Inkjet Printing PLED Display And Related Studies

Organic light emitting diode(OLED) displays have the advantages of low energy consumption, wide view-angle, high switching speed and high contrast, that it shows great potential in the display market. Polymer light emitting diode(PLED) displays have drawn more attention due to their adjustable colors in the whole range of visible light, easy solution-processing, flexibility and large area fabrication. Spin-coating method is undoubtedly the most convenient solution process with good film quality. However, it is normally used for fabricating monochrome displays, but not for patterning polymer materials of different colors on a single substrate to realize full-color display. Furthermore, spin-coating is also material-wasting. Among other patterning technologies for polymer materials, inkjet printing has many advantages such as simple and efficient processing, high material utilization, easy integration into Roll-to-Roll processing and high automation. Thus it is particularly suitable for fabricating large-size, flexible displays with high through-put. Inkjet printing is considered to be one of the most hopeful technology for future PLED fabrication.The deposition of high quality thin films is critical in fabricating PLEDs. However, in inkjet printing, a ring-shaped film is often formed on the substrate, known as the “coffe ring” effet. With the objective of printing good quality thin films of a blue light-emmitting polymer, which is named as PFSO, we have systemically studied how the solvent type, ink concentration, substrate temperature and other printing parameters affect the final film morphology. Experimental results show that these factors significantly influence the contact line pinning time and the capillary flow intensity inside the droplet after contact line pinning. Delay or restrain the pinning of contact line pinning is advantageous to the formation of dot-like film. After the contact line pinning, the solute can be deposited uniformly only when solvent evaporation rate is in conformity with the solute migration rate, otherwise the “coffe ring” forms. Ultimately, by optimizing the solvent type and ink concentration, we have achieved homogeneous films by printing single-solvent ink on non-patterned susbtrates.In the process of inkjet printing, different factors such as droplet stability, depositon accuracy, film uniformity and device structure are critical in fabricating high performance PLED displays. By carefully adjusting the printing parameters, stable droplets are successfully deposited into the pixels with high accuracy without overflow. Furthermore, we have simplified the device structure and printing process by designing line-shaped pixels. The film uniformity in pixels, as a result, is improved significantly. Together with the thickness optimization of the buffer layer and light-emitting layer, the efficiency of the printed devices is greatly improved, approaching that of the reference devices. By scaling up our process, we successfully fabricated a 3-inch blue PLED display of 120 ppi without defects.The wettability of liquid-solid interface is an important scientific problem faced by all-solution processing of displays, which can ultimately affect the uniformity and resolution of inkjet-printed thin films. Regulation of the wettability at liquid-solid interfaces is not only interesting in the perspective of scientific reseach, but is also useful in practical applications. One of the extreme case of wettability and that has attracted many attention in recent years is the phenomenon of superhydrophobicity on solid surfaces. Based on an all-solution process, we fabricated a free-standing, flexible, transparent, and fluorescent superhydrophobic composite film. By regulating the hierarchical flowerlike micro/super-nanostructures, which are self-assembled from compound 1, droplets on the film is translated from the Wenzel state to the Cassia state. The as-prepared composited film shows superhydrophobicity with a CA above 150 o, and is robust to water of the whole pH range. Moreover, the composite film exhibits good environmental stability. The fluorescence from the composite film extends the potential application of the film into the optoelectronic field.