Masayuki Chikamatsu

Solution-processed n-type organic thin-film transistors with high field-effect mobility

We report the performance of solution-processed n-type organic thin-film transistors (OTFTs) based on C60 derivatives. Long-chain alkyl-substituted C60, C60-fused N-methylpyrrolidine-meta-C12 phenyl (C60MC12), was used as a semiconducting layer. The C60MC12-thin-film transistor shows high electron mobility of 0.067cm2∕Vs in saturation regime. From the result of x-ray diffraction analysis, the C60MC12 active layer forms highly ordered crystalline film. We found that self-assemble ability of long alkyl chains plays an important role for fabrication of highly ordered crystalline film, leading to achievement of high electron mobility in solution-processed n-type OTFTs.

Ambipolar organic field-effect transistors based on a low band gap semiconductor with balanced hole and electron mobilities

The authors have demonstrated the thin-film properties and the ambipolar transport of a delocalized singlet biradical hydrocarbon with two phenalenyl radical moieties (Ph2-IDPL). The organic field-effect transistors (OFETs) based on Ph2-IDPL exhibit ambipolar transport with balanced hole and electron mobilities in the order of 10−3cm2∕Vs. The Ph2-IDPL film is an organic semiconductor with a low band gap of 0.8eV and has small injection barriers from gold electrodes to both the highest occupied molecular orbital and the lowest unoccupied molecular orbital. A complementary metal-oxide-semiconductor-like inverter using two identical Ph2-IDPL based ambipolar OFETs shows a sharp inversion of the input voltage with high gain.

Highly polarized polymer light-emitting diodes utilizing friction-transferred poly(9,9-dioctylfluorene) thin films

Polarized polymer light-emitting diodes (PLEDs) have been constructed utilizing friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films. The friction transfer technique allows oriented PFO to be deposited directly onto an indium tin oxide anode without an alignment layer such as polyimide. Polarized absorption and photoluminescence spectra revealed that the polymer backbones are highly aligned in the friction direction. We fabricated PLEDs consisting of friction-transferred PFO as an emissive layer, vacuum-deposited bathocuproine as an electron transport and hole-blocking layer, and a vacuum-deposited LiF∕Al cathode. Highly polarized blue emission with an integrated polarization ratio of 31 and a luminance of up to 300cd∕m2 was observed from the PLEDs.

Effects of intrinsic layer thickness on solar cell parameters of organic p‐i‐n heterojunction photovoltaic cells

We fabricated organic p‐i‐n heterojunction photovoltaic cells of a zinc phthalocyanine (ZnPc)∕1:1 codeposition (ZnPc:C60)∕C60 structure. We investigated the effects of the intrinsic (i-) layer thickness on the photovoltaic properties. The thickness was changed from 0 nm (=p‐n heterojunction) to 50 nm (=alli-layer) with the total thickness of 50 nm. While the short-circuit photocurrent increased with increasing the thickness, the fill factor showed the opposite tendency. Therefore, the power conversion efficiency showed a maximum (1.5%) at the thickness of 10 nm under air mass 1.5 global solar conditions. Device simulation based on idealized equivalent circuit of a solar cell demonstrates that the i-layer thickness is concerned in the series resistance of the cells.

Degradation mechanism of CH3NH3PbI3 perovskite materials upon exposure to humid air

Low stability of organic-inorganic perovskite (CH3NH3PbI3) solar cells in humid air environments is a serious drawback which could limit practical application of this material severely. In this study, from real-time spectroscopic ellipsometry characterization, the degradation mechanism of ultra-smooth CH3NH3PbI3 layers prepared by a laser evaporation technique is studied. We present evidence that the CH3NH3PbI3 degradation in humid air proceeds by two competing reactions of (i) the PbI2 formation by the desorption of CH3NH3I species and (ii) the generation of a CH3NH3PbI3 hydrate phase by H2O incorporation. In particular, rapid phase change occurs in the near-surface region and the CH3NH3PbI3 layer thickness reduces rapidly in the initial 1 h air exposure even at a low relative humidity of 40%. After the prolonged air exposure, the CH3NH3PbI3 layer is converted completely to hexagonal platelet PbI2/hydrate crystals that have a distinct atomic-scale multilayer structure with a period of 0.65 ± 0.05 nm. We ...

Highly efficient polarized polymer light-emitting diodes utilizing oriented films of β-phase poly(9,9-dioctylfluorene)

Uniaxially oriented films of β-phase poly(9,9-dioctylfluorene) (PFO) were realized by a friction-transfer technique followed by thermal annealing and vapor treatments. Absorption and photoluminescence (PL) spectra show the characteristics of β-phase: an additional absorption peak at 433nm and redshifted PL peaks compared with those of the usual nematic (N) phase. We fabricated polarized polymer light-emitting diodes utilizing oriented films of β-phase PFO. Highly polarized β-phase emission with an integrated polarization ratio of 51 was observed from the devices. The efficiency of the devices based on β-phase reached 2.0cd∕A, which is two times higher than that based on N-phase.

Simple push coating of polymer thin-film transistors

Organic semiconductors may be processed in solution under ambient conditions; however, liquid manipulation on hydrophobic surfaces is difficult, which may hinder development of devices. Here, a push-coating technique is used to produce large-area semiconducting polymer films over hydrophobic surfaces.

Light up-conversion from near-infrared to blue using a photoresponsive organic light-emitting device

A photoresponsive organic light-emitting device combining blue-emitting organic electroluminescent (EL) diode with titanyl phthalocyanine as a near-infrared (IR) sensitive layer was fabricated. By irradiating near-IR light to the device, blue emission occurred in the lower drive voltage (between 5 and 12 V). The result indicates that the device acts as a light switch and/or an up-converter from near-IR light (1.6 eV) to blue (2.6 eV). The EL response times of rise and decay using a near-IR light trigger were 260 and 330 μs, respectively. At a higher voltage (above 12 V), enhancement of blue emission was observed with near-IR light irradiation. The ON/OFF ratio reached a maximum of 103.

Doping effect of solution-processed thin-film transistors based on polyfluorene

We report the fabrication of solution-processable organic thin-film transistors (OTFTs) based on poly(9,9′-dioctylfluorene-alt-bithiophene) (F8T2) doped with an electron-acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The 8% doping of the F8T2 film was found to result in an increased hole mobility, up to 0.01 cm2 V−1 s−1, and in an improved on/off ratio, as well as in a low off-current, of the order of 10−11 A. The doped F8T2 device was also found to exhibit a better threshold voltage and reduced hysteresis behavior. These improvements in performance are attributed to the formation of the F8T2-F4TCNQ complex, which results in better hole injection and improved device stability.

Mg-doped C60 thin film as improved n-type organic semiconductor for a solar cell

Mg doping effect on a [2-methoxy, 5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV)/C60 bilayer solar cell has been investigated. We find that the Mg doping occurs during the deposition of the Mg top electrode onto the C60 film and that the solar cell property is markedly improved by the doping. The power conversion efficiency of the Mg-doped device under 100 mW/cm2 white light illumination is 0.54%, which is approximately 400 times larger than that of the nondoped device.