Jung-Ching Hsu

Non-volatile memory devices based on polystyrene derivatives with electron-donating oligofluorene pendent moieties.

We report bistable non-volatile memory devices based on polystyrene derivatives containing pendent electron-donating mono-, di-, and tri(9,9-dihexylfluorene), which are denoted as poly(St-Fl), poly(St-Fl(2)), and poly(St-Fl(3)), respectively. The effects of the oligofluorene chain lengths and polymer surface structures on the memory characteristics were explored. Poly(St-Fl)-, poly(St-Fl(2))-, and poly(St-Fl(3))-based devices exhibited a flash memory characteristic with different turn-on threshold voltages of 2.8, 2.0, and 1.8 V, respectively, which was on the reverse trend with the highest occupied molecular orbital levels of -5.86, -5.80, and -5.77 eV. Moreover, the memory device showed a high ON/OFF current ratio of 2.5 x 10(4) and a long retention time of 10(4) s. The possible mechanism of the switching behavior was explained by the space-charge-limited-current theory and filamentary conduction. The larger aggregation domain size of the polymer thin film processed from the mixed solvent of chlorobenzene/N,N-dimethylformamide probably promoted the diffusion of the Al atoms into the polymer film and formed the conduction channel. Thus, it significantly reduced the turn-on threshold voltage on the studied polymer memory devices. The present study suggested that the polymer memory characteristics could be efficiently tuned through the pendent conjugated chain length and surface structures.

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 supramolecular approach on using poly(fluorenylstyrene)-block-poly(2-vinylpyridine):PCBM composite thin films for non-volatile memory device applications.

Supramolecular composite thin films of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) (P(St-Fl)-b-P2VP):[6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) were prepared for write-once-read-many times (WORM) non-volatile memory devices. The optical absorption and photoluminescence results indicated the formation of charge transfer complexation between the P2VP block and PCBM, which led to the varied PCBM aggregated size and memory characteristics. The ITO/PCBM:(P(St-Fl)-b-P2VP)/Al device exhibited the WORM characteristic with low threshold voltage (-1.6 to -3.2 V) and high ON/OFF ratio (10(3) to 10(5)) by tuning the PCBM content. The switching behavior could be explained by the charge injection dominated thermionic emission in the OFF state and field-induced charge transfer in the ON state. The present study provides a novel approach system for tuning polymer memory device characteristics through the supramolecular materials approach.

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.