Chien-Chung Shih

Nanostructured materials for non-volatile organic transistor memory applications

Over the past decades, the demand for organic memory has rapidly increased due to the development of flexible electronics. In this article, state-of-the-art nanostructured materials-based organic memory is reviewed, including nanoparticles, carbon nanotubes, graphene and nanowires. The operation mechanism and the impact of nanostructured materials on the electrical performance are discussed. Moreover, proposed charge-trapping mechanisms using nano-materials are reviewed to provide fundamental understanding as well as further performance improvement.

High Performance Transparent Transistor Memory Devices Using Nano-Floating Gate of Polymer/ZnO Nanocomposites.

Nano-floating gate memory devices (NFGM) using metal nanoparticles (NPs) covered with an insulating polymer have been considered as a promising electronic device for the next-generation nonvolatile organic memory applications NPs. However, the transparency of the device with metal NPs is restricted to 60~70% due to the light absorption in the visible region caused by the surface plasmon resonance effects of metal NPs. To address this issue, we demonstrate a novel NFGM using the blends of hole-trapping poly (9-(4-vinylphenyl) carbazole) (PVPK) and electron-trapping ZnO NPs as the charge storage element. The memory devices exhibited a remarkably programmable memory window up to 60 V during the program/erase operations, which was attributed to the trapping/detrapping of charge carriers in ZnO NPs/PVPK composite. Furthermore, the devices showed the long-term retention time (>105 s) and WRER test (>200 cycles), indicating excellent electrical reliability and stability. Additionally, the fabricated transistor memory devices exhibited a relatively high transparency of 90% at the wavelength of 500 nm based on the spray-coated PEDOT:PSS as electrode, suggesting high potential for transparent organic electronic memory devices.

Nonvolatile memories using the electrets of conjugated rod-coil block copolymer and its nanocomposite with single wall carbon nanotubes

We report high performance pentacene based organic field-effect transistor (OFET) memory devices using the electrets of conjugated rod-coil block copolymers, poly[2,7-(9,9-dihexylfluorene)]-block-poly(stearyl acrylate) (PF-b-PSA) and their nanocomposites with single-wall carbon nanotubes (SWCNT). The self-assembled PF-b-PSA electret, with the PF nanorods covered by the crystalline PSA block, exhibited a distinct hole-trapping capability due to the high electrical field generated in the confined dimension of the nanorods. Thus, it could effectively reduce the current leakage and stabilize data retention with a large memory window (35.8 V) and a high ON/OFF ratio over 104 s. Furthermore, the memory window of the device was further improved to 49.2 V by wrapping well-dispersed single-wall carbon nanotubes (SWCNT) in PF-b-PSA. The bundles of PF nanorods along the SWCNT effectively capture electrons and maintain retention characteristics similar to that of the PF-b-PSA device. This study demonstrated that the self-assembled conjugated rod-coil block copolymers and their nanocomposites could act as charge-storage electrets for high performance OFET memory devices through the precise morphology control.

Stretchable Polymer Dielectrics for Low-Voltage-Driven Field-Effect Transistors

A stretchable and mechanical robust field-effect transistor is essential for soft wearable electronics. To realize stretchable transistors, elastic dielectrics with small current hysteresis, high elasticity, and high dielectric constants are the critical factor for low-voltage-driven devices. Here, we demonstrate the polar elastomer consisting of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP):poly(4-vinylphenol) (PVP). Owing to the high dielectric constant of PVDF-HFP, the device can be operated under less than 5 V and shows a linear-regime hole mobility as high as 0.199 cm2 V-1 s-1 without significant current hysteresis. Specifically, the PVDF-HFP:PVP blends induce the vertical phase separation and significantly reduce current leakage and reduce the crystallization of PVDF segments, which can contribute current hysteresis in the OFET characteristics. All-stretchable OFETs based on these PVDF-HFP:PVP dielectrics were fabricated. The device can still keep the hole mobility of approximately 0.1 cm2/(V s) under a low operation voltage of 3 V even as stretched with 80% strain. Finally, we successfully fabricate a low-voltage-driven stretchable transistor. The low voltage operating under strains is the desirable characteristics for soft and comfortable wearable electronics.

A star polymer with a metallo-phthalocyanine core as a tunable charge storage material for nonvolatile transistor memory devices

We present a strategy for controlling the device performance of organic field effect transistor (OFET) memory devices by using a metallo-phthalocyanine (MPc)-cored star-shaped polystyrene as a charge storage material. MPc-cored four-armed star polymers (M = Cu or Zn) with polystyrene arms of three different number-average molecular weights were prepared by atom transfer radical polymerization and cyclization reactions with metal ions; the density of the MPc cores dispersed in the polymer matrix was dependent on the lengths of polymer arms. The charge carrier mobility of pentacene-based OFET memory devices containing the star polymer varied with the nature of the MPc-cored star polymer layer owing to the presence of the MPc core unit as well as the nanostructures of the polymer thin films. Application of an external gate bias to the OFET device caused significant reversible shifts in the threshold voltage, and the magnitude of the memory shifts was proportional to the weight percentage of MPc cores in the star polymer matrix. The memory device showed a high memory on/off current ratio (>105) and long retention characteristics (>105 s), permitting it to be characterized as a nonvolatile organic memory device; the retention time extended upon increasing the Mn of the star polymer. The MPc-cored star polymer is a promising material designed as a charge storage layer for controlling the performance of organic flash memory devices.

Bio-Based Transparent Conductive Film Consisting of Polyethylene Furanoate and Silver Nanowires for Flexible Optoelectronic Devices

Exploiting biomass has raised great interest as an alternative to the fossil resources for environmental protection. In this respect, polyethylene furanoate (PEF), one of the bio-based polyesters, thus reveals a great potential to replace the commonly used polyethylene terephthalate (PET) on account of its better mechanical, gas barrier, and thermal properties. Herein, a bio-based, flexible, conductive film is successfully developed by coupling a PEF plastic substrate with silver nanowires (Ag NWs). Besides the appealing advantage of renewable biomass, PEF also exhibits a good transparency around 90% in the visible wavelength range, and its constituent polar furan moiety is revealed to enable an intense interaction with Ag NWs to largely enhance the adhesion of Ag NWs grown above, as exemplified by the superior bending and peeling durability than the currently prevailing PET substrate. Finally, the efficiency of conductive PEF/Ag NWs film in fabricating efficient flexible organic thin-film transistor and organic photovoltaic (OPV) is demonstrated. The OPV device achieves a power conversion efficiency of 6.7%, which is superior to the device based on ITO/PEN device, manifesting the promising merit of the bio-based PEF for flexible electronic applications.

High-performance ternary polymer solar cells using wide-bandgap biaxially extended octithiophene-based conjugated polymers

Ternary organic photovoltaics (OPVs) have recently attracted intense research attention since they have been proven as an effective approach to enhance device performance. We herein describe a new strategy to realize high-performance ternary OPVs by using biaxially extended octithiophene (8T)-based wide-bandgap (Eg) conjugated polymers as the third photoactive component. Owing to the π–π transition of the biaxially extended conjugated side-chains, such polymers exhibit intense absorption in the near-ultraviolet region, in addition to the original intra-charge transfer (ICT) feature arising from the main backbone, revealing a new molecular design for wide-Eg polymers. By further tailoring the polymer backbone with p-type moieties such as thiophene (T) or thienothiphene (TT), two wide-Eg (∼2.0 eV) polymers, P8TT and P8TTT, with absorption wavelengths below 650 nm, were prepared, showing high complementary absorption to the spectra of both the state-of-the-art fullerene-(PTB7-Th:PC71BM) and non-fullerene-based (PBDB-T:ITIC) bulk-heterojunction (BHJ) systems. By providing suitable energy levels, P8TTT was demonstrated to enhance the power conversion efficiency (PCE) of its derived fullerene- and non-fullerene-based ternary blends by 7.58% and 6.60%, respectively, with only a small loading amount (10 wt%). This study manifests a new perspective in wide-Eg material design for realizing efficient ternary BHJ systems.

Multi-state memristive behavior in a light-emitting electrochemical cell

Carbohydrate-based block copolymers, such as maltoheptaose-block-polyisoprene (MHPI), were successfully employed as polyelectrolytes in a light-emitting electrochemical cell (LEC) to realize organic multi-state electrical memory. The observation of distinctive multi-state memristive behavior could be attributed to the controlled ion motion in MHPI and the electron capture capabilities introduced by its constituent hydroxyl groups. The newly developed sandwich-structured memory device is based on a LEC platform as the light-emitting electrochemical cell memory (LECM). Furthermore, the fabricated LECM exhibited excellent ternary-state memory behavior with high ON2/ON1/OFF current ratios of 106/105/1 as well as excellent stabilities for each respective state. The study not only illustrates the use of carbohydrate-based materials as promising electrolytes for LECM applications, but also provides a new horizon in the development of organic multi-state electrical memory devices.

Influence of polymeric electrets on the performance of derived hybrid perovskite-based photo-memory devices

Organic-inorganic hybrid perovskite has become one of the most important photoactive materials owing to its intense light-harvesting property as well as its facile solution processability. Besides its photovoltaic applications, a novel photo-programmed transistor memory was recently developed based on the device architecture of a floating-gate transistor memory using a polymer/perovskite blend as the gate dielectric with the non-volatile memory characteristics of decent light response, applicable On/Off current ratio, and long retention time. In this study, we further clarify the influence of polymer matrix selection on the photo-response and memory properties of derived hybrid perovskite-based photo-memory devices. Four different host polymers, polystyrene (PS), poly(4-vinylphenol) (PVPh), poly(methyl methacrylate) (PMMA), and poly(methacrylic acid) (PMAA), were systematically investigated for comparison herein. This revealed that dissimilar chemical interactions existed between the host polymers and perovskite, resulting in the distinct memory behavior of the derived photo-memory devices, attributable to the different morphologies of the hybrid dielectric layers and the different sizes of the distributed perovskite nanoparticles (NPs). The photo-response behavior and the resultant On/Off current ratio increased as the size of the embedded perovskite NPs decreased, due to the enhanced photo-induced charge transfer across the dielectric/pentacene interface, benefiting from the better confinement effect of perovskite NPs in the polymer matrix. These results demonstrate the influence of perovskite NP aggregation at the dielectric/pentacene interface on the resultant memory behavior of the newly developed photo-memory device.

Advances and Challenges of Green Materials for Electronic and Energy Storage Applications: From Design to End-of-Life Recovery

Harnessing biomass to fabricate electronic devices has lately drawn significant research attention because it not only represents a promising strategy for making materials but is also beneficial for the sustainable development of technologies. Numerous recent studies have demonstrated that green materials are promising candidates for synthesizing high-performance materials, such as biopolymers and hierarchical porous carbons. To catch up with this emerging tide, we have here compiled a comprehensive overview of state-of-the-art green materials, with a specific emphasis on recent progress in biodegradable polymeric materials and biomass-based carbon materials, together with their electronics and energy storage applications. Besides, we also assess the performance of end-of-life electronics recycling to highlight the merits of integrating green design with resource recovery.