Universally applicable small-molecule co-host ink formulation for inkjet printing red,green, and blue phosphorescent organic light-emitting diodes

For organic light-emitting diodes (OLEDs), inkjet printing technology is being developed as an alternative to the traditional vacuum evaporation, because of its precise patterning, high-efficient material utilization, large-area compatibility and low-cost. In this work, we report a universal ink formulation of small-molecule co-host and binary solvents for red, green and blue phosphorescent OLEDs. Moreover, the effect of hole-transporting layers on the ink spreading, film uniformity and exciton confinement ability is investigated. Furthermore, a large-area (170 mm × 170 mm) and homogeneous light-emitting film is inkjet-printed. Finally, red, green and blue OLEDs are successfully constructed using these optimized ink formulations on the solvent resistance hole-transporting layer. This work can reduce the complexity to adjust the host materials and solvents for different color inks, and could be applied in large-area and low-cost OLED displays with high resolution.     

Controlled Zr doping for inkjet-printed ZTO TFTs

This study examined the effects of controlled Zr doping on the electrical properties of inkjet-printed zinc-tin oxide (ZTO) thin-film transistors (TFTs). In contrast to previous reports, below a certain doping concentration, improved electrical properties were obtained due to the effectively suppressed oxygen vacancies, reduced trapped electrons, and controlled carrier concentrations. The 0.0025 M Zr-doped inkjet-printed ZTO TFTs showed higher mobility, higher on-to-off current ratio, lower threshold voltage, and lower subthreshold slope of 6.43 cm²/V s, 3.72×10⁸, 3.35 V, and 0.53 V/dec, respectively, compared to the un-doped TFTs. The bias stability of the Zr-doped inkjet-printed ZTO TFT was also improved.     

Zirconia nano-colloids transfer from continuous hydrothermal synthesis to inkjet printing

Water dispersions of nanometric yttria stabilized zirconia (YSZ) particles synthesized by Continuous Hydrothermal Synthesis are transferred into nano-inks for thin film deposition. YSZ nanoparticles are synthesized in supercritical conditions resulting in highly dispersed crystals of 10 nm in size. The rheology of the colloid is tailored to achieve inkjet printability (Z) by using additives for regulating viscosity and surface tension. Inks with a wide range of properties are produced. A remarkable effect of nanoparticles on the ink printability is registered even at solid load < 1%vol. In particular, nanoparticles hinder the droplet formation at low values of the printability while suitable jetting is observed at high Z values, i.e. Z ≈ 20. For the optimized inks, we achieve high quality printing with lateral and thickness resolutions of 70 μm and ca. 250 nm respectively, as well as self-levelling effect with a reduction of the substrate roughness. Densification is achieved at sintering temperatures below 1200 °C.     

Inkjetting dielectric layer for electronic applications

Printed electronics is expected to increase its market share significantly in near future. The emerging applications include e.g. display applications, RFID tags, and photovoltaic applications. A benefit of printing is the additive character of the process, which means that material is deposited only the amount that is needed. Digital printing increases flexibility of the process, because circuits are manufactured directly from a digital file, which removes need of fixed masks or patterned screens for each layout. Formation of a multilayer circuitry requires printing of conductive and insulative layers. This paper focuses on printing of a dielectric layer with an inkjet printer. Six sigma DMAIC approach was applied during the process characterization and analysis. The study began by defining the process parameters and evaluating their importance to the outputs. Highest rated parameters were taken into consideration and a design of experiments was established. Measured values were analyzed and it was observed which parameters had the highest effect on the outputs. The results were further verified and it was observed that electrically the printed structures were successful.     

Crack formation and substrate effects on electrical resistivity of inkjet-printed Ag lines

We investigated electrical properties of inkjet-printed Ag lines by varying the substrate, dimension of lines and sintering temperature. The effect of cracks developed in the lines during drying and sintering was also examined. The results show clear evidence of a strong crack and substrate effect on electrical properties of printed Ag lines. The crack formation is clearly dependent on the dimension of lines. Crack network occurs and becomes more pronounced for wider and thicker lines. Even without the crack formation, the resistance and resistivity of lines on glass are higher than those on polyimide due to residual thermal stresses.     

Advances and challenges of ceramic pigments for inkjet printing

In the last decades, the development of new ceramic pigments has been a very pursuit goal, mainly since the emergence of the inkjet technology applied to ceramic tiles. The digital decoration of industrial ceramics has contributed to a notable reduction of pigment consumption and an aesthetical decoration improvement, which makes more flexible the production processes, enhancing the reproducibility and cost savings. The nanopigment requirements of the inkjet technology demand submicronic particle size, ink rheology, stability, drop resolution, etc. Although the up-down procedure by micromilling contributed firstly to the particle size reduction of existing pigments, novel approaches are developed to obtain directly high-quality particles of suitable size with the aim of providing a higher optical efficiency. Thus, in this work, a comprehensive review about the existing crystalline structures, novel compositions, and synthesis methods as well as new coloring mechanisms is addressed, giving an overview of all these advances while considering the four-color process required for inkjet technology. The inkjet technology has the challenge of evolving towards a sustainable technology by eliminating the use of critical raw materials, removing the use of synthesis aid fluxes, and reducing the energy consumption, to approximate to a circular economy which is predominant in this sector.     

Thin TiO2 films prepared by inkjet printing of the reverse micelles sol–gel composition

The presented study deals with the adaptation and optimization of the reverse micelles sol–gel composition to make it feasible for inkjet printing. The solvent choice and viscosity adjustment issues essential for reliable jetting performance are discussed. The layer morphology and material properties were in detail investigated by optical, scanning electron and atomic force microscopy as well as Raman spectroscopy and XRD analysis. Photocatalytic activity of the printed layers was evaluated on the basis of stearic acid thin film degradation monitored by FTIR spectrometry. The photoexcitation properties of the TiO₂ layers and the ability of their photocurrent generation were investigated by electrochemical methods.     

LCD-based color filter films fabricated by a pigment-based colorant photo resist inks and printing technology

Inkjet printing technology has been applied to the fabrication of monochrome color filter films and has afforded an efficient and economic production process for flat panel displays. We demonstrate the preparation and physical characterization of the red and/or green monochrome films on glass substrates by inkjet printing technology with pigment-based colorant resist inks. For the green films, a thickness of 1.45 μm and the color coordinate value of x = 0.309, y = 0.588 and the brightness of Y = 58.90 in CIE 1931 diagram are attained at the optimal ink amounts of 1400 pl for green droplets. For the red colorant films, an appropriate film thickness of 1.20 μm and the coordinate value of x = 0.572, y = 0.320 and the brightness of Y = 27.1 in chromaticity diagram at the red droplets amounts of 900 pl are obtained. The experimental results exhibit the percentages of optical transmittance of the as-printed green films achieve greater than 85.0% in the spectral range of 515.0 nm and those of the as-printed red films in the subpixel cell can achieve 98.0% in the range of 640.0 nm.     

R2R gravure and inkjet printed RF resonant tag

The fabrication of passive circuitry by gravure and inkjet printing is studied. A chipless inductively coupled RF resonant tag is analyzed as a test structure. A floating-bridge layout is employed to provide high yield when fabricated by roll-to-roll (R2R) printing. The conducting first layer and insulating second layer are R2R gravure printed with silver nanoparticle ink and a thermally cross-linkable dielectric ink, respectively. Above 10 MS/m conductivity is obtained for the first layer, which passes three times through the 5 m long drying unit at 5 m/min speed. The floating bridge is inkjet printed with silver nanoparticle ink and the prototype tag is measured over a reading distance of ca. 2 cm. An equivalent circuit model is presented and the model parameters are optimized to obtain a best fit to the measured frequency response. This indirect measurement provides an estimate of 4.3 μm for the thickness of the dielectric layer sandwiched between the conducting top and bottom layers. Application possibilities for the all-printed RF resonant tag are outlined.