New Color Patterning Techniques for OLED Displays
Organic light emitting devices (OLEDs) are light emitting devices consisting of a stack of organic semiconductors sandwiched by electrodes. Since the first report of a high-efficiency device by Tang and VanSlyke in 1987, OLEDs have attracted considerable attention particularly for utilization in flat panel displays. This is due to several advantages they offer in terms of power consumption, contrast, response speed, viewing angle, and compatibility with flexible displays. Due to considerable progress, the performance of OLEDs, such as their stability and efficiency, can now meet the requirements of some display products. The commercialization of OLED displays however remains limited and is hampered primarily by manufacturing issues. These include low manufacturing yield, high fabrication cost, and low display quality. Those manufacturing issues are largely attributed to difficulties with the color patterning process; i.e. the fabrication process by which arrays of red, green and blue (RGB) OLEDs can be made side-by-side on one substrate in order to obtain a full-color display. Currently, RGB color patterning is done by sequential vacuum deposition of red, green and blue materials through a pre-patterned shadow mask, typically made of a thin metal sheet in what is widely known as the fine metal mask (FMM) technology. However, the technique has several inherent limitations. These limitations include mask deformation, difficulties in mask-to-substrate overlay alignment, and in making masks with micrometer level dimensional accuracy. These limitations reduce the manufacturing yield, display resolution and display aperture ratio (the ratio of the emissive area to the total surface area of a display). Despite the fact that several approaches for addressing these limitations in OLED displays have been proposed, there is still no commercially viable solution.
In this thesis, two novel color patterning techniques for OLED displays are proposed. One approach utilizes laser-patterned polyimide (PI) sheets as shadow masks. The technique takes advantage of the good processability of PI by direct laser ablation, which makes it possible to cut through it to create slits with very high dimensional accuracies. In addition, being a dry process, the laser-patterning of the PI sheets can be done after they (i.e. the sheets) are already stretched and mounted on the metal holder, and thus avoids post-patterning deformation of the masks by the stretching step. The use of laser also makes it possible to pattern the PI sheets after they have already been mounted on the TFT backplane substrates and thus, in a variant of this technique, allows creating the shadow mask in-situ. This in-situ shadow mask patterning technique can be expected to offer further accuracy advantages since the slits are created by laser over the desired TFT locations directly and thus eliminates the need for a subsequent mechanical alignment step. Such approach can be particularly useful for RGB OLED patterning on flexible substrates where the poor mechanical and dimensional stability of substrates pose additional challenges for aligning shadow masks accurately.
The other color patterning approach proposed here is based on the diffusion of a luminescent material from a donor substrate into the organic host material layer of the OLED that is pre-coated on the backplane substrate. The RGB color patterning in this case can be done without the use of shadow masks, and thus, the approach offers significant advantages. The use of a pre-patterned micro stamp as the donor substrate allows the physical contact between the two substrates to be selectively limited to certain areas hence allows limiting this diffusion to only certain areas of the OLED substrate. Such selective diffusion can also alternatively be done through local heating via electric currents, utilizing for that purpose electrodes in the OLED backplane. This eliminates the need for patterned stamps and mechanical alignment.
In this work, these two techniques are introduced and tested with the purpose of assessing their ability to be used for RGB patterning. As part of this investigation, the effect of using these techniques on OLED performance is also studied. Finally, a first-proof-of-concept utilization of the techniques for producing RGB OLEDs fabricated side-by-side on one substrate is demonstrated.