It is taken notice of new research fields named printable, plastic and flexible electronics. This review presents recent development of organic electroluminescent (EL) devices, thin film transistors (TFT) and photovoltaic cells, and also presents the fundamental device structure, development history and others. The printing methods like the large area and high precision micro-contact printing are also described.

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There have been a lot of practical examples of application of printable electronics recently; e.g. OLEDs, RF-IDs, OTFTs, photovoltaics, and sensors. By using printing technology rapid, large area, or low cost process is expected to be applied to such electronics manufacturing. Here we describe part of our printing techniques for fabrication of organic thin film transistors (OTFTs) focusing on printing of solution-processable organic semiconductors, high resolution printing of source and drain electrodes, and electronic papers using printed OTFT arrays. For solution process of crystalline small molecule organic semiconductors micro-contact printing (μCP) technique was applied to print hydrophobic self-assembled monolayer (SAM) patterns, and patterns of hydrophobic SAM and conjugated SAM were made on gate dielectrics so that poly crystalline films of organic semiconductors could be deposited on the channels of TFTs. We developed an offset-based high resolution printing technique using silver nano-particle ink, which could successfully print L⁄S=1μm⁄1μm pattern. All printed 10.5-inch VGA flexible active-matrix electrophoretic display was demonstrated. Also some other printing techniques for passivation layers, interlayer dielectrics, and pixel electrodes are described.

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In recent years, the printing technologies had evolved remarkably due to the improvements of performances on printing press, plate, ink and substrate and they have contributed to better productivity and better reproduction with finer images. Those improvements have led the printing to electronics field to obtain higher value from existing printing fields of graphic arts and packaging printing. The printing method to make electronic devices, such as RFID tag, antenna has been proposed to the markets in order to accomplish shorter lead-time and lower cost than the existing etching method corresponding mass production and mass consumption. This report explains mainly the electro-conductive ink for the RFID tag antenna, including the printable-electronics technology for the circuit formation etc.

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A high color purity hybrid electroluminescent (EL) device based on an organic dye and an inorganic phosphor has been developed. In this device, the organic dye is excited by the luminescence energy of the inorganic phosphor, and exhibits white luminescence from the three primary colors without a color filter. We developed the EL device using organic dyes that consist of coumarin 6 and a 4-(dicyanomethylene)-2-i-propyl-6-(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)-doped phosphor layer. Furthermore, a RGB color device of the organic-dye dispersed inorganic EL device with red luminescence of high color purity was produced using a printing technique.

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Ink trappings were studied using polyethylene terephthalate (PET) film with black inks for offset proofing and synthetic paper. By observing printed matter from reverse side through the PET film, we detected many air bubbles between the ink layer and the PET film. They are classified roughly to two groups, small number of large ones (φ=2-5μm) and many small ones (φ=0.5-1.0μm). Number of small air bubbles(N_{air bubble}, measured immediately after the trapping)decreased with increasing the amount of ink trapped (y) and increased with increasing the ink distribution time. Therefore, the small air bubbles have been yielded by the ink distribution. We also measured many ink peaks (immediately after the trapping) and pinholes (at 24h) on the printed surface. Numbers of ink peaks and pinholes (N_{ink peak} and N_pinhole, respectively) increased with increasing the ink distribution time. Next, we studied effects of nip width on these values (distribution time=2min.; nip width=2, 3 and 4mm). The N_{air bubble} value decreased with increasing nip width contrary to increase the N_ink peak and N_pinhole values. The latter effect can be represented by differences in the values of 2 and 4mm nip widths. At y=2gm^-2, the difference in the N_{air bubble} values is about one third (synthetic paper ink) or a half (offset proofing ink) of the difference in the N_{ink peak} values.

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