| Organic light emitting diode(OLED)devices have become hot research topic because of their flexibility,high contrast-ratio,ultra-thin,roll-to-roll processing.Currently,all the commercial OLED products are fabricated by thermally evaporating in vacuum and achieve good market effect.Compared to the vacuum process,inkjet printing technology has the potential advantages in fabricating large-area and cost-effective display.Firstly,an innovative way to improve linear film's thickness uniformity was discussed.In order to confine the pattern and prevent the leakage current,the emissive ink need to be deposited in the pixel bank during OLED display fabricating.It is easier to achieve higher aperture ratio to print in linear structured substrates than that in traditional dot matrix substrates.The ink tends to climb along the pixel wall due to capillarity during drying in a pixel microstructure,which will result in poor thickness uniformity and inferior devices performance.In order to solve this issue and achieve uniform films,mixed solvents are used to balance the"Marangoni flow" and "Capillary flow".However,in linear structured substrate,ink tends to gather in the middle area due to slow drying rate and longer channel.As a result,the film in middle area is much thicker than that in both ends.In order to obtain flat films in linear direction,a novel linear film printing method has been proposed.Through the printing scheme with variable droplet pitch,the drop spacing at the two ends of the line increases toward the center,so that the ink aggregation in the middle can be suppressed.The film morphology is systematically adjusted from convex to flat through accurately changing the droplet space and number,significantly suppressed the unevenness of display printed with constant droplet spacing.With no need to redesign ink formulation,this novel inkjet technique can be used to achieve uniform films with any ink and on any substrate.Then,the influencing factors on the preparation of electro printing ink were studied,and a high-resolution polymer LED display device was fabricated.For display panel,images quality improves with resolution.Piezoelectric inkjet printer is widely used in the preparation of OLEDs among all the inkjet printing OLED technologies.This kind of printer uses piezoelectric unit to convert electrical energy into vibration energy and then ejects droplets.However,the size of the droplet ejected by this piezoelectric inkjet printer is too large,with a spreading diameter generally above 30 microns.As a result,this kind of printer cannot be used to fabricate high resolution OLED display.The printing technology based on electrohydrodynamic(EHD),is to apply high voltage on the ultra-fine tip,charges the ink in the needle tube,ultrafine droplets or jets that are many times smaller than the tip diameter are formed under the action of an electric field force,greatly reducing droplet size and increasing resolution.This kind of printing equipment have some requirements in inks conductivity,and inks' conductivity directly determine the printing stability of the ink.However,there is rarely reports about using EHD printer to prepare high resolution OLED displays.In this article,we focus on polymer inks'design and printing.Based on blue emitting polymer PFSO,we successfully printed 2 ?m wide linear films using this kind of printing technique.Benefited from small droplet size,the solvent evaporated so fast that there is almost no solvent left until droplet reaches the substrate,which largely weakens the coffee ring effect.What's more,a flat and thick enough film can be achieved by printing multiple times.Finally,a patterned PLED display device with a resolution of 1058 pixel per inch was fabricated.The current efficiency of this PLED fabricated by EHD printing is almost 90%of that fabricated by spin-coating.This EHD printing technology can break traditional printing devices' limitations of resolution,simplify film optimization process,and establish a new way to fabricate high resolution PLED displays. |
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