Takashi Fujihara

Direct laser writing of complementary logic gates and lateral p-n diodes in a solution-processible monolithic organic semiconductor

2010 WILEY-VCH Verlag Gmb Fabrication of p–n structures is a key issue in a number of electronic devices, including rectifying diodes, solar cells, and bipolar transistors. Complementary metal oxide semiconductor (CMOS) technology based on the integration of discrete pand n-channel field-effect transistors (FETs) on a same substrate has been widely used in a variety of electronic applications and enables the fabrication of integrated circuits with low-power dissipation and high operational stability. In the growing field of organic electronics, efficient large-scale organic CMOS integrated circuits as well as lateral p–n diodes have already been realized by thermally evaporating pand n-type semiconducting organic molecules separately through shadow masks. Alternatively, fabrication of organic inverters by inkjet printing, self-assembly, or spin-coating has also been reported and has demonstrated the enormous potential of solution processing for low-cost, large-area, flexible electronics. However, in spite of remarkable recent advances in high-mobility organic FET materials and nanostructuration techniques, there has still been no reliable approach for effectively patterning p–n bipolar and CMOS microstructures made from solutionprocessible organic semiconductors. We have addressed this issue by exploiting the unique charge-transport properties of the solution-processible dicyanomethylene-substituted quinoidal quaterthiophene [QQT(CN)4], which can be converted from an ambipolar p-type-dominant to n-type semiconductor by either thermal annealing or direct laser writing. The chemical structure of the QQT(CN)4 oligomer is shown in the inset of Figure 1a. Quinoidal oligothiophene derivatives generally show a low highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy bandgap and are very promising candidates for n-type or single-component ambipolar organic FETs. We have fabricated top-contact bottom-gate organic FETs under ambient conditions by spin-coating a QQT(CN)4 thin film on top of a polyimide gate-dielectric layer and using chromium drain/source electrodes. The transfer and output characteristics displayed in Figure 1 show that the device based on as-prepared QQT(CN)4 thin film exhibits an ambipolar behavior with a predominant hole-transport character. The charge-carrier field-effect mobilities were extracted from the transfer characteristics in the saturated regions. Hole and electron mobilities were measured to be 2 1 10 3 and 4 1 10 4 cm V 1 s , respectively. Compared to the QQT(CN)4–based device performance obtained using octadecyltrichlorosilane-treated SiO2 as gate-dielectric layer and gold top-contact electrodes, these mobility values are substantially lower. However, the use of a polyimide gate dielectric presents the significant advantage of strongly reducing

Surface waves in photorefractive polymer films

We observed the temporal development of surface waves and investigated their power propagation loss in typical photorefractive polymer films sandwiched between ITO glass substrates. We found that amplified scattered waves generated in a pumped region started to develop into surface waves from a point where they reached the substrate through the self-bending effect. The surface waves propagated over a distance of 1.7 mm, thereby confining the power to a region at a distance of 30 microns from the substrate. Considerable propagation power loss of the surface waves was observed at a low pumping power of the beam; however, the power loss decreased considerably when the beam had high power.

Control of the charge transport properties in a solution-processible ambipolar quinoidal oligothiophene derivative by direct laser writing

We investigate the effects of laser irradiation on the performance of organic field-effect transistors based on the solution-processible quinoidal oligothiophene [QQT(CN)4]. Whereas electron field-effect mobilities are not modified, hole transport can be selectively controlled and even suppressed depending on the laser irradiation conditions. Vertical p-n bipolar structures in QQT(CN)4 realized by direct laser writing are also studied. The results provide essential information for the effective laser patterning of complementary organic logic circuits and suggests the possibility to fabricate by direct laser writing complex three-dimensional bipolar p-n structures in a single QQT(CN)4 thin film.

Simplified procedure for interferometric determination of electro-optic properties of low-Tg photorefractive polymer

A simplified procedure is proposed for the interferometric determination of optical dispersions of refractive-index changes in a low-Tg photorefractive (PR) polymer. A two-level model for the optical dispersions is used in order to separately evaluate the index changes occurring due to molecular birefringence and first-order electronic electro-optic effects. This evaluation is done by estimating the contributions of the two effects in the low-frequency limit of a modulating voltage. The evaluated results are compared with those determined by a PR two-beam coupling experiment.

Strongly electric-field-dependent spatial properties of fanning beam in polymeric medium

The emission angle and output profile of a fanning beam were evaluated in a high-gain polymeric medium utilizing a conventional sandwich cell structure. An abrupt change of the emission angle and a spread of the beam profile were found with an applied electric field. These beam properties were explained in terms of the electric-field-induced Pockels coefficient of the medium.

Effects of transient dark currents on the buildup dynamics of refractive index changes in photorefractive polymers excited by pulsed voltage

We study the influence of a transient dark current on the buildup dynamics of photorefractive index gratings, which are excited right after the onset of a pulsed voltage, by using low glass-transition temperature photorefractive polymers under a heat-assisted condition. We conclude that the development of photorefractive index gratings is majorly controlled by changes in the transient dark current, through the suppression of a saturated photo-induced space-charge electric field. We also show that this influence can be estimated reasonably well by a simple analysis of the measured transient current traces.

Dark current and light illumination effects on grating formation during periodic long-term operation in photorefractive polymers

Photorefractive grating formation dynamics in long-timescale writing and the effects of periodic writing through the control of writing beam irradiation or electric field application were investigated using typical photorefractive polymers. Both dark current and writing beam irradiation affected grating formation dynamics. Dark current in polymers changed the effective trap density over time through deep trap filling and/or detrapping and thus affected grating formation considerably. The writing beam irradiation also affected grating development in the presence of an electric field owing to the accumulation of filled deep traps. However, grating development recovered after the elimination of the electric field freed up the filled deep traps.

Formation of an anti-guide structure and observation of enhanced SHG in photorefractive materials

Abstract In nonlinear-optical materials with photosensitivity and photoconductivity or in photorefractive materials, an anti-guide structure can be formed by light-pumping. Using a Maker-fringe setup, we observed enhancement of second-harmonic generation under irradiation of a pump light. The samples were sensitive to the pump He–Ne laser or Ar+ laser but not to the fundamental Q-switched Nd:YAG laser. SHG based on the anti-guide structure was studied under various conditions.

Absorption spectrum analysis based on singular value decomposition for photoisomerization and photodegradation in organic dyes

In order to analyze the spectra of inseparable chemical mixtures, many mathematical methods have been developed to decompose them into the components relevant to species from series of spectral data obtained under different conditions. We formulated a method based on singular value decomposition (SVD) of linear algebra, and applied it to two example systems of organic dyes, being successful in reproducing absorption spectra assignable to cis/trans azocarbazole dyes from the spectral data after photoisomerization and to monomer/dimer of cyanine dyes from those during photodegaradation process. For the example of photoisomerization, polymer films containing the azocarbazole dyes were prepared, which have showed updatable holographic stereogram for real images with high performance. We made continuous monitoring of absorption spectrum after optical excitation and found that their spectral shapes varied slightly after the excitation and during recovery process, of which fact suggested the contribution from a generated photoisomer. Application of the method was successful to identify two spectral components due to trans and cis forms of azocarbazoles. Temporal evolution of their weight factors suggested important roles of long lifetimed cis states in azocarbazole derivatives. We also applied the method to the photodegradation of cyanine dyes doped in DNA-lipid complexes which have shown efficient and durable optical amplification and/or lasing under optical pumping. The same SVD method was successful in the extraction of two spectral components presumably due to monomer and H-type dimer. During the photodegradation process, absorption magnitude gradually decreased due to decomposition of molecules and their decaying rates strongly depended on the spectral components, suggesting that the long persistency of the dyes in DNA-complex related to weak tendency of aggregate formation.

Optical and charge transport properties of an ambipolar quinoidal oligothiophene derivative

We report on the charge transport and absorption properties of a low bandgap quinoidal oligothiophene derivative [QQT(CN)4]. Organic field-effect transistors based on as-prepared QQT(CN)4 thin films show an ambipolar p-type dominant behavior and can be converted to n-type by thermal annealing at 100°C for 30 minutes. Absorption spectra and atomic force microscopy measurements carried out in both neat films and polymer blends provide new important insights into the role of the degree of aggregation and the intermolecular interactions on the semiconducting and photophysical properties of this organic electronic material.