Toshiko Mizokuro

Doping of functional materials into poly (p-phenylene vinylene) by the vapor transportation method

Poly(p-phenylene vinylene) (PPV) is a promising material, but shows poor processability, such as doping, due to its insolubility and infusibility. Therefore, the development of a standard and easy method of dye doping into PPV is important for device fabrication using PPV. We developed a simple method for the dispersal of dyes into PPV without deformation. Using this method, it was possible to change the color of PPV from yellow to green by doping with the blue dye 1,4-(N,N’-diethylamino)anthraquinone (SV59). The amount of SV59 doped into PPV was ∼2.7wt%. The fluorescence color of PPV could be changed from green to red by 2 min dispersal of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran.

Nanometer scale marker for fluorescent microscopy

To establish a calibration method of optical performance in fluorescence microscopy, we fabricated a fluorescent nanometer-scale marker by combining a dry dye method for polymer film and fine lithography. The marker has a 50 nm line-and-space fluorescent pattern, finer than the optical diffraction limit. A spin-coated poly(methyl methacrylate) thin film on a silicon wafer was densely doped with Rhodamine 6G using a simple vacuum process, named the vapor-transportation method, and then the pattern was formed on the film using electron-beam lithography. The figure accuracy of the fabricated marker was calibrated by electron microscopes. Using this marker, one can quantitatively evaluate the optical properties; i.e., the contrast-transfer function, the point-spread function, magnification, and so on. To show practical use of the marker, we demonstrated the evaluation of a fluorescent microscope system.

Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge re-distribution.

We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure – in the presence of Fermi-level pinning – at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction’s electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices.

Electronic Properties of Cu-Phthalocyanine/Fullerene Planar and Bulk Hetereojunctions on PEDOT:PSS

In this paper, we report UV and X-ray photoelectron spectroscopy studies on layered planar and mixed bulk heterojunctions of Cu-phtalocyanine (CuPc) and C60, a prototypical material pair for organic photovoltaic cells (OPVCs). The respective heterojunctions were formed on poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrates in order to achieve morphologies comparable to those in actual OPVCs. As a result of a CuPc-to-substrate electron transfer, the work function of pristine PEDOT:PSS is reduced from 5.4 to 4.4 eV. The deposition of C60 onto CuPc, however, leads to a work function increase of 0.2 eV. The codeposition of C60 and CuPc to form mixed bulk heterojunctions resulted in an effective anode work function of 4.6 eV. The energy offset between the highest occupied levels of CuPc and C60 was determined as 1.3 eV for both the layered planar and mixed bulk heterojunction. With reported values of the charge transport gap of C60, we estimate the upper limit of the open circuit voltage to be 1.0 eV for both types of heterojunctions. Our results demonstrate that the energy level offsets are independent of particular interface morphology in C60/CuPc heterojunctions grown on PEDOT:PSS, and that differences in device efficiency are due to other effects.

Efficient triplet–triplet annihilation upconversion in binary crystalline solids fabricated via solution casting and operated in air

Binary crystalline solids consisting of a sensitizer (Pt-octaethylporphyrin) and an emitter (9,10-diphenylanthracene (DPA) or its alkyl-strapped derivative (C7-sDPA)) were fabricated by solution casting under rapid drying conditions and found to show efficient triplet–triplet annihilation upconversion by the suppression of component segregation. Microspectroscopic studies of individual crystalline particles of the binary solids revealed threshold intensities as low as 5 mW cm−2, close to solar levels, and UC quantum yields of 20% were possible when using C7-sDPA as an emitter, even in air.

Orientation of α-sexithiophene on friction-transferred polythiophene film.

Controlling the molecular orientation of the conjugated oligomer, α-sexithiophene (6T), is crucial to improve organic optoelectronic device performance. Most 6T molecules evaporated onto quartz and SiO(2)/Si substrates orient nearly perpendicular to the substrate. Here, we report the formation of oriented thin films of 6T on in-plane-oriented polythiophene (PT) films formed by the friction-transfer method. 6T was evaporated onto oriented PT films under vacuum. The films were investigated by polarized optical microscopy, polarized ultraviolet-visible light (UV-vis) absorption spectroscopy, and grazing incidence X-ray diffraction measurement (GIXD). In all spectra, larger absorbance derived from PT and 6T was observed, in parallel polarization to the friction direction, compared to that of orthogonal polarization. These results indicate that the 6T molecular axis is aligned in the friction direction (PT chain direction) of PT films. GIXD also confirmed that the 6T molecular axis was aligned parallel to the PT chain axis. In contrast, 6T molecules evaporated onto quartz and poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated silicon substrates aligned nearly perpendicular to the substrate. These results indicate that oriented PT films induce 6T orientation parallel to the PT chain direction.

Doped-Dye Orientation Relative to Oriented Polyfluorene Host Film

Polyfluorene oriented films produced by a friction-transfer method show polarized blue-light emission. A subsequent doping of some types of fluorescent dyes into the oriented films using a vapor transportation method resulted in polarized emission from the oriented polymer and from the doped dye. Polarized photoluminescence spectra from the polyfluorene films doped with oligothiophenes, quaterthiophene, and sexithiophene showed that these materials exhibit marked dichroism caused by the alignment of the oligothiophene molecules parallel to the polymer chain. We succeeded in extending the wavelength range of the polarized emission by doping fluorescent dye into the films.

Preparation of smooth polymer thin film using spray method under vacuum

Polymers such as poly(methyl methacrylate) (PMMA) doped with N-[9-(2-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-N-ethylethanaminium chloride (Rhodamine B) can be prepared as thin films using a spray method under vacuum. Smooth films without dust and solvent residues can be obtained by annealing the films above the glass transition temperature (Tg) after they have been prepared using a conventional spray method under vacuum.

White Polarized Electroluminescence Devices by Dye Deposition on Oriented Polyfluorene Films

White polarized electroluminescent (EL) devices were produced by deposition of α-sexithiophene (6T), which is an orange-emitting dye, on blue-emitting oriented poly(9,9-dioctylfluorene) (PFO) polymer films produced by the friction transfer method. The results showed that the color of the emitted light changed from blue to white with increasing 6T film thickness. In addition, good polarization of the light was observed, suggesting that 6T molecules are oriented parallel to the direction of the underlying PFO molecules.

Oriented Polyfluorene Films Dye-Doped for Whitening of Polarized Electroluminescent Devices

Whitening of the emission of electroluminescent (EL) devices was performed by doping α-sexithiophene (6T), which is an orange emitting dye, into blue emitting oriented poly(9,9-dioctylfluorene) (PFO) polymer films. These oriented PFO films were produced by means of the friction transfer method and the doping of 6T was performed by means of the vapor transportation method which is a solvent-free process. The excellent orientation of PFO films and of the doped 6T led to very good polarization of the light emitted by these EL devices.