Shizuyasu Ochiai

Performance of poly(3-hexylthiophene) organic field-effect transistors on cross-linked poly(4-vinyl phenol) dielectric layer and solvent effects

Bottom-contact organic field-effect transistors (OFETs) were fabricated using a polymer gate insulator cross-linked poly(4-vinyl phenol) with regioregular poly(3-hexylthiophene) (RR-P3HT) as an active layer from different organic solvents. With this polymer dielectric, a field-effect mobility of 0.084±0.006cm2V−1s−1 was obtained. Solvents and interfacial properties have pronounced effects in determining the crystallinity and device performance of RR-P3HT on the polymer gate layer. Morphology correlation with the charge carrier mobility of RR-P3HT OFETs is investigated. Large nanoscale crystalline island densities of this polymer play an important role in the high charge carrier mobility of devices.

Examining the effect of additives and thicknesses of hole transport layer for efficient organic solar cell devices

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a conducting polymer, has been receiving a great deal of attention for next generation optoelectronic organic devices. In this report, we discuss the effect of additives along with the thickness of PEDOT:PSS layers on the power conversion efficiency of organic solar cell devices. PEDOT:PSS films treated with high boiling point solvents of dimethyl sulfoxide (DMSO) and ethylene glycol (EG) show a significant enhancement in electrical conductivity without compromising flexibility or optical transparency. The conductivity increased from 0.5 to 517 and 724 S/cm after once and thrice treatment with 4 vol. % EG, respectively. The as-prepared and additives-treated PEDOT:PSS films deposited on glass substrates have been investigated by optical spectroscopy, micro-Raman spectroscopy and atomic force microscopy (AFM). The results indicate that structural and morphological changes were induced by the additive processes. By using DMSO and EG treated PEDOT:PSS as a hole transport layer, organic solar cells with a Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:[6,6]-phenyl-C71-butyric acid methyl esters (PCDTBT:PC71BM) bulk heterojunction have been fabricated on indium-tin-oxide (ITO) coated glass substrates. The high power conversion efficiency (PCE) of 5.17%, and 5.69%, were observed for PEDOT:PSS hole transport layers treated with DMSO and EG respectively, even though the devices were prepared in air atmosphere.

Comparison of properties of polymer organic solar cells prepared using highly conductive modified PEDOT:PSS films by spin- and spray-coating methods

Bulk heterojunction (BHJ) solar cells have made great progress over the past decade and consequently are now attracting extensive academic and commercial interest because of their potential advantages: lightweight, flexible, low cost, and high-throughput production. Polymer conductivity is a key factor for improving the performance of electronic and photonic devices. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is promising for use as a next-generation transparent electrode of optoelectronic devices. In this research, we compare the effect of nanomorphology on conductivity, and power conversion efficiency of polymer organic solar cells prepared by the spin- and spray-coating methods. To improve the conductivity of spray-deposited PEDOT:PSS, we modified the PEDOT:PSS films by simple UV irradiation and by UV irradiation with treatment using various solvents such as methanol, ethanol, acetone, acetonitrile, hydrochloric acid, and sulfuric acid to form a hole transport layer (HTL). The active layer of PTB7:PC70BM is spray-coated on top of the PEDOT:PSS layer. The films were examined by optical spectroscopy, micro-Raman spectroscopy, and conductivity measurements. The surface morphology of the deposited films was examined by atomic force microscopy (AFM). The current density–voltage (J–V) characteristics were measured under illumination with simulated solar light at 100 mW/cm2 (AM 1.5G) using an oriel 1000 W solar simulator. The obtained results are expected to have a considerable impact and suggest a bright future for organic polymer solar cells.

Interfacial polarization in silicone oil-polypropylene insulating system

Abstract Silicone oils, PP films and their composed insulating systems polarized by high DC fields at high temperatures showed several TSC peaks in the temperature range from −140 to 150°C and their origins were clarified. The oil-PP insulating systems exhibited two remarkable TSC peaks due to interfacial polarizations at the oil-PP interfaces. These two TSC peaks shifted to the higher temperature side with increasing viscosity of silicone oil. They appeared at 50 and 80 °C for 25 cS (at room temperature) silicone oil. Their TSC analysis revealed the energy required for the release of ions trapped in the interfacial regions, 0.4 ∼ 0.5 eV and the density of trapped ions for the oil-PP system polarized at 4.5 × 10 5 V/cm at 120°C, 2 × 10 12 cm −2 . Measurements of dielectric losses (tanδ) were also carried out on the oil-PP insulating systems and their results were compared with the TSC results.

Performance of Organic Field-Effect Transistors with Poly(3-hexylthiophene) as the Semiconductor Layer and Poly(4-vinylphenol) Thin Film Untreated and Treated by Hexamethyldisilazane as the Gate Insulator

The relationship between the carrier mobility and the structure/morphology of polymer semiconductor molecules on a substrate remains a very important issue in organic electronics. In this paper, the relationship between the crystallinity of drop-cast regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) thin films and polymer substrates was discussed by using heat-crosslinkable poly(4-vinylphenol) (PVP) as the gate insulator in organic field-effect transistors (OFETs). The effect of a self-assembled monolayer (SAM) of 1,1,1,3,3,3-hexamethyldisilazane (HMDS) on crosslinked PVP was also examined and the performance of the OFET was investigated. Results show that not only the orientation and crystallinity of the polymer semiconductor are very important in determining the OFET performance, but also the gate dielectric properties, surface roughness, and the interface properties between the gate and semiconductor layers are important factors.

Performance evaluation of PTB7 : PC71BM based organic solar cells fabricated by spray coating method using chlorine free solvent

The PTB7 : PC71BM polymer based solar cells have been successfully fabricated by spin and spray coating technique using chlorine-free solvent (xylene), which are desirable to reduce environmental issues. The surface morphology of fabricated film characterized by AFM reveals that the surface morphology of the film is uniform and smooth when xylene is used as compared with chlorobenzene. The highest power conversion efficiency (PCE) (5.07 ± 0.6) was achieved using spray-coating technique than that of spin coating technique (PCE of 4.47 ± 0.6). The enhancement in the performance of the polymer solar cell could be attributed to the improved charge carrier transportation due to additive. The combination of chlorine-free solvent and spray-coating method minimize the waste material and reduce the environmental problem in large-area production of organic solar cells (OSCs).

Performance of Organic Field-Effect Transistor Based on Poly(3-hexylthiophene) as a Semiconductor and Titanium Dioxide Gate Dielectrics by the Solution Process

Organic field-effect transistors (OFETs) were fabricated with a high-dielectric-constant and high-permittivity titanium dioxide (TiO2) as a gate insulator and regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) as the electronically active semiconductor. Positive OFET characteristics were obtained with a low threshold voltage (+3 V) and high field-effect mobility (3.73 ×10-3 cm2 V-1 s-1). The dielectric material (TiO2) was prepared by the sol–gel technique and the gate insulator layer was fabricated by spin coating. The RR-P3HT thin films were fabricated by drop casting with different solution concentrations. The thickness of the thin films was measured using the surface profile measuring system. The fabricated thin film structure was analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD), and UV–visible absorption spectra. The X-ray result shows that the drop-cast RR-P3HT thin film has a high crystallinity on the TiO2 surface, which leads to the high field-effect mobility of the OFET. The results show that the OFET performances are not only dependent on the orientation and crystallinity of the polymer semiconductor, but that also the gate dielectric properties, surface roughness, and interface properties between the gate and semiconductor layers are very important for the efficient performance of the OFET.

Interface Traps and Swelling of Polypropylene Films Immersed in Silicone Oils

Biaxially stretched PP (polypropylene) films immersed in silicone oil showed TSC peaks P1 and P2 due to ions trapped in the oil-PP interface region. Their peak temperatures were lowered by oil-PP interaction such as swelling. The equations governing the swelling of a crystalline polymer were deduced from the analogy of the swelling of a cross-linked polymer. The experimental values of the degree of swelling of PP films immersed in silicone oil agreed well with the theoretical ones calculated from the equations. TSC peak temperatures decreased with increasing degree of swelling of PP films. The good correlation between TSC and swelling gave evidence for the modification of surface traps by swelling.

Organic field-effect transistors with crosslinkable poly(vinyl alcohol) insulator and spin-coated/drop-cast poly(3-hexylthiophene-2,5-diyl) semiconductor

Organic field-effect transistors (OFETs) with water-soluble and crosslinkable poly(vinyl alcohol) (PVA) as a gate insulator and poly(3-hexylthiophene) (P3HT) as an active semiconductor were fabricated, and the device performances were investigated. Heat-crosslinked PVA displays better dielectric properties because it contains less water and the OFETs show enhanced on/off ratios and improved stability. P3HT films were fabricated by spin-coating and drop-casting methods and characterized by atomic force microscopy (AFM) and UV–visible absorption spectra. The mobilities of drop-cast P3HT films are one order higher than those of the spin-coated films, which is ascribed to the slower drying of the solvent and more-ordered structure in the films. However, OFETs with drop-cast P3HT show inferior saturation characteristics and lower on/off ratios. This is probably due to higher roughness and thickness of the drop-cast film and nonnegligible carrier transportation in the bulk, which can be demonstrated by the conductance measurements.

Floating-gate type organic memory device with organic insulator film of plasma polymerized styrene

Plasma polymerized styrene (ppS) thin films were prepared and used as gate insulator and tunneling layer in a floating-gate type organic memory device. To investigate feasibility of the ppS thin film for application in non-volatile organic memory, an organic thin film transistor (OTFT) and a floating-gate type organic memory device were fabricated. Current–voltage (I–V) characteristics of the OTFT and floating-gate type organic memory device were comparatively investigated, and hysteresis in the I–V characteristics of both devices was studied. A pseudo-charge-storage phenomenon was found for the ppS insulator thin film in the OTFT and floating-gate type organic memory device. The floating-gate type organic memory device revealed a reasonable hysteresis voltage of 27 V. It was confirmed that the ppS could be applied to fabricate an alternative floating-gate type organic memory device with promising memory function.