Correlation between microstructure and magnetotransport in organic semiconductor spin valve structures

We have studied magnetotransport in organic-inorganic hybrid multilayer junctions. In these devices, the organic semiconductor (OSC) Alq$_3$ (tris(8-hydroxyquinoline) aluminum) formed a spacer layer between ferromagnetic (FM) Co and Fe layers. The thickness of the Alq$_3$ layer was in the range of 50-150 nm. Positive magnetoresistance (MR) was observed at 4.2 K in a current perpendicular to plane geometry, and these effects persisted up to room temperature. The devices' microstructure was studied by X-ray reflectometry, Auger electron spectroscopy and polarized neutron reflectometry (PNR). The films show well-defined layers with modest average chemical roughness (3-5 nm) at the interface between the Alq$_3$ and the surrounding FM layers. Reflectometry shows that larger MR effects are associated with smaller FM/Alq$_3$ interface width (both chemical and magnetic) and a magnetically dead layer at the Alq$_3$/Fe interface. The PNR data also show that the Co layer, which was deposited on top of the Alq$_3$, adopts a multi-domain magnetic structure at low field and a perfect anti-parallel state is not obtained. The origins of the observed MR are discussed and attributed to spin coherent transport. A lower bound for the spin diffusion length in Alq$_3$ was estimated as $43 \pm 5$ nm at 80 K. However, the subtle correlations between microstructure and magnetotransport indicate the importance of interfacial effects in these systems.Comment: 21 pages, 11 figures and 2 table

Facile synthesis and optoelectronic exploration of silylthiophene substituted benzodithiophene polymer for organic field effect transistors

This work reports the synthesis, characterization and organic field effect transistors (OFET) application of a novel conjugated polymer (PBDTDPP) based on silylthiophene substituted benzo[1,2-b:4,5-b']dithiophene (BDT) donor and diketopyrrolopyrrole (DPP) acceptor obtained via Stille polymerization reaction. The polymer exhibits a broad absorption in the UV-visible spectrum ranging from 300 nm to 900 nm with the band edge of the polymer at 1.31 eV. Thermogravimetric analysis of the polymer demonstrates the stability up to 303 degrees C, and the cyclic voltammetry shows the HOMO and LUMO levels at -5.42 and -4.11 eV, respectively. Employing the polymer as an active layer in a bottom gate-top contact based OFET, hole mobility of as high as 9.34 x 10(-2) cm(2) V-1 s(-1) with the On/Off ratio of similar to 10(4) was obtained. This work successfully demonstrates that the DPP and the silylthiophene substituted BDT are promising units to build D-A based copolymer for organic electronics. (c) 2018 Elsevier B.V. All rights reserved.11Nsciescopu

Acceptor Unit Effects for Ambipolar Organic Field-Effect Transistors Based on TIPS-Benzodithiophene Copolymers

Two narrow band gap triisopropylsilyl substituted benzo[1,2-b:4,5-b] dithiophene (TIPS-BDT) derivatives, P1 (1.65 eV) and P2 (1.46 eV) are synthesized for ambipolar organic field-effect transistors and complementary inverters. Two electron acceptor units, heptadecanyl substituted thieno[3,4-c]pyrrole-4,6-dione (TPD) and ethylhexyl substituted diketopyrrolo[3,4-c]pyrrole (DPP) are incorporated to tune the structure and resulting properties of the donor-acceptor type copolymers. Structural modification based on the acceptor unit variation, resulted in comparable electrochemical, optical, microstructural, and charge transporting properties, as well as environmental and operational stability. TIPS-BDT copolymers with TPD acceptor units show comparatively superior performance, with field effect mobility approximate to 10(-3) cm(2)V(-1)s(-1) for both holes and electrons and inverter gain approximate to 18 with poly(methyl methacrylate) gate dielectric.11Nsciescopuskc

Diketopyrrolopyrrole-Based π-Bridged Donor–Acceptor Polymer for Photovoltaic Applications

We report the synthesis, properties, and photovoltaic applications of a new conjugated copolymer (C12DPP-π-BT) containing a donor group (bithiophene) and an acceptor group (2,5-didodecylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione), bridged by a phenyl group. Using cyclic voltammetry, we found the energy levels of C12DPP-π-BT are intermediate to common electron donor and acceptor photovoltaic materials, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. Whereas P3HT and PCBM are exclusively electron donating or accepting, we predict C12DPP-π-BT may uniquely serve as either an electron donor or an acceptor when paired with PCBM or P3HT forming junctions with large built-in potentials. We confirmed the ambipolar nature of C12DPP-π-BT in space charge limited current measurements and in C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT bulk heterojunction solar cells, achieving power conversion efficiencies of 1.67% and 0.84%, respectively, under illumination of AM 1.5G (100 mW/cm²). Adding diiodooctane to C12DPP-π-BT:PCBM improved donor–acceptor inter-mixing and film uniformity, and therefore enhanced charge separation and overall device efficiency. Using higher-molecular-weight polymer C12DPP-π-BT in both C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT devices improved charge transport and hence the performance of the solar cells. In addition, we compared the structural and electronic properties of C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT blends, representing the materials classes of polymer:fullerene and polymer:polymer blends. In C12DPP-π-BT:PCBM blends, higher short circuit currents were obtained, consistent with faster charge transfer and balanced electron and hole transport, but lower open circuit voltages may be reduced by trap-assisted recombination and interfacial recombination losses. In contrast, C12DPP-π-BT:P3HT blends exhibit higher open circuit voltage, but short circuit currents were limited by charge transfer between the polymers. In conclusion, C12DPP-π-BT is a promising material with intrinsic ambipolar characteristics for organic photovoltaics and may operate as either a donor or acceptor in the design of bulk heterojunction solar cells

Interfacial Engineering of Nanoporous Architectures in Ga₂O₃ Film toward Self-Aligned Tubular Nanostructure with an Enhanced Photocatalytic Activity on Water Splitting

The present work demonstrates the formation of self-aligned nanoporous architecture of gallium oxide by anodization of gallium metal film controlled at −15 °C in aqueous electrolyte consisting of phosphoric acid. SEM examination of the anodized film reveals that by adding ethylene glycol to the electrolyte and optimizing the ratio of phosphoric acid and water, chemical etching at the oxide/electrolyte interfaces can be controlled, leading to the formation of aligned nanotubular oxide structures with closed bottom. XPS analysis confirms the chemical composition of the oxide film as Ga₂O₃. Further, XRD and SAED examination reveals that the as-synthesized nanotubular structure is amorphous, and can be crystallized to β-Ga₂O₃ phase by annealing the film at 600 °C. The nanotubular structured film, when used as photoanode for photoelectrochemical splitting of water, achieved a higher photocurrent of about two folds than that of the nanoporous film, demonstrating the rewarding effect of the nanotubular structure. In addition, the work also demonstrates the formation of highly organized nonporous Ga₂O₃ structure on a nonconducting glass substrate coated with thin film of Ga-metal, highlighting that the current approach can be extended for the formation of self-organized nanoporous Ga₂O₃ thin film even on nonconducting flexible substrates

Facile Routes To Improve Performance of Solution-Processed Amorphous Metal Oxide Thin Film Transistors by Water Vapor Annealing

Here, we report on a simple and high-rate oxidization method for producing solution-based compound mixtures of indium zinc oxide (IZO) and indium gallium zinc oxide (IGZO) metal-oxide semiconductors (MOS) for thin-film transistor (TFT) applications. One of the issues for solution-based MOS fabrication is how to sufficiently oxidize the precursor in order to achieve high performance. As the oxidation rate of solution processing is lower than vacuum-based deposition such as sputtering, devices using solution-processed MOS exhibit relatively poorer performance. Therefore, we propose a method to prepare the metal-oxide precursor upon exposure to saturated water vapor in a closed volume for increasing the oxidization efficiency without requiring additional oxidizing agent. We found that the hydroxide rate of the MOS film exposed to water vapor is lower than when unexposed (≤18%). Hence, we successfully fabricated oxide TFTs with high electron mobility (27.9 cm²/V·s) and established a rapid process (annealing at 400 °C for 5 min) that is much shorter than the conventional as-deposited long-duration annealing (at 400 °C for 1 h) whose corresponding mobility is even lower (19.2 cm²/V·s)