Wen-Ya Lee

Selenophene-DPP donor–acceptor conjugated polymer for high performance ambipolar field effect transistor and nonvolatile memory applications

New donor–acceptor conjugated copolymer, PSeDPP, consisting of selenophene (Se) and 3,6-dithiophen-2-yl-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP), was developed for high performance ambipolar field-effect transistors (FETs) and nonvolatile memory applications. The maximum absorption peak and optical band gap of PSeDPP thin film were observed at 849 nm and 1.29 eV, respectively, indicating the strong intramolecular and intermolecular charge transfer. The polymer crystallinity and FET mobility was significantly enhanced as increasing the solvent boiling point and thermal treatment based on the results from four processing solvents of chloroform, chlorobenzene, o-dichlorobenzene and 1,2,4-trichlorobenzene, as evidenced by TEM, XRD, and AFM. The PSeDPP-based FET processed from1,2,4-trichlorobenzene exhibited a dense nanofiber morphology with lamellar chain packing, leading to the relatively high hole and electron mobility up to 1.62 and 0.14 cm2 V−1 s−1, respectively. PSeDPP was also found to exhibit the first transistor memory characteristics for ambipolar conjugated polymers. The retention time of the FET-based nonvolatile memory devices could maintain the high- and low-conductance states longer than 104 s, and the on-off current ratios of 103–104 at the read voltage of 0 V. These results revealed that PSeDPP had potential applications for flexible electronic device applications.

Nonvolatile memory based on pentacene organic field-effect transistors with polystyrenepara-substituted oligofluorene pendent moieties as polymer electrets

We report the nonvolatile memory characteristics of pentacene-based organic field-effect transistors (OFET) using polystyrenepara-substituted with π-conjugated oligofluorenes (P(St-Fl)n (n = 1–3)) as chargeable polymer electrets. Effects of fluorene conjugated length on the surface structure and memory characteristics of pentacene OFET were investigated. Among these polymer electrets, the device with the P(St-Fl) exhibited the highest field-effect mobility of 0.47 cm2 V−1 s−1 due to the largest grain size of pentacene growth. The device with P(St-Fl)3 revealed the largest hysteresis window of 76 V due to it having the longest fluorene conjugation length among the studied electrets. The smallest difference of the HOMO energy level between pentacene and P(St-Fl)3 facilitated the charge transfer from pentacene to the polymer electret. The shifts on the transfer curves in both positive and negative directions could be reversibly controlled when applied an external gate bias of ±100 V for a short time (1 μs), indicating the fast trapping-detrapping ability of the polymer electrets. The devices showed excellent nonvolatile behaviors for bistable switching. The ON and OFF states were maintained over 104 s with the Ion/Ioff current ratio of 105–106. The write-read-erase-read (WRER) cycles could be operated over 100 cycles. This study suggested that surface characteristics, charge transport, and memory characteristics of pentacene-based OFET can be manipulated by polymer electrets with different pendent conjugation length.

A poly(fluorene-thiophene) donor with a tethered phenanthro[9,10-d]imidazole acceptor for flexible nonvolatile flash resistive memory devices.

A conjugated poly(fluorene-thiophene) donor and a tethered phenanthro[9,10-d]imidazole acceptor (PFT-PI) was used as the active layer in flexible nonvolatile resistor memory devices with low threshold voltages (±2 V), low switching powers (∼100 μW cm(-2)), large ON/OFF memory windows (10(4)), good retention (>10(4) s) and excellent endurance against electric and mechanical stimulus.

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.

Electrically bistable memory devices based on all-conjugated block copolythiophenes and their PCBM composite films

We explore the memory device characteristics of the all-conjugated diblock copolythiophenes, poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), and their blends with PCBM. The field-effect transistors prepared from P3HT-b-P3PT showed a significant hysteresis between the forward and backward gate-bias scans in the transfer curve, indicating the occurrence of charge trapping. The charge trapping may have occurred within the amorphous P3PT domains dispersed in the block copolythiophene by preventing charge transport. P3HT525252-bbb-P3PT393939 and P3HT102102-bb-P3PT3737 exhibited dynamic random access memory (DRAM) behavior in the sandwich configuration of ITO/P3HT-b-P3PT/Al, whereas P3HT only showed semiconductor characteristics, suggesting the significant effect of the amorphous P3PT segments on the electrical switching behavior. By blending a small amount (5–10 wt%) of PCBM into P3HT-b-P3PT of different block ratios (P3HT525252-bbb-P3PT393939, P3HT102102-bb-P3PT3737, and P3HT8989-bb-P3PT2323), the memory devices showed a write-once-read-many times (WORM) behavior with the switching voltages of −2.6 to −3.3 V and high ON/OFF ratios (105 to 107). The mechanism associated with the memory characteristics was the charge transfer from the P3HT-b-P3PT donor to the PCBM acceptor, which stabilized the charge separated state and retained the high conductance state for a long time during the ON stage. These experimental results provide a new strategy of designing all-conjugated block copolymers for advanced memory device applications.

Biaxially extended quaterthiophene-thiophene and -selenophene conjugated polymers for optoelectronic device applications

New biaxially extended quaterthiophene (4T) conjugated polymers, including poly(5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′4′′,2′′′]quaterthiophene) (P4T) and their copolymers with thiophene(P4TT), bithiophene (P4T2T), selenophene(P4TSe) and biselenophene (P4T2Se) were synthesized by Stille coupling reactions under microwave heating. The effects of the ring number of thiophene and selenophene moieties on the physical properties and polymer structures were systematically investigated experimentally and theoretically. With the increased ring number of the unsubstituted thiophene and selenophene moieties, the band gaps and the main-chain torsional angles were reduced. However, the side-chain torsional angles were increased with increasing the ring number, and thus significantly affected the carrier transporting characteristics. Among these studied conjugated polymers, the field-effect transistor (FET) based on P4TSe showed the highest hole mobility of up to 4.28 × 10−2 cm2 V−1 s−1 and an on/off ratio of 1.12 × 104. The photovoltaic device prepared from P4TSe/PC71BM exhibited the highest power conversion efficiency (PCE) of 2.6%, which resulted from more balanced hole/electron mobility and a smaller band gap. The above results revealed that the conformation, charge-transporting and optoelectronic device characteristics of biaxially extended 4T-based conjugated copolymers could be manipulated by incorporating the heteroaromatic ring spacer.

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.

Improving the characteristics of an organic nano floating gate memory by a self-assembled monolayer.

We demonstrate a novel approach to improve the characteristics of the gold nanoparticle-based organic transistor memory devices by using self-assembled monolayers (SAM) with different functional groups as interfacial modifier. SAM-based interfacial engineering significantly improved the hysteresis, memory window, and on/off ratio of a nano floating gate memory (NFGM) at zero gate voltage. This NFGM showed a large memory window of up to 190 V and on/off current ratio of 10(5) during writing and erasing with an operation voltage of 100 V of gate bias in a short time, less than 1 s. Furthermore, the devices show excellent nonvolatile behavior for bistable switching. The ON and OFF state can be stably maintained for 10(3) s with an I(on)/I(off) current ratio of 10(6) for a pentafluorophenyl trimethoxysilane modified device. The results suggested the importance of SAM-modified interface for the memory performance of NFGMs.

Morphology and field-effect transistor characteristics of semicrystalline poly(3-hexylthiophene) and poly(stearyl acrylate) blend nanowires

The morphology and charge transport characteristics of semicrystalline poly(3-hexylthiophene) (P3HT) and semicrystalline poly(stearyl acrylate) (PSA) or amorphous poly(methyl methacrylate) (PMMA) with various blending ratios were systematically investigated using different solvents. The P3HT–PSA films prepared from CH2Cl2 formed well-defined P3HT nanowires with an average diameter of 30 nm, which was larger than that (∼14 nm) of the P3HT–PMMA films, as evidenced by TEM and AFM. The P3HT–PSA nanowire based field effect transistors (FET) could achieve a high hole mobility of 3.2 × 10−3 cm2 V−1 s−1 using only 2 wt% P3HT composition. The maximum FET mobility of 7.86 × 10−3 cm2 V−1 s−1 with the on/off ratio of 105 was obtained in the 10 wt% P3HT–PSA blends, which were higher than those of pristine P3HT and P3HT–PMMA devices. The semicrystalline PSA probably facilitated large P3HT crystallites and led to high FET mobility. Also, the P3HT–PSA FET devices showed lower percolation threshold and better ambient stability than the P3HT–PMMA devices. These results indicated that the crystalline non-conjugated polymers played a critical role in the charge transport and air stability of FETs based on conjugated polymer blends.

Oligosaccharide Carbohydrate Dielectrics toward High-Performance Non-volatile Transistor Memory Devices.

Oligosaccharides are one of the most promising biomaterials because they are abundant, renewable, diversified, and biosourced. The use of oligo- or polysaccharides for high-performance non-volatile organic field-effect-transistor memory is demonstrated herein. The charge-storage mechanism is attributed to charged hydroxyl groups that induce stronger hydrogen bonding, thus leading to the stabilization of trapped charges. This study reveals a promising future for green memory devices.