Tg) (255–277 °C) and high char yields (higher than 67% at 800 °C in nitrogen). In addition, the polymer films showed reversible electrochemical oxidation with enhanced NIR contrast, high coloration efficiency (CE), low switching time, and anodic green/purple electrochromic behaviors.

Nonvolatile transistor memory devices using high dielectric constant polyimide electrets

We report on the nonvolatile memory characteristics of pentacene-based organic field-effect transistors (OFETs) using polyimides, PI(6FDA-TPA-CN), PI(DSDA-TPA-CN), and PI(BTDA-TPA-CN), consisting of electron-donating 4,4′-diamino-4′′-cyanotriphenylamine (TPA-CN) and different electron-accepting dianhydrides as polymer electrets. The dielectric constants of PI(BTDA-TPA-CN), PI(DSDA-TPA-CN), and PI(6FDA-TPA-CN) are 3.44, 3.52, and 3.70, respectively, higher than those (∼3) of common polyimides. Among the polymer electrets, the OFET memory device based on PI(6FDA-TPA-CN) exhibits the highest OFET mobility of 0.5 cm2 V−1 s−1 due to the formation of a pentacene film of large grain size by the hydrophobic surface. The OFET memory devices with the configuration of n+Si/SiO2/PI/pentacene/Au show excellent nonvolatile memory behaviors for bistable switching. The stability for ON and OFF states can be maintained for 104 s with a Ion/Ioff current ratio of 104 for PI(6FDA-TPA-CN). Moreover, the higher dipole moment and larger torsion angle result in the more stable charge transfer complex, accompanied by the largest memory window of 84 V for the fabricated device. The write–read–erase–read (WRER) cycles can be operated over 100 cycles. The present study suggests that the high dielectric constant polyimide electrets with the enhanced capabilities for storing the charges have great potential applications for advanced OFET memory devices.

Flexible, optically transparent, high refractive, and thermally stable polyimide–TiO2 hybrids for anti-reflection coating

In this study, flexible and transparent polyimide–nanocrystalline-titania hybrid optical films with a high titania content (up to 50 wt%) and thickness (15 μm) were prepared from new soluble poly(o-hydroxy-imide)s and fabricated for a three-layer antireflection coating. The hydroxyl groups on the backbone of the polyimides could provide a reaction site for organic–inorganic bonding and resulted in homogeneous hybrid solutions by controlling the mole ratio of titanium butoxide/hydroxyl groups. The well-dispersed nanocrystalline-titania was demonstrated by XRD and TEM measurements. The flexible hybrid films with good surface planarity, thermal dimensional stability, tunable refractive index (1.63–1.80 at 633 nm), and high optical transparency were successfully obtained and further utilized to prepare a three-layer anti-reflection coating, indicating its potential optical applications.

A facile approach towards optically isotropic, colorless, and thermoplastic polyimidothioethers with high refractive index

A series of novel optically isotropic, transparent, and thermoplastic polyimidothioethers (PITEs) with high refractive index were prepared via Michael polyaddition from commercially available bismaleimides and dithiols. These polymers were readily soluble in various organic solvents and showed useful thermal stability associated with high glass-transition temperatures. In addition to the useful thermal processing window up to 190 °C, these PITEs also exhibit a high refractive index and ultra-low birefringence of 1.6763 and 0.0002, respectively.