Masaru Nagai

Controllable Organic Resistive Switching Achieved by One-Step Integration of Cone-Shaped Contact

Conductive filaments (CFs)-based resistive random access memory possesses the ability of scaling down to sub-nanoscale with high-density integration architecture, making it the most promising nanoelectronic technology for reclaiming Moores law. Compared with the extensive study in inorganic switching medium, the scientific challenge now is to understand the growth kinetics of nanoscale CFs in organic polymers, aiming to achieve controllable switching characteristics toward flexible and reliable nonvolatile organic memory. Here, this paper systematically investigates the resistive switching (RS) behaviors based on a widely adopted vertical architecture of Al/organic/indium-tin-oxide (ITO), with poly(9-vinylcarbazole) as the case study. A nanoscale Al filament with a dynamic-gap zone (DGZ) is directly observed using in situ scanning transmission electron microscopy (STEM) , which demonstrates that the RS behaviors are related to the random formation of spliced filaments consisting of Al and oxygen vacancy dual conductive channels growing through carbazole groups. The randomicity of the filament formation can be depressed by introducing a cone-shaped contact via a one-step integration method. The conical electrode can effectively shorten the DGZ and enhance the localized electric field, thus reducing the switching voltage and improving the RS uniformity. This study provides a deeper insight of the multiple filamentary mechanisms for organic RS effect.

Thermally induced vertical phase separation and photovoltaic characteristics of polymer solar cells for P3HT/PCBM composites

The occurrence of vertical phase separation has been reported for various spin-cast polymer films, including bulk-heterojunction films of polymer solar cells (PSCs). Focusing on real-space analysis, we conducted a study on the relationship between the morphology and processing conditions of PSCs for typical poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) cells. Our results demonstrated that spin-casting caused a localized reduction in the P3HT concentration in the bulk center. Thermal annealing after cathode formation enhanced the unevenness in concentration and created a multilayered vertical phase-separated morphology in which the P3HT domains were gathered near the electrodes, leaving only PCBM domains at the center of the film. Cells with this morphology had good power conversion efficiency (~3%).

Structure and temperature dependence of magnetic properties in the nanocrystalline Fe74SixB22−xCu1Nb3 alloys

We have examined the structure and magnetic properties in terms of the temperature dependence of magnetic permeability and magnetization for the Fe74Six B22-x Cu1Nb3 (x=8-18) alloys. The temperature dependence of the permeability of the alloys shows various characteristic behaviors depending on the alloy composition (x=8-18) and annealing temperature (450-630° C). The permeability of the samples having coexisting α-Fe(Si) and amorphous phases decreases abruptly near the Curie temperature of the amorphous materials, while the permeability of the samples containing precipitated Fe-B compounds is not appreciably affected by the magnetism in the residual amorphous region. The precipitation of Fe-B compound depends on the composition of the mother alloys. The Fe-B compounds precipitated in the amorphous region upon annealing at high temperature are mainly Fe23B6 for x=14, Fe3B for x=18, and Fe2B for the composition x less than 12.

Effect of solvent and cell composition on morphology and photovoltaic characteristics of polymer solar cells of poly(3-hexylthiophene)/indene-C60 bisadduct composites

This study investigated the solvent effect on the performance of polymer solar cells (PSCs) by focusing on the film morphology. PSCs consisting of poly(3-hexylthiophene) (P3HT) and the fullerene derivative indene-C60 bisadduct (ICBA) were fabricated using two different solvents, chlorobenzene (CB) and chloroform (CF). The short-circuit current density (J sc) changed depending on the solvent and cell composition. When the ratio of ICBA was higher than that of P3HT, a high J sc was obtained from the CB based cells. When the ratio of P3HT was higher than that of ICBA, the CF cells showed a higher J sc than the CB cells. The high-performance cells had a clear microphase-separated morphology while phase separation was limited in the low-performance cells. Solubility parameter analysis suggested that the cell composition changed the interaction parameter of the system and thereby affected the phase separation behavior.