A method to print two materials of different functionality during the same printing step is presented. In printed electronics, devices are built layer by layer and conventionally only one type of material is deposited in one pass. Here, the challenges involving printing of two emissive materials to form polymer light‐emitting diodes (PLEDs) that emit light of different wavelengths without any significant changes in the device characteristics are described. The surface‐energy‐patterning technique is utilized to print materials in regions of interest. This technique proves beneficial in reducing the amount of ink used during blade coating and improving the reproducibility of printed films. A variety of colors (green, red, and near‐infrared) are demonstrated and characterized. This is the first known attempt to print multiple materials by blade coating. These devices are further used in conjunction with a commercially available photodiode to perform blood oxygenation measurements on the wrist, where common accessories are worn. Prior to actual application, the threshold conditions for each color are discussed, in order to acquire a stable and reproducible photoplethysmogram (PPG) signal. Finally, based on the conditions, PPG and oxygenation measurements are successfully performed on the wrist with green and red PLEDs.
In this paper, the inkjet printing of polymeric field-effect transistors (FETs), inverters and active-matrix backplanes will be reviewed. Inkjet printing, which is characterized as an additive and noncontact patterning method, is an efficient method of fabricating organic devices. All-solution-processed FETs were prepared in ambient air by inkjet-printing the liquid sources of a conductor or a semiconductor, and exhibited a high on–off current ratio of more than 105. This stability is attributed to the high ionization potential (5.4 eV) of the fluorene–bithiophene copolymer used in our work. Channel lengths of less than 20 µm were also achieved by depositing an aqueous dispersion of a conducting polymer along a prepatterned strip that exhibited a hydrophobic surface, thus defining the transistor channel. Partially-solution-processed FETs were also obtained by combining conventional vacuum processes and the inkjet printing of solutions. This approach is considered to be efficient for producing actual devices, and flexible active-matrix backplanes were fabricated using this structure. A flexible electrophoretic display has been achieved by laminating an inkjet-printed active-matrix backplane with an electrophoretic device.
Liquid crystal alignment layers are prepared using a noncontact method based on laser ablation. Nonpolarized light from a KrF excimer laser at 248 nm is exposed through a phase mask to etch gratings of period 1.1 μm onto polyimide alignment layers. Twisted nematic cells were prepared using one rubbed and one grating aligned surface, and azimuthal anchoring energies were found from measurements of the twist angles as a function of grating depth. The measured anchoring energies agree with those predicted from the minimization of elastic strain energy when the liquid crystal directors at the surface are aligned parallel to the groove of the grating. This suggests that topographical rather than epitaxial alignment is achieved.
Monodisperse, ordered conjugated polymer nanowires were deposited by friction-transfer technique. Films that comprise such ordered polymer nanowires show a strong optical and electronic anisotropy due to the high compaction of molecule chains in the individual nanowires. Field-effect transistors that were fabricated using poly[9,9-dioctylfluorene-co-bithiophene(F8T2)] nanowires exhibit a field-effect mobility of 3.5×10−2cm2V−1s−1 along the wire direction, which is much higher than the mobility value (5×10−3cm2V−1s−1) obtained in transistors with spin-coated F8T2.
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