Investigation of organic light-emitting diodes fabricated by close-space sublimation

Abstract

For many consumer electronics applications ranging from smartphones to TVs, organic light-emitting diode (OLED) is the display technology of choice because of its self-emissive property, low power consumption and unique physical form factor. However, despite OLED’s widespread appeal in the marketplace, OLED displays are relatively expensive largely because the manufacturing process, based on the conventional vapor thermal evaporation (VTE), has remained costly. Huge capital expenditure and low material utilization yield associated with VTE are the major cost factors. Increasingly, ultra-high resolution OLED displays are needed for advanced applications, which demand more stringent requirements for manufacturing and thus even higher costs. An alternative deposition method to VTE is close-space sublimation (CSS), which has been previously explored as a cost-effective method in terms of shorter processing time, higher material utilization, and lower capital cost due to the compactness of the equipment. Although CSS is conceptually simple involving the pattern-wise transfer of OLED materials from a donor sheet to a receiver substrate, it has yet to be fully established as a manufacturing process because the nature of the transfer process and the property of the transferred organic films have not been sufficiently characterized.

This thesis focuses on the development of a CSS process to produce efficient and stable OLED devices. Major efforts are devoted to the production and characterization of CSS deposited thin films, primarily for the emitting layer. Two different CSS methods were designed for the transfer of the pre-coated organic film on a donor using either resistive or radiation heating, with the former primarily for the characterization of CSS-deposited film and the latter for device fabrication. The morphological, photoluminescent, thermal and electrical properties of CSS-deposited films as a function of CSS process conditions were evaluated. In general, CSS-deposited films can be produced with characteristics comparable to the reference films deposited by VTE. Among a large set of materials evaluated for CSS in this study, several materials were found to exhibit endothermic phase transition behaviors, revealing metastability in as-evaporated VTE films. Furthermore, CSS offered a simple method for the determination of the enthalpies of sublimation and vaporization for organic thin films.

Blue OLEDs were fabricated with a host-dopant EML deposited by CSS with the transport layers deposited by VTE. The performance of these devices was characterized in relation to the composition and morphology of the EML, which can be substantially altered by the CSS process, including the ambient conditions. Achieving uniformly doped EML films turned out to be rather difficult, however gradient-doped EML films were readily produced with CSS by carefully selecting the host-dopant combinations. Blue OLED devices with a gradient-doped EML can realize a slightly higher efficiency with a lower drive voltage and a longer lifetime, compared to reference devices with a uniformly doped EML.

The effects of the ambience of the CSS deposition chamber on OLED degradation were fully evaluated. We found that water present in the donor is detrimental to the OLED lifetime. Upon pre-treatment on the donor, such as heating and HF treatment, moisture accumulated on the donor can be completely eliminated. An in-situ donor preparation was also demonstrated in achieving a long lifetime CSS device without the donor exposure to the laboratory ambient. We also uncovered a serious cause for lifetime degradation in our OLED devices - exposure to volatile chlorinated contaminants present in the deposition chamber. This finding was affirmed by deliberately introducing HCl as the contaminant in the CSS deposition chamber during device fabrication.

CSS deposition with fine shadow masking was simulated to illustrate the achievable patterning capability in the fabrication of high-resolution OLED displays. Our simulation result shows that in order to produce an OLED display with a resolution greater than 2000 ppi, an ultra-thin shadow mask with a thickness of less than 2 µm is needed. We also demonstrated that such a shadow mask can be made using silicon nitride membrane.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Ching Wan Tang (Supervisor)